Catalytic fixed-bed

Traditionally, an average Sherwood number has been determined for different catalytic fixed-bed reactors assuming constant concentration or constant flux on the catalyst surface. In reality, the boundary condition on the surface has neither a constant concentration nor a constant flux. In addition, the Sh-number will vary locally around the catalyst particles and in time since mass transfer depends on both flow and concentration boundary layers. When external mass transfer becomes important at a high reaction rate, the concentration on the particle surface varies and affects both the reaction rate and selectivity, and consequently, the traditional models fail to predict this outcome. 

A catalytic fixed bed reactor is a (usually) cylindrical tube that is randomly filled with porous catalyst particles. These are frequently spheres or cylindrical pellets, but other shapes are also possible. The use of rings or other forms of particles with internal voids or external shaping is on the increase. During single-phase operation, a gas or liquid flows through the tube and over the catalyst particles, and reactions take place on the surfaces, both interior and exterior, of the particles. 

Single-phase catalytic fixed bed reactors are the main reactor type used for large-scale heterogeneously catalyzed gas-phase reactions. Frequently, multitubular... 

The second approach consists in deliberately abandoning certain similarity criteria and checking the effect on the entire process. This technique was used by Gerhard Damkohler (1908-1944) in his trials to treat a chemical reaction in a catalytic fixed bed reactor by means of dimensional analysis. 

This equation holds for adsorption, ion exchange as well as for catalytic systems, which are in a transient operating condition, e.g. during severe catalyst deactivation. For a steady state catalytic fixed-bed operation, eq. (3.287) becomes... 

Thus, the term usc)Cldz comes from the derivative dFIdV, which should be used instead in eq. (3.287) for a variable-volume system. For catalytic fixed beds, in the typical case of a steady-state operation, eq. (3.288) takes the familiar form... 

The energy balance (3.301) is applicable for catalysis, adsorption, and ion exchange. More specifically, in catalysis, where the steady-state condition exists, frequently the accumulation term is zero. In contrast, adsorption and ion exchange operate under unsteady-state condition. The analysis of the energy balance equation for catalytic fixed beds is presented in detail in Section 5.3.4. 

For constant superficial velocity (zero expansion factor) and negligible pressure drop (see the subsection Nonisobaric fixed-bed operation), the general mass and thermal energy continuity equations for the catalytic fixed-bed reactor are... 

Film diffusion resistance All bubbling reactors such as Catalytic fixed-bed G/S reactors BFB and three-phase reactors All fast, noncatalytic G/S reactions such as combustion and gasification ... 

Fig. 4. Schematic drawing of combined capillary in situ cell and catalytic fixed bed reactor [adapted from Clausen et at. (32)].

The countercurrent-flow fixed-bed operation is often used for physical absorption or for gas-liquid reactions rather than gas-liquid-solid processes. Shah [1] gives a comparison between a gas-liquid-solid (catalytic) fixed bed reactor and a gas-liquid-solid (inert) fixed-bed reactor. The major difference between these two types of reactors are the nature and the size of the packing used and the conditions of gas and liquid flow-rates. 

G. Wild, F. Larachi and A. Laurent, The hydrodynamics characteristics of cocurrent downflow and cocurrent upflow gas-liquid-solid catalytic fixed bed reactors the effect of pressure, Revue de l lnstitut Franfais du Petrole, 46 (1991) 467-490. 

Another major chlorinated hydrocarbon is vinyl chloride. For many years acetylene was the sole raw material for the production of vinyl chloride by a catalytic fixed bed vapor-phase process. This process is characterized by high yields and modest capital investment. Nevertheless, the high relative cost of acetylene provided an incentive to replace it in whole or in part by ethylene. The first step in this direction was the concurrent use of both raw materials. Ethylene was chlorinated to di-chloroethane, and the hydrogen chloride derived from the subsequent dehydrochlorination reacted with acetylene to form additional vinyl chloride. 

Third, the line can become unstable during laser writing, and instead of a single line, a periodic pattern of discrete deposits is obtained (233-235). This pattern is analogous to bifurcations in other spatially distributed systems, such as catalytic fixed-bed reactors, and can be analyzed in the same manner (235). 

There are many other interesting and complex dynamic phenomena besides oscillation and chaos which have been observed but not followed in depth both theoretically and experimentally. One example is the wrong directional behavior of catalytic fixed-bed reactors, for which the dynamic response to input disturbances is opposite of that suggested by the steady-state response [99, 100], This behavior is most probably connected to the instability problems in these catalytic reactors as shown crudely by Elnashaie and Cresswell [99]. Recently Elnashaie and co-workers [102-105] have also shown rich bifurcation and chaotic behavior of an anaerobic fermentor for producing ethanol. They have shown that the periodic and chaotic attractors may give higher ethanol yield and productivity than the optimal steady states. These results have been confirmed experimentally [105],... 

Residfining is a catalytic fixed-bed process for the desulfurization and demetallization of residua (Table 9-17) (Hydrocarbon Processing, 1998). The process can also be used to pretreat residua to suitably low contaminant levels prior to catalytic cracking. 

Fig. 1.5. Effect of heat transfer limitation in industrial steam reformers, (a) Scheme of a single catalytic fixed-bed reformer tube with typical heat transfer and operating parameters.

Fig. 1.9. Coupling of dehydrogenation of ethylbenzene to styrene and hydrogen combustion in a catalytic fixed-bed reverse flow reactor [9]. (a, b) Fixed-bed temperature profiles during production and regeneration cycle.

Fig. 1.15. Coupling of propane dehydrogenation and methane combustion in a four-step catalytic fixed-bed process [28].

G. Eigenberger, Catalytic fixed-bed reactors, in Handbook of heterogeneous catalysis, Eds G. Ertl, H. Knozinger,... 

Studies were also performed with an artificial fixed bed composed of an array of microstructured columns made by a plasma etch process. These columns were made porous to increase the surface area to 100 m2, which is not far from the porosity of catalyst particles in fixed beds, and then coated with a catalyst [278]. The performance of such catalytic microcolumns was compared with that of a catalytic fixed bed reactor. When normalized to the metal content, the reaction rates of the columnar and the particle-containing reactor are similar with 6.5 x 10 5 and 4.5 x 10-5 mol/(minm2), respectively. 

When appropriate material systems are not available for model experiments, accurate simulation of the working conditions of an industrial plant on a laboratory- or bench-scale may not be possible. Under such conditions, experiments on differently sized equipment are customarily performed before extrapolation of the results to the full-scale operation. Sometimes this expensive and basically unreliable procedure can be replaced by a well-planned experimental strategy. Namely, the process in question can be either divided up into parts which are then investigated separately (Example 9 Drag resistance of a ship s hull after Froude) or certain similarity criteria can be deliberately abandoned and then their effect on the entire process checked (Example 41/2 Simultaneous mass and heat transfer in a catalytic fixed bed reactor after Damkohler). 

Figure 1. Basic types of catalytic fixed-bed reactors. A) Adiabatic fixed-bed reactor B) Multitubular fixed-bed reactor.

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