Axial flow fixed-bed reactor

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). 

Axial flow fixed-bed reactor (usually downflow) single (Figure 3.1-a.l), multistage (Figure 3.1-a.2), or multitubular (Figure 3.1-a.3). Here, the different configurations are related... 

This is a reversible, exothermic reaction carried out adiabatically in a multistage, fixed-bed reactor with axial flow of fluid and interstage heat transfer for temperature adjustment see Figure 1.4. The catalyst is promoted V205. 

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... 

In Figure 21.1, the first division is with respect to flow arrangement. Traditionally, most fixed-bed reactors are operated with axial flow of fluid down the bed of solid particles. A more recent trend is to use radial flow, either outward, as depicted in Figure 21.1, or inward. In the case of styrene monomer production (reaction (D) in Section 21.1), the purpose is to reduce the pressure drop (-AP) by increasing the flow area for a given bed volume. We restrict attention to axial flow in this chapter. 

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. 

An analysis is made of the factors which pose a limit to representative downscaling of catalyst testing in continuous fixed-bed reactors operated with either gas or gas-liquid flow. Main limiting factors are the axial dispersion and, in the case of gas-liquid operation, also the contacting of the catalyst. The effects of catalyst and reactor geometries are quantified, and boundaries for safe operation are indicated. 

Wang et al. " compared the model predictions to experimental data on the desulfurization of simulated coal gas from a laboratory-scale fixed-bed reactor and from a process development reactor operated on actual coal gas. The solid reactant was formed from cylindrical pellets of zinc and titanium oxides. Data from six experiments using different temperatures, pressures, feed gas flow rates, and feed gas H2S concentrations were available. Product gas concentrations were measured as a function of time, and the axial distribution of sulfur within the reactor was determined at the conclusion of the test. 

Figure 3.14 Axial dispersion in fixed-bed reactors (a) iiquid flow and (b) gas flow [6]. Gray area represents experimental results. (Adapted from [6], Figure 27.24 Copyright 2012, Wiley-VCH GmbH Co. KGaA.)...

Finally, it should be noted that more sophisticated models have been developed, either on a stagewise basis [62], similar to the Deans-Lapidus model for single phase fixed bed reactors, or on a stochastic respectively propabilistic basics [67,66]. Using data from laboratory and full-scale reactors, Schwarz and Roberts [44] have carried out parametric studies to evaluate the accuracy of the axial dispersion model. Their simulation showed that in case of first-order kinetics, dispersion in the liquid phase is frequently not of major importance. Deviations from plug flow become important only for short reactors and a high degree of conversion. 

Figure 3.1 Fixed-bed reactors. Schematic representation of several operating configurations and arrangements (a) Cylindrical with axial flow (a.l-single, a.2-multistage, a.3-multitubular) (b) cylindrical with radial flow (c) spherical reactor (c.l-axial and c2-radial flow).

Experimental data of stearic acid decarboxylation in a laboratory-scale fixed bed reactor for formation of heptadecane were evaluated studied with the aid of mathematical modeling. Reaction kinetics, catalyst deactivation, and axial dispersion were the central elements of the model. The effect of internal mass transfer resistance in catalyst pores was found negligible due to the slow reaction rates. The model was used for an extensive sensitivity study and parameter estimation. With optimized parameters, the model was able to describe the experimentally observed trends adequately. A reactor scale-up study was made by selecting the reactor geometry (diameter and length of the reactor, size and the shape of the catalyst particles) and operating conditions (superficial liquid velocity, temperature, and pressure) in such a way that nonideal flow and mass and heat transfer phenomena in pilot scale were avoided. 

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