Polymeric Facilitated Transport Membranes for Hydrogen Purification
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Polymeric Facilitated Transport Membranes for Hydrogen Purification May-Britt Hägg and Robert Quinn Abstract The most widely used method of hydrogen production, steam methane reforming, yields a product stream consisting mainly of hydrogen (H2) and carbon dioxide (CO2). Purification of this product is currently accomplished using amine-based acid gas scrubbers or pressure swing adsorption technology. Membranes are well suited to bulk CO2 removal and offer a viable alternative to these established technologies. This review considers one type of such membranes, polymeric facilitated transport membranes. These membranes selectively permeate CO2 by means of a reversible reaction between the gas and the membrane material. In addition, the membrane provides a barrier to H2 permeation. The result is removal of the CO2 contaminant and recovery of the H2 product at high pressure, eliminating the need for recompression prior to use or storage. A wide range of polymeric materials have been investigated, including ion-exchange resins, hydrophilic polymers blended with CO2-reactive salts, polyelectrolytes, fixed-site carrier polymers, and biomimetic materials. This review provides a description of the reaction chemistry of facilitated transport, a summary of membrane permselective properties, and suggestions for future research efforts. Keywords: hydrogen, membrane, polymer, transport.
Introduction The most widely used method of hydrogen production is steam reforming of light hydrocarbons, mainly methane.1 This process consists of two basic steps. In an initial reforming step, methane and excess steam react to form carbon monoxide and hydrogen at about 820°C (Reaction 1): CH4 H2O ↔ CO 3H2.
(1)
Additional hydrogen is obtained by the subsequent reaction of CO with steam in the water–gas shift reaction (Reaction 2): CO H2O ↔ CO2 H2.
(2)
The overall steam methane reforming (SMR) process (Reaction 3) yields 4 mol H2
750
and 1 mol CO2 for each mole of CH4 consumed: CH4 2H2O ↔ CO2 4H2.
(3)
The H2 product composition prior to purification depends on the shift process used. In a high-temperature shift reactor operating at an inlet temperature of 350°C, a typical product composition (dry basis, vol%) is 73.9% H 2 , 17.7% CO 2 , 6.9% CH4, and 1.0% CO.2 A second shift process involving a lower-temperature (190–210°C) shift reactor is often used, with a resulting product composition of 74.1% H 2 , 18.5% CO 2 , 6.9% CH4, and 0.1% CO.2 In either case, H2 purification equates to a CO2 removal process. Hydrogen purification in
SMR plants is currently accomplished using an amine-based acid gas scrubber or pressure swing adsorption technology in which a solid adsorbent is cycled between a high-pressure adsorption step and a lowerpressure regeneration (desorption) step.1 Membranes are well suited to bulk CO2 removal and offer a viable alternative to these established technologies. There are various types of membranes that may be considered for H2 purification at SMR plants. Since the product gas is at 450–650°C, only proton-c
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