Note: Descriptions are shown in the official language in which they were submitted.
CA 02787375 2013-07-12
METHOD AND SYSTEM FOR THE SMALL-SCALE PRODUCTION OF LIQUIFIED
NATURAL GAS (LNG) AND COLD COMPRESSED GAS (CCNG) FROM LOW-
PRESSURE NATURAL GAS
TECHNICAL FIELD
[0002] The present invention relates generally to the compression,
refrigeration and liquefaction
of gases, and more particularly to the liquefaction of a gas, such as natural
gas, on a small scale.
BACKGROUND
[0003] There are no commercially viable Small-Scale liquefied natural gas
("LNG") production
facilities anywhere in the world. "Small-Scale" means less than about 10,000
liters/day. Thus,
any existing liquefied natural gas-fueled fleet must depend on deliveries by
tanker truck from
larger-scale LNG plants or from LNG import terminals. The use of tanker trucks
or terminals
increases the cost of the LNG to the end user, because the delivered price
must include the
substantial cost of transporting the LNG from the production or import
location to the customer.
Those transportation costs tend to outweigh the lower production costs of
large-scale LNG
manufacture, where there is a large distance between the LNG source and the
customer. Also, the
transport of LNG from production or import source to end-users requires that
the LNG be as cold
as possible so as to avoid "boil off' (and losses through pressure relief
valves) during the
transport process. Thus, the LNG needs to be produced at its coldest practical
temperature, say,
about 260 F, rather than at warmer temperatures, requiring more energy input.
When LNG is
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dispensed as compressed natural gas ("CNG") to vehicles, at facilities with no
on-site
liquefaction systems, the cold content of the LNG is dissipated in its
conversion (by pumping to
pressure and warming) to CNG, throwing away a significant amount of energy
that was used to
liquefy the LNG at its source. More generally, the standard model for CNG
production and
dispensing (in the absence of an on-site LNG source) requires large
compressors that produce the
CNG on demand, because CNG is not dense enough to allow for any practical way
to store it in
advance of its dispensing to vehicles. Thus, all CNG stations operate on a
"just in time"
production basis, without the ability to produce and store CNG during off-peak
periods. The cost
of "just in time" production is higher because it often includes peak period
"demand charges" for
the electricity used to run the oversized compressors. The present invention
seeks to solve these
and other problems associated with the standard forms of LNG production and
transport, L/CNG
dispensing, and CNG production and dispensing.
[0004] The LNG customer must also maintain a large storage tank so that
deliveries can be
spread out in time. Such tanks produce "boil off' which is generally vented to
the atmosphere,
causing methane emissions and loss of product, further increasing the net cost
of the LNG, to
both the end user and (by way of the emissions) to society at large. Heat gain
to the storage tank,
in the absence of on-site liquefaction, results in LNG that is not the ideal
density for the vehicle's
fuel tank. Re-liquefaction to avoid boil-off or to increase the product's
density is not an option
without an on-site LNG plant.
[0005] Other drawbacks to tanker-delivered LNG include the lack of competition
in the industry,
making the fleet owner excessively dependent on a single supplier. The quality
of the delivered
product may also vary, to the detriment of the fleet that uses the fuel.
[0006] The alternative that is commonly used is on-site Compressed Natural Gas
(CNG)
production, using the local natural gas pipeline as the feed source. However,
such CNG systems
have severe limitations, including the following: CNG, because it is not very
dense, cannot be
stored in large quantities, so it must be made at a high capacity during the
peak vehicle fueling
demand period. Similarly, the on-vehicle storage of CNG is limited by the need
for heavy, high-
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pressure CNG tanks that store relatively little product, compared to the much
denser LNG, and
thus limit the travel range of the CNG vehicle. Also, because of the lack of
CNG storage options,
the typical CNG compressor must be "over-designed" so as to be able to meet
the "just in time"
demand of the local CNG fleet. In other words, if the CNG station is to fill
any significant
number of vehicles, fast enough to compete with standard fueling rates (such
as for diesel
fueling), then the compressor must have a very large throughput capacity, even
if that capacity is
idle during much of the day. The CNG produced is generally warm, due to the
heat of
compression, and must be sent through ambient air coolers to dissipate the
heat gained during
compression. However, that approach still leaves the CNG at some 15-degrees
hotter than
ambient, reaching about 100 F and more. The hotter the CNG, the less dense it
is, limiting the
amount of product that can be dispensed into each vehicle's on-board storage
tank. Moreover, by
operating during the peak fueling demand period, the CNG station is likely
running its large
compressors during the peak electricity demand period, causing it to pay
"demand charges" to
the electric distribution company. The just in time model (without on site
storage) does not allow
for off-peak CNG production.
[0007] The only reason vehicle-grade LNG needs to be produced at the coldest
possible
temperatures is to allow it to "weather" the time it spends in transport
vehicles and storage tanks,
before it is dispensed to the vehicles.
[0008] Therefore, a system for the small-scale production of LNG from low-
pressure pipelines
and stranded wells is needed to overcome the above listed and other
disadvantages of existing
methods of converting low-pressure natural gas to a dense form that is easily
storable and
transportable Also, a method of dense-phase natural gas production, storage
and dispensing is
needed that allows for off-peak production and off-peak power use, and which
results in lower
energy input costs because reduced refrigeration input is required.
SUMMARY
[0009] The disclosed invention relates to a system for the small-scale
production of liquid
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natural gas comprising: a natural gas supply, the natural gas supply being at
a pressure in a range
of about 55 psia to about 350 psia; a prime mover in fluid communication with
the natural gas
supply, and in fluid communication with a third heat exchanger; a multi-stage
compressor in
operational communication with the prime mover; the multi-stage compressor
comprising at
least a first stage compressor, a second stage compressor, and a third stage
compressor, and
where the inlet temperature of fluid entering the first stage compressor is
less than about 40 F,
and where the inlet temperature of fluid entering the second stage compressor
is less than about
40 F; a first inter-cooler in fluid communication with the first stage
compressor; a molecular
sieve in fluid communication with the first inter-cooler and in fluid
communication with the
natural gas supply; a fourth heat exchanger in fluid communication with the
molecular sieve and
in fluid communication with the first stage compressor; a second inter-cooler
in fluid
communication with the second stage compressor; a first heat exchanger in
fluid communication
with the second inter-cooler and in fluid communication with the third stage
compressor; an
after-cooler in fluid communication with the third stage compressor; a second
heat exchanger in
fluid communication with the after-cooler; a main heat exchanger in fluid
communication with
the second heat exchanger, in fluid communication with a phase separator, in
fluid
communication with a gas turbo-expander, and in fluid communication with the
fourth heat
exchanger, where the operational flow rate from the main heat exchanger to the
gas turbo-
expander can be as low as about 1,450 lb/hr during continuous operation; a
first expansion
device in fluid communication with the main heat exchanger; a sub-cooling heat
exchanger in
fluid communication with the first expansion valve;a second expansion device
in fluid
communication with the sub-cooling heat exchanger; a pressure tank in fluid
communication
with the second expansion valve; a four-way valve in fluid communication with
the pressure
tank; the four-way valve in fluid communication with the sub-cooling heat
exchanger and in
fluid communication with the main heat exchanger;the gas turbo-expander in
fluid
communication with the phase separator, and in operational communication with
an expander
driven compressor; the expander driven compressor in fluid communication with
a fifth heat
exchanger; the fifth heat exchanger in fluid communication with second stage
compressor; an
ammonia absorption chiller in fluid communication with the prime mover, in
fluid
communication with the first heat exchanger, in fluid communication with the
second heat
=
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exchanger, in fluid communication with the third heat exchanger, and in fluid
communication
with a cooling tower; a make-up water line in fluid communication with the
cooling tower; and
where the amount of liquid natural gas produced by this system while
continuously running
. during a 24 hour day can be as low as about 6,000 liters per day, where the
system has no more
than two expansion valves; and where the first and second devices are selected
from a group
consisting of a compressor-loaded multi-phase expander turbine, and an
expansion valve.
[0010] The invention also relates to a system for the small-scale production
of cold compressed
natural gas comprising: a natural gas supply, the natural gas having a
pressure in a range of about
55 psia to about 350 psia; a prime mover in fluid communication with the
natural gas supply, and
in fluid communication with a third heat exchanger; a multi-stage compressor
in operational
communication with the prime mover; the multi-stage compressor comprising a
first stage
compressor, a second stage compressor, and a third stage compressor, and where
the inlet
temperature of fluid entering the first stage compressor is less than about 40
F, and where the
inlet temperature of fluid entering subsequent stages of the compressor is
less than 40 F; a first
inter-cooler in fluid communication with the first stage compressor and with a
waste heat driven
chiller; a molecular sieve in fluid communication with the first inter-cooler
and in fluid
communication with the natural gas supply; a fourth heat exchanger in fluid
communication with
the molecular sieve and in fluid communication with the first stage
compressor; a second inter-
cooler in fluid communication with a waste heat driven chiller and the second
stage compressor;
a first heat exchanger in fluid communication with the second inter-cooler, a
waste heat driven
chiller and in fluid communication with the third stage compressor; an after-
cooler in fluid
communication with the third stage compressor and with a waste heat driven
chiller; a second
heat exchanger in fluid communication with the after-cooler and with a waste
heat driven chiller;
a main heat exchanger in fluid communication with the second heat exchanger,
in fluid
communication with a phase separator, in fluid communication with a compressor-
loaded gas
turbo-expander, and in fluid communication with the fourth heat exchanger,
where the
operational flow rate from the main heat exchanger to the gas turbo-expander
can be as low as
about 1450 lb/hr during continuous operation; a first expansion device, such
as a throttle valve or
compressor-loaded multi-phase expander, in fluid communication with the main
heat exchanger;
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a sub-cooling heat exchanger in fluid communication with the first expansion
valve or
compressor-loaded multi-phase expander; a pressure tank in fluid communication
with the
second expansion valve; a four-way valve in fluid communication with the
pressure tank; the
four-way valve in fluid communication with the sub-cooling heat exchanger and
in fluid
communication with the main heat exchanger; the gas turbo-expander in fluid
communication
with the phase separator, and in operational communication with an expander
driven compressor;
the expander driven compressor in fluid communication with a fifth heat
exchanger; the fifth
heat exchanger in fluid communication with one of the stages of a multi-stage
natural gas
compressor; an ammonia or lithium bromide absorption chiller or an adsorption
chiller in fluid
communication with the prime mover, in fluid communication with the first heat
exchanger, in
fluid communication with the second heat exchanger, in fluid communication
with the third heat
exchanger, and in fluid communication with a cooling tower; a make-up water
line in fluid
communication with the cooling tower; and where the amount of cold compressed
natural gas
produced by this system while continuously running during a 24 hour day can be
as low as the
liquid equivalent of about 6,000 liters per day, and where the system has no
more than two
natural gas expansion devices.
100111 In addition, the invention relates to a method of dispensing natural
gas from stored cold
compressed natural gas, the method comprising: dispensing cold compressed
natural gas from a
cold compressed natural gas storage tank, with or without pumping it with a
cryogenic liquid
pump to a higher pressure; pumping the cold compressed natural gas by a
cryogenic liquid pump
to a pressure suitable for compressed natural gas dispensing and storage in on-
vehicle
compressed natural gas storage tanks; recovering cold from the cold compressed
natural gas by
heat exchange with natural gas feeding the natural gas production plant to
replace dispensed
product, such that the incoming, relatively warm, feed-gas warms the pumped-to-
pressure cold
compressed natural gas to a temperature of about -20 F to about 30 F, thus
converting it from
cold compressed natural gas to compressed natural gas; where the refrigeration
content of the
outbound cold compressed natural gas is used to reduce the refrigeration
needed to convert the
incoming feed gas to more cold compressed natural gas or liquid natural gas;
where the now
warmed gas stream (formerly cold compressed natural gas) is cooler than
standard compressed
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natural gas but can be stored in standard, non-cryogenic, on-board vehicle
fuel storage tanks;
thus allowing for a compressed natural gas dispensing facility that can
achieve storability and
off-peak production, and yielding a cooler than normal, and thus denser
dispensed compressed
natural gas, allowing for existing, standard on-vehicle compressed natural gas
tanks to take away
more product (as measured in pounds per cubic foot of fuel tank capacity),
then is achievable
with standard compressed natural gas at the same pressure but as warm as about
100 F.
[0012] Also, the invention relates to a system for the storage, transport, and
dispensing of natural
gas, comprising: means for handling natural gas in a cold compressed natural
gas state where the
natural gas is a non-liquid, but is dense-enough to allow for pumping to
pressure by a cryogenic
liquid pump; a means for optimally balancing the compression and refrigeration
input required to
produce the cold compressed natural gas; and a means for putting the natural
gas into a cold
compressed natural gas state without first putting the natural into a
cryogenic liquid state which
is subsequently pumped to a higher-than critical pressure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The present disclosure will be better understood by those skilled in
the pertinent art by
referencing the accompanying drawings, where like elements are numbered and
labeled alike in
the several figures, in which:
[0014] Figure 1 is a portion of a process diagram of the system;
[0015] Figure 2 is the remainder of the process diagram of the disclosed
system;
[0016] Figure 3 is a flow chart illustrating one embodiment of the disclosed
method;
[0017] Figure 4 is a detailed process flow diagram of one embodiment of the
disclosed method
for the production of LNG;
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[0018] Figure 5 is a process flow diagram of another embodiment of the
disclosed method for
the production of LNG;
[0019] Figure 6 is a process flow diagram of one embodiment of the disclosed
method for the
production of CCNG;
[0020] Figure 7 is one embodiment of the disclosed method for the production
and dispensing of
CNG by way of LNG or CCNG production and storage;
[0021] Figure 8 is a Phase Diagram of methane, which is an analog for the
phase diagram of
natural gas;
[0022] Figure 9 is a flowchart showing one method of the invention; and
[0023] Figure 10 is a flowchart showing another method of the invention.
DETAILED DESCRIPTION
[0024] The disclosed process provides a means to produce, at small-scales, LNG
at or near the
vehicles that will be served by the facility. With on-site liquefaction
inherent in the disclosed
process, the LNG product need not be as cold as the LNG produced at distant,
large-scale
production plants. "Warmer" LNG requires less energy input than colder LNG,
and LNG made
at (6r near) the vehicle fleet it serves will require less energy input for
transporting the product.
Similarly, if the main customer base is CNG vehicles, then the LNG used to
dispense CNG (that
system being known as L/CNG) need not be any colder than required for
adequately storing and
pumping the LNG to the pressure needed for CNG dispensing.
[0025] The inventor, who is an expert in this field, is not aware of any
existing, commercially
viable Small-Scale LNG plants anywhere in the world and is not aware of any
CCNG
production, storage or dispensing systems or of a CNG dispensing systems that
includes CCNG
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production and storage. The smallest LNG plant that he is aware of, in the
state of Delaware in
the US, produces approximately 25,000 gallons (95,000 liters) per day. By
contrast, the proposed
invention will be viable at a production rate of only about 6,000 liters per
day. That "small-scale"
is an essential component of the business model for the invention, namely that
it will provide
vehicle grade LNG to a medium-sized bus or truck fleet, without requiring that
a portion of the
plant's output be shipped to a second and third, off-site fleet. In short,
each small-scale LNG
plant can act as an "appliance" that serves a single customer at a single
location. Such small-
scale LNG plants will also allow stranded gas fields (those not near
pipelines, or too small for
pipeline extensions) to be developed, allowing the produced LNG to be sent to
off-site customers
or to distant pipelines for re-gasification. Also, the invention allows for a
wide range of "LNG
products" from cold LNG (about -245 F and colder), to warm LNG (between
approximately -
160 F to about -240 F), and to CCNG, which is dense-phase non-liquid state
of natural gas that
is colder than natural gas' critical temperature and is at a higher pressure
than its critical
pressure. That range of cryogenic natural gas conditions will have a density
of approximately 15
to 20 pounds per cubic foot for CCNG and above about 25 pounds per cubic foot
for LNG that is
at about -245 F and colder, with variation that depends on the methane and
other hydrocarbon
content of the natural gas.
100261 CCNG is more than a "supercritical" phase of natural gas, with the
single attribute of
having a higher-than critical pressure. CCNG has the second attribute of being
colder than the
critical temperature of natural gas. It is those two attributes, together,
that achieve its relatively
high density (allowing it to be viably stored, much like LNG in readily
available cryogenic
storage tanks), and, most importantly, achieving the densities that allow that
non-liquid (more
than supercritical) phase of natural gas to be pumped to a higher pressure by
standard cryogenic
liquid pumps, as though CCNG were a liquid.
100271 CCNG is not a liquid but will behave much like a liquid, allowing its
pressure to be
raised not only by compression (as is normally used for vapors) but also by
pumping, as is used
for all liquids. The pumping of liquids requires significantly less energy
input than the
compression of gases because liquids are virtually incompressible, allowing
almost all of the
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energy input to accrue to raising the pressure of the liquid. CCNG is
sufficiently incompressible,
much like a liquid, to allow for efficient pumping. Thus a significant benefit
of CCNG is the
ability to raise its pressure (for example, for dispensing as CNG) by merely
pumping it. An
equally important benefit of CCNG is that the energy input required to produce
it is lower than
the energy input required to produce standard LNG, which is the standard form
of dense-phase,
storable and pump-able natural gas. (At temperatures as cold or colder than
about -150 F and as
the pressure of the LNG is raised above about 700 psia, it becomes CCNG.
However, that
methodology of producing CCNG, by pumping a liquid, requires more energy input
than the
methodology disclosed below.)
[0028] The common aspects of the wide-range of cryogenic natural gas
conditions are the
increased density, when compared to pipeline gas and to CNG, and the ability
to pump such
moderate-pressure cryogenic natural gas to any desired outflow pressure from
the storage
container, using standard cryogenic liquid pumps. Those attributes of
storability and "pump-
ability" are the main attributes of standard LNG. However, the present
invention achieves those
attributes at many warmer (and higher-pressure) conditions than for standard
LNG. Those
warmer and higher-pressure conditions require significantly less energy input
than standard (cold
and low-pressure) LNG, because cryogenic processes are more energy "sensitive"
to the depth of
refrigeration than to the pressure under which the gas is refrigerated. Thus,
the present invention
discloses the novel use of CCNG as a phase of natural gas suitable for the
production, storage,
transport, and dispensing of a variety of dense-phase natural gas products,
including (but not
limited to) vehicle-grade fuels. We say "phase," rather than "state" or
"condition," because
CCNG can be identified on a phase diagram of methane (and natural gas) shown
at Figure 8.
Figure 8 locates the CCNG range on the phase diagram for natural gas.
[0029] "Pump-ability" is an important attribute of cryogenic methane because
often the stored
cryogenic methane is dispensed as high-pressure, near-ambient CNG at pressures
of
approximately 3,000 to about 3,600 psia. Pumping LNG to such pressures, at
L/CNG dispensing
sites is routine, but is often wasteful of the refrigeration content of the
LNG if there is no on-site
liquefaction equipment or other cold recovery options. By contrast, the
disclosed method allows
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for the pumping of non-liquid CCNG, and includes cold recovery, as illustrated
in Figure 7 and
described in more detail below.
[0030] It should be noted that some cryogenic liquid pumps would easily
tolerate the
approximately 700-psia inlet pressure that is required for the pumping of
CCNG. Other pumps,
that can only tolerate, say, about 300 psia inlet pressures, can be used to
pump CCNG if the
CCNG is first expanded down to about 300 psia (causing most of it to become a
liquid), and
where that two-phase product of expansion is sent through a commonly available
phase-
separator. The smaller, vapor portion of that expansion can be further
expanded down toward
atmospheric pressure, producing more mostly liquid (suitable for pumping) and
some vapor.
Alternatively, the vapor portion of the first expansion and separation can be
returned (cold) to the
VX Cycle for re-compression. Thus, there are several practical and widely
available techniques
for pumping CCNG, much like a liquid, to any desired higher-pressure.
[0031] The ability to economically produce vehicle-grade LNG, CCNG or CNG
dispensed from
stored LNG or from CCNG will be achieved by at least two aspects of the
invention: a) low
capital costs, and b) high-efficiency. In one embodiment, the disclosed method
offers, in a single
deployment, the option of producing LNG, CCNG and CNG. LNG and CNG have an
existing
and growing vehicle fuel market as well as other non-vehicular uses. At the
moment, the benefit
of CCNG is that it is less costly to produce than LNG, but can be dispensed as
a liquid (as
discussed above) or, after cold recovery, as CNG. The dispensing and on-
vehicle storage of
CCNG as vehicle fuel is a plausible near term concept that only depends on
certifications by US
DOE and/or other such agencies, of the use of on-vehicle, composite, cryogenic
pressure vessels
(such as those that rely on outer wrappings of carbon fiber and other high-
strength fabrics),
which will tolerate the about -150 F and colder and about 700 psia and higher
pressure
conditions of CCNG. Thus, the present invention offers an entirely new form of
on-vehicle fuel ¨
CCNG ¨ that will have nearly the density of LNG, but which will not "boil off'
because, as a
single-phase fluid, any heat gain will only cause its pressure to rise. As
such, an appropriately
designed CCNG vehicle fuel tank will be lighter than an LNG tank, will not
require space above
the liquid for vapor to form, and will contain the product indefinitely,
without "burping"
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methane.
[0032] The invention will allow an about 2,000-gallon/day LNG/CCNG plant to be
constructed
for less than about $2,000,000 The innovative LNG production cycle will yield
approximately
83% LNG/CCNG out of every unit of natural gas that is delivered to the plant
from the local
low-pressure pipeline or stranded well, with only approximately 17% of the
natural gas used as
fuel for the prime mover. That combination of relatively low capital cost and
low fuel use (high-
efficiency) will yield an operating cost and "price per liter/gallon" that
will allow the
LNG/CCNG (or CNG that is dispensed from the stored LNG/CCNG) to be sold at a
discount to
the market price of diesel, accounting for the energy content (BTU) both
fuels.
[0033] That achievement ¨ competitively priced LNG/CCNG/CNG ¨ will allow
natural gas to be
more than just an "alternative fuel" but also an economically viable
alternative fuel.
[0034] The attached process flow diagrams illustrate the invention, which is
known as the
disclosed system. The invention is a unique and innovative variant of the
methane expansion
cycle, which to date, has only been deployed commercially in certain special,
large-scale
configurations, specifically known as "letdown plants". Thus, the system
described here is also
known as Vandor's Expansion Cycle or the "VX Cycle". It should be noted that
the definition of
CCNG offered above was included in US 7,464,557 B2, which was co-invented by
the inventor
of the presently disclosed method. Figure 8 shows the "position" of CCNG on a
phase diagram
for natural gas.
[0035] The disclosed method and system assumes that a low-pressure natural gas
pipeline or
stranded well is available adjacent to the fleet that will use the liquefied
natural gas; that the
natural gas is delivered at a pressure of about 60 psia or greater; at a
temperature of
approximately 60 F; and with a chemical composition that is about 95%
methane, with some N2
and CO2, but otherwise "clean". In the event that the pipeline gas is not as
clean, there are
several known clean up systems that can be integrated with the disclosed
method and system. In
the event that the gas source is at a lower-than about 60-psia pressure, a
small booster
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compressor can be used to raise its pressure, prior to entry into the main
compressor.
Alternatively, the first stage of the main compressor can receive the feed gas
at whatever
pressure above atmospheric that is available.
[0036] The low-pressure pipeline (or stranded gas well) stream is separated
into a fuel stream
that provides fuel to a natural gas fired "prime mover", such as an internal
combustion engine or
a micro-turbine, and into a product stream to be compressed and liquefied. The
use of natural gas
as a fuel in a prime mover (an internal combustion engine or gas turbine) is
well understood and
is not claimed as an innovation. In contexts, such as California, where it may
be difficult to
obtain a permit for a natural gas fired prime mover, the disclosed method can
function with a
motor drive, with electricity delivered by the electrical grid. In that
embodiment, the waste heat
that would drive the chiller would be limited to the heat of compression that
is produced in the
multi-stage compressor. Depending on the configuration of the compressor,
including the
number of stages, the outflow stream from any single compressor stage may be
hotter than about
280 F, which is more than adequate to drive a chiller that can produce
worthwhile low grade
(approximately 42 F) refrigeration.
[0037] The first step in the liquefaction process is the removal of CO2 and
any water from the
pipeline gas stream, in a multiple vessel molecular sieve, which requires
periodic regeneration,
where the regeneration gas (loaded with CO2) is sent to the prime mover for
use as fuel. This
step is well understood in the industry and is not claimed as an innovation.
The cleaner the
pipeline gas the less complex the molecular sieve system and the less frequent
the need for
regeneration. Alternatives to molecular sieves include membrane separation
technology and
refrigerated methanol clean up systems. The disclosed method is neutral as to
which CO2 and
water removal method is optimal for the particular scale and location at which
the invention is
deployed.
[0038] The cleaned, dry natural gas is sent to a multi-stage natural gas
compressor, such as
might be used at CNG stations, but likely smaller, because it will be
operating 24-hours per day
at a steady rate, rather than in the "just in time" mode of most CNG
compressors. A novel aspect
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of the disclosed method and system is the use of a CNG station and/or standard
CNG equipment
to produce liquefied natural gas or CCNG, allowing for the upgrading of
existing CNG stations,
yielding an operating mode that includes off-peak production, on-site storage,
fast fill during
vehicle fueling, and the dispensing of a wider range of natural gas products,
all of which are
colder and denser than standard CNG
[0039] The disclosed method and system will allow existing CNG stations to be
upgraded to
LNG/CCNG production, by using the existing CNG compressors; and it will allow
makers of
existing CNG equipment to participate in the expansion of the vehicle-grade
LNG industry.
Thus, a widely deployed small-scale LNG/CCNG network need not displace all
existing, well
established CNG production and dispensing facilities, allowing for a smooth
transition from low-
density CNG to high-density LNG/CCNG, including the continued dispensing of
CNG, say, to
light-duty vehicles, but where that CNG is as cool as can be tolerated by
existing CNG fuel tanks
(say, about -20 F) as compared to standard CNG which is almost always above
ambient, say, at
about 100 F. In other words, disclosed method allows for the stored LNG/CCNG
to be
dispensed as high-pressure CNG but at cooler temperatures than standard CNG,
resulting in a
denser product delivered to the on-vehicle fuel tank than can be accomplished
with standard
CNG dispensing. Note that the "cold content" of the stored LNG/CCNG does not
need to be
dissipated before it is dispensed to the non-cryogenic on-vehicle fuel tanks.
Rather, the outbound
cryogenic LNG/CCNG is heat exchanged with incoming feed gas, warming the
outbound,
pumped-to-pressure LNG/CCNG to temperatures acceptable by the on-vehicle fuel
tank, and
thus pre-cooling the inbound natural gas feed stream to the VX Cycle
equipment. That aspect of
the disclosed system/method allows for the optimal temperature and density of
the CNG but
without wasting the refrigeration that was used to achieve the storability and
pump ability of the
LNG/CCNG.
[0040] The feed gas to the LNG plant will be compressed, in stages, from,
about 60 psia to about
400 psia. That choice is an essential feature of the invention because
pressures to about 3,600
psia are routinely provided by most CNG compressors. Operating a CNG
compressor at lower
pressures will reduce the compressor's workload and reduce the "heat of
compression" that is
14
CA 02787375 2012-08-22
absorbed by the natural gas. In some embodiments of the disclosed
system/method, especially
where the optimal product is CCNG at about -150 F and colder and stored at
about 700 psia and
greater pressure, the feed gas may be compressed to above about 700 psia. That
increase in
compression work is a relatively minor manner when compared to the energy
savings of not
having to chill the natural gas down toward about -260 F, because for each
degree of lowered
temperature, the energy input required is exponential. By contrast, increasing
the pressure of the
gas from about 400 psia to about 700 psia, a less than about 2:1 pressure
increase, requires only a
modest extra amount of energy input.
[0041] The disclosed system has a preferred compression range of about 375
psia to about 710
psia, yielding a unique balance between compressor work in the front end and
refrigeration
output at the back end of the cycle. Note that the about 710 psia compression
range is required
only when CCNG is the optimal product. If warm LNG is the product, the lower-
pressure range
(about 400 psia) is adequate. Thus, each embodiment and deployment of the
present invention
will be calibrated to balance the refrigeration and compression input required
to produce the
desired product. That front-end compressor work includes the compression of a
low-pressure
recycle stream, whose pressure is directly related to the expansion of the
about 400-to- about
700-psia natural gas stream to approximately 18 psia during the refrigeration
process.
[0042] The single CNG compressor will perform two functions. It will be both
the feed gas
compressor and the recycle compressor. This is possible because the disclosed
method and
system is an "all methane" cycle, where the working fluid (refrigerant) and
the feed stream are
both methane. Both streams will be compressed simultaneously in a single CNG
compressor.
This is a major advance in LNG production, because the only LNG plants that
use methane
cycles are letdown plants, generally found at pipeline gate stations that
serve large urban areas.
However, letdown plants (by definition) do not require compression because
they rely on high-
pressure feed gas, and have the opportunity to send out large quantities of
low-pressure natural
gas into local low-pressure pipelines.
[0043] The disclosed method and system will use a uniquely integrated chiller
to counteract the
CA 02787375 2012-08-22
heat of compression and to pre-cool the CNG immediately after it exits the
compressor's last
stage after-cooler. That unique use of a well-established technology
(absorption/adsorption
chilling) is a second innovation of the invention, and is described in more
detail below. In this
disclosure the word chiller shall mean any non-mechanical chiller, such as an
ammonia
absorption chiller, lithium bromide absorption chiller, desiccant-based
adsorption chiller, all of
which are driven by waste heat rather than a motor.
[0044] Another novel aspect of the disclosed method and system is that the
heat of compression
will be mitigated, and the natural gas will be pre-cooled by refrigeration
from a chiller powered
by waste heat from the prime mover. In some embodiments of the disclosed
method/system, the
higher-grade portion of the heat of compression (from approximately 150 F to
above about 280
F) is used to partially drive the chiller. Any remaining low-grade heat of
compression contained
in the gas stream is then dissipated in an inter- or after-cooler, prior to
further chilling by the
refrigerant produced at the chiller. Thus, the inlet temperature to each stage
of compression
(including the first stage) can be cooler than would be possible with inter-
coolers alone. Those
inlet temperatures can be reduced to at least about 50 F, and preferably down
to about 30 F,
substantially reducing the workload on each stage of compression.
[0045] The CNG compressor's inter-coolers (between stages) and after-cooler
will be integrated
with the chiller as outlined above. Thus, the gas streams that enter each
stage of compression can
be as cool as about 30 F (or colder), increasing the density of the gas and
reducing the workload
on each compressor stage. (No freezing of the gas will occur because water and
CO2 are
removed prior to compression.) Also, the inter-cooler between the first and
second stage of the
multi-stage compressor will heat exchange the CNG stream with the colder
recycle stream,
chilling the CNG on its way to the second stage, and warming the recycle
stream on its way to
the first stage. This is an example of cold recovery from the low-pressure
recycle stream that
leaves the heat exchanger at approximately -30 F.
[0046] The inter-cooler between the second and third stage will be cooled by
the refrigeration
output of the waste-heat driven chiller. The same chiller will cool the CNG
stream in the
compressor's after-cooler, and in a subsequent heat exchanger, down to as cold
as about -22 F.
16
CA 02787375 2012-08-22
[0047] The chiller will be "powered" by the waste heat from the prime mover,
recovering a
significant portion of the approximately 67% of the energy content of the fuel
used by the
engine/turbine that is normally "wasted" by the engine's exhaust and water
jacket or in the
turbine exhaust. That recovered heat will increase the about 32% - 35% thermal
efficiency of the
engine/turbine to a practical efficiency of approximately 43%, through the
refrigeration output
from the absorption chiller. In some embodiments, a portion the refrigeration
output of the chiller
can be used to pre-cool the inlet air to the turbine that drives the cycle,
thus improving the
efficiency of the turbine. The disclosed method and system seeks to use any
recovered
refrigeration at the earliest possible place in the cycle, reducing workload
as soon as possible so
that energy saving cascades through the process. Thus, when a turbine is the
prime mover, the
chiller's refrigeration output will first be used for cooling the inlet air to
the turbine. Any
remaining refrigeration will be used to cool the inlet gas streams to each
compressor stage, with
any remaining deep refrigeration used to cool the last stage outflow gas,
prior to its entry into the
main heat exchanger.
[0048] The integrations between the chiller and the compressor, as outlined
above, will allow the
"heat of compression" to be mitigated in each stage of the compressor and/or
used to drive the
chiller, improving the compressor's efficiency and allowing the CNG to exit
the compression
cycle pre-cooled to as low as about -22 F.
[0049] The pre-cooled CNG (at between approximately 400 psia and about 700
psia) will then
be sent to a heat exchanger where it is further cooled, condensed, and (after
several steps outside
the heat exchanger) is sub-cooled and liquefied to produce liquefied natural
gas, which will be
sent to a cryogenic storage tank at an appropriate pressure (about 65 psia)
and a temperature of
approximately -245 F. Alternatively, the approximately 700 psia natural gas
is cooled to only
about -150 F (or slightly colder) and is stored in a cryogenic storage tank
at that pressure, as
CCNG. As such, the cryogenic "product" of the disclosed method/system is dense
enough (at
approximately 15 pounds per cubic feet) for storage, and suitable for pumping
to any desired
pressure by standard cryogenic liquid pumps, even though the CCNG is not a
liquid. The
17
CA 02787375 2012-08-22
optimum pump choice, especially as to the inlet pressures to the pump, will be
determined by the
cost and efficiency of available equipment by various pump makers. As
discussed above, some
pumps will tolerate higher inlet pressures, while others will require a two-
step approach that first
expands the CCNG to a lower pressure, causing much of it to become a pump-able
liquid, with
the remaining vapor either returned for re-compression or expanded again.
[0050] The chiller will improve the cycle efficiencies in two ways. First, it
will cool the
compressors second-stage inlet stream. Second, it will reduce the "warm end
loss" of the heat
exchanger, turning it into "warm end gain".
[0051] The cooling of the compressor inlet streams will result in
approximately an about 10%
reduction in compressor power usage. This feature alone will increase the
efficiency of the prime
mover from, about 33% to about 36.5%, or approximately 10 kW.
[0052] The chilling of the compressed feed gas will significantly reduce the
stream's heat
content (enthalpy), compared to the heat content of the returning low-pressure
stream. That will
happen because the feed gas will be compressed to nearly about 400 psia, in
one embodiment,
where its behavior is "non-ideal" (similar to a liquid's behavior), while the
low-pressure recycle
stream (at about 18 psia) will behave in a nearly "ideal" manner. Those
conditions will reduce
the expander's refrigeration requirement by approximately 15%, reducing power
demand by
another about 15 kW.
[0053] The total power reduction achieved (10 kW + 15 kW = 25 kW) for the
production of
LNG equals about 20 %. At the scale of the disclosed method and system, that
power reduction
is important. The power required for CCNG production, will be further reduced
by another
approximately, 25%.
[0054] Another novel aspect of the disclosed method and system is that the
three main
components of the "front-end" -- the engine, the chiller, and the CNG
compressor -- will be
linked, each to the other two components, allowing standard CNG equipment to
produce cold,
18
CA 02787375 2012-08-22
moderate pressure CNG which is then further chilled to produce LNG or CCNG
[0055] The disclosed method and system, unique among LNG cycles, will harness
the CNG
compressor's power source for the chilling of the CNG. The same engine or
turbine that powers
the CNG compressor will (through waste heat) power the chiller. Also, the
disclosed method and
system is unique among LNG cycles in that it can produce CCNG, which has many
of the same
attributes as LNG (storability, transportability, pump-ability) but requires
significantly less
energy input.
[0056] That integration of the prime mover, chiller and compressor is
unprecedented for a
variety of reasons, including because all other commercial-scale LNG cycles
are not dependent
on the compression of low-pressure gas to CNG, and the subsequent condensing
and liquefaction
by expansion of the same (cooled) CNG.
[0057] The disclosed system exploits the limitations of low-pressure methane
compression-to-
expansion, without using refrigerants such as N2, as in nitrogen expansion
cycles; or "mixed
refrigerants" as in MR cycles; or hydrocarbons, as in cascade cycles; and
without the
inefficiencies of high-pressure Joule Thompson cycles. The disclosed method
and system will
achieve a good degree of the efficiency available to turbo-expander (letdown)
LNG plants, but at
much smaller scales and at lower capital costs, and without the need for a
high-pressure pipeline
or a low-pressure outflow "sink". Also, the disclosed system builds on the
CCNG principles
advanced in US 7,464,557 B2 by providing a cost-effective way of producing
CCNG and by
enhancing the "cold recovery" innovations in that invention to the cold
recovery from stored
CCNG to dispensed CNG, as outlined above.
[0058] A significant portion of the product stream cannot be liquefied in a
single run through the
process and is sent back to the beginning of the cycle to be re-compressed,
mixed with more
(cleaned) natural gas from the pipeline (or stranded well), pre-cooled by the
chiller and sent
through the heat exchanger for liquefaction or CCNG production. This return
stream (the recycle
stream) gives up its cold in the heat exchanger (a form of cold recovery),
contributing to the
19
CA 02787375 2012-08-22
cooling and condensing of the portion of the stream that ends up as LNG/CCNG.
[0059] Another novel aspect of the disclosed method and system is that known
refrigeration
"producers", such as JT valves and turbo-expanders are integrated at the "back-
end" to convert
the cold CNG produced in the front into LNG. An alternative, and preferred
embodiment uses a
compressor-loaded, (or generator-loaded or brake-loaded), multiphase, turbo-
expander in lieu of
a JT valve, (also known as a JT valve). That device is shown as E2 (for
expander 2) and C5 (for
compressor 5) on Figure 5. Such a multiphase expander is also known as a Euler
Turbine and as
a Radial Turbine, as compared to Axial Turbines (or axial expander). The
multiphase expander
will, like the JT valve shown in Figures 5, 6 and 7, tolerate a reduced-
pressure outflow stream
that is partially a liquid and partially a vapor. However, the multiphase
expander, because it is
doing work (by, for example, being compressor-loaded), will yield more
refrigeration output
than the JT valve. That "extra" refrigeration will manifest in a larger
portion of the outflow
stream being liquid. In turn, the larger liquid portion will absorb more heat
from the main natural
gas stream that moves through the sub-cooling heat exchanger (HX5S on Figures
5, 6 and 7),
because a cold liquid that is to be vaporized will absorb more heat from a
counter-flowing
warmer gas stream than would a cold vapor stream. The extra refrigeration thus
achieved allows
for several efficiency increasing adjustments to the cycle. For example, the
stream that is sent to
the multi-phase expander can be reduced in flow rate (reducing the recycle and
re-compression
duty), and still achieve the required amount of refrigeration. Or, the stream
that is to be liquefied
can be increased in flow rate because of the extra available refrigeration.
Those familiar with the
art of process design will select the optimal flow rates for each stream,
capitalizing on the extra
refrigeration produced by the multi-phase expander, compared to the
refrigeration produced by a
JT valve. Also note that the compressor-load (C5) on the multi-phase expander
will re-compress
the entire outflow from the multi-phase expander to an extent that will
further reduce the
workload of the main compressor. Figure 5 shows the outflow from C5 moving
through several
heat exchangers (described below) and arriving at Cl to be boosted to a high
enough pressure so
as to be able to join the main feed gas stream that enters C2. Note that the
above-described
embodiment for an alternative to a JT valve is not the only alternative. For
example, some axial
expanders (compressor-, or brake-, or generator-loaded) can also tolerate some
degree of liquid +
CA 02787375 2012-08-22
vapor flow, and can be used in lieu of JT valves, producing more refrigeration
than a JT valve
under the same conditions.
[0060] In order to achieve about -250 F LNG at about 65 psia, (or the about -
150 F CCNG at
about 700 psia) significantly more refrigeration is needed than can be
provided by the front-end
chiller. Two sources of refrigeration are at work near the main heat
exchanger.
[0061] The first refrigeration source is a JT valve, also known as a throttle
valve, or preferably,
as illustrated on Figures 5, 6 and 7, a multi-phase compressor-loaded
expander, either radial or
axial. When LNG production is the goal, the pre-cooled CNG at about 400 psia
and about -22 F
is sent through the single heat exchanger where it is cooled to about -170 F
by the other streams
within the exchanger. That combination of approximately 400 psia and about -
170 F allows for
the use of a "plate fin" heat exchanger (rather than a more-expensive coil
wound unit) and yields
a worthwhile amount of refrigeration as described in the next paragraph. Thus,
this novel aspect
includes, in part, the selection of the about 400 psia and the about -170 F
temperature of the
main stream, allowing a commonly available plate fin heat exchanger to
"coordinate" and
integrate the several refrigeration steps. When producing CCNG, the same
equipment will
operate with a pre-cooled CNG stream at just above about 700 psia which is
cooled in the main
plate fin heat exchanger (HX5 on Figure 7) only to about -150 F. The portion
of the about 700
psia stream that moves through the JT valve or (preferably) the multi-phase
expander (E2 on
Figures 5, 6 and 7) will have a refrigeration content that will chill a larger
stream of CNG to
about -150 F, thus requiring smaller recycle streams. In other words, more of
the feed gas will
be delivered as CCNG to the "storage" tank without any increase in energy
input, thus improving
the efficiency of the cycle. That result, is a concrete example of the benefit
of producing CCNG
as compared to producing LNG. In summary, the disclosed method/system allows
for the
optimal "phase" of natural gas, but always achieving storability,
transportability, and pump-
ability.
[0062] A portion of the about -170 F (or about -150 F) stream, at about 400
psia (or just above
about 700 psia when CCNG is the intended product), is sent through the JT
valve or preferably
21
CA 02787375 2012-08-22
the multi-phase, compressor-loaded expander (shown as E2 and C5 on Figures 5,
6 and 7), which
(by pressure letdown) yields approximately -254 F vapor and liquid at a
pressure of only about
19 psia. That cold vapor + liquid stream is used to sub-cool the portion of
the stream (in HX5S
on Figure 5) that is still at about -150 F to about -170 F and about 400 to
about 700 psia,
cooling it to about -251 F and still at about 400 psia if LNG is the intended
product. If CCNG is
the intended product, no sub-cooling is needed, and the purpose of the JT
valve, or preferably the
multi-phase expander, is to provide an optimal amount of refrigeration to
bring the CNG to its
about -150 F (or colder) storage temperature. When LNG is the desired end
product, the sub-
cooled product is dropped in pressure to about 65 psia; forming LNG at about -
250 F, which
can be sent to the storage tank, without any "flash" (vapor) formation. This
is an important point
because if flashing were allowed, the vapor stream would need to be returned
(after cold
recovery) to the CNG compressor. Note that Figures 5, 6 and 7 show no vapor
return stream
because there is no flash produced. However, such a vapor return line may be
included in the
design, allowing any LNG "boil off' from the storage tank to be returned for
re-liquefaction.
Similarly, if the storage tank is designed to hold CCNG, and if the pressure
of the tank increases
significantly, due to heat gains, then such a vapor return line will allow the
high-pressure CCNG
to be returned to the system for re-cooling at approximately 700 psia.
[0063] For the sake of clarity, the sub-cooler 94 is shown in the process flow
diagram as a
separate heat exchanger. However, the sub-cooling task might occur in the
single plate fin heat
exchanger. On Figures 5, 6 and 7, the sub-cooler is noted as HX5S.
[0064] The low-pressure stream that cooled the main product stream in the sub-
cooler will be
sent back toward the beginning of the process as part of the recycle stream.
Prior to its return trip
through the single heat exchanger, the recycle stream will be joined by a
recycle stream from the
second refrigeration source, a two-stage cryogenic methane turbo-expander 110.
The combined
recycle stream, while low pressure, will be cold enough to substantially cool
the main process
stream to say, about -140 F (when CCNG is the goal) to about -170 F when LNG
is the goal.
The balanced use of a cold, low pressure recycle stream to achieve fairly deep
refrigeration of
the "moderate" pressure main stream, is yet another novel aspect of the
disclosed method and
22
CA 02787375 2012-08-22
system. That balance is especially efficient when the intended product is
CCNG, which requires
Only slightly more compression work than LNG but significantly less
refrigeration input. Thus,
the production of CCNG by way of the disclosed method and system allows for
the optimal
phase of vehicle-grade cryogenic methane, achieving all the benefits of
standard (cold) LNG but
with significantly reduced energy input. Indeed, the energy input required to
achieve CCNG will
be nearly as low as the energy input required to produce CNG, but the product
will be
significantly more valuable because of its storability, (allowing for off-peak
production) and
because it will always be dispensed cooler and denser, even when dispensed as
CNG.
[0065] The second source of refrigeration, the turbo expander 110 on Figure 2
and shown as El
on Figure 5, is needed because the letdown effect through a JT valve or the
multi-phase expander
alone does not provide enough refrigeration to produce CCNG or LNG. The
cryogenic methane
expander (El) will convert cold CNG to colder, lower-pressure natural gas by
doing "work". The
work can be recovered in an integrated compressor, shown as C4 on Figures 5, 6
and 7. If
recovered, the "work" output of the expander (several kilowatts) can be
applied toward the re-
compression of the recycle stream, further reducing the workload of the CNG
compressor and
the need to fuel the prime mover. Thus, in a preferred embodiment of the
disclosed method and
system compressor-loaded expanders are located in two places in the cycle, El
and E2 on Figure
5, 6 and 7. The distinction is that cryogenic methane expanders, which can
tolerate large flow
rates, do not tolerate the formation of any significant amount of liquid at
the outflow from the
expander. By contrast, JT valves, the preferred (radial) multiphase-expander,
and specially
designed axial expanders will tolerate liquid formation. Thus, the use of two
expansion devices
balances the optimal characteristics of those devices, with the cryogenic
methane expander
taking the larger flow rate but without any significant liquid outflow, while
the other expander
takes a lower flow rate but tolerating a higher percentage of liquid outflow.
Those two devices,
along with a chiller (which provides pre-cooling) constitute the refrigeration
equipment in the
cycle. Both expanders work on methane (rather than, say N2), which is the same
methane that
becomes the stored CCNG or LNG. That approach limits the need for refrigerants
and provides a
favorable relationship (methane to methane) between the refrigerant and the
gas stream to be
chilled.
23
CA 02787375 2012-08-22
[0066] The methane expander receives that portion of the main stream from the
heat exchanger
(HX5 on Figures 5, 6 and 7) that did not travel toward the JT valve or multi-
phase expander.
[0067] That second stream will leave the heat exchanger at approximately -90
F to about -104
F, and approximately 400 psia and will be expanded in the cryogenic expander
to approximately
40 psia, and thus cooled to approximately -220 F (when LNG is the desired
product); sent back
to the heat exchanger for "reheat" (cooling the other streams in the heat
exchanger); exiting the
heat exchanger at about 39 psia and about -30 F; giving up its "coldness" to
the warm outflow
stream from the compressor that "loads" the expander; entering that compressor
at approximately
35 F and about 38 psia; and returning to the second stage of the main
compressor for further
compression. When CCNG is the desired product the pressure of the gas streams
is slightly
above about 700 psia but the gas streams need not be cooled to colder than
about -150 F.
(However, if the CCNG is to be transported, it may be cooled to a colder
temperature in
anticipation of some heat gain during transport.)
[0068] The JT valve, multi-phase expander and the cryogenic methane expander
all function
well with the about 400-psia to about 700 psia inlet pressures. When LNG is
the desired product,
a higher than about 400-psia pressure might yield slightly more refrigeration
at the JT valve or
multi-phase expander, but not enough to warrant a more expensive heat
exchanger and the need
for more work by the compressor. The about 400 psia is a "comfortable" inlet
pressure for a
small expander. In short the selected conditions constitute a "sweet spot" in
the efficient small-
scale production or LNG yielding an excellent balance between refrigeration
produced, the size
and temperature of the recycle stream, the workload of the compressor, and the
total amount of
LNG produced per unit of fuel required to run the compressor. When CCNG is the
desired
product the work required to produce the extra pressure is more than offset by
the lowered
refrigeration requirement. If the cryogenic expander is most "comfortable"
operating with about
400 psia inlet gas, then that portion of the gas stream that is sent to that
expander can be
withdrawn from the main compressor at that pressure, and the remaining
portions can be further
compressed to just above about 700 psia and directed to the main heat
exchanger as outlined
24
=
CA 02787375 2012-08-22
above. That optimization will require "extra" compression for only a portion
of the throughput of
the compressor.
[0069] The JT or multi-phase expander effect, the sub-cooler and the expander
reheat cycle
outlined above are all known in the industry. What is unique is the
application of those
individual techniques to a small-scale LNG plant in a specific, optimal
manner. The disclosed
method and system uses the main CNG stream as a "working fluid" (refrigerant)
to liquefy a
significant portion of itself, returning a "recycle" portion for re-
compression, but only after
several "cold recovery" steps. Also, the disclosed method and system offers a
wide-range of
cryogenic methane products, all dense enough for cost-effective storage (and
thus, off-peak
production), and pump-ability. At the warm end, CCNG production by the
disclosed method and
system achieves those benefits with the lowest possible energy input, rivaling
the energy input
required for ordinary CNG production.
[0070] The pre-cooling by absorption/adsorption refrigeration captures the
waste heat of the
engine (and/or the heat of compression) and delivers a significant amount of
refrigeration to the
CNG compressor without any additional fuel use. That pre-cooling step is
illustrated on Figure 5
as follows: The chiller is driven by hot water that is heated in HX1 by engine
(ENG) exhaust and
by the hot water from the engine's water jacket; the hot water that is sent to
the chiller is returned
to HX1 for further heating after it gives up its heat content to the chiller;
the waste heat produced
by the chiller is dissipated in a cooling tower (CT); the refrigerant produced
by the chiller
(ammonia or water) is sent to HX4, HX3 and to HX7 (via point A and B), and
returns to the
chiller warmer, and ready for re-chilling. The CNG compressor will be well
within its capacities
in its effort to compress a recycle and feed-gas stream to about 400 psia to
about 700 psia. Fin-
fan coolers Fl, F2, and F3 are shown on Figure 5, allowing the heat of
compression to be
dissipated so that the gas streams can enter HX2, HX3 and HX4 at near ambient,
thus reducing
the cooling load in those heat exchangers. Similarly, F4 dissipates the heat
of compression from
C4, reducing the cooling load in HX7. (Optimally, a single Fin-Fan unit,
receiving multiple gas
steams would be used, rather than many individual units.) The JT valve or
multi-phase expander
and sub-cooler will produce the L,NG/CCNG relatively efficiently because the
product stream
CA 02787375 2012-08-22
sent to that device will be cold enough (about -140 F to about -170 F) to
yield LNG by sub-
cooling or CCNG. That cold stream to the JT valve will be available because
the expander will
produce natural gas as cold as about -220 F at the appropriate flow rate for
either LNG
production or CCNG production. The addition of "compressor loading" to the
cryogenic
methane expander and to the smaller multi-phase expander (C4 and C5,
respectively on Figures
5, 6 and 7) will further reduce the workload on the CNG compressor and the
fuel required by the
prime mover.
[0071] The recycle stream will be lower in volume than found in alternative
LNG cycles because
of the combined effect of the front-end absorption chiller; the moderate
pressure, cold JT valve
or multi-phase expander; the sub-cooler; and the cryogenic methane expander.
This is especially
true when CCNG is the desired product. The smaller recycle stream, will allow
the compressor
to do less work, requiring less power output from the prime mover, which in
turn will use less
fuel, reducing the plant's fuel use relative to the total output of LNG/CCNG
to levels matched
only by much larger LNG plants.
[0072] Figures 1 and 2 show schematic diagrams of one embodiment of the system
for a small-
scale production of LNG from low-pressure pipeline gas. The right side of
Figure 1 connects to
the left side of Figure 2. The approximate temperatures and pressures at
various points are
shown in circles, with the temperature on top, and the pressure at the bottom.
Low-pressure
(about 60 psia or greater) is the feed gas that will be used, in small part as
the fuel for the prime
mover 10, and will in large part be liquefied. A first inlet valve 14 near
point la is the inlet
connection from an adjacent natural gas pipeline (or from another natural gas
source, such as a
"stranded" gas well). A second inlet valve 18 is also an inlet connection from
an adjacent natural
gas pipeline (or from another natural gas source, such as a "stranded" gas
well). This allows for
a portion of the pipeline-delivered natural gas to be directed to the engine
10 during times such
as: during start up of the plant, or to the clean up and liquefaction cycle
beyond point la.
[0073] The prime mover 10 may be an internal combustion engine fueled by
natural gas. A
micro-turbine may also be used as the prime mover 10. The prime mover 10
directly drives a
26
CA 02787375 2012-08-22
multi-stage compressor 34 comprising a first stage 22, second stage 26, and
third stage 30.
Variations on the number of stages are possible, as are methods for
transferring the power of the
prime mover to the compressor. Those variations will not impact the core
methodology of the
disclosed invention and may be selected on the basis of capital costs,
equipment availability, and
other "optimization" factors.
[0074] Waste heat from the prime mover 10 is used to heat the regeneration gas
in the molecular
sieve clean up system, discussed bellow. Waste heat is also used as an energy
source in an
ammonia absorption chiller 38, shown simply as a circle, which provides
cooling to the
compressor's second inter-cooler 82 and after-cooler 86, at the first heat
exchanger 42 and
second heat exchanger 46, which will be discussed in more detail below.
[0075] The waste heat from the prime mover 10 is delivered to the ammonia
absorption chiller
38 by piping that extends the prime mover's jacket water system (not shown for
clarity), which
normally cools the engine. That hot jacket water is further heated by hot
engine exhaust in the
third heat exchanger 54. The engine exhaust gas is then sent to a flue 58 at
about 225 F. A
catalytic converter may be located at the appropriate place in the engine
exhaust outflow system.
A water pump 62 is shown just prior to the hot water's entry into the third
heat exchanger 54.
The pumping of the water with pump 62 to pressure will keep it from boiling.
The hot water
stream and the return stream from the ammonia absorption chiller 38 are shown
as dotted lines
on the process flow diagram.
[0076] The configuration of the ammonia absorption chiller 38, and its
rejection of low-grade
waste heat is a well-known technology. The process flow diagram does not show
the internal
process for the ammonia absorption chiller, but does show a cooling tower 66,
which uses water
as the cooling medium, disposing low-grade waste heat to the atmosphere. That
cooling tower
66, in fluid communication with a make-up water line 67, also helps cool the
compressor's inter-
and after-coolers 80, 82, 86.
[0077] Point 3a' is the location where the inlet natural gas stream from the
pipeline (or stranded
27
CA 02787375 2012-08-22
well), at approximately 60 F and about 55 psia, is mixed with a clean re-cycle
stream (about 80
F, about 55 psia) that arrives at that point from down-stream process points
that will be described
in subsequent sections of this narrative.
[0078] The first significant step in the liquefaction process is the clean up
cycle, which is well
understood by those in the natural gas processing field, especially related to
natural gas that is
delivered from a pipeline, known as "pipeline quality natural gas." Most
pipeline gas contains
some amount of CO2 and water, which need to be removed prior to liquefaction;
otherwise ice
will form down stream in the process, causing the cycle to "freeze up".
[0079] A molecular sieve 70 is configured to remove CO2 and water from the
natural gas in an
adsorbent such as, but not limited to, zeolyte. The molecular sieve 70 does
not remove any heavy
hydrocarbons from the natural gas feed stream. That portion of the clean up
cycle, if required,
occurs near point 16a, and will be discussed below. The molecular sieve 70 may
be a multi-
vessel system that regenerates the adsorbent beds by using heated natural gas
as the "purging"
fluid. The resultant CO2 laden regeneration gas is sent from the molecular
sieve 70 to the prime
mover 10 as fuel.
[0080] The process flow diagram does not show the configuration of the
molecular sieve 70
system, nor the detailed piping and valves that control the delivery of hot
exhaust gas to warm
the regeneration stream, because that technology is well understood and is not
an innovation of
this invention.
[0081] At point 3a, the feed gas stream (at about 68 F, 55 psia) consists of
the cleaned "make
up" stream from the pipeline (or stranded well) and the recycle stream that
joined it at point 2a.
The reason clean recycled gas is mixed with pipeline gas, prior to the
molecular sieve 70, is to
reduce the CO2 and water load on the mole sieve, by "diluting" the stream's
CO2 and water
content. The stream arriving at point 2a is the outflow of the first stage
compressor 22. The
purpose of the first stage compressor 22 and the source of the "flash recycle"
stream that it
compresses will be discussed below. The stream arrives at point 2a after going
through a first
28
CA 02787375 2012-08-22
inter-cooler 80
[0082] The first cooling step in the LNG production process occurs through the
fourth heat
exchanger 74. The fourth heat exchanger 74 allows the about -30 F "flash
recycle stream" to
chill the cleaned gas to about 42 F, as shown at point 3b. The slightly
cooled main gas stream is
mixed with a recycle stream from a natural gas expander's 78 (located on
Figure 2) outflow from
point 17a. That recycle stream is arriving at point 3b at about 35 F. The
combined natural gas
stream, at point 3, now consists of the make up stream from the pipeline, the
flash recycle stream
and expander 78 recycle stream. The temperature of the stream at point 3 will
be about 37 F.
Note that the pressure of the stream drops slightly as it moves through piping
and heat
exchangers.
[0083] The combined stream enters the second stage compressor 26 at about 54
psia for
compression, and leaves the second stage compressor 26 at about 210 psia. The
heat of
compression warms the natural gas stream to about 284 F, as shown at point 4.
[0084] Natural gas at about +284 F and about 210 psia will be called warm CNG.
The warm
CNG is sent to an inter-cooler 82 (which is cooled by water from the cooling
tower 66) and then
on to the first heat exchanger 42 where it is further cooled by the
refrigerant stream from the
ammonia absorption chiller 38. The cooling water inflow and outflow from the
inter- and after-
coolers are not shown, because that aspect of the process is well understood
by those familiar
with gas processing and the workings of gas compressors.
[0085] The natural gas stream exits the first heat exchanger 42 at about 35 F
and about 209 psia,
as shown at point 5. It then enters the third stage compressor 30 for
additional (and final)
compression, leaving the third stage compressor 30 at about 150 F (due to the
heat of
compression) and approximately 404 psia. The warm CNG travels to the after-
cooler 86, exiting
it at about 80 F and then on to the second heat exchanger 46 where it is
further cooled by the
refrigerant from the ammonia absorption chiller 38 to about -22 F. The entire
purpose of the
waste-heat driven ammonia absorption chiller 38 is to chill the natural gas
stream during its trip
29
CA 02787375 2012-08-22
through the second and third stages 26, 30 of the compressor 34, and to
deliver the natural gas,
pre-cooled to about -22 F, to the plant's main heat exchanger 90 (shown on
Figure 2).
[0086] The main heat exchanger 90 is the main heat exchanger for the disclosed
system. The
sub-cooling heat exchanger 94 may be integrated into heat exchanger 90 or may
be a separate
heat-exchanging unit as shown. The pre-cooled CNG enters the heat exchanger
90, traveling
from point 8 toward point 9. However, it is split into two streams, one going
to point 9 and one
to point 16. The stream that moves to point 9 arrives there at about -170 F
as LNG at moderate-
pressure, having been chilled by the counter-flowing stream in the main heat
exchanger 90.
[0087] The moderate-pressure LNG moves from point 9 toward point 13, but is
split into two
streams, one of which moves through the first expansion valve 98 (also known
as a JT valve),
with the other portion moving on toward point 10. The first expansion valve 98
causes the LNG
to become a two-phase (mostly liquid and less than about 30% vapor) stream,
arriving at point
13 at about -254 F, but "letdown" to at a substantially lower pressure of
only 19 psia. This
stream's function is to act as a refrigerant on the main stream that is
chilled to become LNG.
Refrigeration occurs in a sub-cooling heat exchanger 94 as the liquid portion
of the stream
vaporizes and transfers its "coldness" to the about -170 F LNG counter-
flowing through the
sub-cooler. The vaporization of the refrigerant stream does not change its
temperature during that
phase shift from liquid to vapor, allowing the vaporized refrigerant stream to
move on to points
14 and 15 at approximately -253 F, ready to impart further cooling in heat
exchanger 90, as
described below.
[0088] That cryogenic two-phase "refrigerant" stream, described above, is sent
through sub-
cooling heat exchanger 94 (a sub-cooler) where it cools the "product" stream
arriving from point
(about -170 F, about 400 psia) to become LNG, arriving at point 11 at about -
199 F to
approximately -251 F by the time the product reaches point 11. The about 399
psia LNG is then
dropped in pressure through another expansion valve 102 arriving at point 12,
and subsequently
sent to the LNG storage tank 106, at the design pressure of that tank. In the
embodiment shown
in Figures 1 and 2, the tank pressure is about 65 psia. Other storage
pressures will also work. The
CA 02787375 2012-08-22
extent of "sub-cooling" of the stored product is related to pressure at which
the product is stored
in the LNG storage tank. In this context, sub-cooling may be defined as the
extent to which the
stored product is colder than the temperature at which it will boil, at its
storage pressure. Lower
storage pressures require colder LNG in order to prevent boil off and flash
losses, due to heat
gain. Thus, sub-cooling of the stored LNG is a strategy that limits (or
substantially eliminates)
vaporization of the stored LNG due to unavoidable heat gain to the insulated
storage tank.
[00891 Returning to the "refrigerant" stream that exits the sub-cooling heat
exchanger 94, it
arrives at points 14 and at 15 at approximately -253 F and moves on for
additional "cold
recovery" in heat exchanger 90, leaving the main heat exchanger 90 at
approximately -30 F, as
indicated by the values shown at point 18 and 18a. The remaining cold is
further recovered in the
fourth heat exchanger 74, as discussed above. The relatively warm stream
(about 35 F) arrives
at point 18b at just about 17 psia. Thus, the function of the first stage
compressor 22 is to
recompress this (clean) stream so that it can return to the cycle and join the
make up stream after
point 2a, as discussed above.
[00901 Returning to the stream that entered heat exchanger 90, and was split
into two portions,
we can now follow the portion that arrives at point 16. Its trip through heat
exchanger 90 allowed
the about -22 F inflow stream to be chilled by the other streams in the heat
exchanger, so that it
exits heat exchanger 90 at between about -90 F to about -105 F (the "warmer"
the exit stream,
the less energy was spent on cooling it.) This stream is also a
"refrigeration" stream, providing
the bulk of the refrigeration required to cool the product stream. The, say,
about -100 F CNG (at
approximately 400 psia) is sent to a turbo-expander 110 that substantially
cools the stream by
expanding it to about 40 psia, and by having the turbo-expander 110
"compressor loaded" (by an
expander driven compressor 114) so that "work" is performed. It is the
expansion process,
including the work performed, that achieves the dramatic cooling of the CNG.
[00911 The exit stream from the turbo-expander 110 E will be approximately -
220 F and about
40 psia (see point 16b), allowing the natural gas stream to separate into
heavy hydrocarbon
liquids (such as ethane, and butane) and a nearly pure methane stream in a
phase separator 130,
31
CA 02787375 2012-08-22
shown near point 16a. That phase separation will take place if the feed gas
contains any such
heavy hydrocarbons. In that event, the liquid heavies are sent through a pump
134, to increase
the stream's pressure (see point 16h), and then sent into the storage tank 106
to join the main
liquid product of the process, the liquefied natural gas. The exact location
of where the liquid
heavies enter the tank can vary, and is subject to engineering decisions
related to the mixing of
the slightly warmer heavy hydrocarbon liquids with the larger and colder LNG,
that will not
impact the basic aspects of the disclosed system. Note that the small heavies
stream, which is
approximately at -220 F will slightly warm the contents of the LNG tank, even
though it is
receiving LNG at approximately -250 F. On the other hand, if the feed gas to
the cycle contains
very little or no heavy hydrocarbons, such slight warming will not occur. For
feed gas streams
with a higher concentration of heavy hydrocarbons, or where the product LNG is
used by
vehicles that cannot tolerate any significant heavy hydrocarbon content in the
LNG, some
portion of the heavies from the phase separator may be sent as fuel to the
prime mover. In short,
the disclosed system can tolerate a variety of feed gas compositions,
including from pipelines
and stranded wells, and variety of product specifications for the LNG.
[0092] Continuing the process at 16a, the very pure methane stream, at about -
220 F is a
refrigerant stream that helps cool the stream that went from point 8 to 9 and
the stream that went
from point 8 to point 16. In this manner, (and by way of the sub-cooler
previously described), the
pre-cooled (about -22 F) about 400 psia CNG is both a "product" stream
(beyond points 10, 11,
and 12) and a refrigerant stream. This aspect of the disclosed system is a
unique version of a
"methane expansion" cycle and is a core element of the innovation.
[0093] The outflow stream from the turbo-expander 110 leaves the heat
exchanger 90 at about -
30 F and serves to mitigate the heat of compression as the same (about 39
psia) stream is sent
through the expander driven compressor 114 that "loads" the turbo-expander
110. That "cold
recovery" occurs in a fifth heat exchanger 118, allowing the expander 110
recycle stream to enter
the expander driven compressor 114 at a "warm" state of about 35 F, exiting
the expander
driven compressor 114 at about 98 F, and exiting the fifth heat exchanger 118
at about 35 F,
having dealt with the heat of compression. One optimization of the disclosed
system may include
32
CA 02787375 2012-08-22
a water-cooled after-cooler immediately after the expander driven compressor
114, before point
17, allowing the temperature of the stream to be cooler than now shown at
point 17a, all of which
is included in the scope of the disclosed system. Other optimizations will be
obvious to those
familiar with natural gas processing, but without impacting the core aspects
of the innovative
methane expansion cycle disclosed here.
[0094] It is the work performed by the expander driven compressor 114 that
allows the expander
110 recycle stream to be returned to point 3b at about 56 psia, so that it can
enter the second
stage compressor 26 at a moderate pressure, rather than the first stage
compressor 22 at a lower
pressure.
[0095] Figure 3 shows a flowchart showing a disclosed method of the invention.
At act 140 one
configures a prime mover to be in operable communication with a multi-stage
compressor. At
act 144 one configures the prime mover to be in fluid communication with an
ammonia
absorption chiller. At act 148 one configures the ammonia absorption chiller
to be in fluid
communication with the multi-stage compressor. At act 152 the disclosed system
operates the
ammonia absorption chiller using waste heat from a prime mover. At act 156 the
system pre-
cools a first stream of natural gas using cooled fluid from the ammonia
absorption chiller. At act
160 the system cools a first portion of the first stream of natural gas, using
an expansion valve,
into a two-phase stream. At act 164 the system cools a second portion of the
first stream to
liquefied natural gas, using the two-phase stream as a cooling fluid. At act
168 the system
delivers the second portion of the first stream to a pressure tank. At act 172
the system cools a
third portion of the first stream of natural gas in a turbo-expander. At act
176 the system
separates liquid heavies out of the third portion of the first stream of
natural gas. At act 180 the
system delivers the liquid heavies to a pressure tank.
[0096] Figure 4 is one of many possible embodiments of the disclosed system
and method,
showing the key components, flow streams, and approximate temperatures and
pressures for the
production of LNG at about -245 F and about 65 psia. (Temperatures in
Fahrenheit are shown in
the upper part of the circular notations with pressures in psia shown in the
lower portion of each
33
CA 02787375 2012-08-22
circle.)
[0097] As discussed above, several features of the disclosed method and system
can be
optimized. The following are examples of such adjustments and are generally
illustrated on
Figures 5 and 6: a) The Ammonia Absorption Chiller (AAC) shown near point 21
on Figure 4
can be replaced by a Lithium Bromide Absorption Chiller or by a desiccant
based Adsorption
Chiller or any other non-mechanical, waste heat driven chiller, shown as
"chiller" on Figures 5
and 6; b) The JT valve shown near point 13a on Figure 4 can preferably be
replaced by a multi-
phase axial or radial expander that can be compressor-loaded (or brake- or
generator-loaded),
where the compressor is driven by that expander (both on a single shaft) and
where that
compressor acts to recompress some or all of the expander output to a pressure
suitable for
insertion into stage one of the main compressor, shown as E2 and C5 on Figures
5 and 6; c) The
main compressor, shown as Cl, C2 and C3, can have more stages, especially if
the desired
product is CCNG, as illustrated on Figure 6, where the outflow from the last
stage (a fourth stage
on Figure 6) needs to be somewhat higher (to allow for subsequent pressure
drop) than about 700
psia; d) the engine or engine-driven generator, also known as a "gen-set"
(shown as "ENG") can
be replaced with a turbine (mini- or micro- for small scale deployments, or
turbine-driven gen-
set) which is not shown on any figure, and in that event, the heat source to
HX 106 on Figure 4
(or HX1 on Figure 5) will only be the hot turbine exhaust, with no hot water
as a heat source,
(the total heat from the turbine will be as much or greater than the combined
heat from the
engine's exhaust and water jacket); e) The engine or turbine (or gen-sets) can
be replaced by an
electric motor powered of the electric grid, as shown on Figure 6, allowing
the cycle to be
entirely free of emissions; e) The liquid heavies separator shown near point
16h on Figure 4 will
not be needed if the feed gas to the system is pipeline quality, and is not
shown on Figures 5 and
6; f) The molecular sieve (MS) shown near point 3a on Figure 4, and near HX2
on Figures 5 and
6 can be any one of several CO2 and water removal systems, as discussed above;
g) The vapor
return line shown near point 19 on Figure 4 can be from a vehicle's fuel tank
where "flashing"
may occur if LNG is dispensed into a nearly empty (warm) tank, and/or that
vapor can be the
vapor portion of expanded CCNG, prior to its pumping to pressure as CNG, as
discussed above,
and that vapor return line is not shown on Figures 5 and 6 so as to keep the
graphics simple; i)
34
CA 02787375 2012-08-22
The flue shown near point 23 on Figure 4 and near HX1 on Figure 5 would not be
required if the
prime mover were an electric motor (as indicated on Figure 6), in which case
the cycle would be
a zero-emission process. In an attempt to keep Figures 5 and 6 relatively
simple, the streams that
will regenerate the mole sieve are not shown (as they are on Figure 4),
because such mole sieve
regeneration systems are well understood by those versed in the art and
science of gas clean up.
Figure 6 illustrates yet another embodiment of the invention, where the heat
of compression
between compression stages is used to drive the chiller. That option is
especially relevant when
the prime mover is an electric motor (rather than a fueled engine or turbine),
where there is no
availability of hot exhaust gases or hot jacket water to drive the chiller.
100981 It should be noted that Figure 4 shows only one possible set of
temperature and pressure
conditions, and equipment arrangement, with the intention of producing LNG at
a specific
storage temperature and pressure. Other similar conditions and configurations
may be designed
to optimize the LNG production process at warmer temperatures and somewhat
higher pressures,
and in response to site-specific conditions such as (but not limited to) the
chemical composition
of the feed gas, its feed pressure and temperature, the choice of the prime
mover, and the scale of
the plant. Thus, Figures 5, 6 and 7 refrain from noting specific temperatures
and pressures, and
(for LNG production) allowing for very much the same pressure and temperature
conditions as
shown on Figure 4, but also allowing for higher pressures and slightly warmer
conditions, as
discussed below, for the production of CCNG.
100991 When the process shown in Figures 4 is used to produce CCNG, as
illustrated by Figure
6, at least the following adjustments to the process would be made: a) The
pressures at points 8,
9, and 13a (or through HX5 on Figure 6) would be slightly above about 700
psia, allowing for
pressure drop through the process and resulting in the delivery of the CCNG to
its storage tank at
about 700 psia or greater pressure; b) The outflow temperature from the
pressure letdown device
near point 13a (preferably a multi-phase compressor-loaded expander E2 and C5
on Figures 5
and 6) would produce the same about -254 F two-phase stream, but with a
larger liquid portion
than would be produced by a JT valve, and/or requiring a smaller flow rate
through the device,
resulting in a smaller recycle stream; c) the product stream at points 10 to
11 on Figure 4, and
CA 02787375 2012-08-22
shown exiting HX5S on Figures 6, would arrive at the storage tank at about -
150 F or colder, at
a pressure of about 700 psia or greater. The actual configuration of HX 101,
the need (or lack of
need) for HX 101S, and the exact temperatures, pressures and flow rates of the
natural gas
streams though the main heat exchanger array will be determined for each set
of product
conditions by well known thermodynamic simulations by commonly available
software that will
insure that the "cooling curves" of the gas streams do not "cross" (do not
violate the laws of
thermodynamics) and that the cryogenic heat exchanger will perform as
intended.
[00100] The process shown in Figures 4, 5, 6 and 7 can include many of the
adjustments
outlined above, and can be operated in an LNG production mode, producing
various "grades" of
LNG from as cold as approximately -250 F to as warm as approximately -160F,
with a pressure
range of approximately 60 psia to about 500 psia; or the process can produce
various "grades" of
CCNG, from as warm as about -118 F at about 700 psia to as cold as any LNG
product but at
pressures above about 700 psia, yielding non-liquid, high-density, cryogenic
natural gas. Those
various products can be produced during different time slots, or at the same
time, depending on
product demand. For example, if both LNG and CCNG were desired, only a portion
of the feed
gas would be compressed to above about 700 psia, with the remaining portion
moving through
the process at the approximately 400-psia pressure, producing LNG. The about
700-psia portion
would receive less refrigeration and would reach its CCNG storage tank sooner
than the about
400-psia portion destined for an LNG tank. The lowest operating costs,
primarily because of the
reduced energy input requirement, will be for CCNG production. The commercial
viability of
such Small-Scale LNG/CCNG plants may require that the plant operate 24-hours
per day and as
many as 355 days per year. As such, its capacity (measured, for example, in
"gallons" per day)
would match the daily or weekly demand by the vehicle fleet served by the
plant, with the
LNG/CCNG storage tanks acting as a buffer between the hourly/daily production
rate and the
hourly/daily product demand rate. This paragraph discloses one embodiment of
how to make
LNG and CCNG at the same time with the same equipment.
[00101] Figure 8 is a phase diagram for methane and is an analog for the
phased diagram
for natural gas. Although this patent application discusses the invention with
respect to natural
36
CA 02787375 2012-08-22
gas and various compositions of natural gas, one of ordinary skill in the art
will understand that
the disclosed application applies also to methane, a main component of natural
gas. Methane
and natural gas are similar but not identical. Typical natural gas contains
about 94% methane,
3% heavier hydrocarbons and 3% CO2 plus nitrogen as well as small quantities
of water and
sulfur compounds. CO2, water and sulfur are usually removed prior to chilling
the natural gas to
prevent freeze-out. The phase diagram, Figure 8, can apply to natural gas
because it is
qualitative in nature. Specific values for critical pressure and critical
temperatures discussed in
this patent application are for pure methane, however, it will be obvious to
those of ordinary skill
that slightly different values for critical pressure and critical temperature
will be used for natural
gas, the exact values will be dependant on the composition of the particular
natural gas. At the
triple point, the natural gas can exist as a solid, vapor and liquid. A solid-
vapor coexistence
curve 10 extends downwards and leftwards from the triple point. A solid-liquid
coexistence
curve 14 extends generally upwards from the triple point. A liquid-vapor
coexistence curve 18
extends upwards and rightwards from the triple point up to the critical point.
It is generally
accepted that above the critical temperature ("TcRuicAL") and above the
critical pressure
("PciuricAL") for a composition, it exists in a supercritical state. The
region above the critical
temperature and above the critical pressure shall be referred to the as the
supercritical region, and
fluids within that region shall be referred to as supercritical fluids. The
region to the left of the
supercritical region, that is, the region above the critical pressure, and
below the critical
temperature, and to the right of the solid-liquid coexistence curve shall be
referred to as the cold
compressed region in this disclosure, and fluids within that region shall be
referred to as cold
compressed fluids. The cold compressed region is indicated by the hatch marks
in Figure 8.
Fluids in the supercritical region have unique properties, including existing
as a single-phase
fluid. Fluids in the cold compressed region have some of the same
characteristics of supercritical
fluids, including existing as a single-phase fluid. Additionally, fluids in
the cold compressed
region have densities approaching that of LNG. It should be noted that fluids
in the cold
compressed region are not technically in a liquid phase, but are technically
in a gas phase.
[00102] Figure 9 is a flowchart showing another method of the invention.
At act 200 the
system produces cold compressed natural gas, as shown in the phase diagram at
Figure 8. At act
37
CA 02787375 2012-08-22
204, the system stores the cold compressed natural gas. At act 208 the cold
compressed natural
gas is dispensed. The natural gas may be dispensed to vehicles to be used as
fuel for those
vehicles, for instance.
[0103] Figure 10 is a flowchart showing another method of the invention. At
act 212, the cold
compressed natural gas (CCNG) is dispensed from a cold compressed natural gas
(CCNG)
storage system, which could be a stationary or mobile tank, or any other
suitable storage means.
At act 216, the cold is recovered from the cold compressed natural gas (CCNG),
during the
dispensing of the cold compressed natural gas. At act 220, the recovered cold
is used to
refrigerate incoming natural gas, or "feed-gas," (which has been cleaned of is
water and CO2
content in a molecular sieve or other such device, to a level sufficient for
cryogenic processing),
such that the feed-gas replaces a portion of the outgoing LNG/CCNG, and where
the heat content
of that feed-gas warms the CNG that is derived from the pumped-to-pressure
(formerly)
LNG/CCNG. The refrigeration (cooling) of the feed-gas may occur in optimal
steps of
compression and refrigeration, where the "cold recovery" from the outbound
LNG/CCNG
reduces the need for newly generated refrigeration input. In effect, by
storing LNG/CCNG prior
to dispensing it as CNG, refrigeration input is also stored, and can be
recovered during the CNG
outflow, because that CNG that is dispensed from the LNG/CCNG needs to be much
warmer
(above about -20 F) then the stored LNG/CCNG, which is about -150 F or
colder. In this way,
the disclosed system manages to produce a storable and pump-able dense-phase
natural gas
product that can be dispensed as CNG, but which CNG is cooler than standard
CNG (and
therefore denser), and which CNG can be stored in existing, non-cryogenic, on-
vehicle CNG fuel
tanks.
[0104] Returning to Figure 5, we will now describe the method shown as but one
embodiment of
the disclosure. The natural gas to be liquefied enters the process at the
point which is labeled
"NG," which represents a natural gas pipeline or well, and may also represent
other natural gas
sources such as landfill gas (LFG) and anaerobic digester gas (ADG), or
associated gas from oil
wells or any other natural gas source. That "feed gas" needs to be cleaned of
any water content
and CO2 in order to avoid ice formation in the cryogenic portions of the
process. The symbol for
38
CA 02787375 2012-08-22
a Molecular Sieve (MS) or "mole sieve" is shown as the device that removes the
water and CO2
from the feed gas. As noted above, other clean up systems can also be used.
For "pipeline
quality" natural gas, mole sieves will adequately remove the water and CO2
from the feed gas.
For feed gas, such as LFG and ADG that contain other contaminants or large
amounts of water
and CO2, a more complex clean up system will be required. Such systems are
well understood by
those familiar with gas clean up issues.
[0105] After clean up, the feed gas moves on to HX2 where it is pre-cooled by
a portion of the
refrigerant output (shown as stream R) of the Chiller. Also, the feed gas is
blended with a recycle
stream of natural gas that results from pressure letdown later on the process.
That blended stream
enters the second stage of compression and its pressure is increased by a
ratio that may range
from two-to-one to a ratio of four-to-one, depending on the number of
compressor stages
selected. The heat of compression is dissipated in a Fin-Fan cooler (F2). The
now near-ambient
gas stream moves on the HX3 where it is cooled to approximately 30 F by a
portion of the
refrigeration output of the Chiller. Such pre-cooling before each stage of
compression helps
reduce the workload of the compressor.
[0106] Next, the gas stream is compressed in the third (or last stage) of the
compressor to the
approximately 400 psia that is needed for LNG production. The heat of
compression is dissipated
in F3, and final pre-cooling is accomplished in HX4. As discussed above, in
the description of
Figure 4, that pre-cooling can achieve temperatures as cold as about -22 F,
when the Chiller is
an Ammonia Absorption Chiller. The embodiment described in Figure 5 assumes
that the Chiller
is based on Lithium Bromide (absorption) technology or on desiccant based
adsorption
technology, which produce a lower grade of refrigeration but operate with
lower-grade heat
sources. Thus, the gas stream enters HX5 at approximately 52 F, where it is
chilled to cryogenic
temperatures by two refrigeration sources, both of which use the same methane
that is the
product stream as refrigerant streams. A portion of the stream that entered
HX5 leaves that
brazed aluminum, plate fin, cryogenic heat exchanger as a cold stream
(approximately -100 F)
and is expanded in El (which is loaded by C4), producing a colder but lower
pressure outflow
from El, which is sent back to HX5 as a source of refrigeration. The outflow
stream from El
39
CA 02787375 2012-08-22
will be approximately -220 F. The second refrigeration stream is that portion
of the original
stream that entered HX5, which is sent on to a valve (shown near E2) for
further splitting. The
valve sends one portion to E2 which, as described above, is a radial expander,
loaded by
compressor C5, which cause the stream through it to be chilled to
approximately -254 F, and
which stream is a two phase (liquid and vapor) stream. That liquid plus vapor
stream further
chills (in HX5S) the part of the stream that was separated by the valve near
E2. It is the liquid
aspect of the outflow from E2 that delivers the most significant refrigeration
to the product
stream because that liquid is subject to a phase shift, absorbing heat from
the product stream,
which vaporizes the liquid portion that left E2.
[0107] The product stream, having been liquefied by heat exchange from the
outflow from E2 is
then allowed to enter the cryogenic storage tank as LNG. As discussed above,
the LNG's
temperature and pressure can be "designed" for different end uses.
[0108] Meanwhile, the refrigerant stream that caused the liquefaction of the
product stream in
HX5S moves through HX5 to give up any remaining refrigeration to the other
streams in HX5,
and then exits HX5 at colder than zero F but warmer than about -30 F, and is
sent to C5 for
some compression. The purpose of C5 is to "load" E2, so that work can be
performed and
refrigeration produced in E2. The impact of C5 on raising the pressure of that
recycle stream will
vary, depending on design decisions for each deployment. After compression in
C5, that recycle
stream enters HX6, where it is cooled by the remaining refrigeration contained
in the outflow
from E, (which leaves HX5 at colder than zero degrees F).
[0109] The next stop for the cooled and somewhat compressed recycle stream is
to be further
compressed in Cl of the main compressor. That heat of compression is
dissipated in F I, and the
recycle stream is further cooled in HX2 by the refrigerant output of the
Chiller (shown as stream
A'), where the recycle stream is blended with the cleaned process stream.
[0110] Meanwhile, the recycle stream that left El and HX5, and was also used
as a refrigerant in
HX6, is compressed in C4, which loads El. Again, the purpose of C4 is to allow
El to produce
CA 02787375 2012-08-22
work, thus creating refrigeration. After C4 that second recycle stream's heat
of compression is
dissipated in F4. The stream is further cooled in HX7 by Chiller-produced
stream A-B. The
second recycle stream then enter C2 and, along with the clean feed gas and the
recycle stream
that left Cl, is compressed to at least a two-to-one ratio, depending on the
total number of
compression stages selected by the process designer. The combined streams
leave C2 at the
selected pressure (approximately 200 psia or higher) and then move on to F2
for the dissipation
of the heat of compression. (It should be noted that each trip through a heat
exchanger or a Fin-
Fan cooler will cause a, say, about one pound pressure drop, which needs to be
accounted for in
the overall pressure increase ratios at each compressor in the process.) After
F2, the combined
gas stream is pre-cooled in HX3 by the refrigerant output of the Chiller, and
then the gas stream
moves on to C3, which on Figure 5 is the final compression stage.
[0111] Exiting C3, the combined gas stream's heat of compression is dissipated
in F3. The gas
stream is pre-cooled in HX4 and enters HX5 as discussed above. Thus, the gas
stream that enters
HX5 is a product stream that ends up as LNG after leavening HX5S and is two
refrigerant
streams (one cooled by E2 and the second one cooled by El), where the two
refrigerant streams
are recycled through several steps of "cold recovery" and compression.
[0112] Returning to the Chiller on Figure 5, it is seen that its heat source
is hot water that is
heated in HX1 by the hot exhaust of the engine (or turbine) and by the hot
water that cools the
engine. The refrigerant output from the chiller is shown as stream R, which is
used to cool the
natural gas streams in HX2, HX3, HX4 and HX7. A cooling tower (CT) dissipates
waste heat
from the Chiller. Thus, Figure 5, like Figure 4, illustrates the integration
of a waste-heat driven
Chiller with the prime mover, so that the waste heat can be converted to
useful refrigeration. In
Figure 5, the refrigeration is relatively low grade (as compared to the higher-
grade refrigeration
illustrated in Figure 4). That reduction in refrigeration potential is made up
by the increased
refrigeration output of the E2-05 array, as a replacement for the JT valve
shown on Figure 4.
[0113] In other words, Figure 5 is a variation on the principles outlined in
Figure 4. However,
elements of Figure 4 can be combined with elements from Figure 5. For example,
if an AAC
41
CA 02787375 2012-08-22
were used in Figure 5, along with the E2-05 array (substituting for the JT
valve in Figure 4) the
efficiency of process would improve, yielding a higher flow rate of LNG into
storage with the
same energy input at the prime mover, or the same flow rate of LNG into
storage with less
energy input at the prime mover.
[0114] Thus Figure 5 is just one new embodiment of the previously disclosed
process, and many
variations on Figure 4 and 5 are foreseen. Indeed, Figure 6 is one such
variation, which will be
discussed next.
[0115] Figure 6 is yet another embodiment of the disclosed invention,
illustrating the use of an
electric motor as the prime mover (in lieu of a fueled engine or turbine). The
discussion that
follows will assume that the product sent to the storage tank (at the bottom
right of the Figure) is
CCNG. However, Figure 6, with its electric motor prime mover and other
features can also
produce LNG, much like the process discussed in Figures 4 and 5. As in the
discussion of Figure
5, pressures, temperatures and flow rates of the various streams shown on
Figure 6 are not
specified, because the process sown on Figure 6 will function under a wider
range of pressure,
temperatures and flow rates. Instead, the discussion that follows will offer
approximate
conditions as well as preferable conditions.
[0116] As in Figures 4 and 5, the process in Figure 6 begins with the natural
gas feed (from any
source) at point NG, moving on the mole sieve (or any suitable gas clean up
equipment, designed
for the specific chemical composition of the feed gas) and through HX2, where
it is blended with
a recycle stream, cooled by the refrigerant output of the Chiller, and then
sent on to C2 for
compression, as described above in the discussion of Figure 5. However,
because the prime
mover in Figure 6 is an electric motor, which produces very little in the way
of waste heat, the
heat source for the Chiller is the heat of compression produced in the several
stages of the main
natural gas compressor, including (but not limited to) C2, C3 and C4. Other
heat sources may
include the outflow stream of the Mole Sieve, which is often at temperatures
reaching above
about 200 F, or any other waste hat source.
42
CA 02787375 2012-08-22
[0117] Instead of a Fin-Fan cooler at the outflow from C2, C3 and C4, Figure 6
shows that the
heat-bearing gas stream is first sent to HX1 where it heats the hot water that
drives the Chiller.
The temperature of each of the gas streams shown (C-D, E-F, and G-H) need to
be warmer than
the return water from the Chiller, (warmer than approximately 140 F) and at
least one of the
streams needs to be as warm as approximately 167 F. If the gas streams
leaving the several
stages of compression are as warm as those temperatures, they will provide
enough heat to the
Chiller for it to provide the low-grade refrigeration (about 42 F to about 50
F) needed for the
streams moving through HX2, HX3 and HX4. Preferably, (from the Chiller's point
of view), at
least one of streams C-D, E-F or G-H will be hotter than about 167 F, and
most preferably, as
hot as about 203 F, thus yielding a more efficient Chiller output, as
measured in the Chiller's
Coefficient of Performance, also known as COP. As discussed above, the
refrigerant streams are
shown as R, cooling the natural gas streams in HX2, HX3 and HX4. Optionally,
but not shown
on Figure 6, Fin-Fan coolers may be located near points D, F, and H, after the
natural gas
streams leave HX1, but before they move on to HX3 for pre-cooling. (As
mentioned above, such
Fin-Fan cooling can occur in a single, consolidated unit that receives
multiple streams for
cooling, rather than many individual Fin-Fan units.)
[0118] Figure 6 shows a four-stage compressor (as compared to the three-stage
compressors
shown in Figure 4 and 5.), because the outlet pressure after C4 will be higher
than about 700
psia, (approximately 703 psia) in order to allow for pressure drop through HX5
and HX5S, and
thus allow the end product (CCNG) to arrive at the storage tank at a pressure
of about 700 psia or
greater.
[0119] After each stage of compression and with the heat of compression given
up to warm the
hot water that drives the Chiller, the natural gas streams are pre-cooled in
HX3 and then sent on
to HX5 for further cooling as described above. However, when Figure 6
describes the production
of CCNG, the cooling of the product stream in HX5 and HX5S need not result in
a stream that is
colder than about -150 F. (Optionally, any temperature between about -150 F
and about -245
F can be selected.) Thus, when producing the relatively warm CCNG, the flow
rates through E2
and El may be reduced by as much as 25% compared to the equivalent points on
Figures 4 and
43
CA 02787375 2012-08-22
5. In other words, the recycle streams that leave E2 and El, and which must be
re-compressed in
Cl and C2, respectively, will be smaller streams, requiring less "recycle
work" by the
compressor and allowing more of its work output to be applied to the portion
of the gas stream
that ends up in the storage tank as CCNG.
[0120] Thus, when the process illustrated in Figure 6 is used to make CCNG the
disclosed
process satisfies the goals of the invention by producing a dense-phase, non-
liquid, cryogenic,
pump-able phase of natural gas with lower energy input costs than the
production of standard
(temperature and pressure) LNG. Also, the process illustrated in Figure 6 can
produce "warm"
LNG, a dense-phase, liquid, cryogenic, pump-able phase of natural gas with
lower energy input
costs than the production of colder LNG typically produced in other LNG
plants.
[0121] As noted above, the core elements of Figure 6 can be applied to Figures
4 and 5. For
example, even if a fueled prime mover is used (an engine or a turbine),
Figures 4 and 5 can
benefit from the recovery of the heat of compression to help warm the hot
water used by the
Chiller. The extra refrigeration produced in that embodiment would, for
example, be used to cool
the inlet air to the gas turbine (if that is the prime mover), improving the
efficiency of the
turbine, and reducing its fuel demand relative to its power output. Similarly,
the choice of a four-
stage compressor, rather than the three-stage designs shown on Figures 4 and
5, can reduce the
energy input needed by the compressor, but generate a higher capital cost. In
summary, process
engineers can adjust the disclosed process to respond to specific feed gas
sources and to specific
end products sought.
[0122] Turning to Figure 7, the benefits of the disclosure relative to CNG
dispensing is
illustrated as another embodiment. Like in Figure 6, the product sent to the
storage tank can be
any "grade" of LNG (from "warm" to cold) and any grade of CCNG, from as cold
as
approximately -150 F to any colder storage temperature. The production of the
stored product
would, ideally, be designed to be a full time, 24-hours per day process, with
enough on-site
storage capacity to act as a buffer between the production rate and the
dispensing rate of CNG.
The total daily (or weekly) production rate will match the total CNG demand
and any demand
44
CA 02787375 2012-08-22
for off-site use of the product, which would be transported to those off-site
customers by CCNG
tanker truck. (Such CCNG tankers are similar to LNG tankers but with a higher
pressure
tolerance.)
[0123] In most aspects, Figure 7 is similar to Figure 6. The CNG dispensing
and "cold recovery"
aspects of the disclosure constitute the extra information offered on Figure
7. Starting at the
CCNG storage tank, the product is pumped to pressure by "P," an electric motor-
driven
cryogenic liquid pump. (The motor is not shown.) That pump P increases the
approximately 700
psia of the stored CCNG to the desired pressure of the CNG to be dispensed,
which is generally
in the range of 3,000 to 3,600 psia.
[0124] The high-pressure CCNG warms slightly (approximately 2-degrees F) above
its storage
temperature, warming from, say, about -150 F to about -148 F. That "cold
content" is
recovered in HX7, where the high-pressure CCNG is heat exchanged with the pre-
cooled gas
stream that left HX3 at a temperature as cold as about -22 F and as warm as
about 50 F,
(depending on the choice of the Chiller and the available waste heat sources)
and which has not
yet been chilled in HX5. The chilling of that pre-cooled gas stream in HX7
will cause its
temperature to fall to within about 10-degrees of the high-pressure CCNG that
is flowing counter
to it in HX7. Thus, the process gas stream leaves HX7 and enters HX5 at
approximately -138 F,
requiring significantly less refrigeration input from E2 and El to exit HX5S
at about -150 F,
ready for storage.
[0125] At the same time, the high-pressure CCNG is warmed in HX7 by the
process stream,
leaving HX7 as CNG (at 3,000 to 3,600 psia) with a temperature of about -20 F
to about 60 F,
depending on the inlet temperature of the process gas and the relative flow
rates of the process
gas and the high-pressure CCNG. The cool CNG (about -20 F to about 60 F) is
substantially
cooler than standard CNG at above about 100 F, and therefore denser than
standard CNG.
Instead of the approximately 10.5 pounds per cubic feet density of standard
CNG, such cool
CNG, dispensed from CCNG (or "warm" LNG) will have a density of more than
about 13
pounds per cubic feet, substantially increasing the capacity of existing on-
board CNG fuel tanks.
CA 02787375 2012-08-22
[0126] The disclosed process illustrated on Figure 7 would function in the
same way as shown
on Figures 4, 5, and 6, reducing the refrigeration demand in HX5 only when
cold CCNG (or
LNG) is sent out of storage for dispensing as CNG. The program logic of the
process would
adjust the flow rates through E2 and El to reflect the refrigeration delivered
by the high-pressure
CCNG that is destined to become CNG. Thus, Figure 7 illustrates a way to
"store CNG" (as
CCNG or LNG) and to store and recover the refrigeration input required to
produce the stored
CCNG (or LNG), rather than throwing away that refrigeration, as is the case in
all L/CNG
dispensing sites that do not have on-site liquefaction equipment.
[0127] The disclosed process illustrated on Figure 7 responds to the
shortcomings of existing
CNG production models by allowing for a storage mode, by recovering the waste
heat of
compression and any waste heat produced by a fueled prime mover, and by
delivering a cooler
and denser form of CNG than can be attained by standard CNG production
methods.
[0128] The disclosed process illustrated on Figure 7 can also be integrated
with existing CNG
dispensing facilities, upgrading those facilities and improving their
performance as outlined
immediately above. Such a retrofit would utilize the existing compressor, the
prime mover, and
any gas drying apparatus as the core of the upgrade, and utilize the existing
CNG dispensing
apparatus.
ENERGY INPUT COSTS FOR DENSE PHASE NATURAL GAS, RELATIVE TO DENSITY
ACHIEVED
[0129] The main purpose of producing LNG (at any scale), CCNG, or CNG is to
increase the
density of natural gas, making it heavier per cubic foot of volume, thus
increasing the energy
content of the natural gas per a given volume (say, per cubic foot).
Generally, LNG is the densest
form, with CCNG a close second, and CNG the least dense form.
[0130] That range of density, from densest (coldest) LNG to the least dense
(and warmest) CNG
46
CA 02787375 2012-08-22
does not necessarily shed light on the energy input required for each
condition relative to the
density achieved. In other words, most observers would guess that LNG is the
most costly
product, because it requires "expensive" refrigeration, and that CNG is the
least costly product
because it "only" requires compression. However, that "conventional wisdom" is
not accurate.
[0131] The approximate energy input required to make CNG (at 3,600 psia and 90
F, with a
density of 10.65 pounds per cubic foot) from one decatherm of natural gas is
333 kWH. The ratio
of that energy input to the density achieved is 333 10.65 = 31.3.
[0132] By contrast, the VX Cycle will produce LNG (at 65 psia and -245 F, with
a density of
25.6 pounds per cubic foot), from the same decatherm of natural gas, using
approximately 721
kWH of power. That ratio of power to density achieved is 721 25.6 = 28.2,
which is lower than
for CNG. In other words, the VX Cycle will achieve a higher-density product at
a lower energy
input cost (per density achieved) then standard CNG production systems. Stated
differently, VX
Cycle LNG will cost less to produce than CNG, when accounting for what is
achieved.
[0133] More to the point of the CIP, "warm" LNG and CCNG produced by the VX
Cycle are the
most cost-efficient products, per the following:
= LNG (at 500 psia and -158 F, with a density of 20.4 pounds per cubic
foot), from
the same decatherm of natural gas, will require approximately 513 kWH of power
to
produce. The ratio of power to density achieved is 531.2 20.4 = 25.2.
= CCNG (at 700 psia and -150 F, with a density of 19.8 pounds per cubic
foot),
from the same decatherm of natural gas, will require approximately 500 kWH of
power
to produce. The ratio of power to density achieved is 500 19.8 = 25.3.
[0134] The energy input to density ratio of VX Cycle "warm" LNG or CCNG is
approximately
19% lower than the energy input to density ratio required for standard CNG
production. Over the
lifetime of any single facility, especially if the feed-gas is on a pipeline,
where "retail" prices are
the norm, the extra capital cost of VX, compared to a CNG production system,
will be quickly
47
CA 02787375 2012-08-22
offset by the reduced energy input costs.
VX CYCLE "SWEET SPOT"
[0135] Generally, the coldest LNG is approximately -260 F at approximately 50
psia. However,
for most small-scale applications, including for use as a vehicle fuel, LNG
need not be that cold.
(The colder the LNG is the more energy input is required for its production,
but not in a linear
way but "exponentially" because each degree drop in temperature requires an
exponential input
of energy.)
[0136] Coldest LNG (near -260 F) is necessary if the LNG is to be shipped
across the oceans in
LNG tankers, where warmer LNG would boil off quicker. Similarly, regional LNG
production
facilities that produce large amounts of LNG for distribution to individual
customers, delivering
the LNG in cryogenic trailers, need to produce cold LNG in order to avoid boil
off (or
"weathering") during transport and during on-site storage, prior to
dispensing.
[0137] Because the VX Cycle is primarily (but not exclusively) designed for
small-scale LNG
production, at the customer's site, avoiding long-distance transport, it can
aim for warmer LNG
as a product. In other words, the LNG bus or truck that receives the dispensed
LNG does not
"care" if it is -260 F or -240 F, as long as the tank is full. (The LNG is
vaporized and sent to
the engine as gas, so the engine does not "care" what the temperature of the
on-board LNG is.)
[0138] The innovations described in Figures 4, 5, 6, and 7 aim to produce the
warmest possible
dense-phase natural gas products with the least energy input possible. The
"sweet spot" for VX is
a range of dense phase products that are -245 F and warmer (with pressures of
65 psia and
greater), but colder than -118 F and with a pressure that is at least 700
psia. That temperature
range (-118 F to -245 F) and that pressure range (65 psia to above 700 psia)
will yield densities
between 25.6 pounds per cubic foot for the coldest point on that "continuum"
to approximately
15 pounds per cubic foot for the warmest point.
48
CA 02787375 2012-08-22
[0139] That entire range of temperatures, pressures and resultant densities is
pump-able by
cryogenic liquid pumps, even though the warm end of the range is CCNG, a non-
liquid phase of
natural gas.
[0140] That entire range of storable and pump-able products can be achieved by
the VX Cycle at
a ratio of energy input (kWH) to density that is lower than 30, with most of
the conditions on that
continuum achieved by VX at a ratio of less than 26.
[0141] Thus the VX Cycle identifies a wide-ranging sweet-spot for dense-phase
natural gas
production where the density of the VX product is between approximately 19 to
25 pounds per
cubic foot, and where that density is achieved by the optimal balance between
compression and
refrigeration input. Figures 4, 5, 6, and 7 illustrate the systems for
achieving that optimal
balance.
[0142] Below are suggested operational values for 3 proposed VX systems:
1) Production by VX of a non-liquid, dense-phase, cryogenic form of natural
gas,
achieved by the optimal balance of compression and refrigeration, rather than
first
producing LNG and then pumping it to a supercritical (non-liquid) phase.
2) A VX product temperature range between -118 F and -245 F, preferably
between -
150 F and -200 F; with appropriate pressures for those temperature
conditions, between
65 psia to above 700 psia and preferably between 285 psia and above 700 psia;
and a
density range of between 19 pounds per cubic foot to 25.6 pounds per cubic
foot.
3) A VX product range where the ratio of energy input required to convert one
decatherm
of natural gas to a dense-phase cryogenic product that can be pumped by
cryogenic liquid
pumps is less than 30 and preferably less than 28, and most preferably less
than 26.
[0143] The disclosed system has many advantages. Returning to Figures 1 and 2,
the disclosed
system starts with low-pressure pipeline-quality natural gas (or low-pressure
stranded gas) and a
49
CA 02787375 2012-08-22
prime mover 10 (such as, but not limited to an engine), which drives a multi-
stage compressor.
The waste heat of the prime mover is used to heat regeneration gas that
"sweeps" one of several
beds (sequentially) in a standard molecular sieve 70, removing CO2 and water,
and sending the
regeneration gas back to the prime mover. The bulk of the waste heat provides
heat to an
ammonia absorption chiller 38 that produces a significant amount of
refrigeration without any
additional fuel use. The ammonia absorption chiller 38, which is integrated
with a standard
(water) cooling tower 66, helps remove the heat of compression in each stage
of the compressor,
and significantly pre-cools the CNG stream prior to its entry into the main
heat exchanger 90.
The pre-cooled, moderate pressure liquefied/CNG (at about 400 psia) is
separated into two
streams on two occasions, such that one stream becomes the "product" stream,
and the other
streams act as refrigerant streams. The refrigeration is provided by first and
second expansion
valves 98, 102 (via the JT effect), and by a compressor-loaded turbo-expander
110, resulting in
cold, low-pressure recycle streams that need to return to the main compressor
for compression to
about 400 psia. Those recycle stream are used as refrigerants in the main heat
exchanger 90 and
in the sub-cooling heat exchanger 94, with further cold recovery along the
return flow of the
recycle streams. The disclosed system yields clean, cold, low-pressure, sub-
cooled LNG,
suitable for a variety of applications (including as a vehicle fuel). The
disclosed system does not
need complex cascade cycles that use multiple refrigerants and further does
not need a separate
refrigeration cycle (such as are needed in N2 expansion systems, or mixed
refrigerant systems).
The disclosed system does not need to expand high-pressure gas into a low-
pressure pipeline
such as in standard "pressure letdown" cycles at "gate stations". The
disclosed system results in
a ratio of produced product (LNG) to fuel use that will be better than 80 to
20, and possibly in
excess of 85 to 15, depending on further optimizations and the internal
efficiencies of the main
components.
[0144] As outlined in more detail above and below, the disclosed system offers
many advantages
over standard LNG production and to standard CNG production. Broadly, with
regard to LNG
production, the disclosed system may produce a wide-range of LNG products (as
measured by
the temperature, pressure and density of the LNG), but with lower
refrigeration input costs,
which yield lower fuel and operating costs, using readily available equipment.
As in the parent
CA 02787375 2012-08-22
application the disclosed system can operate with low-pressure feed gas, with
only two natural
gas expansion devices, and at production scales as small as 6,000 liters per
day. In summary, the
disclosed system may produce storable, pump-able, and transportable LNG from
low-pressure
feed gas sources, at small production scales and at lower energy input costs
than other systems
facing the same low-pressure and small-scale challenges.
[0145] With regard to CCNG production, the disclosed system offers many
advantages over
standard LNG production and to standard CNG production. The disclosed system
may produce a
new range of dense-phase natural gas products (CCNG) that, while not a liquid,
can be stored
and transported in moderate-pressure cryogenic storage containers, and, most
importantly, can be
pumped by cryogenic liquid pumps to any desired pressure. That range of dense-
phase natural
gas products (CCNG of varying temperatures colder than about -150 F, and
varying pressures
higher than about 700 psia), may be produced with lower refrigeration input
costs, yielding lower
fuel and operating costs, using readily available equipment. As in the parent
application, the
disclosed system can operate with low-pressure feed gas, with only two natural
gas expansion
devices, and at production scales as small as 6,000 liters per day. In
summary, the disclosed
system may produce storable, pump-able, and transportable CCNG from low-
pressure feed gas
sources, at small production scales and at lower energy input costs than other
systems facing the
same low-pressure and small-scale challenges.
[0146] With regard to CNG production, the disclosed system offers a cost-
effective way to
produce dense-phase natural gas (CCNG) during off-peak periods, which can be
pumped by
cryogenic liquid pumps to any desired pressure, for dispensing as cooler-than-
standard (and
denser) CNG, suitable for use in existing on-vehicle CNG fuel tanks, using
readily available
components, only two expansion devices, at scales as small as the equivalent
of 6,000 liquid
gallons per day. In summary, the disclosed system may produce a storable and
pump-able dense-
phase natural gas that can be dispensed as CNG, but without losing the
refrigeration content
inherent in the stored CCNG (as compared to standard L/CNG systems where the
refrigeration
content is lost), and which can be sited at a low-pressure feed gas source, at
production scales
suitable for individual CNG fleets, and which system will have a lower energy
input cost than
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CA 02787375 2012-08-22
=
any L/CNG dispensing system, rivaling the energy input costs of standard CNG
production/dispensing systems, but yielding colder/denser CNG.
[0147] It should be noted that all temperatures and pressures listed are
approximate, and the
disclosed system will work at other selected temperature and pressure values,
but the about 400
psia range of the CNG is a "sweet spot" for a methane expansion cycle. The
heat recovery from
the prime mover 10, and the use of the ammonia absorption chiller 38 is not an
essential element
of the innovation. For example, a high-efficiency gas-fired turbine (for
example, with an adjacent
steam cycle or an organic Rankine cycle) may increase the efficiency of the
prime mover 10 (by
using its waste heat) such that the operation of the ammonia absorption
chiller 38 would not be
viable. In that event, the disclosed system would "spend" more energy on
compressing the CNG,
but by way of a more efficient prime mover, thus causing the total energy use
to be about the
same. Similarly, the main compressor 34 may be, in an alternative embodiment
of the disclosed
system, an electric power driven compressor, especially where low-cost
electricity is available.
The vapor return stream shown on the process flow diagram is to allow any
"flash" from the
liquefied natural gas-fueled vehicle's storage tank to be recycled, rather
than vented. The vapor
return stream may travel within a vapor return line 125. The process flow
diagram shown in
Figures 1 and 2 is for an about 6,000-liter/day plant with a low-pressure
pipeline, for such
customers as LNG vehicles. However, the disclosed system is not limited to
small-scale (pipeline
based) plants. It is unique in its efficiency and relative simplicity and
therefore suitable for small-
scale, low-pressure pipeline sites. However, it will work as well (and more
efficiently) on higher-
pressure gas sources (pipelines, and wells) and at larger scales. The make up
water line 122 on
Figure 1 would come from a standard "city water line". The 4-way valve 126
shown on Figure 2
is merely a "diagram". In reality, those valves will not be in a single
location, as shown. Some
streams may enter other streams through "T" connections without valves. Thus
the 4-way valve
may comprise a single 4-way valve, or a plurality of valves. The flow-rates of
the various
streams are not discussed above because that will vary for each plant, based
on its size. For the
about 6,000-liter/day plant discussed here, the following are approximate gas
flow rates (in
pounds per hour) at typical points in the cycle. The flow rate of LNG (not
including the heavies),
at point 12 in the process flow diagram is approximately 207 lb/h; the make-up
stream from the
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CA 02787375 2012-08-22
pipeline will contain about 327 lb/h, of which approximately 60 lb/h are used
as fuel by the
prime mover; the flow rate at point 9 will be approximately 386 lb/h; the
flash recycle stream at
point 15 will be approximately 179 lb/h; the stream traveling to the expander
toward point 16
will be approximately 1,450 lb/h; the recycle stream at point 17a, having
given up its heavies
content through point 16h, will pass through 17a at approximately 1,398 lb/h;
while the recycle
stream from the sub-cooler, through point 18 and 18a is 179 lb/h. Those flow
rates can vary
depending on factors such as the energy content of the feed gas; the amount of
heavy
hydrocarbons in the feed; the efficiency of the various components, especially
the prime mover
and the cryogenic expander; the desired temperature and pressure of the stored
LNG; and the
level of insulation of all the pipes and cryogenic components. Of course the
above listed values
can be adjusted, modified and tuned by system engineers, dependent on various
factors, such as
but not limited to desired output. The liquid heavies separator 130 (and the
stream of liquid
heavy hydrocarbons) may be in the plant, but may not need to function on those
days when the
make up stream is very low in heavies. However, if the stream is more laden
with heavies, then
some of those heavies could be sent to the engine for fuel, rather than to the
LNG tank. The
above description does not dwell on the type of heat exchangers used, because
those choices are
well understood by gas process engineers and are not relevant to the core
innovations of the
disclosed system. The disclosed system's relatively modest operating pressures
will result in
cost savings on all components, including heat exchangers, when compared to
other cycles that
operate at higher pressures. A discussion of the appropriate insulation of hot
and cold lines, and
the design of valves and sensors are not covered above because those
technologies are well
understood by process engineers.
[0148] It should be noted that the terms "first", "second", and "third", and
the like may be used
herein to modify elements performing similar and/or analogous functions. These
modifiers do
not imply a spatial, sequential, or hierarchical order to the modified
elements unless specifically
stated.
[0149] While the disclosure has been described with reference to several
embodiments, it will be
understood by those skilled in the art that various changes may be made and
equivalents may be
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CA 02787375 2013-07-12
substituted for elements thereof. In addition, many modifications may be made
to adapt a
particular situation or material to the teachings of the disclosure.
Therefore, it is intended that the
disclosure not be limited to the particular embodiments disclosed as the best
mode contemplated
for carrying out this disclosure. The scope of the claims should not be
limited by the
embodiments set out herein but should be given the broadest interpretation
consistent with the
description as a whole.
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