Steady-State Tubular Reactor with Heat Exchange

1 Steady-State Tubular Reactor with Heat Exchange 

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Sec. 8.4 Steady-State Tubular Reactor with Heat Exchange... 

The second issue for cooled tubular reactors is how to introduce the coolant. One option is to provide a large flowrate of nearly constant temperature, as in a recirculation loop for a jacketed CSTR. Another option is to use a moderate coolant flowrate in countercurrent operation as in a regular heat exchanger. A third choice is to introduce the coolant cocurrently with the reacting fluids (Borio et al., 1989). This option has some definite benefits for control as shown by Bucala et al. (1992). One of the reasons cocurrent flow is advantageous is that it does not introduce thermal feedback through the coolant. It is always good to avoid positive feedback since it creates nonmonotonic exit temperature responses and the possibility for open-loop unstable steady states. 

Tubular reactors do not necessarily operate under isothermal conditions in industry, be it for reasons of chemical equilibrium or of selectivity, of profit optimization, or simply because it is not economically or technically feasible. It then becomes necessary to consider also the energy equation, that is, a heat balance on a differential volume element of the reactor. For reasons of analogy with the derivation of Eq. 9.1-1 assume that convection is the only mechanism of heat transfer. Moreover, this convection is considered to occur by plug flow and the temperature is completely uniform in a cross section. If heat is exchanged through the wall the entire temperature difference with the wall is located in a very thin film close to the wall. The energy equation then becomes, in the steady state ... 

The last example is a gas-phase process with a tubular reactor, gas recycle compressor, feed-effluent heat exchanger, condenser and separator. The steady-state design of this process leads to an uncontrollable system if the reactions are highly temperature sensitive. We demonstrate that changing the design produces a much more easily controlled process. We consider a complete plant, not just the reactor in isolation. 

It is stressed again that the occurrence of multiple steady states is due to the feedback of heat. Tubular reactors that are not coupled with a heat exchanger generally do not exhibit this feature — except in very particular situations, as will be shown later. This does not mean that perturbations in the operating... 

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