Catalytic cracking liquid-phase reactions

Catalytic cracking is the cracking of heavy hydrocarbons using catalyst. The polyolefins such as PP and PE are recycled through this method. In the laboratory scale setup, these reactions are carried out in a flow reactor. There are two modes of catalytic treatment, liquid phase contact and vapor phase contact. In first case, the catalyst is in contact with molten polymers and here the catalyst reacts mainly with oligomers. In vapor phase contact, the catalyst is in contact with thermally degraded polymer [27]. 

C. D. Prater, J. Wei, V. W. Weekman, Jr., and B. Gross, A Reaction Engineering Case History Coke Burning in Thermofor Catalytic Cracking Regenerators Costei D. Denson, Stripping Operations in Polymer Processing Robert C. Reid, Rapid Phase Transitions from Liquid to Vapor John H. Seinfeld, Atmospheric Diffusion Theory... 

The new Brownsville, Tex., plant for the manufacture of synthetic liquid fuels from natural gas makes use of this reaction to increase the octane number of its product by as much as 20 units. Synthetic naphtha produced over iron catalyst is highly olefinic and contains substantial amounts of straight-chain isomers with terminal double bonds (8). The shifting of these double bonds toward the center of the molecule may be accomplished by vapor-phase treatment employing synthetic cracking catalyst in the fluid state, under mild catalytic cracking conditions. Oxygenated compounds also present are converted under the isomerization conditions to hydrocarbons and water. 

Tphe rate-limiting processes in catalytic reaction over zeolites remain A largely undefined, mainly because of the lack of information on counterdiffusion rates at reaction conditions. Thomas and Barmby (7), Chen et al. (2, 3), and Nace (4) speculate on possible diffusional limitations in catalytic cracking over zeolites, and Katzer (5) has shown that intracrystalline diffusional limitations do not exist in liquid-phase benzene alkylation with propene. Tan and Fuller (6) propose internal mass transfer limitations and rapid fouling in benzene alkylation with cyclohexene over Y zeolite, based on the occurrence of a maximum in the reaction rate at about 100 min in flow reaction studies. Venuto et al (7, 8, 9) report similar rate maxima for vapor- and liquid-phase alkylation of benzene and dehydro-... 

Several reactor types have been described [5, 7, 11, 12, 24-26]. They depend mainly on the type of reaction system that is investigated gas-solid (GS), liquid-solid (LS), gas-liquid-solid (GLS), liquid (L) and gas-liquid (GL) systems. The first three arc intended for solid or immobilized catalysts, whereas the last two refer to homogeneously catalyzed reactions. Unless unavoidable, the presence of two reaction phases (gas and liquid) should be avoided as far as possible for the case of data interpretation and experimentation. Premixing and saturation of the liquid phase with gas can be an alternative in this case. In homogenously catalyzed reactions continuous flow systems arc rarely encountered, since the catalyst also leaves the reactor with the product flow. So, fresh catalyst has to be fed in continuously, unless it has been immobilized somehow. One must be sure that in the analysis samples taken from the reactor contents or product stream that the catalyst docs not further affect the composition. Solid catalysts arc also to be fed continuously in rapidly deactivating systems, as in fluid catalytic cracking (FCC). 

In the presence of catalysts, heterogeneous catalytic cracking occms on the surface interface of the melted polymer and solid catalysts. The main steps of reactions are as follows diffusion on the surface of catalyst, adsorption on the catalyst, chemical reaction, desorption from the catalyst, diffusion to the liquid phase. The reaction rate of catalytic reactions is always determined by the slowest elementary reaction. The dominant rate controller elementary reactions are the linking of the polymer to the active site of catalyst. But the selectivity of catalysts on raw materials and products might be important. The selectivity is affected by molecular size and shape of raw materials, intermediates and products [36]. 

The extra-framework aluminium species (EFAL) generated by hydrothermal treatment of zeolite Y have a strong effect on catalytic activity and selectivity upon cracking, isomerization and alkylation reactions of hydrocarbons [14-17]. The effect of EFAL species on the liquid phase isomerization of a-pinene has also been studied [18]. 

The use of solid acids has been traditionally biased towards large-scale continuous vapour phase processes such as catalytic cracking and paraffin isomerisations. However, it is increasingly recognised that there is also a great need for solid acid catalysts which are effective in liquid-phase organic reactions such as those employed in many batch-type reactors by fine, speciality and pharmaceutical intermediate chemical manufacturers. This has contributed towards a substantial recent research effort into the development of new solid acid catalysts.86-91... 

Various mesoporous materials such as MCM-41, MCM-48, SBA-1 and KIT-1 have been rendered more acidic by treatment with reagents including ethanolic solutions of AICI3 and A1(N03)3 and slurries of Al(OPri)3 in non-polar solvent (e.g. hexane) followed by calcination of the resulting solid at temperatures of > 800 K to give solid adds.117 These treatments create either framework or nonframework aluminium centres which can act as Lewis add catalytic sites. The materials are more commonly assodated with vapour-phase reactions such as cracking rather than liquid-phase organic reactions.118... 

Reactions of a light gas with a liquid are almost always exothermic. Though very important in industry, endothermic reactions are exclusively gas phase reactions. It seems more economical to produce, in large scale endothermic processes (steam cracking, catalytic reforming and steam reforming), reactive intermediates from which, by exothermic reactions the wide variety of chemicals are synthetized. So, enthalpy of reaction cannot be used as a cri-terium to classify three phase industrial processes and, in the design of such plants, heat removal and temperature control are always important problems, sometimes critical ones. 

Propylene monomer is produced by catalytic cracking of petroleum fractions or the steam cracking of hydrocarbons during the production of ethylene. Conventional processes in liquid phase and in slurry use stirred reactors and a diluent such as naphtha, hexane, or heptane. The reaction takes place typically at a temperature of about 60-80°C and at 0.5-1.5 MPa, and the final product is obtained as a solid suspension of polypropylene in the liquid phase. Isolation of the resin requires a separation step (such as centrifugation) followed by washing the resin free of residual diluent and drying. 

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