Effect of ZIF-8

“The effect of ZIF-8 loading amount on the CO2 removal efficiency is shown in Fig. 11. It is observed that with increasing ZIF-8 concentration, CO2 removal first increases and then decreases after the loading amount was more than 0.4 wt.%. However, such CO2 removal increase is marginal and therefore any further increase in loading is not beneficial.

Fig 11

Fig. 11. The effect of ZIF-8 loading amount on the CO2 removal efficiency of the MDEA + KLys + ZIF-8

The highest CO2 removal efficiency was achieved for 0.4 wt.% of ZIF-8. Although, increasing the loading amount theoretically increases CO2 removal, but it can lead to particle agglomeration which can block the pores of the membrane, resulting in a decrease in the gas-liquid interface area and lower removal rate. In addition, the ZIF-8 loading affects the stability of the nano-absorbent, which can reduce the absorption efficiency of the nano-absorbent (Yu et al., 2019).

The CO2 removal efficiency of MDEA + KLys absorbent and MDEA + KLys + ZIF-8 nano-absorbent along the module is shown in Fig. 12. In general, the CO2 removal efficiency increases with increasing the fiber length, which could be due to an increase in the contact time and area of the gas and liquid phase.

Fig 12

Fig. 12. CO2 removal efficiency of versus module length for MDEA + KLys and MDEA + KLys + ZIF-8 solutions (gas flow = 6L/hr, solvent flow rate =1.5L/hr,Tgas=298K,∅=0.4wt.%,Cs=30wt%)

The CO2 removal efficiency of MDEA + KLys is larger than that of MDEA + KLys + ZIF-8 at the beginning of the module (< 0.2 m), while it gets higher for the rest of the reactor, where it reached to ∼99% for MDEA + KLys + ZIF-8. This increase in CO2 removal efficiency could be attributed to several reasons, including enhancement of mass transfer coefficient, additional reactions of CO2 in the presence of ZIF-8 and zwitterion mechanism. There are several mechanisms that can be considered for the improvement in the mass transfer coefficient, which include hydrodynamic and bubble breaking effects in the presence of nanomaterials (Yu et al., 2019).

In order to further investigate the separation performance of the MDEA + KLys + ZIF-8 nano-absorbent, its CO2 removal efficiency for different gas flow rates were compared with other nano-absorbents and shown in Fig. 13. It is seen that increasing the gas flow rate resulted in the reduction of the residence time of CO2 in the membrane contactor. Furthermore, increasing the gas flow rate also resulted in the reduction of the amount of CO2 molecules that could permeate through the membrane. The MDEA + KLys + ZIF-8 nano-absorbent exhibited higher CO2 removal efficiency compared to other nano-absorbents.

Fig 13

Fig. 13. Effect of gas flow rate on the efficiency of CO2 removal for MDEA + KLys + ZIF-8

(solvent flow rate = 1.5 L/hr, Tgas = 298 K, C0 = 1400 ppm,Cs=30wt%)

Additionally, Fig. 13 reveals that the CO2 removal efficiency of DW-CNT and DW-SiO2 nano-absorbents was almost constant at high gas flow rates, which is due to negligible mass transfer resistance in the gas phase. The CO2 removal of MDEA + KLys + ZIF-8 continues to be higher than the other nano-absorbents at high gas flow rates.

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