Integrated NGL Recovery In the Production Of Liquefied Natural Gas

Abstract

Process for the liquefaction of natural gas and the recovery of components heavier than methane where natural gas is cooled and separated in a first distillation column into an overhead vapor enriched in methane and a bottoms stream enriched in components heavier than methane, where the first distillation column utilizes a liquefied methane-containing reflux stream. This reflux stream may be provided by a condensed portion of the overhead vapor or a portion of totally condensed overhead vapor that is subsequently warmed. The bottoms stream may be separated in one or more additional distillation columns to provide one or more product streams, any of which are partially or totally withdrawn as recovered hydrocarbons. A stream of unrecovered liquid hydrocarbons may be combined with either the condensed portion of the overhead vapor or a portion of totally condensed overhead vapor that is subsequently warmed.

1 . A process for the liquefaction of natural gas and the recovery of components heavier than methane from the natural gas, wherein the process comprises:
 (a) introducing a natural gas feed into a first distillation column and separating in the first distillation column the natural gas feed into an overhead vapor enriched in methane and a bottoms liquid enriched in components heavier than methane;   (b) withdrawing from the first distillation column a first overhead vapor stream enriched in methane and a bottoms liquid stream enriched in components heavier than methane;   (c) cooling the first overhead vapor stream enriched in methane to provide a cooled overhead stream enriched in methane, combining the cooled overhead stream enriched in methane with a liquified natural gas reflux stream to provide a two-phase mixture, and separating the two-phase mixture to provide a liquid reflux stream and a second overhead vapor stream enriched in methane;   (d) introducing the liquid reflux stream into the first distillation column to provide reflux to the distillation column;   (e) introducing the second overhead vapor stream enriched in methane into a main heat exchanger comprising only two tube bundles, said bundles consisting of a warm bundle and a cold bundle, and cooling and liquifying the second overhead vapor stream in the warm bundle to provide a single-phase, liquefied natural gas product; and   (f) subcooling at least a portion of the liquified natural gas product in the cold bundle of the main heat exchanger to provide a subcooled liquified natural gas product, and withdrawing the subcooled liquified natural gas product from the main heat exchanger.   
     
     
         2 . The process of  claim 1 , wherein the liquefied natural gas reflux stream comprises: a portion of the single-phase, liquefied natural gas product, withdrawn from the main heat exchanger between the warm and cold bundles; a portion of the subcooled liquefied natural gas product; and/or liquefied natural gas obtained from processing and/or storing at least a portion of the subcooled liquefied natural gas product. 
     
     
         3 . The process of  claim 1 , wherein the method further comprises the step of:
 withdrawing from the main heat exchanger, between the warm and cold bundles, a first portion of the liquified natural gas product, the liquified natural gas reflux stream comprising said first portion of the liquified natural gas product; and   wherein step (f) comprises subcooling a second portion of the liquified natural gas product in the cold bundle of the main heat exchanger to provide a subcooled liquified natural gas product, and withdrawing the subcooled liquified natural gas product from the main heat exchanger.   
     
     
         4 . The process of  claim 1 , wherein the method further comprises the steps of:
 separating the bottoms liquid stream in one or more additional distillation columns to provide one or more hydrocarbon product streams selected from the group consisting of a residual vapor stream comprising methane, a liquid stream enriched in ethane, a liquid stream enriched in propane, a liquid stream enriched in butane, and a liquid stream enriched in pentane; and   withdrawing as recovered hydrocarbons all or a portion of any of the one or more hydrocarbon product streams.   
     
     
         5 . The process of  claim 4 , wherein step (c) comprises: combining the first overhead vapor stream enriched in methane with a stream of unrecovered hydrocarbons, cooling the combined stream to provide the cooled overhead stream enriched in methane, combining the cooled overhead stream enriched in methane with the liquified natural gas reflux stream to provide the two-phase mixture, and separating the two-phase mixture to provide the liquid reflux stream and the second overhead vapor stream enriched in methane. 
     
     
         6 . The process of  claim 5 , wherein the stream of unrecovered liquid hydrocarbons comprises any of:
 (I) a portion of the liquid stream enriched in ethane,   (II) a portion of the liquid stream enriched in propane,   (III) a portion of the liquid stream enriched in butane,   (IV) a portion of the liquid stream enriched in pentane, and   (V) all or a portion of the residual vapor stream dissolved in a portion of the liquid stream enriched in propane and/or a portion of the liquid stream enriched in butane and/or a portion of the liquid stream enriched in pentane.   
     
     
         7 . The process of  claim 1 , wherein in step (c) the first overhead vapor stream is cooled to a temperature of from −20 to −70° F. (−29 to −57° C.). 
     
     
         8 . The process of  claim 1 , wherein the natural gas feed is cooled prior to being introduced into the first distillation column. 
     
     
         9 . The process of  claim 8 , wherein the natural gas feed is cooled to a temperature of from 10 to −30° F. (12 to −34° C.) prior to being introduced into the first distillation column. 
     
     
         10 . The process of  claim 8 , wherein the cooling of the first overhead vapor stream enriched in methane to provide the cooled overhead stream enriched in methane, and the cooling of the natural gas feed prior to said natural gas feed being introduced into the first distillation column, is carried out using the same refrigerant. 
     
     
         11 . The process of  claim 1 , wherein the liquid reflux stream is introduced into the top of the first distillation column. 
     
     
         12 . The process of  claim 1 , wherein boilup for the first distillation column is provided by vaporizing a portion of the bottoms liquid in a reboiler. 
     
     
         13 . The process of  claim 1 , wherein the cooling and liquefying of the second overhead vapor stream in the warm bundle of the main heat exchanger is effected by indirect heat exchange with a first vaporizing refrigerant provided by reducing the pressure of a first cooled multicomponent liquid refrigerant. 
     
     
         14 . The process of  claim 13 , wherein the first cooled multicomponent liquid refrigerant is provided by cooling a first multicomponent refrigerant in the warm bundle of the main heat exchanger. 
     
     
         15 . The process of  claim 1 , wherein the subcooling of the at least a portion of the liquified natural gas product in the cold bundle of the main heat exchanger is effected by indirect heat exchange with a second vaporizing refrigerant provided by reducing the pressure of a second cooled multicomponent liquid refrigerant. 
     
     
         16 . The process of  claim 15 , wherein the second cooled multicomponent liquid refrigerant is provided by cooling a second multicomponent refrigerant in the warm and cold bundles of the main heat exchanger. 
     
     
         17 . The process of  claim 1 , wherein the flow rate of the cooled overhead stream enriched in methane is greater than the flow rate of the liquified natural gas reflux stream, such that the cooled overhead stream provides the majority of the two-phase mixture. 
     
     
         18 . The process of  claim 17 , wherein the flow rate of the liquified natural gas reflux stream is less than 10% of the sum of the flow rates of the liquified natural gas reflux stream and the cooled overhead stream. 
     
     
         19 . An apparatus for the liquefaction of natural gas and the recovery of components heavier than methane from the natural gas, wherein the apparatus comprises:
 (a) a first distillation column adapated to separate a natural gas feed into an overhead vapor enriched in methane and a bottoms liquid enriched in components heavier than methane and thereby provide a first overhead vapor stream enriched in methane and a bottoms liquid stream enriched in components heavier than methane;   (b) a cooling system adapted to cool the first overhead vapor stream enriched in methane to provide a cooled overhead stream enriched in methane;   (c) a reflux drum adapted to receive a two-phase mixture, formed from the combination of the cooled overhead stream enriched in methane and a liquified natural gas reflux stream, and adapted to separate the two-phase mixture to provide a liquid reflux stream and a second overhead vapor stream enriched in methane;   (d) piping adapted to introduce the cooled overhead stream and the liquified natural gas reflux stream, as separate streams or as a combined stream, into the reflux drum;   (e) piping adapted to introduce the liquid reflux stream into the first distillation column to provide reflux to the distillation column;   (f) a main heat exchanger comprising only two tube bundles, said bundles consisting of a warm bundle and a cold bundle, the warm bundle being adapted to cool and liquify the second overhead vapor stream to provide a single-phase, liquified natural gas product, and the cold bundle being adapted to subcool at least a portion of the liquified natural gas product to provide a subcooled liquified natural gas product;   (g) piping adapted to introduce the second overhead vapor stream into the warm bundle of the main heat exchanger; and   (h) piping adapted to withdraw the liquified natural gas product stream from the cold bundle of the main heat exchanger.   
     
     
         20 . The apparatus of  claim 19 , wherein said piping adapted to introduce the liquified natural gas reflux stream into the reflux drum is adapted to withdraw a portion of the liquefied natural gas product from the main heat exchanger between the warm and cold bundles, to withdraw a portion of the subcooled liquefied natural gas product and/or to withdraw liquefied natural gas obtained from processing and/or storing at least a portion of the subcooled liquefied natural gas product, the liquified natural gas reflux stream comprising said portion of the liquefied natural gas product, said portion of the subcooled liquefied natural gas product and/or said liquefied natural gas obtained from processing and/or storing at least a portion of the subcooled liquefied natural gas product. 
     
     
         21 . The apparatus of  claim 19 , wherein the apparatus further comprises:
 (i) one or more additional distillation columns adapted to separate the bottoms liquid stream in provide one or more hydrocarbon product streams selected from the group consisting of a residual vapor stream comprising methane, a liquid stream enriched in ethane, a liquid stream enriched in propane, a liquid stream enriched in butane, and a liquid stream enriched in pentane; and   (j) piping adapted to withdraw as recovered hydrocarbons all or a portion of any of the one or more hydrocarbon product streams.   
     
     
         22 . The apparatus of  claim 21 , wherein the apparatus further comprises piping adapted to introduce a stream of unrecovered hydrocarbons into the the first overhead vapor stream enriched in methane prior to the resulting combined stream being cooled in the cooling system, wherein the stream of unrecovered liquid hydrocarbons is withdrawn from one of the additional distillation columns and comprises any of:
 (I) a portion of the liquid stream enriched in ethane,   (II) a portion of the liquid stream enriched in propane,   (Ill) a portion of the liquid stream enriched in butane,   (IV) a portion of the liquid stream enriched in pentane, and   (V) all or a portion of the residual vapor stream dissolved in a portion of the liquid stream enriched in propane and/or a portion of the liquid stream enriched in butane and/or a portion of the liquid stream enriched in pentane.   
     
     
         23 . The apparatus of  claim 19 , wherein the apparatus further comprises a precooling system adapted to cool the natural gas feed prior to said feed being introduced into the first distillation column. 
     
     
         24 . The apparatus of  claim 23 , wherein said cooling system, adapted to cool the first overhead vapor stream enriched in methane to provide a cooled overhead stream enriched in methane, and said precooling system, adapted to cool the natural gas feed prior to said feed being introduced into the first distillation column, are adapted to utilize the same refrigerant. 
     
     
         25 . The apparatus of  claim 19 , wherein the main heat exchanger is a wound coil heat exchanger comprising only two wound coil tube bundles, said bundles consisting of said warm bundle and said cold bundle. 
     
     
         26 . The process of  claim 1 , wherein the main heat exchanger is a wound coil heat exchanger comprising only two wound coil tube bundles, said bundles consisting of said warm bundle and said cold bundle.