OCR Text |
Show while at 300°C, the level was dropping below 1 ppm in less than an hour. The amount of toluene left in the sorbent was 1900 ~g/gm at 20°C, 160 ~g/gm for 140°C, and less than 5 ~g /gm was left at 300°C. Even above the boiling point of toluene, 110°C, toluene remained in the sorbent and the rate of evolution was quite high. Because of these results, and the fact that the rate appeared to level off after a high initial decrease, future experiments will focus on the adsorption of toluene on sorbent as a function of temperature. The rate controlling step might possibly be equilibrium desorption/adsorption which is a function of the temperature; hence, at higher temperatures less toluene remains in the solid for the same gas phase concentration than at lower temperatures. Rotary Kiln Results Experiments in the rotary kiln simulator were conducted using the same commercial sorbent contaminated with industrial-grade toluene. The effects of temperature, charge size, and contaminant concentration are discussed. In all cases, except the contaminant concentration study, the sorbent was contaminated with toluene, 1 % by weight and a 4.5 kg. charge was used. For the concentration study, a 2.25 kg. charge was contaminated to 2% and 1 % by weight. The time period for each experiment was 1.5 hours, after which a solid sample was taken. Charge size, as shown in Figure 9 where mass flow rate of toluene, nonnalized to the initial amount of toluene in the sorbent is plotted versus time, appears to effect the toluene evolution rate because of changes in the bed heat transfer as expected; the 4.5 kg. charge took longer to reach the evolution temperature than did the 2.27 kg. charge, explaining the shift in time between the two curves. The amount of toluene left in the solid after 1.5 hours for both experiments was below 5 ~g/gm. The effect of temperature is shown in Figures 10, 11, 12, and 13. For the lower kiln-gas temperature, 500°C, the nonnalized mass flow rate of toluene, carbon dioxide and oxygen flue gas concentrations and kiln gas temperature, are shown in Figure 10 as a function of time. As shown in these graphs, the mass flow rate was initially high, then dropped and rose again, before finally dropping off to below detection limits . Corresponding to the drop in evolution (at approximately 5 minutes), there was an increase in temperature (40°C) and C02 concentration (2%) and a decrease in 02 concentration (2%), suggesting some combustion of toluene at this time. During the run, initially the envelopes in the kiln browned and after several minutes some lazy flames were seen. At 565°C, data shown in Figure 11, the same trend was present, a 55°C temperature increase, 3% C02 increase, and 3% 02 decrease. In this case, the envelopes immediately ignited; the flames went out, then some lazy flames were seen. For 870°C, the highest temperature, no toluene mass flow was found, as seen in Figure 12. The kiln was filled with flame for several minutes during this run; the flames then subsided and no lazy flame was seen. Again, a temperature increase (28°C), C02 concentration increase (3%), and 02 concentration decrease (3.5%) were evident It appears that at each temperature, some combustion of toluene was taking place in the kiln and the degree of combustion increased with temperature. The toluene could have also been converted to soot or other PIes which may have subsequently been partially combusted; future studies will include complete exhaust hydrocarbon measurements. Linak, Wendt and co:-~orkers (6) have shown that significan~ amounts of soot can be fonned under some condItIOns. Using the C02 concentration data and integrating over time, the fraction of toluene burned was found at each temperature and graphed versus temperature, Figure 13. There appears to be a general trend toward increased fraction burned with increasing |