Layer-Cake Liquids

The presence of a liquid layer on the surface of the filter cake will cause solute to diffuse from the top layer of cake into the liquid. Also if disturbed the layer of liquid will mix with the surface layer of filter cake. This effect can be incorporated into the digital simulation by assuming a given initial depth of liquid as an additional segment of the bed which mixes at time t=0 with the top cake segment. The initial concentrations in the liquid layer and top cake segment are then found by an initial mass balance. 

If the object being rotated is a cylindrical container, the contents of fluid and solids exert an equal and opposite force, called centrifugal force, outward to the walls of the container. This is the force that causes settling or sedimentation of particles through a layer of liquid or filtration of a liquid through a bed of filter cake held inside a perforated rotating chamber. 

Nuts and seed offer the widest choice of oils where the physical condition of the oil is important. Nut oils vary from very solid coconut and palm oils to very liquid canola, safflower and sunflower oils. Thus confectioners, bakers, salters, ice-cream makers and snack food manufacturers have a choice of nut oils to fulfill their needs. A single manufacturer of cookies, wafers, tea cakes, sandwich fillings, fruit cakes, layer cakes and pancake mixes has a choice of oils that meet the particular needs. Nuts and oily seed having the highest oil content are least stable. Schley cultivar of pecans, with 68% oil, is less stable than cultivars containing less oil. The iodine value, which measures the degree of unsaturation of the oil is used commercially as a means of predicting shelf-life. The ratio of oleic to linoleic acids is also a measure of oil stability. 

The layer of liquid outside the cake Ri t)centrifugal force with the pressure gradient applies to this layer (equation [17.61] with infinite permeabiUty). 

Concurrent with the liquid-shiny interface moving radially outward, the cake layer builds up with the cake-slurry interface moving radiaUy inward, with radial position given by ... 

Filter aids may be applied in one of two ways. The first method involves the use of a precoat filter aid, which can be applied as a thin layer over the filter before the suspension is pumped to the apparatus. A precoat prevents fine suspension particles from becoming so entangled in the filter medium that its resistance becomes exces-sive. In addition it facilitates the removal of filter cake at the end of the filtration cycle. The second application method involves incorporation of a certain amount of the material with the suspension before introducing it to the filter. The addition of filter aids increases the porosity of the sludge, decreases its compressibility, and reduces the resistance of the cake. In some cases the filter aid displays an adsorption action, which results in particle separation of sizes down to 0.1 /i. The adsorption ability of certain filter aids, such as bleached earth and activated charcoals, is manifest by a decoloring of the suspension s liquid phase. This practice is widely used for treating fats and oils. The properties of these additives are determined by the characteristics... 

Figure 7. Distribution of static pressure p j in liquid and p along the cake thickness and filter plate I, II -boundaries between the cake and sludge at x" and x III, IV-boundaries between cake layers or cake and filter plate at x" and x V- boundary line between the cake and filter plate or free surface of filter plate 1,3-curves Ps,=f(ho,) and p=f(h J at x 2, 4 -curves Ps,=f(hgj.) and P=f(hoc)...

For fine suspended solids with particles that are too small to be separated from the liquid by gravitational or centrifugal methods, a barrier method such as a filter may be used. The liquid is passed through a filter medium (usually a cloth or screen) that provides a support for the solid particles removed from the slurry. In actuality, the pores in the filter medium are frequently larger than the particles, which penetrate some distance into the medium before being trapped. The layer of solids that builds up on the surface of the medium is called the cake, and it is the cake that provides the actual filtration. The pressure-flow characteristics of the porous cake primarily determine the performance of the filter. 

The apparatus is dismantled, and the reaction mixture is filtered quickly by suction through a 1.5-cm. layer of filter aid on an 11-cm. Buchner funnel placed on a 2-1. filter flask. The filter cake is pressed down well and washed with three 50-ml. portions of methylene chloride. The filtrate is immediately protected from moisture by calcium chloride tubes, and the solvent is removed by distillation from a 2-1. flask. After the solution has been concentrated to about 225 ml., the hot liquid is poured into a suitable container (Note 6), and the remaining solvent is removed under reduced pressure (Note 7). The yield is 192-199 g. (81-84%) of a white, crystalline solid which melts at 155-156°. 

The material to be filtered is fed into the vessel under pressure, and separation takes place with the solids being deposited on the leaf surface, and the liquid passing through the drainage system and out of the filter. Cycle times are determined by pressure, cake capacity or batch quantity. Where particularly fine solids must be removed, a layer of precoat material may be deposited on the leaves prior to filtration, using diatomaceous earth, Perlite, or other suitable precoat materials. 

Schneider and Klein(5) have pointed out that the early stages of cross-flow microfiltration often follow such a pattern although the growth of the cake is limited by the cross-flow of the process liquid. There are a number of ways of accounting for the control of cake growth. A useful method is to rewrite the resistance model to allow for the dynamics of polarisation in the film layer as discussed by Fane 6. Equation 8.3 is then written as ... 

The reaction mixture is allowed to cool to 40-50°C and filtered through a 1/2"-layer of Celite in a 500-mL fritted (medium) glass funnel, making sure that the liquid level does not fall below the surface of the filter cake. A total of 300 mL of ethanol in small portions is used... 

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