Deliquoring phase

Figure 4.18 shows state-of-the-art, laboratory scale experimental apparatus capable of automated data acquisition during sequential filtration, washing and deliquoring phases of a filter cycle the level of hardware and software... 

Figure 4.18 Schematic of a state-of-the-art apparatus for investigating the filtration, displacement washing and gas deliquoring phases of the filter cycle. (1) suspension feed vessel (2) wash liquor feed vessel (3) filter cell (4) rotary index table (5) electronic balance (6) pressure regulator. The inset photograph shows fully automated apparatus for obtaining filtration and deliquoring data including facility for transient measurements of cake growth and state.

Gas deliquoring Parameters specific to a gas deliquoring phase including deliquoring pressure, breakthrough pressure or vacuum for the cake, barometric pressure, irreducible cake saturation and gas viscosity. 

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.

As described in Section 6.1, the cycle for a batch filter can comprise one or more cake formation phases followed by any sequential combination of consolidation, displacement washing and gas deliquoring phases. While a complex batch cycle may involve the list of operations shown in Table 6.2, a more typical cycle can be represented by... 

Table 6.2 Potential operations in a relatively complex batch filter cycle with single washing and deliquoring phases (adapted from Purchas and Wakeman, 1986).

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... 

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

The gas deliquoring phase is performed at a constant pressure of 400 kPa and it is required to reduce the cake moisture content to 25%. Assume that the cake height, solids volume fraction (and thus porosity) and specific resistance are constant and the active filter area is = A H = 150 m. The mass of solids in the cakes remains constant throughout gas deliquoring and equal to the value at the end of washing (M = 4527 kg). 

The irreducible saturation = 0.22 is initially specified. The data sequences shown in Table 6.9 are evaluated to provide information for the gas deliquoring phase where ... 

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

Figure 6.12 Mass balance representation of the diaphragm filter press cycle (from FDS). The values shown for filtrate include the masses of liquid and solute produced during the gas deliquoring phase.

Table 6.14 Data sequences for the gas deliquoring phase of a Nutsche filter cycle. ...

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. 

The rotary disc filter cycle as shown in Figure 7.5 is restricted to single cake formation and deliquoring phases (see also Section 1.4.1.6). Due to the vertical... 

As previously described, the cycle for a continuous filter typically comprises a cake formation phase followed by a combination of sequential displacement washing and gas deliquoring phases, potentially in any order. If a cycle is assumed to comprise the sequence filtration-washing-deliquoring and the subscripts /, d and w, respectively denote values for these phases, then the total time (tj) devoted to a cycle is given by... 

The deliquoring phase is again performed at the same level of vacuum as the filtration phase, and equations (7.2) and (7.4) give... 

Time intervals for the gas deliquoring phase are determined by choosing a value between 1 = 0 s and = 30 s corresponds to a distance Xg along the belt. 

The calculation procedure for the gas deliquoring phase is identical to that shown for the rise phase where cake deliquoring is assumed to occur (i.e. equations (7.89)-(7.104)). Noting that the fraction of the drum devoted to deliquoring cj) = 0.2, equations (7.6) and (7.9) give values at the end of the deliquoring phase ... 

Table 7.11 Data sequences for the deliquoring phase of a rotary drum filter cycle where it is assumed that no gas deliquoring occurs during the rise phase.

Figure 7.9 Schematic representation of the example rotary drum filter cycle. Cake formation phase 0 122°, rise phase 122 -> 198°, washing phase 198 252°, deliquoring phase 252 324° and cake discharge 324 360°.

Figure 7.14 Effects of applied vacuum on the deliquoring phase of a horizontal belt filter cycle.

Fraction of filter area devoted to a gas deliquoring phase (f) = 0.2 0.5... 

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