Coking deactivation function

Figure 2. Change of coking deactivation function with time.

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. 

All the previously cited models and works also consider, and some explicitly cite, this assumption—that the catalyst activity varies with time-on-stream (or with coke concentration [12]) in the same manner or with the same deactivation function (VO for all reactions in the network. That is, a nonselective deactivation model is always used. Corella et al. (16) have recently demonstrated that in the FCC process this assumption is not true and that it would be better to use a selective deactivation model. Another work (17) also shows how this consideration, when applied to catalytic cracking, influences the yield-conversion curves. Nevertheless, to avoid an additional complication, we will use in this chapter a nonselective deactivation model with the same a—t kinetic equation and deactivation function (VO for all the cracking reactions of the network. 

The influence of the coke on the kinetics of the main reaction can be accounted empirically by multiplying the kinetic coefficient of eq. (4) by a deactivation function coke content of the catalyst, Cc ... 

Equations for the kinetic mechanisms of coke formation with the exponential form of the deactivation function are obtained by integrating eqs. (6)—(8) ... 

To quantify the deactivation effect of coke on the various reaction rates, a deactivation function (Cc) was defined as the ratio of the reaction rate at a given coke content to the... 

Since the coking reaction is not deactivated by coke formation, its deactivation function equals one. 

The deactivation functions for the isomerisation reactions of n-hexane were shown to be exponential functions of the coke content. The deactivation constant, the parameter of these functions, did not differ significantly for the various isomerisation reactions leading to tertiary carbenium ions. The deactivation constant for the isomerisation to 2,2-di-Me-butane, formed out of a secondary carbenium ion, was larger. 

The deactivation of cracking catalysts by coking with vacuum gas oils (VGO) is studied in relation to the chemical deactivation due to site coverage, and with the increase of diffusional limitations. These two phenomena are taken into account by a simple deactivation function versus catalyst coke content. The parameters of this function arc discussed in relation to feedstock analysis and change of effective diffiisivity with catalyst coke content. 

The experimental studies using industrial feedstock are carried out in a modified MA.T. (micro activity test) [10], The reaction conditions are presented in table 1, The catalyst is NOVA D equilibrium catalyst from Grace Davison Co, The equilibrium catalysts are previously coked under the same reaction conditions to get partially deactivated samples. The method using the conversion versus initial coke content from experiments to determine the deactivation function, is described in [10]. Three different feedstocks are used (table l). 

At low coke content, pore plugging is still negligible and decay is mainly due to site coverage. Consequently, the variation of deactivation function with coke content is only due to chemical deactivation, and it is proportional to the remaining activity ... 

Figure 1. Deactivation function versus initial coke content (O) Aramco, (A) Montmirail and (D)Nigeria.

Coke formation during the catalytic dehydrogenation of butene-1 has been studied in the temperature range 525-600 °C at butene-1 partial pressures of 0.05 to 0.25 bars. Moderate levels of coke deposits led to blocking of the catalyst mesopores and a hyperbolic deactivation function was found to provide the best fit to the data. Increase of temperature caused the deactivation to change from a parallel to a series coking process. 

The decrease in the rate of dehydrogenation caused by coke deposition is decreased monotonically with coke content. The deactivation function 4, expressed as the ratio of reaction rates in the presence and absence of coke was tested... 

The deactivation of catalysts concerns the decrease in concentration of active sites on the catalyst Nj. This should not be confused with the reversible inhibition of the active sites by competitive adsorption, which is treated above. The deactivation can have various causes, such as sintering, irreversible adsorption and fouling (for example coking or metal depositions in petrochemical conversions). It is generally attempted to express the deactivation in a time-dependent expression in order to be able to predict the catalyst s life time. An important reason for deactivation in industry is coking, which may arise from a side path of the main catalytic reaction or from a precursor that adsorbs strongly on the active sites, but which cannot be related to a measurable gas phase concentration. For example for the reaction A B the site balance contains also the concentration of blocked sites C. A deactivation function is now defined by cq 24, which is used in the rate expression. 

Note that in the hexene balance (eq. 1), the yield of coke is neglected as being insignificant when compared to the yield of the isomers. A linear deactivation function (a) is employed for all surface-catalyzed reactions as follows ... 

These deactivation functions do not contain the gas phase composition It Is clear from (2) and (3) that the coke content is a local value. Consequently, in an integral reactor coke is deposited according to a profile, leading to non uniform deactivation Such a situation cannot be predicted when (1) is multiplied by an empirical function of time only Conversely, it is clear from (2) and (9) e.g. that, when a deactivation function like exp(-at) is used to fit experimental data on deactivation, a cannot be a true constant [ref. 5]. 

Fig. 2 Topological model. Deactivation functions versus surface coverage. Main reaction on single sites. Coking on dual sites [Ref 14].

Freshly deposited coke is not inert and growth is possible through polymerization or polycondensation [ref. 17]. It has been shown by Levinter et al. [ref. IS] that coke can easily reach a size sufficient to block mesopores. If a single-ended pore becomes blocked all the sites located behind the blockage become inacccessible, so that a non linear relation is obtained between the deactivation function and the fractional site coverage, even for single site reactions. 

The model was applied to a meso-macro porous structure representative for a chromia-alumina catalyst- For single site coking, instantaneous growth of coke and no diffusional limitations the following deactivation function was derived ... 

Pt- and alumina sites was assumed to remain constant i e. Independent of time or coke content. Coking and hydrogenolysis were shown to occur on the B ulle sites, and their deactivation functions were identical exp(-aQCQ). The deactivation function for the... 

Notice that coking rate equations were derived, so that there was no real need to resort to empirical deactivation functions. That was done because it was not investigated if the deactivation was caused by site coverage or pore blockage or by both mechanisms ... 

Beeckman and Froment [ref 19] integrated the system (28) - (29) with the appropriate boundary conditions for resp parallel and consecutive coking The catalyst was considered to consist of a network of meso- and macropores for which the deactivation function... 

Grwin and Luss [ref- 37 simulated an adiabatic reactor for a first order irreversible reaction with an activation energy exceeding that for the coke formation and an exponential deactivation function in terms of the coke content The results of the... 

Fig. 9. Surface of rate of main reaction. Parallel coking and exponential deactivation function. [Ref. 36].

---