Filters

FILTERS

The method by which filtrate is driven through the filter medium and cake, if present, may be used to classify filters. Four groups may be listed: gravity filters, vacuum filters, pressure filters, and centrifuge.

Each group may be further subdivided into filters employed in either continuous or batch processes, although, because of technical difficulties, continuous pressure filters are uncommon and expensive. The general principles of each group are discussed below. These principles are illustrated by several widely used filters. More extensive surveys can be found in the literature (Salter and Hosking, 1958; Dickey, 1961).

Gravity Filters

Gravity filters employing thick, granular beds are widely used in municipal water filtration. However, the low operating pressures, usually less than 1.03 x  10⁴ N/m², give low rates of filtration unless very large areas are used. Their use in pharmacy is very limited. Gravity filters using suspended media composed of thick felts are sometimes used for clarification on a small scale. On a somewhat larger scale, a wooden or stone tank, known as a nutsche, is used. The nutsche has a false bottom. This may act as the filter medium, although, more com-monly, the bottom is dressed with a cloth. The slurry is added, and the material filters under its own hydrostatic head. The filtrate is collected in the sump beneath the filter. Thorough washing is possible either by simple displacement and diffusion or by resuspending the solids in a wash liquid and refiltering. The nutsche is comparatively difficult to empty, and labor costs are high.

Vacuum Filters

Vacuum filters operate at higher pressure differentials than gravity filters. The pressure is limited naturally to about 8.27 x 10⁴ N/m², which confines their use to the deposition of fairly thin cakes of freely filtering materials. Despite this limitation, the principle has been successfully applied to continuous and com-pletely automatic cake filtration for which the rotary drum filter is most extensively used.

The rotary drum filter is illustrated in Figure 11.3. A typical construction may be regarded as two concentric, horizontal cylinders, the outer cylinder being the septum with a suitable perforated metal support. The annular space between the cylinders is divided by radial partitions producing a number of peripheral compartments running the length of the drum. Each compartment is connected by a line to a port in a rotary valve, which permits the intermittent application of vacuum or compressed air as dictated by the different parts of the filtration cycle. The drum is partially immersed in a bath to which the slurry is fed. The complete cycle of filtration, washing, partial drying, and discharge is completed with each revolution of the drum and usually takes from one to ten minutes. The relative length of each part of the cycle, indicated by the segments superimposed on the figure, will depend on the cake-forming characteristics of the slurry and the importance of the associated operations of washing and

FIGURE 11.3 Rotary vacuum filter.

drying. They may be varied by the depth of immersion and the speed of rotation so that each compartment remains submerged for sufficient time for the for-mation of an adequate cake. Washing and dewatering can be carried out to the standard required during the remaining part of the cycle. The slurry must be effectively agitated during operation, or else sedimentation will cause the preferential deposition of the finer particles, giving a cake of low permeability. Agitation, of course, must not erode the deposited cake. Maintaining a sus-pension of very coarse particles therefore becomes difficult or impossible, and other methods of feeding must be adopted.

Filtration may be followed by a brief period of draining in which air is drawn through the cake, displacing retained filtrate. Washing is usually carried out with sprays, although devices that flood the cake have been used. Dewatering, again achieved by drawing air through the cake, is followed by discharge. A scraper knife, assisted by compressed air, which causes the septum to belly against the cutting edge, is commonly used. Highly cohesive cakes, such as those encountered in the removal of mycelial growth from antibiotic cultures, may be removed by means of a string discharge. A series of closely spaced, parallel strings run on the cloth around the drum. At the discharge section, the strings lift the cake away from the cloth and over a discharge roller after which the strings are led back to the drum.

Other variants of rotary drum filtration include top feed filtration and precoat filtration. As already mentioned, slurries containing coarse particles cannot be effectively suspended by the method described above. Such materials, which give rapid cake formation and fast dewatering, may be filtered by applying the slurry to the top of the drum using a feed box and suitable dams. Sedimentation, in this case, assists filtration.

Precoat filtration using a rotary drum is applied to slurries that contain a small amount of fine or gelatinous material, which plugs and blinds the filter cloth. Filtration is preceded by the deposition of a filter aid on the drum to a depth of up to 4 in. Blinding of the surface layers occurs during filtration, but these layers are removed at the discharge section by a slowly advancing knife so that a clean filtering surface is continually presented to the slurry. The depth of the cut depends on the penetration of the precoat by the slurry solids and is usually of the order of a hundred-thousandth of a meter. This method has allowed the filtration of slurries that could not previously be filtered or that demanded the addition of large quantities of a filter aid.

For filtration on a smaller scale, the nutsche is used. A vacuum is drawn on the sump of the tank, giving a much faster filtration rate than that in a gravity-operated process.

Pressure Filters

Because of the formation of cakes of low permeability, many slurries require higher pressure differentials for effective filtration than that can be applied by vacuum techniques. Pressure filters are used for such operations. They may also be used when the scale of the operation does not justify the installation of continuous rotary filters. Usually, operational pressures of 6.89 x 10⁴ to 6.89 x 10⁵ N/m² are applied across stationary filter surfaces. This arrangement pro-hibits continuous operation because of the difficulty of discharging the cake while the filter is under pressure. The higher labor costs of batch operation are, however, offset by lower capital costs.

FIGURE 11.4 The filter press: plates and frame. (A) Filter plate, (B) frame, and (C) washing plate.

The most commonly used pressure filter is the plate and frame filter press. It consists of a series of alternating plates and frames mounted in line on bars, which provide support and facilitate assembly and cake discharge. Typical plates and frames are shown in Figure 11.4.

The filter cloth is mounted on the two faces of each plate, and the press is assembled by moving the plates and frames together with a hand screw or hydraulic ram. This provides a series of compartments, the peripheries of which are sealed by the machined edges of the plates and frames uniting on the filter cloth, which acts as a gasket. Dripping often occurs at this point, so the press is less suitable for noxious materials. The dimensions of each compartment are determined by the area of the plates and the thickness of the intervening frame. These dimensions and the number of compartments used depend primarily on the volume of slurry to be handled and its solids content. The plate faces are corrugated by grooves or ribs, which effectively support the cloth, preventing distortion under pressure and allowing free discharge of the filtrate from behind the cloth. A section of the assembled filter press is given in Figure 11.5A.

Coincident holes, shown in the top left-hand corner of both plates and frames, provide, on assembly, a channel for the slurry and, simultaneously, enter into each compartment through an entry port in each frame. All compartments, therefore, behave in the same way with the formation of two cakes on the opposing plate faces. Discharge of filtrate after passage through cake, cloth, and corrugations takes place through an outlet in the plate shown diametrically opposite to the frame entry port. Filtration may be continued until the cake entirely fills the compartments or the accumulation of cake gives unsatisfactory rates of filtration.

Washing may be carried out by simply replacing the slurry by wash liq-uids and providing for its separate collection. This method, however, gives inefficient washing due to erosion and channeling of the cake. Where efficient washing is required, special washing plates alternate with the plates described above. These contain an additional inlet, which leads the wash liquid in behind the filter cloth. During washing, the filtrate outlet on the washing plate is closed so that the wash liquid flows through the cloth and first cake in a direction opposite to that taken by the filtrate. The wash liquid then follows the course of the filtrate through the cake and cloth of the opposite plate. A diagrammatic representation of the flow of liquids during washing is given in Figure 11.5B.

FIGURE 11.5 The filter press assembled press showing a frame and two plates. Movement of liquid during (A) filtration and (B) washing.

The development of filter media in sheet form with high wet strength and the ability to retain extremely fine particles extends the application of the plate and frame filter to clarification. Such media occur in various grades and, when used in apparatus similar to that described above, may be used to clarify or sterilize liquids containing a very low proportion of solids. In sterilization by sheet filtration, the operation is carried out in two stages. The solution is first clarified. The very clean filtrate is then passed through the sterilizing sheet under a relatively low pressure. Before the operation, the assembled filter is sterilized by steam. The washing apparatus, assembled with suitable sheets, may also be used for air filtration.

Other filters widely used for clarification are the metafilter and the streamline filter. The former consists of a large number of closely spaced rings, usually made of stainless steel, mounted on a rod. The rod is fluted to provide channels for the discharge of filtrate. The passage of filtrate between the rings is provided by scallops stamped on one side of each ring and maintains a ring spacing of between one and eight hundred-thousandths of a meter. This con-struction provides a robust support for the actual filtering medium. It is mounted in a suitable pressure vessel, and large filters consist of a number of units. For clarification, the filter is first coated by circulating filter aid of the correct grade. The finest materials are suitable for the removal of bacteria. The coat acts as a depth filter. Filter aids may also be added to the liquid to be clarified.

The “streamline” filter employs paper disks compressed to form a filter pack. The filtrate passes through the minute interstices between the disks, leaving any solids at the edge. This is the principle of edge filtration. Other filters, composed of metal plates or wires, operate on the same principle and are used for coarse clarification.

Many small-scale filters consist simply of a fixed, rigid medium, robust enough to withstand limited pressures, mounted in a suitable housing. Such filters, which are also vacuum operated, are used to clarify by depth filtration. Media are composed of sintered metals, ceramics, plastics, or glass. Filters prepared from closely graded and sintered ceramic powders are suitable for the sterilization of solutions by filtration on a manufacturing scale.