Parameters concerning catalyst effectiveness

PARAMETERS CONCERNING CATALYST EFFECTIVENESS IN INDUSTRIAL OPERATIONS... 

The findings from two long term test runs in the SASOL plant relevant to catalyst life under design conditions in a commercial methane synthesis plant have already been published (3). This paper reports further test results from both demonstration units concerning the effect of certain reaction parameters which are the basis for flexibility and operability of the Lurgi methanation scheme. 

This paper is concerned with an attractive possibility involving both zeolite and semiconductor in photocatalytic oxidation. We study here the heterogeneous photocatalytic oxidation of various alkylbenzenes and unsaturated model compounds using Ti02 and zeolite-added Ti02 as catalyst. The influence of reaction parameters and the effect of zeolite addition are analysed. 

Several studies are available concerning the effects of the operating conditions, feed composition, and of catalyst design parameters on the reduction of NO and on the oxidation of SO2 to SO3 over vanadia-based oxide catalysts. The complexity of the SCR chemistry leads to articulate relationships between the above-mentioned parameters and the various reactions involved in the De-NO process, most of which show a certain degree of interdependence. 

The kinetic values obtained using the power law decay function show similar trends as discussed above concerning order of reactivity among the oil lumps and catalyst effects. It is noted however that the three light oil lumps show approximately the same cracking rates. Furthermore, the 95% confidence intervals for the parameter estimates are of similar magnitude showing that either decay model is appropriate for use in the model for the reaction conditions used in this study. 

These tests were performed to establish the limits in flexibility and operability of a methanation scheme. The two demonstration plants have been operated in order to determine the optimum design parameters as well as the possible variation range which can be tolerated without an effect on catalyst life and SNG specification. Using a recycle methanation system, the requirements for the synthesis gas concerning H2/CO ratio, C02 content, and higher hydrocarbon content are not fixed to a small range only the content of poisons should be kept to a minimum. The catalyst has proved thermostability and resistance to high steam content with a resultant expected life of more than 16,000 hrs. 

In Equations 6.61, U denotes the concentration of catalyst present in the reactor (Ck) and u2 the hydrogen pressure (P). As far as the estimation problem is concerned, both these variables are assumed to be known precisely. Actually, as it will be discussed later on experimental design (Chapter 12), the value of such variables is chosen by the experimentalist and can have a paramount effect on the quality of the parameter estimates. Equations 6.61 are rewritten as following... 

While oxidation of S(IV) in solution in the presence of 02 has been known for many years, there has been considerable controversy concerning the rates, mechanisms, and effects of catalysts such as Fe3+ and Mn2+, particularly under atmospheric conditions. However, studies over the past decade carried out in a number of laboratories, particularly those of Hoffmann and coworkers (e.g., Hoffmann and Boyce, 1983 and references therein) Martin and co-workers (1994 and references therein), have identified the various parameters that determine the overall rate of oxidation. As we shall see, the mechanism and kinetics are so complex that past confusion is understandable. 

The mechanical properties of Micelle-Templated Silicas (MTS) are very sensitive items for industrial process applications which might submit catalysts or adsorbents to relevant pressure levels, either in the shaping of the solid or in the working conditions of catalysis or separation vessels. First studies about compression of these highly porous materials have shown a very low stability against pressure. These results concern these specific materials tested. In this study, we show very stable MTS with only a loss of 25% of the pore volume at 3 kbar. The effects of several synthesis parameters on the mechanical strength are discussed. 

In several recent studies an assumption is made concerning the homogeneous-heterogeneous mechanisms of oxidation reactions as a reason for critical effects, in particular in the oxidation of cyclohexane over zeolites [131] and of CO over Pd [132-134] and V [135] catalysts. Berman and Elinek [131] have established in their experiments that cyclohexane oxidation over zeolites follows a mixed homogeneous-heterogeneous mechanism. Studies of the mathematical reaction model written down in accordance with the law of mass action showed that the system can have from one to three steady states. When the steady state is unique, there exists a region of parameters... 

DeNOx reaction involves a strongly adsorbed NH3 species and a gaseous or weakly adsorbed NO species, but differ in their identification of the nature of the adsorbed reactive ammonia (protonated ammonia vs. molecularly coordinated ammonia), of the active sites (Br0nsted vs. Lewis sites) and of the associated reaction intermediates [16,17]. Concerning the mechanism of SO2 oxidation over DeNOxing catalysts, few systematic studies have been reported up to now. Svachula et al. [18] have proposed a redox reaction mechanism based on the assumption of surface vanadyl sulfates as the active sites, in line with the consolidated picture of active sites in commercial sulfuric acid catalysts [19]. Such a mechanism can explain the observed effects of operating conditions, feed composition, and catalyst design parameters on the SO2 SO3 reaction over metal-oxide-based SCR catalysts. 

Catalytic reactions can take place in either the liquid or vapor phase. Liquid phase reactions can be run in either a continuous manner or as a batch process while vapor phase reactions are run only in a continuous mode. In a batch reaction the catalyst, reactants, and other components of the reaction mixture are placed in an appropriate reaction vessel, the reaction is run and the products removed from the vessel and separated from the catalyst. In a continuous system the reactants are passed through the catalyst and the products removed at the same rate as the reactants are added. The applicability of vapor phase processes is limited by the volatility and thermal stability of the reactants and products so such processes are not commonly involved in the preparation of even moderately complex molecules. Because of this, primary attention will be placed here on liquid phase processes with vapor phase systems of secondary importance. A discussion of the different types of reactors used for each of these processes is found in the following chapter. The present discussion is concerned with the effect that the different reaction parameters can have on the outcome of a catalytic reaction. 

In summary, the parameters which are usually considered to affect the performances of oxidation catalysts are almost identical. Accordingly, our results point to a new important effect. The only meaningful difference known between these phases concern the crystalline structure and the oxide texture. This is discussed herafter. 

The kinetics of the ODH of n-butane has been investigated for unpromoted and cesium promoted a-NiMoOa catalysts. The reaction rates of dehydrogenation products as functions of the butane and oxygen partial pressures are described by a kinetic model based on the Mars and van Krevelen mechanism. The effects of Cs on the kinetic parameters can be interpreted on the basis of recently published results concerning the properties of those catalysts. 

In recent years two papers have appeared [11,12] which support an earlier observation made by the present author, namely, that the phenomena occurring in the active phase of a catalyst can have a profound effect upon the dynamic parameters of the model In this study the available information concerning this problem is reviewed and substantiated by the authofs own measurements. 

An analysis of Point 1 above has been given in the work of Weisz and Hicks, previously cited, and a summary of results reported for Point 2 is also available [J.B. Butt and V.W. Weekman, Jr., Amer. Inst. Chem. Engr. Symp. Ser., 70(143), 27 (1975)). To examine the first point, let us look at means that might be available for estimation of the effectiveness factor from observable quantities. Now, normally the experimentalist has at hand information concerning observed rates of reaction, concentrations, catalyst dimension, and temperature—but no values for intrinsic kinetic parameters. Let us define, for reasons that will be seen in a moment, a new parameter, 5... 

---