Abstract
Cleanable surface filters are used for removing dust particles from gases in a wide range of chemical and process industries. Stimulated by the development of advanced power plant technologies, rigid ceramic filter media for high temperature applications have become available in recent years. The important issues to be addressed in specification and operation of ceramic filters at high temperatures are the long term conditioning behaviour and the filter "cleanability". These two aspects of filtration behaviour are inextricably linked and, at the present state of knowledge, very much system-specific, so that experimental work using the dust of interest is essential. This work is concerned with characterising regeneration of a rigid ceramic filter medium using a coupon test method designed to measure both conditioning and cake removal behaviour at ambient conditions. Coupon filter tests have been performed in which a low-density fibrous ceramic medium has been used to filter calcium carbonate dusts, and the conditioning behaviour and cake removal stress distributions have been measured over a range of particle size and "cake loadings", the cake mass per unit area of filtration medium. For reverse flow cleaning, the cake removal stress decreases strongly as the cake areal mass increases to medium-sized cake loadings (appr. 500 g/m2), and less strongly thereafter. Cake removal stresses were also measured using centrifugal acceleration; the results are consistently lower than those for the reverse flow test and nearly independent of cake loading. If the cake detachment stress under reverse flow conditions is taken at the first point of significant cake removal, (the "burst pressure"), the resulting values are in good agreement with median detachment stresses obtained by acceleration. It is likely that cake removal by reverse flow is influenced by "hinging" of cake patches, which remain loosely attached to the surface after their apparent detachment stress has been overcome. While the acceleration and burst pressure tests may give a truer representation of the intrinsic cake adhesion to the filter, the reverse flow test remains the most appropriate for estimation of the cleaning flow requirement. At cleaning pressures which are typical of industrial practice, cake detachment occurs by patch separation. A fracture mechanics interpretation of this phenomenon is advanced, where crack propagation is considered to be initiated at structural irregularities ("flaws") in the interface of medium and cake.