Coking rate constant

HEAVY OIL PRODUCTION AND COKING RATE CONSTANTS, k2 AND kc (RATE CONSTANTS IN MIN.-1, COKE YIELDS IN WT % KEROGEN) ... 

Do not infer from the above discussion that all the catalyst in a fixed bed ages at the same rate. This is not usually true. Instead, the time-dependent effectiveness factor will vary from point to point in the reactor. The deactivation rate constant kj) will be a function of temperature. It is usually fit to an Arrhenius temperature dependence. For chemical deactivation by chemisorption or coking, deactivation will normally be much higher at the inlet to the bed. In extreme cases, a sharp deactivation front will travel down the bed. Behind the front, the catalyst is deactivated so that there is little or no conversion. At the front, the conversion rises sharply and becomes nearly complete over a short distance. The catalyst ahead of the front does nothing, but remains active, until the front advances to it. When the front reaches the end of the bed, the entire catalyst charge is regenerated or replaced. 

Fig. 6. Burning rate constant is independent of origin of coke. Five different modes of coking are represented by different symbols. From Weisz and Goodwin (1963).

A third problem was that of fast coke. This coke also exhibited first-order intrinsic burning, but with a rate constant 17 times that of slow coke at 950°F (783 K). Again a fortunate simplification was found so that the kiln equations would not have to be solved for two kinds of coke burning... 

Heat of reaction for combustion of coke to CO, Btu/lb dutff Heat of reaction for combustion of fast coke, Btu/lb AHx [axAHcoj + 2(1 - a,) AHJ/a k Rate constant for carbon burning, ft /(lb mole)(hr)... 

As the catalyst ages, the rate constants decrease since the number of active sites is reduced by coke formation ... 

Here A represents gas oil, A2 gasoline, and A3 represents coke and dry gases. The rate of disappearance of gas oil, the rate of appearance of gasoline, and the rate of appearance of coke and light gases are given by the equations (7.25), (7.26), and (7.27), respectively. The rate constants can be written in Arrhenius2 form as follows ... 

Parametric Studies. To provide a feel for the relative importance of some of the model parameters, and to understand observed differences in performance between REV and USV catalysts (Figure 1), key parameters such as the cracking and coking intrinsic constants kj and Aj, the heats of reaction AHj, and the order of coke deactivation, n-, were varied. The base case model parameters and the rate... 

Figure 1. The effects of a series of Mo/AljOj catalysts with varying Mo loading (0-10%(m/m)) on the coke selectivity and the hydrodesulfurization activity. The HDS activity has been expressed as a second-order rate constant ( hds)< Process conditions temperature 450 C, pressure 30 bar, H2/oil ratio 900 Nl/kg, WHSV 1.3 kg/(kg h) and run length 180 h.

Note that as a first approximation the effect of hydrogen is not taken into account, which implies that the model will hold only for a limited range of hydrogen pressures. As a driving force for the reaction we use the gas-phase concentration, Cq, of the coke precursor Q, is the equilibrium constant of adsorption of Q on the catalyst surface. The rate constant for coke formation, kc, depends on the amount of coke present on the surface ... 

In Figure 8 we have gathered data for the extent of coke formation as a function of temperature for a series of experiments and model calculations with the CoMo/A O catalyst. Although the rate constants of coke formation increase with temperature, the net effect of VLE and accelerated kinetics again lead to a pronounced maximum. The arguments discussed above also hold here. 

Sample Crystallinity NFAI content Surface area Micropore volume Coking rate Rate constant ... 

The relation between De and coke content has no analytical expression and the results (obtained by computer resolution of equations (6, 9) and r =f((p)), are presented in figure 4 (computer parameters rate constants 950, 530, and 540 s 1 for Aramco, Nigeria and Montmirail respectively, 0=5 and zeolite diameter = 3pm). 

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