Indirect heat transfer

Even if the reactor temperature is controlled within acceptable limits, the reactor effluent may need to be cooled rapidly, or quenched, to stop the reaction quickly to prevent excessive byproduct formation. This quench can be accomplished by indirect heat transfer using conventional heat transfer equipment or by direct heat transfer by mixing with another fluid. A commonly encountered situation is... 

Find a way to overcome the constraint while still maintaining the areas. This is often possible by using indirect heat transfer between the two areas. The simplest option is via the existing utility system. For example, rather than have a direct match between two streams, one can perhaps generate steam to be fed into the steam mains and the other use steam from the same mains. The utility system then acts as a buffer between the two areas. Another possibility might be to use a heat transfer medium such as a hot oil which circulates between the two streams being matched. To maintain operational independence, a standby heater and cooler supplied by utilities is needed in the hot oil circuit such that if either area is not operational, utilities could substitute heat recovery for short periods. 

Indirect heat transfer with the reactor. Although indirect heat transfer with the reactor tends to bring about the most complex reactor design options, it is often preferable to the use of a heat carrier. A heat carrier creates complications elsewhere in the flowsheet. A number of options for indirect heat transfer were discussed earlier in Chap. 2. 

For indirect heat transfer, the heat integration characteristics of the reactor can be broken down into three cases ... 

If indirect heat transfer is used with a large temperature difference to promote high rates of cooling, then the cooling fluid (e.g., boiling water) is fixed by process requirements. In this case, the heat of reaction is not available at the temperature of the reactor effluent. Rather, the heat of reaction becomes available at the temperature of the quench fluid. Thus the feed stream to the reactor is a cold stream, the quench fluid is a hot stream, and the reactor effluent after the quench is also a hot stream. 

The thermal duty here is the opposite of solidification operations. The indirect heat-transfer equipment suitable for one operation is not suitable for the other because of the material-handling rather than the thermal aspects. Whether the temperature of transformation is a definite or a ranging one is of little importance in the selection of equipment for fusion. The burden is much agitated, but the beds are deep. 

FIG. 11-59 Rotating shells as indirect heat-transfer equipment, (a) Plain, (Courtesy of BSF Corp.) (h) Flighted, (Courtesy of BSF Corp.) (c) Tiihed. (d) Deep-finned t)qje, (Courtesy of Link-Belt Co.)... 

FIG. 11-62 Vibratory-conveyor adaptations as indirect heat-transfer equipment, a) Heavy-duty jacketed for bqiiid coolant or high-pressure steam, (h) Jacketed for coolant spraying, (c) Light-duty jacketed construction, (d) Jacketed for air or steam in tiered arrangement, (e) Jacketed for air or steam with Mix-R-Step surface, Coutiesy of Jeffrey Mfg. Co.)... 

Equipment commonly employed for the diying of sohds is described both in this subsection in Sec. 12, where indirect heat transfer devices are discussed, and in Sec. 17 where fluidized beds are covered. Diyer control is discussed in Sec. 8. Excluding fluid beds this subsection contains mainly descriptions of direct-heat-transfer equipment. It also includes some indirect units e.g., vacuum diyers, furnaces, steam-tube diyers, and rotaiy calciners. 

Indirect heat transfer-fluid never comes in contact with product... 

Rotating-drum-type and belt-type heat-transfer equipment forms granular products directly from fluid pastes and melts without intermediate preforms. These processes are described in Sec. 5 as examples of indirect heat transfer to and from the solid phase. When solidification results from melt freezing, the operation is known as flaking. If evaporation occurs, solidification is by diying. 

Reactor heat carrier. As pointed out in Chapter 7, if adiabatic operation is not possible and it is not possible to control temperature by indirect heat transfer, then an inert material can be introduced to the reactor to increase its heat capacity flowrate (i.e. product of mass flowrate and specific heat capacity). This will reduce temperature rise for exothermic reactions or reduce temperature decrease for endothermic reactions. The introduction of an extraneous component as a heat carrier effects the recycle structure of the flowsheet. Figure 13.6a shows an example of the recycle structure for just such a process. 

Other indirect heat-transfer media and the temperature ranges for which they are used are hot water (35-400°F 2-200°C), mercury (600-1000°F 315-540°C), molten inorganic salts (300-1 100°F 150-600°C), and mineral oils (30600°F —1-3 15°C). The properties of these materials are given in the Chemical Engineer s Handbook.8 References 9 and 10 give the properties of some other substances. 

POX does not need indirect heat transfer (across a wall) so the processor is more compact and lightweight (3). 

Figure 13 shows a schematic diagram illustrating the configuration of a surface cooling (indirect heat transfer) crystallizer. Heat can be transferred to a coolant in an external heat exchanger, as shown, or in coils or a jacket... 

FIG. 11-58 Stationary vertical-tube type of indirect heat-transfer equipment with divided solids inside tubes, laminar solids flow and steady-state heat conditions. 

Conveyor-Befc Devices The metal-belt type (Fig. 11-55) is the only device in this classification of material-handling equipment that has had serious effort ejq)ended on it to adapt it to indirect heat-transfer service with divided solids. It features a lightweight construction of a large area with a thin metal wall. Indirect-cooling applications have been made with poor thermal performance, as could be expected with a static layer. Auxiliary plowlike mixing devices, which are considered an absolute necessity to secure any worthwhile results for this service, restrict applications. 

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