Catalytic cracking deposition

In 2001, Masuda et al. (2001) developed a catalytic cracking deposition (CCD) technique for zeolitic pore size reduction to enhance the H2 separation factor of MFI zeolite membranes. After the CCD modification, the zeolite pore size was reduced to about 0.36—0.47 nm, and the H2/CO2 separation factor of MFI zeolite membranes was enhanced from 1.5—4.5 to more than 100. However, the H2 permeance of the modified membrane was only about one-tenth of the fresh membrane. Using the method developed by Masuda et al., Falconer and co-workers modified B-ZSM-5 and SAPO-34 membranes for the enhancement of H2/CO2 separation factor (Hong, Falconer, Noble, 2005). Similarly, the H2/CO2 separation factor of the modified membrane was increased to 48, while the H2 permeance of the B-ZSM-5 membrane was decreased by more than one order of magnitude. 

Mordenite Framework Inverted (MFI) zeolite membranes were synthesized on the inner surface of a-alumina tubes (Pall Corp.) from an aluminum-free precursor solution containing Si02, NaOH, H2O, and template tetrapropylammonium hydroxide (TPAOH) by in situ hydrothermal crystallization at 453 K for 20 h (Tang et al. 2009). The resultant zeolite membrane had a thickness of 2-3 xm. The membrane surface was modified by the in situ catalytic cracking deposition of methyl-diethoxysilane (MDBS) molecules at the sites of [(tSi-0")H ] whereas MDBS vapor was carried by an equimolar H2/CO2 mixture flowing over the membrane surface at a pressure of 1.5 bar and a temperature of 723 K. 

The most dominant catalytic process in the United States is the fluid catalytic cracking process. In this process, partially vaporized medium-cut petroleum fractions called gas oils are brought in contact with a hot, moving, freshly regenerated catalyst stream for a short period of time at process conditions noted above. Spent catalyst moves continuously into a regenerator where deposited coke on the catalyst is burnt off. The hot, freshly regenerated catalyst moves back to the reactor to contact the hot gas oil (see Catalysts, regeneration). 

Coke deposition is essentially independent of space velocity. These observations, which were developed from the study of amorphous catalysts during the early days of catalytic cracking (11), stiU characteri2e the coking of modem day 2eohte FCC catalysts over a wide range of hydrogen-transfer (H-transfer) capabihties. 

Another approach used to reduce the harmful effects of heavy metals in petroleum residues is metal passivation. In this process an oil-soluble treating agent containing antimony is used that deposits on the catalyst surface in competition with contaminant metals, thus reducing the catalytic activity of these metals in promoting coke and gas formation. Metal passivation is especially important in fluid catalytic cracking (FCC) processes. Additives that improve FCC processes were found to increase catalyst life and improve the yield and quality of products. ... 

The most important undesired metallic impurities are nickel and vanadium, present in porphyrinic structures that originate from plants and are predominantly found in the heavy residues. In addition, iron may be present due to corrosion in storage tanks. These metals deposit on catalysts and give rise to enhanced carbon deposition (nickel in particular). Vanadium has a deleterious effect on the lattice structure of zeolites used in fluid catalytic cracking. A host of other elements may also be present. Hydrodemetallization is strictly speaking not a catalytic process, because the metallic elements remain in the form of sulfides on the catalyst. Decomposition of the porphyrinic structures is a relatively rapid reaction and as a result it occurs mainly in the front end of the catalyst bed, and at the outside of the catalyst particles. 

Cracking is an endothermic reaction, implying that the temperature must be rather high (500 °C), with the consequence that catalysts deactivate rapidly by carbon deposition. The fluidized catalytic cracking (FCC) process, developed by Standard Oil Company of New Jersey (1940) (better known as ESSO and nowadays EXXON), offers a solution for the short lifetime of the catalyst. Although cracking is... 

Figure 7.7b shows the essential features of a refinery catalytic cracker. Large molar mass hydrocarbon molecules are made to crack into smaller hydrocarbon molecules in the presence of a solid catalyst. The liquid hydrocarbon feed is atomized as it enters the catalytic cracking reactor and is mixed with the catalyst particles being carried by a flow of steam or light hydrocarbon gas. The mixture is carried up the riser and the reaction is essentially complete at the top of the riser. However, the reaction is accompanied by the deposition of carbon (coke) on the surface of the catalyst. The catalyst is separated from the gaseous products at the top of the reactor. The gaseous products leave the reactor... 

If deactivation is rapid and caused by a deposition and a physical blocking of the surface this process is often termed fouling. Removal of this solid is termed regeneration. Carbon deposition during catalytic cracking is a common example of fouling... 

Some of the principles used in Thermofor catalytic cracking have been applied to a coking operation. Coke itself, instead of catalyst, is the solid circulated. The coke is heated in the regenerator of the unit and more coke is deposited on the hot moving solid in the reactor of the unit. Appropriate proportions of the coke are removed continuously as the process proceeds (84). 

The principles of fluidized solids so successfully used in catalytic cracking are applicable to the coking operation. Again the coke itself is the solid circulated. It is heated in the regenerator and coke is deposited on it in the reactor (24). 

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