Temperature catalyst performance dependent

The catalyst performance depends, as expected on the oxygoi to hydrocarbon feed ratio (Fig. 2) and the temperature (Fig. 3). The olefins selectivity increased widi decreasing oxygen to LPG feed ratio and increasing temperature. The yield of olefins was essentially constant over a range of oxygen to LPG feed ratio and increased mildly with temperature. 

Since NO production depends on the flame temperature and quantity of excess air, achieving required limits may not be possible through burner design alone. Therefore, many new designs incorporate DENOX units that employ catalytic methods to reduce the NO limit. Platinum-containing monolithic catalysts are used (36). Each catalyst performs optimally for a specific temperature range, and most of them work properly around 400°C. 

The results of some initial experiments indicated that the catalytic activity of 1 is strongly temperature-dependent. For instance, at 100 °C, the rates are relatively low but there is an appreciable turnover [turnover frequency (TOE) = 20.5 h" j. Increasing the reaction temperature to 200 °C increases the TOE to 720 h hence, the thermal robustness of the iridium catalyst is pivotal for optimal catalyst performance. The tridentate pincer-type ligands provide a particularly stable platform for metal confinement through covalent Ir—C bonding combined with the terden-tate chelating coordination. 

Catalysts are often implemented in the walls of monolith channels, with overall performance depending on a balance between surface reactivity and flow conditions. Consider a situation that represents a catalytic-combustion monolith such as in a gas-turbine system (e.g., Fig. 17.17), where an individual channel diameter of d = 2 mm and a length of L = 5 cm. The channel walls may be assumed to be isothermal at Tw = 800°C. A CH4-air mixture enters the monolith with a equivalence ratio of(p = 0.3, inlet temperature of Tm — 400°C, and pressure of p = 1 atm. 

Heterogeneous catalysts are not just chemicals in the ordinary sense of the word they are performance chemicals or surface-active materials. Naturally, the performance of the catalyst will depend not so much on the initial composition or surface of it as on the real surface, which is formed and stabilized and then changing dynamically under the prevailing process conditions. Here one has to take into account known and controlled process parameters such as temperature, pressure, reactant concentration, and space velocity, as well as variable factors such as feed composition, and unpredictable or unsuspected factors such as impurities and poisons in the feed [65, 66]. 

The chosen catalytic test reaction was the oxidation of phenol, which yields a mixture of catechol, hydro-quinonc, and 1,4-benzoquinone (Scheme I). The reaction was conducted at atmospheric pressure by continuously adding aqueous H2O2 to a mixture of catalyst, phenol, water, and a solvent (either methanol or acetone) at the reaction temperature (usually 373 K) reaction times were l-4h. Conversions and product sclectivities depended on the composition of this mixture under the best conditions, H2O2 conversion was 100%, phenol conversion 27%, and phenol hydrox-ylation selectivity 91%. The ratio of o />-substituted products (Scheme 1) was usually about unity. It was concluded that catalytic performance depended critically on calcination conditions, i.e. on the completeness of removal of the template. Many facets of the reaction remain to be investigated. 

Performance depends abov all on the in which the catalyst is employed, and this takes place in two dififerent ways, isothermal and adiabatic Hence the same catalyst formula offers greater abrasion resistance and crushing strength characterized by a lower water to ethylbenzene w dght ratio at the reactor inlet and longer life if operation is isothermal The steam ratio is usually 110 1,2 in this case as compared with 1.6 to 2.5 in adiabatic conditions, and the corresponding lives are 5 to 6 years, against 18 months to 2 years. The essential reason for these differences is the lower feed preheat temperature... 

The size of a catalyst support depends on many factors. Predominant among these are flow rate, light-off performance, conversion efficiency, space velocity, back pressure, space availability, and thermal durability. Other factors, such as washcoat formulation, catalyst loading, inlet gas temperature, and fuel management, can also have an impact on the size of a catalyst support. 

The kinetics of olefin polymerization are the subject of several studles>104,153-156,162,182,221,226,240,241,246,252,255,266,28 12 and of an excellent book by Keii.17 The most relevant studies will be discussed below. However, we first note that the precise description of the kinetics of catalytic olefin polymerization under industrially relevant polymerization conditions has proved to be very difficult. For a given catalytic system, one has to identify all possible insertion, chain-release, and chain-isomerization reactions, and their dependence on the polymerization parameters (most importantly, temperature and monomer concentration). Once the kinetic laws for each elementary step have been determined, they have to be combined in one model in order to be able to predict the catalyst performance. This has been attempted for both ethylene and propylene polymerizations. The case of propylene polymerization with a chiral, isospecific zirconocene is shown in Figure 14.162... 

Table 4 also shows that although the products obtmned wnth catalyst B contain lower total sulfur than with catalyst A, the mercaptan sulfiir content was significantly higher. The mercaptans were about 13 percent of the total sulfur at 220°C and increased with temperature to 80 percent at350°C. It can be inferred that catalyst B has higher HDS activity, but it also favors the formation of mercaptans by H2S-alkene recombination reactions. However, a maximum of 300°C was also observed for optimum catalyst performance. The data indicate that the optimum operating temperature is dependent on the feed composition and not the catalyst used (both catalysts were of Co-Mo type). 

The presence of titania also changed the dependence of catalyst performance on activation temperature. As shown in Figures 41 and 43, the amount of each site type on Cr/silica was found to vary with temperature, but not the character of each site itself. That is, raising the activation temperature did not produce new site behavior, but only redistributed the population of chromium within the same site types. When titania is added, however, this statement no longer holds true the site character does change with temperature. 

The fmictions which a catalyst may perform depend upon the nature and complexity of the reactions involved. These functions may be broadly grouped under two headings (1) to increase the rate of a given reaction or, as is usually the case, to lower the temperature at which u reaction will occur at a desirable rate, and (2) to direct a reaction along a particular path when several are possible. The distinction between these two functions is not sharp since it is quite possible for a catalyst to do both. Thus, a selective catalyst is ordinarily one that increases the reaction rate as well as directs the reaction. Industrially, both of these functions are important since it is not only desirable to obtain high yields of a pure product but also to obtain high yields rapidly. In numerous cases, however, the selection of a catalyst for a given process or reaction may depend only on its ability to perform one or the other of these functions, so that it is justifiable to discuss these functions more or less separately. 

Effective Area. - The catalyst consists of active metal dispersed on a substrate material. Therefore, there is a difference between the effective area used in mass transfer calculations of chemical reaction and the effective area used in heat transfer culculations which corresponds to the monolith surface area including surface roughness. The ratio of both effective areas must be defined based on experimental results, as they depend on catalyst type and manufacturing processes. The Thiele number is sometimes used for this same purpose. The relationship between effective area ratio and conversion efficiency is shown in Figure 3. This effective area ratio may be one of the characteristic values of the catalyst, which affects catalyst performance and catalyst temperature. The effective area ratio in the present study is estimated to be 0.3 for mass transfer and 1 for heat transfer based on the experimental data. 

Due to heterogeneity of reaction mixtures containing PbO, the catalyst performance was affected by the order of mixing, temperature, and time of the catalyst preparation [55]. PbO is very soluble in molten phenol (up to 20wt%), and evaporation of phenol from such solution produces lead phenolate Pb(OPh)2. Study of the interactions between catalyst components in molten phenol showed that the phase composition of resulting mixtures depends on the temperature and the QBr PbO ratio (Scheme 12.3). Below 140 °C, a precipitate formed in this reaction contains phases of PbBr2 and Pb(OH)Br (minor component). Pd(acac)2 is unstable in PhOH solution in the presence of a bromide salt and decomposes with the formation of bromide Pd clusters (Pd Br = 6 1) and Pd- -Pd distance... 

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