| dc.description.abstract | Volatile organic compounds (VOCs) are the foremost hazardous pollutants in the air. VOC emissions have significant effects on the environment and human health. Membrane separation, as an excellent technology for VOC removal and recovery, has advantages in low energy consumption, compact size, and easy operation. The membranes with efficient separation performance are required for the membrane separation process. This study focuses on developing the oleophilic membrane on VOC/N2 separation. 15 VOCs such as MTBE, acetone, DMC, methanol, ethanol, propanol, butanol, pentane, hexane, cyclohexane, isooctane, heptane, benzene, toluene, and xylene were selected for membrane performance evaluation.
The oleo gel membrane was developed by immobilizing bis (2-ethylhexyl) phthalate (DEHP) oil in the PEBA matrix. Multiple PEBA/DEHP oleo gel membranes were prepared, and the content of DEHP in the membranes varied from 15 to 75wt.%. The properties of the prepared PEBA/DEHP oleo gel membrane were comprehensively investigated. The physical properties, structures, and morphologies of the PEBA/DEHP membranes were characterized by Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimeter (DSC), thermogravimetric analysis (TGA), and scanning electron microscope (SEM). The gas permeation properties of the prepared PEBA/DEHP membranes were also investigated. DEHP content in the membranes was demonstrated to have significant effects on their VOC permeabilities and VOC/N2 selectivities, and the prepared membranes showed significantly higher VOC permeabilities than the traditional PEBA membrane in binary VOC/N2 separation test. The effects of feed VOC concentration and operating temperature on the VOC/N2 separation performance of the PEBA/DEHP membranes were evaluated with selected VOCs. The membrane showed excellent VOC/N2 separation performance under all conditions with good stability.
Furthermore, the bis (2-ethylhexyl) adipate (DEHA) oil, of which the VOC solubility is higher than that of DEHP oil, was adopted to improve the VOC permeability of the oleo gel membrane. Newly developed PEBA/DEHA membrane showed drastic VOC/N2 performance improvement compared to the PEBA membrane and previously reported PEBA/DEHP oleo gel membranes. The VOC permeability in the membrane was found to follow the miscible blending rules, which enable the acquirement of VOC permeability of the oil by calculation. The effects of feed VOC concentration and operating temperature on membrane VOC/N2 separation performance were comprehensively investigated on various VOCs. Semi-empirical correlations were developed to relate the membrane VOC permeability to operation parameters, which include feed VOC composition and operating pressures. PEBA/DEHA oleo gel membrane showed excellent VOC permeability and VOC/N2 selectivity under all testing conditions. And the effect of the operation condition of the high VOC concentration and high temperature on the VOC/N2 separation performance of PEBA/DEHA oleo gel membrane was less than the pristine PEBA membrane. The membrane was stable during the 35 days test under various conditions.
Another new oleo gel membrane (OLGM) was developed by gelling the DEHA oil in the crosslinked poly dimethyl siloxane (PDMS) matrix. A series of dense membranes were successfully fabricated. PDMS, as the "oil holder", differs from the PEBA in its adjustable and manipulated crosslinking degree. The effects of the crosslinked degree of the PDMS on the content of the oil immobilized in the membrane and membrane VOC/N2 separation performance were investigated. The VOC permeability of the PDMS/DEHA membrane was further improved than the PEBA/DEHA membrane, which is attributed to the better VOC permeabilities in the PDMS than the PEBA. The systematic assessments of membrane separation performance on VOC/N2 were performed with respect to temperature and feed VOC concentration. The VOC permeation in the membrane was proved to follow the sorption-diffusion mechanism. The membrane showed significantly higher VOC permeability and VOC/N2 selectivity than the traditional PDMS membrane and the other previously reported OLGMs.
In the end, two types of the supported liquid membrane were developed in this work for VOC/N2 separation. One is the silicone oil (SO) based supported liquid membrane, which was made by using SO to wet the surface of the Polysulfone (PS) membrane. The as-prepared membranes showed good stability when using pure gas as the feed at elevated pressure. And its VOC/N2 selectivity was higher compared to silicone rubber membranes for VOCs selected from the alcohol group. However, the membrane was unstable in certain VOCs (e.g., DMC, benzene, hexane), which could be attributed to the instability of PS in specific VOCs, and the instability of PS further weakens the immobilization of SO in the pores of the PS membrane. The other type of membrane, supported PDMS/SO oleo gel membrane, was further developed to improve the stability of the membrane in a variable VOCs environment. The PDMS/SO oleo gel was reinforced in the PTFE membrane substrate, which was believed to have better chemical resistance than the PS membrane. It should be noted that the intrinsic property of the PDMS/SO oleo gel on VOC/N2 separation performance was investigated by testing the PDMS/SO oleo gel membranes (OLGMs) preprepared at different PDMS/SO weight ratios. The experiment results showed that the membranes had better VOC/N2 perm-selectivity than the pristine PDMS. After that, the supported PDMS/SO oleo gel membrane showed competitive VOC/N2 separation performance to pristine PDMS, and the membrane was demonstrated stable in a long-term test for over 100 hours. | en |
