Pressure drop in fixed-bed

Pressure Drop. The prediction of pressure drop in fixed beds of adsorbent particles is important. When the pressure loss is too high, cosdy compression may be increased, adsorbent may be fluidized and subject to attrition, or the excessive force may cmsh the particles. As discussed previously, RPSA rehes on pressure drop for separation. Because of the cychc nature of adsorption processes, pressure drop must be calculated for each of the steps of the cycle. The most commonly used pressure drop equations for fixed beds of adsorbent are those of Ergun (143), Leva (144), and Brownell and co-workers (145). Each of these correlations uses a particle Reynolds number (Re = G///) and friction factor (f) to calculate the pressure drop (AP) per... 

There is evidence in the work reported in Chapter 5 on sedimentation 5) to suggest that where the particles are free to adjust their orientations with respect to one another and to the fluid, as in sedimentation and fluidisation, the equations for pressure drop in fixed beds overestimate the values where the particles can choose their orientation. A value of 3.36 rather than 5 for the Carman-Kozeny constant is in closer accord with experimental data. The coefficient in equation 6.3 then takes on the higher value of 0.0089. The experimental evidence is limited to a few measurements however and equation 6.3, with its possible inaccuracies, is used here. 

Figure 5.15. Pressure drop in fixed beds (from Ergun, 1952).

Molerus, 0. (1980), A coherent representation of pressure drop in fixed beds and of bed expansion for particulate fluidized beds, Chem. Eng. Sci., 35(6), 1331-1340. 

The procedure is quite simple in the case of intraparticle effects, since the diffusional modulus is directly proportional to the characteristic particle dimension. Thus the very practical idea of chop em up and run em again is very sound. If there is any influence of intraparticle transport, it should be manifested by a change (normally an increase) in the rate or conversion observed, with all other factors being the same. In principle, one should always be able to minimize intraparticle transport effects by working with catalyst particles of sufficiently small size. There are, however, practical limits associated with doing this, such as high pressure drops in fixed beds or elution in slurry or fluid beds that employ very fine particles (as we will see later). 

Pressure drop in fixed beds is normally interpreted in terms of the Ergun equation [S. Ergun, Chem. Eng. Prog., 48, 89 (1952)]. 

The choice of reaction equipment can be considerably influenced by fouling. As fouling increases, the pressure drop in fixed-bed reactors increases, whereas flui-dized-bed reactors are virtually unaffected. Moreover, fixed-bed reactors must be opened and emptied in order to regenerate the contaminated catalyst, but flui-dized-bed catalysts can usually be regenerated in the reactor. Often it is also possible... 

Commenge, J.-M., Falk, L., Corriou, J.-P, and Matlosz, M. (2002) Optimal design for flow uniformity in microchannel reactors. AIChE /., 48 (2), 345-358. Wirth, K.-E. (2010) Pressure drop in fixed beds. Part L1.6, in VDI-Heat Atlas, Springer, Berlin, New York, Heidelberg. Dietrich, B., W.S., Kind, M., and Martin, H. (2009) Pressure... 

The pressure loss in empty tubes depends on the fHction flictor/, which is a function of the Re number. For the pressure drop in fixed beds, which increases with decreasing particle diameter, the Ergun equation is used. For many processes (adsorption, gas-solid reactions, and heterogeneous catalysis) fixed beds are applied, and a particle size of more than 1 mm is used to avoid an excessive high pressure drop. 

The factors that affect the pressure drop in fixed-beds are ... 

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