Moving bed processes

The catalyst is employed in bead, pellet, or microspherical form and can be used as a fixed bed, moving bed, or fluid bed. The fixed-bed process was the first process used commercially and employs a static bed of catalyst in several reactors, which allows a continuous flow of feedstock to be maintained. The cycle of operations consists of (/) the flow of feedstock through the catalyst bed (2) the discontinuance of feedstock flow and removal of coke from the catalyst by burning and (J) the insertion of the reactor back on-stream. The moving-bed process uses a reaction vessel, in which cracking takes place, and a kiln, in which the spent catalyst is regenerated and catalyst movement between the vessels is provided by various means. 

In the moving-bed process, oil is heated to up to 1,300"F and is passed under pressure through the reactor where it comes into contact with a catalyst flow in the form of beads or pellets. The cracked products then flow to a fractionating tower where the various compounds are separated and collected. The catalyst is regenerated in a continuous process where deposits of coke on the catalyst are burned off. 

Figure 19. Moving bed catalytic crackers (A) Thermoform moving bed process (B) Houdry catalytic cracking process.

B. Pynnonen, Simulated moving bed processing escape from the liigh-cost box , ]. Chromatogr. 827 143-160(1998). 

S. Nagamatsu, K. Murazumi and S. Makino, Cliiral separation of a pharmaceutical intermediate by a simulated moving bed process , ]. Chromatogr. 832 55-65 (1999). 

In the moving bed processes, the preheated feed meets the hot catalyst, which is in the form of beads that descend by gravity to the regeneration zone. As in fluidized bed cracking, conversion of aromatics is low, and a mixture of saturated and unsaturated light hydrocarbon gases is produced. The gasoline product is also rich in aromatics and branched paraffins. 

Reported residue conversion is significantly high for the five types of included reactors and largest for the slurry type of reactor. Besides, the slurry reactor together with the ebullated bed reactor can handle heaviest feedstocks and highest metal contents. Resid conversion requires higher temperatures, and pressure drop is essentially zero in these two reactors. However, product quality is better for the fixed and moving bed processes. 

In fluidized beds, the temperature is uniform within a few degrees even in the largest vessels, but variation of comnposition is appreciable in large vessels, and is not well correlated for design purposes. One currently successful moving bed process is the UOP "Stacked Reactor" platforming where the catalyst is transported and regenerated in a separate zone. When the activity of the catalyst declines fairly rapidly, its variation with time and position must be taken into account by the mathematical formulation. 

Catalytic cracking is a process that is currently performed exclusively over fluidized catalyst beds. The fluid catalytic cracking (FCC) process was introduced in 1942 and at that time replaced the conventional moving bed processes. These early processes were based on acid-treated clays as acidic catalysts. The replacement of the amorphous aluminosilicate catalysts by Faujasite-type zeolites in the early-1960s is regarded as a major improvement in FCC performance. The new acidic catalysts had a remarkable activity and produced substantially higher yields than the old ones. 

The catalyst-oil volume ratios range from 5 1 to 30 1 for the various processes, although most processes are operated at 10 1. However, for moving-bed processes the catalyst-oil volume ratios may be substantially lower than 10 1. 

The UOP Ebex process has been available for license since the 1970s. This process is a rejective simulated moving bed process where the ethylbenzene is the least adsorbed member of the mixed xylenes and is recovered in high purity in the raffinate stream [47]. Other liquid phase simulated moving bed concepts selective for ethylbenzene have been considered. These would ostensibly require less adsorbent circulation per unit feed because ethylbenzene is typically at <20% concentrahon in mixed xylenes [48, 49]. A process is disclosed by Broughton [50] that produces a pure m-xylene stream along with a pure ethylbenzene stream. 

The first parameter A represents the selective pore rate (m /h). For a set volume of adsorbent contained in the Sorbex chambers, there is a known selective pore volume. This selective volume quantity is divided equally among the various adsorbent beds. Since Sorbex process simulates a moving bed process where adsorbent moves counter current to the process flow, the selective pore rate represents the quantity of selective volume that moves with every step or index of the rotary valve. One step of the rotary valve indexes the feed point from one bed to the next sequential bed position. 

Fluidized catalytic processes, in which the finely powdered catalyst is handled as a fluid, have largely replaced the fixed-bed and moving-bed processes, which use a beaded or pelleted catalyst. A schematic flow diagram of fluid catalytic cracking (FCC) is shown in Fig. 4. 

The cracking reaction in all catalytic cracking processes is affected by the following factors (2) catalyst type and inherent activity charge stock characteristics and midboiling point space rate, usually measured in terms of liquid oil volume per volume of catalyst per hour ratio of catalyst to oil, the amount of catalyst in the reaction zone per unit of oil reacted, which in the fixed-bed process becomes the ratio of reciprocal space rate to time on stream, and in the moving-bed process is the ratio of catalyst rate to oil rate temperature and oil partial pressure. 

In the moving-bed process high-activity catalyst is continuously added to the system to replace attrition losses and, in certain cases, to maintain a desirable average activity. 

Clays are a family of crystalline aluminosilicate solids that interact with a variety of organic compounds (Theng, 1974). Acid treatment develops acidic sites by removing aluminum from the structure and often enhances the reactivity of the clay with specific families of organic compounds. The acid sites also catalyze the formation of coke, and Houdry developed a moving bed process that continuously removed the coked beads from the reactor for regeneration by oxidation with air (McEvoy, 1996). 

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