Coating attrition

The discovery (92) that the graphite coating of molecular sieves can dramatically improve their attrition resistance without significantly impairing adsorption performance should allow the extension of moving-bed technology to bulk gas separations (93). 

Air-Suspension Coa.ting. The Wurster process utilizes a cylindrical chamber in which the cores are suspended in a controlled stream of air. Film coatings are appHed by introducing the coating solution into the airstream, where the solvent evaporates quickly. The process is much quicker than film coating however, care must be taken to avoid destmction of the cores by attrition in the air stream. 

Some SCU products are produced without sealant. These are produced under carefully controlled process conditions that have been optimized to minimize the formation of stress fractures ki the sulfur coating (11). However, such products are more prone to attrition damage when handled than SCU products with a sealant coating. 

The maximum contribution of turbulent attrition rate varies in the range 30-40 per cent of total fine numbers with the assumption that impact attrition fragments are not generated by the rubber coated turbine (i.e. similar to the 25 per cent estimate of Evans etal., 1974). 

Mechanical attrition is used to remove most of the spent binder. First, dry attrition or abrasion processes crush lumps to grain size. Mechanical abrasion is then used to separate the binder from the sand grains. Sometimes, sand is pneumatically propelled against a metal target plate. The impact of the sand on the plate scrubs off the clay and resin coating from the sand grains. Fines are separated and removed by dry classification. 

However, in most practical situations, both the particles and the applied coating material abrade as particles slide past each other, and a compromise between long processing times and excessive attrition must be made. 

Kokkoris et al. (1991, 1995) suggested another method to impair catalyst attrition. In a small scale slugging bed they reduced the attrition rate of zeolites by addition of only very small quantities of various fine solid lubricants. In the case of graphite the reduction was up to 30%. The fines were assumed to reduce attrition by forming a protective coating that... 

Tablet properties Tablet properties such as hardness, shape, and intaglia-tion (if required) are important to obtain a good film-coated tablet. The tablet needs to be hard enough to withstand the coating process. If tablet attrition occurs, the tablets will have a rough surface appearance. For tablet shape, a round tablet will be easier to coat than tablets will multiple sides or edges because of the uniformity of the surface. For intagli-ated tablets, the intagliation style and depth should be developed to prevent fill-in or chipping of the intagliation.

This type of mill is used in wet nanofine grinding. The gap between the conical rotor and the stator of an attrition mill can be varied from 30 to 500 pm. Velocities of up to 35 m s-1 and throughputs of up to several tonnes per hour are possible. Colloid mills are used for the homogenization of liquid-liquid or liquid-solid dispersions. Teeth or abrasive coatings are the main grinding tools. 

The fluidized bed reactor can also handle fast, complex reactions, with mixing and temperature control being especially good when stirring is provided. Unfortunately, the extent of back mixing is difficult to assess so that the residence time distribution of the reactants in the reactor is uncertain. In addition, only small catalyst particles can be used, and attrition, with the consequent breakdown and loss of catalyst, is a problem. Finally, a catalyst bed is adequately fluidized over only a comparatively narrow range of flow rates. More information about kinetic reactors can be found in reviews [33,34,50], Applications of the basket-type mixed reactor to liquid-solid systems are discussed by Suzuki and Kawazo [62] and by Teshima and Ohashi [63], and the development of a laminar flow, liquid-solid reactor by Schmalzer et al. [64], In the latter reactor the wall is coated with a catalyst layer. 

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