Phases heterogeneous catalytic processes

Kirillov, V. A., and Stegasov, A. N., On the simulation of heterogeneous catalytic processes with phase transitions in fixed catalyst bed (in Russian). Teoreticheskiye osnoy khim-icheskoi teknhnologii 22,340-345 (1988). 

Heterogeneous catalytic gas-phase reactions are most important in industrial processes, especially in petrochemistry and related fields, in which most petrochemical and chemical products are manufactured by this method. These reactions are currently being studied in many laboratories, and the results of this research can be also used for synthetic purposes. The reactions are usually performed [61] in a continuous system on a fixed catalyst bed (exceptionally a fluidized bed). 

Veser G, Friedrich G, Freygang M, Zengerle R, A Simple and flexible microreactor for investigation of heterogeneous catalytic gas phase reactions, in Reaction kinetics and development of catalytic processes, Froment GF, Waugh KC, Eds., Studies in Surface Science and Catalysis, 122 237-246, Elsevier, Amsterdam, 1999. 

The attrition of solid particles is an unavoidable consequence of the intensive solids motion resulting from the presence of bubbles in the fluidized bed. The attrition problem is especially critical in processes where the bed material needs to remain unaltered for the longest possible time, as in fluidized-bed reactors for heterogeneous catalytic gas-phase reactions. Catalyst attrition is important in the economics of such processes and may even become the critical factor. 

A ratio of Cas to the rate of catalytic reaction under steady-state conditions (r) gives a rough estimation for the reaction time scale. For typical heterogeneous catalytic processes applied for the production of bulk chemicals and petrochemicals, this value is estimated to be 10-2—101 s (Fig. 5). The changes of the reaction rate caused by the side processes of catalyst modification can take considerably longer. This is attributed to the higher capacity of substances in the catalyst bulk phase that can be involved in side interactions and to a slow rate of side processes in comparison with the stages of the catalytic cycle. 

A heterogeneous catalytic process involves more than one phase usually the catalyst is a soHd and the reactants and products are in liquid or gaseous form. Much of the benzene produced in this country today is manufactured from the dehydrogenation of cyclohexane (obtained from the distillation of crude petroleum) using platinum-on-alumina as the catalyst ... 

In the chemicals industry, the majority of the reactions take place at the surfaces of heterogeneous catalysts. The efficiency of a heterogeneous catalytic process is thus largely determined by the quality of the catalysts used, i.e. the exposed surface area of active phase and the stability. It is instructive to show the correlation between the exposed (specific) surface area, and the particle size of the catalytic material. Let us suppose that the active phase (density p in kg/m3) consists of uniform spherical particles. The specific area can be calculated as follows ... 

The first step in heterogeneous catalytic processes is the transfer of the reactant from the bulk phase to the external surface of the catalyst pellet. If a nonporous catalyst is used, only external mass and heat transfer can influence the effective rate of reaction. The same situation will occur for very fast reactions, where the reactants are completely exhausted at the external catalyst surface. As no internal mass and heat transfer resistances are considered, the overall catalyst effectiveness factor corresponds to the external effectiveness factor,... 

Mkrostructured reactors (MSR) for heterogeneous catalytic processes mostly consist of a large number of parallel flow channels. At least one dimension of these channels is smaller than 1 mm, but rarely <100 pm. This leads to an increased heat transfer in the direction of the smallest dimension. The volumetric heat transfer performance in microstructured devices is several magnitudes higher than in conventional reactors. Therefore, even highly exothermic or endothermic reactions can be operated under near isothermal conditions and thermal runaway can be avoided (see Chapter 5). In addition, mass transfer between the bulk phase... 

MS is frequently used to monitor the progress of gas-phase reactions, in particular heterogeneous catalytic processes. In some cases the outlet gas is first fed through a gas chromatograph to separate components and determine their relative concentrations, before analyzing their mass for identification of the species present. 

These heterogeneous catalytic processes (where a surface of a solid phase acts as catalyst), and the enzyme catalysis (where high molecular weight biomolecules are the active catalysts), are both outside the scope of this article. 

For the heterogeneous catalytic process to be effective, the reactants present in the surrounding fluid phase must be transported to the surfece of the solid catalyst, and after the reaction, the products formed must be carried back from the surface to the bulk fluid. The path of the physical rate processes at the particle scale is divided into two parts, as depicted in the 7-step sequence of the continuous reaction model used in microkinetic analysis ... 

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