Types of Fixed-Bed Reactors

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. 

This is also a reversible, exothermic reaction carried out in various types of fixed-bed reactors, involving different arrangements for flow (axial or radial), and temperature adjustment see Figure 11.5. The traditional catalyst is promoted Fe, but more active Ru-based catalysts are finding use, despite the added... 

The reaction chambers used to remove nitrogen oxides from power station flue gases constitute the largest type of fixed-bed reactors as regards reactor volume and throughput, while automobile exhaust purification... 

Schematic diagrams of various types of fixed-bed reactors are shown in Fig. 1 -1. The reactor thus reproduces the industrial reactors on a small scale and it can be used to obtain the required data for reactor scaleup.

In a fixed bed reactor, gas phase reactions are generally carried out using a stationary bed of solid catalyst. In a typical reactor, suitable screens support the bed of catalyst particles, through which the gas phase flows. Gaseous reactants adsorb on the catalyst surface, reactions occur on this surface and reaction products desorb back to the gas phase. Two major types of fixed bed reactor are the conventional axial flow fixed bed reactor and the radial flow fixed bed reactor. These types are shown... 

FIGURE 13.1 Types of fixed bed reactors, (a) Axial flow fixed bed reactor Up or down flow, single or multi-stage, with or without inter-stage cooling, single or multi-tubular, (b) Radial flow fixed bed reactor Radially inward or outward flow, straight or reverse flow (direction of inlet and outlet is same or opposite to each other). 

The methodology illustrated by the above example is fairly general and can be applied to any type of fixed bed reactor. For example, Ranade (1994) used a CFD model to... 

Two types of fixed-bed reactors are used in HDT processes. Fixed-bed reactors having randomly packed... 

Several types of fixed-bed reactor systems were considered for the MTG process development (ref. 9). Perhaps the easiest to scale-up is the two-reactor configuration shown in Fig. 3 this was used for bench-scale studies. Both reactors contained an axial thermowell to monitor temperature profiles. Special precautions (e.g., adiabatic heaters, insulation) were taken to ensure proper accounting of heat effects, although in small reactors it is virtually impossible to be 100% adiabatic (ref. 

Recently, a similar process has been applied by Degussa for the production of L-amino acids. In this case, L-amino acids are obtained by biocatalytic division of synthetically-produced acetyl DL-amino acids by means of enzymes. Unlike the previous type of fixed-bed reactor with carrier-located acylase, the new approach employs the enzyme in soluble form, and uses a membrane for separating the enzyme from the reagent solution. This avoids losses at the immobilizing stage and reduces enzyme consumption (.5). 

In this chapter it is not possible to concentrate on specific cases and processes. Instead, we discuss general models and principles involved in the design and analysis of any type of fixed bed reactor, no matter what the process. 

There are basically two types of fixed-bed reactors (1) multitubular, in which tubes of approximately 1.5 to 4.0 cm in diameter are placed as a bundle within a shell through which a heat exchange fluid is circulated to control the temperature profile within the reactor and (2) adiabatic, in which the catalyst is placed directly inside a reactor (with no a priori limitation to the diameter), and heat... 

Figure 4.3 Basic types of fixed-bed reactor models.

There are different types of fixed bed reactors, the most known are ... 

Nontraditional types of fixed-bed reactors are begmning to be considered. A gas-lift reactor with monolith type catalyst packing has been described for FT synthesis [40]. It is reported that this reactor has been tested at the laboratory scale with a cobalt... 

More recently, Sasol commercialized a new type of fluidized-bed reactor and was also operating a higher pressure commercial fixed-bed reactor (38). In 1989, a commercial scale fixed fluid-bed reactor was commissioned having a capacity similar to existing commercial reactors at Sasol One (39). This effort is aimed at expanded production of higher value chemicals, in particular waxes (qv) and linear olefins. 

The next development was the replacement of injection cooling by interstage heat exchangers, through which the required or released heat of reaction is supplied or removed (Fig. 13C). More stringent requirements led to the development of fixed-bed reactors where the heat exchange surface is integrated in the fixed bed. This type of reactor will be considered in the next section. 

A generalized method to predict the deviations of the different types of fixed-bed catalytic reactor models with respect to an heterogeneous two-dimensional model is presented. Very good agreement with numerically calculated errors is found. The differences in the responses between the one and two-dimensional versions of each type of model are analyzed. The conditions in which the different types of models should be used are discussed. 

Figure 8.10 Types of fixed-bed catalytic reactor Fluidised bed reactors...

In the fixed-bed -process, shown schematically in Fig. 11-12, soluble cobalt salts of fatty acids or naphthenates are pumped with the olefin to the top of the first reactor and flow countercurrent to the synthesis gas. One type of fixed-bed catalyst consists of 2 per cent metallic cobalt on a pumice carrier. Part of the cobalt is converted to carbonyl, leaves the reactor with the overhead product, and is replaced by the cobalt salts in the feed. A high recycle of cooled crude product to the converter mds in controlling the reaction temperature. Unreacted synthesis gas leaving the top of the reactor is coo ed, passed through a packed tower countercurrent to the olefin feed to remove cobalt carbonyl, and recycled to the reactor. 

Another type of analysis is based on the pattern of the concentration front in the bed, essentially the reverse image of the breakthrough curve, particularly when this front passes through the bed with a eonstant velocity. Some of this we mentioned in Chapter 4. Such constant pattern waves are not limited to ion exchange or adsorption, for they are often encountered in the poisoning of fixed-bed reactors, in either isothermal [A. Wheeler and A.J. Robell, J. CataL, 13, 299 (1969) H.W. Haynes, Jr., Chem. Eng. ScL, 25, 1615 (1970)] or nonisothermal operation [H.S. Weng, G. Eigenberger and J.B. Butt, Chem. Eng. Sci., 30, 1341 (1975) T.H. Price and J.B. Butt, Chem. Eng. Sci., 32, 393 (1977)]. 

A fixed bed reactor has many unique and valuable advantages relative to other reactor types. One of its prime attributes is its simplicity, with the attendant consequences of low costs for construction, operation, and maintenance relative to moving bed or fluidized bed operation. Fixed bed reactors require a minimum of auxiliary equipment and are particularly appropriate for use in small commercial units when investments of large sums for control, catalyst handling, and supporting facilities would be economically prohibitive. Another major advantage of this mode of operation is implicit in the use of the term fixed bed reactor (i.e., there are no problems in separating the catalyst from the reactor effluent stream). (In many fluidized bed systems, catalyst recovery can be quite troublesome and require substantial equipment costs.) Another important attribute of fixed bed reactors is the wide variation in... 

While the particle diameter of fixed bed reactors is on the order of 1-5 mm, the particle size used in another type of reactor—fluidized bed reactors—is on the order of 50-200 p,m. Because of the small diameters, the effectiveness factor is close to 1 in most cases. The Ergun equation characterizes the pressure drop across a bed of solids—at low gas velocities (relative to the particle terminal velocity). When the drag force of the upward moving fluid exceeds the weight of the particles, the particles become fluidized—they begin to move up and down and the solids bed itself behaves like a fluid objects that are denser than the bed will fall through the bed while objects that are less dense will be remain at the top. Based on a force balance, the pressure drop across the bed, AF/L, will be equal to the head of solids (neglecting frictional forces) ... 

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