Moving and Fluidized Beds

Thermofor catalytic cracking (TCC) introduced by Mobil in 1943, fluid catalytic cracking (FCC) introduced by Exxon, and several other similar processes used moving or fluidized beds of strong catalyst particles. Catalyst was withdrawn continuously from the bottom of the reactor and lifted in buckets or by an air stream to the top of a regenerator, or kiln, after the residual hydrocarbons had been stripped out with steam. Catalyst vyas then returned to the reactor after regeneration. There was a limit to the capacity of moving bed processes 

Production was significantly increased and only minor changes required in the processing equipment when the more active zeolite catalysts were introduced between 1962 and 1964. Subsequently, modified catalysts with various additives were developed. These were required to accommodate a wider range of feeds, which included some residual fractions with high molecular weight hydrocarbons and higher levels of impurities, and to meet stricter environmental controls. 

Although FCC unit designs were extensively modified following the introduction of zeolite catalysts, the basic flow sheet remained the same. In fact most of the existing units were simply revamped to use the new catalyst and to in- 

Any catalyst dust formed by attrition and lost in the cyclones has to be replaced at regular intervals. It is also necessary to replace a small proportion of the circulating equilibrium catalyst to compensate for gradual permanent deactivation and maintain conversion. About 3% of fresh catalyst is added to a imit on a daily basis to maintain the necessary catalyst inventory. The whole operation is continuous and a unit may be operated for several years without shut down. An important feature of the process is that heat transferred from the regenerator to the reactor by the hot catalyst as a heat transfer agent is an integral part of the energy balance. 

When higher-activity catalysts consisting of zeolites incorporated in a silica-alumina matrix were introduced, first to TCC units in 1962 and then to FCC 

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Of these, fixed-bed adiabatic reactors are the cheapest in terms of capital cost. Tubular reactors are more expensive than fixed-bed adiabatic reactors, with the highest capital costs associated with moving and fluidized beds. The choice of reactor configuration for reactions involving a solid supported catalyst is often dominated by the deactivation characteristics of the catalyst. 

Adsorption is performed most commonly in fixed vertical beds of porous granular adsorbents. Flow of adsorbing fluid usually is down through the bed, that of regenerant usually is upward. Moving and fluidized beds have only a limited application in the field. 

A rather nice story to serve as a case in point is the work of Weekman and his co-workers [2] at Mobil to describe the behavior of fixed, moving and fluidized bed reactors to catalytically crack feed streams under deactivating conditions. Both the main reaction and the deactivation reaction are exceedingly complex and to describe them precisely is well beyond our knowledge to date. The woe is drat these processes are industrially very important and must be used to produce gasoline. So it is prudent and necessary to try to describe them as well as possible in the absence of complete information. 

Moving- and fluidized-bed catalytic reactors Longitudinal Backmixing Distillation column... 

An important development over the past few years is the large increase in those investigators who have been willing to determine the kinetics of deactivation in various reaction systems, mostly by coking, and then incorporate this information into appropriate models to interpret or predict integral reactor performance. Most of this has been done for fixed beds but important examples treat moving and fluidized beds as well. 

Beside continuous horizontal kilns, numerous other methods for dry pyrolysis of urea have been described, eg, use of stirred batch or continuous reactors, ribbon mixers, ball mills, etc (109), heated metal surfaces such as moving belts, screws, rotating dmms, etc (110), molten tin or its alloys (111), dielectric heating (112), and fluidized beds (with performed urea cyanurate) (113). AH of these modifications yield impure CA. 

If deactivation of the catalyst is very short, then moving-or fluidized-bed reactors are required so that the catalyst can be withdrawn continuously, regenerated and returned to... 

Reactors in which the particles are suspended in a fluid and are constantly moving about (fluidized bed and slurry reactors). 

In this chapter, we consider reactors for fluid-solid reactions in which the solid particles are in motion (relative to the wall of the vessel) in an arbitrary pattern brought about by upward flow of the fluid. Thus, the solid particles are neither in ideal flow, as in the treatment in Chapter 22, nor fixed in position, as in Chapter 21. We focus mainly on the fluidized-bed reactor as an important type of moving-particle reactor. Books dealing with fluidization and fluidized-bed reactors include those by Kunii and Levenspiel (1991), Yates (1983), and Davidson and Harrison (1963). 

Cracking catalysts include synthetic and natural sihca-alumina, treated bentonite clay, fuller s earth, aluminum hydrosUicates, and bauxite. These catalysts are in the form of beads, pellets, and powder, and are used in a fixed, moving, or fluidized bed. The catalyst is usually heated and hfted into the reactor area by the incoming oil feed which, in mrn, is immediately vaporized upon contact. Vapors from the reactors pass upward through a cyclone separator which removes most of the entrained catalyst. The vapors then enter the fractionator, where the desired products are removed and heavier fractions are recycled to the reactor. 

The overall removal process would consist of two stages the absorption stage, in which the sorbent is converted to the sulfide form by its reaction with the H2S in the feed gas, and the regeneration stage, wherein the sulfide is converted to an oxide. It thus represents a cyclic process. Fixed-bed, moving-bed and fluidized-bed operations have been considered for the purpose. Kinetic data are necessary for choosing a proper reactor type and for the design... 

These materials are usually used in moving- or fluidized-bed reactors and thus are prone to severe attrition. Furthermore, because they are fluidized their... 

Alternatively, one can resort to the use of moving- or fluidized-bed systems in which the adsorbent, and sometimes also the catalyst, is continuously withdrawn from the reactor to undergo an external regeneration. Ideally, one tries to achieve countercurrent adsorbent flow pattern to optimize utilization of the adsorptive capacity. The major problems of such arrangements are those of solids handling (e.g., gas-tight... 

However, when membrane tubes are inserted in the fluidized-bed reactor, hydrogen is continuously removed from the reaction mixture thus, the main reaction of ethylbenzene dehydrogenation continues to move in the direction of forward reaction. The ethylbenzene conversion and the yield of styrene increase as a result of the selective permeation of hydrogen through the membrane. Both the conversion and the yield exceed those of the industrial fixed-bed reactors and fluidized-bed reactors without membranes. When 16 membrane tubes are used, the selectivity to styrene is expected to be almost 100% due to suppression of by-products such as toluene [Abdalla and Elnashaie, 1995]. A high ethylbenzene conversion (96.5%) along with a high styrene yield (92.4%) is possible under properly selected realistic conditions. 

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