Effects Of Fiber Wettability On The Coalescence Of Droplets In Diesel And Diesel/Water Separation Performance

Ultra-low sulfur diesel and biodiesel technologies have been rapidly developed and promoted.The testing parameter of ISO 16332:2018 for diesel/water separation have been improved accordingly.According to the most stringent test conditions in the standard,the lowest diesel/water interfacial tension（IFT=11±2 m N/m）,the smallest diameter of water droplets(D₉₀≤30μm),it was difficult for traditional filter media for removing water from diesel to meet the application requirements.The coalescer media prepared by papermaking,the core components of filter media,determines the diesel/water separation performance.To meet the application requirements of effectively removing water droplets from diesel with the lowest IFT and the smallest water droplet size,the following topics were explored:the effect of fiber wettability on the coalescence of water droplets and the diesel/water separation performance of filter media.This providing reference for the design of new filter media for diesel/water separation in theory and application.To observe the process of fibers acting on water droplets at microscale,a coalescence dynamic observation platform was designed and set up based on a microfluidic system and a microscopic high-speed camera.The microfluidic system can produce water droplets with a diameter in the range of 4.3-28.2μm.The microscopic high-speed camera can observe the process of fibers,at least with a length of 4mm,acting on water droplets and the diameter of water droplets downstream of coalescer media during testing the diesel/water separation performance.Therefore,the experimental conditions are provided for exploring the mechanism of filter media acting on the coalescence process of water droplets.Aiming at the effect of fiber wettability on the coalescence of water droplets on fiber,5kinds of fibers for papermaking,cellulose fiber,glass fiber,nylon fiber,polyester（PET）fiber,polypropylene（PP）fiber,were used to investigate the ability of fiber capturing water droplets and the coalescence of water droplets on fiber dipping in diesel with high IFT（47.1 m N/m,no surfactant in diesel）and low IFT（10.7 m N/m）.The results showed that in low IFT diesel,the water wettability of 5 above mentioned fibers and the coalescence frequency of water droplets on fiber declined compared to that in high IFT diesel.The contact angle of water droplet on cellulose fiber and PET fiber increased from 56°,78°to 90°,102°respectively.The coalescence frequency of water droplets on those fibers decreased from 70.4%,24.4%to 0.03%,0.52%respectively.The contact angle of water droplet on glass fiber and nylon fiber increased from79°,87°to 118°,117°respectively.The coalescence frequency of water droplets on those fibers decreased from 54.5%,21.2%to 0 respectively.The contact angle of water droplet on PP fiber was 135°in high IFT diesel and PP fiber can’t capture any droplet in low IFT.This caused by the high coalescence efficiency（100%）and the contact frequency（1.5%～70.4%）of droplets on fiber in high IFT diesel.However,the water wettability of 5 kinds of fiber decreased,leading to the coalescence efficiency（0～35.9%）and the contact frequency（0～1.4%）declined.This contributed to the decrease in coalescence frequency.Thus,PET fiber can be used to prepare the coalescer media so that the diesel/water separation performance can be improved.To reveal the effect of intersection fiber on the coalescence of water droplets,chapter 4prepared the intersection fiber layer（cellulose fiber,glass fiber,nylon fiber,PET fiber）with thickness less than 10 times of fiber diameter.The ability of fiber capturing droplets and the coalescence mechanisms of water droplet were analyzed.The results showed that in high IFT diesel,intersection fiber layer can capture water droplets by sieving,interception by multiple fiber and disturbing by multiple fiber compared to that of single fiber.₈7))of 4 kinds of fiber was 21.1%～22.7%.The water droplets on fiber coalesced by colliding directly,wetting on fiber,moving along the fiber axis.In low IFT diesel,the intersection fiber can prevent the droplet escaping from fiber,resulting in₈7))of 4 kinds of fiber increase to 34.7%～83.3%.The droplets on fiber can coalesce by wetting on fiber,lasting contact.To reveal the action of the woven mesh with uniform pore on the coalescence of water droplets with different diameter,chapter 5 explored the ability of water droplet penetrating the2 kinds of PET mesh with different water wettability.The results showed that 2 kinds of PET mesh can interaction with the droplets with diameter less than the pore size of mesh in both of high and low IFT.In high IFT diesel,₀/（9）of 1#PET mesh and 2#PET mesh was 1.3and 1.6 respectively.In low IFT diesel,₀/（9）of 1#PET mesh and 2#PET mesh was 1.3.This indicated that coalescer media should be designed with pore size near the diameter of water droplets.In high IFT diesel,water droplets on mesh coalesced by droplets contacting with each other.In low IFT diesel,droplets can be captured by synergy between fiber and droplet on fiber.The captured droplets contacted with each other and coalesced in mesh pore.R_int of 1#PET mesh was 80.5%,which was higher than R_int of 2#PET mesh.This indicated that the high water wettability fiber can contribute to the ability of mesh capturing the small droplets.Chapter 6 explores the diesel/water separation performance of coalescer media made mainly by one kind of fiber（cellulose fiber,glass fiber,PET fiber）and various fibers（microglass fiber/PET fiber mixture media,（PET layer/glass fiber layer）double-layer media）.The in-situ observation of the dynamic process of water droplets in coalescer media was carried out.The results show that in low IFT diesel,the coalescence of water droplets in the PET coalescer media and cellulose coalescer media was mainly produced by wetting on the fiber surface,while that in glass filter media is mainly produced by contacting with each other for enough time after being sieved.On the same test conditions,the self-made glass/PET mixture media has the highest diesel/water separation efficiency（81.2%）,which was 11.3%higher than that of others self-made media and 38.7%higher than that of the commercial sample.The pressure drop（0.85 k Pa）of the self-made mixture media was 21.5%of that of the commercial sample（3.96 k Pa）.Therefore,diesel/water separation performance could be improved by combining fine fibers with lower wetting ability and fibers with higher wetting ability.Because the former fiber can improve the capture probability and contact time of water droplets and the latter promoting water droplets coalescing.