Heat Transfer in Stirred Tank Reactors

The temperature in stirred tank reactors may be influenced by chemical or physical reactions within the tank. Cooling or heating devices might be required to control the process temperature. In many endothermic processes heat has to be added to raise and maintain the temperature of the bulk. In other exothermic processes heat is removed to avoid hot spots. 

Heating and cooling of the process fluid are accomplished by heat transfer between the process fluid and a heating or cooling media that is circulated 

Heat transfer from the bulk of the tank to the heat transfer medium can be calculated by the standard heat transfer model  

The overall heat transfer resistance (l/Ug) is calculated as the sum of the individual resistances in analogy to Ohm s law. The relationship between the overall resistance and the different contributions is expressed as  

The stirring and the resulting flow pattern inside the tank can be very important for the overall heat transfer resistance, because the performance of the reactor affects the heat transfer coefficient at the process side hg. The other resistances are determined by the materials used and the properties of the cooling/heating media and are thus not influenced by the reactor performance. 

The temperature in stirred tank reactors may be influenced by chemical or physical reactions within the tank. Cooling or heating devices might be required to control the process temperature. In many endothermic processes heat has to be added to raise and maintain the temperature of the bulk. In other exothermic processes heat is removed to avoid hot spots. Heating and cooling of the process fluid are accomplished by heat transfer between the process fluid and a heating or cooling media that is circulated within a closed heat transfer surface. Different types of heat transfer equipment are used in industrial processes such as jackets, external or internal helical coils, as sketched in Fig. 7.10. Heat transfer from the bulk of the tank to the heat transfer medium can be calculated by the standard heat transfer model  

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Experimental investigations of wall heat transfer in stirred tank reactors have been reported by Engeskaug et al [23], Oldshue [65], Hewitt et al [40], among others. 

Engeskaug R, Thorbjprnsen E, Svendsen HF (2005) Wall Heat Transfer in Stirred Tank Reactors. Ind Eng Chem Res 44 4949-4958 FLUENT (1996) FLUENT User s Guide, Release 4.4, Volume 1-4, Fluent Inc, Lebanon (1996)... 

Some modes of heat transfer to stirred tank reactors are shown in Fig. 23-1 and to packed bed reactors in Fig. 23-2. Temperature and composition profiles of some processes are shown in Fig. 23-3. Operating data, catalysts, and reaction times are stated for a number of industrial reaction processes in Table 23-1. 

A reaction A——>P is to be performed in a PFR. The reaction follows first-order kinetics, and at 50 °C in the batch mode, the conversion reaches 99% in 60 seconds. Pure plug flow behavior is assumed. The flow velocity should be 1 m s"1 and the overall heat transfer coefficient 1000Wm 2 K"1. (Why is it higher than in stirred tank reactors ). The maximum temperature difference with the cooling system is 50 K. 

Disadvantages of the organic polymer supports are low mechanical durabUity (e.g., in stirred tank reactors), poor heat-transfer properties, and limited thermal sta-bihty (up to max. 150°C). 

Taking into consideration the fact that fast polymerisation processes are characterised by inequality of chemical reaction time and transfer time ( chem < it is clear that an increase of facilitates the decrease of and both these processes are comparable in duration. The increase in linear flow rate V, i.e., the intensification of heat and mass exchange in the system, is equivalent to a slow dovm of the polymerisation reaction itself, compared with the transfer process. Therefore, the conventional approaches to external heat removal, which normally have such a restrictive effect on conventionally designed fast polymerisation processes implemented in stirred tank reactors, play an essential role at both high V and values when quasi-plug flow tubular turbulent reactors are used. In this case, control of the external temperature can be significantly enchanced due to zone-type catalyst loading. 

Combining Highbie s Penetration theory and Kolmogoroff s theory on isotropic turbulence, it has been shown recently [34l that experimental data on aerated liquids in stirred tank reactors can be correlated for coil as well as wall heat transfer by the equation... 

In stirred-tank reactors, the characteristic time for heat transfer can be related to geometric parameters and fluid properties as follows ... 

Figure 1.19 Heat transfer strategies in stirred-tank reactors, (a) Jacket, (b) internal coils, (c) internal tubes, (d) external heat exchanger, (e) external reflux condenser, and (f) fired heater.

FIGURE 7.3 Evolution of the oscillating reactor and jacket temperatures, electric heater voltage and calculated overall heat transfer coefficient. With permission of VCH Verlagsgesellchaft from Carloff R, Pross A, Reichert K. Temperature oscillation calorimetry in stirred-tank reactors with variable heat-transfer. Chem Eng Technol 1994 17 406 13. 

Carloff R, Pross A, Reichert K. Temperature oscillation calorimetry in stirred-tank reactors with variable heat-transfer. Chem Eng Technol 1994 17 406-413. 

Two complementai y reviews of this subject are by Shah et al. AIChE Journal, 28, 353-379 [1982]) and Deckwer (in de Lasa, ed.. Chemical Reactor Design andTechnology, Martinus Nijhoff, 1985, pp. 411-461). Useful comments are made by Doraiswamy and Sharma (Heterogeneous Reactions, Wiley, 1984). Charpentier (in Gianetto and Silveston, eds.. Multiphase Chemical Reactors, Hemisphere, 1986, pp. 104—151) emphasizes parameters of trickle bed and stirred tank reactors. Recommendations based on the literature are made for several design parameters namely, bubble diameter and velocity of rise, gas holdup, interfacial area, mass-transfer coefficients k a and /cl but not /cg, axial liquid-phase dispersion coefficient, and heat-transfer coefficient to the wall. The effect of vessel diameter on these parameters is insignificant when D > 0.15 m (0.49 ft), except for the dispersion coefficient. Application of these correlations is to (1) chlorination of toluene in the presence of FeCl,3 catalyst, (2) absorption of SO9 in aqueous potassium carbonate with arsenite catalyst, and (3) reaction of butene with sulfuric acid to butanol. 

A continuous flow stirred tank reactor (CFSTR) differs from the batch reactor in that the feed mixture continuously enters and the outlet mixture is continuously withdrawn. There is intense mixing in the reactor to destroy any concentration and temperature differences. Heat transfer must be extremely efficient to keep the temperature of the reaction mixture equal to the temperature of the heat transfer medium. The CFSTR can either be used alone or as part of a series of battery CFSTRs as shown in Figure 4-5. If several vessels are used in series, the net effect is partial backmixing. 

The advantages of continuous tubular reactors are well known. They include the elimination of batch to batch variations, a large heat transfer area and minimal handling of chemical products. Despite these advantages there are no reported commercial instances of emulsion polymerizations done in a tubular reactor instead the continuous emulsion process has been realized in series-connected stirred tank reactors (1, . ... 

Recycling of partially reacted feed streams is usually carried out after the product is separated and recovered. Unreacted feedstock can be separated and recycled to (ultimate) extinction. Figure 4.2 shows a different situation. It is a loop reactor where some of the reaction mass is returned to the inlet without separation. Internal recycle exists in every stirred tank reactor. An external recycle loop as shown in Figure 4.2 is less common, but is used, particularly in large plants where a conventional stirred tank would have heat transfer limitations. The net throughput for the system is Q = but an amount q is recycled back to the reactor inlet so that the flow through the reactor is Qin + q- Performance of this loop reactor system depends on the recycle ratio qlQin and on the type of reactor that is in the loop. Fast external recycle has... 

Therefore, many traditional designs, such as stirred tank reactors, incorporate heat transfer in the process (jacket, external or internal coil, etc.). However, in these devices, there is a significant distance between the heat transfer site and the site of the chemical reaction where heat is released. As a consequence semibatch mode is implemented while batch mode and/or systems are diluted. 

Four mmoles of malononitrile and benzaldehyde were introduced in a batch stirred tank reactor at 323 K with toluene as solvent (30 ml). Then 0.05 g of aluminophosphate oxynitride was added. Samples were analysed by gas chromatography (Intersmat Delsi DI200) using a capillary column (CPSilSCB-25 m). Care was taken to avoid mass or heat transfer limitations. Before the reaction no specific catalyst pretreatment was done. 

Fig. 5.4-23 shows a sketch drawing of a BSC (Brogli et al., 1981). The stirred-tank reactor made of glass (a metal version is also available) is surrounded by a jacket through which a heat-transfer fluid flows at a very high rate the jacket is not insulated. The temperature of the circulation loop is regulated by a cascaded controller so that the heat evolution in the reactor is equilibrated by heat transfer through the reactor wall. The temperature in the loop is adjusted by injection of thermostatted hot or cold fluid. 

Fig. 3.2 shows the case of a jacketed, stirred-tank reactor, in which either heating by steam or cooling medium can be applied to the jacket. Here V is volume, Cp is specific heat capacity, p is density, Q is the rate of heat transfer, U is the overall heat transfer coefficient, A is the area for heat transfer, T is temperature, H is enthalpy of vapour, h is liquid enthalpy, F is volumetric flow... 

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