Pressure leaf filters

Pressure leaf filters are generally employed where fine suspended impurities are to be removed. This operation is made easier by addition of suitable filter aids, e.g. Hyflo-Super-Cel, a diatomaceous inert powder which makes the filter cake porous, and filtration can be continued for longer time till the cake holding capacity is properly utilised. The following need to be considered for the procurement of this type of filter  

Example-The figure below shows a typical pressure leaf steam jacketed filter for liquid sulphur. It is used to filter molten sulphur at 130 C before feeding to the burner of a sulphuric acid plant The filtration ensures steady plant operation and minimises deposit of ash on catalyst bed. 

Vendor should inform the following (for the approval of the purchaser)  

MOC of filter leaves, maximum size of openings in leaves and heating jacket (if required) 

Mechanism for taking out the carriage on which the filter leaves are mounted 

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Pressure Leaf Filters Sometimes called tank filters, they consist of flat filtering elements (leaves) supported in a pressure shell. The leaves are circ ilar, arc-sided, or rectangular, and they have filtering surfaces on both faces. The shell is a cylindrical or conical tank. Its axis may be horizontal or vertical, and the filter type is described by its shell axis orientation. 

FIG. 18-118 Horizontal-tank pressure leaf filter designed for dry cake discharge. Sparkler Filter, Inc.)... 

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. 

The operation of a vertical pressure leaf filter is labor intensive and requires a complex manipulation of valves so present day installations are in most cases fully automated. The operational steps are as follows ... 

The relation between flow and head for a slurry pump may be represented approximately by a straight line, the maximum flow at zero head being 0.0015 m3/s and the maximum head at zero flow 760 m of liquid. Using this pump to feed a slurry to a pressure leaf filter,... 

Pressure leaf filters are designed for final discharge of solids in either a dry or wet state, under totally enclosed conditions, with fully automatic operation. 

Each type of pressure leaf filter features a pressure vessel in which are located one or more filter elements or leaves of circular or rectangular construction. The filter media may be in the form of a synthetic fibre or other fabrics, or metallic mesh. Supports and intermediate drainage members are in coarse mesh with all components held together by edge binding. Leaf outlets are connected individually to an outlet manifold which passes through the wall of the pressure vessel. 

Pressure leaf filters are supplied in a wide range of size and materials of construction. One typical design is the Verti-jet unit with a vertical tank and vertical leaf filter, as shown in Figure 7.12, with rectangular leaves mounted individually but connected to a common outlet manifold. For sluice cleaning either a stationary or oscillating jet system... 

Using this pump to feed a particular slurry to a pressure leaf filter ... 

The chilled oil-naphtha solution containing crystallized wax and the filter aid is then filtered by means of a continuous vacuum filter or an intermittent pressure leaf filter. Increased dewaxed oil yields are realized by applying a cold naphtha displacement wash to the wax cake on the filter. The pour point of the dewaxed oil is usually 25° to 35° F. higher than the filtering temperature. 

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. 

It is believed that the future will see a significant increase in the popularity of pressure-leaf filters in this region. 

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. 

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