Packed Fixed Bed Reactors

Supercritical Fluids Technology in Lipase Catalyzed Processes 

FIGURE 3.3 Schematic diagram of immobilzed lipase PBR 1, substrate reservoir 2, temperature control 3, peristaltic pump 4, water jacket 5, bed of immobilized enzyme 6, three-way valve 7, product reservoir 8, sampling point 9, cooling/heating water 10, recirculation. (From Hita, E., A. Robles, B. Camacho, P. A. Gonzalez, L. Esteban, M. J. Jimenez, M. M. Munio, and E. Molina, 2009, Biochemical Engineering Journal 46 (3) 257-264. With permission.) 

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To be able to compare MSRs with randomly packed fixed-bed reactors, equivalent design criteria must be defined [69,70]. A fixed-bed reactor of cross-section Sbed and height Lbed filled with np spherical particles of diameter dp is compared with a multichannel MSR with a channel diameter dt and the same cross-section and the same height (Sbed = SstrUc/ kbed hstruc). 

When comparing reactors defined as equivalent according to Equation (13), the pressure drop in randomly packed fixed-bed reactors is about two to three times higher than in MSRs, depending on the geometry of the channels, as shown in Figure 8. 

The results in Table V illustrate the influence of sample weight and heating rate on product yields. The data show that increasing sample size reduces the tar yields and increases the char yields. The drastic decrease in tar yields is primarily due to increased vapor residence time in the reaction bed. If the vapor residence time is reduced in the reaction bed by using a fluidized bed or an entrained flow reactor, then secondary decomposition can be significantly reduced, and the effects of sample weight will not be as drastic. Thus, data collected in the loosely packed fixed-bed reactor of the present study may represent an extreme for an operating industrial reactor. 

Even multiple phases such as gas or liquid phases may be simultaneously present in a catalytic reactor. In such a case, the gas molecules are, at first, dissolved in the liquid bulk, after which they diffuse to the surface of the catalyst. This is where the actual reaction takes place. The industrial system is called a three-phase reactor. Three-phase reactors are discussed in Chapter 6 here, we will concentrate on catalytic two-phase reactors, in which a fluid—gas or liquid— reacts on the surface of a solid catalyst. The most commonly encoimtered types of catalytic two-phase reactors that are used industrially are a packed (fixed) bed reactor, a fluidized bed reactor, and a moving bed reactor. 

Fixed-bed reactors in the form of gas absorption equipment are used commonly for noncatalytic gas-liquid reactions. Here the packed bed serves only to give good contact between the gas and liquid. Both cocurrent and countercurrent operations are used. Countercurrent operation gives the highest reaction rates. Cocurrent operation is preferred if a short liquid residence time is required. 

Vanadium phosphoms oxide-based catalysts ate unstable in that they tend to lose phosphoms over time at reaction temperatures. Hot spots in fixed-bed reactors tend to accelerate this loss of phosphoms. This loss of phosphoms also produces a decrease in selectivity (70,136). Many steps have been taken, however, to aHeviate these problems and create an environment where the catalyst can operate at lower temperatures. For example, volatile organophosphoms compounds are fed to the reactor to mitigate the problem of phosphoms loss by the catalyst (137). The phosphoms feed also has the effect of controlling catalyst activity and thus improving catalyst selectivity in the reactor. The catalyst pack in the reactor may be stratified with an inert material (138,139). Stratification has the effect of reducing the extent of reaction pet unit volume and thus reducing the observed catalyst temperature (hot... 

Figure 4-8 shows a continuous reactor used for bubbling gaseous reactants through a liquid catalyst. This reactor allows for close temperature control. The fixed-bed (packed-bed) reactor is a tubular reactor that is packed with solid catalyst particles. The catalyst of the reactor may be placed in one or more fixed beds (i.e., layers across the reactor) or may be distributed in a series of parallel long tubes. The latter type of fixed-bed reactor is widely used in industry (e.g., ammonia synthesis) and offers several advantages over other forms of fixed beds. 

A packed-bed nonpermselective membrane reactor (PBNMR) is presented by Diakov et al. [31], who increased the operational stability in the partial oxidation of methanol by feeding oxygen directly and methanol through a macroporous stainless steel membrane to the PB. Al-Juaied et al. [32] used an inert membrane to distribute either oxygen or ethylene in the selective ethylene oxidation. By accounting for the proper kinetics of the reaction, the selectivity and yield of ethylene oxide could be enhanced over the fixed-bed reactor operation. 

For this purpose, cylindrical channels have been assumed. In randomly packed fixed beds the porosity is about 0.4, from which the relationship dp = 2.25 d is obtained. Since the focus is on heterogeneously catalyzed gas-phase reactions, it is important to not only ensure comparable conditions from a hydrodynamic point of view, but also as far as chemical reaction kinetics is concerned. Therefore, it is assumed that both reactors contain the same amount of catalyst. 

Reactors with a packed bed of catalyst are identical to those for gas-liquid reactions filled with inert packing. Trickle-bed reactors are probably the most commonly used reactors with a fixed bed of catalyst. A draft-tube reactor (loop reactor) can contain a catalytic packing (see Fig. 5.4-9) inside the central tube. Stmctured catalysts similar to structural packings in distillation and absorption columns or in static mixers, which are characterized by a low pressure drop, can also be inserted into the draft tube. Recently, a monolithic reactor (Fig. 5.4-11) has been developed, which is an alternative to the trickle-bed reactor. The monolith catalyst has the shape of a block with straight narrow channels on the walls of which catalytic species are deposited. The already extremely low pressure drop by friction is compensated by gravity forces. Consequently, the pressure in the gas phase is constant over the whole height of the reactor. If needed, the gas can be recirculated internally without the necessity of using an external pump. 

Laboratory reactor for studying three-phase processes can be divided in reactors with mobile and immobile catalyst particles. Bubble (suspension) column reactors, mechanically stirred tank reactors, ebullated-bed reactors and gas-lift reactors belong the class of reactors with mobile catalyst particles. Fixed-bed reactors with cocurrent (trickle-bed reactor and bubble columns, see Figs. 5.4-7 and 5.4-8 in Section 5.4.1) or countercurrent (packed column, see Fig. 5.4-8) flow of phases are reactors with immobile catalyst particles. A mobile catalyst is usually of the form of finely powdered particles, while coarser catalysts are studied when placing them in a fixed place (possibly moving as in mechanically agitated basket-type reactors). 

Two fixed-bed reactors can be used in parallel, one reacting and the other regenerating. However, there are many disadvantages in carrying out this type of reaction in a packed bed. The operation is not under steady state conditions, and this can present control problems. Eventually, the bed must be taken off line to replace the solid. Fluidized beds (to be discussed later) are usually preferred for gas-solid noncatalytic reactions. 

Fixed Bed Reactors. In its most basic form, a fixed bed reactor consists of a cylindrical tube filled with catalyst pellets. Reactants flow through the catalyst bed and are converted into products. Fixed bed reactors are often referred to as packed bed reactors. They may be regarded as the workhorse of the chemical industry with respect to the number of reactors employed and the economic value of the materials produced. Ammonia synthesis, sulfuric acid production (by oxidation of S02 to S03), and nitric acid production (by ammonia oxidation) are only a few of the extremely high tonnage processes that make extensive use of various forms of packed bed reactors. 

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