Adiabatic fixed bed

Pre-Keformer A pre-reformer is based on the concept of shifting reforming duty away from the direct-fired reformer, thereby reducing the duty of the latter. The pre-reformer usually occurs at about 500°C inlet over an adiabatic fixed bed of special reforming catalyst, such as sulfated nickel, and uses heat recovered from the convection section of the reformer. The process may be attractive in case of plant retrofits to increase reforming capacity or in cases where the feedsock contains heavier components. 

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... 

In an adiabatic fixed bed, heat is not exchanged with the environment through the reactor wall. Note that for the derivation of eq. (5.226), it has been assumed that the flow is ideal plug flow and thus the radial dispersion term is eliminated in an adiabatic fixed bed, the assumption of perfect radial mixing is not necessary since no radial gradients exist. 

Note that in an adiabatic fixed bed, the temperature varies from inlet to the outlet of the bed and thus the fluid density, volumetric flow rate, and superficial velocity are not constant. However, the product pus in the above equation is the mass flow rate per unit cross-sectional area of the bed (kg/rn2 s), which is constant throughout the bed length. 

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). 

As we see, for a specific reaction, the higher the inlet concentration, the higher the conversion and the exit temperature. This is a result of the positive effect of the temperature rise, due to the exothermic nature of the reaction, on the rate coefficient and thus on the reaction rate and conversion. Note that for higher inlet CO concentration, the conversion for the isothermal operation is the same, while for the adiabatic operation the conversion is higher for higher inlet concentrations. Furthermore, the conversion in the adiabatic fixed bed is always higher in comparison to the isothermal fixed bed. Of course, these results are such because the reaction is of first order in respect to CO. 

Fe promoted with Al20Jt K20, CaO, and MgO (adiabatic fixed beds)... 

V2Os plus K2S04 on silica (adiabatic, fixed beds) 90% Pt-10% Rh wire gauze, oxidising conditions 90% Pt-10% Rh wire gauze, under net reducing conditions V20 on titania (multitube fixed bed)... 

Catalytic reforming Pt, Pi-Re or Pt-Sn on acidified A1203 or on zeolite in matrix (adiabatic, fixed beds, or moving bed, with interstage heating)... 

Hydrodesulphurisation, hydrodenitrogenation, hydrotreating CoMo/Al203 or NiMo/Al203, sulphided (adiabatic, fixed beds with interstage cooling)... 

H20 + CO - COj + H2 (water-gas shift) Fe304 promoted with Cr203 (adiabatic fixed bed) for a second, lower temperature stage, Cu-ZnO on A1203 C0M0 on support... 

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 this section we check our models against two industrial adiabatic fixed-bed methanators. Along the length of the methanators we calculate the effectiveness factor from the dusty gas model (DGM) and our simplified models (A) and (B). 

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 ... 

A theoretical and experimental study of multiplicity and transient axial profiles in adiabatic and non-adiabatic fixed bed tubular reactors has been performed. A classification of possible adiabatic operation is presented and is extended to the nonadiabatic case. The catalytic oxidation of CO occurring on a Pt/alumina catalyst has been used as a model reaction. Unlike the adiabatic operation the speed of the propagating temperature wave in a nonadiabatic bed depends on its axial position. For certain inlet CO concentration multiplicity of temperature fronts have been observed. For a downstream moving wave large fluctuation of the wave velocity, hot spot temperature and exit conversion have been measured. For certain operating conditions erratic behavior of temperature profiles in the reactor has been observed. 

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. 

The hydrogenation of a liquid feed in an adiabatic fixed bed with hydrogen in substantial excess will now be considered. 

Partial hydrogenation reactions of a liquid feed in an adiabatic fixed bed with hydrogen in high excess are especially sensitive to this kind of instability. Even local flow reductions caused by a sudden obstruction of part of the packed bed can initiate these phenomena. 

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.

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