Catalyst characteristic

The operating conditions for the gas turbine determine many of the desirable characteristics of the catalyst used in the catalytic combustor. The catalyst must be sufficiently active to operate at the lowest possible inlet temperature, preferably below 400 °C, in order to minimize the use of the prebumer. Unless the catalyst temperature is limited by some means, the catalyst must also be able to withstand the combustor outlet temperature of 1300°C. The catalyst must also operate efficiently at the very high gas velocities in the combustor. 

The catalyst lifetime should be at least 1 year (8760 h) because it would be undesirable to change the catalyst more frequently. Turbines are usually inspected every year, and a catalyst could be changed at this point without additional disruption of operations, provided the change is relatively straightforward. Ideally, the catalyst lifetime should be at least 3 years, as gas turbines undergo a major overhaul and detailed inspection every 3 years. 

Poisoning of the catalyst by contaminants in the fuel/air mixture is one of the major factors affecting catalyst lifetime. Sulfur compounds in the natural gas fuel are possible catalyst poisons. Dust or other contaminants in the air ingested by the turbine could be deposited on the catalyst and mask the active sites or could react with and deactivate the catalyst. In coastal areas salt from sea air is a potential catalyst poison. The catalyst should be sufficiently resistant to these airborne contaminants so that performance is maintained for at least one year. 

The catalyst should also be resistant to thermal shock, that is, a sudden increase or decrease in temperature. Rapid temperature changes occur during start-up or shut-down of the turbine. The most serious thermal shocks occur upon sudden loss of the turbine load. If the turbine load is lost (by opening a circuit breaker, for example) the fuel must be shut off immediately to prevent overspeeding and destruction of the turbine. The air continues to flow, however, so the temperatme of the catalyst drops very rapidly. Under these conditions the catalyst temperature can fall 1000°C in 100ms, which poses severe problems for ceramic materials. Most 

The catalytic combustor should be capable of meeting the NO emission levels required in the most strictly regulated areas. As mentioned above, certain areas of Japan and the United States now require NO c emissions to be limited to 5 ppm, so this should be the emissions target for any programme to develop catalytic combustors for gas turbines. 

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In commercial operations, catalyst activity is affected by operating conditions, feedstock quality, and catalyst characteristics. The MAT separates catalyst effects from feed and process changes. Feed contaminants, such as vanadium and sodium, reduce catalyst activity. E-cat activity is also affected by fresh catalyst makeup rate and regenerator conditions. 

Besides improvements in catalyst characteristics [28], the low productivity of a photocatalytic process can also be improved by reactor design. In photocatalytic research on a laboratory scale, the most widely applied reactors are the top illumination or annular reactors containing a suspended catalyst [29]. This type of... 

The reactivity of vanadyl pyrophosphate (VO)2P207, catalyst for n-butane oxidation to maleic anhydride, was investigated under steady and unsteady conditions, in order to obtain iirformation on the status of the active surface in reaction conditions. Specific treatments of hydrolysis and oxidation were applied in order to modify the characteristics of the surface layer of the catalyst, and then the unsteady catalytic performance was followed along with the reaction time, until the steady original behavior was restored. It was found that the transformations occurring on the vanadyl pyrophosphate surface depend on the catalyst characteristics (i.e., on the PfV atomic ratio) and on the reaction conditions. 

The development of an SCR system for vehicle applications requires precise calibration of the amount of urea injected as a function of the quantity of NO emitted by the engine, exhaust temperature and catalyst characteristics. Although model simulations can help in the control, it is necessary to use specific NO sensors which, however, still have problems of sensitivity and transient response. Installing a clean-up catalyst for ammonia would provide more latitude and obtain higher NO conversion ratios without re-emission of ammonia into the atmosphere. 

N. O. Elbashir, P. Dutta, A. Manivannan, M. S. Seehra and C. B. Roberts, Impact of cobalt-based catalyst characteristics on the performance of conventional gas-phase and supercritical-phase Fischer-Tropsch synthesis, Appl. Catal. A, 2005, 285, 169-180. 

The length and the distribution of chain lengths are functions of the temperature, pressure, residence time, catalyst characteristics, and the proportion of ethylene present in the reaction, A measure of this is the mole ratio of ethylene, which measures the weight of ethylene compared to the weight of triethyl aluminum in scales related to their atomic weights. As an example, Table 15-2 shows how the distribution of chain lengths can vary, using different mole ratios of ethylene to triethyl aluminum. 

In addihon to shape selechvity and acid-site strength, other catalyst characteristics that influence the catalyhc performance of SAPO-34 have also been idenhfied. Variahon in the SAPO-34 gel composition and synthesis condihons have been were used to prepare samples with different median particle sizes and Si contents (Tables 15.3 and 15.4) [104]. In these samples the median parhcle size was varied from 1.4 to 0.6 xm, and the Si mole frachon in the product was varied from 0.14 down to 0.016. A comparison of samples B and E (which have similar parhcle size distributions) shows that reducing Si content decreases propane formation and increases catalyst life. A comparison of samples B and C (which have similar Si contents) illushates an increase in catalyst life with a reduchon in parhcle size. 

One of the most studied aspects of catalysis science is the relationship between structure and function. Some general themes are weU estabUshed by now, but specific connections between catalyst characteristics and performance attributes remain elusive in most cases. The crystalline geometry of zeolites makes them relatively more amenable to study by a variety of powerful modern characterization tools, but there remain many key unanswered questions in the catalytic application... 

It is important to understand the catalyst characteristics in detail, which in turn helps to understand the catalyst better and correlate the structure and composition of the catalysts with its performance, so that further improvement of the catalyst is possible. Acidity is an important property which influences the overall activity of the alkylation catalysts and the same was studied for Cui.xZnxFc204 by IR and TPD methods. The changes in acidity with respect to catalyst composition and temperature were studied through pyridine adsorption followed by IR measurements. In situ FTIR spectra of pyridine adsorbed on Cui xZnxFe204 between 100 and 400°C (Figme 23) indicated Lewis acidity is the predominant active centers available on the surface [14]. 

Carbonyl Support Preparation method Catalyst characteristics Reference... 

Hohnium oxide occurs in nature, usually associated with small quantities of other rare-earth oxides. Commercial applications of this compound have not been explored fuUy. It is used in refractories and as a catalyst. Characteristic spectral emission lines of holmium oxide glass are used to cahbrate spectrophotometers. ... 

In preceding sections, fundamental coal chemistry, liquefaction mechanisms, solvent and catalyst characteristics were summarized briefly. In the following three sections, the roles and improvements in solvents and catalysts in multistage liquefaction processes are reviewed in more detail on the basis of recent progress in this area. 

Another important catalyst characteristic is porosity. Particularly when heavy feeds are processed, high pore volumes and pore diameters are required to reduce pore diffusion limitations. These limitations occur when the intrinsic rate of reaction is high compared with the rate of diffusion of the reactants through the catalyst particle to the active surface. The catalyst is then not used effectively, and reaction rates and selectivity become functions of particle size. If the kinetics of the reaction are known, it is possible to estimate from theory the reaction rate or threshold above which a catalyst of known size will begin to exhibit diffusion limitations. 

The catalysts used are Ni, Ru, Rh, Pd, Ir, Pt and Cu. The characteristic experimental results disclosed in recent publications are summarized in Tables 20 and 21. The majority of hydrogenations were carried out in the presence of Rh, Pt and Cu catalysts. Characteristic examples can be seen in equations 37-41, which also illustrate some exceptional behaviors. 

Based on the "idealized objectives" outlined in Table XIIL, we can arrive at the following catalyst characteristics for optimum coke and gas reduction... 

Comparison of coked catalyst characteristics for different-size catalyst particles... 

In addition to the requirements with respect to size, shape, and mechanical stability, the nature of the active phase also has to be adopted when the same catalyst is applied in different reactor concepts mainly due to differing process conditions. Vanadium phosphorous oxide composed of the vanadyl pyrophosphate phase (VO)2P207 is an excellent catalyst for selective oxidation of H-butane to maleic anhydride [44-47]. This type of catalyst has been operated in, for example, fixed-bed reactors and fluidized-bed-riser reactors [48]. In the different reactor types, different feedstock is applied, the feed being more rich in //-butane (i.e. more reducible) in the riser-reactor technology, which requires different catalyst characteristics [49]. 

Several catalyst regeneration experiments were conducted with pyridine as the solvent using different extraction conditions and the catalyst activities were measured. The conditions of catalyst extractions are listed in Table II and the stabilized activities of the catalysts after 5-6 h of activity tests are shown in Figures 2 and 3. The catalyst characteristics were examined only in one run, which are shown in Table III. 

Figure 4. Effects of SC /Pyridine Extraction on Catalyst Characteristics.

Weisz (2) carried experiments on silica- alumina beads (many times the size of an average FCC particle). Heobservedthattheintrinsic cokebuming rate was independent of the coke composition and the catalyst characteristics but dependent on initial coke level and the diffusivity. Weisz (3) inanotherstudy, found that the CO /CO ratio during intrinsic coke burning is only a function of temperature. He also observed that this ratio i s affected by the presence oftrace metals like iron and nickel etc. Even though this study was elaborate, it was limited to only silica-alumina catalysts in the form of beads. 

A remarkable amount of experimental data show that the Mg/Ti catalysts characteristically provide polymers with lower molecular weight as compared to non-supported catalysts38,91 126,121 128). This is true for ethylene and propylene polymerization and, in principle, may be the result of a considerable increase of the constants for the chain transfer rates with monomer k , hydrogen kf and organoaluminum k 1, although experimental data are rather scarce (see Table 6). Spontaneous P-elimination k,Sp is not considered important at normal polymerization temperatures 126,129,130,131). 

The automotive TWC is one of the major achievements of modern research in heterogeneous catalysis. There have been major efforts to elucidate the fundamental reaction pathways and the catalyst characteristics that account for the success of the TWC. The chemistry involved is understood in considerable detail as a result of work with idealized models of the TWC. The mechanisms of the CO oxidation and NO reduction reactions with... 

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