Reactors with Catalyst Packings

Column reactors with static packing elements provide an attractive alternative for conventional packed bed reactors, since they essentially combine the benefits of classical fixed beds and slurry reactors static mixing elements give rise to local turbulence, catalyst separation 

FIGURE 9.3 Velocity profiles in Katapak elements (CFD calculations, water at 20°C). 

The modeling of column reactors is based on verified hypotheses concerning the hydrodynamics and mass transfer conditions. The following fundamental assumptions can be applied to the modeling of column reactors  

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Steam reforming is usually carried out in fired tubular reactors, with catalyst packed inside the tubes and fuel fired on the outside of the tubes to provide the heat of reaction. The product gas mixture contains carbon dioxide and water vapor as well as carbon monoxide and hydrogen and is conventionally known as synthesis gas or syngas. 

Parameter DBD reactor without catalyst DBD reactor with catalyst Packed-bed reactor with catalyst ... 

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... 

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. 

As already shown by Wiese et al. [17] mass transport rates in biphasic catalysis can be dramatically influenced by hydrodynamics in a tube reactor with Sulzer packings. Above all, the volume rate of the catalyst phase in which the substrates are transported by diffusion plays a decisive role in accelerating the mass transport rate. This effect was also investigated for citral hydrogenation in the loop reactor. Overall reaction rates and conversions as a function of the catalyst volume rate can be seen in Fig. 15. 

Fixed-bed catalytic reactors and reactive distillation columns are widely used in many industrial processes. Recently, structured packing (e.g., monoliths, katapak, mella-pak etc.) has been suggested for various chemical processes [1-4,14].One of the major challenges in the design and operation of reactors with structured packing is the prevention of liquid flow maldistribution, which could cause portions of the bed to be incompletely wetted. Such maldistribution, when it occurs, causes severe under-performance of reactors or catalytic distillation columns. It also can lead to hot spot formation, reactor runaway in exothermic reactions, decreased selectivity to desired products, in addition to the general underutilization of the catalyst bed. 

Modeling of Tubular Nonisothermal Nonadiabatic Packed-Bed Reactors with Catalyst Poisoning... 

Other recent work in the field of optimization of catalytic reactors experiencing catalyst decay includes the work of Romero e/ n/. (1981 a) who carried out an analysis of the temperature-time sequence for deactivating isothermal catalyst bed. Sandana (1982) investigated the optimum temperature policy for a deactivating catalytic packed bed reactor which is operated isothermally. Promanik and Kunzru (1984) obtained the optimal policy for a consecutive reaction in a CSTR with concentration dependent catalyst deactivation. Ferraris ei al. (1984) suggested an approximate method to obtain the optimal control policy for tubular catalytic reactors with catalyst decay. 

Fig. 30. Contacting patterns and contactor types for gas-liquid-solid reactors, (a) Co-current downflow trickle bed. (b) Countercurrent flow trickle bed. (c) Co-current downflow of gas, liquid, and catalyst, (d) Downflow of catalyst and co-current upflow of gas and liquid, (e) Multi-tubular trickle bed with co-current flow of gas and liquid down tubes with catalyst packed inside them coolant on shell side, (f) Multi-tubular trickle bed with downflow of gas and liquid coolant inside the tubes, (g) Three-phase fluidized bed of solids with solids-free freeboard, (h) Three-phase slurry reactor with no solids-free freeboard, (i) Three-phase fluidized beds with horizontally disposed internals to achieve staging, (j) Three-phase slurry reactor with horizontally disposed internals to achieve staging, (k) Three-phase fluidized bed in which cooling tubes have been inserted coolant inside the tubes. (1) Three-phase slurry... 

The same trend was found when plotting the reaction rate constants obtained with the crushed monoliths and the powder catalysts as a function of metal loading. The fact that the rate constant decreases with the metal loading is due to lower dispersion when the metal content increases. Now that monolithic catalysts with the same characteristics as powder catalysts were successfully prepared, the next challenge is to compare the performance of these structured reactors with conventional packed bed reactors in a multiphase system and,... 

Packed Bed Membrane Reactor (Inert Membrane Reactor, Inert Membrane Reactor with Catalyst on Feed side)... 

Prrted by the vessel shell. Sand filters, packed columns, and reactors with catalyst beds are typical examples. The beds are often supported by a combination of beam(s), grating, and a circumferential ring which supports the periphery of the grating. The beams are in turn attached to the shell wall by either clips or beam seats. This procedure offers a quick way for analyzing the various support components. 

To compare the catalytic wall reactor with a packed bed, correct criteria must be chosen [18]. For both the reactors, the outer catalyst surface per void (V oy) volume and the space-time must be identical. Under these conditions, the following relationship between the diameter of the microchannel and the particle diameter holds... 

Oxidative coupling of methane using a catalytic-membrane reactor (CMR), catalyst packed-bed reactor (PBR), and catalyst packed-bed membrane reactor (PBMR), have been also compared. The authors conclude that the catalytic activity of PBR and PBMR (using Na-WMn/Si02) were lower than that observed for CMR (with a yield of 34.7%). 

Structure of the disc/flat-sheet membrane reactor (a) catalyst packed on membrane and (b) catalyst coated on membrane with electrodes and external circuit. 

Oxidation Step. A review of mechanistic studies of partial oxidation of propylene has appeared (58). The oxidation process flow sheet (Fig. 2) shows equipment and typical operating conditions. The reactors are of the fixed-bed shell-and-tube type (about 3—5 mlong and 2.5 cm in diameter) with a molten salt coolant on the shell side. The tubes are packed with catalyst, a small amount of inert material at the top serving as a preheater section for the feed gases. Vaporized propylene is mixed with steam and ak and fed to the first-stage reactor. The feed composition is typically 5—7% propylene, 10—30%... 

Rates are moles methanol formed per cubic meter of catalyst packed reactor volume per second Reproduced with permission from Chem. Eng. Comm, 76, pp. 9-33, 1989 Gordon and Breach. 

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. 

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