Countercurrent cooled reactors

In the next section we shall explore the problem of choosing T z) optimally in an effort to see what the maximum performance of a tubular reactor may be. In Sec. 9.7 we shall look at the cocurrent and countercurrent cooled reactors, and other problems of the third sort. For the moment, however, we need... 

It will be convenient to divide this section more formally than the others, according to the following scheme. Under Sec. 9.7.1 we discuss the genera form of the equations when the wall temperature is constant and illustrate this by considering an endothermic cracking reaction. Under Sec. 9,7.2 we shall consider cocurrent and countercurrent cooled reactors and use the ammonia synthesis reactor as an illustration. These correspond to the two subcases of the third type of design problem mentioned in Sec. 9.5. 

Wherever the catalyst volume required for CD shift conversion permits doing so - fOT the above-mentioned gas this is possible up to a throughput of approximately 50000m /h - the two series-connected reaction stages are often replaced by the so-called countercurrent cooling reactor as shown in Fig. 2.20. 

COOL - Three-Stage Reactor Cascade with Countercurrent Cooling... 

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

Fig. 3.23. Heat transfer characteristics of countercurrent cooled tubular reactor...

To illustrate the problem of thermal sensitivity we will analyse the simple one-dimensional model of the countercurrent cooled packed tubular reactor described earlier and illustrated in Fig. 3.25. We have already seen that the mass and heat balance equations for the system may be written ... 

Next we consider the heat balance for the cooling jacket in incremental form as depicted in Figure 7.2. Note first that ours is a case of cocurrent flow in the jacket and reactor. This simplifies the mathematical problem when compared with countercurrent cooling. Countercurrent cooling is physically more efficient, but it transforms the problem mathematically into a more demanding two point boundary value problem which we want to avoid here see problem 3 of the Exercises. 

Develop the model equations for a countercurrent cooling jacket and the same tubular reactor. This will lead to several coupled boundary value problems with boundary conditions at l = 0 and l = Lt. 

Industrial fixed-bed catalytic reactors have a wide range of different configurations. The configuration of the reactor itself may give rise to multiplicity of the steady states when other sources alone are not sufficient to produce the phenomenon. Most well known is the case of catalytic reactors where the gas phase is in plug flow and all diffusional resistances are negligible, while the reaction is exothermic and is countercurrently cooled. One typical example for this is the TVA type ammonia converter [38-40]. 

Cooled Reactor with Co-current or Countercurrent Coolant Flow... 

Figure 6.67 Cooled reactor with countercurrent coolant flow.

The combined stream is preheated to 122°C in a FEHE. A heater (HX3) is installed after the FEHE so that inlet temperature of the coolant stream in REACT2 can be adjusted to satisfy the energy balance when the exit temperature of the coolant stream is specified in this countercurrent tubular reactor. This temperature is 150°C, and the heat load in HX3 is 9.34 x 106 kcal/h. The stream is further preheated to 265°C in the tube side of reactor REACT2 by the heat transfer from the reactions that are occurring in the hot shell side of this vessel. There is no catalyst on the cold tube side, so the feed stream does not react but its temperature is increased. The stream is then fed to reactor REACT 1, which contains 48,000 kg of catalyst. This reactor is cooled by generating steam. The coolant temperature is 265°C (51 bar steam). This vessel contains 3750 tubes, 0.0375 m in diameter, and 12.2 m in length. The overall heat transfer coefficient between the process gas and the steam is 244 kcal h-1 m-2 °C 1. The heat transfer rate is 42 x 106 kcal/h. 

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. 

A countercurrent cooled tubular reactor such as that in Fig. 9.15 is said to be operating autothemically when the heat of reaction is sufficient to raise the temperature of the incoming stream from 7 to Tq. If it is not autothermic it might be necessary to add heat at z = 0 by means of an electric heater and this in fact is often done during start-up. By solving the equations... 

COUNTERCURRENT COOLING IN TUBULAR REACTORS WITH EXOTHERMIC CHEMICAL REACTIONS... 

TABLE 4-7 System of Equations That Must Be Analyzed to Prevent Thermal Runaway in a Plug-Flow Tiibular Reactor with Countercurrent Cooling in a Concentric Double-Pipe Configuration That Is Not Insulated from the Surroundings"... 

Boundary conditions at the inlet to the reactor, wha-e Trx = 0 Conversion of reactant A, x = 0 Inlet temperature of the reactive fluid, Trx = 340 K Outlet temperature of the countercurrent cooling fluid, Tcooi = (guess)... 

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