Displacement washing

In the dispersion model the superficial velocity (u) and pore velocity (v) of the wash liquor are related to the intrinsic properties of the cake by 

In a batch filter the end of washing is normally specified in one of three ways, that is, the final fractional solute recovery (F ), wash ratio (W ) or washing time and the sequence of calculations alters accordingly. 

Known (0 - Values of up to the maximum are chosen to provide intermediate values in the cycle calculations such that 

If the cake saturation (S) is less than 1 at the start of washing, for instance due to a previous gas deliquoring phase (see Section 6.2.4), then it is necessary to correct each chosen or calculated value of W according to 

With t , F and W known, the volume of washings VJ, and hence the total volume filtrate/liquid volume, and wash liquor flow rate are given by 

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The three types of washing are washing of filter cakes by displacement, washing by reslurrying of cakes or sludges, and washing by successive dilution. 

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. 

Thorough displacement washing is possible in a nutsche if the wash solvent is added before the cake begins to be exposed to air displacement of filtrate. If washing needs to oe more effective, an agitator can be provided in the nutsche vessel to reslurry the cake to allow adequate diffusion of solute from the solids. 

O Handling saturated brines or process conditions require elevated temperatures the vessel, filter floor and paddles may be healed i Reslurry washing, being more efficient than in-situ displacement washing, is required. 

The chilled oil-naphtha solution containing crystallized wax and the filter aid is then filtered by means of a continuous vacuum filter or an intermittent pressure leaf filter. Increased dewaxed oil yields are realized by applying a cold naphtha displacement wash to the wax cake on the filter. The pour point of the dewaxed oil is usually 25° to 35° F. higher than the filtering temperature. 

The wax cake is washed continuously and automatically on the filter with cold solvent to displace any retained oil solution. By control of the conditions of solvent composition, dilution, and the manner in which the cake is deposited on the filter, the displacement wash becomes very effective in producing wax of very low oil content. 

P = Poor. F = Fair. G = Good. E = Excellent. Decantation wash always possible. d Displacement wash feasible. Solids product contaminated by precoat material. (Purchas. 1981). 

When the filtrate contains dissolved substances that should not remain in the filter cake, the occluded filtrate is blown out then the cake is washed by pumping water through it. Theoretically, an amount of wash equal to the volume of the pores should be sufficient, even without blowing with air. In practice, however, only 30-85% of the retained filtrate has been found removed by one-displacement wash. Figure 11.3(b) is the result of one such test. A detailed review of the washing problem has been made by Wake-man (1981, pp. 408 51). 

Displacement wash Uranium elution Fresh water Ammonium carbonate Single bed 10,400 2... 

In the forty-ninth phase, a displacement wash of fresh water flows for 2 h, to flush seawater from the bed. 

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