Modeling and Experimental Study of Carbon Dioxide Absorption in a Membrane Contactor
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Membrane gas absorption is a new way of contacting gas and liquid for industrial scale gas purification and offers significant advantages compared to conventional absorption towers. Due to the separation of the phases by a microporous membrane the contactor may be operated without limitations caused by flooding, foaming, channeling and liquid entrainment. Very compact hollow fiber membrane units can be made resulting in significant savings in weight and space required. This dissertation deals with membrane gas absorption in the application of CO2 removal by aqueous alkanolamines, using microporous PTFE hollow fiber membranes. A new lab-scale apparatus was constructed and an extensive experimental study executed to determine the performance of the membrane gas absorber, with aqueous solutions of monoethanolamine (MEA) and methyldiethanolamine (MDEA) as absorbents. The important operation parameters CO2 partial pressure, gas velocity, liquid velocity, temperature and liquid CO2 loading were systematically varied within the range typically experienced in a process for exhaust gas CO2-removal. The results clearly show the change in the absorption rate and the overall mass transfer coefficient related to each of the variables. An important conclusion from the experimental study is that the contribution from the gas phase in the overall mass transfer resistance is negligible for the conditions studied. Membrane mass transfer resistance corresponds to less than 12% of the total, leaving the liquid side as the totally dominating resistance term. It is found that the liquid side mass transfer is limited by component diffusivities except at low partial pressures, where the chemical reaction may be rate-limiting. A comprehensive model for the simulation of the membrane gas absorber was developed. The model explicitly accounts for the rates of mass transfer through the membrane, diffusion and chemical reaction in the liquid phase and the corresponding heat transfer model. The important effect of radial viscosity gradients on the liquid diffusivities was also included. An equilibrium model was developed to calculate liquid speciation and equilibrium partial pressures in the chemical systems CO2/MEA/water and CO2/MDEA/water. The membrane gas absorber model calculates temperature profiles and concentration profiles of all components through the length of a single membrane tube. The total absorption rate in a membrane module is calculated from a mass balance of the gas and the liquid phase. It was observed that the diffusional transport of chemically bound CO2 and other ionic reaction products is an important rate limiting step. This lead to the requirement of new correlations for these component diffusivities, developed from parameter regression on selected experiments. Model predictions of absorption rates and the effects of individual variables agree well with experimental data, with maximum deviations within 15 %. In the range of operation for an industrial contactor with CO2 absorbing in aqueous MEA, the average model deviation is 2.8%. The possibility of utilizing a lab-scale membrane gas absorber as a tool in measuring the kinetics of CO2-alkanolamine reactions is discussed. It has been shown that the sensitivity to reaction kinetics can be significantly improved by reducing the contact time beyond what is possible in the present experimental set-up. This may be achieved in a membrane module with 1-5 cm tube length and a high number of tubes so that absorption fluxes can still be measured with a high level of accuracy. To verify this procedure, experiments were performed in a range with a reasonably good sensitivity to reaction kinetics in the MDEAsystem. The second order rate constant of the CO2-MDEA reaction was regressed from the experimental data resulting in an Arrhenius expression comparable to literature values.