The filter press is one the simplest of all pressure filters; it allows relatively easy cake removal af-ter the individual filtration steps. It consists of a number of flush plates, up to 100 or more, and frames whose width (between 25 and 150 mm) can be matched to the service conditions. The plates and frames form rather flat pressure filtra-tion spaces with a relatively large filtrafiltra-tion area (plate-and-frame filter press, Fig. 68 A). During filtration, the plates and frames must be squeezed by mechanical or hydraulic means in order to seal the filtration spaces (Fig. 69). The faces of the plates are studded or grooved to permit fil-trate drainage.
Figure 68. Filter press – sectional view A) Plate-and-frame filter press
a) Filter cloth; b) Filter plates; c) Hollow frames; d) Mov-able end half plate; e) Fixed end half plate
B) Recessed-plate filter press
a) Fixed end half plate; b) Filter cloth fastening; c) Recessed filter plates; d) Filter cloth; e) Movable end half plate
The plates can also be made with a raised edge (recessed-plate filter press, Fig. 68 B). There are several essential differences between the two types: In the plate-and-frame press, slurry enters through channels in the corners of the plates and frames. The filter cloths can therefore be shaped as flat strips and pulled over the plates; this ar-rangement is advantageous when the cloths are changed. In recessed-plate presses, slurry is in-troduced into a center orifice of the plates. The filter cloths must be made in two parts and in-serted in the slurry feed channels; this arrange-ment involves much more effort. Further, it is harder to vary the cake thickness in recessed-plate devices than in recessed-plate-and-frame presses, where all that is necessary is to replace the frames. The recessed-plate press has, however, the advantage that only about half of the filter elements need be moved to open the press; the equipment also costs much less for a given filtra-tion area. Because of the deflecfiltra-tion of the filter cloths at the edge of the recessed plate, thinner cakes (up to about 35 mm) are generally pro-duced than in plate-and-frame devices. With ei-ther type of press, it must be noted that capillary flow takes place despite the relatively high den-sity of the filter cloths, and this flow – at least in filtration with solvents – causes the undesir-able escape of filtrate through the seals between plates.
The filtrate is either discharged separately from each plate through cocks (open discharge) or collected in a channel (closed discharge), which is preferred when toxic or volatile ma-terials are handled.
Figure 69. Filter press (reproduced with permission of Eberhard Hoesch & S¨ohne)
At cake washing time, valves are changed over and special lines carry wash water only to every other plate, so that the liquor can flow transversely through the cake to the opposite plate. Occasionally the wash water is also de-livered through the slurry channel; this arrange-ment is, of course, not as effective. After wash-ing, the cake can be dewatered by passing steam or (possibly heated) air through it.
For cake discharge, the press is opened and the plates are moved, one by one, through a pre-determined distance. A variety of mechanical devices are now available for this purpose. If the cake is to drop out by itself, it should not adhere too tightly to the filter cloth and should not be too densely filtered. Cake separation can be as-sisted by rapping, pulling out the filter cloth, or other means. An automatic filter press for high throughputs has more than 100 chambers with a filtration area of 1000 m2and more.
Filter presses are generally insensitive to slurry filtration properties, with perhaps one ex-ception: In large presses with long slurry chan-nels, varying sedimentation may cause a slurry of group S (see Table 2) to fill the filtration spaces unevenly. Under some conditions, an undesired classification occurs, with a detrimental effect on washing, and may even lead to pressure dif-ferences between chambers, causing the plates to deform or break. In such cases, it is desirable to provide a hole for pressure equalization or a supporting button.
In addition to filter presses with vertical plates, some designs also feature horizontal plates.
Filtration Cycle (see Chap. 5). Cake filtra-tion in filter presses involves two indistinctly separated stages. At the start of filtration there is no cake, and the hydraulic resistance is there-fore very low, so that the filtration pressure can-not rise very high at this point. The process is similar to constant-volume filtration (see Sec-tion 2.1.2). The pressure increases as the cake thickness grows, until the rated pressure (of the pump) is reached. Constant-pressure filtration then begins, with a declining quantity of filtrate.
The exact sequence of filtration in filter presses is still unclear and cannot be calculated, because not enough research has been done, but a rough estimate suggests that the highest throughput is achieved if
tf=tkZ (70)
where tf is the filtration time, tkis the time for filling and emptying per chamber, and Z is the number of chambers.
On the basis of a cost calculation, the maxi-mum number of chambers is
Z =
tf(CM−Cp) tkCp
12
(71)
where CMis the cost of mechanical equipment (fixed and movable end plates, adjusting device) and Cpis the cost per plate (chamber). For con-ventional designs, Z is found to be around 60.
To determine tf, the total cycle time includ-ing washinclud-ing and other steps, experimental data must be used. Unfortunately, experience has shown that measurements in laboratory-scale fil-ter presses cannot generally be extended to arbi-trary sizes, since there may be discrepancies in the filling of the filtration chambers and thus in the pressure relationships.
Filter presses are commonly charged at pres-sures of up to 2 MPa, more in special cases. Cen-trifugal or diaphragm pumps are preferred for the cake filtration of highly-concentrated slur-ries. The possibility of employing relatively high filtration pressures, along with the mechanical advantages in charging and discharging, lend fil-ter presses some advantages over other types of pressure filter, even though these devices are al-ways operated batchwise:
1) Insensitivity to feed fluctuations
2) Relatively high throughputs, even with dif-ficultly filterable slurries, so that far smaller amounts of filter aids are needed
3) Low cake moisture
4) Denser cakes that are easier to handle 5) Clear filtrate
6) Good separation of different filtrates (wash liquor, etc.)
7) Arbitrary filtration steps (washing, steam or air blowing, etc.)
8) Low maintenance costs, few spare parts, low depreciation
9) Simple operation
10) Comparable equipment costs per square me-ter of filtration area
The drawbacks are obvious: Batch operation;
personnel requirements in cases where cake does
not drop off by itself; relatively long cleaning times.
Applications. Even though they are batch devices, the good washing and low cake mois-ture available in filter presses have kept them in wide use. Contributing factors, along with com-paratively long service times and good reliabil-ity, include mechanical plate movement, used al-most exclusively for the opening and discharge of large units; automatic filter-cloth cleaning with high-pressure water sprays; and the avail-ability of special plate designs. Plates up to 2.6m on a side give far more than 1000 m2of filtration area in one press. The design of the plates and the use of polypropylene as the main plate mate-rial have remedied many of the shortcomings of filter presses, such as plate fractures, corrosion, and plate mass.
Filter presses can be employed almost any-where, even if the mode of operation and the equipment must be adapted to the feeds in ques-tion. Filter presses are used successfully in ei-ther cake or clarifying filtration, depending on the concentration and properties of the solids; in the latter case, filter aids are usually added, or filter beds are employed instead of cloth media.
In the production of chemicals (e.g., dyes), ceramics and raw materials, and in wastewater treatment, nuclear technology and other fields,
cake filtration is applied chiefly to groups M and S of Table 2. Slurry feed rates in these fields de-pend on the widely varying slurry composition and range from 0.2 – 1 m3m−2h−1on the aver-age.
Figure 70. Filter press for sheet filtration – deep bed pressure filter (reproduced with permission of Seitz-Filter-Werke)
Clarification is carried out in sheet filter presses, which resemble plate-and-frame filter presses in design (Figs 70, 71, and 72). Because of the very much lower solids concentration, ser-vice times are much longer (several hours). For this reason, automation is commonly dispensed with. Filtersheets include ordinary precoats and special papers (see page 64 and Chap. 11).
Figure 71. Sheet filter press opened (reproduced with permission of Seitz-Filter-Werke)
Figure 72. Clarifying filter press – schematic operation Top: Filtration with filter aids; Bottom: Sheet filtration
The most important applications of clari-fying filtration include beer, wort, wine, fruit juices, pharmaceutical liquids, or water steriliza-tion (group D of Table 2). Here too, filter loading varies widely, depending both on the properties of the solids being removed and their concentra-tion in the feed and on the nature of the liquid.
They range from 0.05 m3m−2h−1 for viscous varnishes up to 10 m3m−2h−1 for dilute sus-pensions, such as beverages.
Membrane Filter Presses. In a membrane press, the filter plate is coated with an elastic ma-terial (rubber). After filtration, the membranes are pressurized so that they mechanically com-press the filter cake. In this way the cake mois-ture is reduced by ca. 1 % – 20 %, depending on the cake compressibility. Figure 73 shows an ex-ample of a recessed-plate membrane press. A similar expedient is also possible in a plate-and-frame press. The pressing either shortens filtra-tion time or the filtrafiltra-tion pressure can be made smaller, which is an eminent practical advan-tage.
Membrane filter presses can be operated at up to 2 MPa; they produce a uniformly dewatered cake.
Plate Press with Belt Discharge (Auto-matic Press). This type of filter press, orig-inally developed in Russia, combines positive automatic cake discharge with cake compres-sion. The horizontal filter plates are stacked and, with the filter frames, combined into a three-chamber system: filtrate three-chamber, cake cham-ber, and high-pressure water space. The water space is separated from the cake by a membrane.
For cake discharge, a cloth belt on the frame is advanced until the cake from each plate is com-pletely stripped off as the belt is deflected into the next lower plate (Figs. 74 and 75).
Filter Press with Stationary and Rotating Plates. This filter contains alternating station-ary and rotating plates on a shaft (Fig. 76). A velocity difference is produced in the cake bet-ween the plates, leading to breakdown of the cake. A thin layer of filter cake, whose
thick-ness depends on the filter medium employed, is left on the filtration surface. In the extreme case where cake formation is totally prevented, this is pure cross-flow filtration (see Section 2.4). The intermediate states are referred to as dynamic filtration.
Figure 73. Membrane filter press plate assembly a) Filter cloth; b) Slurry corner feed; c) Recessed plate;
d) Membrane recessed plate; e) Press medium; f) Cake;
g) Filtrate; h) Air or water inlet (Pressuring membranes)
If the filter cake is thixotropic, the shearing makes it flowable, so that it can be conveyed by pressure through the system of plates. Accord-ing to studies of some dye suspensions, this filter can collect the same quantity of solids as a con-ventional filter press with roughly 20 times as large a filtration area.