Gas deliquoring

In order to predict gas deliquoring performance on a lull scale filter it is a prerequisite to determine the threshold pressure (pi), the minimum pressure difference that must be applied across a cake to effect any deliquoring whatsoever, and the irreducible saturation (SJ which is the lowest saturation achievable by fluid displacement alone. While the former can be calculated with reasonable confidence, the latter is far more difficult to predict and is best measured in the laboratory test described below. Both quantities are obtainable from the same experiment although the threshold pressure can, on occasion, be a troublesome measurement. The irreducible saturation can sometimes be inferred from the moisture content of a cake discharged from a test filter. 

It is noted that Chapters 6 and 7 describe in detail how p, and u, the superficial gas velocity through the cake, can be predicted from a knowledge of cake, particle and liquid properties for both pressure and vacuum driven deliquoring. 

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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... 

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.

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... 

Drum diameter Drum width Drum submergence Area fraction - washing Area fraction - gas deliquor 0-2 Phase not used Rotational speed ... 

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. 

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

Depending on the type and mode of operation of a batch filter, a cycle may comprise up to two filtration phases followed by a number of compression, gas deliquoring and washing phases, essentially in any order, as well as a cake discharge operation (see also Tarleton and Hancock, 1997 , Tarleton and Wakeman, 1994c, 2005b Wakeman and Tarleton, 1994a,b, 1999). 

Figure 6.1 Schematic diagram of a simple pressure Nutsche filter cycle, (a) Filtration (b) displacement washing (c) gas deliquoring (d) cake discharge by plough.

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... 

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... 

In a batch filter the end of gas deliquoring is normally specified in one of two ways, either by the final cake moisture content (Mg) or a maximum deliquoring time ((Qe) and the sequence of calculations alters accordingly. 

Calculations for gas deliquoring are completed by evaluating the design air rate which allows a vacuum pump or blower to be specified. In the... 

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 ... 

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.

Variations in upstream formulation mean that crystallisation of the j8-form, which is more difficult to filter, can occur in place of the a-form. In each batch, 50 kg of solids are present at a concentration of 6% v/v and it is envisaged that cake formation will occur to a maximum depth of 50 mm. In order to meet product specifications this new filter installation requires a sequential cycle comprising filtration, displacement washing and gas deliquoring. Preliminary... 

For a filter cycle following the sequence filtration - washing - gas deliquoring, and with reference to the procedures outlined in Section 6.2, calculations... 

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. 

With the horizontal belt filter cycle an endless cloth supported on a perforated belt is driven around two rollers and across a sequence of evacuated suction boxes (see Figure 7.1 and Section 1.4.1.3). The feed suspension is introduced toward one end of the filter and processed at constant vacuum to form cake which is subsequently washed and/or gas deliquored theoretically as many... 

Although not explicitly shown, the pressure variant of the drum filter described in Section 1.4.2.6 can be simulated using the equations formulated in Section 7.2 with appropriate modifications to allow for pressure driven gas deliquoring. Details of the latter are shown in Section 6.2.4. 

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... 

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