Note: Descriptions are shown in the official language in which they were submitted.
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Process for Liquefaction of Natural Gas
Field of the Invention
5The present invention relates to a method for liquefying methane-rich gas
and,
more particularly but not exclusively, relates to a method for producing
liquefied
natural gas (LNG).
Background to the Invention
10Liquefaction of natural gas can practically be achieved by:
- evaporation of liquid refrigerants
- work expansion of gases in expansion machines (expanders).
Evaporation of liquid refrigerants gives the lowest power requirement and is
the
basis of the widely used Cascade and Mixed Refrigerant LNG processes.
Expander-based LNG installations are simple, compact, low in weight and can
avoid the importation/preparation/storage of liquid refrigerants. These
characteristics are attractive for smaller scale applications, particularly
offshore,
where low hydrocarbon inventory is desirable from safety considerations.
25However expander processes have certain drawbacks:
- until recently, limited capacity and experience with expanders
- higher power requirement
- higher internal gas flowrates, requiring larger line diameters, etc.
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With most expander-based processes the working fluid (typically nitrogen)
5remains in the vapour phase at the expander outlet.
Partially liquefying the feed gas itself in an expander, having a two-phase
discharge flow, can reduce internal (recycle) gas flows and reduce power
10requirement.
LNG production in a liquefying expander is not a new idea (USP 2,903, 858 ¨
Bocquet).
The present inventors previously disclosed a process (GB Patent 2393504B,
USP 7,234,321) with potentially lower power requirements, wherein a liquefying
expander is combined with a precooling circuit which contains a simple mixed
20refrigerant generated from the feed natural gas.
= Other recent disclosures comprise precooling by a parallel/recycle gas
expander followed by a liquefying expander:
WO 01/44735 (Minta et al) describing production of pressurised liquid
natural gas (PLNG) at -112 C from feed gas compressed to a high
pressure of "above 1600 psia".
US 2006/0213222 (Whitesell) describing production of LNG from a
feed gas entering the process at, or compressed within, the process
to a pressure of "between about 1500 psig to about 3500 psig".
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Summary of the Invention
Relative to the two above-mentioned patents, an inventive step in the present
application consists of identifying operating conditions for the two expanders
5(the precooling expander and the liquefying expander) which allow for
practical
production of atmospheric pressure LNG at about -161 C. Moreover a very high
,
pressure feed gas, which is a feature of the above-mentioned patents, is no
longer required.
10This results in a simplified process with improved thermal efficiency having
a
wide range of potential applications where the raw feed gas has a pressure as
low as 40 bar (4 MPa) .
The present invention facilitates production of LNG from smaller gas fields,
15particularly offshore, due to its simple flow scheme, low power consumption
and non-reliance on storage and use of liquid refrigerants. The liquefaction
process itself generally does not require process columns, for instance for
refrigerant preparation, which may be less easy to operate under such
operating conditions.
Description of the Invention
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According to the invention, there is provided a process for liquefying natural
gas
or other methane-rich gases. The feed gas, generally at a pressure of from 40
(4 MPa) to 100 bar (10 MPa), is liquefied to give LNG product at approx 1 bar
(0.1 MPa) / -161 C by the expander-based plant configuration described above
Sand comprising:
cooling feed gas and recycle gas (mentioned below) in a first step by
means of a first heat exchanger and in a first work expander; the
heat exchanger having an outlet temperature in the range of -50 to
-80 C, preferably -600 to -70 C; the expander having a lower outlet
temperature than that of the heat exchanger; the expander having its
outlet stream reheated in a cold passage of the said heat exchanger
and then recompressed to form part of the above mentioned recycle
gas.
passing the cooled outlet stream from the said first heat exchanger
partly into a hot passage in a second heat exchanger, wherein it is
essentially condensed, and partly into a second work expander, the
said second expander having a lower outlet temperature than the
cold outlet of the second heat exchanger, the second expander outlet
stream containing a significant amount of liquid (typically 10-15% wt);
the expander outlet being separated into a vapour fraction and a
liquid fraction; the vapour fraction being reheated in cold passages in
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said second and first heat exchangers; then recompressed and
returned to the inlet to the process as part of the above mentioned
recycle gas.
5 - reducing the pressure of the above mentioned separated liquid and
of the condensed liquid from the hot passage from second heat
exchanger (both typically around -120 C) to around to atmospheric
pressure; reheating the flashed gas evolved in further cold passages
in the above heat exchangers; removing the liquid for use as LNG
product.
it has been found that the lowest requirement for recycle gas compression
power results from concentrating the extraction of mechanical work into the
pressure range above 10 bar (1 MPa) approx at the outlet of the second
15expander. An advantage of this is that the outlet pressures from the two
expanders can be equalized at around 10 bar (1MPa) , reducing the first heat
exchanger to a three-passage configuration.
Whereas most existing LNG production relies on evaporation of liquid
20refrigerants to cool and condense the natural gas so as to form LNG product
in
a heat exchanger, this invention comprises a liquefaction process with
moderate power requirement in which the necessary refrigeration, is largely
supplied by work expansion of the feed gas itself. Cryogenic liquid
refrigerants
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or other secondary working fluids such as nitrogen are therefore not required.
In this way energy is extracted at a low temperature level which results in
improved thermodynamic efficiency. As a result, a significant proportion of
the
LNG is formed directly in a work extracting expander, in addition to that
formed
5by condensation in an exchanger which is cooled by the reheating of the cold
gas from the said work expander.
Description of Preferred Embodiments
10The invention will be described with reference to the accompanying drawings
in
which Figures 1 and 2 represent flow diagrams illustrating processes in
accordance with the invention.
Figure 1 shows the operating features of the invention. The exact flow sheet
will
15depend upon the feed gas specification, but will generally contain these
basic
elements. Where pressures are stated anywhere in this application as "bar"
these are bar absolute.
The feed natural gas (Stream 1) is passed through a pretreatment stage A in
20which components that would solidify or otherwise interfere with the
downstream liquefaction process, such CO2, H2S, water vapour and mercury
vapour, are removed to the extent necessary to give- appropriate and
conventional maximum concentrations in the pretreated gas (Stream 2).
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Stream 2 is mixed with part (Stream 4) of the recycle gas (Stream 3) to form
Stream 6, which is passed through a passage in heat exchanger 6, leaving as
Stream 7 at a temperature typically in the range -200 to -60 C, preferably -30
to
-50 C. This temperature is typically low enough to condense sufficient NGL to
5meet the specification for the final LNG product. Any condensed hydrocarbons
in separator C are removed as Stream 8. The outlet vapour from C (Stream 9)
is further cooled in a passage in heat exchanger D, exiting as Stream 10 at a
temperature in the range -50 to -80 C, preferably -60 to -70 C. The
remaining
part of the recycle gas (Stream 5) is cooled in gas expander E having an
outlet
10Stream 11 with a temperature lower than the temperature of Stream 10.
= Optionally part or all of the pretreated feed gas may exit pretreatment
stage A
via Stream 2a to join the recycle gas Stream 3 upstream of the point at which
it
is divided into Streams 4 & 5. This option may be convenient when the natural
gas feed Stream 1 has only a small content of heavy hydrocarbon. In such a
15case the pretreated feed gas may be mixed with the whole of the recycle gas
and then the resulting mixture divided to supply heat exchanger B through
Stream 6 and gas expander E through Stream 5.
The pressure of Stream 11 will typically be around 15 bar (1.5 MPa). Stream 11
enters a first cold passage in heat exchanger D, leaving as Stream 12, which
20then passes through a first cold passage in heat exchanger B, emerging
(Stream 13) at a temperature just below the temperature of Stream 6. The ratio
of the flow rate of Stream 4 to the flow rate of Stream 5 is controlled so
that the
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temperature approach between the composite hot and cold sides of heat
exchangers B and D are substantially uniform throughout their lengths.
A large part of Stream 10 is then passed (Stream 14) through a second gas
expander F from which it emerges as Stream 15 at a pressure between 3 bar
5(0.3 MPa) and 20 bar (2 MPa) , preferably between 5 bar (0.5 MPa) and 15 bar
(1.5 MPa) and in a partly liquefied state. Stream 15 then enters vapour-liquid
separator G. The liquid phase from Separator G (Stream 16) is then typically
let
down in a pressure reduction device H such as a valve or a turbine. The outlet
from H (Stream 17), which is typically at or close to atmospheric pressure, is
10delivered into the LNG Tank I. If it is desired to reduce the nitrogen
content of
the product LNG, a conventional nitrogen stripping column (not shown) may be
used, typically employing the sensible heat of Stream 16 for reboiling.
Optionally and preferably a part of Stream 10 flows as Stream 23 through a hot
15side passage in heat exchanger J, wherein it is liquefied by indirect heat
exchange with the vapour from separator G (Stream 18), emerging as Stream
24. This is then typically let down in pressure through pressure reduction
device K, such as a valve or a turbine. The outlet from K is routed either to
vapour-liquid separator G, shown in broken line as Stream 25a, or preferably
as
20Stream 25b to the LNG tank I. This second option helps to reduce
accumulation of nitrogen in the recycle gas. Stream 18, having been heated in
a first cold passage in heat exchanger J, emerges as Stream 19. It is then
further heated in a second cold passage in heat exchanger D, emerging as
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Stream 20, which is then further heated in a second cold passage in heat
exchanger B, emerging as Stream 21 at a temperature slightly below the
temperature of Stream 6.
5Streams 13 and 21 are compressed in recycle compressor N, from which the
outlet Stream 34 is cooled typically with cooling water in cooler 0.
Compressor
N may consist of more than one stage with intercoolers. Streams 13 and 21
will not have the same pressure and may enter at different compressor stages.
The outlet stream from 0 forms the above-mentioned recycle gas Stream 3.
The flashing of Stream 16 across H and the flashing of Stream 24 across K will
result in the evolution of vapour comprising mainly methane together with most
of the nitrogen content of the feed gas. Typically this vapour (Stream 26),
optionally combined with boil-off vapour resulting from heat leak into tank I,
is
15heated in a second cold passage in heat exchanger J to form Stream 27, then
in a third cold passage in heat exchanger D to give Stream 28 and finally in a
third cold passage in heat exchanger B, emerging as Stream 29 at a
temperature slightly below the temperature of Stream 6. A conventional booster
blower (also not shown) may be provided in Stream 26 to ensure that the
20pressure of Stream 29 does not fall below atmospheric pressure. Stream 29
may typically be used as fuel gas.
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Part or all of Stream 29 (Stream 30) optionally may be compressed for return
to the recycle gas in a low pressure compressor L, leaving as Stream 31. This
stream is cooled in cooler M, from which the outlet (Stream 32) joins Stream
21
to form Stream 22, which then enters the suction of recycle compressor N
5instead of Stream 21 alone if this option is not used. A further option is to
withdraw recycle gas (Stream 33) at a convenient point from compressor N
typically for use as gas turbine fuel. It may be convenient to use Stream 29
or
Stream 33 as stripping gas for regeneration of adsorbents in the pretreatment
stage A, prior to their ultimate combustion as fuels.
Figure 2 shows a preferred embodiment of the invention in which expanders E
and F have essentially the same outlet pressure of between 3 bar (0.3 MPa)
and 20 bar (2 MPa), preferably between 5 bar (0.5 MPa) and 15 bar (1.5 MPa).
The outlet stream from expander E (Stream 11) is then combined with Stream
1519 to form Stream 19a, which enters heat exchanger D in place of Stream 19
in
Fig.1. The heat exchangers B and D then have only three passages,
simplifying the construction of the exchanger and the operation of the plant.
Although in most applications it is expected that the Streams 2 and 3 will
have
20temperatures close to ambient temperature, cooling below this level may be
advantageous. It is feasible to cool those streams, and optionally the outlet
streams from compressor intercoolers and aftercoolers, by means of a
mechanical refrigeration cycle or by means of an absorption refrigeration
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system, typically using lithium bromide (LiBr), which could receive its heat
supply from the exhaust of a gas turbine, gas engine or combined cycle or
anything else suitable.
