Coolant flow, countercurrent cocurrent

Figure 20. Influence of the coolant flow direction and flow velocity os on the reaction temperature profile. A) Isothermal, B) Cocurrent flow C) Countercurrent flow.

When exothermic reactions are carried out in fixed-bed reactors, hot spots can develop. Investigate the stability of the fixed-bed reactor described below for (a) cocurrent coolant flow and (b) countercurrent coolant flow when the inlet coolant temperature (7°) is 350 K and higher. Calculate the sensitivity by plotting the maximum temperature in the reactor versus the inlet coolant temperature (the slope of this line is the sensitivity). Which mode of cooling minimizes the sensitivity (This problem is adapted from material provided by Jean Cropley.)... 

Autothermal reactors. Relationship between coolant inlet temperature and reactor top temperature, (a) Countercurrent flow, (b) Cocurrent flow. After Degnan and Wei [1979]. 

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. 

Consider the condensation of a vapor mixture inside the vertical tube as shown in Figure 15.1. Vapor enters the top of the tube and flows downwards. The condensate flows cocurrently with the remaining vapor down the tube. The coolant may flow cocurrently with or countercurrently to the vapor and liquid streams. Condensation on a horizontal tube was illustrated in Figure 15.2. In this case the condensed liquid drips off the tube and eventually collects at the bottom of the heat exchanger. The vapor flow is directed along or, more likely, across bundle of tubes. The design equations are developed in the same way for both types of condenser. 

By feeding the inlet reactant and coolant streams into separate ports of the reactor, it was possible to achieve five different flow schemes, shown in Fig. 3. An adiabatic reactor was simulated by feeding a stream of preheated air and CO to the catalyst containing reaction pass and by sealing off the coolant pass. Similarly, the cocurrent and countercurrent schemes were approximated by flowing coolant and reactant streams either into adjacent ports or into ports which lie at opposite ends of the reactor-heat exchanger respectively. 

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