Coking tendency

For air compressors Operating safety Thermal stability, Volatility Resistance to oxidation Extreme pressure and anti-wear (compressors) properties Low coking tendency (hot reciprocating compressors)... 

Thus the amount of heat that must be produced by burning coke ia the regenerator is set by the heat balance requirements and not directly set by the coke-making tendencies of the catalyst used ia the catalytic cracker or by the coking tendencies of the feed. Indirectly, these tendencies may cause the cracker operator to change some of the heat-balance elements, such as the amount of heat removed by a catalyst cooler or the amount put iato the system with the feed, which would then change the amount of heat needed from coke burning. 

The amount of catalytic coke that is formed depends on the type of catalyst used ia the FCCU, the coking tendency of the feed, the degree of conversion of the feed, and the length of time the catalyst is exposed to the feed (eq. 2) (11). 

The presence of contaminant metals on the equiUbrium catalyst can significantly increase the catalyst coking tendency, which in turn results in an increase in regenerator temperature if all other factors remain unchanged. As one example, if the metals on an FCCU equiUbrium catalyst increased from an equivalent-nickel value of 2000 wt ppm to 3500 wt ppm, the catalyst coke factor would increase 30—50%. If all controllable parameters remained constant, the regenerator temperature would be expected to increase 35—50°C and conversion would drop. 

An RFCC is distinguished from a conventional vacuum gas oil FCC in the quality of the feedstock. The residue feed has a high coking tendency and an elevated concentration of contaminants. 

The reactor temperature required to prevent coke formation varies considerably for the different processes. Table 2.1 summarizes the values calculated assuming thermodynamic equilibrium for 2,2,4-trimethylpentane reforming. Generally, the coking tendency increases in the following order at constant O/C ratio SR > ATR > POx. These calculations demonstrate that at steam to carbon ratios (S/C) > 2 and reaction temperatures > 600 °C, which is very common for hydrocarbon fuel processors, coke seems to be an unstable species especially under the conditions of steam reforming. 

Heating the feed to reactor temperature, usually greater than 500 C, causes an additional problem. Coke is formed by thermal reactions of the feed in the preheater and feed injection system. The coking tendency is more severe for heavy feedstocks which have higher concentrations of coke precursors. This can be avoided by limiting the feed temperature to 300 C or less. 

On the basis of these limited data, it is not clear if generalizations can be made. For instance, would three-ring additives increase values of k, to a greater extent than do two-ring additives Also, the use of A as an independent variable is a matter of convenience. It is not clear if, for example, a feed of pure cumene (with A = 100) has the same coking tendencies as a mixture of decane and naphthalene with the same value of A. Clearly, however, the use of A and the results are unambiguous in the present case. 

The experiments on the USY-zeolite (USYZ) at normal pressure showed that the USYZ had a low activity and deactivated slowly, but its coke contents were not small [6]. This means that the USYZ has a strong coking tendency. Therefore experiments on USYZ under supercritical conditions are interesting. Figure 1 shows the conversion against time on stream at different temperatures each at 1 bar and 60 bar. It can be seen clearly that the conversion levels under supercritical conditions are higher than those at normal pressure in spite of the much larger flow rate at 60 bar. 

Figure 7 shows the relative areas of different acid centres on ZSM-5 in dependences on temperature and pressure. It can be found that the acid centres were less reduced at 623 K under supercritical conditions than those at normal pressure. Especially on Lewis centres the coking tendency is weak. This implies that the coke deposited on Lewis centres may be loosely built and can be easily removed by supercritical fluid. At 673 K the acid centres of ZSM-5 disappeared almost totally. This indicates that coking tendency increases more quickly with increasing temperature than the ability of coke extraction. 

The coke extraction by supercritical fluids is strongly dependent on the type of catalyst. The three-dimensional USYZ is easier accessible for the solvent than the two-dimensional ZSM-5 and the one-dimensional H-modernite. For USYZ there is an optimal temperature, at which the supercritical fluid has the highest ability for coke extraction. For ZSM-5 the coke content and the rest of the acid centres of catalyst are strongly dependent on the temperature. At 623 K the acid centres decreased only about 5%, but at 673 K they were almost totally decimated. Due to the faint coking tendency of ZSM-5 the supercritical fluid plays only a small role for the regeneration of the catalyst. But the supercritical fluid can ameliorate the product distribution of the EBD on ZSM-5. For H-mordenite the conversion of EB is strongly dependent on the temperature in the range of 623 - 673 K because of its one-dimensional channel system. 

A lower capacity is required at critical flow for a valve with less pressure recovery. Although this may not lead to a smaller body size because of velocity and stability considerations, the choice of a more economical body type and a smaller actuator requirement is attractive. The heavy-duty angle valve finds its application generally on flashing-hydrocarbon liquid service with a coking tendency. 

Temperature is the most important operating variable, since it determines both the rate of thermal decomposition and the stability of feedstock and reaction products. High temperature (>600°C) and both vacuum and product dilution favour the production of simple small gaseous molecules, low temperature (<400°C) and increased pressure lead to more viscous liquid products, higher rates of pyrolysis, a higher coking tendency, more secondary products and dehydrogenation. 

Residence Time. The influence of residence time on yields is similar to that of hydrocarbon partial pressure, but smaller. In principle, unsaturated components increase slightly with shorter residence time, depending on the cracking severity. At the same time, saturated components and pyrolysis fuel oil (PFO) decrease. The quality of pyrolysis fuel oil also is influenced by residence time. For constant P E, the ratio of carbon to hydrogen in PFO becomes smaller with decreasing residence time, which has a positive effect on coking tendency besides other parameters. 

The effect of suJfur poisoning on the coking tendency of alumina-supported ruthenium SNG catalyst has been studied. The clean KU/AI2O3 catalyst has exceptional coking resistance, and at 490 C and 25 atm, can tolerate steam to carbon ratios below stoichiometric (steam/carbori=0.6) with light naphtha before a continuous accumulation of carbon will occur. However, at this temperature (appropriate for SNG production), sulfur can adsorb on the active metal surfaces to a level which will cause a slow but steady accumulation of less reactive carbon. The critical sulfur coverage that adversely affected the steam to carbon ratio necessary to prevent continuous coking appears to fall just above one-half the maximum capacity of the catalyst. 

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