Pore-mouth poisoning

This situation is termed pore-mouth poisoning. As poisoning proceeds the inactive shell thickens and, under extreme conditions, the rate of the catalytic reaction may become limited by the rate of diffusion past the poisoned pore mouths. The apparent activation energy of the reaction under these extreme conditions will be typical of the temperature dependence of diffusion coefficients. If the catalyst and reaction conditions in question are characterized by a low effectiveness factor, one may find that poisoning only a small fraction of the surface gives rise to a disproportionate drop in activity. In a sense one observes a form of selective poisoning. 

If a fraction a of the total catalyst surface has been deactivated by poison, the pore-mouth poisoning model assumes that a cylindrical region of length (aL) nearest the pore mouth will have... 

In order to demonstrate the selective effect of pore-mouth poisoning, it is instructive to consider the two limiting cases of reaction conditions corresponding to large and small values of the Thiele modulus for the poisoned reaction. For the case of active catalysts with small pores, the arguments of all the hyperbolic tangent terms in equation 12.3.124 will become unity and... 

Now consider the other extreme condition where diffusion is rapid relative to chemical reaction [i.e., hT( 1 — a) is small]. In this situation the effectiveness factor will approach unity for both the poisoned and unpoisoned reactions, and we must retain the hyperbolic tangent terms in equation 12.3.124 to properly evaluate Curve C in Figure 12.11 is calculated for a value of hT = 5. It is apparent that in this instance the activity decline is not nearly as sharp at low values of a as it was at the other extreme, but it is obviously more than a linear effect. The reason for this result is that the regions of the catalyst pore exposed to the highest reactant concentrations do not contribute proportionately to the overall reaction rate because they have suffered a disproportionate loss of activity when pore-mouth poisoning takes place. 

What is your interpretation of these data Do the data indicate whether homogeneous or pore-mouth poisoning takes place ... 

A comparison of uniform and pore mouth poisoning appears in P7.06.08 as a function of fractional poisoning. At a given fraction, effectiveness is reduced much more by pore mouth poisoning. 

P7.06.07. PORE MOUTH POISONING OF FIRST ORDER REACTION IN A SLAB... 

In pore mouth poisoning, the catalytic activity is completely destroyed in a fraction (3 of the pore from its mouth. At the mouth of the pore, x = L, C = Cs... 

For uniform poisoning, the effectiveness is obtained by simply replacing kv will) kv(l-(3) in the definition of . For pore mouth poisoning the equation for rj is in P7.06.07. These issults are for first older reaction in slab geome try. 

For first order reaction in slab geometry, evaluate the ratio of effectiveness with uniform poisoning, 7)un> and pore mouth poisoning, T)pm, in terms of fractional poisoning and the Thiele modulus. 

Acres et al. (22) have speculated on the modes of phosphorus and lead poisoning in monolithic catalysts, based on data obtained in simulated aging. Conversion vs. time-of-exposure curves for catalysts poisoned by either lead or phosphorus show quite different shapes, which the authors attribute to pore-mouth poisoning for phosphorus, and uniform poisoning... 

A second important poison is AS2O3 but its poisoning effect is much less than that of sulphur [17], The mechanism of AS2O3-poisoning is based on the formation of an alloy with nickel. The arsenic typically originates from the solutions used in carbon dioxide wash of the catalyst or is present as an impurity in some zinc oxide sulphur removal beds. Also silica is mentioned as a pore mouth poison by physically blocking the entrance to the pore system by which the catalyst activity is decreased [18],... 

Activity-versus-time curves shown in Fig. 25 for alumina-supported Ni and Ni bimetallic catalysts show two significant facts (1) the exponential decay for each of the curves is characteristic of nonuniform pore-mouth poisoning, and (2) the rate at which activity declines varies considerably with metal loading, surface area, and composition. Because of large differences in metal surface area (i.e., sulfur capacity), catalysts cannot be compared directly unless these differences are taken into account. There are basically two ways to do this (1) for monometallic catalysts normalize time in terms of sulfur coverage or the number of H2S molecules passed over the catalysts per active metal site (161,194), and (2) for mono- or bimetallic catalysts compare values of the deactivation rate constant calculated from a poisoning model (113, 195). 

FIG, 23-20 Poisoning of a first-order reaction a) uniform poisoning, (b) pore mouth poisoning. 

Pore Mouth Poisoning Flat Plate Pellets. For flat plate type pellets undergoing pore mouth poisoning, the moles of A reacting per pore is given by (7)... 

Figure 1. Dimensionless reactant concentration vs. dimensionless time for pore mouth poisoning NA = i parameter, h...

Pore Mouth Poisoning Flat Plate Pellets. For pore mouth poisoning of flat plate pellets, substitution of equation (6) into equation (16) yields... 

With either pore-mouth or uniform poisoning, fixed bed conversions and production levels are a strong function of the reactant Thiele modulus h, increasing with h for pore mouth poisoning and decreasing with h for uniform poisoning. These trends depend on the constancy of the dimensionless groups and... 

For pore-mouth poisoning, deactivation has a more pronounced effect on production at lower moduli. At high values of h, further increases in h do not gain significant increases in production. Since pellet deactivation times decrease with increases in temperature, a best temperature may exist for maximum production. 

Poisoning Sulphur Silica Alkali Metals (K,Na) Partly coverage of Ni surface. Pore mouth poison Decrease of reaction rate... 

Rostrup-Nielsen found that the intrinsic reaction rate, rj, for methane steam reforming is correlated with the sulphur coverage by equation 6 (2). In the adiabatic prereformer, the sulphur acts as a pore mouth poison and as the reactions are restricted by pore diffusion 2,8), the effective activity of the sulphur poisoned catalyst pellet can be described by an empirical relation, equation 7, between the effective pellet reaction rate, rp, and the average sulphur coverage, 0av (7/... 

Table III Pseudo-homogeneous 1-D plug flow model with pore-mouth poisoning...

Reactor Model A variation of the pseudo-homogeneous 1-D plug flow model with shell progressive mechanism of poisoning (SPM) is proposed. This model accounts for the intraparticle-diffusional resistance using a pore mouth poisoning mechanism. The... 

Figure 7-10b shows simulation results for the ratio of the effectiveness factor with pore mouth poisoning to that without poisoning for a... 

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