Reactor temperature indirect heat transfer

Even if the reactor temperature is controlled within acceptable limits, the reactor effluent may need to be cooled rapidly, or quenched, to stop the reaction quickly to prevent excessive byproduct formation. This quench can be accomplished by indirect heat transfer using conventional heat transfer equipment or by direct heat transfer by mixing with another fluid. A commonly encountered situation is... 

If indirect heat transfer is used with a large temperature difference to promote high rates of cooling, then the cooling fluid (e.g., boiling water) is fixed by process requirements. In this case, the heat of reaction is not available at the temperature of the reactor effluent. Rather, the heat of reaction becomes available at the temperature of the quench fluid. Thus the feed stream to the reactor is a cold stream, the quench fluid is a hot stream, and the reactor effluent after the quench is also a hot stream. 

Reactor heat carrier. As pointed out in Chapter 7, if adiabatic operation is not possible and it is not possible to control temperature by indirect heat transfer, then an inert material can be introduced to the reactor to increase its heat capacity flowrate (i.e. product of mass flowrate and specific heat capacity). This will reduce temperature rise for exothermic reactions or reduce temperature decrease for endothermic reactions. The introduction of an extraneous component as a heat carrier effects the recycle structure of the flowsheet. Figure 13.6a shows an example of the recycle structure for just such a process. 

Reactions with a large heat of reaction as well as reactions that are very temperature-sensitive are carried out in reactors in which indirect heat exchange occurs with a heat transfer medium which is circulated through the fixed bed. 

Comparison of Temperature-control Characteristics of Various Types of Reactors. Several methods of heat removal are used in the various reactors. Heat removal for the most part may be considered to take place either directly, as in the oil- or gas-cooled systems where the catalyst surface is in contact with the cooling medium, or indirectly, as in the fixed or fluid beds where heat must be transferred through the bed to a cooling surface. Admittedly this is an oversimplification, especially in the case of the fixed and fluid beds where some direct heat transfer occurs. 

Fluidized bed (indirect convection), residence time 30-60 s for surface fluid vaporization 15-30 min for internal diffusion 3500 to 4500 kj/kg water evaporated. Allow 1-2 m for disengagement. The dewpoint of the exit gas should be at least 10 °C less than the exit solids temperature of the bed. Gas and particles leave the bed at the same temperature. Partide-gas heat transfer coeffident U = 0.01-0.06 kW/m K. Surface area 20000-100000 m /m bed. For solids with medium sensitivity, such as organics, grains, PVC. Inlet temp about 80-150 °C AT = 50-100 °C. Bed volume 10-60 m /kg water evap/s soUds residence or drying time, 400-1800 s. Mass air/mass water evaporated = 40-100 MJ/kg water evaporated = 2-10 evaporation rate = 0.002-0.5 kg water evaporated/s m. Solids holdup 100-500 kg solids/m See fluidized bed reactors Section 6.30, heat transfer Section 3.4, size enlargement Section 9.4. 

Nonisothermal reactors with adiabatic beds. Optimization of the temperature profile described above assumes that heat can be added or removed wherever required and at whatever rate required so that the optimal temperature profile can be achieved. A superstructure can be set up to examine design options involving adiabatic reaction sections. Figure 7.12 shows a superstructure for a reactor with adiabatic sections912 that allows heat to be transferred indirectly or directly through intermediate feed injection. 

Considering the problems of the possible radioactivity deposition in the turbine blades in the direct GT-MHTGR, and the lower inlet helium temperature (250 C -300 C) in the indirect gas turbine cycle MHTGR, the indirect gas turbine and steam turbine combined cycles (GT-ST-MHTGR) were studied The GT-ST-MHTGR can be used to generate electricity more efficiently The reactor heat is transferred to the secondary... 

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