Filtration pressure leaf-filters

Pressure leaf filters are used to separate much the same lands of slurries as are filter presses and are used much more extensively than filter presses for filter-aid filtrations. They should be seriously considered whenever uniformity of production permits long-time operation under essentially constant filtration conditions, when thorough washing with a minimum of hquor is desired, or when vapors or fumes make closed construction desirable. Under such conditions, if the filter medium does not require frequent changing, they may show a considerable advantage in cycle and labor economy over a filter press, which has a lower initial cost, and advantages of economy and flexibility over continuous vacuum filters, which have a higher first cost. 

Vertical Pressure Leaf Filters are essentially the same as Horizontal Plate Filters except for the orientation of the filter elements which are vertical rather than horizontal. They are applied for the polishing slurries with very lov solids content of 1-5% or for cake filtration with a solids concentration of 20-25%. As with the horizontal plate filter the vertical leaf filters are also well suited for handling flammable, toxic and corrosive materials since they are autoclaved and designed for hazardous environments when high pressure and safe operation are required. Likewise, they may be readily jacketed for applications whenever hot or cold temperatures are to be preserved.The largest leaf filters in horizontal vessels have a filtration area of 300 m and vertical vessels 100 m both designed for an operating pressure of 6 bar. 

Filtration. Filtration in Washington wineries is performed almost exclusively with plate-and-frame filter presses. In some cases, these filters have wide frames so that they are capable of performing diatomaceous earth filtrations. There is only one stainless steel, screened, diatomaceous earth, pressure-leaf filter in use in the Washington wine industry. Two major wineries have found they can utilize a paper septum over a medium pad in their plate-and-frame filters. This allows them to precoat and body feed with diatomaceous earth as the wine is filtered. This accomplishes both a coarse and medium filtration in one movement of the wine. 

Filtration—After an adsorbent has selectively captured the impurities, it must be removed from the oil before it becomes a catalyst for color development or other undesirable reactions. Filtration, the separation method most often used for spent bleaching media removal, is the process of passing a fluid through a permeable filter material to separate particles from the fluid. Examples of the filtration materials used are filter paper, filter cloth, filter screen, and membranes. Filter aid, such as diatomite, perlite, or cellulose, are usually used in conjunction with the permeable filters for surface protection. Traditionally, either plate and frame or pressure-leaf filters have been used for spent bleaching media removal. Currently, self-cleaning, closed filters that operate on an automated cycle are available. 

Horizontal pressure leaf filters. In these filters the leaves may be rectangular leaves which run parallel to the axis and are of varying sizes since they form chords of the shelf or they may be circular or square elements parallel to the head of the shell, and all of the same dimension. The leaves may be supported in the shell from an independent rack, individually from the shell, or from a filtrate manifold. Horizontal filters are particularly suited to dry-cake discharge. 

Vertical pressure leaf filters. These filters have vertical, parallel, rectangular leaves mounted in an upright cylindrical pressure tank. The leaves usually are of such different widths as to allow them to conform to the curvature of the tank and to fill it without waste space. The leaves often rest on a filtrate manifold, the connection being sealed by an O ring, so that they can be lifted individually from the top of the bind repair. A scavenger leaf frequently is installed in... 

Pressure leaf filters are used to separate much the same kinds of slurries as are filter presses and are used much more extensively than filter presses for filter-aid filtrations. They should be seriously considered whenever uniformity of production permits long-time operation... 

A bomb lesi consists of a 6-23 cm1 leaf covered with appropriate filler medium and placed in a smell pressure vessel containing enough sluny to form a desired amount of cake on die leaf. The quantity of filtrate liquid flowing through die cake is measured as a function of time under various pressures and ihe depth of cake formed is observed. This type of lest can provide infomiation for sizing pressure leaf filters or plate and frame fillers. These tests are best conducted by fiber vendors who have experience in translating the results to full-scale equipment. 

In similar manner, excessive stretch caused by the mass of filter cakes in filter press operations could result in the port holes in the fabric moving out of alignment with the corresponding holes in the filter plate, thereby restricting the flow of filtrate out of the press. Likewise, in other filtration systems, such as pressure leaf filters, the same stretch could result in the formation of creases and ultimately mechanical damage to the fabric. 

This is tested in a laboratory generally and it takes considerable time to do so. A quick test is to dip a piece of thick bright white paper sheet into the filtered liquid. Presence of black spots on the paper sheet will indicate unsatisfactory filtration and need for recycling it. This can be used for checking quality of filtered sulphur coming out from a pressure leaf filter. 

A pressure leaf filter is required for the clarification of a vegetable oil, operating at a constant flowrate of 3 m /h. Afler pre-coating the filter medium with a suitable filter aid, filtration ensues up to a maximum pressure difiEerential of 4.5 bar. 

A worked example showing how basic filtration data can be used to assess the optimum cycle time, and hence filter area required to complete a clarification of a v etable oil using precoat filtration on a pressure leaf filter. The filtration is assumed to be constant rate, after having formed a precoat of 2 mm on the pressure leaves prior to clarification. The cleaning and reforming of the precoat takes iproximately 30% of the total cycle time, hence active filtration time is only 70% of any given cycle time. The analysis is based on a 14 h working and 350 days per year. These are easily altered in cells D15 and D16, respectively. 

Many types of filter are supplied for this sort of operation. In chlor-alkali brine treatment, the two most frequently encountered are the pressure leaf filter and the candle filter. Both types are capable of removing submicron particles and producing a filtrate with less than 1 ppm of suspended solids. A leaf filter, as the name implies, contains a number of thin, flat elements that are active on both sides. In chlor-alkali brine plants, the leaves normally are suspended vertically in a tank. The tank may be horizontal, in which case the leaves are circles or rounded squares, or vertical, in which case the leaves are approximately rectangular and of different widths. 

The chilled caustic flows to a second set of centrifuges, also described in Section 9.3.2.5, for removal of the solid salt and then to pressure-leaf filters for final polishing. Section 93.2.6 discusses filtration. 

---