Heat transfer in packed bed reactors

Specchia V, Baldi G, Sicardi S., "Heat transfer in packed bed reactors with one-phase flow", Chem.Eng.Commun. 36I (1980). 

Figure 17.18. Heat transfer in fixed-bed reactors (a) adequate preheat (b) internal heat exchanger (c) annular cooling spaces (d) packed tubes (e) packed shell (f) tube and thimble (g) external heat exchanger (h) multiple shell, with external heat transfer (Walas, 1959).

Extensive experimental determinations of overall heat transfer coefficients over packed reactor tubes suitable for selective oxidation are presented. The scope of the experiments covers the effects of tube diameter, coolant temperature, air mass velocity, packing size, shape and thermal conductivity. Various predictive models of heat transfer in packed beds are tested with the data. The best results (to within 10%) are obtained from a recently developed two-phase continuum model, incorporating combined conduction, convection and radiation, the latter being found to be significant under commercial operating conditions. 

Knowledge of the heat transfer characteristics and spatial temperature distributions in packed beds is of paramount importance to the design and analysis of the packed-bed catalytic or non-catalytic reactors. Hence, an attempt is made in this section to quantify the heat transfer coefficients in terms of correlations based on a wide variety of experimental data and their associated heat transfer models. The principal modes of heat transfer in packed beds consist of conduction, convection, and radiation. The contribution of each of these modes to the overall heat transfer may not be linearly additive, and mutual interaction effects need to be taken into account [23,24]. Here we limit our discussion to noninteractive modes of heat transfer. 

Figure 12.17 Heat transfer in packed beds. Effective thermal conductivity as a function of Reynolds number. Curve 1, Coberly and Marshall curve 2, Campbell and Huntington curve 3, Calderbank and Pogorski curve 4, Kwong and Smith curve 5, Kunii and Smith. (Adapted from G. F. Froment and K. B. Bischoff, Chemical Reactor Analysis and Design, p. 533. Copyright 1979. Used with permission of John Wiley Sons, Inc.)...

Correlations for heat-transfer coefficients for packed beds are useful in designing fixed-bed systems such as catalytic reactors, dryers for solids, and p>ebble-bed heat exchangers. In Section 3.1C the pressure drop in packed beds was considered and discussions of the geometry factors in these beds were given. For determining the rate of heat transfer in packed beds for a differential length dz in m,... 

The model presented here is a significant step forward in the simulation of fixed bed catalytic reactors. It is an early computational fluid dynamics (CFD) model of the continuum type. In recent years supercomputers have led to an increased application of CFD to studies of heat transfer in packed beds. In modeling the fluid flow in the voids confined by the catalyst particles, Nijemeisland and Dixon [2004] investigated the possibility of deriving values for the heat transfer coefficient between the bed and the wall in terms of the local properties of the flow field, but found no statistically valid correlation. They... 

An experimental evaluation of homogeneous continum models of steady state heat transfer in packed beds of low tube/particle diameter ratio has been carried out. It was found that both axial and radial conduction effects were important in such beds for N j 500, which covers the flow range in many industrial reactors. Heat transfer resistance at the wall was significant, but of secondary importance. 

In packed-bed reactors, the catalyst is fully wetted, whereas the heat and mass transfer efficiency is higher than that observed in trickle-bed reactors. However, low operation efficiency may appear due to backmixing of the liquid phase. Moreover, high liquid-phase residence times can result in the occurrence of homogeneous side reactions. 

McGreavy and THORNTON(23) have developed an alternative approach to the problem of identifying such regions of unique and multiple solutions in packed bed reactors. Recognising that the resistance to heat transfer is probably due to a thin gas film surrounding the particle, but that the resistance to mass transfer is within the porous solid, they solved the mass and heat balance equations for a pellet with modified boundary conditions. Thus the heat balance for the pellet represented by equation 3.24 was replaced by ... 

Heat and Mass Transfer in Packed Beds, NWakao S Kaguei Three-Phase Catalytic Reactors, P A Ramachandran R V Chaudari Drying Principles, Applications and Design, by Cz Strumillo T Kudra... 

The first three types (pellets, extrudates and granules) are primarily used in packed bed operations. Usually two factors (the diffusion resistance within the porous structure and the pressure drop over the bed) determine the size and shape of the particles. In packed bed reactors, cooled or heated through the tube wall, radial heat transfer and heat transfer from the wall to the bed becomes important too. For rapid, highly exothermic and endothermic reactions (oxidation and hydrogenation reactions, such as the ox-... 

The effects of diffusional restrictions on the activity and selectivity of FT synthesis processes have been widely studied (32,52,56-60). Intrapellet diffusion limitations are common in packed-bed reactors because heat transfer and pressure-drop considerations require the use of relatively large particles. Bubble columns typically use much smaller pellets, and FT synthesis rates and selectivity are more likely to be influenced by the rate of mass transfer across the gas-liquid interface as a gas bubble traverses the reactor (59,61,62). 

The heat transfer problems in packed-bed reactors are connected with poor radial mixing. These problems are more pronounced in packed-bed reactors with low liquid rates. In such cases, the catalyst will not be completely wetted, resulting in hot spots and in some cases temperature runaway effects. 

Heat transfer between packed beds and the external column wall has been widely studied because of its relevance to the design and operation of wall-cooled catalytic reactors. In the one-dimensional model, which is the basis of Eq. (7.17), the overall heat transfer resistance may be represented as the sum of the internal, external, and wall resistances ... 

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