Wash ratio

For constant-pressure filtration and washing, such as occur on vacuum filters, the filtration time and washing time can be related through the wash ratio if the wash liquor and filtrate are assumed to have similar physical properties. From the equation for incompressible filtration at constant pressure expressed in terms of a, ecffic resistance  

Considering conditions at the end of the filtration operation, the medium resistance may be neglected leading to a simplified and rearranged equation  

If the wai ung liquor is applied under the same pressure difiference and the washing process assumed to proceed in the same way as the filtration, the wailing rate can be written as  

is the volume of wash liquid then the amoimt applied per unit area can be expressed 

The volume of residual filtrate in the saturated cake Vm is proportional to the quantity of cake and proportional to the volume of filtrate so firom Equation 9.3  

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The factors which may make CCD a preferred choice over other separation systems include the following rapidly settling solids, assisted by flocculation relatively high ratio of solids concentration between underflow and feed moderately high wash ratios allowable (2 to 4 times the volume of hquor in the thickened underflows) large quantity of sohds to be processed and the presence of fine-size sohds that are difficult to concentrate by other means. A technical feasibihty and economic study is desirable in order to make the optimum choice. 

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. 18-103. 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... 

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. 

If it were possible to obtain a perfect sing displacement wash, the frac tion remaining would be numerically equal to I minus the wash ratio. This ideal condition is represented by the maximum theoretical line as shown in Fig. 18-103. Since it represents the best that can be done, no data point should fall to the left of this cni ve. Most, but not all, cake-washing curves tend to fall along the heavy solid line shown. In the absence of ac tnal data, one may estimate washing results by using this cni ve. 

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

When pressure reaehes 6-7 bars wash water is pumped through the filter eake at a predetermined wash ratio to displaee the adhering mother solution. 

Wakeman found that the effect of increasing the equilibrium constant in the description isotherm equation is to increase the wash-ratio required. Is this confirmed by simulation and what is the explanation of this effect. 

Fig. 1 Solute concentration variations as a function of wash ratio.

Sampling rate Sample wash ratio Reagent absorbance... 

Application Filter area, m2 Slurry feed characteristics % solids pH t/hr Wash ratio (wt/wt based on dry solids) Solubles recovery % Final cake moisture %... 

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 ... 

Under mass production conditions, all hair samples are washed in an identical manner by the truncated wash procedure. This consists of one 15-min wash with dry isopropanol and three 1/2-h washes with phosphate buffer. Not aU hair samples will have reached plateau conditions by this procedure. To identify those samples which require further washing, we have defined two additional wash kinetic criteria " the curvature ratio, R, and the extended wash ratio, R w These are defined as follows ... 

Percentage Distribution of Extended Wash Ratio Values Using the Truncated Wash Procedure... 

Extended wash ratio Percentage distribution of positive samples ... 

Another aspect of our diagnostic algorithm that is not sufficiently appreciated is that the validity of the extended wash ratio and the safety zone ratio depends on the wash kinetics showing a significant amount of curvature as measured by the... 

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