Variable compressible cakes

Variable Rate and Pressure Filtration for Compressible Cakes... 

Mechanical Squeezing of Cakes. Mechanical squeesing of the cake in the so-called variable chamber filters has been used relatively recendy to lower moisture content of the final cake. This is appHcable only to cakes that are compressible. Many filters are available in which some form of mechanical expression of the cake is used either to foUow a conventional filtration process or to replace it. 

In the past decade adjustments in many of the more subtle variables that affect the feed to a filter ha e begun to be used to control dewatering presses and improve their pertorrnance. These variables allect the perrneabilitv, compressibility, and rheological properties ot the feed and the resulting cake. For example, pll, streaming potential. 

The dynamics of variable-rate and -pressure filtrations can be illustrated by pressure profiles that exist across the filter medium. Figure 7 shows the graphical representation of those profiles. According to this plot, the compressed force in the cake section is ... 

Since for constant pressure filtration, the tjV versus Vdata can be linearized, as shown in Figure 4.15, the resistances of cake and cloth plus cake held up in cloth can be determined. The former value is usually fairly reproducible while the latter is often variable, being particularly sensitive to start up conditions when cloth blinding occurs. Such tests can be rerun at different pressures and the extent of cake compressibility determined. Similarly, a wash cycle can be introduced. 

The effect of pressure shown earlier is modified in most industrial flltrations in which cake compressibility usually lies between 0.1 and 0.8. Furthermore, the resistance of the filter reduces the effects of the respective variables. It has been found, however, that an increase in pressure causes a nearly proportionate increase in the flow rate in the filtration of granular or crystalline solids. Flocculent or slimy precipitates, on the other hand, have their filtration rates increased only slightly by an increase in pressure. Some materials have a critical pressure above which a further increase results in an actual decrease in flow rate. 

Constant-pressure cake filtration with non-Newtonian suspending fiuids has received considerable attention [Kozicld, 1990]. The average ecific resistance has been found to vary considerably as a fimction of the flow behaviour index N even during the filtration of apparently incompressible materials [Shirato et al, 1977], Later papers extended the analysis to compressible filter cakes and constant rate and variable pressure and rate filtrations [Shirato et al, 1980a, b]. 

The complexity of interacting variables is fiirther emphasised by the summary data in Table 6.17. While both the mass of solids processed per batch and the cycle time increase sequentially with formed cake thickness, for the chosen simulation conditions the nominal solids production rate, which is the ratio of these two parameters, passes through a minimum for a filter cycle with cakes initially formed at 30 mm thickness. For cakes formed at the maximum 40 mm thickness the durations of the filtration, compression and gas deliquoring phases are longer, however, these adverse effects are positively counteracted by the greater amount of solids processed per batch which results in the observed improvement in solids production rate. However, higher production rates are obtained when thinner cakes are processed which reinforces the findings presented in Section 6.5.1. 

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