TEMPERATURE EFFECTS IN HOMOGENEOUS REACTORS

Nonuniform temperatures, or a temperature level different from that of the surroundings, are common in operating reactors. The temperature may be varied deliberately to achieve optimum rates of reaction, or high heats of reaction and limited heat-transfer rates may cause unintended nonisothermal conditions. Reactor design is usually sensitive to small temperature changes because of the exponential effect of temperature on the rate (the Arrhenius equation). The temperature profile, or history, in a reactor is established by an energy balance such as those presented in Chap. 3 for ideal batch and flow reactors. 

In an endothermic reaction the temperature decreases as the conversion increases, unless energy is added to the system in excess of that absorbed by the reaction. Both the reduction in concentration of reactants, due to increasing conversion, and the reduction in temperature cause the rate to fall (Fig. 5-1 2). Hence the conversion in a nonisothermal reactor will normally be less than that for an isothermal one. Adding energy to reduce the temperature drop will limit the reduction in conversion. If the 

Successful designs for such processes must provide for the addition 

In an exothermic reaction the temperature increases as the conyersion increases. At low conversions the rising temperature increases the rate more than it is reduced by the fall in reactants concentration. Normally the conversion will be greater than for isothermal operation. However, undesirable side reactions and other factors may limit the permissible temperatures. In these cases successful design depends on effective removal of the heat of reaction to prevent excessive temperatures (hot spots). In general, the same methods are employed as for adding energy in endothermic reactions. 

The increase in rate as an exothermic reaction progresses is hindered by the limit on the conversion The limit for an irreversible reaction is 100%. As the limit is approached, the concentration of reactants and the rate both approach zero, regardless of the temperature level. Hence the curve of reaction rate vs conversion for an exothermic reaction in an adiabatic reactor has a maximum, as shown in Fig. 5- b. At low conversions the relative change in reactants concentration with conversion is small, and the rate increases exponentially because of the dominant temperature effect. At high conversions the reactants concentration approaches zero, and so does the rate. 

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An assumption involving heat losses from the reactor is made in most treatments. The effect of heat transfer on the maximum reaction rates of a homogeneous reactor has been treated by DeZubay and Woodward (14). It was found that a lowering of the reactor surface temperature appreciably lowered the chemical reaction rates. Longwell and Weiss (43) found, for example, a loss equal to 5% of the maximum adiabatic heat liberated reduces the maximum heat release rate by more than 30%, while a 20% heat loss reduces the rate about 85%. One should not assume an adiabatic system without some definite knowledge of the magnitude of the heat losses. 

Heat transfer is an extremely important factor in CVD reactor operation, particularly for LPCVD reactors. These reactors are operated in a regime in which the deposition is primarily controlled by surface reaction processes. Because of the exponential dependence of reaction rates on temperature, even a few degrees of variation in surface temperature can produce unacceptable variations in deposition rates. On the other hand, with atmospheric CVD processes, which are often limited by mass transfer, small susceptor temperature variations have little effect on the growth rate because of the slow variation of the diffusion with temperature. Heat transfer is also a factor in controlling the gas-phase temperature to avoid homogeneous nucleation through premature reactions. At the high temperatures (700-1400 K) of most... 

The pressure and temperature in the reactor were measured by a K-type thermocouple inserted into the reactor and pressure transducer, respectively. The molten salt bath was bubbled with air so to obtain a homogenous temperature inside the bath and effective heat transfer. The temperature in the reactor rose quickly, exceeding 390 C in 40 s. The pressure continued to slowly increase after the temperature reached 400 C, probably due to the gas formation. 

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