ZSM-5 additive

#This invention relates to improved ZSM-5-containing microspheres produced by spray drying kaolin slurries containing ZSM-5 and calcining the product to form attrition-resistant microspheres.

#It is well known that kaolin can be formed into particles, alone or with other ingredients such as particles of zeolitic molecular sieve, to form coherent bodies such as microspheres which, when calcined, are further hardened. For example, essentially catalytically inert microspheres adapted to be used in a selective vaporization process or to be blended with active zeolite particles are produced by spray drying a slurry of hydrous (uncalcined) kaolin arid calcining the resulting microspheres. See U.S. Pat. No. 4,781,818, Reagan, et al. Microspheres consisting of calcined kaolin and impregnated with precious metal have been commercially used to promote CO combustion in fluid catalytic cracking units. See U.S. Pat. No. 4,171,286, Dight, et al. In some cases, the promoter particles are preblended with particles containing an active cracking catalyst component (usually zeolite Y). In other applications, the promoter particles are introduced at a suitable level into the regenerator of an FCC unit, separately from the particles of cracking catalyst. Still another use of microspheres composed of calcined kaolin is as a reactant with caustic or sodium silicate solution to form zeolitic cracking catalyst by so-called in-situ routes. See, for example, U.S. Pat. No. 4,493,902, Brown, et al. Many cracking catalysts are prepared by mixing a slurry of previously formed crystals of zeolite Y in appropriate ion-exchange form with silica sol or silica alumina sol and kaolin followed by spray drying. Spray dried microspheres of calcined clay may also be used as a fluidization additive in FCC units.

#In carrying out various processes in which an aqueous slurry of kaolin is spray dried, it is conventional to disperse the kaolin in the slurry prior to spray drying in order to permit the formation of high solids slurries that are sufficiently fluid to be spray dried. High solids are preferred for economic reasons. Also, higher solids are conducive to the formation of more strongly bonded particles. To disperse kaolin in water, conventional anionic clay dispersants such as sodium condensed phosphate salts, sodium silicates, soda ash, sodium polyacrylate and mixtures thereof are used. Typically, the pH of concentrated dispersed slurries of kaolin are mildly alkaline to neutral, e.g., 6.0 to 8.0, with pH 7 being optimum.

#FCC catalysts are often blends of microspheres containing a catalytically active component (microspheres containing zeolite Y) and additives (microspheres composed of highly calcined kaolin with low surface area, with and without zeolite). During the process of fluid cracking, the catalyst components attrit forming fines. While formation of fines generally is considered undesirable, formation of particles less than 2.6 microns (microfines) is considered particularly undesirable as these can lead to operational problems in some FCC units while fines less than 2 microns can be important contributors to stack opacity problems.

#Both active and inactive catalyst particles attrit. However, microfine production from insufficiently bound additives may be significantly (2 to 5 times) more severe than from active components.

#The attrition resistance of an FCC catalyst or additive can be assessed by a laboratory test (sometimes referred to as a Roller attrition test) described in U.S. Pat. No. 5,082,814, the entire content of which is herein incorporated by reference. The preferred value for an FCC catalyst additive is less than 15% fines generation.