Experimental Investigation of CO2 Transport Through Combined Membrane Absorber and Regenerator
Abstract
Greenhouse gases such as carbon dioxide have been known to contribute significantly to global warming, which in turn has resulted in serious global environmental problems. Carbon dioxide is the main gaseous contaminant in the atmosphere, representing about 80% of greenhouse gases. It is reported that half of the CO2 emissions are produced by industry and power plants using fossil fuels such as coal-combustion power generators. These emissions create the need for low energy-consumption, and efficient technologies for the capture and removal of CO2 from gas mixtures produced by industrial sources.
Conventional gas absorption processes for the removal of CO2 including chemical absorption by alkanolamine solutions suffer from many drawbacks such as flooding, foaming, entraining, channeling, and high capital and operating costs. The effort of this research is to work on the possibilities of enhancing the efficiency of these processes to reduce the effect of their drawbacks by using Hollow fiber membrane Contactor (HFMC) as a new gas separation process.
In this study several membrane contactors such as homemade Polyvinylidenefluoride (PVDF), commercial Polytetrafluoroethylene (PTFE) and Perfluoroalkoxy alkane (PFA) were individually fabricated as an absorption process, the gas mixture of CO2/N2 flowed on one side of a hydrophobic microporous membrane while several liquid absorbent, such as Monoethanolamine (MEA), Diethanolamine (DEA) and Sodium Hydroxide (NaOH) flowed on the other side of membrane for comparison purpose. The CO2 gaseous contaminant diffused from the gas phase to the membrane gas-liquid interface and is absorbed in the liquid.
The Result revealed that homemade PVDF has the highest removal rate and PFA has the lowest removal efficiency, in addition, although the removal performance by NaOH gave better removal efficiency, by contrast, it suffered from poor regeneration, therefore, DEA became more favorable in overall performance because of its higher regeneration rate. The effects of operation parameters such as gas and liquid flow rates and packing ratio on performance of CO2 removal were analyzed. The results reveal that, regardless of the type of the membrane module used and liquid solvent, increase in liquid flow rate and packing ratio and a decrease in gas flow rate, give the best system performance in the absorption process.
The rich solution may be sent to another membrane contactor for stripping to remove the absorbed gases and regenerate the solvent. In the stripping unit the operating parameters such as temperature, gas flow rate and liquid follow rate were examined to investigate their effect on the stripping performance. Results determined that temperature has the focal effect on stripping performance regardless of the type of the solvent, increase in temperature increases stripping efficiency. In addition, higher stripping performance was found to be at high solvent liquid flow rate, low sweep gas flow rate. Using a suitable membrane configuration could be considered as a way to prevent wetting.
The generated lean solution is then recycled to the absorption unit and the CO2 transport in combined absorber and stripper units were evaluated by time. Various membrane modules using several aqueous amine solutions such as MEA, DEA and NaOH at different heat of regeneration were examined to investigate their impact on membrane wetting and overall performance. Results revealed that DEA shows the optimum performance at high heat of regeneration. A mathematical model was applied to predict the CO2 removal in gas liquid membrane contactor. Model results were in good agreement of the experimental data.