Modelling a Membrane Contactor for CO2 Capture

Chemical absorption of CO2 in conventional absorbers is the most mature and preferred technology today for post-combustion CO2 capture. However, two major bottlenecks of this technology are the large solvent regeneration energy requirement and the large equipment size. Due to their much higher surface area to volume ratio, membrane contactors have been suggested as a promising alternative for absorber size reduction. Furthermore, some novel solvents with low regeneration energy requirement compared to the benchmark solvent (30wt% MEA) have been identified recently. However, at the price of high volatility, which could lead to solvent losses/emissions when applied in the conventional absorbers. The use of membrane contactors has also been suggested to overcome this problem. Several studies have been performed on CO2 absorption using membrane contactors, but most of them were conducted at laboratory-scale. Due to the large volume of flue gases coming from power plants, studies focusing on the modelling and design of large-scale membrane contactors are required. The objective of this thesis has been to develop a comprehensive two-dimensional mathematical model for CO2 absorption in aqueous MEA solution in a hollow fiber membrane contactor (HFMC). The developed HFMC model has been applied for simulation of CO2 absorption from a flue gas of an 800 MWe coal-fired power plant. This was intended to study the opportunities and challenges of the modelling, and design of CO2 absorption in large-scale HFMC modules. The HFMC model is based on mass and heat balance equations for the shell, membrane, and tube sides of the HFMC module. Reversible chemical reactions between CO2 and MEA and the heat of CO2 absorption are considered in the model. The radial variations of the diffusion coefficients of the species in the liquid-phase due to the radial viscosity gradient are also implemented in the model. A rigorous equilibrium model is employed to estimate the equilibrium partial pressure of CO2 and the initial chemical speciation in the CO2-MEA-H2O system. The set of partial differential equations in the HFMC model is developed in the programming language MATLAB and solved using the method of lines/finite difference method. The influence of changing the gas-phase velocity, liquid-phase velocity, solvent lean loading, membrane fiber length, and membrane mass transfer coefficient on the CO2 capture performance of HFMC modules were studied. Results reveal that increasing the gas