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Removal and Destruction of Benzene, Toluene, and Xylene from Wastewater by Air Stripping and Catalytic Oxidation Karl T. Chuang,* Shan Cheng, and Shimin Tong Department of Chemical Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6
Catalytic oxidation is an attractive way to destroy trace quantities of benzene, toluene, and xylene (BTX) from wastewater stripping emissions, but there are no data available on the air stripping and oxidation process of BTX. In the work reported here, oxidation of BTX was studied on a hydrophobic catalyst at temperatures ranging from 90 to 150 "C. The catalyst was characterized by ita unique hydrophobic property which facilitates the conversion of BTX with high activity at relatively low temperatures unaffected by water vapor concentrations in the system. An approach based on the Mars-van Krevelen rate mechanism was used to explain the results, revealing a high ratio of surface reduction rate constant to the surface reoxidation rate constant on the hydrophobic catalyst. The air stripping of BTX from wastewater was simultaneously studied in a small pilot-scale column to determine the condition at which low levels of BTX could be effectively stripped from water.
Introduction The complete catalytic oxidation of volatile organic compounds has received much attention in connection with the purification of contaminated water. Studies have suggested the use of a two-step process, combining a stripping column with a catalytic reactor, to tackle the problem (Kosusko et al., 1988). As an alternate to this idea, our previous investigation into the purification of industrial wastewaters containing methanol demonstrated the direct destruction of methanol in wastewater within a single trickle-bed reactor packed with a hydrophobic catalyst operated at low temperature (60-80 "C) near atmospheric pressure (Cheng and Chuang, 1992). A mathematical model was developed to describe the trickle-bed reactor behavior and predict its performance. However, this process, though good for methanol removal, may not be suitable for the elimination of BTX (benzene, toluene, and xylene) from industrial wastewater since the chemically more stable BTX compounds are more difficult to oxidize than methanol (Anderson et al., 1961). In our previous experiment, it was found that the methanol was stripped from water with difficulty due to ita strong molecular interaction with the water molecule. For this reason, a highly active hydrophobic catalyst was developed and used to conduct the direct destruction of methanol in water in a single trickle-bed reactor column, avoiding the separate stripping step. As expected, this hydrophobic catalyst could not effectively oxidize the more stable BTX compounds in the gas phase at temperatures as low as those in the case of methanol oxidation. A 95% conversion of BTX was obtained in our experiment, however, at 130 OC which is the lowest temperature we have found in the available literature. Gangwal et al. (1988) have studied gas-phase catalytic deep oxidation of benzene and n-hexane over a 0.1% Pt, 3% Ni/r-Al@, catalyst at temperatures ranging from 160 to 360 OC. They achieved a 100% conversion of benzene at temperatures higher than 230 "C. They also found that the oxidation of n-hexane was significantly inhibited by the presence of benzene whereas the oxidation of benzene was not affected by the presence of n-hexane. In another study of benzene oxidation, by Vassileva et al. (1989) over a r-A1203 supported catalyst with 30% V205and 0.5% Pd, it was necessary to raise the reaction temperature to 400 "C to achieve a 100% conversion. For the oxidation of xylene, a temperature higher than 250 "C was required according to the resulta from Evaldsson et al. (19891, who used a supported platinum catalyst.
It remains a challenging research topic to discover an appropriate catalyst by which the BTX compounds can be effectively converted into harmless C02and H20 at low temperatures. One of the objectives of our work was to study the kinetic behavior of the hydrophobic catalyst for deep oxidation of BTX in order to cast light on further development of a more active deep oxidation catalyst effective at lower temperatures. The temperature of wastewater is usually far below that required for effective catalytic destruction. Contaminated groundwater, for example, is usually about 5-25 OC whereas our hydrophobic catalyst requires temperatures of 130 "C or higher, which places a heavy energy penalty on the single reactor approach we used for the destruction of methanol. We therefore adopted the more common two-stage method whereby air stripping is followed by catalytic oxidation in the gas phase. The heating of the cold, water-laden air stripping effluent stream still represents a considerable operating cost. The energy required to heat this stream depends upon the amount of air needed in the stripping operation. The air volumes required are dependent upon the stripping efficiency of the columns which in turn is dependent upon both the gas/liquid (G/L) ratio and the water temperature in the column. For example, Lamarre et al. (1983) showed that the removal efficiency of methyl ethyl ketone (MEK) increased dramatically with increased temperature (43% removal at 54 O F and 99% removal at 136 OF). Kittikul et al. (1990) showed that higher air temperatures could also increase the removal efficiency of toluene from water (76.3% at 5 OC and 85.2% at 25 "C). A second objective of this study, therefore, was to establish a basic understanding of air stripping efficiency for benzene, toluene, and xylene with respect to G/L ratios and temperatures and to obtain data for system designs and modeling which will be the subject of future work. Two hypothetical systems for the catalytic destruction of air stripping effluents from contaminated water are pictured in Figure 1. In both systems, the effluent stripping air has to be heated to the temperature at which the effective catalytic deep oxidation can be carried out. As shown in Figure 1, the comparatively simple nonrecycle system loses the heat energy of the effluent hot air stream from the catalytic reactor to the vent. By recycling the effluent hot gas to the stripping column, shown in Figure 1, the stripping column temperature can be elevated, which will reduce the G/L ratio thereby reducing the energy burden of heating the cold air Stripping effluent stream.
0888-5885192/2631-2466%03.OOIO 63 1992 American Chemical Societv
Ind. Eng. Chem. Res., Vol. 31, No. 11, 1992 2467 Fresh Air
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