Coke formation inhibition

The features of this failure case were similar to the previous case materials loss was observed, coke was detected and the internal wall showed a carburised zone in the metallographic cross-section. The outer surface was partially covered with the red scale and one tube had even cracked locally. Near the crack there were deposits in the tube and the microstructure of the steel indicated strong overheating. The new 9%Cr tube obviously failed due to metal dusting, coke formation inhibiting the heat transfer and locally increased temperatures. 

Nickel catalysts are also used for steam methane reforming. Moreover, nickel catalysts containing potassium to inhibit coke formation from feedstocks such as LPG and naphtha have received wide appHcation. 

In technical hydrocarbon reforming processes using platinum catalysts, high hydrogen pressures are usually used to inhibit catalyst poisoning and coke formation as far as possible, for instance a total pressure of several atmospheres to several tens of atmospheres, with a several-fold excess of hydrogen in the reactant mixture. 

MgO is a basic metal oxide and has the same crystal structure as NiO. As a result, the combination of MgO and NiO results in a solid-solution catalyst with a basic surface (171,172), and both characteristics are helpful in inhibiting carbon deposition (171,172,239). The basic surface increases C02 adsorption, which reduces or inhibits carbon-deposition (Section ALB). The NiO-MgO solid solution can control the nickel particle sizes in the catalyst. This control occurs because in the solid solution NiO has strong interactions with MgO and, as indicated by TPR data (26), the former oxide can no longer be easily reduced. Consequently, only a small amount of NiO is expected to be reduced, and thus small nickel particles are formed on the surface of the solid solution, smaller than the size necessary for coke formation. Indeed, the nickel particles on a reduced 16.7 wt% NiO/MgO solid-solution catalyst were too small to be observed by TEM (171). Furthermore, two additional important qualities stimulated the selection of MgO as a support its high thermal stability and low cost (250,251). 

Recovered catalysts should maintain catalytic activity or at least possess regenerate sites for repeated use. Of prime importance, catalysts in the first stage of liquefaction can inhibit coke formation. Coke once produced on the catalyst is difficult to remove without burning, which inevitably converts iron sulfides to oxides and/or sulfates. In designing catalysts, low acidity or polarity may be appropriate so as not to strongly adsorb heavy, polar, or ba-... 

Additives have been shown to effect decreases in coke deposits. Small amounts of organometallic compounds and commercial fuel-oil additives have been shown to decrease coke weight. Metallic compounds in excess can result in increased deposition because of the metal oxide, and some of these compounds have also been reported to cause metal corrosion, as have sulfur compounds. Results of investigations with additives to inhibit coke formation are not conclusive enough at present to justify their acceptance. 

The CANMET hydrocracking process is used for heavy oils, atmospheric residua, and vacuum residua (Table 9-5) (Pruden, 1978 Waugh, 1983 Pruden et al., 1993). The process does not use a catalyst but employs a low-cost additive to inhibit coke formation and allow high conversion of heavy feedstocks (such as... 

CANMET hydrocracking process a hydrocracking process for heavy feedstocks that employs a low-cost additive to inhibit coke formation and allow high feedstock conversion using a single reactor. 

The results in Table III show that the virgin bitumen that contains the asphaltenes produced relatively more gas and nonhydrocarbon products than did the maltenes. This trend with respect to gases and liquids appears to be confirmed by the results of the run with the asphaltene-enhanced bitumen however, appreciable quantities of coke were formed at the reaction conditions used and good material balances on this run were not achieved. Without essentially complete reduction of coke formation by hydropyrolysis, the significance of results for the asphaltene-enhanced bitumen are suspect. Removal of carbon in the form of coke will have an unknown effect on results that may not be attributable to asphaltenes. These results are included principally as negative results to show the dramatic effects that can result if asphaltenes are not fully dispersed and coke formation is not inhibited during hydropyrolysis. 

The Blowdec depolymerization process from Slovakia converts waste plastics into low-sulphur diesel fuel [35]. The main principle is the processing of waste plastics in a hot whirling bed of hot sand in the BLOWDEC reactor (Figure 15.19). The plastic is heated to 430°C. The process allows for simultaneous cracking of hydrocarbons and inhibition of coke formation. The fluidized sand bed products three types of cracking reactions mechano-activation thermal and catalytic (Si02/aluminosilicates). The process is covered under the US Patent 6,165,349. The process enables the economical conversion of mixed waste plastics into liquid hydrocarbons, mainly low-sulphur (25 ppm) diesel... 

However, the addition of an oxidant such as oxygen is not without some trade-off. To help solve the problem of catalyst deactivation due to carbon deposit in an alumina membrane reactor for dehydrogenation of butane, oxygen is introduced to the sweep gas, helium, on the permeate side at a concentration of 8% by volume. The catalyst service life increa.scs from one to four or five hours, but the selectivity to butene decreases from 60 to 40% at 480 C [Zaspalis et al., 1991b]. If oxygen is added to the feed stream entering the membrane reactor in order to inhibit coke formation, the butene selectivity decreases even more down to 5%. 

In this work we report the kinetic behavior of a bifimctional catalyst based on a M/H-MFI zeolite with unique shape selectivity and active for enhancing gasoline octane number. As a test reaction the n-octane transformation was used. The catalyst showed to be subject to inhibition as well as deactivation effects other than the effects of coke formation. To test these effects, the presence of inhibitors and poisons in the feedstock, was also studied [9]. 

It has been pointed out that the coke formation on the metallic surface occurs mainly on the flat planes (large particles) [10]. If we assume that Au at high coverage is deposited on the dense planes of the platinum particles, we can propose that Au acts as a diluent of the platinum ensembles, and hence that coke formation is inhibited. The level of coke and its nature are under study, and the results will be reported in a subsequent paper. 

Under these circumstances, Japan Energy Corporation has been conducted extensive research on the development of a new aromatization catalyst that exhibits high activity and excellent inhibition of coke formation. Based on this fundamental research, an LNA demonstration plant with a capacity of 2,250 BPSD has been operated in 1994. This paper describes the features of the LNA process and its performance. 

Two aspects of these results will be discussed here, namely the low platinum dispersion and the presence of significant amounts of coke on alumina and zeolite catalysts after testing. The latter finding is prohahly due to a combination of test tenq>eratures below 2S0°C and the propensity for toluene to form coke. In any event, coke formation did not seem to inhibit the oxidation reaction to any significant extent. The low platinum diversions, confirmed by XRD measurements, indicate that the active reside outside the zeolite micropores on... 

Potassium is used as a dopant on catalysts for the methanation reaction and ammonia synthesis. Its purpose is to increase the rate of the reaction. Potassium is also used on the steam reforming catalyst, not as a promotor but as a dopant that inhibits catalyst deactivation by coke formation (ref. 1). It is reasonable that the role of potassium as a promotor of reaction rates is to lower some barrier to bond dissociation. Since molecular beam techniques afford a convenient means of measuring changes in barrier heights as well as in shapes of the barrier through measurements of the dissociation probability versus energy, the possible effect of potassium on the dissociation of CH4 is investigated. 

Inhibition of Coke Formation in Ethylene Steam Cracking... 

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