Bhowmik, Sukhendu Bikash; (1991) Physical characteristics of LT shift catalysts. Doctoral thesis (Ph.D.), University College London.

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Abstract

Literature on diffusion and permeability are reviewed with the conclusion that measurements of diffusion under both reacting and non-reacting conditions are invaluable in a reactor design context. When a large number of experiments must be conducted, a factorial design can reduce the overall total number of experiments. The physical structure of industrially produced porous solid catalysts can be partially characterised in terms of BET surface area, porosity, pore size distribution, permeability, effective diffusivity and tensile strength. Various standard techniques and methods of measuring these properties along with advantages and disadvantages are discussed in Chapter 2. The co-precipitation method of catalyst preparation is described in Chapter 3. A total of 85 "green" and "reduced" catalyst samples were produced from a dried powder of co-precipitated copper oxide, zinc oxide and alumina. These were prepared under carefully controlled conditions at ICI's Billingham Catalysis Research Centre varying five key physical production parameters at their low, normal, high and extra high levels. These five production parameters are; (1) the amount of lubricant added (2) the pre-compaction load (3) the calcination temperature (4) the primary particle cut size (5) the pelleting density A non-isobaric, isothermal model of combined diffusion and flow within a single catalyst pellet is developed for a binary gas system. Numerical solutions of this model are presented. An agreement within 20% was found when compared with the experimental results. Under certain limiting conditions, the model became essentially isobaric. Solution of this model confirmed that large pore diameter systems can adequately be described by such a model. The pellet permeabilities and effective diffusivities are determined using a purpose built Wicke-Kallenbach type single pellet test apparatus. These tests are discussed in detail in Chapter 5. Experimental results on the ICI Cu/Zn/Al2O3 LT shift catalyst are presented in Chapter 6 and from these several conclusions are drawn: (i) The amount of lubricant added and magnitude of the pre-compaction load do not significantly affect the properties of the final catalyst pellet. (ii) Alterations in calcination temperature, particle cut size and pellet density cause large changes in pellet permeability, effective diffusivity and tensile strength. These were much greater than the variations in the values of surface area and porosity. For quality control it is vitally important to characterize catalysts in terms of those properties which vary significantly with alterations in pellet production conditions. (iii) Chemical reduction increased diffusivity by a factor more than 3 and, in some catalysts, permeabilities were increased by a factor in excess of 400. This latter result is attributed to the development of microscopic cracks which can be clearly observed on the Scanning Electron Microscope. Pellet tensile strength following reduction was decreased by a factor more than 3. In Chapter 7, experimental results on porous carbon systems are presented and discussed. Finally, overall discussion, conclusions and recommendations for future work are discussed in Chapter 8.

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