Liquid phase cracking reactions

Cenospheres and oil coke are formed as a result of liquid phase cracking reactions, and, with the exception of ash, account for the major portion of the weight of particulate matter emitted when burning heavy fuel oils. Fuel composition and atomization quality appear to be the dominant parameters controlling ceno-sphere formation. 

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]. 

Hydropyrolysis processing is probably best suited for those feedstocks which can best utilize the inhibition effects of hydrogen on polymerization, condensation, and aromatization reactions. Such feedstocks are high molecular weight naphthenic materials which are susceptible to cracking but are easily converted to coke by liquid-phase bimolecular reactions. 

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... 

Coking. The production of coke is accomplished by lengthening the time of liquid-phase cracking (Fig. 19-6) so that polymerization Or condensation products are produced. However, only the most degraded carbonaceous high-boiling parts of the cracking reaction are exposed to... 

In chemical laboratories, small flasks and beakers are used for liquid phase reactions. Here, a charge of reactants is added and brought to reaction temperature. The reaction may be held at this condition for a predetermined time before the product is discharged. This batch reactor is characterized by the varying extent of reaction and properties of the reaction mixture with time. In contrast to the flasks are large cylindrical tubes used in the petrochemical industry for the cracking of hydrocarbons. This process is continuous with reactants in the tubes and the products obtained from the exit. The extent of reaction and properties, such as composition and temperature, depend on the position along the tube and does not depend on the time. 

Homogeneous reactions are those in which the reactants, products, and any catalysts used form one continuous phase (gaseous or liquid). Homogeneous gas phase reactors are almost always operated continuously, whereas liquid phase reactors may be batch or continuous. Tubular (pipeline) reactors arc normally used for homogeneous gas phase reactions (e.g., in the thermal cracking of petroleum of dichloroethane lo vinyl chloride). Both tubular and stirred tank reactors are used for homogeneous liquid phase reactions. 

At pressures between 250 psi to 1000 psi, and temperatures between approximately 750°F and 900°F (398.9°C and 482.2°C), cracking reactions take place in the liquid phase. Gasoline and distillate-type products are formed under these cracking conditions. 

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-... 

This process is shown schematically in Figure 7. The ethylene part of the feed reacts with chlorine in the liquid phase to produce 1,2-di-chloroethane (EDC) by a simple addition reaction, in the presence of a ferric chloride catalyst (9). Thermal dehydrochlorination, or cracking, of the intermediate EDC then produces the vinyl chloride monomer and by-product HC1 (1). Acetylene is still needed as the other part of the over-all feed, to react with this by-product HC1 and produce VCM as in the all-acetylene route. 

Ethylene-Based (C-2> Routes. MMA and MAA can be produced from ethylene as a feedstock via propanol, propionic acid, or melhyl propionate as intermediates. Propanal may be prepared by hydrofonnylalion of ethylene over cobalt or rhodium catalysts. The propanal then reads in the liquid phase with formaldehyde in the presence of a secondary amine and. optionally, a carboxylic acid. The reaction presumably proceeds via a Mannich base intermediate which is cracked to yield methacrolcin. Alternatively, a gas-phase, crossed akin I reaelion with formaldehyde cataly zed by molecular sieves [Pg.988]

Many catalysts used in industry are heterogeneous, e.g. zeolites in the cracking of heavy crude oil. The actual reaction takes place on the surface of the solid, with the substrates and products being in the gas or liquid phase, depending on the type of reaction and reactor being used. Many involve a metal on some kind of support e.g. Pd on charcoal, Ni on alumina. The surface area and porosity of heterogeneous catalysts are important in determining their efficiency. Some of the... 

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). 

TEM observations of the oxidized scale have revealed mullite grains with transgranular cracks, a phenomenon that is not surprising when one considers that the oxidation of SiC produces a volume expansion of 100%. When the reaction product contains a solid as well as a liquid product, as in the present situation, the volume expansion can be accommodated by squeezing out the liquid phase, resulting in a liquid cap on top of the solid reaction products. This has been observed by Luthra and Park,13 and is apparent in the micrograph shown in Fig. 8.5. 

Reactions of organic compounds, especially hydrocarbons, with oxygen in the gas or liquid phase at moderate temperatures (below 150° C), are important both as industrial processes and as natural decomposition phenomena that are to be suppressed if possible. They are chain reactions, but differ from thermal cracking in that they usually requires initiation. An initiator may have been added intentionally or be present as an impurity or early minor product, possibly a hydroperoxide that had accumulated upon prolonged standing in contact with air. 

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