Catalyst Design 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. 

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. 

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. 

Two complementai y reviews of this subject are by Shah et al. AIChE Journal, 28, 353-379 [1982]) and Deckwer (in de Lasa, ed.. Chemical Reactor Design andTechnology, Martinus Nijhoff, 1985, pp. 411-461). Useful comments are made by Doraiswamy and Sharma (Heterogeneous Reactions, Wiley, 1984). Charpentier (in Gianetto and Silveston, eds.. Multiphase Chemical Reactors, Hemisphere, 1986, pp. 104—151) emphasizes parameters of trickle bed and stirred tank reactors. Recommendations based on the literature are made for several design parameters namely, bubble diameter and velocity of rise, gas holdup, interfacial area, mass-transfer coefficients k a and /cl but not /cg, axial liquid-phase dispersion coefficient, and heat-transfer coefficient to the wall. The effect of vessel diameter on these parameters is insignificant when D > 0.15 m (0.49 ft), except for the dispersion coefficient. Application of these correlations is to (1) chlorination of toluene in the presence of FeCl,3 catalyst, (2) absorption of SO9 in aqueous potassium carbonate with arsenite catalyst, and (3) reaction of butene with sulfuric acid to butanol. 

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. 

Principal design parameters of electro-catalysts for PEMFCs... 

Design parameters of the anode catalyst for the polymer electrolyte membrane fiiel cells were investigated in the aspect of active metal size and inter-metal distances. Various kinds of catalysts were prepared by using pretreated Ketjenblacks as support materials. The prepared electro-catalysts have the morphology such as the sizes of active metal are in the range from 2.0 to 2.8nm and the inter-metal distances are 5.0 to 14.2nm. The electro-catalysts were evaluated as an electrode of PEMFC. In Fig. 1, it looked as if there was a correlation between inter-metal distances and cell performance, i.e. the larger inter-metal distances are related to the inferior cell performance. 

But when the contents of Nafion ionomer was increased from 30 to 45 % to find out the better electrode structures, the Pt-Ru/SRaw, which had showed the lowest single cell performance, became the best electro-catalyst. By this result one can conclude that as long as the structure of the electrode can be optimized for the each of new electro-catalysts, the active metal size is a more important design parameter rather than inter-metal distances. Furthermore, when the electro-catalysts are designed, the principal parameters should be determined in the consideration of the electrode structures which affect on the electron conduction, gas permeability, proton conductivity, and so on. 

The inlet methanol molar concentration was determined by the mass of catalyst, S/C ratio, and W/F ratio. Here, steam-to-carbon (S/C) ratio is defined as the ratio of steam molecules per carbon atom in the reactant feed and W/F ratio as the amount of catalyst loading into the channel divided by the amount of methanol molar flow rate. For more information on the design parameters, physical properties, and operating conditions, refer to Jung et al. [12]. 

Reaction kinetics, catalyst handling, mass and heat transfer, corrosion and many other practical industrial chemistry and engineering considerations impact the success of scaleup from lab to commercial for batch processing. Since the starting point for scaleup studies is the ultimate intended commercial unit, the professional should scaledown from the design parameters and constraints of the proposed commercial unit. 

Once the design parameters are identified, sizing of the SCR catalyst follows. Typically, a multilayer SCR reactor is considered with the flue gas directed in a vertical down flow orientation. Some U.S. refiners have installed bypasses around the... 

Table 6.8 shows the optimum economic steady-state design for the hot reactor system when the catalyst cost is 100/kg. The important steady-state design parameters for this hot reaction system are a total catalyst weight of 11,880 kg, a recycle flow of 0.27kmol/s, a tube diameter of 0.0592 m, and a heat transfer area of 401 m2. The design optimization variables used are the same as discussed in Chapter 5. The TAC of the optimum design is 770,000 per year. 

Fig. 7.16. Mass transfer intensification in a) distinct and b) catalyst hybrid adsorbent pellets together with the pertinant design parameters.

In order to design zeolite catalysts, the parameters controlling the other features of the protonic sites and particularly their strength (Figure 1.2) have also to be assessed. 

Cybulski et al. [39] have studied the performance of a commercial-scale monolith reactor for liquid-phase methanol synthesis by computer simulations. The authors developed a mathematical model of the monolith reactor and investigated the influence of several design parameters for the actual process. Optimal process conditions were derived for the three-phase methanol synthesis. The optimum catalyst thickness for the monolith was found to be of the same order as the particle size for negligible intraparticle diffusion (50-75 p.m). Recirculation of the solvent with decompression was shown to result in higher CO conversion. It was concluded that the performance of a monolith reactor is fully commensurable with slurry columns, autoclaves, and trickle-bed reactors. 

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