Catalyst deactivation shell

Figure 10.14 Profiles of the fraction of catalyst deactivated shell-progressive deactivation.

Phosgene synthesis from CO and Cl2 in a multitubular reactor (Fig. 19-225). The activated carbon catalyst is packed inside the tubes with water on the shell side. Reaction by-products include CCl4. The temperature profile in a tube (shown in the figure) is characterized by a hot spot. The position of the hot spot moves toward the exit of the reactor as the catalyst deactivates. 

A trickle-bed reactor was used to study catalyst deactivation during hydrotreatment of a mixture of 30 wt% SRC and process solvent. The catalyst was Shell 324, NiMo/Al having monodispersed, medium pore diameters. The catalyst zones of the reactors were separated into five sections, and analyzed for pore sizes and coke content. A parallel fouling model is developed to represent the experimental observations. Both model predictions and experimental results consistently show that 1) the coking reactions are parallel to the main reactions, 2) hydrogenation and hydrodenitrogenation activities can be related to catalyst coke content with both time and space, and 3) the coke severely reduces the pore size and restricts the catalyst efficiency. The model is significant because it incorporates a variable diffusi-vity as a function of coke deposition, both time and space profiles for coke are predicted within pellet and reactor, activity is related to coke content, and the model is supported by experimental data. 

Catalyst deactivation in large-pore slab catalysts, where intrapaiticle convection, diffusion and first order reaction are the competing processes, is analyzed by uniform and shell-progressive models. Analytical solutions arc provid as well as plots of effectiveness factors as a function of model parameters as a basis for steady-state reactor design. 

The catalyst plays a crucial role in the technology. A typical modern catalyst consists of 0.15-1.5 wt% Pd, 0.2-1.5 wt% Au, 4-10 wt% KOAc on silica spherical particles of 5 mm [8]. The very fast reaction takes place inside a thin layer (egg-shell catalyst). Preferred conditions are temperatures around 150 to 160 °C and pressures 8 to 10 bar. Hot spots above 200 °C lead to permanent catalyst deactivation. The excess of ethylene to acetic acid is 2 1 to 3 1. Because of explosion danger, the oxygen concentration in the reaction mixture should be kept below 8%. Small amount of water in the initial mixture are necessary for catalyst activation. The dilution of the reaction mixture with inert gas is necessary because of high exothermic effect. Accordingly, the reactor is designed at low values of the per-pass conversions, namely 15 - 35% for the acetic acid and 8-10% for ethylene. The above elements formulate hard constraints both for design and for plantwide control. 

The slope of the saturation concentration versus surface area line was about 2.2 X 10 phosphorus atoms per cm of support surface, quite close to the number of atoms per cm of solid surfaces. Therefore, this indicates a monolayer-equivalent coverage of the alumina surface by phosphorus. The fact that there was very little change in the effective dilfusivity upon poisoning by phosphorus compounds suggests that the poisons tend to deposit in a monolayer-like concentration over the surfaces of the poisoned shell. The simple pore-mouth-poisoning mechanism was adapted for catalyst deactivation by phosphorus compounds. ... 

T. Bacaros, S. Bebelis, S. Pavlou and C.G. Vayenas, Optimal catalyst distribution in pellets with shell progressive poisoning the case of linear kinetics, in Catalyst Deactivation 1987 (B. Delmon and G.F. Forment, Eds.), pp. 459-468,1987. 

The experimental data are shown In Figure 11a, together with a dotted line, extrapolating the reaction controlled regime to zero time. On the other hand some model calculations have been performed under the assumption that, at the beginning of the experiment, the reaction rate is in the diffusion controlled regime. Then only a restricted spherical shell close to the bead surface Is participating In the substrate conversion. If superimposed on the reaction a time dependent catalyst deactivation occurs, the reactive shell should move to the center of the catalyst particle where active cells are available to substitute for the deactivated cells closer to the surface. 

The hydroxyl groups formed on the surface of Ce02 nanoparticles results in inhibition of the diffusion of O. It has been proposed that these hydroxyl groups are mostly located in the proximity of Pd which hinder le-oxidation of metallic Pd and hence catalyst deactivated [15]. The eatalyst deactivated in presence of water has been reported to be composed of Pd in proximity of hydroxylated and partially reduced oxide shells. Additionally it has been reported that the presence of thermally stable hydroxyl gronps and expansion in the Ce02 lattice can presumably result in diffusion of the active species and accessibility of the active Pd sites. The effects of water in the feed exhibited more detrimental effects over Pd Ti02/Al203 catalyst where there was an exceptional decrease in methane conversions. 

Metals and alloys, the principal industrial metalhc catalysts, are found in periodic group TII, which are transition elements with almost-completed 3d, 4d, and 5d electronic orbits. According to theory, electrons from adsorbed molecules can fill the vacancies in the incomplete shells and thus make a chemical bond. What happens subsequently depends on the operating conditions. Platinum, palladium, and nickel form both hydrides and oxides they are effective in hydrogenation (vegetable oils) and oxidation (ammonia or sulfur dioxide). Alloys do not always have catalytic properties intermediate between those of the component metals, since the surface condition may be different from the bulk and catalysis is a function of the surface condition. Addition of some rhenium to Pt/AlgO permits the use of lower temperatures and slows the deactivation rate. The mechanism of catalysis by alloys is still controversial in many instances. 

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