Design, catalysts

The principal class of reactions in the FCC process converts high boiling, low octane normal paraffins to lower boiling, higher octane olefins, naphthenes (cycloparaffins), and aromatics. FCC naphtha is almost always fractionated into two or three streams. Typical properties are shown in Table 5. Properties of specific streams depend on the catalyst, design and operating conditions of the unit, and the cmde properties. 

K. Soga and M. Terrano, eds.. Catalyst Design forTailorMadePolyolefins, Elsevier Science, New York, 1994. 

M. L. OcceUi, ed., Eluid Catalytic Cracking II Concepts in Catalyst Design, American Chemical Society, Washington, D.C., 1991. 

Catalyst design is in a primitive stage. There are hardly any examples of tme design of catalysts (42). However, development of improved catalysts has been guided successfully in instances when the central issues were the interplay of mass transport and reaction. An example is catalysts used for hydroprocessing of heavy fossil fuels. 

Since catalyst activity is dependent on how much catalytically active surface is available, it is usually desirable to maximi2e both the total surface area of the catalyst and the active fraction of the catalytic material. It is often easier to enlarge the total surface area of the catalyst than to increase the active component s surface area. With proper catalyst design, however, it is possible to obtain a much larger total active surface area for a given amount of metal or other active material in a supported catalyst than can be achieved in the absence of a support. 

Catalyst Selectivity. Selectivity is the property of a catalyst that determines what fraction of a reactant will be converted to a particular product under specified conditions. A catalyst designer must find ways to obtain optimum selectivity from any particular catalyst. For example, in the oxidation of ethylene to ethylene oxide over metallic silver supported on alumina, ethylene is converted both to ethylene oxide and to carbon dioxide and water. In addition, some of the ethylene oxide formed is lost to complete oxidation to carbon dioxide and water. The selectivity to ethylene oxide in this example is defined as the molar fraction of the ethylene converted to ethylene oxide as opposed to carbon dioxide. 

The pore-size distribution and the nature of the pores in catalyst supports and hence the catalysts derived from them are important properties that significantly affect catalyst performance (16). In most cases, catalyst designers prefer an open-pore stmcture, that is, pores that have more than one opening, and a pore size as uniform as possible in order to obtain maximum utilization of the available pore volume. This can be achieved by careful choice of raw materials and processing conditions. 

One goal of catalyst designers is to constmct bench-scale reactors that allow determination of performance data truly indicative of performance in a full-scale commercial reactor. This has been accompHshed in a number of areas, but in general, larger pilot-scale reactors are preferred because they can be more fully instmmented and can provide better engineering data for ultimate scale-up. In reactor selection thought must be given to parameters such as space velocity, linear velocity, and the number of catalyst bodies per reactor diameter in order to properly model heat- and mass-transfer effects. 

The syndiotactic polymer configuration is not obtained in pure form from polymerizations carried out above 20°C and, thus has not been a serious concern to most propylene polymerization catalyst designers. Eor most commercial appHcations of polypropylene, a resin with 96+% isotacticity is desired. Carbon-13 nmr can be used to estimate the isotactic fraction in a polypropylene sample. Another common analytical method is to dissolve the sample in boiling xylene and measure the amount of isotactic polymer that precipitates on cooling. 

Volume 89 Catalyst Design for Tailor-made Polyolefins. Proceedings of the International... 

Symposium on Catalyst Design forTailor-made Polyolefins, Kanazawa,... 

Sinfelt, J. H. (1987). Catalyst design Selected topics and examples. In Recent developments in chemical process and plant design, Y. A. Liu, H. A. McGee and W. R. Epperly (eds.), John Wiley and Sons, New Yorit, pp. 1-41. 

S. Hosoda, A. Uemura, Y. Shigematsu, I. Yamamoto, and K. Kojima, Studies in Surface Science and Catalyst 89, Catalyst Design for Tailor-Made Polyolefins, (K. Soga, and M. Teruno, eds.), Elsevier Science BV., The Netherlands, p. 365 (1994). 

The catalyst design should be optimized to achieve the following objectives ... 

Davis, K., and Ritter, R. E., FCC Catalyst Design Considerations for Resid Processing—Part 2, Grace Davison Catalagram, No. 78, 1988. 

Automobile catalytic converter. Catalytic converters contain a "three-way" catalyst designed to convart CO to CO2, unbumed hydrocarbons to CO2 and H2O. and NO to N2. The activa components of the catalysts are the precious metals platinum and rhodium palladium is sometimes used as well. 

This in itself draws attention to one of the artistic aspects of the industrial catalyst designer s job. Money values are neither absolute, invariant, nor always logically desirable entities. For example, resource producing nations can increase feedstock prices and they may do so for political rather than for hard, technological reasons. One very important consequence is the fact that a catalytic process that is economic in one year but not in the next is not as attractive as one that can adapt. 

Improve Chemical Engineering. Improvements in catalyst performance inevitably mean that the optimum plant operating condition will be different from that for the unimproved catalyst. Design changes may be needed to obtain the maximum benefit from improved performance. The cost of such changes must be taken into account when assessing the value of catalyst improvement. 

Ongoing research efforts will lead to the arrival of even more efficient and selective metathesis catalysts with specifically tailored properties [196]. Due to the synergistic relationship between catalyst design and subsequent application in advanced synthesis [197], this progress will further expand the scope of metathesis and its popularity amongst the synthetic community. 

The previous sections have shown that desihcation of ZSM-5 zeohtes results in combined micro- and mesoporous materials with a high degree of tunable porosity and fuUy preserved Bronsted acidic properties. In contrast, dealumination hardly induces any mesoporosityin ZSM-5 zeolites, due to the relatively low concentration of framework aluminum that can be extracted, but obviously impacts on the acidic properties. Combination of both treatments enables an independent tailoring of the porous and acidic properties providing a refined flexibility in zeolite catalyst design. Indeed, desihcation followed by a steam treatment to induce dealumination creates mesoporous zeolites with extra-framework aluminum species providing Lewis acidic functions [56]. 

First Concept in Catalyst Design. Shifting Complexation Equilibria for Ion-Exchange by Oxidation of the Organic Chelates... 

Second Concept in Catalyst Design. One-Pot Synthesis of Fe Zeolite Catalysts... 

Third Concept in Catalyst Design. Fenton Detemplation. Mild Organic Template Removal in Micro- and Mesoporous Molecular Sieves... 

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