Dried cake, thickness

Fig. 5.5 (a) Design space forthe primary drying stage of a coffee extract freeze-drying process calculated for different values of dried cake thickness (b) Values ofthe sublimation flux as a function ofthe operating conditions when Lj/... 

For given operating conditions and submergence, the dry cake production rate increases with the speed of rotation (eq. 10) and the limiting factor is usually the minimum cake thickness which can stiU be successfiiUy discharged by the method used in the filter. Equation 11 shows the dependence of the sohds yield on cake thickness ... 

Since (c) is larger than (a) in the previous step, too thick a cake will be formed and it will not wash or dry adequately unless the effective submergence is artificially restricted to yield the design cake thickness. This may be accomplished by proper bridge-block adjustment or by vacuum regulation within the form zone of the filter valve. 

The resistance of the filter plate is negligible. The problem is to determine the filtration, washing and drying times, the cake thickness, the volumes of filtrate, washing liquid and drying air. The solution to this design case is outlined below in steps. 

Equation (5) is equivalent to stating that sublimation and subsequent transport of 1 g of water vapor into the chamber demands a heat input of 650 cal (2720 J) from the shelves. The vial heat transfer coefficient, Kv, depends upon the chamber pressure, Pc and the vapor pressure of ice, P0, depends in exponential fashion upon the product temperature, Tp. With a knowledge of the mass transfer coefficients, Rp and Rs, and the vial heat transfer coefficient, Kv, specification of the process control parameters, Pc and 7 , allows Eq. (5) to be solved for the product temperature, Tp. The product temperature, and therefore P0, are obviously determined by a number of factors, including the nature of the product and the extent of prior drying (i.e., the cake thickness) through Rp, the nature of the container through Kv, and the process control variables Pc and Ts. With the product temperature calculated, the sublimation rate is determined by Eq. (4). 

A slurry containing 40 per cent by mass solid is to be filtered on a rotary drum filter 2 m diameter and 2 m long which normally operates with 40 per cent of its surface immersed in the slurry and under a pressure of 17 kN/m2. A laboratory test on a sample of the slurry using a leaf filter of area 200 cm2 and covered with a similar cloth to that on the drum, produced 300 cm3 of filtrate in the first 60 s and 140 cm3 in the next 60 s, when the leaf was under pressure of 84 kN/m2. The bulk density of the dry cake was 1500 kg/m3 and the density of the filtrate was 1000 kg/m3. The minimum thickness of cake which could be readily removed from the cloth was 5 mm. 

All filters of this type have the distinct advantage of continuous operation and, as a result, the feed and wash liquors may be fed to the equipment at steady rates. Dense, quick settling solids can be handled, and the cake thickness — and the washing and drying times — can be varied independently between quite wide limits. The cake can be flooded with wash liquor, which can be taken away separately and re-used to obtain countercurrent... 

Coating thickness was measured both wet and dry. Wet thickness measurements were taken using a Gardner s Company Precision Hot Cake, which can measure between 125 and 500 pm films with a 2a uncertainty of +10%. Dry film thickness measurements were taken using an ElektroPhysik s Exacto FN gauge, which can measure between 0.1 and 2000pm with a 2a uncertainty of 3%. 

Designating w as the pounds of dry-cake solids per unit volume of filtrate, pc as the cake density expressed as pounds of dry-cake solids per unit volume of wet filter cake, and VF as the fictitious volume of filtrate per unit of filtering area necessary to lay down a cake thickness lF, the actual cake thickness plus the fictitious cake thickness is... 

Mass of dry inert solid per unit area of filtration = (inert solid density)(cake thickness)... 

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