(89c) Effect of Surface Modification on Filtration Performance of Gas-Liquid Coalescing Filters (Invited)

Great attention has been paid to the purification of liquid particles from exhaust streams by fibrous filters which are mostly wettable. However, it seems that non-wettable filters are more advantageous under certain conditions, with superior gas-liquid filtration performance. Hence, the requirement of modifying the surface properties of present wettable filters is increasing, because the fiber surface characteristics can not only alter shapes of liquid particles on the fiber surface, but also influence capture mechanisms of liquid particles carried by the airflow. Especially, the processes of coalescing, transporting and draining of liquid particles are closely related to the fiber surface properties.

The work aimed at experimentally investigating the effect of surface modification on separation of liquid particles by filters from air streams. Three different methods were applied to modify surface properties of glass microfiber filters, which were commonly in use in industries and were selected as the experimental control, being transferred from wettable to non-wettable. First method was using the silicon dioxide (SiO₂) nanoparticles, with a mean diameter of about 100 nm (greatly smaller than the fiber diameters), and they were physically grafted onto fiber surface. The second method was treating filters with fluoropolymer to reduce the surface energy of fibers. The third treating method combined the two strategies above, viz., nano coating followed by polymer fluorination. Therefore, the fibers possessing the best liquid-repellent performance were obtained. All filters were tested with an employment of a filtration performance experimental setup, aiming to evaluate the pressure drop, efficiency and quality factor. The efficiency was derived from both upstream and downstream particle number concentrations, simultaneously utilizing a scanning mobility particle sizer (SMPS) and an aerodynamic particle sizer (APS). The particle sizes taken in this study range from 0.05 to 20 microns.

It was illustrated in the results that the surface modification played an important role in the surface characteristics and filtration performance of fibrous filters. The three methods mentioned above were all able to change the wettability of filters from wettable condition to non-wettable one. The average oil contact angles were 101°, 107°, and 135°, respectively, for these three different kinds of modified filters. It was found that the nanoparticles were uniformly distributed on the fiber surface by microscopic observation. Furthermore, a pronounced increase of equilibrium efficiency was shown from all modified filters, compared to the control. As expected, it was difficult for the modified filters to be wetted by liquid, which resulted in a reduced equilibrium saturation, suggesting a longer operation lifetime for these filters. However, the results also showed that filters modified by the first and third methods presented slightly higher steady-state pressure drop, as the migration and drainage of liquid particles were negatively affected by grafted nanoparticles. On the whole, a filter, first modified by grafting nanoparticles and then treated with fluorination, had the overall best filtration performance in the steady state, with a highest quality factor of 0.84 kPa^-1for the particles measured by the SMPS and of 1.82 kPa^-1 for the particles measured by the APS.