For moving-bed processes

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. 

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. 

When used as part of a commercial operation with gas or liquid mixtures, the single pellets discussed in the context of rate processes are consolidated in the form of packed beds. Usually the beds are stationary and the feed is switched to a second bed when the first becomes saturated. Whilst there are applications for moving-beds, as discussed later, only fixed-bed equipment will be considered, here as this is the most widely used type. 

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. 

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). 

Houdresid catalytic cracking a continuous moving-bed process for catalyti-cally cracking reduced crude oil to produce high octane gasoline and light distillate fuels. 

In the first case, product purities are controlled indirectly by controlling front positions. In distillation columns the front positions are easily controlled with cheap, reliable and fast online temperature measurements on sensitive trays inside the column [27]. A similar procedure was recently proposed for moving-bed chromatographic processes with UV rather than temperature measurement [37]. However, the performance of such an approach is usually limited. Exact product specifications cannot be guaranteed because of this indirect approach. Furthermore, in combined reaction separation processes the relationship between the measured variable and the variable to be controlled is often non-unique, which may lead to severe operational problems as shown for reactive distillation processes [23], It was concluded that these problems could be overcome if in addition some direct or indirect measure of conversion is taken into account. 

A number of inoiganic pollutants are removable by TSA processes. One of the major pollutants requiring removal is S02 from flue gases and from sulfuric acid plant tail gases. The Sulfacid and Hitachi fixed-bed processes, the Sumitomo and BF moving-bed processes, and the Westvaco fluidized-bed process all use activated carbon adsorbents for proven S02 removal (58). Zeolites with high acid resistance, such as mordenite and clinoptilolite, have proven to be effective adsorbents for dry S02 removal from sulfuric acid tail gas (59), and special zeolite adsorbents have been incorporated into the UOP PURASIV S process for this application (54). 

Kloppenburg E. and Gilles E.D., Automatic control of the simulated moving bed process for Cg aromatics separation using asymptotically exact input/output-linearization. J. of Process Control 9 (2000) pp. 41-50. 

Carbon adsorbents have traditionally been used for removal of coloration in the clarified effluent liquor. The general carbon adsorbents, which may be bone char, granular carbon, or powdered carbon products, are used in either a fixed-bed operation or a moving-bed process. These carbons are regenerated at intervals to maintain their effectiveness. 

The commercial processes available for use can be broadly classified as the moving-bed, fluid-bed, and fixed-bed types. The fluid- and moving-bed processes use mixed nonprecious metal oxide catalysts in units equipped with separate regeneration facilities. Fixed-bed processes use predominantly platinum-containing catalysts in units equipped for cycle, occasional, or no regeneration. 

---