Filter leaf is the device which is used in the filtration of the solids containing the suspensions and involves in the separation of the solids from the liquids.
They are applied for the polishing slurries with very low solids content of 1-5% or for cake filtration with a solids concentration of 20-25%.
Leaf Filters are also very 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.
The largest Leaf Filters in horizontal vessels have a filtration area of 300 m2 and vertical vessels 100 m2 both designed for an operating pressure of 6 bar.
Leaf Filters are best selected in the following instances:
- When minimum floor space for large filtration areas is required.
- When the liquids are volatile and may not be subjected to vacuum.
- When there is a risk of environmental hazard from toxic, flammable or volatile cakes specially secured discharge mechanisms may be incorporated.
- When high filtrate clarity is required for polishing applications.
- When handling saturated brines that require elevated temperatures the tank may be steam jacketed.
When the cake may be discharged either dry or as a thickened slurry. They should be selected with care:
- When the cake is thick and heavy and the pressure is not sufficient to hold it on the leaf.
- When coarse mesh screens are used the filtration step must be preceded with a precoat to retain cakes with fine particles. Precoating with a thin layer of diatomite or perlite is not a simple operation and should be avoided whenever possible.
The principle invoves in this type of filtration is surface mechanism and acts as sieve or strainer. Vaccum or presure applied to increase the rate of filtration.
The slurry is pumped under pressure into a vessel that is fitted with a stack of vertical leaves that serve as filter elements. Each leaf has a centrally located neck at its bottom which is inserted into a manifold that collects the filtrate. The leaf is constructed with ribs on both sides to allow free flow of filtrate towards the neck and is covered with coarse mesh screens that support the finer woven metal screens or filter cloth that retain the cake.
The space between the leaves may vary from 30-100 mm depending on the cake formation properties and the ability of the vacuum to hold a thick and heavy cake to the vertical leaf surface. The space is set by the filtrate necks of the leaves at the bottom end and with spacers at the top end brackets.
For fast filtering slurries the space may be doubled by removing every second plate so consequently the cake space doubles but the filtration area is cut in half.
The leaves involve in the filter leaf
There are two types of vessel configuration:
- Vertical vessels
- Horizontal vessels
- In most of the fine chemicals processes the leaves are fitted into vertical vessels whilst horizontal vessels are used in the heavier process industries such as the preparation of sulfur in phosphoric acid plants. The leaves inside horizontal tanks may be positioned either along the tank axis or perpendicular to the axis.
- In order to utilize the tank volume for maximum filtration area the width of the leaves is graduated so they fit to the circular contour of the tank. This also reduces the slurry heel volume that surrounds the leaves.
- The vessels are fitted with highly secured cake discharge openings to ensure safe sealing of the tank under pressure. The cake that accumulates on the leaves may be discharged as a wet thickened sludge or as a dry cake.
- The head cover of vertical vessels is often pivoted so that it is swung away to allow the upwards removal of the leaves in the stack. It is good practice to design a special rig that will support a leaf that is removed from the vessel.
- Special quick opening bolts are fitted around the cover so that tightness is secured during operation but enable easy opening when access to the stack is required.
Horizontal and verticle tanks or vessels in the filter leafs
The operation of a leaf Filter is labor intensive and requires a complex manipulation of valves so present day installations are in most cases fully automated.
- The precoating stage is done only in the following cases:
- When a clear filtrate is required immediately after the filtration cycle commences otherwise recirculation must be employed until a clear filtrate is obtained.
- Once the precoating stage is completed the process slurry is pumped into the filter, the forming cake is retained on the leaves and the filtrate flows to further processing.
- When the solids are fine and slow to filter a body-aid is added to the feed slurry in order to enhance cake permeability. However, it should be kept in mind that the addition of body-aid increases the solids concentration in the feed so it occupies additional volume between the leaves and increases the amount of cake for disposal.
- Likewise, for all those applications when the cake is the product, precoat and filter-aid may not be used since they mix and discharge together with the cake.
- Once the filtration cycle is completed air or gas is blown into the vessel.
- At this point the remaining heel slurry is evacuated back to the feed tank by a special dip pipe that is located at the very bottom of the vessel so that the vessel is empty from slurry.
- The air then continues to pass through the cake until the captive moisture is reduced to a minimum and the cake is in practical terms considered to be dry.
- At this point the air pressure is released, the cake outlet is opened and the leaf stack is vibrated to discharge the cake. The cake outlet opening must be interlocked with a pressure sensor to avoid opening under pressure.
- On some filters the cloth or mesh screen may be back washed with water after cake discharge to dislodge and remove any cake residue that adhered to the medium.
- The leaf filters is satisfactory, if the solids content of the slurry is not too high about 5% that is dilute suspension.
- The cloth or woven mesh screens that cover the leaves of horizontal tanks may be accessed easily once the stack is pulled out of the vessel.
- This allows thorough washing of the medium with high impact jets manually in case that the cake bridges between the leaves.
- On vertical tanks the head cover must be unbolted and removed in order to access the leaf stack.
- Mechanically simple since there are no complex sealing glands or bearings.
- High headroom is required for dismantling the leaves on vertical vessels.
- Large floor space is required for discharging the cake on horizontal vessels.
- The leaf Filter requires attention on a regular basis to safety devices and automation features that accompany modern filters.
- The space of the filter should have a hoisting device and sufficient headroom to lift each leaf and move it horizontally to a location adjacent to the filter tank. It is recommended to have a special rig that will hold the leaf for maintenance. Space must also be allocated for the cover which may be either hinged or removed.
The major components that require attention are:
- The filter tank must conform to an Unfired Pressure Vessel code, such as ASME, and checked periodically as required by the safety regulations.
- The pressure relief valve that is located on the top of the tank must be checked for emergency functioning.
- The "o"-rings that seal between the leaves necks and the filtrate collecting manifolds.
- The large diameter caulking gasket of the dished top head cover. The ends must be cut in an angle to ensure a perfect seal.
- The hinged head cover locking bolts.
- The cleanliness of the filtrate sight glass that monitor on-line or visually enables inspection of the filtrate clarity.
- The interlock that disables opening the cake discharge when the vessel is still under pressure.
- The maintenance hoist above the filter must pull out the leaves vertically so that they will not hit the tank wall.
- The condition of the filter medium, cloth or mesh screen, must be done periodically to ensure that they are not damaged.
- The vent on top of the head must be checked for free evacuation of air.
- The filter must not be overfilled with cake since this causes the leaves to bend so they must be checked periodically.
- C.v.s subrahmanyam et al., pharmaceutical engineering principles and practices-filtration, 2001, page no:267.