Coking inhibitors

Simulated data are shown in Figure 1 with solid lines and the parameter values that best fit experimental data in Table 1. The specific reaction rate constant x(l) has the same value for all cases and this is consistent with the fact that each deactivation run was done starting with fi esh catalyst. On the other hand, parameter x(2) reflects some type of equilibrium state reached for free active catalytic sites. It takes very small values for a poison as pyridine, intermediate values for an inhibitor like CS2, and values close to one for pure n-octane and for n-octane mixed with the smallest amount of benzene. In larger amounts, benzene seems to act as coke inhibitor, which is consistent with the fact that it does not change activity in a significant manner but it does change the distribution of reaction products. 

Modern catalysts usually contain live dements a basic active component (Fe303 etc.), a stabilizer (Cr2Oj. A1203. MgO etc.) a coke inhibitor (K,0 etc.), an initiator (CuO, V,Os, AgO etc.), and a binder which imparts certain mechanical properties to the system (calcium alwninate etc.). 

The dehydrogenation of the mixture of m- and -ethyltoluenes is similar to that of ethylbenzene, but more dilution steam is required to prevent rapid coking on the catalyst. The recovery and purification of vinyltoluene monomer is considerably more difficult than for styrene owing to the high boiling point and high rate of thermal polymerization of the former and the complexity of the reactor effluent, which contains a large number of by-products. Pressures as low as 2.7 kPa (20 mm Hg) are used to keep distillation temperatures low even in the presence of polymerization inhibitor. The finished vinyltoluene monomer typically has an assay of 99.6%. 

Additive inhibitors have been developed to reduce the contaminant coke produced through nickel-cataly2ed reactions. These inhibitors are injected into the feed stream going to the catalytic cracker. The additive forms a nickel complex that deposits the nickel on the catalyst in a less catalyticaHy active state. The first such additive was an antimony compound developed and first used in 1976 by Phillips Petroleum. The use of the antimony additive reportedly reduced coke yields by 15% in a commercial trial (17). 

Chlorination of Methane. Methane can be chlorinated thermally, photochemicaHy, or catalyticaHy. Thermal chlorination, the most difficult method, may be carried out in the absence of light or catalysts. It is a free-radical chain reaction limited by the presence of oxygen and other free-radical inhibitors. The first step in the reaction is the thermal dissociation of the chlorine molecules for which the activation energy is about 84 kj/mol (20 kcal/mol), which is 33 kJ (8 kcal) higher than for catalytic chlorination. This dissociation occurs sufficiendy rapidly in the 400 to 500°C temperature range. The chlorine atoms react with methane to form hydrogen chloride and a methyl radical. The methyl radical in turn reacts with a chlorine molecule to form methyl chloride and another chlorine atom that can continue the reaction. The methane raw material may be natural gas, coke oven gas, or gas from petroleum refining. 

Silica gels will shatter in the presence of free water and are chemically attacked by many corrosion inhibitors. The chemical attack permanently destroys the silica gels. The other desiccants are not as sensitive to free water and are not chemically attacked by most corrosion inhibitors. However, unless the regeneration temperature is high enough to desorb the inhibitor, the inhibitor may adhere to the desiccants and possibly cause coking. 

In addition, organics also may be found in boiler section iron oxide deposits taken from large process industry boiler plants. Here, the organics are more likely to be hydrocarbons, such as oil, tar, or petroleum coke, rather than inhibitor treatments. The various mixtures that form all become very insoluble with age and quite dense. 

Nickel aluminate, a spinel, has long been known to trap nickel. Metals like arsenic(19), antimony(20-21) and bismuth(20) are known to passivate transition elements and can be used to decrease and coke make. Sulfur is also a known inhibitor for nickel therefore, higher sulfur-containing crudes may be a little less sensitive to nickel poisoning. In our work we also found that nickel at low concentrations is actually a slight promoter of the cracking reaction when incorporated into a molecular sieve (Figure 17). 

Effect of Aromatics on Formation of Coke and Carbon. Conventional liquid fuels contain widely differing levels of aromatics gasoline usually contains more than diesel. The studies show that these compounds cause more rapid deactivation than linear alkanes alone. A related result is that the steady state conversion of diesel fractions is also reduced in the presence of aromatics—i.e., these compounds act as kinetic inhibitors, limiting the production of H2. 

As already discussed, if a catalyst is deactivated via coking, it could be regenerated so that its initial catalytic activity is restored. The treatment at high temperatures in an oxygen-rich atmosphere can burn off the coke deposited and the catalyst may regain its activity. Moreover, if the reduced activity is a temporary event caused by an inhibitor, die removal of the inhibiting substance of the feed can restore the catalyst to its initial potential. 

Fe(CN)6]3, as the free acid, has been used as a corrosion inhibitor for metal surfaces 55 K2[Fe(CN)6] has also been reported as being effective for this purpose.56 Both hexacyano ions act as combustion inhibitors for aromatic polymers.57 In the production of carbon monoxide by coke gasification for use in blast furnaces, additives are needed to enhance the reduction of any carbon dioxide formed back to the monoxide. Trisoxalatoiron has been found effective for this... 

Coke Formation as a Function of Mole Fraction of Inhibitor.. . 330... 

In order to determine whether the relationships discovered for coke formation from cumene hydroperoxide might reflect a more general behavior concerning coke formation in cracking reactions, experiments were undertaken with a light East Texas gas oil (LETGO). Such gas oils contain a large variety of inhibitors. 

The coke studies lead to the interesting result that the nondesorbable coke, which does not deactivate the catalyst and is produced by inhibitors, is probably not produced by a mechanism involving the cracking sites. 

It follows that regeneration may consist of either (i) removal of IS sometimes poisons, most often inhibitors or fouling agents, e.g., coke (hydrogenation catalysts, e.g., selective hydrogenation of pyrolysis gasoline) or (ii) redispersion of the active species (platinum catalysts) or (iii) both (hydrodesulfurization or catalytic reforming catalysts). 

Use and exposure Toluene is a clear, colorless liquid with an aromatic odor. It is a natural constituent of crude oil and is produced from petroleum refining and coke-oven operations. It is used in household aerosols, nail polish, paints and paint thinners, lacquers, rust inhibitors, adhesives, and solvent-based cleaning agents. Toluene is also used in printing operations, leather tanning, and chemical processes. Benzene and other PAHs are common... 

For polymers whose pyrolysis products are not inhibitors for the flaming reaction, Van Krevelen established a simple empirical correlation between the limiting oxygen index and the coke residue ... 

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