Monolithic reactors channel design

In this chapter, first, the existing correlations for three-phase monolith reactors will be reviewed. It should be emphasized that most of these correlations were derived from a limited number of experiments, and care must be taken in applying them outside the ranges studied. Furthermore, most of the theoretical work concerns Taylor flow in cylindrical channels (see Chapter 9). However, for other geometries and flow patterns we have to rely on empirical or semiempirical correlations. Next, the modeling of the monolith reactors will be presented. On this basis, comparisons will be made between three basic types of continuous three-phase reactor monolith reactor (MR), trickle-bed reactor (TBR), and slurry reactor (SR). Finally, for MRs, factors important in the reactor design will be discussed. 

Jet injectors may also be combined with monolith reactors. Monoliths are usually tube reactors with channeled flow. The reaction occurs at the gas-liquid interface as well as on the channel wall, which are usually catalytic or coated with catalytic material. Monoliths can be made into vertical (similar to bubble column) or horizontal tubes, airlift devices (whereby the riser would a monolith), or even into a mechanically stirred device. Usually, however, monoliths are designed like bubble columns or airhft reactors (Broekhuis et al., 2001). 

Hydraulic diameters of monolithic channels range between ca. 3x 10 m and 6x 10 m [8]. Combination of such small diameter channels leads to surfece areas per reactor volume in the order of 10 m /m (which is 10 m /m for PBRs) and void fractions up to 75% (which is -40% for PBRs). As shown in Figure 1.10 [9], these design properties allow monolith reactors to operate with pressure drops that are up to three orders of magnitude less than those observed in PBRs. 

Under most operating conditions and reactor/catalyst designs, nonnegligible sources of internal and external mass transfer limitation will exist. Actually, as it is discussed in Section 8.3.5, mass transfer-controlled conditions correspond to the monolith design associated with lower pressure drop (channel length). 

In structured reactors, the structural units are repeated. In monoliths, the units are usually simple channels with a square cross section, though many variations are encountered. In Sulzer-t)q)e packings, the units are combinations of corrugated sheets. In microreactors, the basic unit can have any of several shapes. What structured reactors have in common is that a precise design is made, starting from a description of one single unit. 

Channels filled with a single-particle string have much better solid flow characteristics than a packed bed, and so application of monoliths as moving-bed reactor internals is appealing. This structured moving-bed reactor design opens a wide range of applications. 

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