Heat Transfer in Reactors

Values of overall coefficients of heat transfer are collected in Tables 17.10-17.12. Two sets of formulas for tank-side film coefficients are in Tables 17.13 and 17.14. They relate the Nusselt number to the Reynolds and Prandtl numbers and several other factors. In the equation for jacketed tanks, for example, 

HEAT TRANSFER IN REACTORS 615 Typical Data for ICI Quench Converters of Various Sizes 

B - Gas exit to heat recovery C - Gas exit D — Direct by-pass 

E - Gas from externa start up heater F — Quench gas inlets G - Pyrometer 

Since most of the literature in this area is relatively old, practitioners apparently believe that what has been found out is adequate or is kept confidential. Table 17.14 has the recommended formulas. 

The use of a jacket surrounding the reactor vessel is probably the most common method for providing heat transfer because it is relatively inexpensive in terms of equipment capital cost (see Fig. 1.10a). If heating is required, steam is condensed in the jacket or a hot heat transfer fluid stream is fed to the jacket. If cooling is required, a cooling medium is fed to the jacket. For moderate reactor temperatures (between 50 and 80°C), cooling water at 30°C is typically used. For lower temperature reactors, a cold refrigeration stream (brine) is used. 

For reactor temperatures between 80 and 130°C, a tempered water or oil cooling medium is used. Plain cooling water should not be used because the large temperature difference between the reactor and the cooling medium leads to dynamic control problems. This is illustrated quantitatively in Chapter 2. It occurs because the temperature difference can be changed by only a small amount, which means that the heat removal rate cannot be changed much. Therefore the magnitude of the dynamic upsets that can be handled is quite limited. 

For reactor temperatures above 130°C, steam can be generated in the jacket at a suitable pressure (to provide a 30-50°C temperature differential between the steam and the reactor see Fig. 1.10b). Reactors operating at very high temperatures usually employ a molten salt for heat removal. 

Jacket Cooling Circulating Water System Heat Exchanger and Bypass 

The final heat removal scheme discussed here is called autorefrigeration or evaporative cooling (Fig. 1.10/). The pressure in the reactor is adjusted so that the liquid can boil if 

Process Pressure (bar) Effluent ammonia (%) TPD/m Catalyst life (yr) 

J—housing g-support ring J—Inlet pipe connection dlKhars pipe connection 4—geTe —catalyst discharge pipe connection —porcelain balls  

6 layer I tolls 6 opttonol addilional loyers. of progressively smaller tolls for iteproired distritohon ood scoit removol 

ICI methanol reactor, showing internal distributors. C, D and E are cold shot nozzles, F = catalyst dropout, L = thermocouple, and O catalyst input, (b) ICI methanol reactor with internal heal exchange and cold shots, (c) Fixed bed reactor for gasoline from coal synthesis gas dimensions 10 x 42 ft, 2000 2-in. dia tubes packed with promoted iron catalyst, production rate 5 tons/day per reactor, (d) Synthol fluidized bed continuous reactor system for gasoline from coal synthesis gas, 

Reynolds numbers varies as the cube root of N. Accordingly it appears that the coefficient is proportional to the 0.22 exponent of the power input to the stirred tank, 

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In this chapter, correlations for heat transfer in reactors are presented, and the requirements for stable operation are discussed. The continuous stirred-tank reactor is treated first, since it is the simplest case, and uniform temperature and concentration are assumed for the fluid in the tank. For a homogeneous reaction in a pipeline, there are axial gradients of temperature... 

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