Principle of Plate and Frame Filter Press
The mechanism of this
filter is surface filtration. The slurry enters the frame by pressure and flows
through the filter medium. The filtrate is collected on the plates and sent to
the outlet. Several frames and plates are used so that surface area increases
and consequently large volumes of slurry can be processed simultaneously with
or without washing.
Construction of Plate and Frame Filter Press
The construction of a
plate and frame filter press is shown in Fig.1. The filter press is made of two
types of units, plates and frames.
Fig.1: Frame and Press Filter
The components of this
filter are as follows.
- Frame: It maintains the slurry reservoir and has an inlet (eye) for slurry.
- Filter medium: It is for solid retention.
- Plate: This equipment along with section-supports the filter medium, receives the filtrate and has an outlet (eye).
- Assembly of plate and frame filter press.
The plate and frame are
usually made of aluminium alloy. Sometimes these are also coated for protection
against corrosive chemicals and made suitable for steam sterilization. The frame
contains an open space inside wherein the slurry reservoir is maintained for
filtration and an inlet to receive the slurry. The plate has a studded or
grooved surface to support the filter cloth and an outlet. The filter medium
(usually cloth) is fitted between plate and frame. Frames of different thicknesses
are available and a frame with optimum thickness is chosen. Selection is mainly
based upon the thickness of the cake formed during filtration. The plate,
filter medium, frame, filter medium and plate are arranged in the sequence as
shown in Fig.1. and clamped in a supporting structure. Several plates and
frames are used to have a large surface area. These filtration units are
operated in parallel. Channels for the slurry inlet and filtrate outlet can be
arranged by the fitting inlet to the plates and frames, these join together to
form a channel. In some types, only one inlet channel is formed, while each
plate is having individual outlets controlled by valves.
Working of Plate and Frame Filter Press
The working of the frame
and plate process can be described in two steps, namely filtration and washing
of the cake (if desirable). The working of a plate and frame press is shown in
Fig.2. Slurry enters the frame from the feed channel and passes through the
filter medium onto the surface of the plate. The solids form a filter cake and
remain in the frame. The thickness of the cake is usually half the frame
thickness as filtration occurs on each side of the frame. Thus, two filter
cakes are formed, which meet eventually in the centre of the frame. The optimum
thickness of filter cake for any slurry depends on the solid content in the
slurry and the resistance of the filter cake. The filtrate drains between the
projections on the surface of the plate and escapes from the outlet. With time
resistance of the cake increases and the filtration rate decreases. It is
advisable to stop the process at a certain point rather than continuing at very
low flow rates. On completion of the filtration, the press is emptied and the
cycle is restarted.
Washing:
Generally washing the filter cake is necessary and for that, the ordinary plate
and frame press is not suitable. The cake built-up at the centre of the frame
brings flow to a standstill. Thus, water washing of cake using the same
channels used for the filtrate is inefficient. A modification of the plate and
frame press is another option. In modified form, an additional channel is
included for washing, Fig.2.(d). In half the wash plate there is a connection
from the wash water channel to the surface of the plate.
Fig.2: Filtration Operation with Wash Water Facility
The steps of the
filtration are as follows:
- Slurry enters the plates and filtration proceeds ordinarily until the frames are filled with cake.
- To wash the filter cake, the outlets of the washing plates are closed.
- Wash water is pumped into the washing channel. The water enters through the inlets onto the surface of the washing plates.
- Water passes through the filter cloth and enters the frame which contains the cake. The water washes the cake, passes through the filter cloth and enters the plate down the surface.
- Finally washed water escapes through the outlet of that plate.
Thus, the provision of
special washing plates makes it possible for the wash water to flow over the
entire surface of the washing plate which makes the uniform resistance of the cake
to the flow and the entire cake is washed efficiently. The water wash is
efficient only when the frames are full of filter cake. If the cake does not
fill the frame the wash water causes the cake to break on the washing plate
side of the frame and the washing will be less effective. It is essential to
allow the frames to become filled with the cake. This helps in emptying the
frames as well as in washing the cake correctly.
Special Provisions:
A provision of a glass tube (sight glass) to observe contamination can be made
at the outlet on each plate. This permits the inspection of the quality of the
filtrate. The filtrate goes through the control valve to an outlet channel. The
filtration process from each plate can be seen. In the case of a broken cloth,
the faulty plate can be isolated and filtration can be continued with one plate
less. A provision of filter sheets composed of asbestos and cellulose capable
of retaining bacteria can be used for sterile filtration with a whole filter
press and filter medium previously sterilized. Usually, steam is passed through
the assembled unit for sterilization. Examples include a collection of
precipitated antitoxin, removal of precipitated proteins from insulin liquors
and removal of cell broth from the fermentation medium. Heating/cooling coils
can be incorporated into the press for the filtration of viscous liquids.
Advantages of Plate and Frame Filter Press
- Simple in operation, convenient in maintenance, and safe in operation with multiple safety devices. A variety of materials such as cast iron (for handling common substances), bronze (for smaller units), stainless steel (to avoid contamination), hard rubber or plastics (to avoid metal) and wood (for lightness) can be used.
- It provides a large filtering area in relatively small floor space.
- It is versatile equipment whose capacity can be varied according to the thickness and the number of frames used. Surface area can be increased by employing chambers up to 60.
- This press has a sturdy construction that makes use of a pressure difference of 2000 kilopascals to be used.
- Cake washing is very efficient.
- Operation and maintenance are low as it has no moving parts and the filter cloths can be easily renewable.
- The presence of all external joints makes the plate to be isolated in case of any leaks and the contamination of the filtrate can be avoided.
- It produces dry cake in the form of a slab.
Applications of Plate and Frame Filter Press
- Filter presses are used in a huge variety of different applications, from dewatering slurries to purification. At the same time, filter press technology is widely established for ultrafine dewatering as well as filtrate recovery.
- Solid-liquid separation in alcohol, chemical, metallurgy, pharmaceutical, light industry, coal mining, food-stuff, textile, environmental protection, energy source and other industries.
Disadvantages of Plate and Frame Filter Press
- It is a batch filter so there is a good deal of ‘down-time’, which is non-productive.
- The filter press is expensive. The emptying time, the labour involved and the wear and tear of the cloth results in a high-cost process.
- The operation is critical as the frames should be full otherwise inefficient washing results in difficulty to remove cake.
- The filter press is used for slurries containing less than 5% solids. So high costs make it imperative that this filter press is used for expensive materials such as a collection of precipitated antitoxin and removal of precipitated proteins from insulin liquors.
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