LNG Production Process Field of the Invention

This invention relates to a process for liquefying a methane-rich gas and more particularly but not exclusively relates to a method of producing liquefied natural gas (LNG) comprising a gas expander and a liquid pressure reduction machine or turbine.

Background to the Invention

A market is emerging for small-scale liquefaction of natural gas for local use, for instance as a transportation fuel, especially for heavy goods vehicles.

As these small liquefaction units are usually required to operate without the presence of an attendant, liquefaction processes requiring evaporation and storage of liquid hydrocarbon refrigerants are not very suitable, particularly on grounds of safety.

Consequently, liquefaction processes based on use of methane or nitrogen gas expanders have been preferred for these small unmanned liquefiers. However such gas expander liquefaction processes based on use of a methane or nitrogen expander generally have a low thermal efficiency, requiring substantially more compression power per unit of LNG produced when compared with large base-load LNG plants.

Summary of the Invention According to the present invention there is provided a process for the production of a methane-rich liquid which comprises

1 ) providing a methane-rich gas stream having a pressure of from 40 bar to 120 bar (4 MPa to 12 MPa),

2) cooling said gas stream in a first heat exchanger by at least 10°C,

3) dividing the gas stream from step 2 into at least first and second parts, 4) passing said first part through a gas expansion machine or turbine having an outlet pressure of from 3 bar to 20 bar (0.3 MPa to 2MPa) and an outlet temperature of from -70 to -130°C,

5) passing the outlet stream from step 4) successively through a second heat exchanger and the first heat exchanger, and

6) at least partially recycling said stream from step 5), wherein:

7) the second part from step 3) is cooled in the second heat exchanger to a temperature in the range -70 to -130°C, at which it is substantially condensed,

8) passing the liquid fraction of the cooled gas stream from the second heat exchanger through a pressure reduction machine or turbine having an outlet pressure in the range 1 bar to 20 bar (0.1 MPa to 2 MPa),

9) separating the two phase stream from said pressure reduction machine or turbine into a methane-rich liquid product and a vapour. The methane-rich gas stream in Step 1 ) typically has a temperature of from -10 to 60°C.

The process optionally includes the steps of 10) passing the outlet vapour stream from step 9) successively through the second heat exchanger and first heat exchanger, and optionally

1 1 ) at least partially recycling said stream from step 10). The present invention describes a natural gas liquefaction process comprising a step of expanding a methane rich gas in an expansion machine or turbine and a second step of reducing the pressure of a methane rich liquid through a pressure reduction machine or turbine, thereby materially reducing the compression power required per unit of LNG produced compared with a process comprising a gas expansion machine or turbine alone.

Small scale LNG production units frequently are required to deliver the liquefied product at above atmospheric pressure, and accordingly the invention described herein produces liquid typically at a pressure of around 3 bar (0.3MPa). The invention may however be adapted to produce LNG product at nearer to atmospheric pressure, as outlined in the Description below. Where pressures are referred to, these are absolute values. Description of Preferred Embodiments Reference is made to the accompanying Figure!

A feed stream of methane rich gas (1 ) with acid gases and water vapour substantially removed, a pressure typically between 40 bar and 120 bar (4 MPa and 12 MPa) and a temperature generally between -20 and 60°C, typically between 10 and 50°C, is mixed with a recycle gas (10) described below. The resulting mixture (2) is cooled by at least 10°C in a passage of a first heat exchanger (A). For instance, if the first heat exchanger inlet temperature is 42°C, the corresponding outlet from the exchanger would be around -7°C, if the inlet temperature is 0°C the outlet would be around -20°C and if -10°C the outlet would be around -24°C. Thus as the feed plus recycle temperature drops, so does the duty of the first heat exchanger.

The cooled mixture (3) is then divided. One part (4) is admitted to an expansion machine or turbine (B), having outlet pressure typically between 3 bar and 20 bar (0.3 MPa and 2 MPa) and outlet temperature typically between -80 and -110°C. This stream (5) is reheated in a first cold passage of a second heat exchanger (C) and then further reheated (6) in a first cold passage of heat exchanger (A). The reheated stream (7) having a temperature close to the temperature of the inlet gas stream to the first heat exchanger (2) is mixed with compressed flash gas (21 ) described below and flows (8) to a recycle compressor (D). The compressor outlet stream (9) is cooled (10) in a recycle cooler (E) to form the above mentioned recycle gas.

Some variations of the above described flow scheme are foreseen, as instances:

1. the feed gas (1 ) may be introduced by mixing with stream (8) or at an intermediate pressure in compressor (D);

2. the recycle gas (10) may be cooled separately from the feed gas (1) in a second hot passage in heat exchanger (A) and then divided, one part flowing to expansion turbine (B) and another part being combined with the cooled feed gas (3);

3. if the feed gas (1 ) has a significant content of C3+ hydrocarbon, this may in part be removed by a separator typically provided downstream of heat exchanger (A); 4. where a external demand exists for medium pressure fuel gas, this may be extracted from streams (7) or (8);

5. any part or all of the feed and/or compressor suction and/or recycle streams may be cooled separately, or after mixing (2), typically to a temperature of between 10 to -20°C before entering heat exchanger A, by means of an absorption chiller or a mechanical refrigeration cycle or a combination thereof.

Another part (11) of the above mentioned cooled mixture (3) is cooled in a passage of second heat exchanger (C), emerging as a condensed liquid or cooled supercritical fluid at a temperature (12) typically between -80°C and - 110°C. This stream flows to a pressure reduction machine or turbine (F), emerging (13) at a pressure between 1 bar and 10 bar (0.1 MPa and 1.0 MPa).

This two-phase stream flows to vapour/liquid separator (G). The vapour (15) is reheated in a second cold passage of heat exchanger (C) and then is further reheated (16) in a second cold passage of heat exchanger (A) emerging (17) at a temperature close to the temperature of the mixture of feed gas and recycle gas (2). Stream (17) may optionally be discharged from the process (18), for instance for use as a fuel, or compressed all or in part (19) in a flash gas compressor (H). The compressed stream (20) is cooled (21) in a flash gas cooler (I) and mixed with recycle gas (7) as described above. The liquid phase (14) may comprise the liquefied methane rich product from the process. This liquefied methane stream may be further depressurised in a valve or expansion machine or turbine, for instance into an atmospheric pressure storage tank. The resulting low pressure flash gas may be reheated in a third cold passage of heat exchanger (C) and then further reheated in a third cold passage of heat exchanger (A) emerging at a temperature close to the temperature of the mixture of feed gas and recycle gas (2).

The aforementioned first and second heat exchangers may be combined into a single heat exchange unit such as a brazed aluminium plate-fin core construction, such single heat exchange unit containing internal streams corresponding to streams 3), 6), 11) and 16) in Fig. 1 with a stream

corresponding to stream 4) taken out of the heat exchange unit at an intermediate point as also shown in Fig.1.

A typical set of values of pressure and temperature for streams in Figure 1 are as below.

Name 1 2 3 4 5 6

Pressure (bar) 65.0 65.0 64.6 64.6 12.0 11.9 Temperature (C) 45.0 42.5 -6.9 -6.9 -90.6 -32.9

Name 7 8 9 10 11 12

Pressure (bar) 11.7 11.7 65.3 65.0 64.6 64.4 Temperature (C) 39.5 39.8 80.3 42.0 -6.9 -88.6

Name 13 14 15 16 17 18

Pressure (bar) 3.00 3.00 3.00 2.98 2.95 2.95 Temperature (C) -147.1 147.1 147.1 -32.9 39.5 39.5

Name 19 20 21

Pressure (bar) 2.95 11.8 11.7

Temperature (C) 39.5 98.0 42.0

Any part or all of the feed (1), recycle gas (10), mixture thereof (2) and/or compressor suction streams may be cooled by means of external refrigeration system, such as an absorption refrigeration cycle or a mechanical refrigeration cycle. In the case of an absorption refrigeration cycle, the heat requirement of the refrigeration system may be supplied by the exhaust heat of a gas engine or a gas turbine, such gas engines or turbines typically being used for supplying power to the process compressors. The said cooling of either feed and/or recycle streams and/or compressor suction streams by means of an external refrigeration system may be combined with removal of carbon dioxide and/or other impurities from the feed gas.