Catalyst deactivation pore volume distribution

Fleisch et al. (1984) measured the catalyst surface area and pore volume changes that occurred after severe deactivation of a 100- to 150-A pore catalyst. The results of these measurements are shown in Table XXVIII for various positions in the reactor bed. Catalyst surface area and pore volume are substantially reduced in the top of the bed due to the concentrated buildup of metals in this region. The pore volume distribution of Fig. 44 reveals the selective loss of the larger pores and an actual increase in smaller (<50-A) pores due to the buildup of deposits and constriction of the larger pores. Fleisch et al. (1984) also observed an increase in the hysteresis loop of the nitrogen adsorption-desorption isotherms between fresh and spent catalysts, which reflects the constrictions caused by pore... 

The route of catalyst deactivation via a cyclic metal impregnation and deactivation method has produced significant improvements in approaching realistic vanadium and nickel profiles over the catalyst particles. From electron microprobe analyses of Ni and V loaded catalyst it has been established that after pore volume saturation, Ni and V are rather homogeneously distributed over the catalyst. 

Pore size. The pore size distribution of the catalyst matrix plays a key role in the catalytic performance of the catalyst. An optimum pore size distribution usually helps in a balanced distribution of smaller and larger pores, and depends on feedstock type and cracking conditions. The pore size distribution of the matrix changes when another component is added e.g. by adding 35-40% kaolin to a silica-alumina gel, a pore structure with a significant amount of micropores can be obtained. Figure 27.9 Pore volume. Pore volume is an indication of the quantity of voids in the catalyst particles and can be a clue in detecting the type of catalyst deactivation that takes place in a commercial unit. Hydrothermal deactivation has very little effect on pore volume, whereas thermal deactivation decreases pore volume. 

The catalysts were impregnated with 5000 ppm V by the traditional Mitchel pore volume impregnation method and by the cyclic deactivation method. With the pore volume method (PV) the vanadium is distributed homogeneously over the catalyst. With the cyclic deactivation method (CD), the vanadium profile over the particle is as in commercial practice. 

Nam, Eldridge and Kittrell studied the pore size distribution for vanadia/alumina catalysts for the removal of NOx by reaction with ammonia. The pore size distributions are found to change dramatically as sulfur poisons the de-NOx reaction. The smallest pores (<10 nm in radius) are found (by porosimetry) to be filled first. As a result the surface decreases by up to 90% with 12% sulfur content, although the pore volume decreased by only 20%. The associated de-NOx activity decreased substantially. It was proposed that ammonium sulfate, bisulfate, or aluminum sulfate formed on the surface to deactivate the catalyst. 

The coking and regeneration of a reforming catalyst was studied by physical characterization methods (pore volume, tortuosity, porosity, carbon distribution) as well as by kinetic investigations on the reaction rate of coke bum-off. For temperatures of industrial relevance for the Pt/Re-A Os catalyst, i.e. below 550°C (deactivation), the bum-off rate is determined by the interplay of chemical reaction and pore diffusion limitation by external mass transfer can be excluded. Based on the kinetic parameters, the process of the regeneration of a technical reactor is discussed. 

Specific surface area and pore volume of the slightly fouled spent catalyst particle were substantially higher than those of the heavily fouled catalyst. It is likely that no fully deactivated catalyst be also withdrawn. Considerable changes in the physical dimensions of the catalyst pellets were also noticed. Particle length distribution was significantly altered with a remarkable increase in the percentage of smaller particles with a length lower than 2 mm. 

The most common characterization techn iques used in refineries to monitor the changes in catalyst activity during commercial operation are textural properties (surface area, pore volume, average pore diameter, and pore size distribution) determined by nitrogen adsorption/desorption metals content (mainly Ni and V) by atomic absorption and carbon content by combustion. There are more advanced characterizations techniques that are mostly employed by researchers for more detailed studies of catalyst deactivation such as Nuclear Magnetic Resonance (NMR), x-ray Photoelectron Spectroscopy... 

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