Intraparticle deactivation

S. Zmcevic, The intraparticle deactivation of Ni-Si02-Al203 by thiophene in benzene hydrogenation, Chem.Eng.Sci., 39(1984)1245. 

The intraparticle deactivation of nonisothermal pellets has been analyzed by Ray (21) using a pore mouth poisoning, slab geometry model. Heat flux at the boundary of the active and inactive portions of the particle (Figure 2a) was computed via Bischoff s ( ) asymptotic solution for large Thiele modulus. An effective diffusional modulus for this case can be defined as ... 

Additional theoretical investigations of the intraparticle deactivation problem, which unfortunately we cannot treat in detail here, have been reported by Luss and co-workers (28,29) on the modification of selectivity upon poisoning, and by Hegedus (30) on the combined influence of interphase and intraparticle gradients on deactivation It is of interest that deactivation in certain instances can actually have beneficial results on selectivity and in the long run the problem may be to achieve the best balance between diminished activity and enhanced selectivity such results are reminiscent of those pertaining to deactivation of bifunctional catalysts (19,20) ... 

The interrelation between the process mechanisms and the deactivation kinetics under conditions of intraparticle resistance is discussed. Object of examination are processes in which the formation of blocking agents is a constituent part of the reaction scheme. 

The phenomena relevant to HDM catalyst deactivation are intrinsic reaction kinetics, restrictive intraparticle diffusion and (changing) catalyst porous texture. 

Preliminary HDM catalyst deactivation simulations using the reaction kinetics of model compound vanadyl-tetraphenylporphyrin indicate that reliable metal deposition profiles and catalyst life-time predictions can be made provided that intrinsic reaction kinetics and restrictive intraparticle diffusion are introduced in the catalyst deactivation model. 

Another deactivation factor is the deposited substances that hinder intraparticle diffusion of reactants. The vanadium deposition is supposed to have a significant effect on the effective diffusivity in the catalyst pores. The decline of the effective diffusivity is expressed by the following equations for sulfur Ds and vanadium Dy, respectively ... 

Deactivation of large-pore slab catalysts where intraparticle convection, diffusion and first order reaction are competing mechanisms was analyzed by uniform and shelLprogressive models. For each situation, analytical solutions for concentration profiles, effectiveness factor and enhancement factor due to convection were developed thus providing a sound basis for steady-state reactor design. 

However, it is apparent from the results of Sagara et al., and fi om subsequent experiments that nonisothermality has a profound influence upon the development of intraparticle profiles in the presence of deactivation, as will be seen below. 

Not shown here, but of importance, is the fact that the magnitude of the ratio (KeffDeff) is very significant in determining both the level of deactivation and the thermal response of individud catalyst pellets. Particularly for exothermic reactions, this will define where the final intraparticle exotherm resides, and since it is a ratio of quantities, it is difficult to discern the contribution of individual factors in general. This is shown in detailed calculations by Lee et al. 

As a first estimate, it would appear that most combinations of deactivation mechanism and intraparticle transport processes have been investigated, at least theoretically. We still would benefit from additional experimental studies in this area, especially in systems where thermal effects are of importance. We also tend to forget that sometimes important physical properties such as porosity and effective difiusivity change significantly with time-on-stream. The basics of such studies, however, are well-established fi om work dating back even twenty plus years, as seen here, so unfortunately research of this kind is probably not considered very glamorous. In addition, although the topic has not been discussed here, more work on reaction/difiusion/deactivation in biochemical systems, in many instances similar to that done for chemical systems, is much needed and is an important area for future study. 

The porosity of the beads used is the result of a lot of optimization, and is formed both by macropores with pore diameters exceeding 0.1 pm and by micropores with a pore diameter less than 20 nm. The micropores give the high BET surface area, whereas the macropores assure a high intraparticle mass transfer rate as well as a resistance against deactivation by poisoning. 

COMBINED EFFECTS OF CATALYST DEACTIVATION AND INTRAPARTICLE DIFFUSION ON THE DYNAMICS OF A FIXED REACTOR... 

In both cases the enhancement of conversion due to intraparticle convection is higher for some intermedite value of Thiele modulus. The reactor performance is worse in the case of shell-progressive deactivated catalysts for cc=0-1 however, the enhancement due to convection is higher in this case. 

The model reaction of p-xylene hydrogenation was chosen in order to provide the mild conditions of the experiments in both gas and capillary condensed phases, and to avoid the influence of side reaction and catalyst deactivation. The recycle type of gradientless reactor was used that provides uniform temperature and concentration profiles within all the catalyst packing. The catalyst particles (0.25-0.50 mm) provide a negligible intraparticle limitation of mass- and heat-transfer. 

The catalyst particles cannot be very small. The intraparticle diffusion effects can be significant. The catalyst pore-mouth plugging can cause rapid deactivation. 

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