Process for producing liquefied natural gas

Abstract

Process for producing liquid methane with a reduced content of higher hydrocarbons comprising providing a stream 1 of natural gas containing ethane and higher hydrocarbons cooling the resulting mixtur

Description

Field of the Invention 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. Background Natural gases typically comprise methane with 3 or more percent of hydrocarbons with higher molecular weights. 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. 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. 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. Summary of the Invention The aim of the invention is production of liquefied natural gas with a reduced content of higher hydrocarbons. The term ‘higher hydrocarbon’ anywhere in this application shall mean any hydrocarbon other than methane. 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. Where pressures are stated anywhere in this application as ‘bar’, these are bar absolute. 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 Drawing 1 / 2 and the equipment tags and stream numbers shown thereon): - providing a stream of natural gas [1] at a pressure of between 30 and 150 bar and at ambient temperature; - passing stream [1 ] to pretreatment [A], having outlet stream [2]; - cooling stream [2] in a first hot passage of refrigeration heat exchanger [B], having outlet stream [3]; - passing stream [3] to a gas expander turbine [C], having outlet stream [4] with a pressure of between 3 bar and 30 bar and comprising a vapor phase and a liquid phase; - passing stream [4] to vapor / liquid separator [D]; - providing vapor / liquid separator [D] with a vapor outlet stream [5]; - heating stream [5] successively in a cold passage of heat exchanger [E] having outlet stream [6], and in a cold passage of heat exchanger [B] having outlet stream [7] with near-ambient temperature; - compressing stream [7] to a pressure of between 30 and 150 bar [8] in compressor [F]; - cooling stream [8] in cooler [G], having outlet stream [9] with nearambient temperature; - cooling stream [9] successively in a second hot passage of refrigeration heat exchanger [B] having outlet stream

[10] , and in a hot passage of heat exchanger [E] having outlet stream

[11] with a close temperature approach to stream [5]; - applying a source of external refrigeration [J] to the heat exchanger [B]; - reducing the pressure of stream

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

[12] having a lower fractional content of higher hydrocarbons than stream [2]; - providing vapor / liquid separator [D] with a liquid outlet stream

[13] ; - reducing the pressure of stream

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

[14] containing the balance of the higher hydrocarbon content of stream [2]. The pressure reduction valves [H] and [I] may be replaced with liquid pressure reduction turbines. The Applicant has found that the process described can unexpectedly result in a reduction of the higher hydrocarbon content of the said first liquefied natural gas product

[12] to around 0.1 mol% (typically equivalent to >95% removal). The application respectfully submits that this outcome is inventive and novel. 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 in stream [7a], Description of a Particular Embodiment There is provided as follows a description of a particular embodiment of the invention; reference is made to Drawing 2 / 2 and the equipment tags and stream numbers shown thereon. The flow rates, compositions, pressures and temperatures of the relevant streams are shown in the accompanying Table 1. 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 pretreatment unit [A], The higher hydrocarbon content of the pretreated gas is 3.4 mol%. The pretreated gas [2] flows to a first hot passage of a heat exchanger [B], leaving as cooled stream [3] at -51 degC. The cooled gas is them mixed with a first stream [10a] of recycled methane-rich gas. The resulting mixture [3a] flows to gas expander [C]. The expander outlet stream [4] has a pressure of 10.0 bar, a temperature of -124 degC and a vapor fraction of 0.72. Stream [4] flows to first vapor-liquid separator [D], having a vapor outlet [5] with a higher hydrocarbon content of 0.11 mol%. The stream [5] is heated in a cold passage of heat exchanger [E] to form stream [6], and is then is mixed with a second stream

[20] of recycled methane-rich gas. The resulting mixture [6a] then further heated in a cold passage of heat exchanger [B], leaving at near-ambient temperature as stream [7] with a pressure of 9.1 bar and a temperature of 30 degC. The reheated stream [7] is compressed to a pressure of 90 bar in first recycle compressor [F] to form stream [8], which then cooled to 33 degC [9] in recycle cooler [G], The recycled stream [9] is then cooled in a second hot passage of heat exchanger [B] to form stream

[10] , which is then divided into two parts, the first part [10a] forming the above-mentioned first stream [10a] of recycled methane-rich gas. The second part [10b] is further cooled in a hot passage of heat exchanger [E] by heat exchange with stream [5] to form stream

[11] , which is then let down in pressure through valve [H] to form two-phase stream

[12] , Stream

[12] flows into second vapor-liquid separator [K], which has vapor outlet stream

[15] and liquid outlet stream

[16] , having a pressure of 1.05 bar and a temperature of-163 degC. Stream

[16] constitutes the first liquefied methane product of the liquefaction process, having a higher hydrocarbon content of 0.13 mol%. The molar flow rate of stream

[16] is 50.9% of the molar flow rate of the pretreated feed gas [2] but contains only 1.9% of its molar flow rate of higher hydrocarbons, therefore representing a reduction in higher hydrocarbon content of (50.9-1.9) / 50.9*100 = 96.3%. The liquid outlet stream

[13] from vapor-liquid separator [D] is let down in pressure through valve [I] to form two-phase Stream

[14] , Stream

[14] flows into third vapor-liquid separator [L], which has vapor outlet stream

[17] and liquid outlet stream

[18] , having a pressure of 1.05 bar and a temperature of-161 degC. Stream

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

[21] of the first liquefied methane product amounting to 60% of stream

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

[22] of the first liquefied methane product is combined with stream

[18] to form a second export stream

[23] having a higher content of higher hydrocarbon than the pretreated feed gas [2]. The vapor streams

[15] and

[17] are combined to form stream

[19] , which is compressed by second recycle compressor [M] to form the said second stream

[20] of recycled methane-rich gas. 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 heat exchanger [B]. TABLE 1 Stream 1 2 3 3a 4 5 6 6a 7 Vapour Fraction 1.000 1.000 1.000 1.000 0.719 1.000 1.000 1.000 1.000 Pressure (bar) 90 89.5 89.2 88.9 10 9.8 9.4 9.4 9.1 Temperature (C) 33 33 -51.32 -51.65 -124.1 -124.6 -54.32 -53.69 29.86 Molar Flow (kgmole / h) 1019 1018 1018 2455 2455 1770 1770 2236 2236 Mass Flow (kg / h) 17007 16965 16965 41065 41065 29452 29452 37522 37522 Molecular Weight 16.69 16.66 16.66 16.73 16.73 16.64 16.64 16.78 16.78 Comp Mole Frac (CO2) 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 Comp Mole Frac (Nitrogen) 0.0040 0.0041 0.0041 0.0371 0.0371 0.0489 0.0489 0.0605 0.0605 Comp Mole Frac (Methane) 0.9614 0.9620 0.9620 0.9484 0.9484 0.9501 0.9501 0.9387 0.9387 Comp Mole Frac (Ethane) 0.0304 0.0304 0.0304 0.0131 0.0131 0.0010 0.0010 0.0008 0.0008 Comp Mole Frac (Propane) 0.0021 0.0021 0.0021 0.0009 0.0009 0.0000 0.0000 0.0000 0.0000 Comp Mole Frac (i-Butane) 0.0003 0.0003 0.0003 0.0001 0.0001 0.0000 0.0000 0.0000 0.0000 Comp Mole Frac (n-Butane) 0.0003 0.0003 0.0003 0.0001 0.0001 0.0000 0.0000 0.0000 0.0000 Comp Mole Frac (i-Pentane) 0.0001 0.0001 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 Comp Mole Frac (n-Pentane) 0.0002 0.0002 0.0002 0.0001 0.0001 0.0000 0.0000 0.0000 0.0000 Comp Mole Frac (n-Hexane) 0.0005 0.0005 0.0005 0.0002 0.0002 0.0000 0.0000 0.0000 0.0000 Comp Mole Frac (Benzene) 0.0005 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 Stream 8 9 10 10a 10b 11 12 13 14 Vapour Fraction 1.000 1.000 1.000 1.000 1.000 0.000 0.349 0.000 0.266 Pressure (bar) 89.7 89.5 89 89 89 88.7 1.1 9.8 1.1 Temperature (C) 72.67 33 -51.32 -51.32 -51.32 -119.6 -162.7 -124.6 -160 Molar Flow (kgmole / h) 2236 2236 2236 1436 799.8 799.8 799.8 685.2 685.2 Mass Flow (kg / h) 37522 37522 37522 24100 13420 13420 13420 11614 11614 Molecular Weight 16.78 16.78 16.78 16.78 16.78 16.78 16.78 16.95 16.95 Comp Mole Frac (CO2) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 Comp Mole Frac (Nitrogen) 0.0605 0.0605 0.0605 0.0605 0.0605 0.0605 0.0605 0.0065 0.0065 Comp Mole Frac (Methane) 0.9387 0.9387 0.9387 0.9387 0.9387 0.9387 0.9387 0.9439 0.9439 Comp Mole Frac (Ethane) 0.0008 0.0008 0.0008 0.0008 0.0008 0.0008 0.0008 0.0442 0.0442 Comp Mole Frac (Propane) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0032 0.0032 Comp Mole Frac (i-Butane) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0005 0.0005 Comp Mole Frac (n-Butane) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0005 0.0005 Comp Mole Frac (i-Pentane) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0002 0.0002 Comp Mole Frac (n-Pentane) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0003 0.0003 Comp Mole Frac (n-Hexane) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0008 0.0008 Comp Mole Frac (Benzene) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 Stream 15 16 17 18 19 20 21 22 23 Vapour Fraction 1.000 0.000 1.000 0.000 1.000 1.000 0.000 0.000 0.000 Pressure (bar) 1.05 1.05 1.05 1.05 1.01 9.4 1.05 1.05 1.05 Temperature (C) -163.2 "163.2 "160.6 "160.6 "162.3 "51.25 "163.2 "163.2 "161.4 Molar Flow (kgmole / h) 281.9 517.9 184.8 500.4 466.7 466.7 309.7 208.2 708.6 Mass Flow (kg / h) 5057 8361 3014 8601 8069 8069 5000 3362 11962 Molecular Weight 17.94 16.15 16.31 17.19 17.29 17.29 16.15 16.15 16.88 Comp Mole Frac (CO2) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 Comp Mole Frac (Nitrogen) 0.1584 0.0071 0.0220 0.0008 0.1044 0.1044 0.0071 0.0071 0.0027 Comp Mole Frac (Methane) 0.8416 0.9916 0.9779 0.9314 0.8955 0.8955 0.9916 0.9916 0.9491 Comp Mole Frac (Ethane) 0.0000 0.0013 0.0001 0.0605 0.0001 0.0001 0.0013 0.0013 0.0431 Comp Mole Frac (Propane) 0.0000 0.0000 0.0000 0.0043 0.0000 0.0000 0.0000 0.0000 0.0031 Comp Mole Frac (i-Butane) 0.0000 0.0000 0.0000 0.0007 0.0000 0.0000 0.0000 0.0000 0.0005 Comp Mole Frac (n-Butane) 0.0000 0.0000 0.0000 0.0007 0.0000 0.0000 0.0000 0.0000 0.0005 Comp Mole Frac (i-Pentane) 0.0000 0.0000 0.0000 0.0002 0.0000 0.0000 0.0000 0.0000 0.0001 Comp Mole Frac (n-Pentane) 0.0000 0.0000 0.0000 0.0004 0.0000 0.0000 0.0000 0.0000 0.0003 Comp Mole Frac (n-Hexane) 0.0000 0.0000 0.0000 0.0010 0.0000 0.0000 0.0000 0.0000 0.0007 Comp Mole Frac (Benzene) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

Claims

1 A process for producing liquefied natural gas comprising:- providing a stream of natural gas [2] at a pressure of between 30 and 150 bar and at ambient temperature;- cooling stream [2] in a first hot passage of refrigeration heat exchanger [B], having outlet stream [3];- passing stream [3] to a gas expander turbine [C], having outlet stream [4] with a pressure of between 3 bar and 30 bar and comprising a vapor phase and a liquid phase;- passing stream [4] to vapor / liquid separator [D];- providing vapor / liquid separator [D] with a vapor outlet stream [5];- heating stream [5] successively in a cold passage of heat exchanger [E] having outlet stream [6], and in a cold passage of heat exchanger [B] having outlet stream [7] with near-ambient temperature;- compressing stream [7] in compressor [F] to form stream [8] having a pressure of between 30 and 150 bar;- cooling stream [8] in cooler [G], having outlet stream [9] with nearambient temperature;- cooling stream [9] successively in a second hot passage of refrigeration heat exchanger [B] having outlet stream [10], and in a hot passage of heat exchanger [E] having outlet stream [11] with a close temperature approach to stream [5];- reducing the pressure of stream [11] through valve [H] to form a first liquefied natural gas product [12] having a lower fractional content of higher hydrocarbons than stream [2];- providing vapor / liquid separator [D] with a liquid outlet stream [13];- reducing the pressure of stream [13] through valve [I] to form a second liquefied natural gas product [14] containing the balance of the higher hydrocarbon content of stream [2].2 A process for producing liquefied natural gas comprising:- providing a stream of natural gas [2] at a pressure of between 30 and 150 bar and at ambient temperature;- cooling stream [2] in a first hot passage of refrigeration heat exchanger [B], having outlet stream [3];- providing a first stream [10a] of methane-rich recycle gas;- combining streams [3] and [10a] to form stream [3a];- passing stream [3] to a gas expander turbine [C], having outlet stream [4] with a pressure of between 3 bar and 30 bar and comprising a vapor phase and a liquid phase;- passing stream [4] to a first vapor / liquid separator [D];- providing vapor / liquid separator [D] with a vapor outlet stream [5];- heating stream [5] in a cold passage of heat exchanger [E] having outlet stream [6];- providing a second stream [20] of a methane-rich recycle gas;- combining streams [6] and [20a] to form stream [6a];- heating stream [6a] in a cold passage of heat exchanger [B] having outlet stream [7] with near-ambient temperature;- compressing stream [7] in compressor [F] to form stream [8] having a pressure of between 30 and 150 bar;- cooling stream [8] in cooler [G], having outlet stream [9] with nearambient temperature;- cooling stream [9] in a second hot passage of refrigeration heat exchanger [B] having outlet stream [10];- separating the said first stream [10a] of methane-rich recycle gas, forming stream [10b];- cooling stream [10b] in a hot passage of heat exchanger [E] having outlet stream [11] with a close temperature approach to stream [5];- reducing the pressure of stream [11] through valve [H] to form stream [12];- passing stream [12] into second vapor-liquid separator [K] having a vapor outlet stream [15] and liquid outlet stream [16];- providing first vapor / liquid separator [D] with a liquid outlet stream [13];- reducing the pressure of stream [13] through valve [I] to form stream [14];- passing stream [14] into third vapor-liquid separator [I] having a vapor outlet stream [17] and liquid outlet stream [18];- separating a fraction stream [21] of stream [16] to form the first liquefied methane product of the process, having a lower content of higher hydrocarbon than stream [2];- forming stream [22] as the balance of stream [16];5 - combining streams [18] and [22] to form the second liquefied methaneproduct of the process, having a higher content of higher hydrocarbon than stream [2];- combining streams [15] and [17] to form stream [19];- compressing stream [19] in compressor [M] to at least the same10 pressure as stream [6] to form the said second stream [20] of methane-rich recycle gas.3 A process as claimed in either preceding claim in which part or all of the product having a reduced fractional content of higher hydrocarbons is15 exported from the process as a vapor.

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