Redispersion

Oxidative redispersion of Pd and Cu aggregates in zeolite NaHY has been reported (76,143,205). First the metal (M = Pd or Cu) is oxidized to oxide particles  

In the case of Pd/NaY, the reaction given by Eq. (11) predominates below 350°C. The formation of palladium oxide is confirmed with TPR by its characteristic reduction profile near 0°C and the concomitant formation of water. 

The second step involves reaction of zeolite protons with the metal oxides  

For relatively small particles of PdO or CuO and high proton concentration in their vicinity, the reaction given by Eq. (12) is virtually complete at 400°C (76,205). The reaction is driven by the formation of water and the migration of the ions to sites with high electronegative charge density. The ions then have the same oxidation states and locations as they had after original calcination. For Cu/NaY this has been demonstrated by electron 

Several methods have been published to remedy this situation. One strategy aims at bringing the primary dispersed metal ions back into the supercages, where their mobility is much higher (207). Treatment of partially redispersed Pd/HY with NH3 transforms naked Pd ions in sodalite cages ions in supercages  

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Anotlier standard metliod is to use a (high-speed) centrifuge to sediment tire colloids, replace tire supernatant and redisperse tire particles. Provided tire particles are well stabilized in tire solvent, tliis allows for a rigorous purification. Larger objects, such as particle aggregates, can be fractionated off because tliey settle first. A tliird metliod is (ultra)filtration, whereby larger impurities can be retained, particularly using membrane filters witli accurately defined pore sizes. 

The pressure filter with tubular elements has also been used as a thickener, when the cake, backwashed by intermittent reverse flow, is redispersed by an agitator at the bottom of the vessel and discharged continuously as a slurry. In some cases the filter cake builds up to a critical thickness and then falls away without bio whack. 

The protein precipitate is washed with water, redispersed at pH 7, and then spray dried. Typical commercial soy protein isolates contain greater than 90% cmde protein, dry wt basis. 

Cleaning is frequently aided mechanically. Foam balls scour the center of tubes, and hoUow-filter systems can be back-flushed. HoUow fibers and membranes attached to rigid supports can be back-pressured, thereby eliminating the pressure drop that holds redispersed films on the membrane surface. 

Oxidation and chlorination of the catalyst are then performed to ensure complete carbon removal, restore the catalyst chloride to its proper level, and maintain full platinum dispersion on the catalyst surface. Typically, the catalyst is oxidized in sufficient oxygen at about 510°C for a period of six hours or more. Sufficient chloride is added, usually as an organic chloride, to restore the chloride content and acid function of the catalyst and to provide redispersion of any platinum agglomeration that may have occurred. The catalyst is then reduced to return the metal components to their active form. This reduction is accompHshed by using a flow of electrolytic hydrogen or recycle gas from another Platforming unit at 400 to 480°C for a period of one to two hours. 

Thermal Degradation and Sintering Thermally iaduced deactivation of catalysts may result from redispersion, ie, loss of catalytic surface area because of crystal growth ia the catalyst phase (21,24,33) or from sintering, ie, loss of catalyst-support area because of support coUapse (18). Sintering processes generally take... 

The latter have obseivations during mass transfer. Coalescence Rates The droplets coalesce and redisperse at rates... 

Stirred Vessels Gases may be dispersed in hquids by spargers or nozzles and redispersed by packing or trays. More intensive dispersion and redispersion is obtained by mechanical agitation. At the same time, the agitation will improve heat transfer and will keep catalyst particles in suspension if necessaiy. Power inputs of 0.6 to 2.0 kW/m (3.05 to 10.15 np/1,000 gal) are suitable. 

Bubble Reactors In bubble columns the gas is dispersed by nozzles or spargers without mechanical agitation. In order to improve the operation, redispersion at intei vals may be effected by static mixers, such as perforated plates. The liquid may be clear or be a slurry. 

Pectin [9000-69-5] Mr 25,000-100,000, amorphous. Dissolved in hot water to give a 1% soln, then cooled, and made about 0.05M in HCl by addition of cone HCl, and ppted by pouring slowly, with vigorous stirring into two volumes of 95% EtOH. After standing for several hours, the pectin is filtered onto nylon cloth, then redispersed in 95% EtOH and stood overnight. The ppte is filtered off, washed with EtOH/ElaO, then Et20 and air dried. 

Good housekeeping, partieularly to avoid a devastating seeondary explosion, following redispersion of any aeeumulations of eombustible dust. 

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