CA1062602A - Process for the liquefaction of natural gas - Google Patents
Process for the liquefaction of natural gasInfo
- Publication number
- CA1062602A CA1062602A CA305,842A CA305842A CA1062602A CA 1062602 A CA1062602 A CA 1062602A CA 305842 A CA305842 A CA 305842A CA 1062602 A CA1062602 A CA 1062602A
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- Prior art keywords
- natural gas
- line
- passed
- refrigerant
- nitrogen
- Prior art date
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Abstract
ABSTRACT
Natural gas is liquefied by cooling it under pressure in stages comprising cooling a natural gas stream containing some nitrogen by passing it in heat-exchange relationship with at least one refrigerant circulating in at least one circuit, so that the natural gas liquefies; passing the liquefied natural gas to a nitrogen stripper; separating nitrogen from the liquefied natural gas, removing a gas mixture comprising at least nitrogen and methane from the top of the nitrogen stripper, and removing liquefied natural gas from the nitrogen stripper as a product.
Natural gas is liquefied by cooling it under pressure in stages comprising cooling a natural gas stream containing some nitrogen by passing it in heat-exchange relationship with at least one refrigerant circulating in at least one circuit, so that the natural gas liquefies; passing the liquefied natural gas to a nitrogen stripper; separating nitrogen from the liquefied natural gas, removing a gas mixture comprising at least nitrogen and methane from the top of the nitrogen stripper, and removing liquefied natural gas from the nitrogen stripper as a product.
Description
~062602 The invention relates to a method of liquefying a natu~l gas, more particularly the invention i8 concerned with the liquefaction of natural gas containing some nitrogen.
This application is a divis ion of Canadian Patent Application S.N. 273, 609, filed Màrch 9, 1977 .
There is herein disclosed a method of cooling natural gas which comprises cooling a natural gas stream by pasæing it in heat-exchange relationship with a first refrigerant circulating in a first circuit and successively pas~ing the natural ga~ stream in heat-exchange relationship with at least a ~econd refrigerant circulating in at least a second circuit, wherein the first refrigerant is of a composition which differs ;
from the composition of the second refrigerant.
~ lthough it i9 pOSS ible and sometimes attractive to use a single component refrigerant, such as for example propane, in a suitable embodiment of the above method,at least one of the refrigerants is a so-called multi-component or mixed refrigerant, for example, a mixture of at least ethane and butane or, for example, a mixture of at least nitrogen, methane and ethane.
The present invèntion provides a method of improved economy, for liquefying natural gas containing nitrogen. In the invention a nitrogen stripper is successfully employed rather than a two-stage end flash system. By stripping the bulk of the nitrogen from the natuE~ gas, a liquefied natural gas product of higher temperature is obtained, so that the liquefied natural ga3 can leave the cold heat-exchanger at a higher temperature.
This gain in temperature is about 1.5~C for feed gas containing 3/O nitrogen, and this corresponds approximately with a saving of 1.5% on total compression energy.
Using the cold gaseous mixture of at least nitrogen .'' ~
lO~;ZtjO2 and methane leaving the nitrogen stripper for cooli~g a gaseous fraction of the second mixed refrigerant, before the latter is expanded to a lower pressure, has the advantage that the refrigeration energy requirement of the second cooling circuit is reduced, so that the total energy necessary for liquefying a given quantity of natural gas is reduced as well.
According to the present invention there is provided a method of liquefying a natural gas by cooling it under pressure in stages comprising cooling a natural gas stream containing some nitrogen by passing it in heat-exchange relationship with a first mixed refrigerant circulating in a first circuit and with a second mixed refrigerant circulating in a second circuit, so that the natural gas liquefies, passing the liquefied natural gas to a nitrogen stripper, separating nitrogen from the liquefi-ed natural gas in the nitrogen stripper' removing a gas mixture comprising at least nitrogen and methane from the top of the nitrogen stripper, removing liquefied natural gas from the nitro-gen stripper as a product, and cooling a gaseous fraction of said second mixed refrigerant, before it is expanded to a lower pressure, with said gas mixture from the nitrogen stripper.
In a particular embodiment the liquefied natural gas is expanded to a lower pressure to produce vapour before passing to the nitrogen stripper.
A gaseous fraction of the second mixed refrigerant is suitablyexpanded to a lower pressure before circulating it in the second circuit, and the gaseous fraction is cooled with the gas mixture from the nitrogen stripper before the expanding.
Before the liquefaction of the natural gas, it is necessary to remove water from the natural gas. In this re-spect, the water i9 suitably removed from the natural gas byprecooling it. This may comprise raising the pressure of a ,; . ~ ' , :
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mixed refrigerant in a compressor cooling the refrigerant 80 that it condenses partially, in a separator separating the liquid fraction from the gaseous fraction, branching off part of the liquid fraction of the refrigerant, in an ex-pansion device reducing the pressure of the branched-off part of the liquid fraction of the refrigerant and allowing it to evaporate at least largely to produce cold, which cold is used to precool natural gas in order to remove water from :~
the natural gas~
Further details and embodiments of the process according to the invention will be explained with refer- >
ence to the drawing, showing a flow sheet of the proces~.
Referring to the drawing, natural gas, from which C2 and water have been removed, is supplied through a line 1.
This natural gas, which is at a relatively high : .
pressure of, for example, 50 bar and at a temperature :
of, for example, 20 degrees centigrade, is passed through a coil 2 of a heat-exchanger 4. In the coil 2 2D the temperature of the natural gas is lowered. The cooled natural gas leaving the coil 2 is passed through ,.
a line 3 to a phase separator 5, wherein condensed :
heavier hydrocarbon components are separated from the gas. The condensed heavier hydrocarbons are removed from the phase separator 5 through a discharge line 6.
Together with the condensate some lighter hydrocarbons, such as methane, ethane and propane are removed from the separator 5 through the discharge line 6. The natural gas leaves the phase separator 5 as a vapour and is passed through a line 7 to a coil 8 of the heat- :
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~o~z~oz exchanger 4, wherein the temperature of the natural gas is lowered rurther. From coil 8 the cooled natural gas, which contains a small quantity Or liquid, is passed through a line 9 to a coil 10 o~ a heat~exchanger 11.
In coil 10 the temperature of the natural gas is re-duced to a lower value, so that more liquid is formed.
From coil 10 the natural gas is passed through a line 12 to a phase separator 13. In phase separator 13 condensed hydrocarbons are separated from the natural gas. The condensed hydrocarbons, which contain, for example, mainly methane, ethane, propane and butane as well as some pentane, are removed from the separatcr 13 through a line 14.
Natural gas vapour containing mainly methane, ethane and nitrogen leaving the phase separator 13 is passed through a line 15 to a coil 16 of heat-exchanger 11, wherein the temperature Or the natural gas is ~;
lowered further and the natural gas is fully condensed.
From coil 16 the natural gas is passed to an expansion device 17. In expan~ion device 17 the pressure of the condensed natural gas is reduced, whereafter it is passed via a line 18 to a coil 19 of heat-exchanger 11.
In coil 19 the condensed natural gas is cooled further to a temperature of, for example, minus 147 degrees centigrade. From coil 19 the liquefied natural gas is passed through a line 20 to a heat-exchanger 22 -in a nitrogen stripper 21. In the heat-exchanger ~2 the liquefied natura~ gas is cooled further. From heat-exchanger 22 the liquefied natural gas is passed to an expansion device 23. In expansion device 23 the liquefied natural gas is expanded to a lower pressure, so that some vapour is formed, and then it is passed through a line 24 to a distribution device 25 in the stripper 21, whereln liquid and vapour are separated.
The liquid natural gas fraction le~ving distribution 10 device 25, passes through the stripper 21 to an outlet 26 and from outlet 26 via line 27 to storage. In stripper 21 nitrogen is separated from tne liquefied natural gas. A methane/nitrogen vapour mixture con-taining some traces of ethane leaves the top of the stripper 21 via a line 29 and is passed to a heat-ex~hanger 30. In h~t-exchanger 30 the temperature of this gas mixture is raised and then this gas mixture -is passed through a line 31 to a heat-exchanger 32 in which the temperature of the said gas mixture is raised further. Finally, the said gas mixture is passed from heat-exchange~ 32 through a line 33 to a suitable location to be used, for example, as a fuel gas.
The condensate leaving the phase separator 5 is passed via line 6 and an expansion device 150 to a demethanizer 151 which is provided with a reboiler 152.
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A heavy hydrocarbon stream, com~rising ethane and componert~ heavier than ethane i~ removed as liquid rrom the bottom of the demethanizer 151 and i8 paS8ed to a refrigerant make-up unit tnot shown) via a line 153. Part Or the liquid ~aving the demethanizer 151 through line 153 is recirculated through the re-boiler 152. Methane, possibly with some ethane~ leaves the demethanizer 151 as overhead vapour and is passed through a line 154 to be intermixed with a gas stream flowing in a line 157.
The C2-h~idrocarbons produced in the refrigerant make-up unit are passed via a line 141 ~ the heat-exchanger 32 to be lowered in temperature. From heat- ~ -exchanger 32 the said C2-hydrocarbons are passed via a line 142 to storage. The C3-hydrocarbons produced in the refrigerant make-up unit are passed via a line 143 to the heat-~xchanger 32. In heat-exchanger 32 the said C3-hydrocarbons are lowered in temper-ature and then passed via a line 144 to storage.
The condensed hydrocarbons leaving the phase separator 13 via line 14 can be passed via a line 158 to an expansion device 159. Another part of said condensed hydrocarbons can be passed via a pump 160 to the gas flowing in line 15 to be intermixed there-wîth.
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In expansion device 159 the liquid is expanded to a lower pressure and then it is passed to a de-methanizer 162 which i8 provided with a reboiler 163.
A hydrocarbon stream comprising mainly ethane, propane and butane is removed as liquid rrom the bottom Or the demethanizer 162 and is passed as ra~ natural gas liquid via a line 164 to storage ror rurther treatment.
Part of the liquid leaving the demethanizer 162 ~rough line 164 i6 recirculated through the reboi~r 163.
Methane, possibly with some ethane, leaves the de-methanizer 162 as overhead vapour and is passed via the line 157 to a coil 165 in heat-exchanger 11. In coil 165 the gas is cooled and condensed and then it ~-is passed via a line 166 to ~he gas rlOwing in line 18 ~ :
to be intermixed therewith.
The quantity of liquid to be passed to the de-methanizer 162 can be controlled at will by manipulating the expansion device 159. In so doing the extraction of natural gas liquids from the natural gas feed stream can be regulated. All condensate ~rom separator 13 which is not sent to demethanizer 162 is passed via pump 160 to line 15.
In the above natural gas liquefaction system two separate cooling circuits are used, In the first cooling .circuit a so-called mixed refrigerant, ror example a suitable mixture of methane, ethane, propane, butane lQ6;~;0Z
and pentane, is supplied in gaseous condition through a line 34 to a compressor 3L. In compressor 35 the pressure Or the mixed refrigerant is raised. From compressor 35 the mixed refrigerant is passed through a line 36 to a heat-exchanger 37, which is cooled ~or example by water.
In heat-exchanger 37 the mixed re~rigerant is cooled to such a degree that partial condensation of the mixture occurs. From heat-exchanger 37 the partially condensed mixed re~rigerant is passed through a line 38 to a phase separator 39 in which condensed mixed refrigerant is separated from gaseous mixed refrigerant. The gaseous I mixed refrigerant is passed from phase separator 39 through line 40 to a compressor 41. In compressor 41 the pressure of the gaseous mixed refrigerant i8 raised further. Fr,om compressor 41 said gaseous mixed re-frigerant is passed through a line 42 to a heat-exchanger 43.
The condensed mixed refrigerant leaves the phase separator ~9 ~ia outlet 44 and is passed to a pump 45.
In pump 45 the pressure of the condensed mixed re-frigerant is raised to such a level that it can be passed through a line 46 to line 42 and be added to the gaseous mixed re~rigerant leaving the compressor 41.
In the heat-exchanger 43, which is, for example, cooled b.y water, the mixed refrigerant is cooled and partially ~ondensed and from heat-exchanger 43 the cooled mixed ~,,, ' , ., .~
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rerrigerant is passed through a line 47 to a phase ~eparator 48.
The condensed mixed refrigerant leaving the phase separator 48 is passed through a line 49 to a coil 50 Or the heat-exchanger 4. The gaseous mixed refrigerant leaving the phase separator 48 is passed through a line 51 to a coil 52 of the heat-exchanger 4.
In coil 50 the condensed mixed refrigerant is cooled and is then passed from coil 50 via a line 53 to !
an expansion device 54. In e.xpansion device 54 the cooled ~ .
~iquid refrigerant is expanded to a lower pressure. The liq~id, possibly together with a small portion o~
vapour, is passed from expansion device 54 through a line 55 and is injected through a distribution device 56 into heat-exchanger 4, wherein it combines with a mixed re-frigerant stream which enters heat-exchanger 4 via a distribution device 64. The combined mixed refri~rant stream flows downward over the coils 50, 52, 100 and 2 to cool the contents of these coils. During this process the largest part of the mixed refrigerant eYaporate ~he mixed refrigera~t, which îs largely in gaseous condition and contains only a small portion o~ liquid, leaves the heat-exchanger 4 vla a line 57 to be passed to a ph~se separator 58. In phase separator 58 liquid mixed refrigerant is separated from gaseous m;xed re-frigerant. The separated liquid mixed rerrigerant is .
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removed from phase separator 58 via an ou~et 76 to be injected, after its pressure has been raised, into, for example, line 36 J or into, for example, separator 39.
Via a line 59 so-called "make-up refrigerant" i5 added to the rerrigerant passing through line 57 to compensate for re~rigerant lost during the process. Gaseous re-frigerant is passed from phase separator 58 via line 34 to compressor 35 to repeat the cycle as described in the above.
The mixed refrigerant passing through coil 52 is lowered in temperature and condensed in said coil and is then passed to a further coil 60 Or heat-exchanger 4.
In coil 60 the condensed mixed refrigerant is cooled further and then it is passed via a line 61 to an expansion device 62. In expansion device 62, the rerrigerant iæ expanded to a lower pressure so that some vapour is ~ormed and then it is passed via a line 63 to a distribut~n device 64. From distribution device 64 the refrigerant, which is largely in liquid condition, flows downward over the coils 60, 101 and 8 to cool the contents o~ these coils and further down-ward over the coils 50, 52, 100 and 2 to cool the contents of these coils until it reaches the bottom of the heat-exchanger.
During this process the refrigerant evaporates largely. Finally, the refrigerant leaves the heat-lO~Z60Z
exchanger 4 via the line 57 to be passed to phase separator 58.
Some Or the condensed refrigerant leaving the : -phase separator 48 via the line 49 is branched orf ';
arld is passed through a line 65 to an expansion device 66. In expansion device 66 the refrigerant i8 -:
expanded to a lower pr(essure so that some gas is formed and is then pas;ed through a line 67 to a distribution device 68 of a heat-exchanger 69 which is provided with a coil 70. The refrigerant, which is largely in liquid condition, leaving the distribution device 68 flows downward over the coil 70 to cool the contents of the coil 70. During this process the re- ~, frigerant evaporates largely and finally leaves t,he heat-exchanger 69 through a line 71.
Via line 71 the refrigerant is passed to an ex-pansion device 72 in which the refrigerant is expanded to a lower pressure. Then the refrigerant~ which is largely in gaseous condition, is passed from the ex-pansion device 72 to the phase separator 39 via aline 73 to be combined with the refrigerant arriving from the compressor 35. In heat-exchanger 69 natural gas to be lique~ied is precooled in order to remove a quantity of water which is present in the natural gas.
25. Fo,r this purpose the natural gas is supplied thro'ugh a line 74 to the coil 70 and passed through coil 70.
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The natural gas precooled in coil 70, leaves the coil 70 via a line 75 and is passed to a phase separator (not shown) in which condensed water is removed rrom the natural gas. Then the partly dried natural gas is passed to a conventional drier (not shown) to remove the remaining water from the natural gas. Thiæ con-ventional drier is, for example, of the kind containing a suitable desiccant. From the conventional drier the natural ~s is passed to line 1 in order to be liquefied in the m~anner as described in the above.
In the second cooling circuit a so-called mixed re-frigerant is circulating as well. The composition of the mixed re~rigerant circulating in the second cooling circuit is, however, difrerent from the mixed re-~rigerant circulating in the first cooling circuit.The mixed refri~erant circulating in the second cool-ing circuit is, for example, a mixture of ethane, methane and nitrogen.
In the second cooling circuit gaseous mixed re-20 rrigerant i8 supplied through a line 80 to a com-pressor 81.
In the compressor 81 the pressure of the mixed refrigerant is raised and then it is passed through a line 82 to a heat-exchanger 83, which is cooled, ror example, by water. In heat-exchan~er 83 the mixed re-rrigerant is cooled and then it is passed via a line 84 . . .,. . - .
: .
106Z6(~2 to a kr,ock-out vessel 85. From knock-out vesseJ 85 the mixed refrigerant is passed via a line 86 to a com-pressor 87.
In compressor 87 the pressure Or the mixed re-frigerant is raised ~urther and.then it is pas~ed via a line 88 to a heat-exchanger 8~l which is cooled, for example, by water. From heat-exchanger 89, the mixed refrigerant is passed via a line 90 to a coil 100 of the heat-exchanger 4. In coil 100 the temperature of O the mixe~ refrigerant is lowered. From coil 100, the mixed rerrigerant i~ passed to a coil 101 in which it is cooled ~urther ~nd partially condensed.
From coil 101 the cooled mixed refrigerant is passed through a line 102 to a phase separator 106.
In the phase separator 106 the gaseou~ refrigerant is separated ~rom liquid refrigerant. From phase separator 106, the liquid re~rigerant is paæsed through a line 110 to a coil 111 o~ the heat-exchanger 11. In coil 111 the re~rigerant is cooled further and then it is passed through a line 112 to an expansion device 113. In ex-pansion device 113 the refrigerant is expanded, where-&fter it is passed through a line 114 and is injected through a distribution device 115 into the heat-exchanger 11, wherein it combines with a mixed re-~rigerant stream which enters heat-exchanger 11 via a distribution device 132.
.
106~602 The combined mlxed refrigerant stream is pa~sed over the coils 111, 127, 165, lÇ and 10 which cau~es cooling of the contents Or these coils. During the passage Or the refrigerant over the coil~ 111, 127, 165, 16 and 10 the refrigerant evaporates at least partly.
Finally~ the refrigerant reaches the lower part of the heat-exchanger 11 and then it is passed through a line 116 to a heat-exchanger 117.
In heat-exchanger 117, the refrigerant cools the contents of a coil 109 of the heat-exchanger 117. Then the refrigerant ~ passed through a line 118 to a knock-out vessel 119. Finally, the refrigerant,which îs in gaseous condition, is passed from knock-out vessel 119 ~hrough the line 80 to the compressor 81 to repeat the cycle.
Gaseous refrigerant leaves the phase separator 106 through a line 125. From line 125 part o~ the said gaseous refrigerant is pasæed through a line 126 to a coil 127 in the heat-exchanger 11 in which it is cooled
This application is a divis ion of Canadian Patent Application S.N. 273, 609, filed Màrch 9, 1977 .
There is herein disclosed a method of cooling natural gas which comprises cooling a natural gas stream by pasæing it in heat-exchange relationship with a first refrigerant circulating in a first circuit and successively pas~ing the natural ga~ stream in heat-exchange relationship with at least a ~econd refrigerant circulating in at least a second circuit, wherein the first refrigerant is of a composition which differs ;
from the composition of the second refrigerant.
~ lthough it i9 pOSS ible and sometimes attractive to use a single component refrigerant, such as for example propane, in a suitable embodiment of the above method,at least one of the refrigerants is a so-called multi-component or mixed refrigerant, for example, a mixture of at least ethane and butane or, for example, a mixture of at least nitrogen, methane and ethane.
The present invèntion provides a method of improved economy, for liquefying natural gas containing nitrogen. In the invention a nitrogen stripper is successfully employed rather than a two-stage end flash system. By stripping the bulk of the nitrogen from the natuE~ gas, a liquefied natural gas product of higher temperature is obtained, so that the liquefied natural ga3 can leave the cold heat-exchanger at a higher temperature.
This gain in temperature is about 1.5~C for feed gas containing 3/O nitrogen, and this corresponds approximately with a saving of 1.5% on total compression energy.
Using the cold gaseous mixture of at least nitrogen .'' ~
lO~;ZtjO2 and methane leaving the nitrogen stripper for cooli~g a gaseous fraction of the second mixed refrigerant, before the latter is expanded to a lower pressure, has the advantage that the refrigeration energy requirement of the second cooling circuit is reduced, so that the total energy necessary for liquefying a given quantity of natural gas is reduced as well.
According to the present invention there is provided a method of liquefying a natural gas by cooling it under pressure in stages comprising cooling a natural gas stream containing some nitrogen by passing it in heat-exchange relationship with a first mixed refrigerant circulating in a first circuit and with a second mixed refrigerant circulating in a second circuit, so that the natural gas liquefies, passing the liquefied natural gas to a nitrogen stripper, separating nitrogen from the liquefi-ed natural gas in the nitrogen stripper' removing a gas mixture comprising at least nitrogen and methane from the top of the nitrogen stripper, removing liquefied natural gas from the nitro-gen stripper as a product, and cooling a gaseous fraction of said second mixed refrigerant, before it is expanded to a lower pressure, with said gas mixture from the nitrogen stripper.
In a particular embodiment the liquefied natural gas is expanded to a lower pressure to produce vapour before passing to the nitrogen stripper.
A gaseous fraction of the second mixed refrigerant is suitablyexpanded to a lower pressure before circulating it in the second circuit, and the gaseous fraction is cooled with the gas mixture from the nitrogen stripper before the expanding.
Before the liquefaction of the natural gas, it is necessary to remove water from the natural gas. In this re-spect, the water i9 suitably removed from the natural gas byprecooling it. This may comprise raising the pressure of a ,; . ~ ' , :
lO~iZf~OZ
mixed refrigerant in a compressor cooling the refrigerant 80 that it condenses partially, in a separator separating the liquid fraction from the gaseous fraction, branching off part of the liquid fraction of the refrigerant, in an ex-pansion device reducing the pressure of the branched-off part of the liquid fraction of the refrigerant and allowing it to evaporate at least largely to produce cold, which cold is used to precool natural gas in order to remove water from :~
the natural gas~
Further details and embodiments of the process according to the invention will be explained with refer- >
ence to the drawing, showing a flow sheet of the proces~.
Referring to the drawing, natural gas, from which C2 and water have been removed, is supplied through a line 1.
This natural gas, which is at a relatively high : .
pressure of, for example, 50 bar and at a temperature :
of, for example, 20 degrees centigrade, is passed through a coil 2 of a heat-exchanger 4. In the coil 2 2D the temperature of the natural gas is lowered. The cooled natural gas leaving the coil 2 is passed through ,.
a line 3 to a phase separator 5, wherein condensed :
heavier hydrocarbon components are separated from the gas. The condensed heavier hydrocarbons are removed from the phase separator 5 through a discharge line 6.
Together with the condensate some lighter hydrocarbons, such as methane, ethane and propane are removed from the separator 5 through the discharge line 6. The natural gas leaves the phase separator 5 as a vapour and is passed through a line 7 to a coil 8 of the heat- :
.,., , -~
~o~z~oz exchanger 4, wherein the temperature of the natural gas is lowered rurther. From coil 8 the cooled natural gas, which contains a small quantity Or liquid, is passed through a line 9 to a coil 10 o~ a heat~exchanger 11.
In coil 10 the temperature of the natural gas is re-duced to a lower value, so that more liquid is formed.
From coil 10 the natural gas is passed through a line 12 to a phase separator 13. In phase separator 13 condensed hydrocarbons are separated from the natural gas. The condensed hydrocarbons, which contain, for example, mainly methane, ethane, propane and butane as well as some pentane, are removed from the separatcr 13 through a line 14.
Natural gas vapour containing mainly methane, ethane and nitrogen leaving the phase separator 13 is passed through a line 15 to a coil 16 of heat-exchanger 11, wherein the temperature Or the natural gas is ~;
lowered further and the natural gas is fully condensed.
From coil 16 the natural gas is passed to an expansion device 17. In expan~ion device 17 the pressure of the condensed natural gas is reduced, whereafter it is passed via a line 18 to a coil 19 of heat-exchanger 11.
In coil 19 the condensed natural gas is cooled further to a temperature of, for example, minus 147 degrees centigrade. From coil 19 the liquefied natural gas is passed through a line 20 to a heat-exchanger 22 -in a nitrogen stripper 21. In the heat-exchanger ~2 the liquefied natura~ gas is cooled further. From heat-exchanger 22 the liquefied natural gas is passed to an expansion device 23. In expansion device 23 the liquefied natural gas is expanded to a lower pressure, so that some vapour is formed, and then it is passed through a line 24 to a distribution device 25 in the stripper 21, whereln liquid and vapour are separated.
The liquid natural gas fraction le~ving distribution 10 device 25, passes through the stripper 21 to an outlet 26 and from outlet 26 via line 27 to storage. In stripper 21 nitrogen is separated from tne liquefied natural gas. A methane/nitrogen vapour mixture con-taining some traces of ethane leaves the top of the stripper 21 via a line 29 and is passed to a heat-ex~hanger 30. In h~t-exchanger 30 the temperature of this gas mixture is raised and then this gas mixture -is passed through a line 31 to a heat-exchanger 32 in which the temperature of the said gas mixture is raised further. Finally, the said gas mixture is passed from heat-exchange~ 32 through a line 33 to a suitable location to be used, for example, as a fuel gas.
The condensate leaving the phase separator 5 is passed via line 6 and an expansion device 150 to a demethanizer 151 which is provided with a reboiler 152.
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A heavy hydrocarbon stream, com~rising ethane and componert~ heavier than ethane i~ removed as liquid rrom the bottom of the demethanizer 151 and i8 paS8ed to a refrigerant make-up unit tnot shown) via a line 153. Part Or the liquid ~aving the demethanizer 151 through line 153 is recirculated through the re-boiler 152. Methane, possibly with some ethane~ leaves the demethanizer 151 as overhead vapour and is passed through a line 154 to be intermixed with a gas stream flowing in a line 157.
The C2-h~idrocarbons produced in the refrigerant make-up unit are passed via a line 141 ~ the heat-exchanger 32 to be lowered in temperature. From heat- ~ -exchanger 32 the said C2-hydrocarbons are passed via a line 142 to storage. The C3-hydrocarbons produced in the refrigerant make-up unit are passed via a line 143 to the heat-~xchanger 32. In heat-exchanger 32 the said C3-hydrocarbons are lowered in temper-ature and then passed via a line 144 to storage.
The condensed hydrocarbons leaving the phase separator 13 via line 14 can be passed via a line 158 to an expansion device 159. Another part of said condensed hydrocarbons can be passed via a pump 160 to the gas flowing in line 15 to be intermixed there-wîth.
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In expansion device 159 the liquid is expanded to a lower pressure and then it is passed to a de-methanizer 162 which i8 provided with a reboiler 163.
A hydrocarbon stream comprising mainly ethane, propane and butane is removed as liquid rrom the bottom Or the demethanizer 162 and is passed as ra~ natural gas liquid via a line 164 to storage ror rurther treatment.
Part of the liquid leaving the demethanizer 162 ~rough line 164 i6 recirculated through the reboi~r 163.
Methane, possibly with some ethane, leaves the de-methanizer 162 as overhead vapour and is passed via the line 157 to a coil 165 in heat-exchanger 11. In coil 165 the gas is cooled and condensed and then it ~-is passed via a line 166 to ~he gas rlOwing in line 18 ~ :
to be intermixed therewith.
The quantity of liquid to be passed to the de-methanizer 162 can be controlled at will by manipulating the expansion device 159. In so doing the extraction of natural gas liquids from the natural gas feed stream can be regulated. All condensate ~rom separator 13 which is not sent to demethanizer 162 is passed via pump 160 to line 15.
In the above natural gas liquefaction system two separate cooling circuits are used, In the first cooling .circuit a so-called mixed refrigerant, ror example a suitable mixture of methane, ethane, propane, butane lQ6;~;0Z
and pentane, is supplied in gaseous condition through a line 34 to a compressor 3L. In compressor 35 the pressure Or the mixed refrigerant is raised. From compressor 35 the mixed refrigerant is passed through a line 36 to a heat-exchanger 37, which is cooled ~or example by water.
In heat-exchanger 37 the mixed re~rigerant is cooled to such a degree that partial condensation of the mixture occurs. From heat-exchanger 37 the partially condensed mixed re~rigerant is passed through a line 38 to a phase separator 39 in which condensed mixed refrigerant is separated from gaseous mixed refrigerant. The gaseous I mixed refrigerant is passed from phase separator 39 through line 40 to a compressor 41. In compressor 41 the pressure of the gaseous mixed refrigerant i8 raised further. Fr,om compressor 41 said gaseous mixed re-frigerant is passed through a line 42 to a heat-exchanger 43.
The condensed mixed refrigerant leaves the phase separator ~9 ~ia outlet 44 and is passed to a pump 45.
In pump 45 the pressure of the condensed mixed re-frigerant is raised to such a level that it can be passed through a line 46 to line 42 and be added to the gaseous mixed re~rigerant leaving the compressor 41.
In the heat-exchanger 43, which is, for example, cooled b.y water, the mixed refrigerant is cooled and partially ~ondensed and from heat-exchanger 43 the cooled mixed ~,,, ' , ., .~
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rerrigerant is passed through a line 47 to a phase ~eparator 48.
The condensed mixed refrigerant leaving the phase separator 48 is passed through a line 49 to a coil 50 Or the heat-exchanger 4. The gaseous mixed refrigerant leaving the phase separator 48 is passed through a line 51 to a coil 52 of the heat-exchanger 4.
In coil 50 the condensed mixed refrigerant is cooled and is then passed from coil 50 via a line 53 to !
an expansion device 54. In e.xpansion device 54 the cooled ~ .
~iquid refrigerant is expanded to a lower pressure. The liq~id, possibly together with a small portion o~
vapour, is passed from expansion device 54 through a line 55 and is injected through a distribution device 56 into heat-exchanger 4, wherein it combines with a mixed re-frigerant stream which enters heat-exchanger 4 via a distribution device 64. The combined mixed refri~rant stream flows downward over the coils 50, 52, 100 and 2 to cool the contents of these coils. During this process the largest part of the mixed refrigerant eYaporate ~he mixed refrigera~t, which îs largely in gaseous condition and contains only a small portion o~ liquid, leaves the heat-exchanger 4 vla a line 57 to be passed to a ph~se separator 58. In phase separator 58 liquid mixed refrigerant is separated from gaseous m;xed re-frigerant. The separated liquid mixed rerrigerant is .
106Z6~)Z
removed from phase separator 58 via an ou~et 76 to be injected, after its pressure has been raised, into, for example, line 36 J or into, for example, separator 39.
Via a line 59 so-called "make-up refrigerant" i5 added to the rerrigerant passing through line 57 to compensate for re~rigerant lost during the process. Gaseous re-frigerant is passed from phase separator 58 via line 34 to compressor 35 to repeat the cycle as described in the above.
The mixed refrigerant passing through coil 52 is lowered in temperature and condensed in said coil and is then passed to a further coil 60 Or heat-exchanger 4.
In coil 60 the condensed mixed refrigerant is cooled further and then it is passed via a line 61 to an expansion device 62. In expansion device 62, the rerrigerant iæ expanded to a lower pressure so that some vapour is ~ormed and then it is passed via a line 63 to a distribut~n device 64. From distribution device 64 the refrigerant, which is largely in liquid condition, flows downward over the coils 60, 101 and 8 to cool the contents o~ these coils and further down-ward over the coils 50, 52, 100 and 2 to cool the contents of these coils until it reaches the bottom of the heat-exchanger.
During this process the refrigerant evaporates largely. Finally, the refrigerant leaves the heat-lO~Z60Z
exchanger 4 via the line 57 to be passed to phase separator 58.
Some Or the condensed refrigerant leaving the : -phase separator 48 via the line 49 is branched orf ';
arld is passed through a line 65 to an expansion device 66. In expansion device 66 the refrigerant i8 -:
expanded to a lower pr(essure so that some gas is formed and is then pas;ed through a line 67 to a distribution device 68 of a heat-exchanger 69 which is provided with a coil 70. The refrigerant, which is largely in liquid condition, leaving the distribution device 68 flows downward over the coil 70 to cool the contents of the coil 70. During this process the re- ~, frigerant evaporates largely and finally leaves t,he heat-exchanger 69 through a line 71.
Via line 71 the refrigerant is passed to an ex-pansion device 72 in which the refrigerant is expanded to a lower pressure. Then the refrigerant~ which is largely in gaseous condition, is passed from the ex-pansion device 72 to the phase separator 39 via aline 73 to be combined with the refrigerant arriving from the compressor 35. In heat-exchanger 69 natural gas to be lique~ied is precooled in order to remove a quantity of water which is present in the natural gas.
25. Fo,r this purpose the natural gas is supplied thro'ugh a line 74 to the coil 70 and passed through coil 70.
.
. lQ6Z60Z
The natural gas precooled in coil 70, leaves the coil 70 via a line 75 and is passed to a phase separator (not shown) in which condensed water is removed rrom the natural gas. Then the partly dried natural gas is passed to a conventional drier (not shown) to remove the remaining water from the natural gas. Thiæ con-ventional drier is, for example, of the kind containing a suitable desiccant. From the conventional drier the natural ~s is passed to line 1 in order to be liquefied in the m~anner as described in the above.
In the second cooling circuit a so-called mixed re-frigerant is circulating as well. The composition of the mixed re~rigerant circulating in the second cooling circuit is, however, difrerent from the mixed re-~rigerant circulating in the first cooling circuit.The mixed refri~erant circulating in the second cool-ing circuit is, for example, a mixture of ethane, methane and nitrogen.
In the second cooling circuit gaseous mixed re-20 rrigerant i8 supplied through a line 80 to a com-pressor 81.
In the compressor 81 the pressure of the mixed refrigerant is raised and then it is passed through a line 82 to a heat-exchanger 83, which is cooled, ror example, by water. In heat-exchan~er 83 the mixed re-rrigerant is cooled and then it is passed via a line 84 . . .,. . - .
: .
106Z6(~2 to a kr,ock-out vessel 85. From knock-out vesseJ 85 the mixed refrigerant is passed via a line 86 to a com-pressor 87.
In compressor 87 the pressure Or the mixed re-frigerant is raised ~urther and.then it is pas~ed via a line 88 to a heat-exchanger 8~l which is cooled, for example, by water. From heat-exchanger 89, the mixed refrigerant is passed via a line 90 to a coil 100 of the heat-exchanger 4. In coil 100 the temperature of O the mixe~ refrigerant is lowered. From coil 100, the mixed rerrigerant i~ passed to a coil 101 in which it is cooled ~urther ~nd partially condensed.
From coil 101 the cooled mixed refrigerant is passed through a line 102 to a phase separator 106.
In the phase separator 106 the gaseou~ refrigerant is separated ~rom liquid refrigerant. From phase separator 106, the liquid re~rigerant is paæsed through a line 110 to a coil 111 o~ the heat-exchanger 11. In coil 111 the re~rigerant is cooled further and then it is passed through a line 112 to an expansion device 113. In ex-pansion device 113 the refrigerant is expanded, where-&fter it is passed through a line 114 and is injected through a distribution device 115 into the heat-exchanger 11, wherein it combines with a mixed re-~rigerant stream which enters heat-exchanger 11 via a distribution device 132.
.
106~602 The combined mlxed refrigerant stream is pa~sed over the coils 111, 127, 165, lÇ and 10 which cau~es cooling of the contents Or these coils. During the passage Or the refrigerant over the coil~ 111, 127, 165, 16 and 10 the refrigerant evaporates at least partly.
Finally~ the refrigerant reaches the lower part of the heat-exchanger 11 and then it is passed through a line 116 to a heat-exchanger 117.
In heat-exchanger 117, the refrigerant cools the contents of a coil 109 of the heat-exchanger 117. Then the refrigerant ~ passed through a line 118 to a knock-out vessel 119. Finally, the refrigerant,which îs in gaseous condition, is passed from knock-out vessel 119 ~hrough the line 80 to the compressor 81 to repeat the cycle.
Gaseous refrigerant leaves the phase separator 106 through a line 125. From line 125 part o~ the said gaseous refrigerant is pasæed through a line 126 to a coil 127 in the heat-exchanger 11 in which it is cooled
2~ ~nd condensed. From coil 127 the condensed refrigerant is passed to a coil 128 in which it is cooled rurther.
From coil 128 the refrigerant is passed vi~. a line 129 ~ -to an expansion device 130 in which the refrigerant ~-is expanded to a lower pressure. From expansion device 130, the refrigerant, which is now largely in liquid condition is pa~sed via a line 131 to-a distribution ' .' . '?
....
~ 06Z6~Z
device 1~2. From distribution device 132, the re-rrigerant is passed downward over the coils 19, 128 J 16, 165; 10, 127 and 111 to the bottom part o~ the heat-exchanger 11. During the passage of the rerrigerant, the latter cools the contents Or the said coils.
Finally, the refrigerant leaves the heat-exchanger 11 through the line 116.
~ Part of the g~seous rerrigerant leaving the phase se?arator 106 via the line 125 is branched off and is passed via a line 135 to the heat-exchanger 30. In heat-exchanger 30 the refrigerant is cooled against gaseous mixture leaving the nitrogen stripper via the line 29. From heat-exchanger 30 the refrigerant is~
passed via a line 136 to an expansion device 137.
In expansion device 137 the refrigerant is ex-panded to the pressureof the refrigerant leaving the expansion device 130. Finally, both streams of refrigerant are mixed and led via line 131 to the distribution device 1~2 to be injected into the heat-exchanger 11.
Part of the mixed refrigerant passing through line 90 is recycled. For this purpose, part of the gas stream is branched off and is passed via a line 138 to the coil 109 of the.heat-exchanger 117. In coil 109 the temperature of the mixed re~rigerant is lowered and then it is passed from coil 109 through a line 139 .
lO~:;Z602 to line 102 to be intermixed with the mixed rerrigerant passing through line 102 to be passed to the phase separator 106.
At 120 make-up re~rigerant is added to the re~rigerant circuit to compensate I~or losses of refrigerant circulating in the circuit.
.
From coil 128 the refrigerant is passed vi~. a line 129 ~ -to an expansion device 130 in which the refrigerant ~-is expanded to a lower pressure. From expansion device 130, the refrigerant, which is now largely in liquid condition is pa~sed via a line 131 to-a distribution ' .' . '?
....
~ 06Z6~Z
device 1~2. From distribution device 132, the re-rrigerant is passed downward over the coils 19, 128 J 16, 165; 10, 127 and 111 to the bottom part o~ the heat-exchanger 11. During the passage of the rerrigerant, the latter cools the contents Or the said coils.
Finally, the refrigerant leaves the heat-exchanger 11 through the line 116.
~ Part of the g~seous rerrigerant leaving the phase se?arator 106 via the line 125 is branched off and is passed via a line 135 to the heat-exchanger 30. In heat-exchanger 30 the refrigerant is cooled against gaseous mixture leaving the nitrogen stripper via the line 29. From heat-exchanger 30 the refrigerant is~
passed via a line 136 to an expansion device 137.
In expansion device 137 the refrigerant is ex-panded to the pressureof the refrigerant leaving the expansion device 130. Finally, both streams of refrigerant are mixed and led via line 131 to the distribution device 1~2 to be injected into the heat-exchanger 11.
Part of the mixed refrigerant passing through line 90 is recycled. For this purpose, part of the gas stream is branched off and is passed via a line 138 to the coil 109 of the.heat-exchanger 117. In coil 109 the temperature of the mixed re~rigerant is lowered and then it is passed from coil 109 through a line 139 .
lO~:;Z602 to line 102 to be intermixed with the mixed rerrigerant passing through line 102 to be passed to the phase separator 106.
At 120 make-up re~rigerant is added to the re~rigerant circuit to compensate I~or losses of refrigerant circulating in the circuit.
.
Claims (3)
1. A method of liquefying a natural gas by cooling it under pressure in stages comprising cooling a natural gas stream containing some nitrogen by passing it in heat-exchange relationship with a first mixed refrigerant circulating in a first circuit and with a second mixed refrigerant circulating in a second circuit, so that the natural gas liquefies; passing the liquefied natural gas to a nitrogen stripper; separating nitrogen from the liquefied natural gas in the nitrogen stripper;
removing a gas mixture comprising at least nitrogen and methane from the top of the nitrogen stripper, removing liquefied natural gas from the nitrogen stripper as a product, and cooling a gaseous fraction of said second mixed refrigerant, before it is expanded to a lower pressure, with said gas mixture from the nitrogen stripper.
removing a gas mixture comprising at least nitrogen and methane from the top of the nitrogen stripper, removing liquefied natural gas from the nitrogen stripper as a product, and cooling a gaseous fraction of said second mixed refrigerant, before it is expanded to a lower pressure, with said gas mixture from the nitrogen stripper.
2. A method according to claim 1 including the step of expanding a gaseous fraction of said second mixed refrigerant to a lower pressure before circulating it in said second circuit; said gaseous fraction being cooled with said gas mixture from the nitrogen stripper before said step of expanding.
3. A method according to claim 1 or 2 wherein said liquefied natural gas is expanded to a lower pressure to produce vapour before passing to said nitrogen stripper.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA305,842A CA1062602A (en) | 1976-04-21 | 1978-06-20 | Process for the liquefaction of natural gas |
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB1611276A GB1572898A (en) | 1976-04-21 | 1976-04-21 | Process for the liquefaction of natural gas |
| CA273,609A CA1053568A (en) | 1976-04-21 | 1977-03-09 | Process for the liquefaction of natural gas |
| CA305,842A CA1062602A (en) | 1976-04-21 | 1978-06-20 | Process for the liquefaction of natural gas |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1062602A true CA1062602A (en) | 1979-09-18 |
Family
ID=27164962
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA305,842A Expired CA1062602A (en) | 1976-04-21 | 1978-06-20 | Process for the liquefaction of natural gas |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1062602A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102564059A (en) * | 2012-02-19 | 2012-07-11 | 中国石油集团工程设计有限责任公司 | Twin-stage multi-component mixed refrigerant refrigeration natural gas liquefaction system and method |
-
1978
- 1978-06-20 CA CA305,842A patent/CA1062602A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102564059A (en) * | 2012-02-19 | 2012-07-11 | 中国石油集团工程设计有限责任公司 | Twin-stage multi-component mixed refrigerant refrigeration natural gas liquefaction system and method |
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