The present invention relates generally to a natural gas
liquefaction process and an apparatus for the same. The
invention particularly relates to the initial liquefaction of
natural gas from the field. More particularly the present
invention relates to a method and process for operating a
liquefaction plant in a more efficient and economical manner.
Even more particularly, the present invention relates to the
use of nitrogen as the refrigerant in the liquefaction of
natural gas, more particularly, to a modification of or an
improvement in the nitrogen expander cycle process which is
used in the liquefaction of natural gas feed stock whereby
the supply of nitrogen that is used to effect cooling of the
natural gas feed is divided into two or more portions in
which each portion effects cooling of the natural gas in a
different operation and/or in different parts of the
installation in which the overall process is conducted and at
different temperatures and pressures. The present invention
particularly relates to split nitrogen flow cycles whereby
the different portions of the nitrogen refrigerant are passed
through different expanders which are arranged in parallel
with each other.
Although the present invention will be described with
particular reference to process cycles for the liquefaction
of natural gas in which nitrogen is used as the refrigerant,
it is to be noted that the scope of the present invention is

not restricted, to the described embodiment or embodiments but
rather the scope of the present invention is more
extensive so as to include other methods and
applications of the process using nitrogen, and to
the use of other gases in the improved application or in

other applications than those specifically described,
natural gas which is obtained in the form of a gas from
gas and oil fields occurring in nature, is discharged from
the earth to form a natural gas feed which requires
processing before it can be used commercially, the natural
gas feed enters a processing facility and is processed
through a variety of operations in different installations
to finally emerge as liquid natural gas (LNG) in a form
which is suitable for use. The liquid gas is subsequently
stored and transported to another suitable site for
revaporisation and subsequent use. In the processing of
the natural gas feed the gas emerging from the naturally
occurring field oust be first pretreated to remove or
reduce the concentrations of impurities or contaminants,
such as for example carbon dioxide and water or the like,
before it is cooled to form LNG in order to reduce or
eliminate the chances of blockage to equipment used in the
processing occurring and to overcome other processing
difficulties. One example of the impurities and/or
contaminants are acid gases such as carbon dioxide and
hydrogen sulphide. After the acid gas is removed in an
acid gas removal installation, the feed gas stream is dried
to remove all traces of water. Mercury is also removed
from the natural feed gas prior to cooling. Once all of
the contaminants or unwanted or undesirable materials are
removed from the feed gas stream it undergoes subsequent
processing, such as cooling, to produce LNG.
Cooling of the natural gas feed may be accomplished by a
number of different cooling process cycles, such as for
example, the cascade cycle where refrigeration is provided
by three different refrigerant cycles, i.e. by using
methane, ethylene and propane in sequence. Another cooling
process cycle uses a propane preaooled, mixed refrigerant
cycle which involves the use of a multicomponent mixture of
hydrocarbons, e.g. propane/ethane/methane and/or nitrogen

in one cycle and a separate propane refrigeration cycle in
another cycle to provide precooling of the mixed
refrigerant and natural gas. a further cooling process
involves the use of a nitrogen expander cycle in which, in
its simplest form, a closed loop is employed in which
nitrogen gas is first compressed and cooled to ambient
conditions with air or water cooling and then further
cooled by counter-current exchange with cold low pressure
nitrogen gas. The cooled nitrogen stream is then expanded
through a turbo-expander to produce a cold low pressure
stream. The cold nitrogen gas is used to cool the natural
gas feed and the high pressure nitrogen stream. The work
produced in the expander by the nitrogen expanding is
recovered in a nitrogen booster compressor connected to the
shaft of the expander. Thus, in this process cold nitrogen
is not only used to liguefy the natural gas by cooling it
but the cold nitrogen is also used to precool or cool
nitrogen gas in the same exchanger. The precooled or
cooled nitrogen is then subsequently further cooled by
expansion to form the cold nitrogen refrigerant.
Improvements to the simple nitrogen cycle have been
disclosed whereby the high pressure nitrogen refrigerant is
divided into two portions where one portion is
isentropically expanded in a turbo-expander and a second
portion is iseanthalpically expanded through a valve to
produce, in some applications, liquid refrigerant. The
objective of this improvement is to avoid large separations
between the heating and cooling curves which are evidence
of thermodynamic inefficiencies and higher power
requirements for the refrigeration loop. The field of
application for this type of modification has typically
been for religuefying low temperature, low pressure boil-
off gases from LNG storage vessels which may contain high
nitrogen content in the gas during transportation of the
LNG or during offloading operations or when the vessel is
in restricted areas where venting of LKG is prohibited.

such as in major population centres and the like. However,
the operating parameters for reliquefying boil-off gases
are completely different to the operating parameters for
producing LNG from field gases.
One such different operating parameter is that the cooling
curves for boil-off gases are a different shape to that
encountered for the liguefaction of natural gas in base-
load plants or peak-shaving plants where the natural gas
feed is usually available at high pressure and ambient
temperature resulting in a different shape of cooling
curve. The known modifications to the nitrogen cycle to
essentially reliquefy boil-off gas from LNG that has
previously been made elsewhere do not result in the same
reductions in power requirements that the present invention
provides, firstly due to the better matching of the cooling
curve for the high pressure, ambient temperature feed
stream and secondly as a result of expanding the second
portion of the refrigerant isentropically through a turbo
expander rather than isenthalpically through a valve which
results in higher thermodynamic irreversibilities, and thus
consumes more power which is opposite to the present
invention which consumes less power.
Other improvements to the simple nitrogen refrigeration
cycle are also known from the air separation industry
whereby the high pressure nitrogen refrigerant is similarly
divided into two portions where one portion is
isentropically expanded in two turbo-expanders in seauence
with reheating of the refrigerant from the first expander
against feed gas before expanding in the second expander,
Tne second portion of the refrigerant is expanded
isenthalpically as discussed above through a valve and the
objectives as before are to reduce the separations of the
heating and cooling curves and hence minimise the power
requirements for the refrigeration cycle. When this known
modification is applied to the liquefaction of high

pressure natural gas at ambient temperature it does not
result in the same reductions in power that the present
invention would obtain due to the better matching of the
cooling curve end reduction in the thermodynamic
irreversibilities associated with the expansion of the
second portion of refrigerant isenthalpically through a
valve.
The present invention is a further modification of or an
improvement in the use of the nitrogen expander cycle and
involves the use of a single phase refrigerant which is a
gas which is wholly nitrogen or a gas which is a major
portion of nitrogen mixed with minor amounts of other
suitable gases,such as methane, or is any other gas which
could be used as a single phase refrigerant when cooled by
expansion in a turbo-expander. However, in operation of
the present invention, it is usual to use a gas which is
substantially wholly nitrogen.
Although the nitrogen expander cycles of the prior art are
usually only considered for small scale LRG plants or boil-
off gas raliquefaction because the power consumption of
using this refrigeration system is generally greater than
for other cooling cycles, thus making operating costs for
LNG produced by this method more expensive than when using
other refrigeration systems, the nitrogen expander cycle
has a number of inherent advantages when compared to the
conventional mixed refrigeration cycle. These advantages
include the use of a safe non-flammable refrigerant as
opposed to the use of large amounts of flammable
hydrocarbons which are necessary when using the mixed
refrigerant process. Another advantage Includes the easy
replenishment of the nitrogen refrigerant which is readily
available and easily obtained since fresh nitrogen
refrigerant is readily extracted from the atmosphere at the
plant site whereas with the mixed refrigerant processes
relatively large amounts of each of the components of the

mixed refrigerant cycle must be obtained either from the
natural gas feed by being extracted from the natural gas
feed, fractionated into the various components and
independently stored, and then recombined in the correct
proportions to replenish the refrigerant or be brought to the
site and stored until needed. When sufficient natural gas
liquids are not present in the natural gas feed stream, the .
different components of the mixed refrigerant must be
imported, all of which adds to the cost of using this form of
refrigerant and to the overall cost of the process, and hence
the final cost of the LNG itself. Additionally, storage
facilities are required for each of the components of the
mixed refrigerant system which contributes to the size and
complexity of the overall installation and results in
additional operating costs and safety problems.
A further advantage of using nitrogen as the refrigerant or
as the major part of the refrigerant relates to the physical
size and layout of the installation in that conventional
mixed refrigerant processes require a large number of
individual equipment items associated with the propane
precooling loop and other ancillary services to the basic
mixed refrigerant loop to be located at widely spaced apart
locations to allow room for piping and valves and to reduce
the risk of fire and to avoid other safety hazards whereas
processes using nitrogen do not present the same fire risks

or safety hazards as nitrogen is not combustible and also
less individual equipment items are required and what items
are required can be located much closer together which
reduces the physical size and complexity of the overall
installation. The reduction in size, complexity, safety
hazards and fire risks in the LNG installation using nitrogen
refrigerant results in the possibility of nitrogen
refrigerants being able to be used in off-shore
installations, if it were not for the high power consumption
of operating plants usin+g the nitrogen refrigerant cycles.

Nitrogen expander cycles have not as yet mat with
widespread use or acceptance for LNG production from
natural gasfields because of the high power consumption of
using such refrigerants due to the inherent inefficiencies
of using nitrogen as the refrigerant. One inherent
inefficiency results from the warming curve of the nitrogen
refrigerant not being able to be closely aligned to or
matched with the cooling curve of the feed gas being used
to produce the LNG. Any divergence between the two curves
results in inefficiencies due to waste or excess work being
done by the refrigeration cycle. Attempts to match the
curve by splitting the nitrogen into two portions after the
first nitrogen flow cooling phase and passing one portion
through a valve have only resulted in small reductions in
power consumption. Furthermore/ such nitrogen expander
cycles have only been used for small scale liquefaction of
natural gas from boil-off after the initial liquefaction,
where the liquefaction can be performed at higher
temperatures and the gas consists mainly of lighter
hydrocarbon portions. Furthermore, in previous nitrogen
flow cycles advantage has not been taken of the work of the
nitrogen as in the present invention where the work
collected in the expander is used in the compressor.
Therefore, if the power consumption disadvantages of using
the nitrogen recycle process could be overcome, it would be
possible to enjoy the inherent advantages of using these
processes and moreover if it were possible to use a
nitrogen expander cycle more efficiently it could be
possible to produce LNG from field gas more efficiently and
at a lower cost which would mean that reserves of natural
gas that hitherto before could not be used to produce LNG
economically could now be used as the LKG could be made
more cheaply. Also it would mean that LNG production
facilities could be located off-shore.
Therefore, it is an aim of the present invention to provide

a modified nitrogen expander cycle or other process using
nitrogen as a refrigerant which results in the production of
LNG more economically and efficiently so as to render the
production of LNG to be more viable in existing plants or to
be able to commission new plants for making LNG, or to locate
LNG plants in places where it has not previously been
possible, such as for example off-shore.
However, it is to be noted that the present invention is not
limited to the liquefaction of natural gas using a modified
nitrogen expander cycle but it can equally apply to the
refrigeration of any feedstream in which there are large
separations between the cooling and warming curves of the
feedstock and refrigerant respectively when the simple
nitrogen cycle is used as the refrigerant.
Accordingly, the present invention provides a natural gas
liquefaction process comprising the steps of passing natural
gas through a series of heat exchangers in countercurrent
relationship with a single phase refrigerant gas circulated
through a cooling cycle, substantially isentropically
expanding portions of refrigerant to different cooling
temperatures at which said refrigerant portions are supplied
to respective heat exchangers for cooling the natural gas
through corresponding temperature ranges, whereby the warming
curve for the refrigerant comprising all said portions has
sections of different gradients, discharging cooled natural

gas from a final heat exchanger at an exit temperature in the
range -160°C to -140°C, and supplying to the final heat
exchanger of said series a refrigerant portion at a cooling
temperature and in an amount selected in the range of 20 to
50% of the circulated refrigerant so that the part of the
refrigerant warming curve relating to the final heat
exchanger is closely matched to and has substantially the
same slope as the part of the natural gas cooling curve
extending over the temperature range from said exit
temperature to -100°C.
The present invention also provides an apparatus for
liquefying natural gas by the above process by cooling with a
single phase refrigerant consisting substantially of
nitrogen, said apparatus comprising a series of heat
exchangers, and a compressor having an inlet connected to
receive warmed refrigerant from the heat exchangers and an
outlet connected to deliver refrigerant to further compressor
means driven by turbo expanders through which portions of
compressed refrigerant are isentropically expanded and cooled
to different temperatures for use with different heat
exchangers, said turbo expanders having refrigerant outlets
connected to respective heat exchangers for delivering the
different cooled portions of the refrigerant to respective
heat exchangers for passage therethrough in countercurrent
relationship with the natural gas, the warming curve for the
refrigerant comprising sections having different gradients

and a part of the refrigerant warming curve relating to
passage through a final heat exchanger being closely matched
to and having substantially the same gradient as a part of
the natural gas cooling curve extending over the same
temperature range of the final heat exchanger.
In one aspect, the present invention provides a method of
treating a feed material to produce a commercial product by
liquefaction of the feed material using a single phase
refrigerant, said method comprising dividing the refrigerant
into two or more supply portions, supplying a first portion
of the refrigerant to a first heat exchanger for cooling the
feed material to an intermediate temperature and supplying a
second portion of the refrigerant to a second heat exchanger
for cooling the feed material to a further temperature such
that the temperature of cooling of the second portion is
lower than the temperature of cooling of the first portion
whereby the warming curve of the refrigerant of the first and
second supply portions comprise at least two discrete
portions having different gradients so that the combined
warming curve of the refrigerant is more closely matched to
the cooling curve of the feed material so as to minimize
thermodynamic inefficiencies and hence power requirements
involved in operation of the method.
According to another aspect of the present invention there

is provided a method of treating a natural gas feed
material to produce a commercial LNG product by
liquefaction of the feed material using a single phase
refrigerant comprising at least mainly nitrogen, said
method comprising dividing the refrigerant into at least
two portions, supplying each portion of the refrigerant to
a different heat exchanger for cooling the feed material
over different temperature ranges, such that the
temperature of cooling of each portion of refrigerant in
the heat exchanger is different, so that the combined
warming curve of the refrigerant made up of the warming
curves of the various portions of refrigerant exhibit
discrete gradients corresponding to the various portions of
refrigerant so that the combined warming curve of the
refrigerant can be selectively adjusted to closely match
the cooling curve of the feed material so as to minimise
thermodynamic inefficiencies and hence power requirements
in the operation of the method to produce the commercial
product by selectively altering the relative portions of
each refrigerant portion to each other when dividing the
refrigerant into the at least two portions.
Typically, there are two, three, four or more portions of
refrigerant. More typically/ the proportions are divided
from 15% to 85% of the total flow, in the event that the
refrigerant is divided into two portions, the ratios are
preferably 50% to 60% for the first portion and 50% to 20%
for the second portion. More typically, the larger first
portion is supplied to the first exchanger such that the
temperature of cooling of the second portion is less than
the temperature of cooling of the first portion. More
typically, the stream of lesser volume is passed to the
colder of the exchangers or the coldest exchanger, even
more typically to an exchanger which is colder than the
exchanger to which the stream of greater volume is passed.
A further modification of the present invention relates to

dividing the nitrogen refrigerant stream into three
separate streams, in this embodiment which is a further
variation on the split nitrogen flow process there are
three expanders in parallel with each other with splits of
approximately 20/50/30% by volume of the total volume of
the nitrogen refrigerant. The coldest level (30%) runs at
an outlet pressure of 11.7 bar or similar to other
embodiments described, while the warmer levels (50% and
20%) run at a different outlet pressure of 19.4 bar. The
high pressure feed to the third (warmest) expander is
precooled to 10°C by a conventional refrigeration or
chilled water system, however, the system can be configured
to run without it at slightly higher power requirements.
In this embodiment where the refrigerant is returned to or
forms the main refrigerant stream there are three separate
parallel streams, each stream having one of the three
expanders in parallel. The three streams are returned to
separate exchangers, The warming/cooling curve of this
arrangement shows that the two curves are more closely
aligned and match with each other in the region from about
-100°C to about 20°C, particularly in the region about
-80°c to about -40°C, in addition to matching of the curves
below about -100°C.
Typiaally, the present invention provides a significant
improvement in the simple nitrogen expander cycle process
for the liquefaction of gases, particularly natural gas,
and more particularly when producing LNG. The improvement
in efficiency of the simple nitrogen refrigeration cycle as
applied to the liquefaction of natural gas is achieved
through modification of the closed loop refrigeration cycle
to allow closer alignment or matching of the warming curve
of the nitrogen refrigerant with the cooling curve of the
natural gas, or of the combination of natural gas and
nitrogen refrigerant which is to say the process of the
present invention is operated by adapting or changing the
warming curve of the nitrogen refrigerant to more closely

approximate the cooling curve of the feed gas being
processed when the cooling carve of the nitrogen
refrigerant used for the precooling step ia also taken into
account.
More typically, the present invention provides e
significant improvement to the simple nitrogen expander
cycle process for the liquefaction of gases including
natural gas. The method of the present invention comprises
dividing the refrigerant into two portions after initial
precooling in the first exchanger whereby the first portion
is expanded at near to isentropic conditions in a turbo-
expander to provide cooling of the natural gas to about
-95°C and also to provide further cooling of the second
portion of the refrigerant such that when this second
portion is also isentropically expanded in a second turbo-
expander it provides final cooling of the natural gas
stream to the required temperature of about -140°C to
-160°C to form LNG suitable for the next stage of
processing which is reduction of the nitrogen content of
the LNG if required. The division of the refrigerant into
two portions at two different temperature levels allows the
close matching of the warming curve of the nitrogen
refrigerant to the cooling curve of the natural gas feed
and cooling curve of the nitrogen refrigerant when being
precooled.
Typically, in the operation of the simple nitrogen expander
cycle all of the high pressure nitrogen refrigerant is
first cooled to an intermediate temperature by the low
pressure nitrogen refrigerant at a colder temperature and
then the cooled high pressure nitrogen is expanded in a
turbo-expander to form a cold low pressure nitrogen stream
to further cool the natural gas to the required temperature
to form LNG which is from about -1400C to about -160°C
The intermediate temperature is selected to be low enough
such that when the nitrogen is expanded in the turbo-

expander the temperature of the cold low pressure nitrogen,
gas thus produced by the expansion is just sufficiently low
enough to subcool the natural gas to the required
temperature of about -140°c to -160°C. At this temperature
which exists at the cold end of the heat exchanger the
warning curve of the nitrogen is almost coincident with the
cooling curve of the feed gas and accordingly there is a
dose approximation of both curves at this temperature
which is the lowest temperature required of the cooling
process. Thus, this sets the lowest temperature of the
heat exchange process.
The warming curve of the nitrogen refrigerant is
essentially a straight line having a slope which is
adjusted by varying the circulation rate of nitrogen
refrigerant until a close approximation is achieved between
the warming curve of the nitrogen refrigerant and. the
cooling curve of the feed gas at the warm end of the
exchanger. This sets the upper limit o£ operation of the
liquefaction process. Thus, by using this method it is
possible to obtain relatively close approximations at both
the warm and cold ends of the heat exchanger between the
different curves. However, because of the different shapes
of the respective curves in the intermediate portion of
each it is not possible to maintain a close approximation
between the two curves over the entire temperature range of
the process, i.e. the two curves diverge from each other in
their intermediate portions. Although the nitrogen
refrigerant warming curve approximates a straight line, the
cooling curve of the feed gas and nitrogen is of a complex
shape and diverges markedly from the linear warming curve
of the nitrogen refrigerant. The divergence between the
linear warming curve and the complex cooling curve is a
measure of and represents thermodynamic inefficiencies or
lost work in operating the overall process. Such
inefficiencies or lost work are partly responsible for the
higher power consumption of using the nitrogen refrigerant

cycle compared to other processes such as the mixed
refrigerant cycle.
Typically, operation of the present invention, hereinafter
referred to as the split flow nitrogen expander cycle,
results in reduction of the thermodynamic inefficiencies or
lost work when using this improved method. Such reductions
are achieved by dividing the warming curve for the nitrogen
refrigerant into a number of discrete sections each having
different slopes so that the warming curve of the nitrogen
refrigerant is more closely matched to the cooling curve of
the feed gas and nitrogen so that the temperature differences
and hence thermodynamic losses between the two are minimized.
In one example of the present invention described below and
illustrated with reference to Figure 2, the warming curve is
divided into two discrete sections by splitting the supply of
compressed and cooled nitrogen used in the process into two
parts. The first supply part is expanded in a turbo-expander
to a lower pressure at a lower temperature and provides
cooling to an intermediate temperature. The second supply
part is cooled further and then expanded in a second turbo-
expander to a lower pressure at a still lower temperature and
provides cooling of the natural gas to the lowest temperature
required of the liquefaction process. The flow rate of the
second supply part is chosen so that the slope of the warming
curve of the nitrogen is approximately the same as that of
the cooling curve for subcooling natural gas in the cold end

of the heat exchanger. This maintains close temperature
approaches or approximation throughout the exchanger. The
second supply part of the nitrogen refrigerant is warmed in
the heat exchanger to the same temperature as that achieved
in the expansion of the first supply part of the nitrogen in
the first expander i.e. to the intermediate temperature. In
this example the two turbo-expanders are located in parallel
arranged streams.
In a typical example of the present invention both of the
nitrogen supply streams are expanded to the same pressure
which allows the streams to be recombined at the intermediate
temperature level, hence simplifying the heat exchanger
arrangement. The combined streams are now reheated as before
in the simple nitrogen expander cycle and the increased mass
flow of the combined stream compared to that of the second
supply part of refrigerant results in a reduced slope of the
warming curve of the refrigerant in the remainder of the heat
exchangers. The flow rate of the second supply part of
nitrogen is chosen to give a feasible temperature approach at
the warm end of the first exchanger. As illustrated by a
comparison between Figures 1 and 2 the split flow nitrogen
expander cycle of Figure 2 increases significantly the
average internal temperature at which the heat exchanger is
operated and more closely matches the warming curve of the

refrigerant to the cooling curve of the feed gas and nitrogen
as compared to the simple cycle, especially at or towards the
cold end of the heat exchanger.
Typically, further improvements to the split nitrogen cycle
include combining other known enhancements with the simple
cycle of the present invention. Such enhancements include
adding a separate precooling refrigeration cycle (e.g.
propane, ammonia absorption or freon) to the nitrogen cycle
which increases the efficiency of the simple cycle. The use
of two expanders to expand the cooled nitrogen serially in
two stages with reheating of the cold gas from the first
expander before expanding in the second expander also
increases the efficiency of the simple cycle.
The present invention will now be described by way of example
with reference to the accompanying drawings in which:
Figure 1 is a plot of nitrogen refrigerant warming curve as a
comparison of the LNG/nitrogen cooling curve for the simple
nitrogen expander cooling cycle in accordance with the prior
art showing the divergence of the two curves from each other
in their respective intermediate portions, which divergence
represents wasted energy.
Figure 2 is a plot similar to Figure 1 of the nitrogen
refrigerant warming curve compared to the LNG/nitrogen
cooling curve using the split nitrogen flow expander cycle of

the present invention showing a closer matching of the two
curves to each other, particularly in the respective
intermediate portions, which demonstrates a saving in energy.
Figure 3 is a plot of the nitrogen warming curve compared to
the LNG/nitrogen cooling curve in accordance with the present
invention when using further embodiments of the split flow
nitrogen expander cycle involving the use of a precooling
refrigeration system and serial expanders showing even
greater matching of the two curves with respect to each other
over almost the entire curves, which results in further
energy savings.
Figure 4 is a flowchart of the split flow nitrogen expander
cycle process operated in accordance with the present
invention from which the plot of Figure 2 is derived.
Figure 5 is a flowchart in accordance with which the split
flow nitrogen cycle process of the present invention having a
small precooling refrigeration system and reheating expander
steps is operated from which the plot of Figure 3 is derived.
Figure 6 is a flow chart of the split flow nitrogen cycle
process in accordance with the present invention having a
full precooling refrigeration system such that one part of
the nitrogen refrigerant is not used in the first exchanger

and accordingly cold nitrogen is returned to the suction of
the compressor.
Embodiments of the present invention will now be described.
Example 1
One embodiment of the present invention will now be
described with reference to Figure 4 which shows one
example of the present invention as applied to the
liquefaction of a lean natural gas feed stream. Turning
first to the cooling of the natural gas feed to produce
LNG, it can be seen that a compressed natural gas feed
stream at about ambient temperature, denoted by reference
numeral 1, comprising predominantly methane, is treated in
a conventional pretreatment plant A to remove water, earbon
dioxide and mercury contaminants. Various pretreatment
arrangements are known and the exact pretreatment necessary
depends on the precise composition and the level and nature
of undesirable contaminants or impurities present in the
natural gas feed, frretreatments for removing the
contaminants and impurities are in accordance with
techniques well known to those skilled in the art.
The treated feed, stream 2, emerging from pretreatment
plant A is then passed to and cooled in heat exchanger
device 100 and then in other heat exchangers 101 to 103 in
turn to more or less liguafy the gas feed to produce liquid
LNG The heat exchangers comprise one or more separate
heat exchangers and use the main stream of nitrogen
refrigerant as the coolant. More specifically, the stream
of cooled feed gas 3 emerging from heat exchanger 100 is
passed serially through heat exchanger 101 where it is
cooled to -84CC and on emerging from exchanger 101 as
stream 4 is passed through heat exchanger 102. The
liquefied feed 5 emerging from heat exchanger 102 is then
further cooled to approx -149°C with a smaller stream of
nitrogen refrigerant at a temperature of about -152°C in

heat exchanger 103. The subcooled high pressure LNG
stream 7, exiting from heat exchanger 103 then flows
directly to storage, after reducing the pressure through a
valve or other suitable means, or if necessary, via a
conventional nitrogen rejection unit B where nitrogen is
removed in the flash gas resulting from the letdown of
pressure of the LNG, depending on the level of nitrogen in
the feed and/or the LN3 specification required for storage
and subsequent use or transportation to a remote site for
subsequent use. Thus, natural gas feed is introduced in
the form of a gas as stream 1 and is discharged as a LNG in
the form of a liquid as stream 7.
The nitrogen refrigeration cycle which transforms gas
stream 2 to liquid stream 7 will now be described starting
with warm nitrogen stream 22 which has been exhausted of
all or most of its cooling properties by absorbing heat
from the feed gas. The warm nitrogen, stream 22, exhausted
of its cooling properties is at the lowest pressure of the
cycle of about 10 bar, and is fed to and recompressed in a
multistage compressor unit 105 provided with intercooling
and aftercooling stages to produce compressed stream 23 at
about ambient temperature. Operation of compressor unit
105 consumes almost all of the power required by the
nitrogen expander cycle. Stream 23 is divided into 2
streams 24 and 25 which are fed to compressors 108,109
respectively so that each stream is boosted in pressure
from about 30 bar to about 55 bar by compressors 108 and
109 to form streams 26 and 27 respectively. Compressors
108,109 are attached to expanders 106 and 107 respectively
and recover the majority of the work produced by the
expanders 106,107 (to be described in detail below).
Alternatively, compressors 106 and 109 can be replaced with,
a single compressor driven by both expanders 106 and 107,
such as for example being connected to a common shaft. The
compressed nitrogen streams 26,27 are combined into a
single stream 28 which is then cooled in aftercooler 110 to

ambient conditions to produce stream 29 which flows to
exchanger 100 as stream 10. In exchanger 101, stream 10 is
precooled to -20°C by the countercurrent passage of
nitrogen refrigerant stream 21 through exchanger 100 to
form stream 22 which is now exhausted of ita cooling
properties. Stream 10 emerges as stream 11 from exchanger
100.
The close approach or approximation of the refrigerant
warming curve to the feed cooling curve made possible by
operating the system in accordance with the present
invention is achieved in this example by splitting the
compressed nitrogen refrigerant stream 11 which exits from
heat exchanger 100 into two main portions/ stream 13 and
stream 12. One portion which is stream 13 comprising
approx 35% of the main flow of nitrogen refrigerant from
stream 11 is precooled in heat exchanger 101 to form stream
14 a temperature of approximately -84CC by the counter flow
of nitrogen refrigerant from stream 20 to stream 21.
Stream 14 exiting from neat exchanger 101 is then combined
with a small stream of nitrogen, stream 31, which was split
off from stream 29 as stream 30 when stream 10 was formed.
Stream 30 had been previously precooled to approx -120°C in
heat exchanger device 104 using cold natural gas/nitrogen
reject stream 8, produced by the nitrogen rejection unit B
through which stream 6 was passed in installations where
this unit is provided. The combined cold stream, stream 15,
formed from streams 31 and 11 is then expanded at close to
isentropic conditions in expander 107 at a pressure of
approx 11 bar to form a very cold stream 1$ of nitrogen
refrigerant. The resulting cold stream, 16 which is at a
temperature of approx -152°C is used to subcool the high
pressure LNG in exchanger 103. The flow rate of stream 15
is chosen to provide a close approach of the refrigerant
warming curve to the LNG cooling curve in the regions below
about -lOO0c in accordance with the present invention.
Stream 16 emerges from heat exchanger 103 as stream 17

which is combined with stream 18 from expander 106 to form
stream 19 which is used to provide cooling of the natural
gas feed stream 5 in heat exchanger 102 as described
previously. The combination of stream 18 with stream 17
will be described in more detail later.
The modification of the present invention over the
conventional nitrogen expander cycle and other previous
modifications of this cycle resides mainly with stream 12
and how this stream is processed. The second main portion
divided from stream 11, which is stream 12, is the larger
portion of the nitrogen refrigerant stream 13 and is about
65% of the main flow of refrigerant and is fed to expander
106 and expanded in expander 106. it is to be noted that
stream 11 from which stream 12 was derived bad been
precooled to a temperature of approx -20°C in heat
exchanger 100. Stream 12 is considerably further cooled in
expander 105. The resulting cold stream, stream 18, exits
from expander 106 at a temperature of approx -1040c and is
combined with stream 17 which is also at approx -1040C and
is used to cool the natural gas feed in exchangers 102,101
and 100 in turn. Stream 19 is responsible for the close
approximation of the refrigerant warming curve to the LNG
cooling curve in the regions above about -100°c in
accordance with the present invention.
The cold nitrogen refrigerant stream 20 turning into stream
21 by passing through exchanger 101 is also used to precool
the low temperature nitrogen stream 13 turning into stream
14 in exchanger 101 and the combined nitrogen stream 10 as
it is precooled to -20°C in exchanger 100. Stream 18
provides the greater amount of cooling of the process of
the present invention.
With particular reference to Figure 2 it can be seen that
in contrast to the essentially straight line of the
refrigerant warming curve of the simple nitrogen cycle as

shown in Figure 1, splitting the nitrogen cycle into two
supply portions, streams 12 and 13, at two different
temperature levels allows the combined cooling curve of the
natural gas and toe nitrogen to be notched more closely by
the warming curve of the nitrogen refrigerant/ especially
at the low temperature end of the cooling curve of the
nitrogen refrigerant such ae at temperatures below -100°c.
This is demonstrated by a comparison of Figures 1 and 2
which compares the warming curves for the simple nitrogen
cycle process with that of the split flow nitrogen cycle
process of the present invention. The closer temperature
approaches of the split flow nitrogen cycle result in
smaller thermodynamic irreversibilities or exergy losses"
and provides a substantial reduction in power requirements
for the split flow nitrogen cycle operated in accordance
with the present invention.
Thus, it can be readily seen that splitting the nitrogen
refrigerant from stream 11 to streams 12 and 13 after
passing through exchanger 100 and returning these two
streams at a different place in the cycle, by recombining
the streams 17 and 18 to form stream 19 prior to exchanger
102, provides the advantages of the present invention.
Example 2
A further improvement in power consumption for the split
flow nitrogen cycle of the present invention can be
obtained by the use of a further embodiment of the present
invention which involves the use of a small preceding
refrigeration cycle and a third expander to further modify
the shape of the nitrogen refrigerant warming curve to
match the cooling curve even more closely. Figure 5 shows
an example of the split flow nitrogen expander cycle
provided with the modifications of this example mentioned
above. The matching of the two curves using this
embodiment is shown in Figure 3.

This embodiment will now be described with particular
reference to Figures 3 and 5. It is to be noted that the
reference numerals of Figure 5 are unique to this embodiment,
and may or may not be used to refer to the same features in
Figures 4 and 6. As in the previous example, lean natural
gas l is treated and then liquefied by exchange with cold
nitrogen gas and flows to storage via a conventional nitrogen
rejection unit B if required. Thus, streams 1 through to 8
are as previously described in Example 1, with stream 7 being .
the LNG which goes to storage and stream 8 being a flash gas
derived from nitrogen rejection unit B which is passed to and
through exchanger 109 for producing compressed fuel gas. The
modification of this embodiment relates to exchanger 100 and
the presence of a precool refrigeration system 114 and to
having three expanders, 106, 107, 108. The cooled and
compressed nitrogen, stream 10, is precooled to a temperature
of -30°C in heat exchanger 100 by exchange against a
combination of nitrogen refrigerant stream 21 and a separate
refrigeration package 114. This refrigeration package 114 is
a conventional refrigeration cycle using propane, freon or
ammonia absorption cycles and consumes a relatively small
amount of power, such as for example about 4% of total power
consumed by the main nitrogen cycle compressors 105. In heat
exchanger 100 not only is the feed gas stream 2 being cooled

but also nitrogen refrigerant stream 10 is also being cooled.
This is the first change from Example 1.
The precooled nitrogen stream 11 emerging from heat exchanger
100 is split into two portions as in Example 1 and the
smaller portion, stream 13, is further cooled in heat
exchangers 101 and 102 against the counter flow of nitrogen
refrigerant in streams 19 and 23 to a temperature of
approximately -82°C. Stream 15 is then combined with a small
stream of nitrogen, stream 36, which has been precooled to
approx -120°C in exchanger device 109 using cold natural

gas/nitrogen reject streams, stream 8, produced by the
nitrogen rejection unit B where this unit is required. The
combined cold stream, stream 16, is then expanded at close
to isentropic conditions in expander 108 at a pressure of
approx 11 bar. The resulting cold stream, stream 17, at a
temperature of approx -152°C is used to subcool the high
pressure LUG in exchanger 104. The flow rate of stream 17
is chosen to give a close approach of the LNG cooling and
nitrogen warming curves, in the regions below -100°c,
The larger portion of the nitrogen refrigerant stream,
stream 12, is expanded to a pressure of approx 15 bar in
expander 105 after preceding to a temperature of
approximately -30°C as described previously in Example 1.
The resulting cold stream, stream 22, at a temperature of
approx -99°0 is used to cool natural gas feed in exchangers
102,103. This stream is reheated in exchangers 102 and 103
to a temperature of approximately -75°C and then expanded
to a pressure of approx 10.5 bar in expander 107. The
resulting cold stream, stream 25, at a temperature of
approx -91°C is combined with stream 1B also at approx
-91°C and is used to cool natural gas feed in exchangers
102,101 and 100. The cold nitrogen is also used to precool
the low tamp nitrogen stream 13 in exchangers 101 and 102
and nitrogen stream 10 is preceded to -30°C in exchanger
100 using stream 21 and a conventional refrigeration
package unit 114. Thus, stream 12 is in effect divided
from the main refrigerant stream, passed sequentially
through expanders 106 and 107 before returning to the main
refrigerant stream. Therefore, in this embodiment there
are two streams in parallel with one of the streams being
passed through two expanders in series. This is tne second
modification of this example.
The wanned nitrogen, stream 37 , is recompressed in. a
multistage compressor unit 105 with intercooling and
aftercooling and then boosted in pressure to approx 55 bar

by compressors 111, 112 and 113 which are attached to
expanders 106, 107 and 108 and recover the majority of the
work produced by the expanders, Alternatively, the
compressors 111, 112 and 113 may be combined in one
compressor driven by expanders 106, 107 and 108 attached to
a common shaft. The compressed nitrogen stream 33 is
cooled in aftercooler 110 to ambient conditions and flows
as stream 10 to exchanger 100 and refrigeration package 114
where it is precooled to -30°C as described above.
Example 3
A modification of the arrangement of Figure 5 is shown in
Figure 6. The modification of Figure 6 relates to stream
21 of Figure 5. Stream 21 of Figure 5 is passed from
exchanger 101 to exchanger 100 from which it emerges as
stream 37 which is passed to compressor 105. In the
modification of this example as shown in Figure 6, stream
21 exiting exchanger 101 is not passed through exchanger
100 but rather is connected directly to compressor 105.
All the precooling for the high pressure nitrogen stream 10
and natural gas feed stream 2 to -30°C is now performed by
the refrigerant package 114. Thus, stream 21 of Figure 6
as it enters compressor 105 corresponds to stream 37 of
Figure 5 as it enters compressor 105, However, as stream
21 of Figure 0 does not pass through exchanger 100 it does
not gain heat and accordingly is at a lower temperature
than stream 37. Therefore, less work is required to
compress and cool the nitrogen refrigerant of stream 21 to
form stream 26 in the embodiment of Figure 6 is required
than in the embodiment of Figure 5 and accordingly the
embodiment of Figure 6 is more energy efficient in
operation and requires less power to operate which in turn
results in more economical production of LNG. Operation in
accordance with this embodiment is otherwise the same as
for the embodiment of Figure 5.

Comparison of alternative cycles
The relative performances of the nitrogen expander cycle as
shown in Figure 1, the embodiments of the split flow
nitrogen expander cycle of the present invention as shown
in Figure 2, and the two versions of the split nitrogen
expander cycle with precooling and reheat expander as shown
in Figure 3 were simulated for a trial production of 2600
tonnes/day of LNG from a lean natural gas feed at a supply
pressure of 55 bar and temperature of 30°C.
For comparison purposes, the flow sheet for the simple
nitrogen cycle of the prior art used heat exchangers
equivalent to exchangers 100, 101 and 102 only, omitted
streams 12,18 and compressor/expander 106,108, i.e. did not
have a split nitrogen flow of two parallel streams and did
not have two compressors/expanders in parallel which is a
characteristic feature of the present invention.
Table 1 compares the power requirements and nitrogen cycle
operating conditions of the four alternative nitrogen
cycles. For completeness the power requirements are also
compared to the Mixed Refrigerant cycle using a figure of
35 MW as being typical of current propane precooled mixed
refrigerant processes.



From the above results it can be seen that the use of the
split nitrogen expander cycle results in a power reduction
o£ 21.1 Mw against the simple nitrogen expander cycle with
the addition of one expander to the cycle. At a discharge
pressure of 55 bars for the nitrogen compression system the
optimum expansion ratio for the expander in the simple
cycle results in a compressor suction pressure of
approximately 5.6 bara to obtain the minimum power
consumption. Another effect of the split nitrogen expander
cycle is to increase the optimum pressure for that cycle to
approx 10 bara. This can be expected to have several
benefits including lower circulating refrigerant volumes
and hence piping diameter, higher single phase heat
transfer coefficients and expansion ratios for the nitrogen
expanders that can be achieved with a single expander
stage. The higher expansion ratio for the simple nitrogen
cycle may require the expansion to be achieved in two
expander stages which further adds to the cost.
The modifications to the split nitrogen expander cycle
shown in Figure 5 relating co the use of a third expander
result in a further power reduction of 6.8 MW for the
nitrogen cycle compressors due to the addition of the third
expander and the small precool refrigeration cycle

requiring approx 1.8 Mw of power giving a net reduction of
5 MW.
If a larger preaooling refrigeration cycle is used as shown
in Figure 6 such that all the cooling duty for the natural
gas and nitrogen from ambient to -30°C is performed by a
separate refrigeration system, even further power
reductions occur. In this case the suction of the nitrogen
compressor operates at approximately -36°C. The duty for
the precooling refrigeration eyetarn increases to 8 MW,
however the duty of the nitrogen compressor falls to 33.1
Mw giving a further reduction of 3 MM overall.
With particular reference to Figure 2 and 4 operation of
the process of the present invention will now he described.
In heat exchanger 100 natural gas feed 2 is precooled to a
temperature of about -20°C. At the same time, cool
nitrogen stream 10 is further cooled in heat exchanger 100
to about -20°C. Both of the natural gas feed 2 and
nitrogen stream 10 are cooled by the action of nitrogen
stream 21. The cooling curve of the combined natural gas
feed and nitrogen stream 10 is shown in Figure 2 together
with the warming curve of the nitrogen refrigeration stream
21. At the warmest and of exchanger 100 it can be readily
seen that both the nitrogen warming curve and the
LNG/nitrogen cooling curve are about coincident whereas
when the LNG/nitrogen is at about -20°C the nitrogen
refrigerant is at about -380c.
Beat exchanger 101 reduces the temperature of the natural
gas feed stream 3 which exits as stream 4 and the nitrogen
refrigerant stream 13 which exits as stream 14 from about
-20°O to about -840C by the action of nitrogen refrigerant
stream 20.
In heat exchanger 102 the LOTS gas stream 4 is reduced from
a temperature of about -84°c to about -100°C by the action

of refrigerant stream 19.
The slope of the nitrogen refrigerant warming curve from
about 30°C to about -105°C is of constant gradient due to the
same amount of refrigerant being passed through each of heat
exchangers 102, 101 and 100 in turn.
In heat exchanger 103 the temperature of the natural gas feed
stream 5 is reduced from about -100°C to about -149°C by-
nitrogen refrigerant stream 16. As the mass flow rate of
nitrogen refrigerant stream 16 is less than that of streams
19, 20 and 21 the slope of the nitrogen refrigeration warming
curve over this temperature range is different to that of
streams 19, 20 and 21. In the described example the gradient
of the nitrogen refrigerant warming curve in exchanger 103 it
is greater than that in exchangers 102, 101 and 100 and is
more closely aligned to the gradient of the LNG cooling curve
from about -105°C to -152°C. Therefore, by judiciously
adjusting the circulation rate of nitrogen refrigerant stream
17 coming from expander 107 and passing through heat
exchanger 103 it is possible to minimize the energy losses of
the split flow nitrogen cycle at the lower end of the
temperature range by more closely aligning the warming curve
of the nitrogen refrigerant to that of the LNG cooling curve
in the same temperature range. Accordingly, less energy is
required to operate the overall process and in particular in

compressors 105 because less energy is being wasted in
exchangers 1G3, 102 and 101 when compared to the simple
nitrogen expander cycle shown in Figure 1 and more energy is
recovered in the isentropic expansion of stream 15 in
expander 106 and expander 107 is operated at a higher inlet
temperature producing more work than in the simple cycle.
Thus, by having a split flow of the nitrogen refrigerant it
is possible to have two expanders in parallel and the
relative ratio of the flow in each of the splits of the

flow can be selectively adjusted by passing more or less
through each expander, with reference to Figure 2, it can
be seen that the same amount of refrigerant passes through
exchangers 100, 101 and 102 and hence the slope of the
warming curve of Figure 2 between -105°C and 30°C is
constant. Because of the split in flow less refrigerant is
passed through exchanger 103 than through the remaining
exchangers and hence the gradient of the nitrogen
refrigerant warming curve corresponding to passage through
exchanger 103 to change the temperature from -105°C to
-152°c is different.
with particular reference to Figure 3, the effect of having
a third expander can be readily seen by the changes to the
gradient of the warming curve in the region from about
-100°C to about -80°C where a closer fit to the cooling
curve of the LNG/nitrogen is possible by selectively
adjusting the relative ratios of the flows through the
expanders.
Also with particular reference to Figure 3, the effect of
the precool refrigeration system 114 can be seen by the
change in gradient of the warming curve. In the region
above about -40°C the slope of the warming curve due to the
passage of stream 21 through exchanger 100 by itself would
result in a temperature cross in exchanger 100 indicating
that stream 21 by itself cannot provide sufficient cooling
to cool streams 2 and 10 to -30°C. The multistage
precooling refrigeration system provides the extra cooling
required (indicated by the horizontal portions of the
warming curve) at typically 3 temperature levels to
maintain the separation of warming and cooling curves.
Advantages of the present invention include that split
nitrogen expander cycle operates entirely in the single
phase gas region, which allows the elimination of all
compressor suction drums, phase separators and refrigerant

accumulators required in the mixed refrigerant process.
The single phase of the refrigerant eliminates the flow
distribution problems associated with two phase flow in
heat exchanger devices and allows the use of conventional
aluminium plate fin exchangers without the associated phase
separators and distribution systems normally required or
offers an alternative to the highly specialised and
expensive spiral wound heat exchangers conventionally used
in mixed refrigerant process plants.
The described arrangement has been advanced by explanation
and many modifications may be made without departing from
the spirit and scope of the invention which includes every
novel feature and novel combination of features herein
disclosed.
Those skilled in the art will appreciate that the invention
described herein is susceptible to variations and
modifications other than those specifically described, it
is understood that the invention includes all such
variations and modifications which fall within the spirit
and scope.

WE CLAIM:
1. A natural gas liquefaction process comprising the
steps of passing natural gas through a series of heat
exchangers in countercurrent relationship with a single
phase refrigerant gas circulated through a cooling
cycle, substantially isentropically expanding portions
of refrigerant to different cooling temperatures at
which said refrigerant portions are supplied to
respective heat exchangers for cooling the natural gas
through corresponding temperature ranges, whereby the
warming curve for the refrigerant comprising all said
portions has sections of different gradients,
discharging cooled natural gas from a final heat
exchanger at an exit temperature in the range -160°C to
-140°C, and supplying to the final heat exchanger of
said series a refrigerant portion at a cooling
temperature and in an amount selected in the range of 20
to 50% of the circulated refrigerant so that the part of
the refrigerant warming curve relating to the final heat
exchanger is closely matched to and has substantially
the same slope as the part of the natural gas cooling
curve extending over the temperature range from said
exit temperature to -100°C.
2. A process as claimed in claim 1, wherein the
refrigerant is substantially nitrogen.

3. A process as claimed in claim 1 or 2, wherein the
refrigerant portion supplied to the final heat exchanger
is substantially isentropically expanded to a
temperature of about -152°C.
4. A process as claimed in any preceding claim,
wherein the refrigerant exits the final heat exchanger
at a temperature of about -104°C.
5. A process as claimed in any preceding claim,
wherein the refrigerant portion supplied to the final
heat exchanger is cooled, before being expanded, by heat
exchange with the isentropically expanded refrigerant,
the refrigerant portion supplied to and having flowed
through the final heat exchanger being combined with
another refrigerant portion to form a combined cooling
stream, said other refrigerant portion being expanded
substantially isentropically to the approximate
temperature of the refrigerant with which it is
combined, the natural gas and the cooled refrigerant
portion being cooled through a temperature range
including the range of -80°C to -40°C, by the combined
cooling stream in a part of said series of heat
exchangers upstream of said final heat exchanger, the
amount of said other refrigerant portion being so
selected in the range of 50 to 80% of the circulated
refrigerant that the refrigerant warming curve is

closely matched with the combined cooling curve of the
natural gas and refrigerant over said temperature range
of -80°C to -40°C.
6. A process as claimed in claim 5, wherein the
natural gas and the cooled refrigerant portion are
cooled through a temperature range of -80°C to -60°C, by
the combined cooling stream in the part of said series
of heat exchangers upstream of said final heat
exchanger.
7. A process as claimed in claim 5 or 6, wherein the
amount of said other refrigerant portion is so selected
in the range of 50 to 80% of the circulated refrigerant
that the refrigerant warming curve is closely matched
with the combined cooling curve of the natural gas and
refrigerant over said temperature range of -80°C to -
60°C.
8. A process as claimed in any preceding claim,
wherein the refrigerant portions are expanded in
respective turbo expanders and are recombined before one
portion is admitted to a heat exchanger.
9. A process as claimed in any preceding claim,
wherein one refrigerant portion is passed through one
heat exchanger and then passed to another heat
exchanger, and another refrigerant portion is passed
through said other heat exchanger and is subsequently

recombined with said one portion to form a common
refrigerant stream.
10. A process as claimed in any preceding claim,
wherein the refrigerant is divided into two portions,
and the refrigerant portion supplied to the final heat
exchanger is about 35% of the total flow of refrigerant.
11. A process as claimed in any preceding claim,
wherein the refrigerant is divided into two portions,
the first portion is passed through a single turbo
expander and the second portion is passed through two
turbo expanders in series, said first and second
portions being in parallel and then being recombined
before passing through a further one of said heat
exchangers.
12. A process as claimed in any preceding claim,
wherein the refrigerant portions are substantially
isentropically expanded to a pressure of about 55 bar.
13. A process as claimed in any preceding claim,
wherein the refrigerant portions are substantially
isentropically expanded to a pressure of about 11 bar.
14. A process as claimed in any preceding claim,
wherein the refrigerant stream is divided into three
portions in a ratio of about 10% to 30% for the first
portion, from about 30% to 70% for the second portion
and from about 20% to 40% for the third portion and the

refrigerant portions are expanded in expanders arranged
in parallel relationship with each other.
15. A process as claimed in claim 14, wherein the
refrigerant stream is divided into three portions in a
ratio of about 20%/50%/30% by volume for the first,
second and third portions respectively.
16. A process as claimed in claim 14 or 15, wherein the
refrigerant portion supplied to the final heat exchanger
is expanded to about 11.7 bar.
17. A process as claimed in any preceding claim,
wherein the refrigerant is cooled in a precool
refrigeration system before being divided to form said
portions.
18. Apparatus for liquefying natural gas by the process
as claimed in any of claims 1 to 17 by cooling with a
single phase refrigerant consisting substantially of
nitrogen, said apparatus comprising a series of heat
exchangers, and a compressor having an inlet connected
to receive warmed refrigerant from the heat exchangers
and an outlet connected to deliver refrigerant to
further compressor means driven by turbo expanders
through which portions of compressed refrigerant are
isentropically expanded and cooled to different
temperatures for use with different heat exchangers,
said turbo expanders having refrigerant outlets

connected to respective heat exchangers for delivering
the different cooled portions of the refrigerant to
respective heat exchangers for passage therethrough in
countercurrent relationship with the natural gas, the
warming curve for the refrigerant comprising sections
having different gradients and a part of the refrigerant
warming curve relating to passage through a final heat
exchanger being closely matched to and having
substantially the same gradient as a part of the natural
gas cooling curve extending over the same temperature
range of the final heat exchanger.
19. A natural gas liquefaction process substantially as
herein described, particularly with reference to the
accompanying drawings.

A process for producing a liquefied natural product such at
LNG is described where a single phase nitrogen refrigerant
is used in such a way that the refrigerant stream is
divided into at least two separate portions which are
passed through separate turbo-expanders before being
admitted to separate heat exchangers so that the warming
curve of the refrigerant more closely matches the cooling
curve of the product being liquefied so as to minimise
thermodynamic inefficiencies and hence power requirements
involved in operation of the method.