$11\mathrm{.}6\mathrm{.}1$ Exothermic Reactions

Conversions higher than those shown in Figure E$11-4\mathrm{.}1$ can be achieved for adiabatic operations by connecting reactors in series with interstage cooling.

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In Figure $11-5,$ we show the case of an exothermic adiabatic reaction taking place in a PBR reactor train. The exit temperature from Reactor $1$ is very

high, $800K,$ and the equilibrium conversion, $x_e,$ is low as is the reactor conversion $x_1,$ which approaches $x_e.$ Next, we pass the exit stream from adiabatic

reactor $1$ through a heat exchanger to lower the temperature back to $500K$ where $x_e$ is high but $x$ is still low. To increase the conversion, the stream then

enters adiabatic reactor $2$ where the conversion increases to $x_2,$ which is followed by a heat exchanger and the process is repeated.

The conversion$-$temperature plot for this scheme is shown in Figure $11-6.$ We see that with three interstage coolers, $88\%$ conversion can be achieved,

compared to an equilibrium conversion of $35\%$ for no interstage cooling.

Figure $11-6$ Increasing conversion by interstage cooling for an exothermic reaction. Note: Lines and curves are approximate.