Catalytic coking

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). 

Equation 6 relates the catalytic coke yield (as a fraction of the feed) to the delta coke, to the conversion, and to the catalyst residence time. 

At high metals levels, the coking characteristics of a cracking catalyst can be greatly increased that is, the ratio of contaminant coke to catalytic coke can be quite high. The effect of the contaminant metals on the coke response is affected not only by the level of metals but also by the type of catalyst and the use of a metals passivator. Catalysts, which contain effective metals traps to inhibit the contaminant effects, do produce much less contaminant coke than catalyst without metal traps. 

The Conradson test (ASTM D-189) measures carbon residue by evaporative and destructive distillation. The sample is placed in a preweighed sample dish. The sample is heated, using a gas burner, until vapor ceases to burn and no blue smoke is observed. After cooling, the sample dish is reweighed to calculate the percent carbon residue. The test, though popular, is not a good measure of the cokeforming tendency of FCC feed because it indicates thermal, rather than catalytic, coke. In addition, the test is labor intensive and is usually not reproducible, and the procedure tends to be subjective. 

For a given catalyst and feedstock, catalytic coke yield is a direct function of conversion. However, an optimum riser temperature will minimize coke yield. For a typical cat cracker, this temperature is... 

Catalytic coke is a byproduct of the cracking of FCC feed to lighter products. Its yield is a function of conversion, catalyst type, and hydrocarbon/catalyst residence time in the reactor. 

Feed/catalyst injection. A well-designed injection system provides a rapid and uniform vaporization of the liquid feed. This will lower delta coke by minimizing non-catalytic coke deposition as well as reducing the deposits of heavy material on the catalyst. 

Catalytic Coal Liquid (CCL) process, 6 848 Catalytic coke yield, 11 705 Catalytic constant, 10 254 Catalytic converter(s), 10 31, 39 40 26 719. See also Three-way catalytic converter... 

Lower dilution levels did not allow sufficient depolymerlzatlon and higher dilution caused excessive depolymerlzatlon In the aged solutions. Pillared clays prepared from aged dilute solutions had an enhanced microstructure which showed an Increased activity for selectively cracking large molecules to the light cycle oil range. This microstructure Is lost In the presence of steam which also reduces the formation of catalytic coke. Addition of rare earth zeolite to pillared clay can partially overcome the effects of this loss of microstructure. 

This observation was not so obvious on coke yields because the coke production is a contribution of mnltiple mechanisms and reactions. Thus, the coke yields are quite similar, probably because the catalytic coke is decreased while the contaminant coke is increased. The coke remarks are also observed on the CPS samples taking into account that the dehydrogenation degree is not strongly affected by the extended ReDox cycles, becanse the lower catalysts decay is limiting the effect of the required mass of catalyst (C/0 ratio). Thus, the small decrement of the coke yield on the CPS samples is possibly related to the descent of the catalyst (less specific area) leaving less available space for coke adsorption and less activity for catalytic coke production. It is clear that prolonging the deactivation procednres is not beneficial as far as the metal effects are concerned. 

The catalytic coke produced by the activity of the catalyst and simultaneous reactions of cracking, isomerization, hydrogen transfer, polymerization, and condensation of complex aromatic structures of high molecular weight. This type of coke is more abundant and constitutes around 35-65% of the total deposited coke on the catalyst surface. This coke determines the shape of temperature programmed oxidation (TPO) spectra. The higher the catalyst activity the higher will be the production of such coke [1],... 

The TPO profiles obtained were analyzed by deconvoluting them using Gaussian peaks and GRAMS 32 software. The peaks obtained were assumed to represent the four different types of coke in the spent catalyst catalytic coke, contaminant coke, occluded coke, and additional coke (Conradson carbon). 

Table 10.4 illustrates the TPO data for each of the samples tested, standardized in coke (g of coke associated with each peak)/total coke on the catalyst (g). In this table it is observed how the total coke yield of these samples is inversely proportional to the catalyst deactivation severity. Similarly, it is noted that catalytic coke increases with catalyst activity, but it is not a direct function of conversion. 

When the zeolite surface area is plotted as a function of catalytic coke the correlation improves. The best correlation between the physicochemical properties of the catalyst and catalytic coke is the one involving an amount of aluminums in the framework estimated from the unit cell size by Equation 10.1 [1], as it is evidenced in Figure 10.2. 

It is possible to conclude from the TPO spectra that the signal with the peak of highest intensity located between 610°C and 617°C, for the case of light feedstock and low CCR, can be attributed to catalytic coke. This type of coke can reach values up to 89% of the total coke on the catalyst. In addition, it can be also concluded that catalytic coke yields are a direct function of the number framework aluminums in the zeolite structure. 

Table 10.6 summarizes data from the TPO profiles for the three samples with and without metals. This table clearly shows how the signal C increase as the concentration of nickel and vanadium increases and supports the hypothesis that this peak corresponds to contaminant coke. It is possible to support the theory that a higher content of vanadium in the catalyst results in a loss of activity because the peak area B, previously attributed to catalytic coke, decreases strongly with vanadium levels. 

Slurry hydroconversion process a process in which the feedstock is contacted with hydrogen under pressure in the presence of a catalytic coke-inhibiting additive. 

Based on Voorhies time-on-stream theory (4), catalytic coke is a function of catalyst contact time ... 

Reduce the catalytic coke formation of the catalyst in order to allow some room for the additional coke formed by these poisons. In this approach we accept that the extra coke formed is unavoidable, and that we need to compensate by improving the coke selectivity of the catalyst. 

Further reduction of "catalytic" coke and fuel gas production in order to allow more room for resid processing... 

The catalytic coking reaction may require dual catalyst sites, whereas small size reactants which are less subjective to coking may be able to access the sites covered by the coke. 

A comprehensive study on coke deposition in trickle-bed reactors during severe hydroprocessing of vacuum gas oil has been carried out. On the basis of results obtained with different catalysts on the one hand, and analytical and catalytic characterisation of the coke deposits on the other, it is argued that coke is formed via two parallel routes, viz. (i) thermal condensation reactions of aromatic moieties and (ii) catalytic dehydrogenation reactions. The catalyst composition has a large impact on the amount of catalytic coke whilst physical effects (vapour-liquid equilibria, VLE) predominate in determining the extent of thermal coke formation. The effect of VLE is related to the concentration of heavy coke precursors in the liquid phase under conditions which promote oil evaporation such as elevated temperatures. A quantitative model which describes inter alinea the distinct maximum of coke deposited as a function of temperature is presented. 

The catalytic coke is formed via dehydrogenation reactions catalysed by the MoSj phase. Higher Mo loads will not lead to further reduction of thermal coke and to enhanced amounts of catalytic coke. The latter aspect explains the increasing coke formation going from 0,2 to 10 Mo/100 A1203. 

In order to arrive at a sound description of the amount of coke deposited it is essential to build on the insight that two routes to coke exist, viz. catalytic and thermal coke. For the rate of formation of catalytic coke we assume that a simple Langmuir type kinetic expression suffices. 

Again CL is the concentration of coke precursors. is the hydrogen pressure. However, we now distinguish between the concentration in the liquid phase and the vapour phase and use equation (3) for both phases separately. The reason is that we are dealing with thermal reactions which take place both in the gas and in the liquid phase and depend on the respective concentrations and residence times. For the catalytic coke — equation (1) — deposition is related to the adsorbed phase and the driving force in the gas phase suffices to arrive at a proper description. For the latter, actual residence times are not important either as the contact time with the catalyst is the relevant parameter. 

Catalytic coke is strongly hydrogen deficient, has a well developed graphitic structure and is widely spread over the surface, thus covering up the active sites to a large extent. 

We continue to rely extensively on the two-step (initiation - propagation or autocatalytic) model 4) to evaluate data on coking rates. Two rate constants are involved fc for the deposition of coke on a "clean" surface, i.e., with no coke around and k2 when coke is deposited adjacent to another coke deposit. The former rate constant is for an initiation step (or "non-catalytic" coking), while the latter is for the propagation step (or coking catalyzed by the presence of the coke "product") hence, typically, k2 > ki. A third parameter used in the model is M, which represents the maximum amount of coke which can be deposited on the catalyst. In terms of these three parameters, the coke level expected in a pulse reactor after the passage of R amount of reactant is given by ... 

Overall, evaluation of catalysts on resid feedstocks requires sophisticated and well integrated catalyst deactivation, catalyst stripping and cracking systems. It is important to determine not only the coke yield, but each of its components (Catalytic coke, contaminant coke, CCR coke and stripper (soft) coke). This paper provides details on how each of the components of the coke yield may be experimentally determined using catalyst metallation by cyclic deactivation, catalyst strippability measurements and modified catalytic cracking techniques. 

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