Removal of VOCs from air by absorption and stripping in hollow fiber devices

Large volumes of exhaust air streams contaminated with volatile organic compounds (VOCs) such as toluene, xylene, acetone etc. are discharged into the atmosphere by various industrial facilities. Common technologies for VOC emission abatement e.g., activated carbon adsorption, absorption in a liquid, incineration and catalytic oxidation, have many strengths as well as considerable limitations. In this study, a regenerative absorption-based removal of VOCs from N{dollar}sb2{dollar} in an inert, nonvolatile, organic liquid flowing in compact hollow fiber devices has been developed. The process eliminates flooding, loading and entrainment encountered in conventional absorption devices.; Contaminated air/N{dollar}sb2{dollar} stream was fed through the tube side of the hollow fiber module; a suitable inert absorbent liquid having a high solubility for the VOCs and extremely low vapor pressure was pumped countercurrently over the outside (shell side) of the fibers. The absorbent liquid was regenerated by vacuum in a separate hollow fiber membrane-based stripper. Two types of hollow fiber membranes were studied: one having microporous wall and the other having a nonporous VOC-permeable coating on the outer surface of a microporous hollow fiber. Criteria for nondispersive operation have been developed for each case. Experiments were conducted for the absorption of acetone, methylene chloride, toluene and methanol from the respective VOC-N{dollar}sb2{dollar} gas mixture using two different inert absorbent liquids, silicone oil and Paratherm{dollar}rmsp{lcub}TM{rcub}.{dollar} Theoretical models have been developed from first principles to simulate the behavior of absorption as well as the combined absorption-stripping process.; Highest mass transfer coefficient was obtained for toluene absorption followed by methylene chloride, acetone and methanol absorption. Studies with multicomponent VOC-N{dollar}sb2{dollar} gas mixtures also showed that percent toluene removal was highest followed by methylene chloride, acetone and methanol. The behaviors of VOC mass transfer coefficients have been illustrated as a function of the gas and liquid flow rates. A comprehensive characterization of different resistances making up the overall resistance in VOC absorption has been carried out to develop a predictive capability and compare two types of fibers. Relative absorption performance between the two types of fiber for a given VOC was dependent on the diffusivity of the VOC in the absorbent liquid used. VOC absorption characteristics were determined and compared for the two absorbents used. The absorbent-filled porous membrane was found to contribute significantly to the total mass transfer resistance. Continuous absorption-stripping experiments employing recycling of the absorbent liquid via regeneration in a hollow fiber stripper were also accomplished. The overall performance of the combined process appears to be controlled by stripping due to the low temperature and lower membrane surface area in the stripper. The difference between only absorption and combined absorption-stripping results was more pronounced for VOC-absorbent system having higher Henry's law constant and diffusivity. Simulated results obtained from the mathematical models agree well with the experimental results for absorption as well as for combined absorption-stripping.