Mixing internal heat transfer surface

Figure 17.9. Stirred tank reactors, batch and continuous, (a) With agitator and internal heat transfer surface, batch or continuous, (b) With pumparound mixing and external heat transfer surface, batch or continuous, (c) Three-stage continuous stirred tank reactor battery, (d) Three-stage continuous stirred tank battery in a single shell.

Lower cost per fL of heat-transfer surface. Replaceable straight tubes allow for easy internal cleaning. Full tube bundle minimizes shell-side bypassing. No packedjoints or internal gaskets, so hot and cold fluids cannot mix due to gasket failure. 

The aspect ratio of a pressure vessel, defined as the ratio of the internal length to the internal diameter, is also important for improved mixing and heat transfer. As important as these factors may be, they must often be balanced with the cost of the pressure vessel. As the diameter of a vessel increases, the thickness of the wall must increase proportionally for safe operation at the same pressure. The heat transfer rate can become limited because of the thicker wall and because of the decreased surface to volume ratio of vessels with larger diameters. 

Internal baffles are sometimes used to improve mixing and heat transfer. In fact such baffles are often designed with internal channels for cooling fluid so they not only alter the bulk fluid dynamics, but also add heat-transfer surface. The presence of baffles, however, provide additional surface and, in some cases, areas which are prone to wall polymer formation. 

Good mixing and heat transfer can be obtained without internal baffles if marine propellers are mounted at an angle off the vertical. Such a reactor system is described by Pettelkou and Ehrig (8). They have also suggested non-geometric scale-up to preserve heat transfer surface as reactor volume increases. This Bayer patent (8) claims considerable success in the polymerization of chloroprene which is a system quite prone to flocculation, wall polymer formation and excessive reaction rates. 

Heat Transfer Surfaces. When the process requires heat addition to or removal from the process fluid, the mixing tank must be equipped with appropriate heat transfer surfaces. Liquid motion supplied by the mixer enhances the heat transfer coefficient. Commonly used heat transfer surfaces, shown in Figure 6-8, include jackets, internal helical coils, and internal baffle coils. A jacket can be a tank outside the main tank, baffled, half-pipe, or dimpled. Each of these heat transfer surfaces can also be used in combination with a single coil or multiple heating coils installed within the space between the impeller and the tank wall. A suitable heat transfer fluid must be supplied on the service side of the heat transfer surfaces. 

These resistances are illustrated in Figure 9.51. The subscript i in Equation (9.83) refers to the coefficient at the inside wall of the mixing vessel the subscript j refers to the jacket side. The other terms are the wall resistance and the fouling resistances for either side. A similar equation can be written for an internal coil or other device. In situations where both a jacket and an internal device are used, the overall coefficients for each type of surface should be calculated separately, and the two g s should be added to obtain the overall heat-transfer capability. 

Spray (indirect convection) residence time 3 to 30 s gas velocity 0.2 m/s thermal efficiency 50% adiabatic efficiency 100% solid temperatnre = adiabatic satnration temperatnre volnmetric heat transfer coefficient 0.13 to 0.18 kW/m K 1.8 to 2.7 kg steam/kg water evaporated. Ap = 1.5 to 5 kPa. See size redaction sprays, Section 16.11.8.2 spray reactor, Section 16.11.6.12 heat exchange. Section 16.11.3.12 and size enlargement. Section 16.11.9.4. Flash/transported (indirect convection) 175 to 630°C, gas velocity 3 to 30 m/s or 2.5 to 3 times the terminal velocity of the particles gas reqnirement 1 to 5 Nm /kg solid or 1 to 10 kg air/kg solid exit air temperatnre 20°C greater than exit dry solid temperatnre 4000 to 10,000 kJ/kg water evaporated. See transported slnrry, transfer Une reactors. Section 16.11.6.9. Heat transfer coefficient for gas drying h = 0.2 kW/m -K. Flnidized bed (indirect convection) residence time 30 to 60 s for surface fluid vaporization 15 to 30 min for internal diffnsion 3500 to 4500 kJ/kg water evaporated. See fluidized bed reactors. Section 16.11.6.27 heat transfer. Sections 16.11.3.4 and 16.11.3.8 size enlargement. Section 16.11.9.5 and mixing. Section 16.11.7.1. Tray/gas flow through the bed 0.24 to 3.3 g water evaporated/s m tray area. Residence time 2 to 8.5 h superficial air velocity 0.2 to 1 m/s steam 2 to 6.8 kg steam/kg water evaporated. Fan power 1.6 to... 

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