Process for Producing Liquefied Natural Gas

Abstract

A process for liquefying methane-rich gases comprising: (a) providing a stream of methane-rich feed gas containing higher hydrocarbons comprising C.sub.5+ hydrocarbons and/or aromatic compounds at a feed gas pressure of from 40 bar to 120 bar; (b) providing a stream of methane-rich recycle gas at a recycle gas pressure of from 40 bar to 120 bar; (c) mixing the feed gas with a first part of the recycle gas to form a mixture; (d) passing the resulting mixture to a first gas expander having an outlet, the first expander outlet having a first gas expander outlet pressure of between 3 bar and 50 bar and less than the feed gas and recycle gas pressures, to form a first gas expander outlet stream comprising a mixture of vapor and a condensed liquid containing the higher hydrocarbons; (e) separating the first gas expander outlet stream into a liquid stream and a vapor stream; (f) reheating and compressing the first vapor stream to a first vapor stream pressure of from 40 bar to 120 bar to form a first constituent of the recycle gas; (g) cooling a second part of the recycle gas to a temperature higher than an outlet temperature of the first gas expander; and, (h) liquefying said cooled second part of the recycle gas to form liquefied methane, wherein a content of C5+ hydrocarbons is about 0.1 mol % or less in the liquefied methane and a content of aromatic compounds is below 1 mol ppm in the liquefied methane.

In an embodiment, the cooled second part of the recycle gas is completely or substantially liquefied in step (h). In another embodiment, the second part of the recycle gas is liquefied to form liquefied methane and a second vapor stream in step (h), and the second vapor stream is reheated and compress in a step (i) to a second vapor stream pressure of from 40 bar to 120 bar to form a second constituent of the recycle gas.

Claims

1. A process for liquefying methane-rich gases, the process comprising: (a) providing a stream of methane-rich feed gas containing higher hydrocarbons comprising C.sub.5+ hydrocarbons and/or aromatic compounds at a feed gas pressure of from 40 bar to 120 bar; (b) providing a stream of methane-rich recycle gas at a recycle gas pressure of from 40 bar to 120 bar; (c) mixing the feed gas with a first part of the recycle gas to form a mixture; (d) passing the resulting mixture to a first gas expander having an outlet, the first gas expander outlet having a first gas expander outlet pressure of between 3 bar and 50 bar and less than said feed gas and recycle gas pressures, to form a first gas expander outlet stream comprising a mixture of vapor and a condensed liquid containing said higher hydrocarbons; (e) separating the first gas expander outlet stream into a liquid stream and a first vapor stream; (f) reheating and compressing said first vapor stream to a first vapor stream pressure of from 40 bar to 120 bar to form a first constituent of said recycle gas; (g) cooling a second part of said recycle gas to a temperature higher than an outlet temperature of said first gas expander; (h) liquefying said cooled second part of the recycle gas to form liquefied methane and a second vapor stream, wherein a content of C.sub.5+ hydrocarbons is about 0.1 mol % or less in the liquefied methane and a content of aromatic compounds is below 1 mol ppm in the liquefied methane; and, (i) reheating and compressing said second vapor stream to a second vapor stream pressure of from 40 bar to 120 bar to form a second constituent of said recycle gas.

2. The process according to claim 1, comprising cooling the mixture of feed gas and the first part of the recycle gas in a heat exchanger before passing the mixture to the first gas expander.

3. The process according to claim 1, comprising heating or cooling the first gas expander outlet stream in a heat exchanger prior to separation to modify the quantity of said higher hydrocarbons in the liquid.

4. The process as claimed in claim 1, wherein the methane-rich feed gas pressure and the methane-rich recycle gas pressure are each from 50 bar to 100 bar and/or the first gas expander pressure is from 5 bar to 30 bar.

5. The process as claimed in claim 1, comprising at least partially cooling the second part of the recycle gas by heat exchange with the second vapor stream prior to compressing said second vapor stream.

6. The process as claimed in claim 1, wherein the methane-rich gas is natural gas.

7. A process for liquefying methane-rich gases, the process comprising: (a) providing a stream of methane-rich feed gas containing higher hydrocarbons comprising C.sub.5+ hydrocarbons and/or aromatic compounds at a feed gas pressure of from 40 bar to 120 bar; (b) providing a stream of methane-rich recycle gas at a recycle gas pressure of from 40 bar to 120 bar; (c) mixing the feed gas with a first part of the recycle gas to form a mixture; (d) passing the resulting mixture to a first gas expander having an outlet, the first gas expander outlet having a first gas expander outlet pressure of between 3 bar and 50 bar and less than the feed gas and recycle gas pressures, to form a first gas expander outlet stream comprising a mixture of vapor and a condensed liquid containing said higher hydrocarbons; (e) separating the first gas expander outlet stream into a liquid stream and a first vapor stream; (f) reheating and compressing said first vapor stream to a vapor stream pressure of from 40 bar to 120 bar to form a first constituent of said recycle gas; (g) cooling a second part of said recycle gas to a temperature higher than an outlet temperature of said first gas expander; and, (h) completely or substantially liquefying said cooled second part of the recycle gas to form liquefied methane, wherein a content of C.sub.5+ hydrocarbons is about 0.1 mol % or less in the liquefied methane and a content of aromatic compounds is below 1 mol ppm in the liquefied methane.

8. The process according to claim 7, comprising cooling the mixture of feed gas and the first part of the recycle gas in a heat exchanger before passing the mixture to the first gas expander.

9. The process according to claim 7, comprising heating or cooling the first gas expander outlet stream in a heat exchanger prior to separation to modify the quantity of said higher hydrocarbons in the liquid.

10. The process as claimed in claim 7, wherein the methane-rich feed gas pressure and the methane-rich recycle gas pressure are each from 50 bar to 100 bar and/or the first gas expander pressure is from 5 bar to 30 bar.

11. The process as claimed in claim 7, comprising at least partially cooling the second part of the recycle gas by heat exchange with the second vapor stream prior to compressing said second vapor stream.

12. The process as claimed in claim 7, wherein the methane-rich feed gas is natural gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The invention will be further described with reference to the accompanying drawings in which FIG. 1 represents a flow diagram illustrating a process in accordance with the invention, FIG. 2 represents a flow diagram illustrating a process in accordance with another embodiment of the invention, and FIG. 3 represents a flow diagram illustrating a process in accordance with yet another embodiment of the invention. Throughout the drawing figures, like reference numerals identify like elements of the drawings.

[0021] The exact flow sheet will depend 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.

[0022] The feed natural gas (1) is passed through a pretreatment stage A in which components such as acid gases, water vapor and mercury may be removed to produce a pre-treated gas (2).

[0023] The pre-treated gas is mixed with a first part (4) of a recycle gas (3), described below, comprising typically 30% to 60% of the total recycle gas flow on a molar basis. In the resulting mixture the ratio of the molar flow of the recycle gas to the molar flow of feed gas is typically in the range of 0.5 to 2. The resulting mixture (5), after optionally cooling (6) in cooler B, flows to a gas expander machine C at a pressure of between 40 and 120 bar, more typically between 50 and 100 bar.

[0024] The outlet from expander C, stream (7), having a pressure of between 3 bar and 50 bar and more typically between 5 bar and 30 bar, and may contain a condensate comprising C.sub.5+ and/or aromatic compounds. Stream (7) may optionally be further cooled in cooler D (stream 8) so as to increase the amount of condensate formed.

[0025] The partially condensed stream (7 or 8) is separated into a liquid (9) and a vapor (10) in separator E. Typically stream 9 contains lighter hydrocarbons in addition to the aforesaid condensed heavy hydrocarbons. This stream will typically be removed from the process for use as fuel, or may be separated into lighter and heavier fractions, with the lighter fraction optionally recycled. In a further option Separator E may form the upper part of a demethanizer column. All these options for separation and subsequent processing of Stream 9 do not form part of the invention.

[0026] The vapor (10) from separator E is typically reheated in a first cold passage of heat exchanger F and the stream (11) compressed in compressor G to a pressure of 40 to 120 bar (stream 12) and then cooled in cooler H to form a first constituent of the aforementioned recycle gas (3).

[0027] A second part (Stream 13) of the recycle gas (3) is cooled (14) in a hot passage of heat exchanger F and is then passed into a liquefaction unit N shown in dashed outline. The products of the liquefaction unit are liquefied methane (LNG) and a vapor stream (23). In the liquefaction unit the stream (14) is divided. A first part (15), which typically comprises 25% to 35% of Stream 14, is further cooled in a hot passage of heat exchanger I, to form a methane-rich condensate or dense phase (16), which may be depressurized in a valve or turbine J (Stream 17) to produce LNG product.

[0028] While the example is based on a liquefaction unit N generally in accordance with WO 2012/172281, other types of liquefaction units could be substituted. In particular, a liquefaction unit which achieved complete liquefaction of the said second part of the recycle gas (14) so that the second vapor stream (23) is zero could be employed.

[0029] To provide the most part of the necessary cooling in heat exchanger I, a second part (18) is expanded in a second gas expander K. Any liquid in the expander outlet (19) is separated (20) in separator L and depressurized through valve or turbine M to produce additional LNG product (21).

[0030] The vapor from separator L (22) is reheated in a cold passage of heat exchanger I and stream (23) reheated in a second cold passage of heat exchanger F. Stream (24) is then compressed in compressor G to a pressure of from 40 to 120 bar to form a second constituent of the aforementioned recycle gas (stream 3).

[0031] According to the invention the pressure of stream (24) may be higher or lower than the pressure of stream (11).

[0032] An example of the removal of heavy hydrocarbon and aromatic material is provided in Table 1. The benzene concentration of the feed (2) of 1000 mol ppm is reduced to 1 mol ppm in stream (10). Stream (10) has a composition close to the composition of the LNG product.

TABLE-US-00001 TABLE 1 Stream No. 2 4 5 6 7 8 9 10 mol CO.sub.2 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 fraction N.sub.2 0.010399 0.017629 0.015034 0.015034 0.015034 0.015034 0.000567 0.015644 CH.sub.4 0.806366 0.935888 0.889394 0.889394 0.889394 0.889394 0.206702 0.918189 C.sub.2H.sub.6 0.101516 0.038661 0.061224 0.061224 0.061224 0.061224 0.215712 0.054708 C.sub.3H.sub.8 0.052817 0.007219 0.023588 0.023588 0.023588 0.023588 0.332095 0.010575 i-C.sub.4H.sub.10 0.006795 0.000283 0.002621 0.002621 0.002621 0.002621 0.054901 0.000416 n-C.sub.4H.sub.10 0.012252 0.000290 0.004584 0.004584 0.004584 0.004584 0.103162 0.000426 i-C.sub.5H.sub.12 0.002574 0.000016 0.000934 0.000934 0.000934 0.000934 0.022530 0.000023 n-C.sub.5H.sub.12 0.002986 0.000011 0.001079 0.001079 0.001079 0.001079 0.026281 0.000016 N C.sub.6H.sub.14 0.001544 0.000001 0.000555 0.000555 0.000555 0.000555 0.013681 0.000001 M- 0.000412 0.000000 0.000148 0.000148 0.000148 0.000148 0.003648 0.000000 cyclopentane Benzene 0.001000 0.000001 0.000359 0.000359 0.000359 0.000359 0.008853 0.000001 cyclohexane 0.000206 0.000000 0.000074 0.000074 0.000074 0.000074 0.001824 0.000000 n-C.sub.7H.sub.16 0.000515 0.000000 0.000185 0.000185 0.000185 0.000185 0.004565 0.000000 M- 0.000206 0.000000 0.000074 0.000074 0.000074 0.000074 0.001826 0.000000 cyclohexane toluene 0.000103 0.000000 0.000037 0.000037 0.000037 0.000037 0.000913 0.000000 n-C.sub.8H.sub.18 0.000206 0.000000 0.000074 0.000074 0.000074 0.000074 0.001826 0.000000 n-C.sub.9H.sub.20 0.000103 0.000000 0.000037 0.000037 0.000037 0.000037 0.000913 0.000000 H.sub.2O 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 C. 30.0 30.0 29.5 10.0 63.3 68.3 68.3 68.3 bar abs 65.0 64.9 64.9 64.8 14.0 13.9 13.9 13.9 kmol/h 5480 9786 15266 15266 15266 15266 618 14648 vapor fraction 1 1 1 1 0.968 0.960 0 1 mol C.sub.5+ 0.009854 0.000042 fraction aromatic 0.001102 0.000001

[0033] FIG. 2 represents a flow diagram illustrating a process in accordance with another embodiment of the invention wherein a heat exchanger I completely or substantially liquefies the second part of the recycle gas stream 14 to form liquefied methane 17.

[0034] FIG. 3 represents a flow diagram illustrating a process in accordance with yet another embodiment of the invention wherein there is no liquid in stream 19. Under such conditions, the vapor/liquid separator L and the valve M of FIG. 1 are redundant.