Heat transfer packed beds

VOC emissions from printing and chemical plants are oxidized in reverse flow reactors that couple reaction with regenerative heat transfer. The concept here is to maintain a catalyst zone in the center of a packed bed with inert heat-transfer packing on either side. 

N. Wakao, Heat and Mass Transfer in Packed Beds, Gordon Breach, New York, 1982. 

A. Heat or mass transfer in packed bed for gases and liquids... 

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... 

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. 

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... 

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. 

There are two basic types of packed-bed reactors those in which the solid is a reactant and those in which the solid is a catalyst. Many e.xaniples of the first type can be found in the extractive metallurgical industries. In the chemical process industries, the designer normally meets the second type, catalytic reactors. Industrial packed-bed catalylic reactors range in size from units with small tubes (a few centimeters in diameter) to large-diameter packed beds. Packed-bed reactors are used for gas and gas-liquid reactions. Heat transfer rates in large-diameter packed beds are poor and where high heat transfer rates are required, Jluidized beds should be considered. ... 

Superior surface utilization in mass and heat transfer, allowng shorter packed bed heights. Turn-down performance is superior over 2-in. and 3) -in. Pall rings. 

A certain amount of information on particle-to-liquid and bed-to-wall heat transfer is available for single-phase fluid flow through a packed bed. It is not clear, however, to what extent this information can be applied to... 

The heat-transfer properties of the suspended-bed operations appear superior to those of the packed-bed operations. In particular, the former have a high effective thermal conductivity, and the development of uneven temperature distributions is therefore less likely. 

Experimental results for fixed packed beds are very sensitive to the structure of the bed which may be strongly influenced by its method of formation. GUPTA and Thodos157 have studied both heat transfer and mass transfer in fixed beds and have shown that the results for both processes may be correlated by similar equations based on. / -factors (see Section 10.8.1). Re-arrangement of the terms in the mass transfer equation, permits the results for the Sherwood number (Sh1) to be expressed as a function of the Reynolds (Re,) and Schmidt numbers (Sc) ... 

Dixon, A. G. and Cresswell, D. L., Theoretical prediction of effective heat transfer parameters in packed beds, AIChE /., 25, 663-676 (1979). 

The heat and mass transfer phenomena associated with packed-bed reactors are described in... 

Correlations for heat transfer in packed-beds are still being developed. The current state of the art is represented by... 

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