Filtration cake compressibility

Filtration and compression take place with the press closed and the belt stationary the press is then opened to allow movement of the belt for cake discharge over a discharge roUer of a small diameter. This allows washing of the belt on both sides (Fig. 15). Cycle times are short, typically between 10 and 30 minutes, and the operation is fully automated. Si2es up to 32 m are available and the maximum cake thickness is 35 mm. 

The following example helps to illustrate the use of the equations presented up to this point. An aqueous slurry was filtered in a small laboratory filter press with a pressure drop of 0.5 atm and at a temperature of 20 C. After 10 minutes, 4.7 liters of filtrate were obtained after 20 minutes, 7.0 liters were collected. From experiments at other pressures, it was determined that the cake compression coefficient was s = 0.4. We wish to determine the volume of filtrate expected after 30 minutes from a filter press having a filtering area 10 times greater than the laboratory press if the filtration is to be performed at 1.5 atm pressure. The liquid temperature will be 55 °C. We also wish to determine the rate of filtration at the end of the process. 

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. 

In conclusion, the following experiments on filtration-washing-deliquoring should be performed to produce data (viscosity of liquids, effective solid concentration, specific cake resistance, cake compressibility, etc.) that are necessary to evaluate times of individual steps of filtration at an industrial scale, i.e. to obtain the proper basis for scale-up of filtration processes measure the filtrate volume versus time make marks on your vacuum flask and take down the time when the filtrate level reaches the mark => no more experiments are needed for preliminary evaluations of filtration properties of slurries initially fines pass the filter medium => recirculate them to the slurry,... 

Cross-flow is the usual case where cake compressibility is a problem. Cross-flow microfiltration is much the same as cross-flow ultrafiltration in principle. In practice, the devices are often different. As with UF, spiral-wound membranes provide the most economical configuration for many large-scale installations. However, capillary devices and cassettes are widely employed, especially at smaller scale. A detailed description of cross-flow microfiltration had been given by Murkes and Carlsson [Crossflow Filtration, Wiley, New York (1988)]. 

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. 

Variable Rate and Pressure Filtration for Compressible Cakes... 

The development so far is also based on constant a or on homogeneous cakes whose properties do not change with time or operating pressure. In fact, most cakes are compressible to some degree. As more fluid passes and more cake deposits, the older layers of solid are subject to frictional drag. The particles continue to move toward the filter surface and even to penetrate it, and the cake becomes denser in that direction. In other words, the cake is compressible, and the value of a varies. Significant cake compression is more likely in constant-rate filtration, where the pressure increases continuously. 

Alternatively, compression-permeability experiments can be performed. A filter cake at a low pressure drop and atm pressure is built up by gravity filtering in a cylinder with a porous bottom. A piston is loaded on top and the cake compressed to a given pressure. Then filtrate is fed to the cake and a is determined by a differential form of Eq. (14.2-9). This is then repeated for other compression pressures (Gl). 

For an initial determination of filtration performance the procedures described in Steps 2-9 are adequate. If data are required for filter sizing and simulation, then Steps 2-9 need to be repeated at a range of different constant pressures/vacua to establish any variation of cake resistance and solids concentration and thus cake compressibility (see also Section 4.7). It is likely that more sophisticated equipment, such as that described in Section 4.6, will give more reliable results. 

The equations for c may need to be modified when considering a batch filtration. Unless care is taken the entire batch of suspension can be filtered and the experiment can be continued with the result that undesirable cake deliquoring, and sometimes cake compression, occur. As seen in Figure 4.2 these phenomena manifest themselves on a t/F vs. V plot as a sharp deviation at longer filtration times, and hence larger volumes of filtrate. Should cake deliquoring occur then both and need to be adjusted in order to calculate correct values for specific cake resistance and the volume fraction of cake soUds (Q ) as the mass of wet cake recorded at the end of an experiment will be too low. When the volume of filtrate at the transition from cake formation to gas deliquoring is... 

Where cake compression or deliquoring occurs towards the end of a test equation (4.15) must be modified by substituting (mAtr for av No similar correction can easily be applied to equation (4.14) to account for cake deliquoring as neither the cake thickness nor the mass of solids deposited is usually known as a function of the filtration time. 

The principal objective of an expression test is to determine the compression deliquoring characteristics of a cake. However, the nature of the test allows both filtration and compression characteristics to be determined when the starting mixture is a suspension (i.e. where the solids are not networked or they are interacting to a significant extent). Cake formation rate, specific resistance and solids volume fraction data can be determined for the filtration phase while analysis of a subsequent consolidation phase allows the calculation of parameters such as consolidation coefficient, consolidation index and ultimate solids concentration in the cake. Repeated use of the expression test over a range of constant pressures allows the evaluation of scale-up coefficients for filter sizing and simulation as described in Section 4.7. 

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