Note: Descriptions are shown in the official language in which they were submitted.
CA 02919209 2016-01-22
WO 2015/017357
PCT/US2014/048522
PROCESS FOR LIQUEFACTION OF NATURAL GAS
PRIORITY CLAIM OF EARLIER NATIONAL APPLICATIONS
[0001] This application claims priority to U.S. Application No.
61/860,319 filed July 31,
2013 and U.S. Application No. 14/338,982 filed July 23, 2014.
BACKGROUND OF THE INVENTION
[0002] Liquefied natural gas or LNG is natural gas (predominantly
methane, CH4) that
has been converted to liquid form for ease of storage or transport. The
liquefaction process
involves removal of certain components, such as dust, acid gases, helium,
water, and
heavy hydrocarbons, which could cause difficulty downstream in the
liquefaction process.
The natural gas is condensed into a liquid at ambient pressure. It is
typically liquefied at
ambient pressure at ¨101 C (maximum transport pressure set at around 25 kPa
(3.6 psi) and
then cooled to ¨162 C by using a Joule-Thompson expansion or through the use
of a
subcooler. By raising the pressure of liquefaction, the latent duty is
reduced, improving the
efficiency of the liquefaction cycle. LNG achieves a higher reduction in
volume
than compressed natural gas (CNG) so that the energy density of LNG is 2.4
times greater
than that of CNG and 60% of that of diesel fuel. This makes LNG cost efficient
to transport
over long distances where pipelines do not exist.
[0003] Specially designed cryogenic sea vessels (LNG carriers) or
cryogenic road tankers
are used for its transport. The natural gas fed into the LNG plant is treated
to remove
water, hydrogen sulfide, carbon dioxide and other components that will freeze
(such as
benzene) under the low temperatures needed for storage or be destructive to
the liquefaction
facility. Also, hydrocarbons heavier than methane are removed for higher value
uses. LNG
typically contains more than 90% methane. It also contains small amounts
of ethane, propane, butane, some heavier alkanes, and Nitrogen. The
purification process can
be designed to give almost 100% methane.
[0004] The cost of building an LNG liquefaction plant has steadily
increased by five
times more as compared to ten years ago, largely due to high raw material
prices as well as
other factors. Due to these high costs it is desirable to develop more
efficient processes and
equipment for producing LNG.
- 1 -
CA 02919209 2016-01-22
WO 2015/017357
PCT/US2014/048522
[0005] Pipeline natural gas typically contains levels of H20, CO2, and
other materials
which are perceived to require removal prior to liquefaction. Due to their
freeze points,
during the cooling process, these materials will tend to foul the heat
exchangers and lead to
blockages. Therefore, in order to solve this problem, the industry will
typically use PSA,
solvent scrubbers and membranes in order to remove these contaminants prior to
liquefaction.
While cost effective in large scale units, the additional costs in small scale
units tends to drive
up the total cost of ownership for the production of LNG. The issue therefore
is to extract
these contaminants without adding large amounts of capital cost.
SUMMARY OF THE INVENTION
[0006] The invention provides a process and a system for producing a LNG
product.
There are several variations of the process and the system. The process, which
uses several
purification devices, chillers and a column, includes multiple steps.
[0007] In an embodiment of the process, at least a portion of the
supply of natural gas is
at least partially purified with removal of water and other contaminants. The
next step is to
feed a stream of LNG as well as the supply of natural gas to a column such as
a quench
tower. The first section of the process uses latent heat in the LNG to cool
the feed incoming
natural gas. The first section is run with excess liquid such that the
volatile components in the
natural gas as condensed and frozen into the cooling liquid.
[0008] The chilled gas may be a quantity of the product from the
process, liquefied
natural gas. A portion of at least one contaminant condenses, solidifies or
dissolves. The next
step may be to filter out these contaminants followed by further cooling to
the necessary
temperature to produce LNG. In another embodiment of the invention, solids may
end up in
the end product.
[0009] The process involves liquefaction of natural gas, comprising
cooling a natural gas
stream to a temperature from 00 to ¨100 C to produce a cooled natural gas
stream, sending
the cooled natural gas stream to a quench unit, sending a quantity of
liquefied natural gas to
the quench unit to be combined with the cooled natural gas steam to produce a
bottoms
stream comprising an intermediate cooled natural gas stream comprising solid
impurities and
a top stream comprising methane and incondensable impurities, The bottoms
stream is then
sent to a unit to remove solid impurities to a produce a purified bottoms
stream; and the
purified bottoms stream is then cooled to produce liquefied natural gas. Prior
to the cooling of
- 2 -
CA 02919209 2016-01-22
WO 2015/017357
PCT/US2014/048522
the natural gas stream, at least a portion of water is removed as well as
other contaminants.
The natural gas stream may be cooled to a temperature of ¨25 to ¨75 C or ¨55
to ¨60 C.
The solid impurities that are removed include carbon dioxide, C5 and C6
hydrocarbons and
water. The incondensable impurities are selected from the group consisting of
nitrogen,
helium, oxygen and hydrogen. Typically, the bottoms stream is at a temperature
from ¨75 to
¨120 C and preferably it is at a temperature from ¨90 to ¨100 C. The
invention also
comprises a system for producing liquefied natural gas from a supply of
natural gas,
comprising a device for supplying a stream of natural gas, a separation device
to remove
water and other impurities from the stream of natural gas to produce a
partially dried stream
of natural gas, a means for feeding the partially dried stream of natural gas
to a chilling
device and then to a quench tower, a means for feeding a quantity of a chilled
liquid to the
quench tower where the chilled liquid and said partially dried stream of
natural gas are
present in said quench tower and a means to vent a purge stream from said
quench tower; a
means to transport a combination of the chilled liquid and the partially dried
stream of natural
gas to a filter unit; a means to transport a purified stream from the filter
unit to a chiller to
produce liquefied natural gas and a means to the said liquefied natural gas to
a storage device.
The LNG may then be transported by ship or truck to a customer.
[0010]
The invention overcomes the issue of fouling or blocking of the heat exchange
device used to liquefy the natural gas by undertaking the majority of the
cooling duty of the
natural gas stream through direct contact with sub cooled LNG or saturated
LNG. The natural
gas stream is introduced to a quench unit and sending a quantity of liquefied
natural gas to
the quench unit to be combined with the natural gas stream, a bottoms stream
comprising
solid impurities in a liquefied hydrocarbon is produced and a top stream
comprising methane
and incondensable impurities. The bottoms stream is then sent to a unit to
remove solid
impurities to a produce a purified bottoms stream. The bottoms stream may be
cooled to the
liquefied natural gas temperature prior to or after purification. Without a
reduction in
pressure, this cooling of the natural gas will produce a sub cooled liquefied
natural gas. A
portion of the impurities, such as water, may be removed by first passing the
gas through
other impurity removal devices in order to enable some of the cooling to be
undertaken in an
indirect heat exchange device. In such a case, the natural gas stream may be
cooled to a
temperature of ¨25 to ¨75 C prior to introduction into the quench device. The
solid
impurities that are removed include carbon dioxide, C5 and C6 hydrocarbons and
water. The
- 3 -
CA 02919209 2016-01-22
WO 2015/017357
PCT/US2014/048522
incondensable impurities are selected from the group consisting of nitrogen,
oxygen, helium
and hydrogen. Typically, the bottoms stream is at a temperature from ¨75 to
¨120 C and
preferably it is at a temperature from ¨90 to ¨100 C. The top stream may be
condensed
using either an indirect liquefaction process, such as a heat exchanger, or
through direct
contact with a portion of the sub cooled liquefied natural gas. Further, by
running this top
stream liquefied with excess sub cooled natural gas, a feed stream of sub
cooled or saturated
LNG may be provided to the quench unit. A means to transport a purified stream
from the
filter unit to a chiller to produce liquefied natural gas and a means to the
said liquefied natural
gas to a storage device. The LNG may then be transported by ship or truck to a
customer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 illustrates a flow scheme for the production of liquefied
natural gas from
natural gas through indirect condensation
[0012] FIG. 2 illustrates an alternative flow scheme of the production
of liquefied natural
gas from natural gas through direct condensation
[0013] FIG. 3 illustrates a flow scheme in which LNG is generated from the
quench
condensate
DETAILED DESCRIPTION
[0014] The invention solves the problem by undertaking the liquefaction
in a way which
does not lead to blockage of the condenser. Therefore, contaminants can be
introduced into
this unit, and then removed as a solid from the resulting LNG. While it is
feasible to purify
the natural gas to a level that allows for production of LNG, the present
invention provides a
process to reduce overall costs of production.
[0015] Low cost precleaning of the natural gas stream may also take
place as needed,
reducing the load on the filters. In an embodiment, the design would spray
supercooled LNG
into the top of a column, and introduce the warmer natural gas feed into the
bottom where the
supercooled LNG and warmer natural gas feed would mix. While the two fluids
would be
thermally in equilibrium at the top of the column, at the bottom there may be
more than a
38 C (100 F) temperature difference in temperature between the liquid and the
gas. The
thermal equilibrium will occur partway down the column. Given that at the top
of the
- 4 -
CA 02919209 2016-01-22
WO 2015/017357
PCT/US2014/048522
column, liquefaction of the methane rich gas is taking place through contact
with a methane
rich liquid, the liquid must be at a colder temperature than the gas phase.
[0016] This quenching operation at the bottom of the column
preferentially extracts the
contaminants into the liquid phase where they can be extracted. The lighter
components rise
up the column, where they in turn condense. Some of the nitrogen, helium,
hydrogen, argon
and oxygen may exit the top of the column as a small purge stream while some
of these gases
may form part of the product. A small portion of the methane stream will also
escape into the
purge stream. All other components, including carbon dioxide and water, are
condensed into
a 'dirty' LNG then passes through a series of filters to be cleaned and then
cooled to the
necessary storage tank temperature of ¨161 C (-257 F) by using Joule-Thompson
expansion
or a subcooler. From this tank, liquid is pumped to the top of the column in
order to condense
the feed. Periodically, the hydrocarbon/carbon dioxide filter cake would be
removed and
either combusted or used in other applications. The unit therefore would
consist of a low cost
front end bed (dryer or similar unit), precooler, wash column, filter,
subcooler, storage tank
and pump.
[0017] US 6,637,240 described a method for making nitrogen gas using a
tank of liquid
nitrogen. Cool air is introduced into the bottom, and liquid nitrogen is
introduced at the top.
Akin to this unit, the temperature difference at the bottom of the column
produces the same
sort of cryogenic quench. Further the bottoms (a crude dirty liquid oxygen) is
viewed as
waste and not subsequently subcooled in order to generate both a product as
well as the
quenching liquid. In other cryogenic nitrogen patents, the crude liquid oxygen
is sub cooled
through a Joule Thompson effect and used to provide liquid to the top of the
column.
However, it achieves this though indirect heat exchange with the top gases and
not by direct
contact. Due to the higher temperature difference between the freeze point of
the solids and
the internal surfaces of the column, the solids do not foul the column. It is
only with low
temperature differential during freezing that the two solids can form a
cohesive bond. One
key factor is that fouling does not take place in the quench because of the
high temperature
difference between the solid surfaces and the incoming natural gas. Adhesion
during the
freezing stage only takes place if there is a low temperature difference. The
high temperature
difference therefore precludes this adhesion taking place and the formed
solids will be
washed away with the liquid.
- 5 -
CA 02919209 2016-01-22
WO 2015/017357
PCT/US2014/048522
[0018] FIG. 3 shows a simplified flow sheet of the process of the
present invention. In an
example of the process, 11.34 kg (25 lb) mol/hr of natural gas is scrubbed of
a portion of the
water (but not to LNG specifications) and introduced into a prechiller cooling
it to ¨58 C.
This cooled natural gas is then introduced into a quencher and cooled using 17
kg (37.5 lb)
mol/hr of sub cooled LNG that is at ¨162 C, the standard temperature for LNG.
The majority
of the feed stream is condensed. The quencher is fed from 62 which is a
partial condenser.
The quencher itself is fed with a saturated liquid from the partial condenser
62. While there
may be one column, in the embodiment shown in FIG. 3, there are two columns.
[0019] A purge stream containing the incondensable contaminants exits
from the top of
the column as well as a portion of the methane. A bottom stream at ¨100 C
contains all of
the impurities left by the feed scrubber. Due to the cold temperature, these
are in the solid
form. A simple mesh filter would then be used to remove these particles, and
the liquid would
then be further chilled to ¨161 C (-257 F) for storage. During operation,
subcooled LNG
would be extracted from the storage tank and used in the quench chiller.
Simulations
suggested that a simple closed nitrogen refrigeration cycle would provide
sufficient cooling
for this process using 309 hp of power in order to condense 9071 kg (10
tons)/day of LNG.
Rather than requiring steam, solvents and other front end purification
processes, this unit
would run using higher power, a quench tower and a simple filtration device.
[0020] FIG. 1 presents one embodiment of the invention which does not
generate LNG
though direct condensation, nor does it use the bottoms liquid as a source of
the product. A
natural gas stream 2 is introduced into a simple PSA 6 wherein a portion of
the water is
removed producing a dry natural gas stream 8 with a dew point below ¨50 C and
a purge
stream 10 containing the removed moisture. Stream 8 is then cooled to ¨50 C in
an indirect
heat exchanger 12 to produce a chilled dry natural gas stream 14. Stream 14 is
then
introduced into the quench column 16 wherein it is cooled to ¨100 C through
direct contact
with a primary cooling liquid and a substantial portion of the heavy
components within
stream 14 condense and solidify into the dirty heavy bottom liquid 18. A
portion of the
primary cooling liquid is evaporated during this process and mixes with the
light components
of stream 14 and exits the quench column 16 as the chilled medium gas 20.
Stream 20 is then
further cooled through direct contact in a secondary cooler 22 with a
secondary cooling
liquid, removing further impurities such as C2 and C3 to producing a medium
bottom liquid
24 which is fed to the quench column 16 to provide a portion of the primary
cooling liquid.
- 6 -
CA 02919209 2016-01-22
WO 2015/017357
PCT/US2014/048522
The solids in stream 18 are removed through the use of a filter 28 producing a
heavy liquid
30 and a solid waste stream 32. Stream 30 is then raised in pressure in
recycle pump 34 and
fed as heavy cooling liquid 36 to quench column 16 where it forms part of the
primary
cooling fluid. The remaining uncondensed portion of stream 20 exits the
secondary cooler 22
as a light chilled gas 26 and is fully condensed in a liquefier 38 and fed as
a light liquid
stream 40 to a storage tank 42. When required, a draw stream 44 is removed
from tank 42,
reduced in pressure by regulator 46 and delivered as liquefied natural gas 48.
Meanwhile, a
light liquid draw stream 50 is withdrawn from tank 42, raised in pressure in
pump 52 in order
to be fed to secondary cooler 22 as a light cooling liquid 54. The LNG may be
transported by
ship, truck or other transport means.
[0021] FIG. 2 is an example of the process which generates LNG though
direct
condensation, but does not uses the bottoms liquid as a source of the product.
In this example,
A natural gas stream 2 is introduced into a simple PSA 6 wherein a portion of
the water is
removed producing a dry natural gas stream 8 with a dew point below ¨50 C and
a purge
stream 10 containing the removed moisture. Stream 8 is then cooled to ¨50 C in
an indirect
heat exchanger 12 to produce a chilled dry natural gas stream 14. Stream 14 is
then
introduced into the quench column 16 wherein it is cooled to ¨100 C through
direct contact
with a primary cooling liquid and a substantial portion of the heavy
components within
stream 14 condense and solidify into the dirty heavy bottom liquid 18. A
portion of the
primary cooling liquid is evaporated during this process and mixes with the
light components
of stream 14 and exits the quench column 16 as the chilled medium gas 20.
Stream 20 is then
further cooled through direct contact in a secondary cooler 22 with a
secondary cooling
liquid, removing further impurities such as C2 and C3 to producing a medium
bottom liquid
24 which is fed to the quench column 16 to provide a portion of the primary
cooling liquid.
The solids in stream 18 are removed through the use of a filter 28 producing a
heavy liquid
and a solid waste stream 32. Stream 30 is then raised in pressure in recycle
pump 34 and
fed as heavy cooling liquid 36 to quench column 16 where it forms part of the
primary
cooling fluid. The remaining uncondensed portion of stream 20 exits the
secondary cooler 22
as a light chilled gas 26 and is fully condensed in a liquefier 38 and fed as
a light liquid
30 stream 40 to a storage tank 42. When required, a draw stream 44 is
removed from tank 42,
reduced in pressure by regulator 46 and delivered as liquefied natural gas 48.
Meanwhile, a
light liquid draw stream 50 is withdrawn from tank 42, raised in pressure in
pump 52 in order
- 7 -
CA 02919209 2016-01-22
WO 2015/017357
PCT/US2014/048522
to be fed to secondary cooler 22 as a light cooling liquid 54. a secondary
light liquid draw
stream 56 is removed from tank 42, raised in pressure in pump 58 to form light
condensing
fluid 60 and introduced into direct condenser 62. The light chilled gas 26 is
also introduced
into the direct condenser 62 where the majority of the stream is condensed
forming light
condensate 64. Stream 64 is cooled even further in exchanger 66 to form stream
40. Any
incondensable gases leave direct condenser 62 as purge 68.
[0022] FIG. 3 does generate LNG though direct condensation and uses the
bottoms liquid
as a source of the product. In this example, natural gas stream 2 is
introduced into a simple
PSA 6 wherein a portion of the water is removed producing a dry natural gas
stream 8 with a
dew point below ¨50 C and a purge stream 10 containing the removed moisture.
Stream 8 is
then cooled to ¨50 C in an indirect heat exchanger 12 to produce a chilled dry
natural gas
stream 14. Stream 14 is then introduced into the quench column 16 wherein it
is cooled to
¨100 C through direct contact with a primary cooling liquid and a substantial
portion of the
heavy components within stream 14 condense and solidify into the dirty heavy
bottom liquid
18. A portion of the cooling is evaporated during this process and mixes with
the light
components of stream 14 and exits the quench column 16 as the chilled medium
gas 20.
Stream 20 chilled medium gas 20 is instead sent to direct condenser 62 to
produce the
medium bottom liquid 24. Medium bottom liquid 24 which is fed to the quench
column 16 to
provide a portion of the primary cooling liquid. The solids in stream 18 are
removed through
the use of a filter 28 producing a heavy liquid 30 and a solid waste stream
32. Heavy liquid
stream 30 is cooled in exchanger 66 to form stream 40 and be fed to tank 42.
The remaining
uncondensed portion of stream 20 exits the secondary cooler 22 as a light
chilled gas 26 and
is fully condensed in a liquefier 38 and fed as a light liquid stream 40 to a
storage tank 42.
When required, a draw stream 44 is removed from tank 42, reduced in pressure
by regulator
46 and delivered as liquefied natural gas 48. Meanwhile, a light liquid draw
stream 50 is
withdrawn from tank 42, raised in pressure in pump 52 in order to be fed to
secondary cooler
22 as a light cooling liquid 54.
[0023] Those experienced in the art will recognize that columns 62, 22
and 16, while
shown as separate units could be combined into one single column or their
roles could be
split across multiple different columns.
- 8 -
CA 02919209 2016-01-22
WO 2015/017357
PCT/US2014/048522
SPECIFIC EMBODIMENTS
[0024] While the following is described in conjunction with specific
embodiments, it will
be understood that this description is intended to illustrate and not limit
the scope of the
preceding description and the appended claims.
[0025] A first embodiment of the invention is a process for liquefaction of
natural gas,
the process comprising a) sending the cooled natural gas stream to a quench
unit; b) sending a
quantity of liquefied natural gas to the quench unit to be combined with the
cooled natural
gas steam to produce a bottoms stream comprising a liquid stream comprising
solid
impurities and a top stream comprising methane and incondensable impurities;
and c) cooling
the bottoms stream to produce liquefied natural gas. An embodiment of the
invention is one,
any or all of prior embodiments in this paragraph up through the first
embodiment in this
paragraph wherein the natural gas stream is cooled to a temperature from 00 to
¨100 C to
produce a cooled natural gas stream. An embodiment of the invention is one,
any or all of
prior embodiments in this paragraph up through the first embodiment in this
paragraph
further comprising sending the bottoms stream to a unit to remove the solid
impurities to a
produce a purified bottoms stream. An embodiment of the invention is one, any
or all of prior
embodiments in this paragraph up through the first embodiment in this
paragraph wherein a
portion of the bottom stream is returned to the quench unit. An embodiment of
the invention
is one, any or all of prior embodiments in this paragraph up through the first
embodiment in
this paragraph wherein before the natural gas stream is cooled, at least a
portion of water
within the natural gas stream is removed. An embodiment of the invention is
one, any or all
of prior embodiments in this paragraph up through the first embodiment in this
paragraph
wherein the natural gas stream is cooled to a temperature of ¨25 to ¨75 C. An
embodiment
of the invention is one, any or all of prior embodiments in this paragraph up
through the first
embodiment in this paragraph wherein the natural gas stream is cooled to a
temperature of
¨55 to ¨60 C. An embodiment of the invention is one, any or all of prior
embodiments in
this paragraph up through the first embodiment in this paragraph wherein the
solid impurities
are selected from the group consisting of carbon dioxide, C5 and C6
hydrocarbons and water.
An embodiment of the invention is one, any or all of prior embodiments in this
paragraph up
through the first embodiment in this paragraph wherein the incondensable
impurities are
selected from the group consisting of nitrogen, oxygen and hydrogen. An
embodiment of the
- 9 -
CA 02919209 2016-01-22
WO 2015/017357
PCT/US2014/048522
invention is one, any or all of prior embodiments in this paragraph up through
the first
embodiment in this paragraph wherein the bottoms stream is at a temperature
from ¨750 to
¨120 C. An embodiment of the invention is one, any or all of prior embodiments
in this
paragraph up through the first embodiment in this paragraph wherein the
bottoms stream is at
a temperature from ¨90 to ¨100 C.
[0026] A second embodiment of the invention is a system for producing
liquefied natural
gas from a supply of natural gas, comprising a) a device for supplying a
stream of natural gas;
b) a separation device to remove water from the stream of natural gas to
produce a partially
dried stream of natural gas; c) a means for feeding the partially dried stream
of natural gas to
a chilling device and then to a column; d) a means for feeding a quantity of a
chilled liquid to
the column where the chilled liquid and the partially dried stream of natural
gas are present in
the column and a means to vent a purge stream from the quench tower; e) a
means to
transport a combination of the chilled liquid and the partially dried stream
of natural gas to a
particle removal unit; f) a means to transport a purified stream from the
filter unit to a chiller
to produce liquefied natural gas; and g) a means to send the liquefied natural
gas to a storage
device. An embodiment of the invention is one, any or all of prior embodiments
in this
paragraph up through the second embodiment in this paragraph wherein the
separation device
removes additional contaminants from the stream of natural gas. An embodiment
of the
invention is one, any or all of prior embodiments in this paragraph up through
the second
embodiment in this paragraph wherein the column is a quench tower. An
embodiment of the
invention is one, any or all of prior embodiments in this paragraph up through
the second
embodiment in this paragraph wherein the chilled liquid is a quantity of
liquefied natural gas.
[0027] Without further elaboration, it is believed that using the
preceding description that
one skilled in the art can utilize the present invention to its fullest extent
and easily ascertain
the essential characteristics of this invention, without departing from the
spirit and scope
thereof, to make various changes and modifications of the invention and to
adapt it to various
usages and conditions. The preceding preferred specific embodiments are,
therefore, to be
construed as merely illustrative, and not limiting the remainder of the
disclosure in any way
whatsoever, and that it is intended to cover various modifications and
equivalent
arrangements included within the scope of the appended claims.
[0028] In the foregoing, all temperatures are set forth in degrees
Celsius and, all parts and
percentages are by weight, unless otherwise indicated.
- 10 -
