Lumping scheme deactivation

In this chapter the following topics will be reviewed KINPTR s start-of-cycle and deactivation kinetics, the overall program structure of KINPTR, the rationale for the kinetic lumping schemes, the model s accuracy, and examples of KINPTR use within Mobil. As an example, the detailed kinetics for the C6 hydrocarbons are provided. 

A deactivation function is needed to account for the catalyst activity decay due to coke deposition on the catalyst. The various forms of this function were discussed in Chapter 2 where can be a function of catalyst time-on-stream or more appropriately as a function of coke content on the catalyst. The kinetic constant, k, is the overall gas oil cracking rate constant which is the sum of ki and kj from the 3-lump scheme. Substituting this value for rA in equation (3.1) results in ... 

Different orders of deactivation will be used in the modeling. A simplification will be that the activity function for all components is the same. Using the reaction scheme given in figure 2 and the reaction rates as described by equation 1, in combination with a plug flow reactor model, mass balances can be derived for the different lumped groups, resulting in ... 

The calculation of the composition and activity profiles in the reaction stage in an adiabatic fixed bed is carried out by simultaneous solution of the following equations 1) mass conservation equations for each one of the lumps of the kinetic scheme, eq. (8) and for water, eq. (9), assuming plug flow 2) the heat conservation equation for the bed+gas system, eq. (10) and for the reactor wall, eq (11) 3) and the kinetic equation for deactivation, eq. (4). In a previous paper [13] the suitability of this set of equations for describing the behaviour of the adiabatic reactor has been proven. 

Due to the similarity of the catalyst and of the characteristics of the deactivation in the transformation of methanol and of ethanol, eq. (5) has been taken as a basis for the establishment of possible deactivation equations for the transformation of ethanol into hydrocarbons. In eq. (5), X is the mass fraction (based on the organic components in the reaction medium) of the lumps of the kinetic scheme that can be considered coke precursors. This is the way in which the composition of the reaction medium is commonly expressed in the literature for the kinetic study of the processes of transformation of methanol on a HZSM-5 zeolite [8,12-14,16] and on a SAPO-34 [9]. In eq. (5), activity, a, is defined as the ratio between reaction rate at t time and at zero time ... 

A set of (n-1) equations such as eq. (15), corresponding to (n-1) lumps of the kinetic scheme, must be solved, together with the deactivation kinetic equation. A program in FORTRAN has been developed for this calculation, which uses the LSODE subroutine from the DSSP library. The composition of the remaining lump is calculated by difference to unity. 

The fitting of the kinetic model calculated to the experimental results is shown in Figures 1 and 2, where the evolution with time on stream of the mass fraction (by mass unit of organic components in reaction medium) of each lump of the kinetic scheme has been plotted. As is observed in Figure 1, deactivation attenuates as water content in the feed increases, at 350 °C. 

It has been proven that the deactivation of the HZSM-5 zeolite used in the transformation of aqueous ethanol into hydrocarbons in the 350-450 °C range is explained by a similar kinetic scheme to that already established for the transformation of aqueous methanol. The kinetic equation, eq. (13), is applicable to all the steps of the kinetic scheme except to ethanol dehydration, which is not affected by catalyst deactivation. Eq. (13) takes into account the effect of concentration of ethene and of the lump of olefins and gasoline in the reaction medium on the deactivation, although ethene is the main coke precursor on the basis of the values of the kinetic constants. The water present in the reaction medium plays an important... 

In the above expression, [TOC j is the asymptotic residual organic carbon, which cannot be oxidized (Rcl) further. Agreement between the experimental data for combined thermolysis, catalytic, and non-catalytic WO and the model prediction is shown in Figure 5.5. From this plot one can conclude that the oxidation progress is terminated once the catalyst is deactivated due to the adsorption of carbonaceous intermediates on its surface. However, for practical design purposes one should use the lump kinetic approach based on the triangular reaction scheme such as depicted in Figure 5.1. It is believed that the rate laws can be expressed mostly by a simple power function. 

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