Contact Heat Transfer

One disadvantage of fluidized heds is that attrition of the catalyst can cause the generation of catalyst flnes, which are then carried over from the hed and lost from the system. This carryover of catalyst flnes sometimes necessitates cooling the reactor effluent through direct-contact heat transfer hy mixing with a cold fluid, since the fines tend to foul conventional heat exchangers. 

While a superstructure based on the structure in Fig. 16.26 allows for many structural options, it is not comprehensive. Wood, Wilcox, and Grossmanr showed how direct contact heat transfer by mixing at unequal temperatures can be used to decrease the number of units in a heat exchanger network. Floudas, Ciric, and Grossman showed how such features can be included in a heat exchanger network superstructure. Figure 16.27 shows the structure from Fig. 16.26 with possibilities for direct contact heat transfer included. In the... 

Figure 16.27 Possibilities for direct contact heat transfer can be added to the superstructure.

Aqueous solutions of propylene glycol display excellent antifree2e properties and are therefore valuable as low temperature heat-transfer fluids. For apphcations involving indirect food contact, heat-transfer fluids formulated with the USP grade product are preferred, since there could be inadvertent contact with a food product. These fluids are commonly used in the brewing and dairy industries as well as in refrigerated display cases in retail grocery stores. 

Contactive (Direct) Heat Transfer Contactive heat-transfer equipment is so constructed that the particulate burden in solid phase is directly exposed to and permeated by the heating or cooling medium (Sec. 20). The carrier may either heat or cool the solids. A large amount of the industrial heat processing of sohds is effected by this mechanism. Physically, these can be classified into packed beds and various degrees of agitated beds from dilute to dense fluidized beds. 

Although direc t contactive heat transfer can provide high temperatures and heat concentrations and at the same time be small in size, its use may not always be preferable because of undesired side effec ts such as diying, contamination, case hardening, shrinkage, off color, and dusting. 

This section describes equipment for heat transfer to or from solids by the indirect mode. Such equipment is so constructed that the solids load (burden) is separated from the heat-carrier medium by a wall the two phases are never in direct contact. Heat transfer is by conduction based on diffusion laws. Equipment in which the phases are in direct contact is covered in other sections of this Handbook, principally in Sec. 20. 

The static mixer is also useful for direct contact heat transfer between fluids, nvo phase contacting, and other useful applications such as mass transfer. 

The spray-filled tower, Figure 9-100, is also an atmospheric type, containing no fill other than the water sprays and no fans. The water-air contact comes about due to the spray distribution system [144], This design is often used where higher water temperatures are allowed, and the situations where excessive contaminants building up in the water would cause fouling of other direct contact heat transfer surfaces. 

Example 10-26. Determine Contact Stages Actually Required for Direct Contact Heat Transfer in Plate-Type Columns... 

John C. Berg, Andreas Acrivos, and Michel Boudart, Evaporation Convection H. M. Tsuchiya, A. G, Fredrickson, and R. Aiis, Dynamics of Microbial Cell Populations Samuel Sideman, Direct Contact Heat Transfer between Immiscible Liquids Howard Brenner, Hydrodynamic Resistance of Particles at Small Reynolds Numbers... 

Figure 19.5 Nonisothermal mixing carries out direct contact heat transfer.

H. M. Tsuchiya, A. G. Fredrickson, and R. Aris Direct Contact Heat Transfer between Immiscible Liquids... 

The design of devices to promote cocurrent drop flows for heating or cooling a two-phase system, or for direct-contact heat transfer between two liquids, is difficult. The study by Wilke et al. (W7) is typical of the approach frequently used to analyze these processes. Wilke et al. described the direct-contact heat transfer between Aroclor (a heavy organic liquid) and seawater in a 3-in. pipe. No attempt was made to describe the flow pattern that existed in the system. The interfacial heat-transfer coefficient was defined by... 

Nuclear accident scenarios, direct contact heat transfer in, 13 268 Nuclear age, 25 391 Nuclear applications hafnium, 13 89 nitrides in, 17 219... 

Direct Contact Heat Transfer between immiscible Liquids Samuel Sideman... 

The main mechanisms of heat transfer in a hydrocarbon are thermal radiation and direct flame contact. Heat transfer to personnel will cause burns. Heat transfer to equipment and structures can lead to failure of hydrocarbon containing equipment, which can further feed the fire. 

Data of direct contact heat transfer are not abundant. The literature has been reviewed by Fair (1972) from whom specific data will be cited. 

The principles of direct-contact heat transfer, treated in the discussions to follow, are presently almost exclusively applied to water cooling and humidification of air. It should be noted, however, that these very same principles can be applied to the cooling or heating of just about any insoluble gas or liquid. 

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