Reactor solid catalysed reactions

Type of Reaction and Application. An increased emphasis on gas-solid reactions has been evident for about a decade. Three of the papers in this symposium treat gas-solid reactions, two (13,18) dealing with coal combustion and the other (11) with catalyst regeneration. Of the four papers which consider solid-catalysed gas-phase reactions, one (15) deals with a specific application (production of maleic anhydride), and one (12) treats an unspecified consecutive reaction of the type A B C the other two (14,16) are concerned with unspecified first order irreversible reactions. The final paper (17) considers a relatively recent application, fluidized bed aerosol filtration. Principles of fluid bed reactor modeling are directly applicable to such a case Aerosol particles disappear by adsorption on the collector (fluidized) particles much as a gaseous component disappears by reaction in the case of a solid-catalysed reaction. 

Chapter 2 covers the basic principles of chemical kinetics and catalysis and gives a brief introduction on classification and types of chemical reactors. Differential and integral methods of analysis of rate equations for different types of reactions—irreversible and reversible reactions, autocatalytic reactions, elementary and non-elementary reactions, and series and parallel reactions are discussed in detail. Development of rate equations for solid catalysed reactions and enzyme catalysed biochemical reactions are presented. Methods for estimation of kinetic parameters from batch reactor data are explained with a number of illustrative examples and solved problems. 

For catalytic reactor models, it is not necessary to specify solids mass fluxes and solids axial dispersion coefficients. To illustrate the genesis of models for solid catalysed reactions. 

There are several commercial gas-solid catalysed reactions in which heat transfer plays a significant, if not dominant, role in limiting the reactor productivity, lowering the process selectivity and reducing the life of the catalyst. Among these include the oxidation of ethylene, benzene, C hydrocarbons and methanol, the ammoxidation of propylene, methanol synthesis (Lurgi), the hydrochlorination of methanol and steam reforming of natural gas and naphtha. 

So far, the discussion has been in terms of an empty tubular reactor. Similar arguments can be applied to a reactor packed with catalyst particles. The design of reactors for carrying out solid-catalysed gas reactions is discussed fully in Chap. 4. 

The experimental results are presented for the esterification of dodecanoic acid (C12H24O2) with 2-ethylhexanol (CsHisO) and methanol (CH4O), in presence of solid acid catalysts (SAC). Reactions were performed using a system of six parallel reactors (Omni-Reacto Station 6100). in a typical reaction 1 eq of dodecanoic acid and 1 eq of 2-ethylhexanol were reacted at 160 °C in the presence of 1 wt% SAC. Reaction progress was monitored by gas chromatography (GC). GC analysis was performed using an interScience GC-8000 with a DB-1 capillary column (30 m x 0.21 mm). GC conditions isotherm at 40 °C (2 min), ramp at 20 °C min" to 200 °C, isotherm at 200 °C (4 min). Injector and detector temperatures were set at 240 °C. Reaction profiles were measured for both non-catalysed and catalysed reactions. 

In order to be able to represent the behaviour of fluidized bed reactors with confidence, one must have a thorough understanding of the bed hydrodynamics and of the reaction kinetics. Almost all of the reactions carried out in fluidized beds are either solid-catalysed gas phase reactions or gas-solid reactions. (We will not consider here homogeneous gas phase reactions, reactions in liquid fluidized beds or reactions in three phase fluidized beds.) While the chemical kinetics can often be highly complex. 

Heterogeneously catalysed reactions are two-, three-, or even more than three-phase operations. Solid catalyst and gaseous and liquid reactants are brought in contact to achieve the desired conversion. Some of the reactor types that are used are briefly presented here for background information with generalized remarks on their advantages and disadvantages. 

The concept of extractive reaction, which was conceived over 40 years ago, has connections with acid hydrolysis of pentosans in an aqueous medium to give furfural, which readily polymerizes in the presence of an acid. The use of a water-immiscible solvent, such as tetralin allows the labile furfural to be extracted and thus prevents polymerization, increases the yield, and improves the recovery procedures. In the recent past an interesting and useful method has been suggested by Rivalier et al. (1995) for acid-catalysed dehydration of hexoses to 5-hydroxy methyl furfural. Here, a new solid-liquid-liquid extractor reactor has been suggested with zeolites in protonic form like H-Y-faujasite, H-mordenite, H-beta, and H-ZSM-5, in suspension in the aqueous phase and with simultaneous extraction of the intermediate product with a solvent, like methyl Aobutyl ketone, circulating countercurrently. 

Brogli, F. et al., Runaway Reactions, 1981, Paper 3/M, 5-6, 10 Unstable plant-scale operation in the catalysed cyclisation by sodium hydroxide to cyclopropanecarbonitrile was investigated using a bench scale calorimeter. Crust formation on the reactor wall, which caused the erratic operation, was eliminated by using liquid alkali instead of solid. 

Microwave-promoted palladium-catalysed processes have found wide general application (see Chapter 2). A Larock-type heteroannulation of an iodoaniline and an internal alkyne has been employed in the synthesis of substituted indoles9 (Scheme 3.7). The microwave conditions were carefully optimised using a focused microwave reactor. Application of microwave heating provided clear advantages in reaction rate and yield over conventional thermal conditions. It is interesting to note that fixed microwave power input provided improved yields over constant temperature conditions (variable microwave power input). This chemistry was successfully extended to a solid-phase format (Rink amide resin)10. 

There are indeed significant fundamental and practical differences between classical organic reactions (either stoichiometric or homogeneously catalysed ones) and those catalysed by solids and especially zeolites (Table 2.1). It is also the case when one compares the relatively simple transformations generally studied by the specialists in Heterogeneous Catalysis and the transformation of complex molecules involved in the synthesis of Fine Chemicals. The operating conditions are very different high temperature, gas phase, fixed bed reactors on the one hand low... 

The characteristics of the solid particles of catalyst (size, mechanical resistance, etc.) have to be adapted to the reactor. In many organic reactions catalysed by acid zeolites, the catalytic act is concentrated in the outer rim of the crystals and decreasing the zeolite particle size generates a significant gain in activity. However, the use of small particles in batch reactors causes serious drawbacks in the separation of the zeolite from the reaction mixture for the recovery of reaction products and the eventual reuse of the catalyst. Also, small particles cannot be used in fixed bed reactors because of excessive pressure drops. 

Nitroalkanols are intermediate compounds of /1-amino alcohols that are used extensively in many important syntheses. They are obtained by Henry s reaction through the condensation of nitroalkanes with aldehydes. Different nitro compounds have been reacted with carbonyl compounds in reactions catalysed by alkaline earth metal oxides and hydroxides/621 Among the catalysts examined, MgO, CaO, Ba(OH)2, and Sr(OH)2, exhibited high activity for the reaction of nitromethane with propionaldehyde. The yields were between 60 % (for MgO) and 26 % [for Sr(OH)2] at 313 K after 1 h in a batch reactor. The study of the influence of the pretreatment temperature of the solid showed that for MgO and CaO a... 

Another classification of chemical reactors is according to the phases being present, either single phase or multiphase reactors. Examples of multiphase reactors are gas liquid, liquid-liquid, gas solid or liquid solid catalytic reactors. In the last category, all reactants and products are in the same phase, but the reaction is catalysed by a solid catalyst. Another group is gas liquid solid reactors, where one reactant is in the gas phase, another in the liquid phase and the reaction is catalysed by a solid catalyst. In multiphase reactors, in order for the reaction to occur, components have to diffuse from one phase to another. These mass transfer processes influence and determine, in combination with the chemical kinetics, the overall reaction rate, i.e. how fast the chemical reaction takes place. This interaction between mass transfer and chemical kinetics is very important in chemical reaction engineering. Since chemical reactions either produce or consume heat, heat removal is also very important. Heat transfer processes determine the reaction temperature and, hence, influence the reaction rate. 

Other types of reactors, such as trickle bed or bubble (slurry) reactors, are used only in special cases for reactions of liquids with gases catalysed by solids. 

This reactor is used with solid or liquid catalysts and with liquid reactants, or for gas/liquid reactions in the presence of solid catalysts. Batch reactors are also frequently used, as closed systems with circulation of the gas, for reactions of gases catalysed by solids. Nowadays, relatively few detailed kinetic studies are performed using batch reactors. A common use of these reactors is for the rapid screening of catalysts particularily in high pressure/high temperature reactions. 

Some examples of heterogeneous hydrogenation of alkenes catalysed by T12-H2 complexes in either solid-gas or solid-liquid systems have been reported. The first of such reactions regards the hydrogenation of ethylene catalysed by crystals of the iridium(III) complex [(triphos)Ir(H)2(C2H4)]BPh4 in a tubular flow reactor... 

Figure 4. Stirredflow reactor for the study of a GPTR catalysed by a solid or of a CVD reaction.

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