Catalyst operating points

In Chapter 1 we emphasized that the properties of a heterogeneous catalyst surface are determined by its composition and structure on the atomic scale. Hence, from a fundamental point of view, the ultimate goal of catalyst characterization should be to examine the surface atom by atom under the reaction conditions under which the catalyst operates, i.e. in situ. However, a catalyst often consists of small particles of metal, oxide, or sulfide on a support material. Chemical promoters may have been added to the catalyst to optimize its activity and/or selectivity, and structural promoters may have been incorporated to improve the mechanical properties and stabilize the particles against sintering. As a result, a heterogeneous catalyst can be quite complex. Moreover, the state of the catalytic surface generally depends on the conditions under which it is used. 

Improved Filtration Rate Filterability is an important powder catalyst physical property. Sometimes, it can become more important than the catalyst activity depending on the chemical process. When a simple reaction requires less reaction time, a slow filtration operation can slow down the whole process. From a practical point of view, an ideal catalyst not only should have good activity, but also it should have good filtration. From catalyst development point of view, one should consider the relationship between catalyst particle size and its distribution with its catalytic activity and filterability. Smaller catalyst particle size will have better activity but will generally result in slower filtration rate. A narrower particle size distribution with proper particle size will provide a better filtration rate and maintain good activity. 

The choice of cyclone modification, from an operating point of view, becomes a balance of incremental profit from increased conversion, versus catalyst makeup charges, and from a capital cost point of view, the price of either of the cyclone modifications, which must be depreciated. In many instances, there is an additional background time element, involving ongoing development of more attrition resistant and/or active catalyst. 

To elucidate the use of TMS systems for the isomerizing hydroformylation, PC was chosen as the solvent for the rhodium catalyst, because the best selectivity to n-nonanal of 95% with a conversion on trans-4-octene of also 95% was achieved in this solvent under single-phase conditions. Dodecane was used as a non-polar solvent for the extraction of the product and p-xylene served as the mediator between the catalyst and the product phase [24]. Appropriate operation points for the reaction within this solvent system were determined by cloud titrations. 

Ethylbenzene (boiling point 136°C, density 0.8672, flash point 21°C) is a colorless liquid that is manufactured from benzene and ethylene by several modifications of the older mixed liquid-gas reaction system using aluminum chloride as a catalyst (Friedel-Crafts reaction). The reaction takes place in the gas phase over a fixed-bed unit at 370 C under a pressure of 1450 to 2850 kPa. Unchanged andpolyethylated materials are recirculated, making a yield of 98 percent possible. The catalyst operates several days before requiring regeneration. 

From the previous analysis, we conclude that a robust plantwide control structure will fix the combined isobutane + recycle (Fj) and fresh butene flow (F0), as illustrated in Figure 9.4. The desired production rate and selectivity could be achieved in a 3-m3 reactor, operated at 268 K. The operating point shows low sensitivity to errors in the manipulated variable Fj (Figure 9.5). This design seems to ensure feasible operation even if the temperature decreases to 260 K (Figure 9.6) or the catalyst activity becomes 40% of the initial value (Figure 9.7), irrespective of the purity of the butene feed stream (Figure 9.8). 

The problem we will approach in this section is stated as For the operating conditions presented in Table 9.5, find the reactor of minimum volume such that a feasible operating point exists. Additionally, feasibility should be preserved for any temperature in the range T0 AT, and any catalyst activity in the range cp0 Aq> . 

Catalyst fusion is essential to bring and keep the py-rosulfatc-vanadium oxide system into a homogeneous state which is the basis for operating the system at the eutectic in the ternary phase diagram. The reaction mechanism and the fact that the operation point of the... 

When the catalyst operates under intraparticle diffusion control, the point selectivity can be expressed in terms of the net number of moles of product A2 effectively produced inside the pellet per unit time, divided by the net number of moles of reactant Ai consumed inside the pellet per unit time. Thus, we may write... 

The value of the upper stable operating point is fairly insensitive to variations in gas velocity and the degree of conversion this is valid not only for one piece of catalyst but equally well for a large collection of particles. This can be understood as follows. The mass and heat transfer coefficients are related by the Chilton-Colbum analogy ... 

The discussion to this point has emphasized kinetics of catalytic reactions on a uniform surface where only one type of active site participates in the reaction. Bifunctional catalysts operate by utilizing two different types of catalytic sites on the same solid. For example, hydrocarbon reforming reactions that are used to upgrade motor fuels are catalyzed by platinum particles supported on acidified alumina. Extensive research revealed that the metallic function of Pt/Al203 catalyzes hydrogenation/dehydrogenation of hydrocarbons, whereas the acidic function of the support facilitates skeletal isomerization of alkenes. The isomerization of n-pentane (N) to isopentane (I) is used to illustrate the kinetic sequence associated with a bifunctional Pt/Al203 catalyst ... 

After that the liquid reactant feed and the hydrogen gas are supplied to the reactor in the desired ratio. The reaction starts, the reaction mixture heats up and at reaching a temperature somewhat below the boiling point of the solvent at the set reactor pressure, the evaporation will become so high that a stable operating point is reached. Solvent vapours are condensed and returned to the reactor, the liquid phase leaves the top of the catalyst bed via an overflow. For stopping the reaction the catalyst bed is washed out with pure solvent and the catalyst is kept covered with liquid. 

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