Limiting cake growth

Removal of Cake by Mass Forces. This method of limiting cake growth employs mass or electrophoretic forces on particles, acting tangentially to or away from the filter medium. Only mass forces are considered here because the electrophoretic effects have been discussed previously. 

Cross-Flow Filtration in Porous Pipes. Another way of limiting cake growth is to pump the slurry through porous pipes at high velocities of the order of thousands of times the filtration velocity through the walls of the pipes. This is ia direct analogy with the now weU-estabHshed process of ultrafiltration which itself borders on reverse osmosis at the molecular level. The three processes are closely related yet different ia many respects. 

The early stages of cross-flow microfiltration often follow such a pattern. However, the growth of the cake is limited by the cross-flow of the process liquid. There are several ways of accounting for the control of cake growth. A useful method is to rewrite (16.6.1) as ... 

There are several ways of limiting cake growth and these are classified into five groups as follows ... 

Cross-flow filtration and the other methods for limiting cake growth are stiU undergoing development and will gradually find wider application in industry. 

This is a method of limiting cake growth by pumping the slurry through porous tubes at high velocities, so that the ratio of the axial flow velocity to filtration velocity through the tube walls is of the order of thousands. This is in direct analogy with the now well-established process of ultra-filtration applicable to much finer solids, which itself borders with reverse osmosis on the molecular level. It is therefore appropriate to review briefly the latter and then to follow on with ultra-filtration and, finally, with the relatively recently explored cross-flow filtration in porous tubes. 

Thirdly, there are those chapters which only needed minor updating and amendments. These include Characterization of Particles Suspended in Liquids, Efficiency of Separation of Particles from Fluids, Hydrocyclones, Separation by Centrifugal Sedimentation, Filtration Fundamentals, Methods for Limiting Cake Growth, Pressure Filtration, Particle-Huid Interaction, Thermodynamics of Solid-Liquid Separation. 

The comparison between measured and calculated values shows good agreement. The measured order of magnitude and cake growth dynamics seem to be correct. Nonetheless, the evolution of the measured thickness with the deposited mass is nonlinear. This is due to the modification of the deposit structure with time which is not taken into account in the model. The limitation of the model is... 

Equation (6.39) is only valid when the wetting front in the substrate is smooth, which depends on the (pore) homogeneity of the substrate. Tiller and Tsai [52] showed that there is an optimum pore size of the substrate which produces the maximum pressure drop across the cake. This is shown by Eqn. (6.37). A smaller pore size gives a larger capillary pressure AP,but also a smaller substrate permeability Ki. As a consequence, local differences in growth kinetics may arise, which limit the minimum layer thickness and are a source of defects. 

Baked goods. The growth of moulds, yeasts and sometimes bacteria often limits the shelf-life of cakes and pastries. Many products, especially those partially baked, currently require frozen storage. Control of gas composition has much to offer in the preservation of such foods provided that the water activity can be kept low enough. 

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