Process for producing liquefied natural gas

Abstract

A process for producing liquid methane with a reduced content of higher hydrocarbons, comprising the steps of: providing a stream of natural gas containing a mixture of ethane and higher hydrocarbons; cooling the mixture in a first heat exchanger passage; passing the cooled mixture to a gas expander; passing an outlet stream from the gas expander to a separator having a vapor outlet stream and a liquid outlet stream; compressing the vapor outlet stream; cooling the compressed vapor stream in a second heat exchanger passage to form a first liquefied methane product having a reduced content of higher hydrocarbons, wherein the liquid outlet stream from the separator forms a second liquefied methane product having an increased content of higher hydrocarbons.

Description

[0001] Description

[0002] Process for Producing Liquefied Natural Gas

[0003] Cross-Reference to Related Application

[0004] [1] This application claims priority to Great Britain Patent Application No. GB2318454.2 filed December 02, 2024, the entire disclosure of which is incorporated by reference.

[0005] Field of the Invention

[0006] [2] The invention relates to a method for production of liquefied natural gas, particularly production of liquefied natural gas with a reduced content of higher hydrocarbons.

[0007] Background of the Invention

[0008] [3] Natural gases typically comprise methane with 3 or more percent of hydrocarbons with higher molecular weights than methane.

[0009] [4] Liquefied natural gas for certain applications, including use as an aerospace propellant, advantageously have a reduced content of hydrocarbons higher in molecular weight than methane. [5] Treatment of liquefied natural gases for the purpose of reducing the content of higher hydrocarbons typically requires provision of a cryogenic fractionation facility with significant power demand. As consumption of aerospace propellants is typically intermittent, the provision and operation of such a fractionation facility is inconvenient and expensive.

[0010] As an alternative, a liquefied natural gas (LNG) production process which can provide a part of its production with a low content of higher hydrocarbons as and when required may be of significant interest, especially if the incremental investment cost and energy demand for such intermittent production are relatively low.

[0011] Brief Description of the Drawings

[0012] [6] Fig. 1 is a flow sheet depicting an embodiment of a process for production of natural gas with a reduced content of higher hydrocarbons.

[0013] [7] Fig. 2 is a flow sheet depicting a particular embodiment of a process for production of natural gas with a reduced content of higher hydrocarbons.

[0014] Summary of the Invention

[0015] [8] The invention provides a method for production of liquefied natural gas with a reduced content of higher hydrocarbons. [9] The term “higher hydrocarbon” anywhere in this application shall mean any hydrocarbon other than methane.

[0016]

[0010] The term “expander” anywhere in this application describes a process duty only. More than one expander machine or rotor in series may be required for an individual process duty.

[0017]

[0011] Pressures stated anywhere in this application as “bar” refer to bar absolute.

[0018]

[0012] Accordingly, there is provided as follows a description of a process and apparatus for production of natural gas with a reduced content of higher hydrocarbons, illustrating the main aspects of the invention (reference is made to Fig. 1 and the equipment tags and stream numbers shown therein): providing a stream of natural gas [1] at a pressure of between 30 bar and 150 bar and at an ambient temperature; passing the natural gas stream [1] to a pretreatment unit [A], having a first outlet stream [2]; cooling the first outlet stream [2] in a first hot passage of a first refrigeration heat exchanger [B], having a second outlet stream [3]; passing the second outlet stream [3] to a gas expander turbine [C], having a third outlet stream [4] with a pressure of between 3 bar and 30 bar and comprising a vapor phase and a liquid phase; passing the third outlet stream [4] to a first vapor / liquid separator [D]; providing the vapor / liquid separator [D] with a fourth, vapor outlet stream [5]; heating the fourth vapor outlet stream [5] successively in a cold passage of a second heat exchanger [E] having a fifth outlet stream [6], and in a cold passage of the first refrigeration heat exchanger [B] having a sixth outlet stream [7] with a near-ambient temperature; compressing the sixth outlet stream [7] in a compressor [F] to a pressure of between 30 bar and 150 bar in a seventh outlet stream [8]; cooling the seventh outlet stream [8] in a recycle cooler [G] having an eighth outlet stream [9] with near-ambient temperature; cooling the eighth outlet stream [9] successively in a second hot passage of the first refrigeration heat exchanger [B] having a ninth outlet stream

[0010] , and in a hot passage of the second heat exchanger [E] having a tenth outlet stream

[0011] with a close temperature approach to a temperature of the fourth, vapor outlet stream [5]; applying a source of external refrigeration [J] to the first refrigeration heat exchanger [B]; reducing a pressure of the tenth outlet stream

[0011] through a first valve [H] to form a first liquefied natural gas product

[0012] having a lower fractional content of higher hydrocarbons than a fractional content of higher hydrocarbons of the first outlet stream [2]; providing the vapor / liquid separator [D] with an eleventh liquid outlet stream

[0013] ; and, reducing a pressure of the eleventh liquid outlet stream

[0013] through a second valve [I] to form a second liquefied natural gas product

[0014] containing a balance of the higher hydrocarbon content of the first outlet stream [2],

[0019]

[0013] The pressure reduction valves [H] and [I] may be replaced with liquid pressure reduction turbines.

[0020]

[0014] The Applicant has found that the process described can unexpectedly result in a large reduction of the higher hydrocarbon content of the first liquefied natural gas product

[0012] to around 0.1 mol% (typically equivalent to >95% removal). The Applicant respectfully submits that this outcome, achieved by the agency of a partially liquefying gas expander, is both novel and inventive.

[0021]

[0015] The invention includes a variant according to which part or all of the product with reduced fractional content of higher hydrocarbons is exported from the process as a vapor stream [7a] from the sixth outlet stream [7].

[0022] Description of a Particular Embodiment

[0023]

[0016] There is provided as follows a description of a particular embodiment of the invention, wherein reference is made to Fig. 2 and the equipment tags and stream numbers shown thereon. The embodiment of Fig. 2 incorporates some structural elements shown in Fig. 1 (where identified by the same reference numerals and letters), and the flow rates, compositions, pressures, and temperatures of the relevant streams are shown in the accompanying Table 1 , below.

[0024]

[0017] An inlet stream of natural gas [1] has a pressure of 90 bar and a temperature of 33 degC. Acid gases, benzene, and water vapor are removed in a pretreatment unit [A], The higher hydrocarbon content of the pretreated gas is 3.4 mol%.

[0025]

[0018] A pretreated first feed gas outlet stream [2] flows to a first hot passage of a first refrigeration heat exchanger [B] , leaving as a cooled second outlet stream [3] at -51 degC. The cooled gas is then mixed with a first partial recycle stream [10a] of recycled methane-rich gas. A resulting first mixture [3a] flows to a gas expander turbine [C] having an expander outlet. A third outlet stream [4] has a pressure of 10.0 bar, a temperature of -124 degC, and a vapor fraction of 0.72.

[0026]

[0019] The third outlet stream [4] flows to first vapor / liquid separator [D], having a fourth vapor outlet stream [5] with a higher hydrocarbon content of 0.11 mol%.

[0027]

[0020] The fourth vapor outlet stream [5] is heated in a cold passage of a second heat exchanger [E] to form a fifth outlet stream [6], and is then is mixed with a second recycle stream

[0020] of recycled methane-rich gas. A resulting second mixture [6a] is then further heated in a cold passage of the first heat exchanger [B], leaving at near-ambient temperature as a sixth outlet stream [7] with a pressure of 9.1 bar and a temperature of 30 degC.

[0021] The reheated sixth outlet stream [7] is compressed to a pressure of 90 bar in first recycle compressor [F] to form a seventh outlet stream [8], which then cooled to 33 degC in a recycle cooler [G] to form an eighth recycled outlet stream [9],

[0028]

[0022] The eighth recycled outlet stream [9] is then cooled in a second hot passage of the first heat exchanger [B] to form a ninth outlet stream

[0010] , which is then divided into two parts, a first part forming the above-mentioned first partial recycle stream [10a] of recycled methane-rich gas. A second partial recycle stream [1 Ob] is further cooled in a hot passage of the second heat exchanger [E] by heat exchange with the fourth vapor outlet stream [5] to form a tenth outlet stream

[0011] , which is then let down in pressure through a first valve [H] to form a first two-phase liquified natural gas product stream

[0012] .

[0029]

[0023] The first natural gas product stream

[0012] flows to a second vapor-liquid separator [K], which has a twelfth vapor outlet stream

[0015] and a thirteenth liquid outlet stream

[0016] , having a pressure of 1 .05 bar and a temperature of - 163 degC. The thirteenth liquid outlet stream

[0016] constitutes a first liquefied methane product of the liquefaction process, having a higher hydrocarbon content of 0.13 mol%.

[0030]

[0024] Taking into consideration that the concentrations of higher hydrocarbon are 3.4 mol% in the pretreated feed gas [2] and 0.13 mol% in the thirteenth liquid outlet stream

[0016] , a large reduction in higher hydrocarbon in concentration with reference to stream

[0016] of (3.4 - 0.13) / 3.4*100 = 96.2% is achieved through the agency of a partially liquefying gas expander.

[0025] The eleventh liquid outlet stream

[0013] from the vapor-liquid separator [D] is let down in pressure through a second valve [I] to form a second two-phase liquified natural gas product stream

[0014] ,

[0031]

[0026] The second product stream

[0014] flows into a third vapor-liquid separator [L], which has a fourteenth vapor outlet stream

[0017] and a fifteenth liquid outlet stream

[0018] , having a pressure of 1 .05 bar and a temperature of -161 degC. The fifteenth liquid outlet stream

[0018] constitutes the second liquefied methane product of the liquefaction process, having a higher hydrocarbon content of 6.8 mol%.

[0032]

[0027] A fraction (first export) stream

[0021] of the first liquefied methane product amounting in this example to 60% of the thirteenth liquid outlet stream

[0016] is exported from the process as liquefied methane with a low content of higher hydrocarbon.

[0033]

[0028] A balance

[0022] of the first liquefied methane product is combined with the fifteenth liquid outlet stream

[0018] to form a second export stream

[0023] having a higher content of higher hydrocarbon than the pretreated feed gas [2],

[0034]

[0029] The twelfth and fourteenth vapor outlet streams

[0015] and

[0017] , respectively, are combined to form a sixteenth vapor stream

[0019] , which is compressed by a second recycle compressor [M] to form a second stream

[0020] of recycled methane-rich gas.

[0030] A source of external refrigeration [J], which may be in the form of an evaporative refrigeration process or an expander-based refrigeration process, is applied to the first heat exchanger [B],

[0035] Table 1 below summarize the compositions of the various streams described above with reference to Fig. 2.

[0036] TABLE 1

[0037]

[0038]

Claims

ClaimsWhat is claimed is:

1. A process for producing liquefied natural gas comprising the steps of:(a) providing a first feed gas outlet stream [2] of natural gas at a pressure of between 30 bar and 150 bar and at an ambient temperature;(b) cooling the first outlet stream [2] in a first hot passage of a first refrigeration heat exchanger [B], having a second outlet stream [3];(c) passing the second outlet stream [3] to a gas expander turbine [C], having a third outlet stream [4] with a pressure of between 3 bar and 30 bar and comprising a vapor phase and a liquid phase;(d) passing the third outlet stream [4] to a vapor / liquid separator [D];(e) providing the vapor / liquid separator [D] with a fourth vapor outlet stream [5];(f) heating the fourth vapor outlet stream [5] successively in a cold passage of a second heat exchanger [E] having a fifth outlet stream [6], and in a cold passage of the first refrigeration heat exchanger [B] having a sixth outlet stream [7] with a near-ambient temperature;(g) compressing the sixth outlet stream [7] in a compressor [F] to a pressure of between 30 bar and 150 bar in a seventh outlet stream [8];(h) cooling the seventh outlet stream [8] in a recycle cooler [G] having an eighth outlet stream [9] with near-ambient temperature;(i) cooling the eighth outlet stream [9] successively in a second hot passage of the first refrigeration heat exchanger [B] having a ninth outlet stream [10], and in a hot passage of the second heat exchanger [E] having a tenth outlet stream [11] with a close temperature approach to a temperature of the fourth vapor outlet stream [5];(j) reducing a pressure of the tenth outlet stream [11] through a first valve [H] to form a first liquefied natural gas product [12] having a lower fractional content of higher hydrocarbons than a fractional content of higher hydrocarbons of the first outlet stream [2];(k) providing the vapor / liquid separator [D] with a twelfth liquid outlet stream [13]; and,(l) reducing a pressure of the twelfth liquid outlet stream [13] through a second valve [I] to form a second liquefied natural gas product [14] containing a balance of the higher hydrocarbon content of the first outlet stream [2],2. The process as claimed in claim 1 , comprising pretreating the first outlet stream [2] of step (a) in a pretreatment unit before step (b).

3. The' process as claimed in claim 1 , comprising applying a source of external refrigeration [J] to the first heat exchanger [B].

4. A process as claimed in claim 1 wherein part or all of the product having a reduced fractional content of higher hydrocarbons is exported from the process as a vapor.

5. A process for producing liquefied natural gas comprising:(a) providing a first stream of natural gas [2] at a pressure of between 30 and 150 bar and at ambient temperature;(b) cooling the first stream [2] in a first hot passage of a first refrigeration heat exchanger [B], having a second outlet stream [3];(c) providing a first partial recycle stream [10a] of methane-rich recycle gas;(d) combining the second outlet stream [3] and the first partial recycle stream [10a] to form a resulting first mixture stream [3a];(e) passing the first mixture stream [3a] to a gas expander turbine [C], having a third outlet stream [4] with a pressure of between 3 bar and 30 bar and comprising a vapor phase and a liquid phase;(f) passing the third expander outlet stream [4] to a first vapor / liquid separator [D];(g) providing the vapor / liquid separator [D] with a fourth vapor outlet stream [5];(g) heating the fourth vapor outlet stream [5] in a cold passage of a second heat exchanger [E] having a fifth outlet stream [6];(h) providing a second recycle gas stream [20] of a methane-rich recycle gas;(i) combining the fifth outlet stream [6] and the second gas recycle stream [20] to form a resulting second mixture stream [6a];(j) heating the second mixture stream [6a] in a cold passage of the first heat exchanger [B] having a sixth outlet stream [7] with near-ambient temperature;(k) compressing the sixth outlet stream [7] in a compressor [F] to form a seventh outlet stream [8] having a pressure of between 30 and 150 bar;(l) cooling the seventh outlet stream [8] in a cooler [G], having an eighth outlet stream [9] with near-ambient temperature;(m) cooling the eighth outlet stream [9] in a second hot passage of the first refrigeration heat exchanger [B] having a ninth outlet stream [10];(n) separating the first partial recycle stream [10a] of methane-rich recycle gas to form a second partial recycle stream [10b];(o) cooling the second partial recycle stream [10b] in a hot passage of the second heat exchanger [E] having a tenth outlet stream [11] with a close temperature approach to the fourth vapor outlet stream [5];(p) reducing a pressure of the tenth outlet stream [11] through a first valve [H] to form a first product stream [12];(q) passing the first gas product stream [12] into a second vapor / liquid separator [K] having a twelfth vapor outlet stream [15] and a thirteenth liquid outlet stream [16];(r) providing a first vapor / liquid separator [D] with an eleventh liquid outlet stream [13];(s) reducing a pressure of the eleventh liquid outlet stream [13] through a second valve [I] to form a second liquid product stream [14];(t) passing the second liquid product stream [14] into a third vapor / liquid separator [I] having a fourteenth vapor outlet stream [17] and a fifteenth liquid outlet stream [18];(u) separating a fraction (first export) stream [21] of the thirteenth liquid methane product stream [16] to form a first liquefied methane product of the process, having a lower content of higher hydrocarbons than the first feed gas outlet stream [2];(v) forming a balance stream [22] as a balance of the thirteenth liquid outlet stream [16];(w) combining the fifteenth liquid outlet stream [18] and the balance stream [22] to form a second liquefied methane product export stream [23] of the process, having a higher content of higher hydrocarbons than the first feed gas outlet stream [2];(x) combining the twelfth and fourteenth vapor outlet streams [15] and [17] to form a sixteenth vapor stream [19]; and,(y) compressing the sixteenth vapor stream [19] in a compressor [M] to at least the same pressure as the pressure of the fifth outlet stream [6] toform the said second recycle gas stream [20] of methane- rich recycle gas.

6. The process as claimed in claim 5, comprising pretreating the stream [2] of step (a) in a pretreatment unit before step (b).

7. The' process as claimed in claim 5, comprising applying a source of external refrigeration [J] to the heat exchanger [B].

8. A process as claimed in claim 5 wherein part or all of the product having a reduced fractional content of higher hydrocarbons is exported from the process as a vapor.

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