Packings heat transfer

Fractionation efficiency, 6,11,195-196, 209, 365 fractionator problems, 195-196, 209 Fractionator problems (FCCU product fractionation), 18f3-200 flooding, 188-189 load observation, 189-191 liquid loading sensitivity, 189 capacity comparison, 189,192-193 heattransfer coefficients, 193-194 structured packing heat transfer, 193 height equivalent to theoretical stage,... 

After compression and removal of impurities, the air is cooled ia heat exchangers and expanded to low pressure through a turbiae, to recover energy, or through a valve. Liquid air, which forms at about 80 K, is separated via a distillation column. The column as well as the heat exchangers and the associated piping are placed within a cold box, which is packed with iasulation to minimise heat transfer (qv) between streams and to protect the system from the ambient air external to the cold box. 

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. 

The packed-bed-to-fluid heat-transfer coefficient has been investigated by Baumeister and Bennett [Am. In.st. Chem. Eng. J., 4, 69 (1958)], who proposed the equation... 

Functional Definitions Heat-transfer equipment can be designated by type (e.g., fixed tube sheet, outside packed head, etc.) or by... 

The disadvantage is that volumetric efficiency is usually much less than conventional trays or packed contactors. Applications are usually limited to cases when only a few transfer units or a single eqiiihbriiim stage is required. Since many of these applications tend to be in heat-transfer sei vice, the following discussion will be in terms of thermal properties and thermal measures of performance. 

Some modes of heat transfer to stirred tank reacdors are shown in Fig. 23-1 and to packed bed reactors in Fig. 23-2. Temperature and composition profiles of some processes are shown in Fig. 23-3. Operating data, catalysts, and reaction times are stated for a number of industrial reaction processes in Table 23-1. 

Stirred Vessels Gases may be dispersed in hquids by spargers or nozzles and redispersed by packing or trays. More intensive dispersion and redispersion is obtained by mechanical agitation. At the same time, the agitation will improve heat transfer and will keep catalyst particles in suspension if necessaiy. Power inputs of 0.6 to 2.0 kW/m (3.05 to 10.15 np/1,000 gal) are suitable. 

Equipment suitable for reactions between hquids is represented in Fig. 23-37. Almost invariably, one of the phases is aqueous with reactants distributed between phases for instance, NaOH in water at the start and an ester in the organic phase. Such reac tions can be carried out in any kind of equipment that is suitable for physical extraction, including mixer-settlers and towers of various kinds-, empty or packed, still or agitated, either phase dispersed, provided that adequate heat transfer can be incorporated. Mechanically agitated tanks are favored because the interfacial area can be made large, as much as 100 times that of spray towers, for instance. Power requirements for L/L mixing are normally about 5 hp/1,000 gal and tip speeds of turbine-type impellers are 4.6 to 6.1 i7i/s (15 to 20 ft/s). 

The Rowe-Claxton empirical equation has been found to conform to many experimental studies of heat transfer in a packed bed, such as the reactor typically used in the catalytic processes described earlier. It is first necessary in this situation to define die voidage of the system, AV, where... 

Yagi and Wakao (1959) used mass transfer measurement results to estimate the heat transfer coefficient at the tube wall. Material was coated on the inner surface of the packed tubes and the dissolution rate was measured. 

As can be seen in the table above, the upper two results for heat transfer coefficients hp between particle and gas are about 10% apart. The lower three results for wall heat transfer coefficients, h in packed beds have a somewhat wider range among themselves. The two groups are not very different if errors internal to the groups are considered. Since the heat transfer area of the particles is many times larger than that at the wall, the critical temperature difference will be at the wall. The significance of this will be shown later in the discussion of thermal sensitivity and stability. 

Fill Packing Specially designed baffling used to provide a large surface area for heat transfer. Two classes of materials are used splash bars of wood, metal transite or plastic and film pack (cellular fill). The splash type cools the water as the droplets bounce down a series of bars in the air stream film packing converts droplets into a thin film. 

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