Mechanical deactivation

Azevedo, H., Bishop, D., and Cavaco-Paulo, A. 2002a. Possibilities for Recycling Cellulases After Use in Cotton Processing. Part I Effects of End-Product Inhibition, Thermal and Mechanical Deactivation, and Cellulase Depletion by Adsorption. Appl. Biochem. Biotechnol., 101, 61-75. 

Mechanical deactivation is due to strong stresses of packed catalysts beds during start ups, shut-downs and catalyst regeneration. 

In the last decade, there has been an increase in the research effort in this field. Coke characterization has been included in many papers where deactivation is a major issue. Several characterization techniques have been used to study coke deposits and to obtain information regarding reaction mechanism, deactivation mechanism, and regeneration conditions. One of the most widely used techniques is temperature-programmed oxidation. Because of its simplicity and utility, this technique has been widely accepted and used in the characterization of coke in a large variety of catalytic systems. 

A kinetic model for n-butane isomerization over sulfated zirconia catalysts is proposed involving two types of active sites on the catalyst surface and a bi-molecular reaction mechanism. Deactivation kinetics are included in which the two different active sites deactivate at different rates. The proposed model more accurately captures the activity trends observed experimentally with respect to time on stream behavior compared to a single site model with deactivation. 

We can summarize these effects of moving zones by saying that for the reactant precursor mechanism (either poisoning or coking), the bed is deactivated from front to back, and for the product precursor mechanism deactivated from back to front. The similarity of such behavior to some corresponding reaction schemes involving individual catalyst particles is striking, as will be shown in Chapter 7. 

Mechanical deactivation is the physical destruction of the catalyst due to thermal shock, breakage, and catalyst support collapse. This is a result of improper operation of the process system. 

The lubricant oxidation mechanism is free-radical in nature and the additives act on the kinetic oxidation chain by capturing the reactive species either by decomposition of the peroxides, or by deactivation of the metal. 

The components in catalysts called promoters lack significant catalytic activity tliemselves, but tliey improve a catalyst by making it more active, selective, or stable. A chemical promoter is used in minute amounts (e.g., parts per million) and affects tlie chemistry of tlie catalysis by influencing or being part of tlie catalytic sites. A textural (structural) promoter, on tlie otlier hand, is used in massive amounts and usually plays a role such as stabilization of tlie catalyst, for instance, by reducing tlie tendency of tlie porous material to collapse or sinter and lose internal surface area, which is a mechanism of deactivation. 

Shift Conversion. Carbon oxides deactivate the ammonia synthesis catalyst and must be removed prior to the synthesis loop. The exothermic water-gas shift reaction (eq. 23) provides a convenient mechanism to maximize hydrogen production while converting CO to the more easily removable CO2. A two-stage adiabatic reactor sequence is normally employed to maximize this conversion. The bulk of the CO is shifted to CO2 in a high... 

Hydrosdylation can also be initiated by a free-radical mechanism (227—229). A photochemical route uses photosensitizers such as peresters to generate radicals in the system. Unfortunately, the reaction is quite sluggish. In several apphcations, radiation is used in combination with platinum and an inhibitor to cure via hydro sdylation (230—232). The inhibitor is either destroyed or deactivated by uv radiation. 

Catalysts in this service can deactivate by several different mechanisms, but deactivation is ordinarily and primarily the result of deposition of carbonaceous materials onto the catalyst surface during hydrocarbon charge-stock processing at elevated temperature. This deposit of highly dehydrogenated polymers or polynuclear-condensed ring aromatics is called coke. The deposition of coke on the catalyst results in substantial deterioration in catalyst performance. The catalyst activity, or its abiUty to convert reactants, is adversely affected by this coke deposition, and the catalyst is referred to as spent. The coke deposits on spent reforming catalyst may exceed 20 wt %. 

ActivatedL yer Loss. Loss of the catalytic layer is the third method of deactivation. Attrition, erosion, or loss of adhesion and exfoHation of the active catalytic layer aU. result in loss of catalyst performance. The monolithic honeycomb catalyst is designed to be resistant to aU. of these mechanisms. There is some erosion of the inlet edge of the cells at the entrance to the monolithic honeycomb, but this loss is minor. The peUetted catalyst is more susceptible to attrition losses because the pellets in the catalytic bed mb against each other. Improvements in the design of the peUetted converter, the surface hardness of the peUets, and the depth of the active layer of the peUets also minimise loss of catalyst performance from attrition in that converter. 

Catalyst Selection. The choice of catalyst is one of the most important design decisions. Selection is usually based on activity, selectivity, stabiUty, mechanical strength, and cost (31). StabiUty and mechanical strength, which make for steady, long-term performance, are the key characteristics. The basic strategy in process design is to minimize catalyst deactivation, while optimizing pollutant destmction. 

The paper-impregnation drying oven exhausts contain high concentrations (10—20% LEL) of alcohols and some resin monomer. Vinyl resins and melamine resins, which sometimes also contain organic phosphate fire retardants, may be used for air filters. The organic phosphates could shorten catalyst life depending on the mechanism of reduction of catalyst activity. Mild acid leaching removes iron and phosphoms from partially deactivated catalyst and has restored activity in at least one known case. 

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. 

Deactivation in Process The active surface of a catalyst can be degraded by chemical, thermal, or mechanical factors. Poisons and... 

Hartwick, W Power Requirement and Associated Effects of Reciprocating Compressor Cylinder Ends, Deactivated by Internal By-Pas.sing, ASMS 75-DGP-9, New York, NY American Society of Mechanical tine neens, 1975. 

Similar approaches are applicable in the chemical industry. For example, maleic anhydride is manufactured by partial oxidation of benzene in a fixed catalyst bed tubular reactor. There is a potential for extremely high temperatures due to thermal runaway if feed ratios are not maintained within safe limits. Catalyst geometry, heat capacity, and partial catalyst deactivation have been used to create a self-regulatory mechanism to prevent excessive temperature (Raghaven, 1992). 

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