Jacketed vessels, design heat transfer

Three different principles govern the design of bench-scale calorimetric units heat flow, heat balance, and power consumption. The RC1 [184], for example, is based on the heat-flow principle, by measuring the temperature difference between the reaction mixture and the heat transfer fluid in the reactor jacket. In order to determine the heat release rate, the heat transfer coefficient and area must be known. The Contalab [185], as originally marketed by Contraves, is based on the heat balance principle, by measuring the difference between the temperature of the heat transfer fluid at the jacket inlet and the outlet. Knowledge of the characteristics of the heat transfer fluid, such as mass flow rates and the specific heat, is required. ThermoMetric instruments, such as the CPA [188], are designed on the power compensation principle (i.e., the supply or removal of heat to or from the reactor vessel to maintain reactor contents at a prescribed temperature is measured). 

This group normally operates under low shear conditions and is broken down by impeller design and movement. Design can also include a jacketed vessel to facilitate heat transfer. 

To illustrate some of the design and control issues, a vessel size (DR = 2 m, VR = 12.57 m3, jacket heat transfer area Aj = 25.13 m2) and a maximum reactor temperature (7j) ax = 340 K) are selected. The vessel is initially heated with a hot fluid until the reaction begins to generate heat. Then a cold fluid is used. A split-range-heating/ cooling system is used that adds hot or cold water to a circulating-water system, which is assumed to be perfectly mixed at temperature Tj. The setpoint of a reactor temperature controller is ramped up from 300 K to the maximum temperature over some time period. 

Similarly, when moving from the pilot plant to manufacturing, a process engineer will either choose an existing vessel or specify the design criteria for a new reactor. A necessary condition for operation with a specified reactor temperature profile is that the required jacket temperature is feasible. We have therefore chosen to focus on heat transfer-related issues in scale-up. Clearly there are other scale-up issues, such as mixing sensitive reactions. See Paul [1] for several examples of mixing scale-up in the pharmaceutical industry. 

Agttation. The purpose of agitation is to keep the microorganisms in suspension, to maintain uniformity to eliminate concentration gradients and hot spots, and to improve heat transfer to the cooling jacket. Rules for the design of agitation systems are covered in Chapter 10. In vessels of a 1000 gal or more, a power input of about 10 HP/1000 gal and impeller tip speeds of 15-20 ft/sec are adequate, but the standard fermenter described in Table 19.15 is supplied with about four times this power. 

Some dryers also provide heat energy to the powder mass by a jacketed vessel, thereby increasing overall heat transfer. Moisture can be removed via vacuum or hot air fluidization depending on the design of the dryer allowing for improved evaporative drying and vapor mass transfer. Fig. 13 shows the relationship between power input (W) and first-order drying rate constant in a microwave fluid-bed processor. ... 

Fig. 1.2 Design of the vessel wall for heat transfer [102]. A - Jacketed vessel... 

The drying here is achieved by means of exposing the product to the surface area of the jacketed vessel. The jacket is a shell of metal (usually carbon steel) welded onto a stainless steel vessel body. This design can include a heated shaft for increased surface area exposure. The heat transfer medium used here is generally steam, hot oil, or hot water. Ports must be provided so as to vent the evaporated vapors being removed from the product. 

Most of the factors in Table 4.1 apply equally to batch and flow vessels. The throughput in a well-designed CSTR is small compared to the internal circulation and does not affect quantities such as power, mixing time, and heat transfer to the jacket. The heat transfer factors in Table 4.1 are discussed in Chapter 5. 

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