Effect of Liquid Viscosity on the Performance of Non-Porous Membrane Contactor for CO2 Capture

Abstract

CO2 capture and storage has been proposed as the main option for reduction of CO2 emissions in the short term, and one promising technology is the membrane contactor. Amine based solvents have proved to be effective absorbents of CO2, and so called 3rd generation absorbents stand out due to their reduced energy requirement in the amine regeneration step. The main disadvantage is that they are viscous fluids, and the effect of the viscosity has appeared to be an issue to the overall performance of the capture system due to the decrease of the CO2 diffusion coefficient. This thesis investigates the effect of viscosity on the performance of a membrane contactor. Aqueous solutions of two types of absorbents, monoethanolamine (MEA) and NaOH, have been made with various viscosity and tested in a membrane contactor setup, where sucrose has been used as a viscosifier. A composite membrane made of a thin layer of Teflon AF2400 coated on porous polypropylene has been used in the membrane contactor. The solutions have been characterized in terms of viscosity, surface tension and contact angle towards the membrane, as well as their change in properties when loaded with CO2. The results of increasing the viscosity of the solutions have shown a significant reduction of the overall mass transfer coefficient, and this effect is enhanced at higher CO2 concentrations in the gas stream. This is believed to be due to the lower diffusion coefficient of CO2 in solution, and local saturation of absorbent at the membrane/liquid interface. Comparison of membrane contactor experiments and gas permeation experiments also indicated that the effect of liquid on the overall mass transfer only became notable at higher viscosities. The experimental results have also been compared to a resistance in series model, finding that it underestimated the effect of viscosity.