Pressure filtration capacity

The same moisture content of the produced cake can be obtained in shorter dewatering times if higher pressures are used. If a path of constant dewatering time is taken, moisture content is reduced at higher pressures with a parallel increase in cake production capacity. This is an advantage of pressure filtration of reasonably incompressible soHds like coal and other minerals. 

Pressure Filtration. High pressure drops have a twofold effect, ie, on capacity and on displacement dewatering which often follows. 

The test results reported show the advantages of pressure filtration quite clearly, ie, the dry cake production capacity obtained with the test soHds (coal suspensions) was raised 60 or 70% and the final moisture content of the cake reduced by as much as 5 to 7% by increasing the pressure drop from 60 to 200 kPa. Further increases in the operating pressure bring about less and less return in terms of capacity and moisture content. 

Significant improvements were made in the 1980s and early 1990s in high capacity, automated variable volume filters that incorporate automatic pressure filtration, expression, washing, and air displacement. Some of the large plate-and-frame automatic presses can operate at up to 2 MPa (ca 285 psig), with up to 100 chambers (25,26). 

Alternative 2 consists of preliminary treatment followed by dual-media pressure filtration, and two-stage air stripping (Figure 8.4). The preliminary treatment step for iron removal would be exactly the same as specified under Alternative 1. The filters would be recommended to remove suspended matter and particulate iron prior to the air strippers. The required filtration capacity could be provided with either a duplex system of two 60-in.-diameter filters or a triplex system of three 42-in-diameter filters. 

The material to be filtered is fed into the vessel under pressure, and separation takes place with the solids being deposited on the leaf surface, and the liquid passing through the drainage system and out of the filter. Cycle times are determined by pressure, cake capacity or batch quantity. Where particularly fine solids must be removed, a layer of precoat material may be deposited on the leaves prior to filtration, using diatomaceous earth, Perlite, or other suitable precoat materials. 

LP is the hydraulic conductivity coefficient and can have units of m s-1 Pa-1. It describes the mechanical filtration capacity of a membrane or other barrier namely, when An is zero, LP relates the total volume flux density, Jv, to the hydrostatic pressure difference, AP. When AP is zero, Equation 3.37 indicates that a difference in osmotic pressure leads to a diffusional flow characterized by the coefficient Lo Membranes also generally exhibit a property called ultrafiltration, whereby they offer different resistances to the passage of the solute and water.14 For instance, in the absence of an osmotic pressure difference (An = 0), Equation 3.37 indicates a diffusional flux density equal to LopkP. Based on Equation 3.35, vs is then... 

Figure 3.10 shows the details of the numerical-solving algorithm for the monodimensional. This numerical transposition has the capacity of being related with any available software. In Fig. 3.10, we can note that only the case of constant filtrate rate has been presented. Otherwise, when we operate at constant pressure, the filtrate rate decreases with the time due to the continuous clogging phenomenon. To simulate a constant pressure filtration, some changes in the computing program of Fig. 3.10 are necessary these modifications are shown in Fig. 3.11. It is easily observable that here the stop criterion has been completed with the decreasing of the solid concentration in the recycled suspension.

The THF filtrate, which also contains cyclohexane soluble vehicle, was concentrated by roto-evaporation to remove the THF, 5 mL of THF was added back to the slurry, and this THF slurry was dripped into 150 mL of stirred cyclohexane. The flask that contained the concentrated THF solution was sonicated with 10-30 mL of cyclohexane in order to clean it thoroughly. Since the filter/holder has a capacity of 100 mL, only two-thirds of the 150-mL cyclohexane suspension was added. Approximately half of the cyclohexane suspension in the filter/holder was pressure-filtered to allow addition of the remaining suspension. Five hundred mL of cyclohexane was then added to reservoir R, and the sample was pressure-filtered. The residue was then removed, vacuum-dried, and weighed. Since the THF-insoluble material was also cyclohexane-insoluble, combining the weight of the THF-insoluble residue and the insoluble residue obtained from the cyclohexane filtration allows the calculation of a cyclohexane solubility value. The results from the modified procedure on one sample will thus give both THF and cyclohexane solubility values. The modified pressure-filtration procedure takes 2 to 2 1/2 hours. 

Cake thickness is an important factor in determining the capacity and design of a batch filter and it determines the cycle of operation. In constant-pressure filtration, the integration of Equation 58.1, with the medium resistance neglected, yields... 

The first advantage of any continuous filter is the absence of or reduction in down-times which means gains in capacity. Bearing in mind the effects of compressibility of cakes discussed in section 12.2.1.1, the effect of higher driving force on capacity can be seen in equation 12.1 which is derived for constant-pressure filtration from the well-known law of Darcy. The effect of resistance of the filter medium is neglected here and the cake is assumed to be incompressible. 

A further obvious distinction is between vacuum filters (negative pressure) and pressure filters (positive pressure) (see Figure 17.7). As the vacuum filters are limited to a maximum practical pressure differential of about 6.8—8.2 x lO Nm (10 to 12 p.s.i.), theoretically they have a lower capacity. Since most pressure filters, however, rely on a thicker cake and, therefore, a greater cake resistance, the net result is not always so much in favour of pressure filtration. It is important, therefore, actually to check these figures and not assume higher throughput with a higher pressure. 

The fundamental case for pressure filters may be made using equation 10 for dry cake production capacity Y (kg/m s) derived from Darcy s law when the filter medium resistance is neglected. Eor the same cycle time (same speed), if the pressure drop is increased by a factor of four, production capacity is doubled. In other words, filtration area can be halved for the same capacity but only if is constant. If increases with pressure drop, and depending how fast it increases, the increased pressure drop may not give much more capacity and may actually cause capacity reductions. 

Drum Filters. The rotary dmm filter, also borrowed from vacuum filtration, makes relatively poor use of the space available in the pressure vessel, and the filtration areas and capacities of such filters cannot possibly match those of the disk pressure filters. In spite of this disadvantage, however, the pressure dmm filter has been extensively developed. 

Feed Slurry Temperature Temperature can be both an aid and a limitation. As temperature of the feed slurry is increased, the viscosity of the hquid phase is decreased, causing an increase in filtration rate and a decrease in cake moisture content. The limit to the benefits of increased temperature occurs when the vapor pressure of the hquid phase starts to materially reduce the allowable vacuum. If the hquid phase is permitted to flash within the filter internals, various undesired resiilts may ensue disruption in cake formation adjacent to the medium, scale deposit on the filter internals, a sharp rise in pressure drop within the filter drainage passages due to increased vapor flow, or decreased vacuum pump capacity. In most cases, the vacuum system should be designed so that the liquid phase does not boil. 

Plate pr esses. Sometimes called sheet filters, these are assemblies of plates, sheets of filter media, and sometimes screens or frames. Thev are essentially modified filter presses with practically no cakeholding capacity. A press may consist of many plates or of a single filter sheet between two plates, the plates may be rectangular or circular, and the sheets may lie in a horizontal or vertical plane. The operation is similar to that of a filter press, and the flow rates are about the same as for disk filters. The operating pressure usually does not exceed 138 kPa (20 psig). The presses are used most frequently for low-viscosity liqmds, but an ordinaiy filter press with thin frames is commonly used as a clarifier for 100-Pa s (1000-P) rayon-spinning solution. Here the filtration pressure may be 6900 kPa (1000 psig). 

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