Significance of gas velocity change during the transport of CO₂ through hollow fiber membrane contactors

A comprehensive 2D mathematical model was developed for the physical and chemical absorption of CO₂ from natural gas containing high percentage of CO₂. Unlike previous mathematical models, the model considers change in the axial gas velocity as CO₂ is being absorbed from the gas mixture. The model was validated with the experimental data and compared with previous model results for a gas mixture containing 10% CO_2, and then was expanded to account for higher percentages of CO₂. For 10% CO₂, the model predictions showed a slight difference between the previous model and the experimental data for the physical absorption of CO₂. However, there was a much improved agreement between the model predictions and the experimental data for the case of chemical absorption using 0.005M MEA. Although this difference might be small for low content of CO₂ in gas mixtures, the model results showed that the decrease in gas velocity becomes significant for higher content of CO₂, particularly if high absorption rate of CO₂ was achieved and thus, maintaining a high percent of removal of CO₂ due to the increase in residence time. This is a major contrast with the previous model behavior where the percent removal of CO₂ continuously decreased with the introduction of more CO₂ in the gas mixture. Furthermore, the model results showed that the effect of bulk flow contribution for gas mixtures containing high content of CO₂ is insignificant for systems where the gas phase mass transfer resistance is small when compared to that of the liquid phase.