Adiabatic fixed-bed reactors

Fig. 3. Multiple fixed-bed configurations (a) adiabatic fixed-bed reactor, (b) tubular fixed beds, (c) staged adiabatic reactor witb interbed beating (cooling),...

The final step in the methanol-to-gasoline process can be carried out in an adiabatic, fixed-bed reactor using a zeolite catalyst. A product mixture similar to ordinary gasoline is obtained. As is typical of polymerizations, a pure reactant is converted to a complex mixture of products. 

Xiao, W.-D., and Yuan, W.-K., Modelling and simulation for adiabatic fixed-bed reactor with flow reversal. Chem. Eng. Sci. 49(21), 3631-3641 (1994). 

ILLUSTRATION 12.8 PRODUCTION OF SULFUR TRIOXIDE IN AN ADIABATIC FIXED BED REACTOR... 

Typically, the prereforming process is performed in an adiabatic fixed-bed reactor upstream of the main reformer. In the pre-reformer, higher... 

Then, assume that the reaction takes place in a fixed bed of 1.61 m diameter and 16.1 m height, under contact time of 5 min, and the inlet temperature of gas being 50 °C, for different CO inlet concentration (several runs). Estimate the conversion of CO in an isothermal and adiabatic fixed-bed reactor and under the following assumptions isobaric process, negligible external mass transfer resistance, and approximately constant heat capacity of air (cp = 1 kJ/kg K) and heat of reaction (AH = -67,636 cal/mol). The inlet temperature of the reaction mixture is 50 °C and its composition is 79% N2 and approximately 21% 02, while the inlet CO concentration varies from 180-4000 ppm (mg/kgair) (for each individual ran). 

Methanators are usually adiabatic fixed-bed reactors. The kinetics of methanization are described by the reverse ry, f// and fjjj of the reactions /, II, and III. Hence there are two exothermic reactions I and II and one endothermic one, III. Carbon dioxide and methane are the key components here. 

In some cases a plant may have a pre-reformer. A pre-former is an adiabatic, fixed-bed reactor upstream of the primary reformer. It provides an operation with increased flexibility in the choice of feed stock it increases the life of the steam reforming catalyst and tubes it provides the option to increase the overall plant capacity and it allows the reformer to operate at lower steam-to-carbon ratios166. The hot flue gas from the reformer convection section provides the heat required for this endothermic reaction. 

Aniline can also be produced when phenol is subjected to gas-phase ammonolysis at 200 bar and 425°C in an adiabatic, fixed-bed reactor. This is the Halcon/Scientific Design process. The chemistry is ... 

The phenomenon of multiplicity and propagating fronts in adiabatic fixed bed reactors has received much attention in the literature and is the subject of a rather exhaustive treatment [1-6]. Unlike the adiabatic operation, the nonadiabatic case enjoyed far less attention and many questions are still to be answered. Hence, the principal interest in this work was to investigate experimentally the theoretically the characteristic features of multiplicity and propagating fronts created under different conditions in a nonadiabatically operated packed bed reactors and to make a comparison with the adiabatic operation. 

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

Adiabatic fixed-bed reactors constitute the oldest fixed-bed reactor configuration. In the simplest case they consist of a cylindrical jacket in which the catalyst is loosely packed on a screen support and is traversed in the axial direction (Fig. 9A). To avoid catalyst abrasion by partial fluidization, random catalyst packings arc always traversed from top to bottom. If fixed-beds composed of monolith catalyst sections are used, the flow direction is arbitrary. 

Reference is made in Section 10.1.2.3 to the importance of uniform flow into and through adiabatic fixed-bed reactors. This is not easy to achieve, particularly with low-pressure-loss monolith reactors, and requires a careful design of the inflow hood. On account of the low pressure loss, unfavorable flow conditions in the outflow hood may also aflcct the flow behavior through the catalyst bed. 

Purely adiabatic fixed-bed reactors are used mainly for reactions with a small heat of reaction. Such reactions are primarily involved in gas purification, in which small amounts of noxious components are converted. The chambers used to remove NO, from power station flue gases, with a catalyst volume of more than 1000 m3, are the largest industrial adiabatic reactors, and the exhaust catalyst for internal combustion engines, with a catalyst volume of ca. 1 L, the smallest. Typical applications in the chemical industry include the methanation of traces of CO and CO2 in NH3 synthesis gas, as well as the hydrogenation of small amounts of unsaturated compounds in hydrocarbon streams. The latter case requires accurate monitoring and regulation when hydrogen is in excess, in order to prevent complete methanation due to an uncontrolled temperature runaway. 

Figure 13. Development of fixed-bed reactors. A) Single-bed adiabatic packed-bed reactor B) Adiabatic reactor with interstage gas Iced (ICI concept) C) Mullibed adiabatic fixed-bed reactor with interstage heat exchange.

A completely different design of an autothermal reactor has been proposed and developed by Boreskov and Matros [41-43], Fig. 24A gives a sketch of the basic concept, showing an adiabatic fixed bed reactor with fecd/cxit tubes and two valves for periodic flow reversal. Before start of operation the fixed bed has again to be heated above the ignition temperature of the catalytic reaction. If the reactor is then fed with cold feed from one side the cold feed gas is heated up... 

In practice catalytic reforming is usually carried out in fixed bed reactors (see Fig. 2.2). Because the reactions are endothermic, heat has to be introduced. A conventional scheme is based upon a series of three or four adiabatic fixed bed reactors with interstage heating. Part of the hydrogen produced is recycled to maintain high hydrogen partial pressures. The product mixture from the last reactor is cooled and separated into a gas phase and a liquid phase. The latter is purified by distillation. 

An extensive analysis of the behaviour of different types of non-adiabatic fixed bed reactor models is carried out and the importance of the heterogeneous one and two-dimensional models III-0 and III-T is stressed. Although in these models the heat and mass transfer phenomena are correctly taken into account, they... 

Styrene is produced by dehydrogenation of ethylbenzene in an adiabatic, fixed-bed reactor. Although Sheel and Crowe [29] list ten reactions and several products, the major reaction is the conversion of ethylbenzene to styrene, according to the following equation. 

The use of two-phase homogeneous continuum models in packed bed modelling has often been avoided due to the computational difficulties. Recently, Paspek and Varma (15) have found a two-phase model to be necessary to describe an adiabatic fixed-bed reactor, while Dixon and Cresswell (16) have shown that the effective parameters of the one-phase model may be interpreted in terms of the more fundamental parameters of a two-phase model, thus demonstrating more clearly their qualitative dependencies on the operating and design characteristics of the bed. When two phases and several species are involved, the computational advantages of the cubic Hermite method may be anticipated to be high. 

T. H. Price and J. B. Butt [Chem. Eng. Sci., 32 (1977) 393] investigated this reaction in an adiabatic, fixed-bed reactor. Using the following data ... 

Example 10.2.1 illustrates the simultaneous solution of the mass and energy balances for an adiabatic, fixed-bed reactor with no fluid density changes and no transport limitations of the rate, that is, rj = 1. Next, situations where these simplifications do not arise are described. 

Fig. 8.3. Adiabatic fixed-bed reactor single fluid phase.

Advantages of the adiabatic fixed-bed reactor are its simplicity, the high catalyst load per unit volume, little catalyst attrition, and little backmixing. Disadvantages are the large pressure drop, difficult temperature control, and the long diffusion distances. 

Some of the many processes using adiabatic fixed-bed reactors with gaseous reactants are the production of methanol and ammonia, and hydrotreating of naphtha. 

Fig. 7 Three single-stage adiabatic fixed-bed reactor types.

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