Continuous catalytic hydrogenation liquid-phase

Column reactors can contain a draft tube - possibly filled with a packing characterized by low pressure drop - or be coupled with a loop tube, to make the gas recirculating within the reaction zone (see Fig. 5.4-9). In recent years, the Buss loop reactor has found many applications in two- and three-phase processes About 200 Buss loop systems are now in operation worldwide, also in fine chemicals plants. This is due to the high mass-transfer rate between the gas and the liquid phase. The Buss loop reactor can be operated semibatch-wise or continuously. As a semibach reactor it is mostly used for catalytic hydrogenations. 

Fixed-bed reactors Trickle-flow reactor (TFR) This is a tubular flow reactor with a concurrent down-flow of gas and liquid over a fixed-bed of catalyst (Figure 3.10). Liquid trickles down whereas the gas phase is continuous. This reactor is mainly used in catalytic applications. Typical application examples of this reactor type are the following HDS of heavy oil fractions and catalytic hydrogenation of aqueous nitrate solutions. 

All the catalytic hydrogenations (2-pentyl-2-nonenal or cinnamaldehyde) were carried out in a 200-ml static reactor under atmospheric pressure with a continuous flow of hydrogen and at temperatures ranging from 50°C to 120°C (23). Activity and selectivity values were obtained by gas phase chromatography analysis of the liquid mbdure on a Cp SIL5 capillary column. 

Catalytic reductions can be carried out in batches or in continuous processes, in the liquid phase or in the vapor phase. This method has many advantages over other methods of reduction, particularly for lai e-volume production. With low-cost hydrogen, as is the case when by-product hydrogen is available from other installations or when large hydrocarbon-steam units are installed, this process cannot be matched by other methods of reduction in so far as economics and quality of product are concerned. 

Although cinchona alkaloids and especially cinchonidine, Cnd, proved to be the most effective chiral modifier for the catalytic system of Pt-alumina, in the liquid phase enantioselective hydrogenations of the carbonyl group in pyruvic acid esters, efforts to understand the mechanism of action of this catalyst system has continued to the present. The efforts may be divided into two categories finding natural modifiers other than cinchona alkaloids and examining new effective amino alcohols, which are modeled after the structure of known cinchona modifiers. 

This problem has been adapted with permission from the late Professor C. N. Satterfield of MIT. R. H. Price and R. B. Schiewetz [Ind Eng. Chem., 49, 807 (1957)] studied the catalytic liquid-phase hydrogenation of cyclohexene in a laboratory-scale semibatch reactor. A supported platinum catalyst was suspended in a cyclohexene solution of the reactant by mechanical agitation of the solution. Hydrogen was bubbled through the solution continuously. The reactor is described in their words as follows ... 

Three phase catalytic slurry reactors are characterized by a continuous liquid phase in which a gas phase is dispersed and a solid (catalyst) is suspended. They are commonly used for catalytic hydrogenation, oxidation, halogenation or polymerization reactions such as edible oil hydrogenation, olefin oxidation or hydroformylation etc. But also fermenters can be included into this category of multiphase reactors. 

The nonstationary (transient) operation of chemical reactors is traditionally applied in kinetic research in order to reveal reaction mechanisms. Pulses and step changes can be introduced in continuous reactors, and concentration changes at the reactor outlet are monitored by on-line or off-line analysis. The method is applicable to both gas- and liquid-phase systems. Isotope exchange can be commenced wherein H2/D2 (hydrogen/deuterium) experiments can reveal the role of hydrogen in a catalytic process. Some examples of catalytic isomerizations are displayed in Figure 9.22. A reaction network for hydrocarbon isomerization is shown in Figure 9.22. 

Catalytic hydrogenation requires a catalyst such as nickel, copper, platinum, molybdenum, or tungsten. These catalysts usually are supported on other materials and are especially prepared for the type of reduction to be carried out. Reduction conditions vary widely, depending on the nature of the nitro compound and the catalyst. Reduction may be carried out in solvent in the vapor phase or in the liquid phase. Aniline can be made by continuous vapor-phase reduction of nitrobenzene at 350 to 460°C at nearly atmospheric pressure. Some reductions, on the other hand, are run at 1000 to 4000 psi. 

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