Mass Transfer Studies on Catalyst-Aided CO2 Desorption in a Post Combustion CO2 Capture Plant

Abstract

Two solid acid catalysts of contrasting characteristics were studied on their effects on the liquid-side mass transfer performance for the desorption of CO2 from CO2-loaded 5M monoethanolamine (MEA) solvent. The characteristics of the catalysts were both physical (surface area of catalyst which increases the interfacial area for mass transfer) and chemical (the catalyst acid sites to catalyze the desorption of CO2). Protonated zeolite socony mobile-5 (HZSM-5) representing a higher Brφnsted/Lewis acid site ratio in a catalyst and γ-Al2O3 representing lower Brφnsted/Lewis acid site ratio were used. The experiments were conducted in a 1.07 m height x 0.051m diameter packed column in full cycle operation. The packing structure consisted of a 0.51 m bed height of 6 mm inert marble randomly mixed with a specific mass of catalyst and a 50.8 mm Sulzer LDX structured packing acting as top and bottom support beds. 5M monoethanolamine (MEA) was selected as the baseline solvent for the study and further tested on an optimum 7M MEA + 2M Methyl diethanolamine (MDEA) blend. The effect of several other operating variables on desorption performance was also analyzed. The catalyst physical characteristics in terms of the added interfacial area which enhances desorption was higher for HZSM-5 compared to γ-Al2O3. The mass transfer coefficient increased by an average of 16% and 22% using γ-Al2O3 and HZSM-5 catalyst respectively with the baseline solvent at a CO2 desorption temperature of 85oC. These results are consistent with the ability of HZSM-5 as a proton acceptor to facilitate 𝑀𝐸𝐴𝐶𝑂𝑂− breakdown throughout the loading range of desorption as compared with γ- Al2O3, an amphoteric oxide, which substitutes the role of 𝐻𝐶𝑂3 − to release CO2 with lower energy penalties at leaner CO2 loadings. The results using the blended solvent showed a better performance compared to the baseline. The tertiary solvent, MDEA, introduces other preferential desorption ions that further facilitate 𝑀𝐸𝐴𝐶𝑂𝑂−and 𝐻𝐶𝑂3 − breakdown. This constitutes a 12% and 14.5% increase in the mass transfer coefficient using H-ZSM-5 and γ-Al2O3, respectively compared to the baseline solvent. The percentage increase implies a reduction in the size of the desorber column which further translates to lower capital cost.