Gas deliquoring phase

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). 

Using relevant values from the end of the previous washing phase, i.e. 

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  

Knowing the value of 9, equation (6.73) is used to give the actual deli-quoring time (t )  

The value of at pi = 26.85 is then determined by interpolation. Noting that by definition pl = 100 and pl = 100 -pi, the averaged dimensionless air flow rate (ul ), superficial air velocity (uj and design air rate are respectively given by equations (6.80), (6.81) and (6.83) such that 

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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. 

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... 

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 6.9 Data sequences for the gas deliquoring phase of a diaphragm press cycle. ...

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.

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. 

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... 

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 ... 

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

Fraction of filter area devoted to filtration phase 0.05 Fraction of filter area devoted to gas deliquoring phase = 0.47 0.49 Drum diameter D = 0.5->2 m Drum width = 0.5->2 m Additional limitations hplD- 4... 

Number of segments devoted to filtration phase f= I - 5 Number of segments devoted to a washing phase = 1->19 Number of segments devoted to a gas deliquoring phase j— 1 19... 

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