(396j) An Experimental and Modeling Study of Membrane for Syngas Purification

As the awareness on global warming is growing, electricity generation from the Integrated Gasification Combined Cycle (IGCC) power plants seems to be a favorable alternative for power generation with simultaneous carbon capture (CO₂).  In this case, CO₂ needs to be separated from syngas for the H₂ purification, where the purified syngas is the feed to the Gas Turbine (GT) for power generation.  For the CO₂ capture in these power plants, conventional absorption process using Selexol as the solvent has been considered by DOE/NETL as the baseline technology.  Recent data published in the DOE baseline document suggests that incorporating this technology for the CO₂ capture and sequestration in the IGCC power plants would result in an increase in the Cost of Electricity (COE) by 37%.  Syngas from the low temperature Water Gas Shifter (WGS) reactor is at pressures close to 50 bar and at temperatures close to 235 ^oC.

This work describes the transport properties of our CO₂-selective membranes under conditions (pressure from 2 bar to 15 bar and temperature 107 ^oC) and the potential it has for operating at higher pressures (even higher than 20 bar) for CO₂ capture from syngas and apply it to the process that is integrated with the IGCC power plants.  These CO₂-selective facilitated transport membranes are composed of amine carriers dispersed in the crosslinked polyvinylalcohol (PVA) matrix.  CO₂ permeability and CO₂/H₂ selectivity at different pressures investigated are discussed.  These membranes have been used for the H₂purification in a project funded by the Office of Naval Research.

This study deals with the modeling and cost study of a promising syngas purification process for IGCC plants with the CO₂capture using the membranes mentioned above.  A two-stage process has been considered for this application.  For both the stages, the effects of operating parameters like the feed pressures and temperatures on the overall economics were analyzed to obtain an optimal condition of operation.  We have also studied the effects of membrane performance, selectivity and permeance on the overall increase in the COE and the capture cost.

A CO₂ permeability of 1287 Barrers and a CO₂/H₂ ideal selectivity of 87 at about 15 bar pressure were obtained from our group.  From a study in our group on the transport properties at lower pressures (2 bar), a CO₂ permeability of 5000 Barrers and CO₂/H₂ ideal selectivity of 250 was obtained.  Based on the transport results, our cost calculations show an increase in the COE close to 15% and a capture cost about $17/tonne of CO₂ removed for the two-stage membrane based process for achieving about 90% CO₂ capture (>95% purity) and greater than 99% H₂ recovery.  This process operates at pressures close to 23 and 2 bar on the feed side and the sweep side of the Stage 1, respectively.  The membrane with a CO₂ permeance of 162 GPU, CO₂/H₂ ideal selectivity of 49 and with a thickness of 5 microns was considered.  The transport properties of the membrane at 23 bar have been estimated from a study on the membrane performance-property relationship, and they will be verified experimentally.  The feed pressure on the Stage 2 is close to 3 bar and the sweep side at atmospheric pressure.  The Stage 2 membrane has a thickness of 5 microns with a CO₂ permeance of 1000 GPU and a CO₂/H₂ selectivity of 250.  This study shows the potential our membrane based technology can have for the IGCC application.