OCR Text |
Show react for a specified residence time, r. The PSR (Perfectly Stirred Reactor) code by Glarborg et. al [21] and CHEMKIN [22] were used to perform the calculations. The temperature of the reactor was specified so that we could determine some interesting features such as the residence time required to achieve 99.99% destruction of the chlorinated hydrocarbon. Alternatively, the energy equation can be solved and behavior such as extinction of chlorinated-hydrocarbon/air mixtures can be examined [e.g. 14]. RESULTS AND DISCUSSION Calculations were performed over a wide range of temperature, residence time, equivalence ratio, and pressure to examine the effect of these parameters on destructive efficiency. The chlorinated hydrocarbon chloroethane was considered in this initial investigation because its chemical kinetics are probably the least complicated to treat of the C2 chlorinated hydrocarbons. Regulatory requirements in the United States dictate that 99.99% of the hazardous component be destroyed by incineration [23]. We performed calculations to determine the relationship between residence time and temperature at 99.99% destructive efficiency (DE). Figure 1 shows the residence time and temperature at 99.99 % DE for a stoichiometric mixture of chloroethane-air at 1 atm. To obtain each point, we performed a series of calculations in which the residence time was fixed and the reactor temperature was varied to achieve a DE of 99.99%. It is interesting to note from the plot that a residence time of 1 sec requires a temperature of 1110 K to achieve 99.99% DE. The plot also shows that if one wants to reduce in residence time by a factor of 10 and still have 99.99% DE, the reactor temperature must be raised by about 8sK. It is useful to plot the calculated results in Arrhenius form to determine an overall activation energy (Fig. 2). The activation energy is quite high, about 66 kcal/mole (the temperature dependences of the ignition delay time for most hydrocarbons exhibit an activation energies of around 40 kcal/mole). The effect of equivalence ratio (~) on destructive efficiency (DE) is shown in Fig. 3. The equivalence ratio was defined assuming that the final combustion products are carbon dioxide, water, and hydrogen chloride. As seen from the plot, the equivalence ratio must be near stoichiometric to maximize the destructive efficiency. The destructive efficiency decreases rapidly for rich equivalence ratios (~ > 1) since there is insufficient oxygen to oxidize the chloroethane. Changing the equivalence ratio has the complicating effect of changing both the chloroethane to 02 ratio and the chloroethane to N2 ratio. For example as the equivalence ratio is reduced, the ratio of oxygen to chloroethane is increased and the chloroethane is further diluted by nitrogen. To separate these two effects, we investigated the effect of dilution alone in Fig. 4. Note that a dilution of 0% nitrogen corresponds to a chloroethane-02 mixture. The results show that the destructive efficiency (DE) increases significantly with increasing dilution until the dilution reaches that corresponding to air. It is very interesting to use the model to investigate the effects of pressure which can be difficult to examine experimentally (Fig. 5). For this set of calculations, the residence time was 0.1 sec, the reactor temperature was 1200 K, and the equivalence ratio was one. The results -4- |