Catalyst, activity

The catalyst activity in the dark zone is similarly defined by fill (tii.n d, l d,n 1 n dyct c d,n 

If the catalyst acts in the fizz zone or in the dark zone, becomes smaller than r f n or respectively. As shown in Fig. 8.24, the behavior of qy corresponds to that of r y. Both catalyst activities have positive values in the super-rate region, decrease with increasing pressure in the plateau region, and finally both become negative above 3 MPa. This indicates that the catalyst acts as a positive catalyst in 

The effects of the catalyst on burning rate and flame reaction indicate that the super-rate burning phenomena observed in the combustion of HMX-CMDB propellants are fundamentally the same as the combustion phenomena of catalyzed double-base propellants. This implies that the lead catalysts act on the combustion of HMX to produce super-rate burning. 

As was mentioned catalysts are necessary for luminol chemiluminescence in aqueous media. When a rapid stream of carbon dioxide is passed through a basic solution of luminol containing hydrogen peroxide and manganous chloride/ sodium chloride as catalyst, the intensity of the emitted chemiluminescence light passes through 4 maxima before the reaction stops (having then reached a pH of 8-9 [31]. 

This phenomenon very probably is caused by the successive formation of four catalytically active forms of manganese complexes differing in their coordination spheres [47]. When hemin is used instead of manganese under otherwise the same reaction conditions, there is only a single maximum in the intensity. 

Likewise, the phosphorus-31 coordination chemical shift (JdP reori/coord) of the chelate-phosphorus reflects fhe properties of fhe intact steering hgand which are transmitted by the nickel into fhe metallocycle. The intact yhde -dependent chemical shift behavior is mirrored in fhe activity profile of fhe catalyst system [12]. 

on the other hand, fhe intact yhde hgand MCjlTd 1 is retained [Eq. (10), top], the turnover increases in fhe sequence formyl-, acetyl-, benzoyl-mefhylene-tri-phenylphosphorane, at 10 bar and approximately 100°C, to around 0.5 xIO mole reacted ethene per mole nickel per hour. This corresponds to a catalyst activity of 1.4x10 ° g PE or 1.4 tons PE produced per mole of catalyst wifhout using any cocatalyst or noncoordinating anions. 

Coscia et al 1961, Olcay 1963, Pepper and Fleming 1978, Rahman and Pepper 1988) 

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The preparative method for the Pd(0) catalyst active in these regioselective eliminations under mild conditions is crucial. The very active catalyst is prepared by mixing equimolar amounts of Pd(OAc) or Pd(acac)2 and pure n-... 

Step 1 The acid catalyst activates the anhydride toward nucleophilic addition by protonation of the carbonyl oxygen... 

Dialkylaminoethyl acryhc esters are readily prepared by transesterification of the corresponding dialkylaminoethanol (102,103). Catalysts include strong acids and tetraalkyl titanates for higher alkyl esters and titanates, sodium phenoxides, magnesium alkoxides, and dialkyitin oxides, as well as titanium and zirconium chelates, for the preparation of functional esters. Because of loss of catalyst activity during the reaction, incremental or continuous additions may be required to maintain an adequate reaction rate. 

Friedel-Crafts catalysts are electron acceptors, ie, Lewis acids. The alkylating ability of ben2yl chloride was selected to evaluate the relative catalytic activity of a large number of Lewis acid haUdes. The results of this study suggest four categories of catalyst activity (200) (Table 1). 

SHica—alumina has been studied most extensively. Dehydrated sHica—alumina is inactive as isomerisation catalyst but addition of water increases activity until a maximum is reached additional water then decreases activity. The effect of water suggests that Brmnsted acidity is responsible for catalyst activity (207). SHica—alumina is quantitatively at least as acidic as 90% sulfuric acid (208). 

This reaction is affected by the steam-to-carbon ratio, temperature, and pressure, as well as catalyst activity. 

Vanadium phosphoms oxide-based catalysts ate unstable in that they tend to lose phosphoms over time at reaction temperatures. Hot spots in fixed-bed reactors tend to accelerate this loss of phosphoms. This loss of phosphoms also produces a decrease in selectivity (70,136). Many steps have been taken, however, to aHeviate these problems and create an environment where the catalyst can operate at lower temperatures. For example, volatile organophosphoms compounds are fed to the reactor to mitigate the problem of phosphoms loss by the catalyst (137). The phosphoms feed also has the effect of controlling catalyst activity and thus improving catalyst selectivity in the reactor. The catalyst pack in the reactor may be stratified with an inert material (138,139). Stratification has the effect of reducing the extent of reaction pet unit volume and thus reducing the observed catalyst temperature (hot... 

Fluidized-bed reaction systems are not normally shut down for changing catalyst. Fresh catalyst is periodically added to manage catalyst activity and particle size distribution. The ALMA process includes faciUties for adding back both catalyst fines and fresh catalyst to the reactor. 

Selectivity is primarily a function of temperature. The amount of by-products tends to increase as the operating temperature is raised to compensate for declining catalyst activity. By-product formation is also influenced by catalyst impurities, whether left behind during manufacture or otherwise introduced into the process. Alkaline impurities cataly2e higher alcohol production whereas acidic impurities, as well as trace iron and nickel, promote heavier hydrocarbon formation. 

Reductive alkylations and aminations requite pressure-rated reaction vessels and hiUy contained and blanketed support equipment. Nitrile hydrogenations are similar in thein requirements. Arylamine hydrogenations have historically required very high pressure vessel materials of constmction. A nominal breakpoint of 8 MPa (- 1200 psi) requites yet heavier wall constmction and correspondingly more expensive hydrogen pressurization. Heat transfer must be adequate, for the heat of reaction in arylamine ring reduction is - 50 kJ/mol (12 kcal/mol) (59). Solvents employed to maintain catalyst activity and improve heat-transfer efficiency reduce effective hydrogen partial pressures and requite fractionation from product and recycle to prove cost-effective. 

Catalyst Particle Size. Catalyst activity increases as catalyst particles decrease in size and the ratio of the catalyst s surface area to its volume increases. Small catalyst particles also have a lower resistance to mass transfer within the catalyst pore stmcture. Catalysts are available in a wide range of sizes. Axial flow converters predorninanfly use those in the 6—10 mm range whereas the radial and horizontal designs take advantage of the increased activity of the 1.5—3.0 mm size. 

At conditions of high temperature and low pressure, for sufficient catalyst activity and acceptable reaction rates, equiUbrium conversions maybe as low as 5%, necessitating recycle of large amounts of unreacted propylene (101). 

Catalyst Effectiveness. Even at steady-state, isothermal conditions, consideration must be given to the possible loss in catalyst activity resulting from gradients. The loss is usually calculated based on the effectiveness factor, which is the diffusion-limited reaction rate within catalyst pores divided by the reaction rate at catalyst surface conditions (50). The effectiveness factor E, in turn, is related to the Thiele modulus,

first-order rate constant, a the internal surface area, and the effective diffusivity. It is desirable for E to be as close as possible to its maximum value of unity. Various formulas have been developed for E, which are particularly usehil for analyzing reactors that are potentially subject to thermal instabilities, such as hot spots and temperature mnaways (1,48,51). 

Analysis of a method of maximizing the usefiilness of smaH pilot units in achieving similitude is described in Reference 67. The pilot unit should be designed to produce fully developed large bubbles or slugs as rapidly as possible above the inlet. UsuaHy, the basic reaction conditions of feed composition, temperature, pressure, and catalyst activity are kept constant. Constant catalyst activity usuaHy requires use of the same particle size distribution and therefore constant minimum fluidization velocity which is usuaHy much less than the superficial gas velocity. Mass transport from the bubble by diffusion may be less than by convective exchange between the bubble and the surrounding emulsion phase. 

The alkoxy titanate compounds formed by reaction of one mole of tetraalkyl titanate with one mole of a dialkanolamine are excellent esterification catalysts for the manufacture of phthalate-based plasticizers (112). If a 1 1 molar mixture of alkanolamine and water is used ia place of the alkanolamine, oligomeric titanate complexes are formed, which have high catalyst activity and can be used as thixotropic additives to paints and other aqueous coating formulations (113). 

During regeneration the coke is burned off the catalyst. The techniques employed are fairly sophisticated so as to maintain the platinum and any other active metals ia a well dispersed form and to restore the original catalyst activity. Regeneration usually takes several days. 

The methanation reaction is carried out over a catalyst at operating conditions of 503—723 K, 0.1—10 MPa (1—100 atm), and space velocities of 500—25,000 h . Although many catalysts are suitable for effecting the conversion of synthesis gas to methane, nickel-based catalysts are are used almost exclusively for industrial appHcations. Methanation is extremely exothermic (AT/ qq = —214.6 kJ or —51.3 kcal), and heat must be removed efficiently to minimise loss of catalyst activity from metal sintering or reactor plugging by nickel carbide formation. 

Catalyst Activity. Of utmost importance in the design of most catalysts is activity, which is a measure of the ability of a catalyst to effect conversion of the reactant(s) to the desired product(s) under specified conditions. In industrial applications, catalyst activity is usually discussed in terms of the percent conversion of a reactant under given conditions of temperature, pressure, and contact time. 

Since catalyst activity is dependent on how much catalytically active surface is available, it is usually desirable to maximi2e both the total surface area of the catalyst and the active fraction of the catalytic material. It is often easier to enlarge the total surface area of the catalyst than to increase the active component s surface area. With proper catalyst design, however, it is possible to obtain a much larger total active surface area for a given amount of metal or other active material in a supported catalyst than can be achieved in the absence of a support. 

Surface Area. This property is of paramount importance to catalyst performance because in general catalyst activity increases as the surface area of the catalyst increases. However because some reaction rates are strongly dependent on the nature of the stmcture of the catalytic surface, a linear correlation of catalyst activity with surface area should not be expected. As the catalyst surface area increases, for many reactions the selectivity of the catalyst is found to decrease. If the support material is completely inert to the reactants and products, this effect may be diminished somewhat. 

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