8 10 12 Pressure (bar)
that affects the selection of pressure is a high-temperature limitation. In this case the pressure is chosen to keep the temperature in the base of the column, where the temperature is the highest, below this maximum. Pressure affects only the vapor-liquid equilibrium in conventional distillation.
However, in reactive distillation, pressure affects both chemical kinetics and vapor-liquid equilibrium. Therefore, the optimum pressure may not correspond to the minimum attainable while still using cooling water in the condenser. For example, the optimum pressure for the base case is 8 bar, as we will demonstrate. The corresponding reflux-drum temperature is 353 K (80 °C, 176 °F), which is well above the temperature that could be achieved using 305 K (32 °C, 90 °F) cooling water.
Because relative volatilities are constant in the base case, there is no effect of pressure on the ease or difficulty of separation. The system with relative volatilities that do change with temperature will be explored in Section 2.5. In this situation a mismatch can occur between temperatures that are favorable for reaction and temperatures that are favorable for separation.
In the constant relative volatility case, pressure only affects the specific reaction rates and the chemical equilibrium constant. Increasing pressure increases temperatures. Figure 2.8 shows the effects of temperature on the chemical kinetics. Because the reaction is exothermic, the chemical equilibrium constant decreases with increasing temperature. The backward specific reaction rate increases more quickly with increasing temperature than the forward specific reaction rate because of its larger activation energy.
Figure 2.11 Effect of pressure on composition profiles.
These curves indicate that low pressure and the resulting low temperatures would result in low reaction rate, which would require large holdups on reactive trays. Alternatively, for a fixed number of trays and a fixed holdup, the concentrations of the reactants in the reactive zone would have to be large at low temperatures. This would require a large vapor boilup and reflux flowrate to keep the reactants from escaping out of the top or bottom.
In contrast, high pressure and temperatures would result in a lower chemical equilibrium constant, which would also require large vapor boilup and reflux flowrates.
Figure 2.9 shows the effect of the operating pressure. There is an optimum pressure at which the energy consumption is minimized, as shown in the top left graph. The impurities of reactants in the two products reflect the changes in fractionation as the vapor and liquid rates change with pressure. Less fractionation permits more of the nonadjacent component to go into the product; that is, there is more B in the distillate product (mostly C) and more A in the bottoms product (mostly D) at the smaller vapor and liquid rates.
Figure 2.10 displays temperature profiles. The higher the pressure is, the higher the temperatures throughout the column. Figure 2.11 provides the composition profiles at three different pressures. The solid lines are for the optimum case with a pressure of 8 bar.
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