Jacket for heat transfer

Figure 10-93A. Typical vessel external jackets for heat transfer.

There are two distinct aspects of the design of jackets for heat transfer. These are the thermal design and the physical design. The thermal design falls into three parts ... 

Convection. In these mixers an impeller operates within a static shell and particles are moved from one location to another within the bulk. bbon Tjpe. Spiral or other blade styles transfer materials from one end to the other or from both ends to the center for discharge (Fig. 37b). This mixer can be used for dry materials or pastes of heavy consistency. It can be jacketed for heating or cooling. Blades can be smoothly contoured and highly pohshed when cleanliness is an important process requirement. 

Exothermicity. The catalytic reactions are often exothermic bond-forming reactions of small molecules that give larger molecules. Consequendy, the reactors are designed for efficient heat removal. They may be jacketed or contain coils for heat-transfer media, or the heat of reaction may be used to vaporize the products and aid in the downstream separation by distillation. 

Most of the correlations for heat transfer from the agitated hquid contents of vessels to jacketed walls have been of the form ... 

For heat transfer fluids inside reactor jackets or other process vessels with agitation to fluids in vessels (Figure 10-93A), the heat transfer is expressed as... 

L = length of straight tube for heat transfer (or, nominal tube length), ft or, length of path, ft or, length or thickness of coil or jacket, ft or, superficial liquid mass velocity, lb/ (hr) (ft ) or, equivalent length of pipe, ft or, thickness of insulation, in. 

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... 

Here, Um is the film heat transfer coefficient between the reactor and the reactor wall, Uj is the film heat transfer coefficient between the reactor wall and the jacket. Am is the area for heat transfer between the reactor and the wall and Aj is the area for heat transfer between the wall and the jacket. 

Figure 10.15. Some mixers and blenders for powders and pastes, (a) Ribbon blender for powders, (b) Flow pattern in a double cone blender rotating on a horizontal axis, (c) Twin shell (Vee-type) agglomerate breaking and liquid injection are shown on the broken line, (d) Twin rotor available with jacket and hollow screws for heat transfer, (e) Batch muller. (f) Twin mullers operated continuously, (g) Double-arm mixer and kneader (Baker-Perkins Inc.), (h) Some types of blades for the double-arm kneader (Baker—Perkins Irtc.).

Figure 17.19. Reactors for the oxidation of sulfur dioxide (a) Feed-product heat exchange, (b) External heat exchanger and internal tube and thimble, (c) Multibed reactor, cooling with charge gas in a spiral jacket, (d) Tube and thimble for feed against product and for heat transfer medium, (e) BASF-Knietsch, with autothermal packed tubes and external exchanger, (f) Sper reactor with internal heat transfer surface, (g) Zieren-Chemiebau reactor assembly and the temperature profile (Winnacker- Weingartner, Chemische Technologie, Carl Hanser Verlag, Munich, 1950-1954).

If the reactor does not operate adiabatically, then its design must include provision for heat transfer. Figure 1.4 shows some of the ways in which the contents of a batch reactor may be heated or cooled. In a and b the jacket and the coils form part of the reactor itself, whereas in c an external heat exchanger is used with a recirculating pump. If one of the constituents of the reaction mixture, possibly a... 

The two steady-state heat-transfer coefficients, hr and hj, could be further described in terms of the physical properties of the system. The solution-to-wall coefficient for heat transfer, hT in Equation 8.8, is strongly dependent on the physical properties of the reaction mixture (heat capacity, density, viscosity and thermal conductivity) as well as on the fluid dynamics inside the reactor. Similarly, the wall-to-jacket coefficient for heat transfer, hj, depends on the properties and on the fluid dynamics of the chosen cooling liquid. Thus, U generally varies during measurements on a chemical reaction mainly for the following two reasons. 

Compute the process-side heat-transfer coefficient. The correlations for inside (process-side) heat-transfer coefficient in an agitated tank are similar to those for heat transfer in pipe flow, except that the impeller Reynolds number and geometric factors associated with the tank and impeller are used and the coefficients and exponents are different. A typical correlation for the agitated heat-transfer Nusselt number (ANu = htT/k) of a jacketed tank is expressed as... 

Temperature control for laboratory reactors is typically easy because of high heat transfer area-reactor volume ratios, which do not require large driving forces (temperature differences) for heat transfer from the reactor to the jacket. Pilot- and full-scale reactors, however, often have a limited heat transfer capability. A process development engineer will usually have a choice of reactors when moving from the laboratory to the pilot plant. Kinetic and heat of reaction parameters obtained from the laboratory reactor, in conjunction with information on the heat transfer characteristics of each pilot plant vessel, can be used to select the proper pilot plant reactor. 

Based on initial heat flow calorimetry studies, a process development engineer must choose the appropriate reactor vessels for pilot plant studies. A pilot plant typically has vessels that range from 80 to 5000 L, some constructed of alloy and others that are glass lined. In addition some vessels may have half-pipe coils for heat transfer, while others have jackets with agitation nozzles. A process drawing for a typical glass-lined vessel is shown in Figure 4. In Sections 3.1.4.1 and 3.1.4.2 we review fundamental heat transfer relationships in order to predict overall heat transfer coefficients. In Section 3.1.4.3 we review experimental techniques to estimate heat transfer coefficients in process vessels. 

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