Chemical and Biomolecular Engineering - Theses
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ItemThe use of nanoporous carbon membranes for high temperature carbon dioxide separation( 2009)Technology designed to capture and store carbon dioxide (CO2) will play a significant role in the near-term reduction of CO2 emissions and is considered necessary to slow global warming. Nanoporous carbon (NPC) membranes, made from the pyrolysis of a polymer, may be the new generation of gas separation membranes suitable for CO2 capture. The research work presented in this thesis has assessed the suitability of NPC membranes for CO2 capture from natural gas (methane, CH4) and air-blown synthesis gas purification (nitrogen, N2) at high operating temperatures and in the presence of condensable and non-condensable impurities. Three different types of NPC membranes, namely supported, modified-supported and unsupported, have been manufactured. The elimination of cracking within the NPC membranes, which is a significant concern for the commercialisation of these membranes, was most successful for supported NPC membranes made from polyfurfuryl alcohol (PFA). Characterisation results of the produced NPC membranes showed a pore size range suitable for gas separation via molecular sieving (3 - 5A). The characterisation results also indicated a significant increase in porosity with pyrolysis temperature. The membrane performance results supported these findings with a significant increase in permeance observed with increasing pyrolysis temperature Mixed gas performance measurements revealed an increase in the CO2/CH4 selectivity over the pure gas CO2/CH4 permselectivity due to the competitive adsorption of the more polar CO2. Mixed gas performance measurements also showed an increase in CH4 permeance as the operating temperature was increased from 35 to 200oC, which can be related to an increase in the rate of diffusion. However, the selectivity decreased with increasing operating temperature due to the smaller changes in the CO2 permeance. These smaller changes in CO2 permeance are related to the stronger adsorption of this gas to the carbon surface at lower operating temperatures as confirmed by gas adsorption experiments and molecular modelling simulation. The exposure of the NPC membranes to condensable and non-condensable impurities typically found in natural gas and synthesis gas saw a small reduction in permeance and a minimal reduction in selectivity, particularly at elevated operating temperatures. Overall, NPC membranes have good potential as a technology suitable for CO2 capture as they traditionally have a better performance than their polymeric counterparts and can be operated at higher temperatures with only a slight impact on performance in the presence of impurities. However, the challenges of cracking and aging are considerable and have not as yet been fully resolved. The very recent emergence of new membrane technologies that combine the performance of a NPC membrane with the durability of a polymeric membrane are certainly now attracting more attention and may well reveal themselves as the best of both worlds.