Cold membrane nitrogen rejection process and system

Abstract

An approach for separating a gaseous mixture includes a multi-stage membrane system in which a rubbery membrane is operated at a low temperature. Various streams are cooled and heated in a multi-fluid heat exchanger. In specific configurations, the multi-fluid heat exchanger is cooled by using no fluids other than fluids derived from the permeate and/or residue generated in the first membrane stage.

What is claimed is: 
     
         1 . A membrane separation process, comprising:
 cooling a feed containing methane and nitrogen in a main heat exchanger to produce a cooled feed at a temperature below 0° C.;   processing the cooled feed in a first membrane stage to produce a first permeate and a first residue;   heating at least a portion of the first residue in the main heat exchanger to produce a heated first residue stream,   introducing the heated first residue stream into a second membrane stage to produce a second permeate and a second residue,   collecting fluids derived from the first permeate as a methane-rich product gas, and   recycling fluids derived from the second permeate back to the first membrane stage.   
     
     
         2 . The process of  claim 1 , wherein no fluids are used to cool the main heat exchanger other than the fluids derived from the first permeate and/or the fluids derived from the first residue. 
     
     
         3 . The process of  claim 1 , wherein the collected fluids derived from the first permeate are derived by processing at least a portion of the first permeate in a third membrane stage so as to obtain a third permeate and a third residue, the third permeate being the methane-rich product, the fluids derived from the second permeate being the second permeate. 
     
     
         4 . The process of  claim 3 , wherein at least a portion of the first permeate is warmed in the main heat exchanger before being processed in the third membrane stage. 
     
     
         5 . The process of  claim 3 , wherein the feed contains at least methane, nitrogen and water, water in the feed is removed in a multi-bed adsorption dryer, and adsorbent in the multi-bed adsorption dryer is regenerated using the second or third residue. 
     
     
         6 . The process of  claim 1 , wherein the recycled fluids derived from the second permeate are warmed in the main heat exchanger before being recycled back to the feed. 
     
     
         7 . The process of  claim 1 , further comprising the step of processing the second permeate in a third membrane stage so as to obtain a third permeate and a third residue, wherein the third permeate is recycled back to the first membrane stage, the first permeate is the methane-rich product, and the recycled fluids derived from the second permeate is the third permeate. 
     
     
         8 . The process of  claim 7 , further comprising the steps of:
 compressing the second permeate before the second permeate is processed in the third membrane stage; and   compressing the third permeate.   
     
     
         9 . The process of  claim 8 , wherein the third permeate is warmed in the main heat exchanger before or after being compressed. 
     
     
         10 . The process of  claim 7 , wherein the feed contains at least methane, nitrogen and water, water in the feed is removed in a multi-bed adsorption dryer, and adsorbent in the multi-bed adsorption dryer is regenerated using the second residue or third residue. 
     
     
         11 . The process of  claim 1 , wherein the collected fluids derived from the first permeate are derived by warming the first permeate in the main heat exchanger. 
     
     
         12 . The process of  claim 1 , wherein the main heat exchanger is a multi-fluid heat exchanger. 
     
     
         13 . The process of  claim 1 , further comprising the step of heating the fluids derived from the first permeate in the main heat exchanger. 
     
     
         14 . The process of  claim 1 , further comprising the step of heating fluids derived from the second residue in the main heat exchanger. 
     
     
         15 . The process of  claim 1 , further comprising the step of controlling a temperature of the cooled feed via a bypass valve. 
     
     
         16 . The process of  claim 1 , wherein the first permeate is the methane-rich product gas and the second permeate is recycled back to the first membrane stage. 
     
     
         17 . The process of  claim 1 , further comprising the step of compressing the recycled fluids derived from the second permeate and collected fluids derived from the first permeate, wherein each of the recycled fluids derived from the second permeate and the collected fluids derived from the first permeate the recyclable stream is at a sub-ambient temperature. 
     
     
         18 . A membrane separation system, comprising:
 a feed conduit for directing a flow of feed gas comprising methane and nitrogen;   at least first and second membrane separation stages, each of the membrane stages comprising membranes selective for methane over nitrogen, a feed inlet, a permeate outlet, and a residue outlet, wherein each of the membrane stages is adapted and configured to produce a respective permeate and a respective residue, the feed inlet of the first membrane separation stage is in downstream flow communication with the feed conduit, and the feed inlet of the second membrane separation stage is in downstream flow communication with the residue outlet of the first membrane separation stage; and   a multi-fluid heat exchanger, wherein:
 the first membrane separation stage and the multi-fluid heat exchanger are adapted and configured for heating the retentate produced by the first membrane separation stage at the multi-fluid heat exchanger; and 
 the feed conduit and the multi-fluid heat exchanger are adapted and configured for cooling the flow of feed gas at the multi-fluid heat exchanger. 
   
     
     
         19 . The membrane separation system of  claim 18 , wherein no fluids are used to cool the main heat exchanger other than fluids derived from the permeate or residue produced by the first membrane stage. 
     
     
         20 . The membrane separation system of  claim 18 , further comprising a bypass valve adapted and configured for controlling a temperature of a cooled feed introduced to the first membrane stage by allowing a portion of the flow of feed gas to bypass the multi-fluid heat exchanger, wherein the portion of the flow of feed gas that bypasses the multi-fluid heat exchanger is combined with a remaining portion of the flow of feed gas that is cooled at the multi-fluid heat exchanger before the combined portions are fed to the first membrane stage. 
     
     
         21 . The membrane separation system of  claim 18 , further comprising a conduit for recycling the second permeate back to the feed that is in downstream fluid communication with the permeate outlet of the second membrane stage and in upstream flow communication with the feed inlet of the first membrane stage, wherein the multi-fluid heat exchanger is adapted and configured to warm the permeate from the second membrane separation stage. 
     
     
         22 . The membrane separation stage of  claim 21 , further comprising a compressor in fluid communication between the permeate outlet of the second membrane stage and the feed inlet of the first membrane stage. 
     
     
         23 . The membrane separation system of  claim 18 , further comprising a conduit for recycling the permeate from the second membrane stage back to the feed that is in downstream fluid communication with the permeate outlet of the second membrane stage and in upstream flow communication with the feed inlet of the first membrane stage, wherein said at least first and second membrane separation stages further comprise a third membrane separation stage and the feed inlet of the third membrane stage is in downstream flow communication with the permeate outlet of the first membrane stage. 
     
     
         24 . The membrane separation system of  claim 23 , further comprising a compressor in fluid communication between the permeate outlet of the second membrane stage and the feed inlet of the first membrane stage and a compressor in downstream fluid communication with the permeate outlet of the third membrane stage. 
     
     
         25 . The membrane separation system of  claim 18 , wherein said at least first and second membrane separation stages further comprise a third membrane separation stage, the feed inlet of the third membrane stage is in downstream flow communication with the permeate outlet of the second membrane stage, and said system further comprises a conduit for recycling the permeate from the third membrane separation stage back to the feed that is in downstream flow communication with the permeate outlet of the third membrane stage and in upstream flow communication with the feed inlet of the first membrane stage. 
     
     
         26 . The membrane separation system of  claim 25 , further comprising a compressor in fluid communication between the permeate outlet of the second membrane stage and the feed inlet of the third membrane stage and a compressor in fluid communication between the permeate outlet of the third membrane stage and the feed inlet of the first membrane stage. 
     
     
         27 . The membrane separation system of  claim 18 , wherein said system is configured to produce a first permeate having a nitrogen content that is less than 4 times a nitrogen content of the feed. 
     
     
         28 . The membrane separation system of  claim 18 , wherein said system is configured to cool the flow of feed gas to a temperature below 0° C.