Cake volume, wash ratio

Depending on the desired reduction of the solute or mother fiquor in the cake, the wash ratio required can be read off the R curve directly. This multiplied by the void volume in the cake gives the wash liquid volume needed. One, therefore, needs either the R curve or the washing curve applicable. Note that using mass balance, the R curve can be calculated from the washing curve by integration. Experimental values are superior but, if unavailable, some predictive models exist for this. 

Practical experience has shown that the most convenient and best means of expressing R is in terms of the solute concentrations in the washed cake hquid the feed liquid (or unwashed cake liquid), and the cake wash hquid. Furthermore, the wash ratio N may also be expressed either as a volume or weight ratio. 

V = volume of cake wash/nnit area/cycle N = wash ratio... 

This expression can be represented graphically in dimensionless form to simplify its use. It is generally expressed as the so-called filtration number , defined as follows E, = /iR, / 2APT3 jr x . The filtration number, E, is dimensionless and varies from zero at Rf = 0 to a large value when there is an increase in the viscosity of the sludge and Rf or a decrease in pressure drop, auxiliary time, specific cake resistance and the ratio of cake volume to filtrate volume. It may be assumed in practice that F(, = 0 to 10. If washing and drying times are constant and independent of filtration time, they may be added directly to the auxiliary time. In... 

Experimental wash curves represented as fraction of solute remaining versus the wash ratio j (ratio of wash to void volume of cake) can be plotted semilogarithmically as in Fig. 14.11 (the solid line). No experimental point will fall on the left of the maximum theoretical curve (the dotted line), which represents perfect displacement. 

The following conditions and specifications are assumed slurry contains 40% solids by weight solute in the liquid is 2% final cake moisture is 25% wash ratio (wash volume per void volume) is 1.5 ... 

Cake Washing Wash efficiency data are most conveniently represented by a semilog plot of percent remaining B as a function of wash ratio N as shown in Fig. 18-116. Percent remaining refers to that portion of the solute in the dewatered but unwashed cake which is left in the washed and dewatered cake. Since a cake-washing operation involves the displacement of one volume of liquid by another volume, the removal of solute is related to the ratio of the volume of washing fluid divided by the volume of liquid in the cake. This ratio is defined as wash ratio N. 

Wash ratio j a ratio equaling the total volume of wash liquid exiting a cake to the volume of void of the cake. 

An aqueous su ension containing 10 wt% of solids is filtered at a constant pressure difference of 4 bar on a 0.4 m area filter. The volume filtered is 0.02 m. The filter cake is then given a wash with water at the same pressure for 30 s. Calculate the wai iing rate and the wash ratio. 

By repeating the experiment several times but using a different volume of wash liquid each time, the variation of the amount of soluble species in the cake can be plotted as a function of the amount of wash liquid permeated through the cake (i.e. a wash curve). The amount of wash liquid used is estimated from a knowledge of the amount of residual liquid in the cake just before washing is started which can, for instance, be determined by measurements in accordance with Step 9 in Section 4.1.1. The wash liquid amount should be varied from about 0.5 up to 3.5 times the residual amount (i.e. between 0.5 and 3.5 wash ratios). 

To plot a wash curve it is necessary to maifipulate the data recorded in the experiment. The mass of liquid in the cake M = 412.7 - 276 = 136.7 g equates to a volume of 136.7/0.998 = 137 cm and represents a wash ratio W = 1. The complete list of wash ratios shown in Column (3) of Table 4.3 is obtained by dividing each value in Column (1) by 137. 

Figure 10.7 also gives the fraction of solute remaining in cake R as computed with equation 10.7 from the washing curve. In the present example, wash ratio n corresponding to R = 0.05 is sought and the result is about n = 1.75. The total theoretical amount of wash liquid needed is, therefore, the void volume times the wash ratio 0.36 x 10 x 1.75 = 0.63 x 10 m whilst the washing time is 0.63 x 10 /3.60 x 10 " = 1.75 s. 

These filters (Fig. 9) have the advantages of low cost, near indestructibility, and ease of internal inspection. They have the lowest liquid volume-to-area ratio, which makes them most efficient for the washing of filter cakes. Because of this low ratio, they will also have the smallest unfiltered heel remaining at the end of the cycle. 

Attractive features of the leaf and plate filters are their ease or operation and maintenance, the large filtration area to vessel volume ratio, the high filtration rates that can be achieved, and their ability to form homogeneous and compact cakes that can be efficiently washed with relatively small wash volumes. There is a minimum number of seals (i.e. potential leak points) between feed and filtrate. Most designs enable filtration, washing, drying and discharging in a closed system. 

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