Rate of coking

The rate of coke burning for coke deposited on a zeolite-containing catalyst has been reported to be first order with respect both to coke concentration and oxygen partial pressure (23) ... 

Over 25 years ago the coking factor of the radiant coil was empirically correlated to operating conditions (48). It has been assumed that the mass transfer of coke precursors from the bulk of the gas to the walls was controlling the rate of deposition (39). Kinetic models (24,49,50) were developed based on the chemical reaction at the wall as a controlling step. Bench-scale data (51—53) appear to indicate that a chemical reaction controls. However, flow regimes of bench-scale reactors are so different from the commercial furnaces that scale-up of bench-scale results caimot be confidently appHed to commercial furnaces. For example. Figure 3 shows the coke deposited on a controlled cylindrical specimen in a continuous stirred tank reactor (CSTR) and the rate of coke deposition. The deposition rate decreases with time and attains a pseudo steady value. Though this is achieved in a matter of rninutes in bench-scale reactors, it takes a few days in a commercial furnace. 

In single-stage units which do not produce kerosene or other critical stocks, flash zone temperatures may be as high as 750 - 775 F. The principal limitation is the point at which cracking of distillates to less valuable gas or the rate of coke formation in the furnace tubes becomes excessive. 

Interestingly, one can easily deduce an expression for the relative rate of coke formation as compared to that of methanation. The rate of initial coke formation depends on the combination probability of carbon atoms and hence is given by... 

The relative rate of coke versus methane formation then follows from... 

We can equate the rate of decrease in available active sites to the sum of the rates of coke buildup on the catalyst for each adsorbed intermediate by differentiating Eq. (29) ... 

As discussed previously, the rate of coke buildup can be related to the concentration of components in the gas phase [Eq. (26)]. An example for the coking of ring isomerization sites by cyclohexane intermediates is... 

Stocks of higher mid-boiling point are more readily decomposed and necessitate the use of milder conditions for a given rate of coke production. This is attained in the fixed-bed units by the use of lower oil partial pressures and catalyst activities, and in the moving-bed units by higher space rates. 

The rate of coke burning is given in equation (7.28) and the value of the preexponential factor and activation energy are... 

Calculate the rate of coke formation Rcf from equation (7.43) and Ta from equation (7.31). 

Coke Formation as a Function of Pressure The amount of coke formed is independent of total pressure (Pa + Pi) in the range tested (76 to 44 cm. Hg). This is shown by the data for cumene hydroperoxide at 420°C. given in Table XI. The length of the runs was 4000 sec. and the mole fraction of cumene hydroperoxide used is indicated. The fact that the rate of coke formation does not depend on pressure means that the rate of coke deposition must be independent of the absolute concentration of cumene hydroperoxide and depend only on the mole fraction present. 

Coke combined with active sites is not included, as the type of coke under consideration does not deactivate the catalyst. The coke steps are symmetrical with the cracking steps. This leads to an equation for the rate of coke formation analogous with Eq. (11). 

Steam is generally added for at least two reasons first, it helps obtain quickly and then maintain the desired starting temperatures second, it reduces the rate of coke collected on the inner surface of the reactor coil. Steam reacts slowly with deposited coke nickel and iron on the coil surfaces, or present in the coke, catalyze this reaction. 

At intervals often varying from 1 week to several months, a pyrolysis unit must be shut down in order to clean (or decoke) the coils. Such decokings sometimes require 1-2 days to complete. Obviously, as the rates of coking increase, the number of decokings per year also increases, causing the annual production rate of ethylene to decrease. Furthermore, utility and labor costs for each decoking are relatively expensive. 

The addition of steam to the entering feed provides several advantages, including heat sink which helps maintain higher temperatures and results in lower partial pressure of the hydrocarbons. The decreased partial pressure helps to minimize undesirable reactions. Increased amounts of steam result in increased steam-coke reactions, which result in the production of carbon oxides and in slower rates of coke formation on the metal surfaces. Typical steam requirements are shown in Table V. 

Adopting these approximations, and assuming that the rate of coke burning is given by... 

Clearly, burning should be performed at a temperature high enough to give a sharp bum front, but alone this is still not sufficient to reduce the coke to an acceptable level in a short time, for two reasons. First, the rate of coke burning decreases markedly when the center of the burn front passes out of the bed and, second, coke consists of a mixture, some of whose components bum much more slowly than the others. Consequently, it is common to switch to more severe conditions for a period of secondary burn following the primary bum. 

The reason why the minimum steam ratio goes down with temperature is not known with certainty. One possibility is that the competing reactions of carbon production and consumption have such kinetics that the rate of coke consumption increases faster with temperature than the rate of coke generation, which suggests that the carbon-steam reaction has a higher activation energy than the methane cracking and carbon monoxide disproportionation reaction. 

During experiments performed at constant feed rates, the conversion changes as a consequence of the deactivation of the catalyst. This implies that a single experiment can not distinguish between the influence of coke and that of conversion on the reaction kinetics and the rate of coke formation. 

The effect of the H2/oil ratio on the coke content of a NtV/SiOj catalyst (low HDS activity, thermal coke predominates) is shown in Figure 6. The distinct maximum of the coke deposited with the H2/oil ratio is apparent from both experiment and theory. Detailed analysis of the model output indicates that at low gas rates the VGO feedstock is mainly in the liquid phase throughout the reactor, whilst at the highest gas rates the reactor is operated in the gas phase already at the reactor inlet. In both limits the amount of coke deposited is modest. Intermediate gas rates (1000 Nl/kg), however, lead to much higher rates of coke... 

Model compound testing was used to study the role of coke in the deactivation of hydrotreating catalysts. The approach used was to pick one of the compounds used in the initial adsorption experiments that gave a measurable increase in coke over a period of a few days and to study whether the rate of coking was affected by the presence of a sulfur or nitrogen compound that could be used to measure activity (HDS and HDNt respectively). If the rate of coking was unaffected, the activity was measured as a function of time, It would then be possible to relate activity to coke content. 

The coke precursor chosen was pyrene. As shown in Fig. 6, it was found that the amount of coke deposited on a NiMo catalyst increased, at first rapidly and later more slowly, to about 14% after 150 hours. Adding DBT to the pyrene feed did not affect the rate of coking, whereas indole in the feed changed the rate and extent of coking significantly, which is not surprising in the light of the results obtained in the initial adsorption experiments. 

---