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Absorption of VOCs by Solubilization with Nonionic Surfactants and Microemulsions

Author: LiuLian
Tutor: TianSenLin
School: Kunming University of Science and Technology
Course: Environmental Engineering
Keywords: Microemulsion Surfactant Solubilization Absorption Volatile organic compounds Toluene
CLC: X701
Type: Master's thesis
Year: 2009
Downloads: 123
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Abstract

Atmospheric emission of volatile organic compounds (VOCs) from industrial facilities causes serious environmental problems and financial losses. The VOC emissions have been subjected to more and more stringent legislation. Absorption is one of the common air pollution recovery techniques with high removal efficiency, widely application and maturity process. This separation operation can be in the elimination of gas pollutants while recycling. But for hydrophobic components such as toluene, their poor solubility in water decreased their absorption efficiency. Therefore, absorbent was modified or improved to enhance toluene solubility. VOCs i.e. toluene can be solubilized into microemulsion formulated with special surfactant and cosurfactant spontaneously as the oil phase. The microemulsions were used to absorb toluene from VOCs-containing gas stream by formation of microemulsion, thus the apparent solubility of aqueous solution was increased. In this paper, the five parts were discussed as following:①screening of absorption systems from the phase diagrams;②the gas-liquid equilibrium relationship between toluene and absorbent;③the mass transfer, kinetics and influencing factors of absorption process;④scrubbing of toluene containing exhaust gas in a packed tower using microemulsion as absorbent;⑤desorption of saturated absorbent. The main results obtained in this study are as the follows:(1) It is feasible to use the nonionic surfactant/toluene/water microemulsion systems for solubilization absorption of toluene.(2) The formation of microemulsions could be stabilized by the nonionic surfactants and toluene. The results indicated that Tween-60 was superior to other Tween surfactants, and TX-100 was better than the other TX surfactants with cosurfactant free; the optimum value of Hydrophile-Lipophile Balance (HLB) was 15. The performances of cosurfactants followed the order:n-butylamine> alcohol> butyric acid. It was found that an affirmative effect of the shorter length of carbon chain for alcohol, the cosurfactant with linear chains are better than those with the branched chains, and the carbon number of cosurfactants must less than 6, otherwise it can not form into microemulsion. The experiment results showed that HLB value was more suitable than Bansal, Shah, O’Connell (BSO) equation for microemulsion systems. The effect of salt (NaCl) concentration on the system is inconspicuous and the lower temperature favor enhancing the solubilization capacity.(3) The surfactant solutions have remarkable solubilization ability when the concentration of surfactants above critical micelle concentration (CMC), and the solubilization curve obviously change at CMC point; the solubilization of microemulsions was superior to surfactant solutions, and the value of WSR was increased. The value of CMC was low of Tween series, that is, the micelles can be formed at very low concentration of surfactant solution. It is propitious to solubilization absorption of waste gas containing toluene.(4) The mass transfer and kinetics for the absorption of toluene in Tween-20 microemulsion systems have been studied in a double stirred reactor at 30℃, the liquid phase stirring speed was 120 r/min, and the concentrations of Tween-20 solutions ranged from 8.157 to 40.783 mol/m3. The result indicated thatκL=9.183×10-5 m/s, kG=7.502×10-7 mol/(m2·s), the vast majority of resistance exists in liquid film, the gas film resistance can be ignored; The absorption of toluene appears as a fast pseudo-first-order reaction with respect to Tween-20, and k 2=0.00122 m3/(mol·s). The maximum relative deviation was 12.904%, therefore, experimental values were in a good agreement with the values from the calculated. With the increase of the concentration of Tween-20 the absorption rate of toluene was enlarged accordingly, especially when the concentration of Tween-20 exceed CMC value; the volume of absorbent only impact absorption capacity instead of absorption rate with constant gas-liquid surface area; Tween-20 solution has a significant effect on the kinetics at the various concentrations, especially at low temperatures and adding cosurfactants.(5) The scrubbing of toluene containing exhaust gas in a packed tower using microemulsion as absorbent carried out. The influence factors (spray flow rate, toluene concentration in the inlet gas, the concentration and component of absorbent) on the removal of toluene was discussed. The results showed that absorption treatment of toluene with microemulsion has significant effect; Tween-20 was used as surfactant, the optimum spray flow rate was 40 L/h (liquid gas ratio was 1:5), and with the concentration of toluene increased, the removal rate enhanced respectively; added cosurfactant can improve the removal rate and absorbed of toluene, Tween-20/n-butylamine/toluene/water microemulsion was the optimum system for solubilization absorption of toluene. The removal of toluene in the exhaust gas can achieve 65% with low concentration.(6) Desorption was carried out in a straight distillation method, and the proportion of desorption were all above 98%. Thus, the method of straight distillation is one of the feasible recycle technologies for saturation absorbent.Therefore, nonionic surfactant microemulsions have a significant absorption enhancement for toluene, which have great prospects in air pollution treatment. Using microemulsion as absorbent has overcomed the high cost of traditional absorbent, the volatilization loss of solvent, and the prominent problem of secondary pollution. This research has proposed a feasible treatment of VOCs emission.

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