Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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MIXED-REFLUX FOR HEAVIES REMOVAL IN LNG PROCESSING
FIELD OF THE INVENTION
[0001] The present invention relates generally to methods for liquefying
natural gas.
More particularly, but not by way of limitation, embodiments of the present
invention
include systems and methods for removing heavy hydrocarbons from natural gas
using
mixed-refluxed heavy removal columns.
BACKGROUND OF THE INVENTION
[0002] Natural gas is an important resource widely used as energy source or as
industrial
feedstock used in, for example, manufacture of plastics. Comprising primarily
of
methane, natural gas is a mixture of naturally occurring hydrocarbon gases and
is
typically found in deep underground natural rock formations or other
hydrocarbon
reservoirs. Exact composition of natural gas may vary from source to source.
Typically,
natural gas is transported from source to consumers through pipelines that
physically
connect a reservoir to a market. Because natural gas is sometimes found in
remote areas
devoid of necessary infrastructure (i.e., pipelines), alternative methods for
transporting
natural gas must be used. This situation commonly arises when the source of
natural gas
and the market are separated by great distances, for example a large body of
water.
Bringing this natural gas from remote areas to market can have significant
commercial
value if the cost of transporting natural gas is minimized.
[0003] One alternative method of transporting natural gas involves converting
natural gas
into a liquefied form through a liquefaction process. In its liquefied form,
natural gas has
a specific volume that is significantly lower than its specific volume in its
gaseous form.
Thus, the liquefaction process greatly increases the ease of transporting and
storing
natural gas, particularly in cases where pipelines are not available. For
example, ocean
liners carrying LNG tanks can effectively link a natural gas source with a
distant market
when the source and market are separated by large bodies of water. Converting
natural
gas to its liquefied form can have other economic benefits. For example,
storing LNG
can help balance out periodic fluctuations in natural gas supply and demand.
In
particular, LNG can be more easily "stockpiled" for later use when natural gas
demand is
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low and/or supply is high. As a result, future demand peaks can be met with
LNG from
storage, which can be vaporized as demand requires.
[0004] Several methods exist for liquefying natural gas. Some methods produce
a
pressurized LNG (PLNG) product that is useful, but requires expensive pressure-
containing vessels for storage and transportation. Other methods produce an
LNG
product having a pressure at or near atmospheric pressure. In general, these
non-
pressurized LNG production methods involve cooling a natural gas stream
through
indirect heat exchange with one or more refrigerants and then expanding the
cooled
natural gas stream to near atmospheric pressure. In addition, most LNG
facilities employ
one or more systems to remove contaminants (e.g., water, mercury and mercury
components, acid gases, and nitrogen, as well as a portion of ethane and
heavier
components) from the natural gas stream at different points during the
liquefaction
process.
[0005] In order to store and transport natural gas in the liquid state, the
natural gas is
typically cooled to ¨240 F to ¨260 F at near-atmospheric vapor pressure.
Liquefaction
of natural gas can be achieved by sequentially passing the natural gas at an
elevated
pressure through a plurality of cooling stages whereupon the gas is cooled to
successively
lower temperatures until liquefaction temperature is reached. Cooling is
generally
accomplished by indirect heat exchange with one or more refrigerants such as
propane,
propylene, ethane, ethylene, methane, nitrogen, carbon dioxide, or
combinations of the
preceding refrigerants (e.g., mixed refrigerant systems).
[0006] Natural gas is primarily comprised of methane, but may also include
small
amounts of heavy hydrocarbon components. In some cases, the heavy hydrocarbon
components may be utilized as natural gas liquid ("NGL") that includes
components such
as, but not limited to, ethane, propane, normal butane and iso-butane. Heavier
heavy
hydrocarbon components will often require at least partial removal as they
freeze in LNG
streams if present in sufficiently high concentrations. Examples of heavier
heavy
hydrocarbon components may include, but are not limited to, benzene,
cyclohexane,
toluene, ethylbenzene, xylene isomers, and certain isomers of: pentane,
hexane, heptane,
octane, nonane, and decane, and the like.
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[0007] Some conventional LNG facilities employ a refluxed heavies removal
column in
order to enhance heavies removal as compared to facilities employing non-
refluxed
heavies removal column. In general, hydrocarbon reflux must meet appropriate
quality
and quantity standards to achieve effective and efficient removal of heavy
hydrocarbons.
At least one cascade liquefaction process utilizes a debutanizer to provide
reflux to the
heavies removal column. Thus, "lean" natural gases sources lacking adequate
amounts of
C2-C4 hydrocarbons may not be compatible with certain cascade liquefaction
processes
requiring a refluxed heavies removal column because of difficulty of
generating
sufficient quantities of reflux stream with satisfactory composition. Still,
lean natural
gases may contain significant amounts of C6+ hydrocarbons that can freeze
and/or
deposit in downstream cryogenic liquefaction equipment.
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BRIEF SUMMARY OF THE DISCLOSURE
[0008] The present invention relates generally to methods for liquefying
natural gas.
More particularly, but not by way of limitation, embodiments of the present
invention
include systems and methods for removing heavy hydrocarbons from natural gas
using
mixed-refluxed heavy removal columns.
[0009] One example of a method for liquefying a natural gas stream comprises:
(a)
cooling at least a portion of the natural gas stream in an upstream
refrigeration cycle of a
liquefaction process to produce a cooled natural gas stream; (b) separating
via a first
distillation column the cooled natural gas stream into a first top fraction
and a first
bottom fraction, wherein the first distillation column is a heavies removal
column and the
top fraction does not freeze in a subsequent downstream step of the
liquefaction process;
(c) separating via a second distillation column the first bottom fraction into
a second top
fraction and a second bottom fraction; (d) separating via a third distillation
column the
second bottom fraction into a third top fraction and a third bottom fraction;
(e) combining
at least a portion of the second top fraction and a portion of the third top
fraction to form
a mixed-reflux stream; and (f) introducing the mixed-reflux stream into the
first
distillation column.
[0010] Another example of a method for liquefying a natural gas stream
comprises: (a)
cooling at least a portion of the natural gas stream in an upstream
refrigeration cycle of a
liquefaction process to produce a cooled natural gas stream; (b) separating
via a first
distillation column the cooled natural gas stream into a first top fraction
and a first
bottom fraction, wherein the first fraction does not freeze in a subsequent
downstream
step of the liquefaction process; (c) separating via a second distillation
column the first
bottom fraction into a second top fraction and a second bottom fraction,
wherein the
second top fraction at least a portion of a reflux stream; (d) optionally
separating via a
third distillation column the second bottom fraction into a third top fraction
and a third
bottom fraction, wherein the third top fraction forms a portion of the reflux
stream; and
(e) introducing the reflux stream into the first distillation column.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0011] A more complete understanding of the present invention and benefits
thereof may
be acquired by referring to the follow description taken in conjunction with
the
accompanying drawings in which:
[0012] FIG. 1 is a simplified flow diagram of a cascade refrigeration process
for LNG
production compatible with a mixed-reflux heavies removal system according to
one or
more embodiments.
[0013] FIG. 2 is a flow diagram of one aspect of the mixed-reflux heavies
removal
system compatible with the cascade refrigeration process shown FIG. 1.
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DETAILED DESCRIPTION
[0014] Reference will now be made in detail to embodiments of the invention,
one or
more examples of which are illustrated in the accompanying drawings. Each
example is
provided by way of explanation of the invention, not as a limitation of the
invention. It
will be apparent to those skilled in the art that various modifications and
variations can be
made in the present invention without departing from the scope or spirit of
the invention.
For instance, features illustrated or described as part of one embodiment can
be used on
another embodiment to yield a still further embodiment. Thus, it is intended
that the
present invention cover such modifications and variations that come within the
scope of
the invention.
[0015] The present invention provides systems and methods related to heavy
hydrocarbon removal during liquefaction of natural gas ("LNG process").
According to
one or more embodiments, the present invention processes lean natural gas by
generating
a mixed-reflux stream for a heavies removal column. As used herein, "lean
natural gas"
is a natural gas comprising relatively low concentrations of C2-C4 components.
For
example, a natural gas stream may be considered lean if its concentration of
C2-C4 is too
low to provide sufficient reflux in some conventional reflux heavies removal
columns.
As used herein, a "mixed-reflux" is a process stream combined from multiple
downstream locations which may be particularly useful, for example, when a
reflux
stream from a single downstream location does not meet certain desirable
characteristics.
These characteristics may include, but are not limited to, sufficient flow
rates, suitable
composition (due to the overall leanness of the feed natural gas), and the
like. In some
embodiments, the mixed-reflux may be obtained from a combination of overhead
streams
from downstream elements (e.g., debutanizer, condensate stabilizer, etc.).
Moreover,
feed streams to both the debutanizer and the condensate stabilizer may be
originally
sourced in a product stream from the heavies removal column itself
[0016] The heavies removal system according to one or more embodiments
integrates
external heat and refrigerant sources contained within an LNG or gas plant to
enhance
thermal and separation efficiency as well as overall operating flexibility and
stability.
This design also allows independent adjustment of refrigerant and heat
sources, which in
turn, allows adjustments for wider variations in feed composition and promotes
greater
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turn down capacity. Moreover, as compared to many conventional systems and
methods,
advantages of certain embodiments of liquefying natural gas methods and
systems
described herein include, but are not limited to, one or more of the
following:
= may be implemented into existing LNG liquefaction processes and
facilities
without significant departure from known and/or previously-approved process
designs or equipment design criteria for heavies removal,
= provides sufficient quantity of reflux of appropriate composition to
heavies
removal columns when lean feed gases are fed,
= extends operable range of heavies removal column to broader range of feed
gas compositions of commercial interest.
Other advantages will be apparent from the disclosure herein.
[0017] The present invention can be implemented in a facility used to cool
natural gas to
its liquefaction temperature to produce liquefied natural gas (LNG). The LNG
facility
generally employs one or more refrigerants to extract heat from the natural
gas and reject
to the environment. Numerous configurations of LNG systems exist and the
present
invention may be implemented in many different types of LNG systems.
[0018] In one embodiment, the present invention may be implemented in a mixed
refrigerant LNG system. Examples of mixed refrigerant processes can include,
but are
not limited to, a single refrigeration system using a mixed refrigerant, a
propane pre-
cooled mixed refrigerant system, and a dual mixed refrigerant system.
[0019] In another embodiment, the present invention may be implemented in a
cascade
LNG system employing a cascade-type refrigeration process using one or more
predominately pure component refrigerants. The refrigerants utilized in
cascade-type
refrigeration processes can have successively lower boiling points in order to
facilitate
heat removal from the natural gas stream being liquefied. Additionally,
cascade-type
refrigeration processes can include some level of heat integration. For
example, a
cascade-type refrigeration process can cool one or more refrigerants having a
higher
volatility through indirect heat exchange with one or more refrigerants having
a lower
volatility. In addition to cooling the natural gas stream through indirect
heat exchange
with one or more refrigerants, cascade and mixed-refrigerant LNG systems can
employ
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one or more expansion cooling stages to simultaneously cool the LNG while
reducing its
pressure.
Cascade LNG Process
[0020] In one embodiment, the LNG process may employ a cascade-type
refrigeration
process that uses a plurality of multi-stage cooling cycles, each employing a
different
refrigerant composition, to sequentially cool the natural gas stream to lower
and lower
temperatures. For example, a first refrigerant may be used to cool a first
refrigeration
cycle. A second refrigerant may be used to cool a second refrigeration cycle.
A third
refrigerant may be used to cool a third refrigeration cycle. Each
refrigeration cycle may
consider a closed cycle or an open cycle. The terms "first", "second", and
"third" refer to
the relative position of a refrigeration cycle. For example, the first
refrigeration cycle is
positioned just upstream of the second refrigeration cycle while the second
refrigeration
cycle is positioned upstream of the third refrigeration cycle and so forth.
While at least
one reference to a cascade LNG process comprising 3 different refrigerants in
3 separate
refrigeration cycles is made, this is not intended to be limiting. It is
recognized that a
cascade LNG process involving any number of refrigerants and/or refrigeration
cycles
may be compatible with one or more embodiments of the present invention. Other
variations to the cascade LNG process may also be contemplated. In another
embodiment, the mixed-reflux heavies removal system of the present invention
may be
utilized in non-cascade LNG processes. One example of a non-cascade LNG
process
involves a mixed refrigerant LNG process that employs a combination of two or
more
refrigerants to cool the natural gas stream in at least one cooling cycle.
[0021] Referring first to FIG. 1, an example cascade LNG facility in
accordance with the
concept described herein is illustrated. The LNG facility depicted in FIG. 1
generally
comprises a propane refrigeration cycle 30, an ethylene refrigeration cycle
50, and a
methane refrigeration cycle 70 with an expansion section 80. FIG. 2
illustrates an
embodiment of mixed-reflux heavies removal system capable of being integrated
into the
LNG facility depicted in FIG. 1 through conduits A-I. Those skilled in the art
will
recognize that FIGS. 1-2 are schematics only and, therefore, many items of
equipment
that would be needed in a commercial plant for successful operation have been
omitted
for sake of clarity. Such items might include, for example, compressor
controls, flow and
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level measurements and corresponding controllers, temperature and pressure
controls,
pumps, motors, filters, additional heat exchangers, valves, and the like.
These items
would be provided in accordance with standard engineering practice.
[0022] While "propane," "ethylene," and "methane" are used to refer to
respective first,
second, and third refrigerants, it should be understood that the embodiment
illustrated in
FIG. 1 and described herein can apply to any combination of suitable
refrigerants. The
main components of propane refrigeration cycle 30 include a propane compressor
31, a
propane cooler/condenser 32, high-stage propane chillers 33A and 33B, an
intermediate-
stage propane chiller 34, and a low-stage propane chiller 35. The main
components of
ethylene refrigeration cycle 50 include an ethylene compressor 51, an ethylene
cooler 52,
a high-stage ethylene chiller 53, a low-stage ethylene chiller/condenser 55,
and an
ethylene economizer 56. The main components of methane refrigeration cycle 70
include
a methane compressor 71, a methane cooler 72, and a methane economizer 73. The
main
components of expansion section 80 include a high-stage methane expansion
valve
and/or expander 81, a high-stage methane flash drum 82, an intermediate-stage
methane
expansion valve and/or expander 83, an intermediate-stage methane flash drum
84, a low-
stage methane expansion valve and/or expander 85, and a low-stage methane
flash drum
86.
[0023] The operation of the LNG facility illustrated in FIG. 1 will now be
described in
more detail, beginning with propane refrigeration cycle 30. Propane is
compressed in
multi-stage (e.g., three-stage) propane compressor 31 driven by, for example,
a gas
turbine driver (not illustrated). The stages of compression may exist in a
single unit or
two or more separate units mechanically coupled to a single driver. Upon
compression,
the propane is passed through conduit 300 to propane cooler 32 where it is
cooled and
liquefied through indirect heat exchange with an external fluid (e.g., air or
water). A
portion of the stream from propane cooler 32 can then be passed through
conduits 302
and 302A to a pressure reduction means, illustrated as expansion valve 36A,
wherein the
pressure of the liquefied propane is reduced, thereby evaporating or flashing
a portion
thereof The resulting two-phase stream then flows through conduit 304a into
high-stage
propane chiller 33A where it can cool the natural gas stream 110 in indirect
heat
exchange means 38. High stage propane chiller 33A uses the flashed propane
refrigerant
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to cool the incoming natural gas stream in conduit 110. Another portion of the
stream
from propane cooler 32 is routed through conduit 302B to another pressure
reduction
means, illustrated as expansion valve 36B, wherein the pressure of the
liquefied propane
is reduced in stream 304B.
[0024] The cooled natural gas stream from high-stage propane chiller 33A flows
through
conduit 114 to a separation vessel, wherein water and in some cases a portion
of propane
and/or heavier components are removed, typically followed by a treatment
system 40, in
cases where not already completed in upstream processing, wherein moisture,
mercury
and mercury compounds, particulates, and other contaminants are removed to
create a
treated stream. The stream exits the treatment system 40 through conduit 116.
Thereafter, a portion of the stream in conduit 116 can be routed through
conduit A to a
mixed-reflux heavies removal system illustrated in FIG. 2, which will be
discussed in
detail shortly. The stream 116 then enters intermediate-stage propane chiller
34, wherein
the stream is cooled in indirect heat exchange means 41 through indirect heat
exchange
with a propane refrigerant stream. The resulting cooled stream in conduit 118
can then
be recombined with a stream in conduit B exiting mixed-reflux heavies removal
system
illustrated in FIG. 2, and the combined stream can then be routed to low-stage
propane
chiller 35, wherein the stream can be further cooled through indirect heat
exchange
means 42. The resultant cooled stream can then exit low-stage propane chiller
35
through conduit 120. Subsequently, the cooled stream in conduit 120 can be
routed to
high-stage ethylene chiller 53.
[0025] A vaporized propane refrigerant stream exiting high-stage propane
chillers 33A
and 33B is returned to the high-stage inlet port of propane compressor 31
through conduit
306. An unvaporized propane refrigerant stream exits the high-stage propane
chiller 33B
via conduit 308 and is flashed via a pressure reduction means, illustrated
here in FIG. 1 as
expansion valve 43. The liquid propane refrigerant in high-stage propane
chiller 33A
provides refrigeration duty for the natural gas stream 110. Two-phase
refrigerant stream
can enter the intermediate-stage propane chiller 34 through conduit 310,
thereby
providing coolant for the natural gas stream (in conduit 116) and stream
entering
intermediate-stage propane chiller 34 through conduit 204. The vaporized
portion of the
propane refrigerant exits intermediate-stage propane chiller 34 through
conduit 312 and
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enters the intermediate-stage inlet port of propane compressor 31. The
liquefied portion
of the propane refrigerant exits intermediate-stage propane chiller 34 through
conduit 314
and is passed through a pressure-reduction means, illustrated here as
expansion valve 44,
whereupon the pressure of the liquefied propane refrigerant is reduced to
flash or
vaporize a portion thereof The resulting vapor-liquid refrigerant stream can
then be
routed to low-stage propane chiller 35 through conduit 316 and where the
refrigerant
stream can cool the methane-rich stream and an ethylene refrigerant stream
entering low-
stage propane chiller 35 through conduits 118 and 206, respectively. The
vaporized
propane refrigerant stream then exits low-stage propane chiller 35 and is
routed to the
low-stage inlet port of propane compressor 31 through conduit 318 wherein it
is
compressed and recycled as previously described.
[0026] Still referring to FIG. 1, a stream of ethylene refrigerant in conduit
202 enters
high-stage propane chiller 33B, wherein the ethylene stream is cooled through
indirect
heat exchange means 39. The resulting cooled ethylene stream can then be
routed in
conduit 204 from high-stage propane chiller 33B to intermediate-stage propane
chiller 34.
Upon entering intermediate-stage propane chiller 34, the ethylene refrigerant
stream can
be further cooled through indirect heat exchange means 45 in intermediate-
stage propane
chiller 34. The resulting cooled ethylene stream can then exit intermediate-
stage propane
chiller 34 and can be routed through conduit 206 to enter low-stage propane
chiller 35. In
low-stage propane chiller 35, the ethylene refrigerant stream can be at least
partially
condensed, or condensed in its entirety, through indirect heat exchange means
46. The
resulting stream exits low-stage propane chiller 35 through conduit 208 and
can
subsequently be routed to a separation vessel 47, wherein a vapor portion of
the stream, if
present, can be removed through conduit 210, while a liquid portion of the
ethylene
refrigerant stream can exit separator 47 through conduit 212. The liquid
portion of the
ethylene refrigerant stream exiting separator 47 can have a representative
temperature
and pressure of about -24 F (about -31 C) and about 285 psia (about 1,965
kPa).
[0027] Turning now to the ethylene refrigeration cycle 50 in FIG. 1, liquefied
ethylene
refrigerant stream in conduit 212 can enter an ethylene economizer 56, wherein
the
stream can be further cooled by an indirect heat exchange means 57. The
resulting
cooled liquid ethylene stream in conduit 214 can then be routed through a
pressure
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reduction means, illustrated here as expansion valve 58, whereupon the
pressure of the
cooled predominantly liquid ethylene stream is reduced to thereby flash or
vaporize a
portion thereof. The cooled, two-phase stream in conduit 215 can then enter
high-stage
ethylene chiller 53. In high-stage ethylene chiller 53, at least a portion of
the ethylene
refrigerant stream can vaporize to further cool the stream in conduit 121
entering an
indirect heat exchange means 59. The vaporized and remaining liquefied
ethylene
refrigerant exits high-stage ethylene chiller 53 through conduits 216 and 220,
respectively. The vaporized ethylene refrigerant in conduit 216 can re-enter
ethylene
economizer 56, wherein the stream can be warmed through an indirect heat
exchange
means 60 prior to entering the high-stage inlet port of ethylene compressor 51
through
conduit 218.
Ethylene is compressed in multi-stage (e.g., three-stage) ethylene
compressor 51 driven by, for example, a gas turbine driver (not illustrated).
The stages of
compression may exist in a single unit or two or more separate units
mechanically
coupled to a single driver.
[0028] The cooled stream in conduit 120 exiting low-stage propane chiller 35
can
thereafter be split into two portions, as shown in FIG. 1. At least a portion
of the cooled
natural gas stream can be routed through conduit E while a remaining portion
of the
cooled natural gas stream in conduit 121 can be routed to high-stage ethylene
chiller 53,
where it is cooled via indirect heat exchange means 59 of high-stage ethylene
chiller 53.
The cooled natural gas stream in conduit E may be routed to a mixed-reflux
heavies
removal system according to one or more embodiments (see FIG. 2) where the
stream is
flashed through a pressure reduction means, illustrated here as expansion
valve 612,
before it is fed into heavies removal column 610 (described in more detail
later).
[0029] The remaining liquefied ethylene refrigerant exiting high-stage
ethylene chiller 53
in conduit 220 can re-enter ethylene economizer 56 and undergo further sub-
cooling by
an indirect heat exchange means 61 in ethylene economizer 56. The resulting
sub-cooled
refrigerant stream exits ethylene economizer 56 through conduit 222 and
subsequently
passes a pressure reduction means, illustrated here as expansion valve 62,
whereupon the
pressure of the refrigerant stream is reduced to vaporize or flash a portion
thereof The
resulting, cooled two-phase stream in conduit 224 enters low-stage ethylene
chiller/condenser 55.
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[0030] A portion of the cooled natural gas stream exiting high-stage ethylene
chiller 53
can be routed through conduit C to the mixed-reflux heavies removal system in
FIG. 2,
while another portion of the cooled natural gas stream exiting high-stage
ethylene chiller
53 can be routed through conduit 122 to enter indirect heat exchange means 63
of low-
stage ethylene chiller/condenser 55. The cooled natural gas stream in conduit
C may be
routed to a mixed-reflux heavies removal system according to one or more
embodiments
(See FIG. 2) where the stream is flashed through a pressure reduction means,
illustrated
here as expansion valve 613, before combining with the stream in conduit E.
The portion
of the cooled natural gas stream in conduit 122 can then be combined the first
column
vapor stream exiting the mixed-reflux heavies removal system in conduit D
and/or may
be combined with a stream exiting methane refrigeration cycle 70 in conduit
168. The
resulting composite stream can then enter indirect heat exchange means 63 in
low-stage
ethylene chiller/condenser 55.
[0031] In the low-stage ethylene chiller/condenser 55, cooled stream (which
can include
stream in conduit 122 and optionally streams in conduits D and 168) can be at
least
partially condensed and, often, subcooled through indirect heat exchange with
the
ethylene refrigerant entering low-stage ethylene chiller/condenser 55 through
conduit
224. The vaporized ethylene refrigerant exits low-stage ethylene
chiller/condenser 55
through conduit 226, which then enters ethylene economizer 56. In the ethylene
economizer 56, vaporized ethylene refrigerant stream 226 can be warmed through
an
indirect heat exchange means 64 prior to being fed into the low-stage inlet
port of
ethylene compressor 51 through conduit 230. As shown in FIG. 1, a stream of
compressed ethylene refrigerant exits ethylene compressor 51 through conduit
236 and
subsequently enters ethylene cooler 52, wherein the compressed ethylene stream
can be
cooled through indirect heat exchange with an external fluid (e.g., water or
air). The
resulting cooled ethylene stream is introduced through conduit 202 into high-
stage
propane chiller 33B for additional cooling as previously described.
[0032] The condensed and, often, subcooled liquid natural gas stream exiting
low-stage
ethylene chiller/condenser 55 in conduit 124 can also be referred to as a
"pressurized
LNG-bearing stream." This pressurized LNG-bearing stream exits low-stage
ethylene
chiller/condenser 55 through conduit 124 prior to entering main methane
economizer 73.
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In the main methane economizer 73, methane-rich stream in conduit 124 can be
further
cooled in an indirect heat exchange means 75 through indirect heat exchange
with one or
more methane refrigerant streams (e.g., 76, 77, 78). The cooled, pressurized
LNG-
bearing stream exits main methane economizer 73 through conduit 134 and routes
to
expansion section 80 of methane refrigeration cycle 70. In the expansion
section 80, the
pressurized LNG-bearing stream first passes through high-stage methane
expansion valve
or expander 81, whereupon the pressure of this stream is reduced to vaporize
or flash a
portion thereof The resulting two-phase methane-rich stream in conduit 136 can
then
enter into high-stage methane flash drum 82, whereupon the vapor and liquid
portions of
the reduced-pressure stream can be separated. The vapor portion of the reduced-
pressure
stream (also called the high-stage flash gas) exits high-stage methane flash
drum 82
through conduit 138 to then enter into main methane economizer 73, wherein at
least a
portion of the high-stage flash gas can be heated through indirect heat
exchange means 76
of main methane economizer 73. The resulting warmed vapor stream exits main
methane
economizer 73 through conduit 138 and is then routed to the high-stage inlet
port of
methane compressor 71, as shown in FIG. 1.
[0033] The liquid portion of the reduced-pressure stream exits high-stage
methane flash
drum 82 through conduit 142 to then re-enter main methane economizer 73,
wherein the
liquid stream can be cooled through indirect heat exchange means 74 of main
methane
economizer 73. The resulting cooled stream exits main methane economizer 73
through
conduit 144 and then routed to a second expansion stage, illustrated here as
intermediate-
stage expansion valve 83 and/or expander. Intermediate-stage expansion valve
83 further
reduces the pressure of the cooled methane stream which reduces the stream's
temperature by vaporizing or flashing a portion thereof The resulting two-
phase
methane-rich stream in conduit 146 can then enter intermediate-stage methane
flash drum
84, wherein the liquid and vapor portions of this stream can be separated and
exits the
intermediate-stage flash drum 84 through conduits 148 and 150, respectively.
The vapor
portion (also called the intermediate-stage flash gas) in conduit 150 can re-
enter methane
economizer 73, wherein the vapor portion can be heated through an indirect
heat
exchange means 77 of main methane economizer 73. The resulting warmed stream
can
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then be routed through conduit 154 to the intermediate-stage inlet port of
methane
compressor 71, as shown in FIG. 1.
[0034] The liquid stream exiting intermediate-stage methane flash drum 84
through
conduit 148 can then pass through a low-stage expansion valve 85 and/or
expander,
whereupon the pressure of the liquefied methane-rich stream can be further
reduced to
vaporize or flash a portion thereof. The resulting cooled, two-phase stream in
conduit
156 can then enter low-stage methane flash drum 86, wherein the vapor and
liquid phases
are separated. The liquid stream exiting low-stage methane flash drum 86
through
conduit 158 can comprise the liquefied natural gas (LNG) product at near
atmospheric
pressure. This LNG product can be routed downstream for subsequent storage,
transportation, and/or use.
[0035] A vapor stream exiting low-stage methane flash drum (also called the
low-stage
methane flash gas) in conduit 160 can be routed to methane economizer 73,
wherein the
low-stage methane flash gas can be warmed through an indirect heat exchange
means 78
of main methane economizer 73. The resulting stream can exit methane
economizer 73
through conduit 164, whereafter the stream can be routed to the low-stage
inlet port of
methane compressor 71.
[0036] The methane compressor 71 can comprise one or more compression stages.
In
one embodiment, methane compressor 71 comprises three compression stages in a
single
module. In another embodiment, one or more of the compression modules can be
separate but mechanically coupled to a common driver. Generally, one or more
intercoolers (not shown) can be provided between subsequent compression
stages.
[0037] As shown in FIG. 1, a compressed methane refrigerant stream exiting
methane
compressor 71 can be discharged into conduit 166. A portion of the compressed
methane
refrigerant stream exiting compressor 71 through conduit 166 can be routed
through
conduit F to the mixed-reflux heavies removal system in FIG. 2, while another
portion of
the compressed methane refrigerant can be routed to methane cooler 72,
whereafter the
stream can be cooled through indirect heat exchange with an external fluid
(e.g., air or
water) in methane cooler 72. The resulting cooled methane refrigerant stream
exits
methane cooler 72 through conduit 112, wherein a portion of the methane
refrigerant can
be routed through conduit H to the mixed-reflux heavies removal system in FIG.
2, while
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the remaining portion of the methane refrigerant stream can be directed to and
further
cooled in propane refrigeration cycle 30.
[0038] Upon cooling in the propane refrigeration cycle 30 through heat
exchanger means
37, the methane refrigerant stream can be discharged into conduit 130 where it
may be
combined with methane-rich gas in conduit G from the mixed-reflux heavies
removal
system and subsequently routed to main methane economizer 73, wherein the
stream can
be further cooled through indirect heat exchange means 79. The resulting sub-
cooled
stream exits main methane economizer 73 through conduit 168 and then combined
with
stream in conduit 122 exiting high-stage ethylene chiller 53 and/or with
stream in conduit
D prior to entering low-stage ethylene chiller/condenser 55, as previously
discussed.
[0039] The liquefaction process described herein may incorporate one of
several types of
cooling means including, but not limited to, (a) indirect heat exchange, (b)
vaporization,
and (c) expansion or pressure reduction. Indirect heat exchange, as used
herein, refers to
a process wherein a cooler stream cools the substance to be cooled without
actual
physical contact between the cooler stream and the substance to be cooled.
Specific
examples of indirect heat exchange means include heat exchange undergone in a
shell-
and-tube heat exchanger, a core-in-shell heat exchanger, and a brazed aluminum
plate-fin
heat exchanger. The specific physical state of the refrigerant and substance
to be cooled
can vary depending on demands of the refrigeration system and type of heat
exchanger
chosen.
[0040] Vaporization cooling refers to the cooling of a substance by
evaporation or
vaporization of a portion of the substance at a constant pressure. During
vaporization,
portion of the substance which evaporates absorbs heat from portion of the
substance
which remains in a liquid state and hence, cools the liquid portion. Finally,
expansion or
pressure reduction cooling refers to cooling which occurs when the pressure of
a gas,
liquid or a two-phase system is decreased by passing through a pressure
reduction means.
In some embodiments, expansion means may be a Joule-Thomson expansion valve.
In
other embodiments, the expansion means may be either a hydraulic or gas
expander.
Because expanders recover work energy from the expansion process, lower
process
stream temperatures are possible upon expansion.
Mixed-Reflux Heavies Removal Column
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[0041] Referring to FIG. 2, an example of mixed-reflux heavies removal system
in
accordance with the concepts described herein is illustrated. Some of the main
distillation columns of the mixed-reflux heavies removal system include
heavies removal
column 610, nominal debutanizer 620, and condensate stabilizer 630. Other
components
of the mixed-reflux heavies removal system may include heavies removal column
reboiler 606, debutanizer reboiler 625, stabilizer feed drum 640, stabilizer
reboiler 637,
mixed reflux drum 650, valves and/or expanders (e.g., 612, 613), pumps (e.g.,
629, 677,
691, 692), various conduits (described in more detail later), and the like.
[0042] At least a portion of the natural gas stream withdrawn from conduit 116
in FIG. 1
can be routed to the mixed-reflux heavies removal system depicted in FIG. 2
through
conduit A. Referring to FIG. 2, the natural gas stream in conduit A enters the
warm fluid
inlet of reboiler 606 to form a heating pass 607 and provide reboiler heat
duty to the
heavies removal column 610. Alternatively, some embodiments may utilize other
known
methods, such as a thermosyphon reboiler or direct heat through a stripping
gas or
heating gas introduced directly to the heavies removal column, to provide heat
duty for
the heavies removal column 610. One or more kettle, thermosyphon or pump
around
exchangers may be provided for the heavies removal column exchangers as well
as one
or more heating or stripping gas streams. Consequently, the reboiler 606 can
produce
heated vapor fraction in conduit 608A, heated liquid fraction in conduit 608B,
and cooled
and/or partially condensed natural gas stream (conduit B). Alternatively, some
embodiments may utilize other known methods to provide for boilup vapor to the
heavies
removal column 610. The use of natural gas in conduit A as heating medium as
described herein is one of a number of possible embodiments. The heavies
removal
column 610 contains a chimney or trap-out tray wherein lighter composition
streams are
directed to the upper regions of the distillation column while heavier
composition streams
are routed to the lower portions of the distillation column. The reboiler 606
supplies heat
by a controlled slip stream of warm upstream feed gas (conduit A). This flow
of warm
feed gas is adjusted by a temperature controller on the reboiler heating inlet
stream
(conduit A). Once cooled and/or partially condensed, a portion of the natural
gas stream
is withdrawn from the warm side outlet of the reboiler 606 and routed back
into the main
liquefaction process (i.e., process illustrated in FIG. 1) through conduit B.
A reboiler
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inlet stream exits the heavies removal column 610 at the chimney or trap-out
tray and is
routed through conduit 614 to the reboiler 606. A liquid bottoms product
stream (or
"liquid bottom stream") in conduit 601 exits the heavies removal column 610.
The liquid
bottom stream is subjected to a pressure reduction means, illustrated here as
expansion
valve 699, to form a flashed or expanded two-phase stream 601A.
[0043] As shown in FIG. 1, a portion of a methane-rich stream exiting a high
stage
methane compressor through conduit 166 can be withdrawn through conduit F or H
and
routed to the mixed-reflux heavies removal system depicted in FIG. 2. In some
embodiments, a portion of the methane-rich stream in conduit F or H may be a
methane
compressor discharge stream. As shown in FIG. 2, the methane-rich stream in
conduit F
or H can enter the warm fluid inlet of a cooling pass 605 of preheater 615 to
provide heat
duty to the preheater 615. The methane-rich stream in conduit F or H and the
flashed or
expanded two phase stream 601A undergo indirect heat exchange to produce a
cooled
portion of the methane-rich stream and a heated liquid stream in conduit 602.
The
resulting cooled portion of the methane-rich stream can be routed back to the
main
liquefaction process through conduit G to various possible destinations, such
as stream
130, depending on the temperature and pressure of the gas in conduit G. The
heated
stream exiting preheater 615 is introduced as feed to the nominal debutanizer
620.
[0044] In the illustrated embodiment, a debutanizer overhead stream in conduit
603
provides nominal C4- recovery while a debutanizer bottom stream 604 provides
nominal
C5+ rejection. A portion of the C4- stream returns as part of the mixed-reflux
to the
heavies removal column 610. A non-condensed vapor portion of the C4-
eventually
returns to an appropriate compressor stage inlet of the methane compression
loop of the
main liquefaction process via conduit I. Alternatively, the stream in conduit
I may be
routed to fuel (not illustrated). The C5+ stream in conduit 604 is eventually
removed
from mixed-reflux heavies removal system as a byproduct condensate stream
(conduit J).
[0045] Still referring to FIG. 2, prior to reaching mixed reflux drum 650, the
debutanizer
overhead vapor stream in conduit 603 is routed to a partial condenser 660,
wherein the
vapor is partially condensed (with air or water cooling). The partially-
condensed stream
in conduit 616 flows from partial condenser 660 to debutanizer reflux drum 628
from
which liquid reflux is returned to the debutanizer in conduit 662 using reflux
pump 629.
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Vapor from debutanizer reflux drum 628 flows in conduit 661 to combine with
liquid in
conduit 632 from the overhead of the condensate stabilizer 630. A condensate
stabilizer
630 is installed downstream of the nominal debutanizer 620. A combined stream
671 of
vapor stream from debutanizer overhead stream in conduit 661 and condensate
stabilizer
overhead stream in conduit 632 is eventually collected in the mixed-reflux
drum 650.
Prior to reaching mixed-reflux drum 650, the condensate stabilizer overhead
stream in
conduit 631 is routed to a condenser 635 where it is condensed (with air or
water cooling)
to form a condensed overhead stream 636. A portion of the liquid in conduit
636 is
returned as reflux in conduit 634 using reflux pump 692 to the condensate
stabilizer 630.
Another portion of the liquid in conduit 636 is pumped by recycle pump 691 in
conduit
632 to combine with the debutanizer reflux drum 628 overhead vapor in conduit
661 and
with the resultant combined two-phase stream flowing in conduit 671. If the
flow in
conduit 671 is in excess then a portion of the liquid in conduit 632 may be
optionally
withdrawn in conduit 633 to a product storage of the LNG facility as shown in
FIG. 2.
Other strategies for control of inventory, or levels, in the heavies removal
system may be
added as needed but are not illustrated in this particular embodiment. As
shown, the
combined two-phase stream in conduit 671 is cooled and further condensed by a
propane
refrigerant (provided from the main liquefaction process but not illustrated
in FIG. 1) in
indirect heat exchangers 670 and 675 through cooling passes 672 and 673,
respectively,
to provide a cooled combined two-phase stream in conduit 674. A reflux drum
bottom
stream 676 exiting the mixed-reflux drum 650 is pumped to a mixed-reflux
subcooler 680
via pump 677. The combined stream in conduit 676 is subcooled typically using
high
stage ethylene refrigerant (provided from the main liquefaction process but
not illustrated
in FIG. 1) in exchanger means 681 of the mixed-refluxed subcooler 680 prior to
delivery
to the heavies removal column 610 as mixed-reflux in conduit 611. As
previously
mentioned, this mixed-reflux rate provides both adequate volumetric flow for
packing
irrigation and molar flow/composition to sufficiently absorb the heavy
components in the
feed gas found in conduit 609 which is fed into the heavies removal column
610.
Heavies Removal Column
[0046] In the illustrated embodiment shown in FIG. 2, the heavies removal
column 610 is
a packed column with two or more sections of varying diameters. Above the main
feed
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point C, the heavies removal column 610 is typically larger in diameter than
below the
main feed point C and consequently requires a relatively high reflux rate for
proper
irrigation of the packing. For "lean" natural gas, the overhead flow 603 of
the nominal
debutanizer 620 may be of insufficient flow to support the irrigation
requirements of the
heavies removal column 610. As mentioned earlier, some feed gases may contain
low
concentrations of C2-C4 components which results in inadequate flow of the C4-
overhead stream in conduit 603 from the nominal debutanizer 620. Without
adequate
flow, sufficient reflux to the heavies removal column 610 cannot be supplied.
Generally,
the heavies removal column 610 must have a reflux of sufficient quantity and
of
appropriate composition to: (1) remove (e.g., by absorption) enough C6+
heavies to
prevent downstream freezing and (2) properly irrigate random packing in the
heavies
removal column 610. The latter aspect of proper irrigation supports the former
aspect of
proper reflux flow rate and composition. Performance of the heavies removal
column
610 can depend greatly on the irrigation rate per unit cross sectional area of
the packing.
In some embodiments, an irrigation rate typically between 0.5 to 1.5 gallons
per minute
per square foot of column cross sectional area may be required to ensure
proper liquid
distribution within the heavies removal column. The appropriate minimum
irrigation rate
may depend on a number of factors including, but not limited to, packing and
column
dimensions, internal vapor and liquid flow rates, and packing type of the
heavies removal
column. Therefore, a mixed-reflux stream 611 that includes a mixture of the
overhead
streams from nominal debutanizer 620 and condensate stabilizer 630 is used.
[0047] Feed temperature to the heavies removal column 610 and column pressure
may be
monitored and controlled to insure that vapor-to-liquid density ratios and
other
vapor/liquid behavior within the columns are appropriate. Temperature control
may also
be required to maintain relative constancy of the liquid fraction of the
heavies removal
column feed stream C. In some cases, the heavies removal column 610 may flood
in the
bottom section (if feed temperature too low) or go off specification in terms
of heavies
removal in the top section (if feed temperature is too high). In some
embodiments,
advanced regulatory control techniques may be employed to stabilize the feed
temperature and other aspects of the column's operation (not illustrated). In
some
embodiments, the heavies removal column 610 may have multiple feeds based on
the
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overall process optimization of the LNG plant and not just a single main feed
as
represented in FIG. 2. Additional feed nozzle(s) may be provided on the
heavies removal
column 610 to accommodate varying feed gas compositions such as, for example,
a C8+
rich liquid stream which could be collected upstream as result of condensation
from a
feed gas with C8+ levels in excess of normal.
Debutanizer
[0048] In some embodiments, the nominal debutanizer 620 may be a frayed or
packed or
both. In the embodiment shown in FIG. 2, the feed stream in conduit 602 to the
nominal
debutanizer 620 is a two-phase stream in which the vapor and liquid phases may
be fed to
different trays if a feed separator (not illustrated) is included in the mixed-
reflux heavies
removal system. In some embodiments, the column pressure of nominal
debutanizer 620
should be set as low as possible for separation efficiency but sufficiently
high such that
the vapor portion of the overhead product (which cannot be condensed against
low-stage
propane refrigerant) can be returned to the main liquefaction process for
recompression.
Heat duty is provided to the nominal debutanizer 620 by a debutanizer reboiler
625 that
typically uses hot oil as the energy supply.
[0049] Still referring to FIG. 2, the nominal debutanizer 620 is typically
operated with a
sufficiently high pressure in the partial condenser 660 such that after
condensation using
two (or more) levels of propane refrigeration, a residual vapor stream in
conduit I is
typically returned in a controlled fashion to a high stage methane compressor.
As
mentioned earlier, the condensed debutanizer overhead stream is then routed to
a
debutanizer reflux drum 628, wherein the stream can be separated into a vapor
stream in
conduit 661 and a liquid stream in conduit 662. The liquid stream 662 is
pumped back to
the nominal debutanizer 620 via reflux pump 629 while the vapor stream 661 is
combined with a portion of a condensed overhead stream in conduit 632 prior to
being
routed to a series of indirect heat exchangers (670 and 671). A portion of the
condensed
condensate stabilizer overhead stream in conduit 634 is routed back to the
condensate
stabilizer 630 as reflux while another portion of the condensed overhead
stream in
conduit 633 may be directed to product storage for inventory control within
the heavies
removal system. In some embodiments, the operating pressure may be selected
sufficiently high to accommodate condensation of an adequate reflux flow rate
by air
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cooling for the primary condensation in the partial condenser 660. The nominal
debutanizer 620 may be designed with a specific reflux rate and reboiler duty
for optimal
number of trays and feed locations, such that desired specifications of
debutanizer
overhead stream in conduit 603 and debutanizer bottom stream in conduit 604
are
achieved. In some embodiments, advanced regulatory control may also be
provided for
the debutanizer, including decoupled controllers for top pressure control and
debutanizer
reflux drum 628 level control. In some embodiments, hot oil flow to the
debutanizer
reboiler 625 may be used to stabilize a controlled temperature on a
temperature-sensitive
tray near the bottom of the column.
Condensate Stabilizer
[0050] In some embodiments, the condensate stabilizer 630 may be trayed or
packed or
both. The feed stream in conduit 604 to the condensate stabilizer 630 is two-
phase. The
vapor and liquid phases may be fed to different trays if a feed separator drum
has been
included. .The condensate stabilizer 630 is designed with certain reflux rate
and reboiler
duty that are compatible with an optimal number of trays and feed location,
such that
desired specifications for the top and bottom products are achieved. Coupled
to the
condensate stabilizer 630 is a stabilizer reboiler 637 typically using hot oil
as the energy
supply. In some embodiments, advanced regulatory control may be provided for
the
condensate stabilizer 630, including feed forward of upstream flow to adjust
both the
reflux flow controller and bottom temperature controller.
[0051] While at least one embodiment described is a mixed-reflux heavies
removal
system comprising process streams resulting from a condensation of bottom
stream of the
heavies removal column, this is not intended to be limiting. In some
embodiments, the
present invention comprises a mixed-reflux comprising two or more mixed
process
streams one of which is resulting from a condensation of the overhead vapor
stream of
the heavies removal column.
[0052] Furthermore, in some embodiments, the reflux to the heavies removal
column
610 may arise solely from the overhead stream of the nominal debutanizer, that
is, from a
single source. These embodiments may be particularly useful when the
composition of
the natural gas is such that the debutanizer can be operated to simultaneously
achieve
specifications of flow rate and composition for the reflux to the heavies
removal column
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610 while producing a debutanizer bottoms product (e.g., condensate product)
with
acceptable properties (e.g., Reid Vapor Pressure). In such embodiments, the
condensate
stabilizer 630 may not be required. The ultimately noncondensed vapor portion
of the
debutanizer (in conduit I) may be required to return to the main liquefaction
process at a
lower pressure stage of the methane compression.
[0053] In closing, it should be noted that the discussion of any reference is
not an
admission that it is prior art to the present invention, especially any
reference that may
have a publication date after the priority date of this application. At the
same time, each
and every claim below is hereby incorporated into this detailed description or
specification as a additional embodiments of the present invention.
[0054] Although the systems and processes described herein have been described
in detail,
it should be understood that various changes, substitutions, and alterations
can be made
without departing from the spirit and scope of the invention as defined by the
following
claims. Those skilled in the art may be able to study the preferred
embodiments and
identify other ways to practice the invention that are not exactly as
described herein. It is
the intent of the inventors that variations and equivalents of the invention
are within the
scope of the claims while the description, abstract and drawings are not to be
used to
limit the scope of the invention. The invention is specifically intended to be
as broad as
the claims below and their equivalents.
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REFERENCES
[0055] All of the references cited herein are expressly incorporated by
reference. The
discussion of any reference is not an admission that it is prior art to the
present invention,
especially any reference that may have a publication data after the priority
date of this
application. Incorporated references are listed again here for convenience:
1. US 8,257,508
2. US 7,600,395
3. US 2012/0118007
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