Multi-component Refrigerant For The Liquefaction Of Natural Gas

Abstract

The efficiency of mixed refrigerant gas liquefaction process, for example liquefaction of natural gas using a refrigerant consisting of components extracted from the natural gas, may be improved by controlling the composition, and hence the temperature/enthalpy profiles, of the refrigerant streams in each heat exchange stage. Coarse control is effected by adding portions of a refrigerant mixture extracted from the previous heat exchange stage, and fine control is effected by adding pure liquefied gases.

Description

FIELD OF THE INVENTION

The invention relates to a gas liquefaction process employing a refrigerant consisting of a mixture of gases of different boiling points.

DESCRIPTION OF THE PRIOR ART

Known processes of this type employ a single multi-component refrigerant sometimes consisting of components extracted from the gas undergoing liquefaction. The composition of the refrigerant is determined so that, in a series of fractional condensation steps, the liquid separated in each heat exchange stage matches the refrigeration requirement of the stage immediately following. The thermodynamic efficiency of such processes is limited by the number of heat exchange stages economically justifiable and the limited flexibility in the composition of any single liquid fraction. This latter feature is due to the interdependence of all the condensed fractions each of which is a part of a single circulating refrigerant.

SUMMARY OF THE INVENTION

According to the present invention there is provided a process for the refrigeration of a fluid feedstock comprising passing the fluid feedstock through a series of heat exchange stages operating at successively lower temperatures, cooling at least two of the heat exchange stages with portions of expanded refrigerating liquid in course of vaporization, each portion of refrigerating liquid resulting from partial liquefaction of a compressed gaseous mixture in a higher temperature heat exchange stage, and adjusting the composition of at least one of said portions of refrigerating liquid by addition thereto of portions of liquefied gas.

The present invention also provides a process for refrigerating a fluid feedstock wherein said fluid feedstock is a gaseous mixture and comprising compressing said fluid feedstock to form said compressed gaseous mixture.

Preferably the process also comprises recompressing at least some of the vapors produced by vaporization of said portions of refrigerating liquid and continuously replacing at least some of said compressed gaseous mixture with said compressed vapors.

The portions of liquefied gas may be portions of liquefied gaseous mixtures, in which case each portion of liquefied gaseous mixture may be obtained from the portion of refrigerating liquid to be vaporized in a higher temperature heat exchange stage.

Alternatively the portions of liquefied gas may be portions of one or more substantially pure liquefied gases.

Low boiling point components may be removed from the feedstock by expanding it into a column and effecting fractional distillation therein between a reboiler and a condenser. The reboiler may be heated by passing the feedstock through the reboiler prior to its expansion into the column. The condenser may be cooled by part of a portion of expanded refrigerating liquid to be vaporized in a lower temperature heat exchange stage. The composition of the portion of refrigerating liquid introduced into said condenser may be adjusted by adding thereto portions of one or more substantially pure gases.

A bleed may be withdrawn from one or more of said portions of refrigerating liquid to maintain the overall composition of the compressed gaseous mixture.

The invention will be further described, by way of example, and with reference to the accompanying drawings in which

FIGS. 1, 2 and 3 are diagrams of three different process for the liquefaction of natural gas.

In the liquefaction process illustrated in FIG. 1 the natural gas is cooled in a heat exchanger 4, cooled and partially condensed in a heat exchanger 6 and separated into liquid and vapor fractions in a separator 12. The vapor fraction withdrawn from the separator 12 is condensed by passage through a heat exchanger 8 and sub-cooled by passage through heat exchangers 10 and 11 to a temperature such that there is a minimum flash on expansion through an expansion valve 18 to the storage pressure. The liquid fraction withdrawn from the separator 12 is separated in a column system (not shown) into its components for mixed refrigerant make-up purposes.

Refrigeration is provided by a mixed refrigerant comprising hydrocarbons extracted from the natural gas undergoing liquefaction and added nitrogen. The mixed refrigerant is compressed in a compressor 15, cooled in a cooler 16 and separated in a separator 17. The vapor fraction withdrawn from the separator 17 is cooled and partially condensed by passage through the heat exchanger 4 and then passed to a separator 5. The vapor fraction withdrawn from the separator 5 is cooled and partially condensed by passage through the heat exchanger 6 and then passed to a separator 7. The vapor fraction withdrawn from the separator 7 is cooled and partially condensed by passage through the heat exchanger 8 and then passed to a separator 9. The vapor fraction withdrawn from the separator 9 is condensed in the heat exchanger 10, sub-cooled in the heat exchanger 11, expanded through an expansion vlave 20 and returned through the heat exchangers 11, 10, 8, 6 and 4 to provide cooling.

The liquid fraction withdrawn from the separator 9 is sub-cooled in the heat exchanger 10, expanded through an expansion valve 21, and vaporized in the heat exchanger 10. The liquid fraction withdrawn from the separator 7 is sub-cooled in the heat exchanger 8, expanded through an expansion valve 23 and vaporized in the heat exchanger 8. The liquid fraction withdrawn from the separator 5 is sub-cooled in the heat exchanger 6, expanded through an expansion valve 24 and vaporized in the heat exchanger 6. The liquid fraction withdrawn from the separator 17 is sub-cooled in the heat exchanger 4, expanded through an expansion valve 13 and vaporized in the heat exchanger 4.

In each case the vapor fractions resulting from the vaporization of the expanded refrigerating liquid in the heat exchangers 4, 6, 8 and 10 is returned to the compressor 15.

The overall efficiency of the basic cycle described above is, in accordance with the invention, improved by adjusting the compositions of the individual condensed fractions to obtain better matching of the temperature/enthalpy profiles of the heat exchangers.

A coarse adjustment of the temperature/enthalpy profiles in the heat exchangers 4, 6, 8 and 10 is effected by withdrawing controlled amounts of refrigerant by way of flow lines 26, 27 and 28 and adding them to the refrigerant in flow lines 29, 30 and 31 respectively. In each case the quantity of liquid refrigerant added is individually controlled to provide optimum conditions in the heat exchanger in which the liquid is ultimately vaporized. Fine control of the temperature/enthalpy profiles in the heat exchangers may be obtained by introducing one or more substantially pure components preferably at a point just before the refrigerant enters a particular heat exchanger. In this process controlled amounts of substantially pure components are introduced via lines 32 and 33 into the refrigerant just before it enters the heat exchanger 8. It will be appreciated that fine control of the temperature/enthalpy profile of any heat exchanger employed in the system may be carried out by introducing controlled amounts of substantially pure components just before the expanded refrigerant is fed into that particular heat exchanger.

In order to provide substantially pure components for fine control of the expanded refrigerant fractions whilst maintaining the overall composition of the refrigerant substantially constant, a bleed is taken from one or more of the separated condensed liquids via flow lines 40, 41 and 42, passed to a column separation system (not shown) in which it is separated into the required substantially pure components, excess quantities of these components being returned to the refrigerant via the normal make-up system.

In the gas liquefaction process illustrated in FIG. 2 the natural gas feedstock is introduced by way of flow line 51 and partially liquefied by passing it successively through heat exchanger 52 and 53. The condensate withdrawn from the heat exchanger 53 is separated in a separator 54 and the liquid fraction withdrawn therefrom separated in a column separation system (not shown) into its pure components for use for mixed refrigerant make-up purposes.

The vapor fraction withdrawn from the separator 54 is partially liquefied in a heat exchange 55 and passed through the reboiler 56 of a degasification column 57 to provide re-boil therefor. It is then passed through a heat exchanger 58, expanded by passage through an expansion valve 59 and fed to the degasification column 57. In the column 57 the partially expanded mixture is separated between the reboiler 56 and a condenser 60 to provide most of the hydrocarbons at the bottom of the column 57 and nitrogen with some lighter hydrocarbons at the top. The hydrocarbon liquid withdrawn from the bottom of the column 57 is passed successively through a series of heat exchangers 58, 61, and 62 where it is cooled in such a way that there is a minimum flash as it is expanded through an expansion valve 63 to the storage pressure. The nitrogen-rich vapor fraction withdrawn from the top of the degasification column 57 is cooled by passage through the heat exchangers 61 and 62, expanded through an expansion valve 64 and then passed successively through the heat exchangers 62, 61, 65, 53 and 52 to provide cooling. The product withdrawn from the heat exchanger 52 is suitable for use as a fuel gas.

The mixed refrigerant is compressed in a compressor 70, partially liquefied in a cooler 71 and then passed into a separator 72. The vapor fraction withdrawn from the separator 72 is partially liquefied in the heat exchanger 52 and then fed into a separator 73. The liquid fraction from the separator 72 is passed through the heat exchanger 52, expanded by passage through an expansion valve 74 and then passed by way of a refrigerant return line 68 through the heat exchanger 52 where it vaporizes and provides cooling. The vapor fraction discharged from the heat exchanger 52 by way of the refrigerant return line 68 is returned to the compressor 70.

The vapor fraction withdrawn from the separator 73 is partially liquefied in the heat exchanger 53 and then passed to a separator 59. The liquid fraction withdrawn from the separator 73 is passed through the heat exchanger 53, expanded through an expansion valve 75 and then returned by way of a flow line 50 to the heat exchanger 53 where it vaporizes and provides cooling. The vapor fraction is withdrawn from the heat exchanger 53 by way of a flow line 69 and passed to the refrigerant return line 68 at a point in advance of the heat exchanger 52.

The vapor fraction withdrawn from the separator 59 is partially liquefied in the heat exchanger 65 and passed to a separator 76. The liquid fraction from the separator 59 is passed through the heat exchanger 65 after which it divides into two parts, one of which is expanded by passage through an expansion valve 77 while the other is expanded by passage through an expansion valve 78. The expanded refrigerant from the valve 77 is returned by way of a refrigerant return line 81 to the heat exchanger 65 to provide cooling and the expanded refrigerant from the valve 78 passed through the heat exchanger 55 where it vaporizes and provides cooling. The vapors withdrawn from the heat exchangers 65 and 55 are fed by way of a flow line 79 into the flow line 50 where they join the refrigerant from the expansion valve 75 prior to being fed into the heat exchanger 53.

The vapor fraction from the separation 76 is cooled by passing it successively through the heat exchangers 61 and 62 and then expanded by passage through an expansion valve 80. The expanded refrigerant is then returned through the heat exchangers 62 and 61 to join the refrigerant return line 81 at a point in advance of the heat exchanger 65.

The liquid fraction from the separator 76 is cooled by passage through the heat exchanger 61 after which part is expanded by passage through an expansion valve 88 while the other part is expanded by passage through an expansion valve 87. The expanded refrigerant from the expansion valve 88 is fed into a refrigerant return line 83 while the other part from the expansion valve 87 is fed into the condenser 60 of the degasification column 57 where it vaporizes and provides cooling. The refrigerant withdrawn from the condenser 60 is passed through the heat exchanger 58 and fed into the refrigerant return line 83.

The overall efficiency of the basic cycle described above is improved by adjusting the compositions of the individual condensed fractions to obtain better matching of the temperature/enthalpy profiles of the heat exchangers. A coarse adjustment of the temperature/enthalpy profiles in the heat exchangers 53, 55, 65 and 61 is effected by withdrawing controlled amounts of refrigerant by way of flow lines 93, 94 and 95 and adding them to the refrigerant in flow lines 91, 92 and 90 respectively. In each case the quantity of liquid refrigerant added is individually controlled to provide optimum conditions in the heat exchanger in which the liquid is ultimately evaporated.

Fine control of the temperature/enthalpy profiles in the heat exchangers may be obtained by introducing one or more substantially pure components at a joint just before the expanded refrigerant enters a particular heat exchanger. Controlled amounts of substantially pure components are introduced via lines 97 and 98 into line 81 just before the refrigerant enters the heat exchanger 65. Similarly, substantially pure components are introduced by way of lines 106 and 99 to control the temperature/enthalpy profile in the condenser 60 and into lines 82 and 89 to control the temperature/enthalpy profile in the heat exchanger 55. It will be appreciated that fine control of the temperature/enthalpy profile of any heat exchanger employed in the system may be carried out by introducing controlled amounts of substantially pure components just before the expanded refrigerant is fed into that particular heat exchanger.

In order to provide the substantially pure components for fine control of the expanded refrigerant fractions whilst maintaining the overall composition of the refrigerant substantially constant, a bleed is taken from one or more of the separated condensed liquids via lines 103, 104 and 105, passed to a column separation system in which it is separated into the required substantially pure components, excess quantities of these components being returned to the refrigerant via the normal make-up system.

Boil-off gas emerging from the storage tank is cold and its refrigeration may be utilized by passing it through the heat exchangers 65, 53 and 52.

In the gas liquefaction process illustrated in FIG. 3 the natural gas feed stock is introduced by way of flow line 111 and is joined by the vapor refrigerant leaving separator 153. The natural gas refrigerant mixture is partially liquefied by passing it through the heat exchanger 112 after which it is transferred to the separator 113. The vapor fraction from the separator 113 is partially liquefied by passage through the heat exchanger 114 and then transferred to the separator 115. The liquid fraction from the separator 113 is passed through the heat exchanger 114, expanded by passage through an expansion valve 116 and then returned to the heat exchanger 114 to provide cooling. The vapor fraction leaving the heat exchanger 114 is fed into the refrigerant return line 117 at a point in advance of the heat exchanger 112.

The vapor fraction from the separator 115 is divided into two parts one of which is partially liquefied in a heat exchanger 118 and then passed to a separator 119 while the other is partially liquefied by passage through a heat exchanger 120 and then passed to the reboiler 121 of a column 122. The partially liquefied gaseous mixture provides heat for the reboiler 121 and is then passed through a heat exchanger 123, expanded by passage through an expansion valve 124 and passed to the column 122 where it is separated between the reboiler 121 and a condenser 125.

The liquid fraction from the separator 115 is passed through the heat exchanger 118 after which it divides into two parts, one of which is expanded by passage through an expansion valve 126 and the other by passage through an expansion valve 127. The expanded liquid emerging from expansion valve 126 is passed through the heat exchanger 118 where it provides cooling and the expanded liquid from the expansion valve 127 is fed to the heat exchanger 120 to provide cooling. The vapor fractions leaving the heat exchangers 118 and 120 are combined and passed by way of flow line 129 into a flow line 110 where they join the refrigerant from expansion valve 116 prior to being fed into the heat exchanger 114.

The vapor fraction from the separator 119 is fed into the column 122 by way of flow line 179 and is separated in the column 122 together with the expanded mixture entering the column 122 after expansion through valve 124. The vapor fraction comprising nitrogen and some lighter hydrocarbons withdrawn from the top of the column 122 is fed by way of a flow line 130 successively through the heat exchangers 131 and 132 in which it is liquefied and subcooled, expanded by passage through an expansion valve 133 and then returned through the heat exchangers 132, 131, 118, 114 and 112 to provide cooling. The expanded nitrogen rich fraction emerging from the heat exchanger 112 may be used as a fuel gas.

The liquid hydrocarbon fraction from the bottom of the column 122 is passed successively through a series of heat exchangers 123, 131, and 132 where it is cooled in such a way that there is a minimum flash on expansion through an expansion valve 140 to the storage pressure.

The liquid fraction withdrawn from the separator 119 is cooled by passage through the heat exchanger 131 and then divided into two parts. One part is expanded by passage through an expansion valve 141 and returned through the heat exchanger 131 to provide cooling therefor, while the other part is expanded by passage through expansion valve 142 and then used to provide cooling for the condenser 125 of the column 122. The vapor leaving the condenser 125 is passed through the heat exchanger 123 and first combined with the part emerging from the heat exchanger 131 after which they join the liquid fraction emerging from the expansion valve 126.

Vapor returning by way of flow line 117 is recompressed in a compressor 150, partially liquefied by passage through a cooler 151 and then fed to a separator 153. The vapor product leaving the separator 153 rejoins the gaseous mixture in the flow line 111. The liquid product withdrawn from the separator 153 is fed through the heat exchanger 112, expanded by passage through the expansion valve 154 and then returned through the heat exchanger 112 to provide cooling.

The overall efficiency of the basic cycle described above is improved by adjusting the compositions of the individual condensed fractions to obtain better matching of the temperature/enthalpy profiles of the heat exchangers. A coarse adjustment of the temperature/enthalpy profiles in the heat exchangers 114, 118, 120 and 131 is effected by withdrawing controlled amounts of refrigerant by way of flow lines 170, 171 and 172 and adding them to the refrigerant in flow lines 161, 162 and 174 respectively. In each case the quantity of liquid refrigerant added is individually controlled to provide optimum conditions in the heat exchanger in which the liquid refrigerant is ultimately evaporated.

Fine control of the temperature/enthalpy profiles in the heat exchangers may be obtained by introducing controlled quantities of one or more substantially pure components at a point just before the expanded refrigerant enters a particular heat exchanger. Controlled amounts of substantially pure components are introduced via lines 180 and 181 just before the expanded refrigerant enters the heat exchanger 118. Similarly, substnatially pure components are introduced by way of lines 182 and 183 to control the temperature/enthalpy profiles in the condenser 125 and into lines 184 and 185 to control the temperature/enthalpy profiles in the heat exchanger 120. It will be appreciated that fine control of the temperature/enthalpy profile of any heat exchanger employed in the system may be carried out by introducing controlled amounts of substantially pure components just before the refrigerant is fed into that particular heat exchanger.

In order to provide the substantially pure components for fine control of the expanded refrigerant fractions whilst maintaining the overall composition of the refrigerant substantially constant, a bleed is taken from one or more of the separated condensed liquids via lines 190, 191 and 192, passed to a column separation system in which it is separated into the required substantially pure components, excess quantities of these components being returned to the refrigerant via the normal make-up system.

Boil-off gas emerging from the storage tank is cold and its refrigeration may be utilized by passing it through the heat exchangers 118, 114 and 112. The invention is not restricted to the details of the foregoing examples.

Claims

We claim:

1. A process for the liquefaction of a natural gas feedstock including the steps of (1) passing said feedstock through a series of at least two heat exchange stages operating at successively lower temperatures, (2) circulating a compressed multicomponent refrigerant in closed circuit and in indirect heat exchange relation with said feedstock in each of said stages, (3) compressing and partially condensing said refrigerant to obtain separated first liquid and first vapor phases, (4) flowing said first liquid phase cocurrently with said feedstock in said first heat exchange stage, (5) separating said liquid phase into first and second portions, (6) expanding said first portion for counter current heat exchange with said feedstock, said first vapor phase and said first separated liquid, (7) partially condensing and flowing said first vapor phase through said first stage and separating said partially condensed first vapor phase into second liquid and vapor phases, (8) combining said second portion of said first liquid phase with said second separated liquid phase to define a second stream for cocurrent separate flow with said feedstock and said second separated vapor phase in said second heat exchange stage, (9) and separating said combined second stream into first and second portions for similar manipulation as said first and second portions of said first liquid stream.

2. A process according to claim 1, in which at least one substantially pure liquefied gas is introduced into at least one of the expanded portions of liquid refrigerant prior to its introduction into a chosen heat exchange stage, whereby the temperature-enthalpy profile of the chosen heat exchange stage is improved.

3. A process according to claim 2, additionally comprising taking a bleed from at least one of said refrigerant liquid portions and then separating the bleed to form the substantially pure liquefied gas.

4. A process according to claim 1, in which the partial liquefaction producing the least volatile portion of refrigerant liquid is performed by passing the compressed circulating refrigerant mixture through a water cooler, and the other partial liquefactions are performed by cooling in said heat exchange stages.

5. A process according to claim 1, in which after the successive partial liquefactions have been performed, the remainder of the refrigerant mixture is expanded and used to cool any heat exchange stage through which the expanded liquid portions have not been passed.

6. A process according to claim 1, in which vaporized liquid natural gas is passed through at least some of the heat exchange stages.

7. A process according to claim 1 comprising removing low boiling point products from the feedstock by expanding the feedstock into a column and effecting fractional distillation therein between a boiler and a condenser.

8. A process according to claim 7 comprising heating the reboiler by passing the feedstock through the reboiler prior to its expansion into the column.

9. A process according to claim 7 comprising cooling said condenser with at least part of one of the portions of expanded refrigerating liquid to be vaporized in a lower temperature heat exchange stage.

10. A process according to claim 9 comprising adjusting the composition of the portion of refrigerating liquid introduced into said condenser by adding thereto portions of one or more substantially pure gases.