| With the rapid development of both the society and economy, the increasing type and concentration of alkanes in air have posed severe effects on human health and the environment. There are various techniques for the removal of organic waste gases, such as condensation, absorption, membrane separation, adsorption and catalytic combustion. Among these methods, adsorption is widely used due to its easy operation, high purification efficiency, low energy requirement as well as non-secondary pollution. However, traditional fixed bed adsorbers filled with granular adsorbents usually have high mass transfer resistance, low mass transfer rate and bed utilization. Therefore, how to design and prepare a novel adsorbing material that can reduce mass transfer resistance and enhance adsorption rate for treating alkane pollutants in air have been recognized as a cutting-edge research topic. In this paper, the fabrication and characterization of microfibrous zeolite membrane 5A/PSSF and microfibrous entrapped activated carbon(MEAC) composites were investigated. Adsoption dynamics and mass teansfer mechanism of methane and ethane in granular fixed beds and structured fixed beds based on microfibrous composites were analyzed and discussed.Firstly, the preparation process of microfibrous zeolite membrane 5A/PSSF was studied. The PSSF support was fabricated by wet lay-up papermaking process and sintering process. Zeolite membrane 5A/PSSF was synthesized via in situ hydrothermal synthesis. The properties of zeolite membrane were characterized by XRD, SEM and N2 adsorption/desorption. Meanwhile, single component adsorption dynamics of ethane in 5A zeolite fixed bed was investigated by exploring the effects of volume flow rate, bed height and bed structure on adsorption breakthrough curves. The LUB theory and Yoon-Nelson model were used to analyze and fit breakthrough curves of ethane. The results showed that a continuous and uniform 5A zeolite membrane was successfully synthesized on the surface of PSSF support. The thickness of the zeolite membrane was about 1.61 μm, and the specific surface area of the composite was 118 m2/g. The breakthrough time of ethane in granular fixed bed decreased with increasing flow rate or decreasing bed height. With the increasing filling proportion of zeolite membrane in the structured fixed bed, breakthrough time and the LUB value decreased, while the bed utilization and adsorption rate increased. The adsorption rate constant of ethane in the structured fixed bed(3.5 cm 5A+1.5 cm 5A/PSSF) increased 63% compared with that in the granular fixed bed under the same conditions. Yoon-Nelson model was valid for studying the adsorption dynamics of ethane in 5A zeolite fixed bed.Secondly, the preparation process of microfibrous entrapped activated carbon composite was investigated. Microfibrous entrapped activated carbon composite with three-dimentional network structure was prepared by wet lay-up papermaking process and sintering process. The properties of MEAC were characterized by SEM and N2 adsorption/desorption. Besides, adsorption performance of activated carbon and MEAC composite were also evaluated by investigating the effects of different operating conditions(volume flow rate, bed height and bed structure) on the breakthrough curves of ethane. LUB values and bed utilization were obtained by the LUB theory, and Yoon-Nelson model was used to fit the breakthrough curves by linear regression analysis. The results showed that the specific surface area of MEAC was 457 m2/g, and the proportion of entrapped activated carbon was 64.9%. The specific surface areas of activated carbons before and after entrapment were 712 m2/g and 786 m2/g, respectively, and the pore distribution of the two activated carbons were basically the same. The breakthrough time of ethane was prolonged with decreasing flow rate or increasing bed height in traditional activated carbon fixed bed. MEAC composite, like 5A/PSSF, can obviously improve contact efficiency and enhance mass transfer. The adsorption rate constant of ethane constantly increased as the loading ratio of MEAC composite in the structured fixed bed increased. The LUB value of the structured fixed bed(3 cm AC+2 cm MEAC) decreased 8% compared with that of granular fixed bed, and the bed utilization and adsorption rate increased 2% and 56%, respectively. Single component adsorption dynamics of ethane in activated carbon fixed bed could be well described by Yoon-Nelson model.Finally, binary component adsorption dynamics of methane and ethane in activated carbon fixed bed under different conditions was investigated. The result showed that methane appeared earlier at the outlet of fixed bed than ethane, indicating adsorption capacity of activated carbon for ethane was bigger than that for methane. There is no overshoot phenomenon for the adsorption of methane and ethane, suggesting no competitive adsorption existed between methane and ethane under the experimental conditions. When operating conditions changed, breakthrough curves of methane and ethane showed the same tendency as that of ethane during single component adsorption. With the increase of MEAC loading, the steepness of breakthrough curves was more upright, and the adsorption rate constant increased, while saturated time of fixed bed was shortened. LUB values for methane adsorption were bigger than those for ethane, but the bed utilization for methane adsorption was lower than that for ethane. Compared with granular fixed bed for methane adsorption, the LUB value of the structured fixed bed(11 cm AC+4 cm MEAC) decreased 11%, while bed utilization and adsorption rate constant increased 12% and 54%, respectively. And for ethane adsorption, the LUB value decreased 36%, while the bed utilization and adsorption rate constant increased 9% and 80%, respectively. Moreover, the length of unused bed for ethane adsorption constantly decreased and the bed utilization increased when the filling proportion of MEAC increased. The results predicted by Yoon-Nelson model were in very good agreement with the experimental breakthrough curves of the mixture. |
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