Countercurrent cooling, temperature profiles

Techniques for approaching optimum temperature profiles for exothermic reaction, (a) Adiabatic operation of reactors with interstage cooling, (b) Countercurrent heat exchange. (Adapted from Chemical Reaction Engineering, Second Edition, by O. Levenspiel. Copyright 1972. Reprinted by permission of John Wiley and Sons, Inc.)... 

Figure 5-21 Temperature profiles for cocuirent and countercurrent cooled PFTR with feed temperature To and coolant feed temperature Too.

Figure 5-22 Temperature profiles in a wall-cooled reactor with countercurrent feed cooling.

The conversion and selectivity of the reaction can be decisively influenced by the design and the operation of the heat transfer circuit. The most obvious, although technically most complex solution, is to arrange different heat transfer circuits so as to achieve a stepwise approximation of an optimum temperature profile. The purposeful utilization of the temperature change of the heat transfer medium flowing through the reactor is technically simpler, and will be discussed here in connection with cocurrent or countercurrent cooling of a fixed-bed reactor with an exothermic reaction. 

The cooling fluid is fed at point A or at point B for cocurrent and countercurrent operation, respectively. Next, the reactor temperature profiles from the two modes of operation... 

In most applications of trickle-flow reactors, the conversions generate heat that causes a temperature rise of the reactants, since the industrial reactors are generally operated adiabatically. In the cocurrent mode of operation, both the gas and the liquid rise in temperature as they accumulate heat, so there is a significant temperature profile in the axial direction, with the highest temperature at the exit end. When the total adiabatic temperature rise exceeds the allowable temperature span for the reaction, the total catalyst volume is generally split up between several adiabatic beds, with interbed cooling of the reactants. In the countercurrent mode of operation, heat is transported by gas and liquid in both directions, rather than in one direction only, and this may increase the possibility of obtaining a more desirable temperature profile over the reactor. 

Figure 4-14 Examples of two different steady-state solutions for the reactive and cooling fluid temperature profiles in a countercurrent concentric double-pipe configuration with exothermic chemical reaction in the inner tube. In both cases, the inlet temperatures of the reactive and cooling fluids are 340 K and 322 K, respectively, (a) Thamally well-behaved reactor with 18% outlet conversion of reactants to products, (b) Thamally well-behaved reactor with 49% outlet conversion of reactants to products.

Fig. 6.6. Schematic drawing, typical temperature profile, and operating curve (temperature/am-monia concentration plot) for four important converter types, (from [471]) a Internal cooling, countercurrent flow (TVA-converter) b Internal cooling, cocurrent flow (NEC-converter) c Quench cooling d Indirect cooling (heat exchange)...

The most important converter type using internal cooling with countercurrent flow in cooling tubes is the TVA-converter. A schematic drawing of the converter and the corresponding temperature- and temperature/conversion profile is shown in Fig. 6.6a. Feed gas enters at the top and passes in the annulus between the pressure shell and the basket to the bottom of the converter. In this way the pressure vessel is cooled (the feed gas is used as shell cooling gas ) so that a lower design temperature is applicable for the expensive pressure vessel. 

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