Activated pellets

An important aspect concerning catalytically active membrane reactors, is the distribution of the active phase within the membrane system. Modem modification techniques (van Praag et al. 1989, Lin, de Vries and Burggraaf 1989) allow control over the catalyst distribution and preferential deposition of the active phase at different places in the membrane (top layer/support) system. Studies on conventionally used plate-shaped and cylindrically-sha-ped catalytically active pellets (Vayenas and Pavlou 1987a, b, Dougherty and Verykios 1987) have shown that nonuniformly activated catalysts (catalysts with nonuniform distribution of active sites according to a certain profile)... 

Reduction Experiments. The reduction experiments were carried out with 50 ml of activated pellets. The cultures were agitated at 50 rpm and 37°C. Quinones were added at a concentration of 0.2 mM. Samples of the supernatant were taken at defined intervals and their absorption measured at 288 nm and 360 nm on a Perkin-Elmer 557 Photospectrometer. 

Surprisingly, the synthesis rate is not influenced strongly by pellet diameter, in a size range where marked selectivity changes occur (Table V). This reflects reaction kinetics that are negative order in the diffusion-limited reactant (CO). These types of kinetics delay the inevitable decrease in catalyst productivity that ultimately accompanies reactant transport limitations and that occurs on Co catalysts only for larger and more active pellets. 

TABLE 12.2-2 Typical Adsorption Data on Molecular-Sieve Adsorbent Products (Capacity Expressed in Weight Percent on Basis of Activated Pellet)... 

If the thickness of the active layer is 10% of the pellet diameter, how would the effectiveness factors compare with those for a uniformly active pellet Assume a first-order reaction and spherical pellets, and compare over a wide range of reaction rates. How would the reaction rates per particle compare over a wide range of rates, expressed as (p sphere ... 

Fig. 6.15 SEM images of cycled CaO-based sorbents a sectioned spent sorbent particle, b re-activated/pelletized sorbent pellets, c morphology of the re-activated and calcined sorbent containing 10 % cement as a binder, and d morphology of the re-activated sorbent (containing 10 % cement as a binder) after 30 carbonation/calcination cycles. Reprinted from Ref. [47] with the permission from Dr. Ben Anthony, Copyright 2012...

The temperature at which the activation is ignited depends on the reactivity of the hydrocarbon and on the state of the catalyst. Often the activation starts in the hotter part of the tube and the activation zone moves backwards to the colder inlet of the tube. The mechanism probably involves back diffusion of hydrogen from one activated pellet to the neighbouring inactivated pellet as illustrated in Figure 4.3. 

Endex is a 50% active pelletized concentrate in a polymer caurrier. It can be used in a wide range of cnamndity and engineering plastics, combining quality and efficiency with benefits of versatility and cost effectiveness. 

Table 4 shows, the sources are available with physical sizes ranging from 1 mm x 1 mm up to 3 mm x 3 mm. They are produced from firmly compressed selenium pellets of cylindrical shape. The activities range up to 3 TBq or 80 Ci, which is the maximum allowed loading of the GammaMat SE portable isotope transport and working container, as well as the Source Projector M-Se crawler camera. 

This argument is sound if we imagine Che number of active sites per unit surface area to remain constant as the pore size is changed. However, if the number of active sites per unit pellet volume remains constant, will be independent of d, and then 1/ d. Neither of these re-... 

Pressure pellets sink when placed in water, whereas under the proper conditions, floating pellets can be produced through the extmsion process. That is accomphshed when the feed mixture contains high levels of starch that expands and traps air as the cooked pellets leave the barrel of the extmder. This gives the pellets a density of less than 1.0. Eloating pellets are desirable for species that come to the surface to feed since the aquaculturist can visually determine that the fish are actively feeding and can control daily feeding rates based on observed consumption. 

Many methods for the conversion of acid copolymers to ionomers have been described by Du Pont (27,28). The chemistry involved is simple when cations such as sodium or potassium are involved, but conditions must be controlled to obtain uniform products. Solutions of sodium hydroxide or methoxide can be fed to the acid copolymer melt, using a high shear device such as a two-roU mill to achieve uniformity. AH volatile by-products are easily removed during the conversion, which is mn at about 150°C. A continuous process has been described, using two extmders, the first designed to plasticate the feed polymer and mix it rapidly with the metal compound, eg, zinc oxide, at 160°C (28). Acetic acid is pumped into the melt to function as an activator. Volatiles are removed in an extraction-extmder which follows the reactor-extmder, and the anhydrous melt emerges through a die-plate as strands which are cut into pellets. 

Phosphates are the principal catalysts used in polymerization units the commercially used catalysts are Hquid phosphoric acid, phosphoric acid on kieselguhr, copper pyrophosphate pellets, and phosphoric acid film on quartz. The last is the least active and has the disadvantage that carbonaceous deposits must occasionally be burned off the support. Compared to other processes, the one using Hquid phosphoric acid catalyst is far more responsive to attempts to raise production by increasing temperature. 

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). 

Two or more soHd catalyst components can be mixed to produce a composite that functions as a supported catalyst. The ingredients may be mixed as wet or dry powders and pressed into tablets, roUed into spheres, or pelletized, and then activated. The promoted potassium ferrite catalysts used to dehydrogenate ethylbenzene in the manufacture of styrene or to dehydrogenate butanes in the manufacture of butenes are examples of catalysts manufactured by pelletization and calcination of physically mixed soHd components. In this case a potassium salt, iron oxide, and other ingredients are mixed, extmded, and calcined to produce the iron oxide-supported potassium ferrite catalyst. 

The active phase, which is soHd at room temperature, is comprised of mixed potassium and sodium vanadates and pyrosulfates, whereas the support is macroporous siUca, usually in the form of 6—12 mm diameter rings or pellets. The patent Hterature describes a number of ways to prepare the catalyst a typical example contains 7 wt % vanadium pentoxide, 8% potassium added as potassium hydroxide or carbonate, 1% sodium, and 78 wt % siUca, added as diatomaceous earth or siUca gel, formed into rings, and calcined in the presence of sulfur dioxide or sulfur trioxide to convert a portion of the alkah metal salts into various pyrosulfates (81,82). 

ActivatedL yer Loss. Loss of the catalytic layer is the third method of deactivation. Attrition, erosion, or loss of adhesion and exfoHation of the active catalytic layer aU. result in loss of catalyst performance. The monolithic honeycomb catalyst is designed to be resistant to aU. of these mechanisms. There is some erosion of the inlet edge of the cells at the entrance to the monolithic honeycomb, but this loss is minor. The peUetted catalyst is more susceptible to attrition losses because the pellets in the catalytic bed mb against each other. Improvements in the design of the peUetted converter, the surface hardness of the peUets, and the depth of the active layer of the peUets also minimise loss of catalyst performance from attrition in that converter. 

Usually they are employed as porous pellets in a packed bed. Some exceptions are platinum for the oxidation of ammonia, which is in the form of several layers of fine-mesh wire gauze, and catalysts deposited on membranes. Pore surfaces can be several hundred mVg and pore diameters of the order of 100 A. The entire structure may be or catalytic material (silica or alumina, for instance, sometimes exert catalytic properties) or an active ingredient may be deposited on a porous refractory carrier as a thin film. In such cases the mass of expensive catalytic material, such as Pt or Pd, may be only a fraction of 1 percent. 

Specific. surface of solid spheres of 0.1 mm (0.0039 in) dia is 0.06 mVml (18,300 ftVfF) and a porous activated alumina pellet has about 600 mVml (1.83 X 10 ftVfF). Other considerations aside, a large surface is desirable because the rate of reaction is proportional to the accessible surface. On the other hand, large specific surface means pores of small diameter. 

Diethylamine [109-89-7] M 73.1, b 55.5, d 0.707, n 1.38637, pK 11.38. Dried with LiAlH4 or KOH pellets. Refluxed with, and distd from, BaO or KOH. Converted to the p-toluenesulfonamide and crystd to constant melting point from dry pet ether (b 90-120 ), then hydrolysed with HCl, excess NaOH was added, and the amine passed through a tower of activated alumina, redistd and dried with activated alumina before use [Swift 7 Am Chem Soc 64 115 1942]. 

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