Catalytic shutdown

Liu, Ruettinger, Farrauto, and coworkers—loss of catalytic activity during shutdown/restart cycles linked to formation of carbonates. Liu and coworkers435 at Engelhard reported on the loss of catalytic activity when Pt/ceria catalysts were exposed to shutdown/restart cycles. Utilizing DRIFTS spectroscopy, the authors... 

These activities may introduce many hazards, such as contaminants, materials of repair corrodible, combustible or catalytic in the given environment, blocked vents, open valves etc. into the restarted plant, while shutdown and startup are, in any event, the most dangerous periods. Many examples of reactive hazards thus introduced are to be found in [1], Mutatis mutandis, this is also true of the laboratory this Handbook contains many incidents consequent upon stopping a reaction and/or its agitation to sample, change cooling bath, etc. 

Dust explosions (ASTM E789) that can occur during catalytic reactor shutdown and cleaning are due to the production of finely divided solids through attrition. Many catalyst dusts can bum explosively in air. Thus, control of dust generated by catalyst attrition is essential (Mody and Jakhete, 1988). 

A number of refinery processes require the use of a fixed-bed catalyst These processes include catalytic reforming, hydrodesulfurization, hydrotreating, hydro-cracking, and others. These catalysts become inactive in six months to three years and are eventually replaced in the reactors with fresh catalyst during a unit shutdown. Many of these catalysts contain valuable metals which can be recovered economically. Some of these metals, such as platinum and palladium, represent the active catalytic component other metals such as nickel and vanadium are contaminants in the feed which are deposited on the catalyst during use. After valuable metals are recovered (a service usually performed by the outside companies), the residuals are expected to be disposed of as solid waste. 

It was thought that the formation of inactive cobalt clusters such as Co4(CO)i2, formed by dimerisation of the remaining cobalt carbonyl species after release of the cyclopentenone product, were responsible for the shutdown of the catalytic cycle when dicobalt octacarbonyl was employed.51 Krafft and co-workers were able to show that Co4(CO)12 can actually be exploited as a catalytic species in the PK reaction and were able to obtain excellent yields if cyclohexylamine was introduced as an additive alongside the metal cluster.57,58 The use of metal clusters as catalysts for the reaction has been extended to involve mixed metal clusters.59... 

Large quantities of butadiene have become available over the past 30 yr, mostly as a by-product from the thermal cracking of naphtha and other heavy hydrocarbons. This marked shift has resulted in the shutdown of all on-purpose catalytic dehydrogenation units for butadiene production in North America, Western Europe, and the Far East. 

Production of light olefins by the catalytic dehydrogenation of light paraffins must be able to maintain reasonable per-pass conversion levels and high olefin selectivity. Very importantly, it must be able to produce olefins in high yields over long periods of time without shutdowns. 

Gas-catalytic reactions. Temperature and pressure drop across bed are usually key variables. When a hot spot develops, it usually develops at the front end of the bed and gradually moves through the bed. It may take three to four weeks to travel through the full bed. If the hot spot is 100-200 °C above normal, then usually carbon is deposited and the catalyst is irrevocably damaged. Temperature control is critical for exothermic reactions. Ap rapidly increases emergency shutdown . Pressure surge possible shutdown /[runaway reactor]. ... 

Leakage currents can be short circuited [64] or biased by placing an opposite current via electrodes placed in the piping [40] or reacted at sacrificial electrodes located in the manifolds [65]. These concepts have been used in the manifolds of MBC electrolyzers of Chlorine Engineers and also in the design of the CME and BiTac cells. Corrosion of catalytic cathodes used in bipolar water electrolyzers during shutdowns is prevented by cathodic protection [9]. 

Reversible poisons are those which can be removed from the catalyst without a shutdown, and the catalyst recovers the lost catalytic performance. An irreversible or permanent poison cannot be eliminated from the catalyst, and the unit must be shut down in order to replace the catalyst. 

Preventing masking of active surface of catalysts in a fixed-bed reactor by filtering the incoming gas streams containing dust particles. This also minimizes pressure drop in catalyst beds and frequent shutdown of the catalytic reactor for screening of the bed is not required. 

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