| Volatile organic compounds (VOCs), a major source of air pollution with large amounts emitted, are detrimental to the environment and public health. Development of an economically feasible method to control them has become a high priority. Absorption has been widely used for chemical separation, allowing for the reuse and recycling of pollutants by removing gaseous contaminants; it has been frequently applied in air pollution control.However, the selection of a suitable absorbent is a significant problem. When hydrophobic VOCs, such as benzene, toluene, ethylbenzene, and xylenes (BTEX), are to be removed from industrial exhaust gases, an absorption reactor filled with pure water is not efficient because of the low solubility of hydrophobic VOCs in water. Oils are simple long-chain hydrocarbons, and surfactants, which have a hydrophilic head and a hydrophobic tail, and can be dissolved in water to form micelles. Hydrophilic groups attach to water groups,while hydrophobic tails aggregate in the middle of the micelle. Because hydrophobic VOCs are trapped in the hydrophobic tails, their solubility in the surfactant is much greater than it is in pure water. Microemulsions, which are composed of surfactants and cosurfactants, are soluble in water. The solubilization capacity of organic compounds in the microemulsions is greater than it is in the surfactants. In this paper, the following seven activities are reported and discussed: (1) the appropriate solubilization-absorption microemulsion system was selected by the phase-diagram method; (2) the surface-chemical properties of the solubilization-absorption microemulsion system were investigated; (3) the gas-liquid equilibrium relationship between toluene and absorbent was studied; (4) the mass transfer,kinetics and factors influencing the solubilization-absorption process were analyzed; (5) a mathematical model of the solubilization-absorption mass-transfer reaction was established;(6) a packing-absorption tower was designed, solubilization-absorption tests were performed on it, and the solubilization-absorption process was simulated by Aspen Plus ; (7)desorption of saturated absorbent was researched.(1) Stabilized microemulsions could be formed spontaneously by cation surfactants,cosurfactant and toluene, and it is feasible to use the microemulsion systems for the solubilization-absorption of toluene.(2) Using the phase-diagram microemulsion area as an index,the appropriate microemulsion solubilization system was selected. The results indicated that the quaternary ammonium salt surfactants were superior to other surfactants, and cetyltrimethylammonium bromide (CTAB) was better than the other surfactants. The performances of cosurfactants followed the order: short-carbon-chain quaternary ammonium salts > acids > alcohols. The experimental results showed that cetyltrimethylammonium bromide (TBAB) was the best cosurfactant, that the optimum value of the mass ratio of surfactant to cosurfactant was 1:1,and that the mass ratio of surfactant to cosurfactant was suitable for the Bansal-Shah-O'Connell (BSO) equation for microemulsion systems design. The effect of salt (NaCl) concentration on the system was negligible, and higher temperatures increased the solubilization capacity.(3) Using the drop volume method and the paracetamol-induced hepatotoxicity method, the surface tension and solubilization ability of solubilization-absorption microemulsion systems were investigated, and the critical micelle concentration and solubilization ratios of solubilization systems were calculated. The cosurfactant could reduce the surface tension and the value of critical micell concentration (CMC). When toluene was solubilized, it would participate in the formation of a microemulsion as the oil phase. And the value of the microemulsion CMC was one-fourth that of the surfactant solution. The surfactant solutions have remarkable solubilization ability when their concentrations are above CMC, and the solubilization curve obviously changes at the CMC point; the solubilization of microemulsions was superior to that of surfactant solution and the value of solubilization ratio (SR) was increased. The value of solubilization was higher for the CTAB/TBAB microemulsion, that is, the micelles can solubilize more toluene in the same mass. This is advantageous for the solubilization-absorption of waste gas containing toluene.(4) Based on the departing theory for toluene in surfactant solutions and the curve for solubilization of toluene, it was found that Henry's constants changed as the concentrations changed. The relationship between Henry's constant and the concentration of surfactants was deduced. Fitting the experimental data and the calculated theoretical values, we obtained a very good correlation. This validated the equation of van't Hoff for the relationship between temperature and surfactant concentration.(5) The mass transfer and kinetics for the absorption reaction of toluene in CTAB/TBAB microemulsion systems were studied in a double stirred reactor at 303K. The gas phase stirring speed was 250 revs.·min-1, and the concentrations of CTAB/TBAB microemulsion ranged from 0.44 to 137 mol·m-3. The results indicated that kL=3.754×10-5 m·s-1, and kG = 9.251 ×10-7 mol·m-2·s-1·Pa-1. Almost all of the resistance exists in the gas film, and the liquid film resistance can be ignored. This reaction system belongs to the fast pseudo-first-order reaction regime systems, and k2=4.4149.251×10-7 mol·m-2·s-1 . The maximum relative deviation was 8.7%; therefore, the experimental values of the enhancement factor were in good agreement with the calculated values. With increasing concentration of CTAB/TBAB, the absorption rate of toluene was increased accordingly,especially when the concentration of CTAB exceeded CMC; the volume of absorbent impacted only the absorption capacity, not the absorption rate, at a constant gas-liquid surface area.(6) A simplified and approximate mass-transfer-rate model based on film theory was developed and used to predict the rates of toluene absorption by the CTAB/TBAB microemulsion. The results obtained from the model are in agreement with experimental values. The maximum relative deviation was less than 20% when the model was corrected by the solubilization coefficient.(7) The scrubbing of toluene-containing exhaust gas in a packing tower using microemulsion as absorbent was investigated. The influence of a number of factors (spray flow rate, toluene concentration in the inlet gas, concentration and component of absorbent)on the removal of toluene was studied. The results showed that the influence of these factors on the absorption of toluene with microemulsions was significant. When the CTAB/TBAB microemulsion was used as absorbent, the optimum spray flow rate was 30 L·h-1 (liquid gas ratio was 3:10). As the concentration of toluene increased, the absorption capacity also increased. Addition of cosurfactant improved the removal rate and absorption of toluene. The CTAB/TBAB/toluene/water microemulsion was the optimum system for solubilization-absorption of toluene. The removal of toluene from the exhaust gas reached 80% at low concentration.(8) The absorption of organic waste gases in the packing tower was simulated by Aspen Plus. The greatest relative error of the absorprocess is 6.63%, and the average relative error of it was 1.06%. Through comparing the calculated results with the measured ones, the method proposed is demonstrated to be feasible and accurate.(9) Desorption was carried out by a straight distillation method, and the proportion of desorption was always above 98%. Thus, the method of straight distillation is a feasible recycling technology for saturated absorbent.In conclusion, cation surfactant microemulsions are effective absorbers of toluene and have favorable prospects in air-pollution treatment. Using microemulsions as absorbents can overcome the high cost of traditional absorbents, the volatilization loss of solvent, and the prominent problem of secondary pollution. This research has proposed a feasible treatment of VOCs emissions and has provided a theoretical basis and basic data. |
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