Compression deliquoring

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. 

Figure 4.20 Example of the plots used to evaluate scale-up coefficients for filtration and cake consolidation (compression deliquoring).

Figure 5.12 Interactive graphical display screen for the expression analyses shown in Figure 5.11 where the correct choice of transition from filtration to compression deliquoring has been made. The data are the same as those in the worked example shown...

Compression deliquoring Parameters specific to a consolidation phase including compression pressure, consolidation index, two scale-up coefficients for cake solids concentration and two scale-up coefficients for consolidation coefficient. 

Filter/press type Compression deliquoring Washing Gas deliquoring... 

Figure 6.3 Schematic diagram of the horizontal diaphragm press cycle (side view of one chamber shown and cake discharge omitted), (a) Filtration via pump (b) filtration via diaphragm (c) compression deliquoring (d) displacement washing (e) gas deliquoring. Plate and frame and recessed plate press cycles are similar but the filtration phase using the diaphragm (b) and the compression deliquoring phase (c) are omitted.

Denoting as the cake moisture content at the end of compression deliquor-ing, the corresponding cake solids volume fraction and specific resistance are... 

Table 6.7 Data sequences for the consolidation (compression deliquoring) phase of a diaphragm press cycle.

The mass of solids in the cakes remains constant throughout washing and is equal to the value at the end of compression deliquoring (M = 4527 kg). The mass of liquid in the cakes, as given by equation (6.63), also remains the same as the density of filtrate and wash are equal (M = 3920 kg). The data sequences shown in Table 6.8 provide information for the washing phase where ... 

Filtration phase durations Compression deliquoring phase duration Washing phase duration Gas deliquoring phase duration... 

Gradients on Characteristic Plots for cake filtration, s m, and cake compression deliquoring, s" ° ... 

Volume of cake at start of compression deliquoring, m Volume of filtrate at start of filtration, m ... 

Compression deliquoring Membrane fracture Loss of air pressure a. Difficulty in maintaining air pressure b. Wetter cakes... 

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,... 

Tiller, F.M. Horng, L.L. Hydraulic deliquoring of compressible filter cakes. Part I. Reverse flow in Alter presses. AIChE J. 1983,29, 297-305. 

FIGURE 22.39 Frank M. Tiller, et al., Hydraulic Deliquoring of Compressible Filter Cakes, Part I Reverse Flow in Filter Presses, AIChE Journal, vol. 29, no. 2, AIChE, p. 298 Figure 1, Figure 2 p. 299, Figure 3, March 1983. 

Typical uses Deliquoring of finer particle suspensions forming compressible filter cakes. 

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. 

During the filtration process cake progressively grows upwards from the filter medium surface, liquid is expelled from the press and the piston moves downwards in the direction of the medium. When the particles become sufficiently networked (which can be from the very start of the test), or enough cake has formed for the piston to touch the top of the cake, further deliquor-ing is achieved by consolidation where liquid is expelled as the porosity of the cake decreases. Consolidation continues until an equilibrium state is reached such that the particle structure/cake is sufficiently strong to withstand any tendency to compress under the applied piston loading. 

While valuable information on settling, filtration and cake post-treatments such as washing and gas deliquoring can be obtained from individual tests, in order to subsequently simulate filter performance it is usually necessary to evaluate so-called scale-up coefficients from sequences of tests. These empirical coefficients principally relate to cake formation (compressibility) and compression dehquoring (consolidation), as it is currently impossible to predict either from a knowledge of fundamental solid and liquid properties. Many filter cakes are compressible to some extent, and increases in filtering pressure lead to less porous and more resistant cakes. For these systems data are needed which relate the specific resistance, oc, a measure of cake structure such as solids volume fraction, and where appropriate the modified consolidation coefficient, Q, to variations in the plied pressure difference Ap. It is conventional practice to assume that Q and Q are solely functions of Ap. 

The methodologies presented in this chapter illustrate how characterising parameters for cake formation, compression and gas deliquoring, washing and... 

The two equipment simulation modules provide calculation sequences for more than 20 types of vacuum and pressure filters, potentially involving combinations of cake formation, compression, gas deliquoring and washing. Batch filters include single and multi-element leaf filters, filter presses and diaphragm and tube presses while continuous filters include the horizontal belt, drum, disc, table and tilting pan filters. The user is able to define filter... 

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