References trickle beds

Numerous studies have been made of the hydrodynamics and other aspects of the behavior of gas/liquid/sohd systems, in particular of trickle beds, and including absorption and extrac tiou in packed beds. Some of the hterature is reviewed in the references at the end of this subsection. 

In this section, four other papers may be finally referred to, which, in addition to liquid holdup, deal with various other aspects of liquid flow in trickle-bed operation. 

It is well known that trickle-flow operation is characterized by comparatively poor heat-transfer properties, this being one of the disadvantages of this type of operation. Schoenemann (S4), for example, refers to the difficulties of controlling temperature in trickle-bed reactors. 

In trickle beds, the gas-to-liquid, kigaGL, and liquid-to-particle, kfaLS, coefficients are used to represent the effect of the external mass transfer resistances. The interfacial areas aGl and <2ls refer to the effective mass transfer surface per unit volume of empty reactor. Due to the fact that the coefficients kig and klL cannot be easily estimated independently from the corresponding interfacial areas aGL and aLS respectively, by simple experimental techniques, correlations are normally reported for the products kigaGL and k,a]S (Smith, 1981). 

The literature contains a number of references to other flow regime maps however, there is no clear advantage of using one map versus another. Wall effects can also have a major effect on the hydrodynamics of trickle bed reactors. Most of the data reported in the literature are for small laboratory units of 2-in diameter and under. 

Trickling and Pulsing Transition in Cocurrent Downflow Trickle-Bed Reactors with Special Reference to Large-Scale Columns... 

A summary of reactor models used by various authors to interpret trickle-bed reactor data mainly from liquid-limiting petroleum hydrodesulfurization reactions (19-21) is given in Table I of reference (37). These models are based upon i) plug-flow of the liquid-phase, ii) the apparent rate of reaction is controlled by either internal diffusion or intrinsic kinetics, iii) the reactor operates isothermally, and iv) the intrinsic reaction rate is first-order with respect to the nonvolatile liquid-limiting reactant. Model 4 in this table accounts for both incomplete external and internal catalyst wetting by introduction of the effectiveness factor r)Tg developed especially for this situation (37 ). 

X. Cocurrent gas-liquid flow in fixed beds Downflow in trickle bed and upflow in bubble columns. Literature review and meta-analysis. Analyzed both downflow and upflow. Recommendations for best mass- and heat-transfer correlations (see reference). [95]... 

The following discussion is concerned with processes in which both gas and liquid phases are present the third phase referred to is that of the catalyst itself. In trickle-bed processes the gas phase is continuous in bubble columns and loop reactors the gas phase is discrete. 

Tubular fluidized and fixed bed fermenters are deviations from the simple bubble column fermenter. Often utilized in producing beer and ciders, these fermenters contain immobilized microorganisms or microbial films on support surfaces. Microbes lost with the product are continuously replenished by adding fresh microorganisms into the packed bed fermenters. In the fixed bed case, slow downward flow of the medium significantly reduces the shear removal (mobilization) of the microbes from the support materials and increases the residence time in the packed column. This is a typical characteristic of the trickle bed fermenter for continuous operation. Readers are referred to the packed bed reactor entry in this volume for a more... 

The CD-ROM includes all the material on trickle bed reactors from the first edition of this book, A comprehensive example problem for trickle bed reactor design is included. See Profes.sjonal Reference Shelf R12.2. 

These reactions take place normally in a trickle bed downflow reactor at high hydrogen pressures (1500 to 4000 psi) and temperatures in excess of 600°F. The catalyst is frequently referred to as dual function since it promotes both cracking and hydrogenation. The catalyst base material is usually silica-alumina with metal oxides from groups VI and VIII (nickel, cobalt, molybdenum, tungsten). The active forms are the metal sulfides. Fractionation provides the waxy lubes cuts which are subsequently dewaxed and hydrofinished. 

The name trickle-bed reactor is usually applied in reference to a fixed bed in which a liquid phase and a gas phase flow concurrently throughout a bed of catalyst. By far the most important application, and hence much of the work, on these reactors has been in the hydrotreating of heavy feedstocks in the petroleum industry (hydrocracking, hydrodesulfurization, hydrodenitrogenation). However, this seems a very versatile processing method, and has not been exploited nearly to its potential in other areas such as waste water treatment—at least as the scientific literature would indicate. 

There have been many studies of the hydrodynamics of trickle beds that describe the different flow regimes and give empirical correlations for the pressure drop, liquid holdup, and the partial wetting of the catalyst. Only a few of these studies are discussed here, since extensive reviews are available [18-21]. A recent review [20] includes over 170 references. 

This review puts its focus on high-throughput preparation of heterogeneous catalysts, that is, solid-state materials that are apphed in fixed-bed reactors for gas-phase reactions and in trickle-bed or stirred-tank reactors for liquid or gas-hquid reactions, respectively. Other fields of catalysis are not discussed since very different catalytic systems are used. We refer to the following reviews for homogeneous catalysis (2, 3), where combinatorial catalysis deals mainly with variation of ligands and for electrochemical catalysis [4,5], where catalysts are prepared as arrays of thin films in electrochemical cells. 

The topic of trickle beds involves also many novel investigations, some of which can be found in reference (52). 

Consider a simple trickle-bed for which one can assume uniform gas and liquid distributions, a nonvolatile liquid, trickling flow, isothermal pellets, and complete wetting. For an isothermal reactor with a uniform distribution of the gas and liquid phases, there will be no radial gradients of concentration. Although axial dispersion is more important in trickle-beds than in fixed-beds because of relatively low fluid velocities, we will assume that the dispersion is negligible. Under the assumptions, one can write for the gas phase (refer to Figure 12.3) ... 

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