Sweep gas for membrane-based dehydration modules

Abstract

An air dehydration membrane module is provided with a sweep gas which is taken from the waste gas of a pressure swing adsorption (PSA) unit. No additional compressor is required, other than the compressor forming part of the PSA unit. In another embodiment, the sweep gas includes the combination of dried product gas, taken from the dehydration membrane module, and a supplemental gas, which may be ambient air, or permeate gas from an air separation membrane, or waste gas from a PSA unit. An air ejector combines the streams, without the use of an additional compression step, and the combined gas is used as a sweep stream for the dehydration module. The invention also includes the method of selecting an optimum point at which the sweep gas is injected into the module.

1 . A system for drying a feed gas and for producing a purified product gas, comprising:
 a) an air dehydration membrane module having a feed end and a product end,   b) a pressure swing adsorption (PSA) unit, the PSA unit being connected to receive product gas from the dehydration membrane, the PSA unit providing a product stream and a waste stream,   c) a conduit for directing said waste stream from said PSA unit to a point in a vicinity of the product end of the dehydration membrane module, wherein said waste stream comprises a countercurrent sweep stream for the dehydration membrane module.   
     
     
         2 . The system of  claim 1 , further comprising a buffer, connected to receive the waste stream from the PSA unit, the buffer being connected to said conduit. 
     
     
         3 . The system of  claim 1 , wherein the system includes no pressurization device for the waste stream outside of the PSA unit. 
     
     
         4 . A method for making a dry component gas from a relatively wet feed stream, the method comprising:
 a) passing a feed gas stream through a dehydration membrane module, the module having a feed end and a product end, to produce a dry product gas,   b) directing the dry product gas into a PSA unit so as to separate the dry product gas into a purified product gas and a waste stream,   c) conveying said waste stream from the PSA unit into the dehydration membrane, the waste stream being conveyed to a point in a vicinity of the product end such that the waste stream can flow countercurrently to the feed gas stream and can function as a sweep gas for the dehydration module.   
     
     
         5 . The method of  claim 4 , wherein step (c) includes conveying the waste stream into a buffer, and conveying the waste stream from the buffer to the dehydration membrane. 
     
     
         6 . The method of  claim 4 , wherein the waste stream is not further pressurized outside of the PSA unit. 
     
     
         7 . The method of  claim 4 , wherein step (c) includes selecting an injection point along the module, for the sweep gas, according to the following steps:
 d) determining a water vapor concentration of said sweep gas,   e) calculating a water vapor concentration of said feed gas stream flowing within the module, the calculating step being performed for a plurality of locations along the module, and   f) injecting the sweep gas at a location along the module such that the water vapor concentration of said feed gas stream at said location is greater than or equal to the water vapor concentration of said sweep stream.   
     
     
         8 . A gas drying system, comprising:
 a) an air dehydration membrane module having a feed end and a product end,   b) an air ejector unit connected to receive dried gas from the product end of the dehydration membrane module,   c) the air ejector unit also being connected to receive a supplemental gas from a source, and   d) a conduit for directing gas ejected from the air ejector unit to a point in a vicinity of the product end of the dehydration membrane module, wherein said ejected gas comprises a countercurrent sweep stream for the dehydration membrane module.   
     
     
         9 . The system of  claim 8 , wherein the source of the supplemental gas is selected from the group consisting of ambient air, permeate gas from an air separation membrane, and vent gas from a pressure swing adsorption unit. 
     
     
         10 . The system of  claim 8 , wherein the air ejector unit includes no pressurization device. 
     
     
         11 . A method for drying a wet gas, the method comprising:
 a) passing a feed gas stream through a dehydration membrane module, the module having a feed end and a product end, to produce a dry product gas,   b) entraining a portion of the dry product gas with a supplemental gas from a source, so as to produce a combined gas, and   c) conveying said combined gas into the dehydration membrane, the combined gas being conveyed to a point in a vicinity of the product end such that the combined gas functions as a sweep stream which flows countercurrently to the feed gas stream of the dehydration module.   
     
     
         12 . The method of  claim 11 , wherein the entraining step comprises passing a portion of the dry product gas into an air ejector, while passing the supplemental gas into the air ejector, wherein the air ejector has an output comprising said combined gas. 
     
     
         13 . The method of  claim 12 , wherein the air ejector contains no pressurizing device. 
     
     
         14 . The method of  claim 11 , further comprising selecting said source from the group consisting of ambient air, permeate gas from an air separation membrane, and vent gas from a pressure swing adsorption unit. 
     
     
         15 . The method of  claim 11 , further comprising selecting a location for said point according to the following steps:
 d) determining a water vapor concentration of said sweep stream,   e) calculating a water vapor concentration of said feed gas stream flowing within the module, the calculating step being performed for a plurality of locations along the module, and   f) injecting the sweep stream at a location along the module such that the water vapor concentration of said feed gas stream at said location is greater than or equal to the water vapor concentration of said sweep stream.   
     
     
         16 . A method of providing a sweep gas for a dehydration membrane module, the module having a feed end and a product end, wherein a feed gas stream containing water vapor flows through the module such that some of the water vapor permeates through a membrane in the module, the method comprising:
 a) providing a sweep stream for the dehydration membrane module,   b) determining a water vapor concentration of said sweep stream,   c) calculating a water vapor concentration of said feed gas stream flowing within the module, the calculating step being performed for a plurality of locations along the module, and   d) injecting the sweep stream at a location along the module such that the water vapor concentration of said feed gas at said location is greater than or equal to the water vapor concentration of said sweep stream, the injecting being performed such that the sweep stream flows countercurrently to a flow of said feed gas.   
     
     
         17 . The method of  claim 16 , further comprising selecting said sweep stream from the group consisting of ambient air, permeate gas from an air separation membrane, and vent gas from a pressure swing adsorption unit.