Filtration In Pharmaceutical Engineering

Filtration In Pharmaceutical Engineering Introduction

Filtration may be defined as the separation of a solid from a fluid using a porous medium that retains the solid but allows the fluid to pass. Filtration is usually more expensive than sedimentation, but if has the advantage that it is applicable without regard to density differences and it allows enhanced separation.

The liquid produced after filtering is called filtrate, while the solid remaining in the filter is called residue (retentate, filtrate) or filter cake. The filtering device or the material of the filter is called the filter medium. In such cases, periodical or steady residue removal has to be ensured in the course of filtering operations.

The term fluid includes liquids and gases, so that both may be subjected to filtration. The suspension of solid and liquid to be filtered is known as the “slurry”. The porous medium used to retain the solids is described as the filter medium; the accumulation of solids on the filter is referred to as the filter cake, while the clear liquid passing through the filter is the filtrate.

Filtration In Pharmaceutical Engineering Objectives

• To eliminate contaminant particles to recover dispersing fluid.
• To recover solid particles by eliminating the dispersing fluid.
• To maintain the safety of parenteral solutions by eliminating particulate matter from parenteral solution.
• To provide high-quality water for workers as well as for the processing of pharmaceuticals.
• To separate the reaction mixture from the final product after a chemical reaction.
• To sterilize the solutions containing heat-sensitive drugs by using bacteria-proof filters.

Filtration Applications In Pharmaceutical Engineering

Filtration is used to separate particles and fluid in a suspension, where the fluid be a liquid, a gas, or a supercritical fluid. Depending on the application, either one or both of the components may be isolated.

Filtration, as a physical operation, is very important in chemistry

• Materials of different chemical compositions.
• A solvent is chosen which dissolves the component, while not dissolving the other.
• By dissolving the mixture in the chosen solvent, one component will go into the solution and pass through the filter, while the other will be retained.
• This is one of the most important techniques used by chemists to purify compounds.
• Filtration is also important and widely used as one of the unit operations of pharmaceutical technology.
• It may be simultaneously combined with other unit operations to process the feed stream, as in the bio-filter, which is a combined filter and biological digestion device.
• Production of sterile products: In sterile manufacturing, there is a need for a pure and
  particle-free air which is fulfilled by the HEPA filters. Also for sterilization of
  solutions containing heat-sensitive drugs bacteria-proof filters are used.
• Eye drops are sterilized by the filtration.
• In the production of drugs after a chemical reaction, the final drug is separated from the reaction mixture by filtration.
• Filtration is an essential step in oral liquid formulations such as elixirs, aromatic waters, syrup, etc.
• Waste solids must be separated from the waste liquid before its disposal

Filtration Mechanisms

The mechanisms whereby particles are retained by the filter are of significance only in the early stages of liquid filtration, as a rule. Once a preliminary layer of particles has been deposited, the filtration is effected by the filter cake, the filter medium serving only as a support.

1. Straining: The simplest filtration procedure is “straining”, in which, like sieving, the pores are smaller than the particles, so that the latter are retained on the filter medium
2.  Impingement: If the filter medium is cloth with a nap or is porous, then particles get entangled in the mass of fibers. It occurs due to the smaller size of particles than the pores.
3. Entanglement: If the filter medium consists of a cloth with a nap or a porous felt, then particles become entangled in the mass of fibers. Usually, the particles are smaller than the pores, so impingement may be involved.
4. Attractive Forces: In certain circumstances, particles may collect on a filter medium as a result of attractive forces. The ultimate in this method is the electrostatic precipitator, where large potential differences are used to remove the particles from air streams. In practice, the process may combine the various mechanisms, but the solids removal is affected normally by a straining mechanism once the first complete layers of solids has begun to form the cake on the filter medium.

Filtration Theories

The flow of liquid through a filter follows the basic rule that governs the flow of any liquid through the medium offering resistance.

The rate of flow may be expressed as:

Rate= Driving force/Resistance

The rate of filtration may be expressed as volume (liter) per unit time (dv/dt). The driving
force is the pressure difference between the upstream and downstream of the filter. The resistance is not constant. It increases with an increase in the deposition of solids on the filter medium. Therefore filtration is not steady state.

The rate of flow will be greatest at the beginning of the filtration process since the resistance is minimal. Once the filter cake is formed, its surface acts as a filter medium, and solids continuously deposit adding to the thickness of the cake

Resistance to movement = Press upstream-downstream /Length of capillaries

1. The Hagen – Poiseuille law is applicable in the case of laminar, frictional, and temporally constant flow. According to this, filtration can be considered a flow flowing through parallel capillaries.

The average rate is one-half of the maximum rate:

V = V_max /2 = r⁴ πΔp/8ηL

Where,

V is the rate of flow

r is the radius of capillary,

h is the viscosity of the liquid,

L is the length of the capillary.

Δp is the pressure difference across the filter,

η is the viscosity of the filtrate,

2. Darcy studied the flow of liquids through granular media at constant pressure and established that the filtration rate is,

dV/dt= BAΔp_i/η L

Where,

V is the volume of filtrate,

t is the duration of filtration,

B is the permeability constant of the filter bed,

A is filter surface,

Δp_i a drop of pressure on the filter bed,

η dynamic viscosity of filtrate,

L width of the filter bed

3. The Kozeny-Carman relation applies to laminar flow passing through agglomerated particles. According to the model, the permeability constant is:

Where,

ε porosity,

η viscosity,

k Kozeny-Carman constant,

l_i width of sludge cake,

a_f specific surface of particles.

Factors Affecting Rate Of Filtration

1. Permeability coefficient:

The constant (K) represents the resistance of both the filter medium and the filter cake. As the thickness of the cake increases, the rate of filtration will decrease. Also, the surface area of the particles, the porosity of the cake, and the rigidity or compressibility of the particles could affect the permeability of the cake.

2. Area of filter medium:

The total volume of filtrate flowing from the filter will be proportional to the area of the filter. The area can be increased by using larger filters. In the rotary drum filter, the continuous removal of the filter cake will give an infinite area for filtration.

3. Pressure drop:

The rate of filtration is proportional to the pressure difference across both the filter medium and filter cake.

The pressure drop can be achieved in several ways:

• Gravity: A pressure difference could be obtained by maintaining a head of slurry above the filter medium. The pressure developed will depend on the density of the slurry.
• Vacuum: The pressure below the filter medium may be reduced below atmospheric pressure by connecting the filtrate receiver to a vacuum pump and creating a pressure difference across the filter.
• Pressure: The simplest method is to pump the slurry into the filter under pressure.
• Centrifugal force: The gravitational force could be replaced by centrifugal force in particle separation,

4. The viscosity of filtrate:

It would be expected that an increase in the viscosity of the filtrate will increase the resistance of flow so that the rate of filtration is inversely proportional to the viscosity of the fluid.

This problem can be overcome by two methods:

• The rate of filtration may be increased by raising the temperature of the liquid, which lowers its viscosity. However, it is not practicable if thermolabile materials are involved or if the filtrate is volatile.
• Dilution is another alternative but the rate must be doubled.

5. Thickness of filter cake:

The rate of flow of the filtrate through the filter cake is inversely proportional to the thickness of the cake. Preliminary decantation may be useful to decrease the amount of the solids.

6. Pore size of filter media:

The rate of filtration is directly proportional to the pore size of the filter media. The liquid having coarse particles requires a coarse filtering media to remove them. So, the rate of filtration is increased when a coarse filter medium is used for filtration.

7. Temperature of the liquid:

Temperature plays an important role in rate of filtration. The viscosity of liquid is reduced due to an increase in the temperature. So, the speed of the filtration will increase with the increase in temperature.

Filter Media

Filter medium is a surface where solids are deposited during filtration forming a cake and which also provides mechanical support for the filter cake.

Ideal Properties of Filter Media:

• It should be chemically inert.
• It should have high retention power.
• It should have sufficient mechanical strength.
• It should not absorb dissolved substances.
• It should be resistant to corrosive action.

Selection of Filter Media Depends on:

• Size of particles to be filtered.
• Amount of liquid to be filtered.
• Nature of product to be filtered.

Filter media may be either flexible or inflexible:

1. Flexible: Flexible For the filtration of chemically aggressive fluids and for filtration at elevated temperatures or large mechanical stresses. Flexible media also may be non-metallic barriers consisting of cloth or unwoven fibers. Such non-metallic media may be made of asbestos, glass, cotton, wool, or poly-vinyl chloride.
2. Inflexible: Inflexible media include rigid disks, slabs, canisters, and sheets made by molding and sintering powdered ceramics, metals, glass, or synthetic materials. Such media also may consist of beds of unconsolidated particles of stone, coal, charcoal, coke, diatomaceous earth, sand, or clay.

Different Types of Filter Media Used

• Woven materials: Woven materials are made up of cotton, silk, glass metal, etc. Synthetic fibers are more resistant to chemicals as compared to natural fibers.
• Perforated sheet metal: The stainless steel plates have pores that act as channels as in the case of metafilter.
• Bed of granular solid: Build up on supporting medium: In some processes a bed of graded solids may be formed to reduce the resistance to the flow. e.g. graval, asbestos.
• Prefabricated porous solid unit: Porous solids are prefabricated into a single unit and are being increasingly used for their convenience and effectiveness, e.g. sintered glass filter.
• Membrane filter media: These are basic tools for micro-filtration, useful in the preparation of sterile solutions. These filters are made by casting various esters of cellulose, or from nylon, Teflon, and polyvinyl chloride. The filter is a thin membrane with millions of pores per square centimeter of filter surface.

Filter Aids

Filter aid can be defined as an agent consisting of solid particles that improve filtering efficiency (as by increasing the permeability of the filter cake) and that is either added to the suspension to be filtered or placed on the filter as a layer through which the liquid must pass.

These are fine, chemically inert powders used in filtration to maintain high flow rates while giving brilliant clarity. The objective of the filter aid is to prevent the medium from becoming blocked and to form an open, porous cake, so reducing the resistance to the flow of the filtrate. The particles must be inert, insoluble, incompressible, and irregularly shaped.

Filter Aid Mechanism of Action:

Filter aids impart rigidity and porosity to the cake due to their peculiar irregular shape, low surface area and narrow particle size distribution. The rigid structure provides support for the compressible particles in the slurry.

Filter Aid Ideal Properties :

• It should be chemically inert to the liquid being filtered
• It should be free from impurities.
• It should have low specific gravity, so that filter aids remain suspended in liquid.
• It should be recoverable.
• It should form a porous cake.
• It should be insoluble in liquids

The common filter aids are diatomaceous earth (DE), perlite, cellulose and others. Diatomaceous earth (DE) is the skeleton of ancient diatoms.

They are mined from ancient seabed, processed, and classified to make different grades of filter aids.

• DE is the most commonly used filter aid today. However, the crystalline type DE is a suspicious carcinogen and inhalation needs to be avoided during handling.
• There are different grades of commercial DE. A finer grade may be employed to increase the clarity of the filtrate.
• The smaller the filter aid particle size, the smaller the process particles can be removed.
• However, the filtration rate is lower. There is always a balance between initial filtrate clarity and filtration rate.
• The particle size captured by various filter aids may also vary because of liquid viscosity, surface charge, etc.
• Perlite is another important mineral filter aid. It is a particular variety of naturally occurring glassy volcanic rock, characterized by onion-like, splintery breakage planes.
• After crushing and heating, this rock will expand explosively to about ten times its original volume.
• Diatomaceous earth and perlite are silica-based minerals. There are several other special materials used as filter aids, including asbestos, cellulose, agricultural fibers, sawdust, rice hull ash, paper fibers etc.
• Cellulose can be used for filtration systems that cannot tolerate silica. The filterability of cellulose is much worse than DE or perlite but cellulose can be incinerated as well as provides better cake integrity

Filter Aid Disadvantages: 

• Sometimes coloring active substances get adsorb on the filter aids.
• Rarely, filter aids cause contamination such as soluble iron salts.
• Liquid retained in the pores of filter cake is getting lost.

Classification Of The Filtration Equipment

Equipment is are classified as follows:

1. Based on the application of external force:
   • Pressure filters: Plate and frame filter press and MetaFilter.
   • Vacuum filters: Filter leaf.
   • Centrifugal filters.
2. Based on the operation of the filtration:
3. Continuous filtration: Discharge and filtrate are separated steadily and uninterrupted
4. Discontinuous filtration: Discharge of filtered solids is intermittent. Filtrate is removed continuously. The operation must be stopped to collect the solids.
5. Based on the nature of filtration:
6. Cake filters: Remove large amounts of solids (sludge or crystals).
7. Clarifying filters: Remove small amounts of solids.
8. Cross-flow filters: Feed of suspension flows under pressure at a fairly high velocity across the filter medium.

Equipments Of Pharmaceutical Interest

• Sand filters
• Filter presses: chamber, plate, and frame filters (non-washing/washing; closed delivery/open delivery).
• Leaf filters.
• Edge filters: Streamline and meta filters.
• Rotary continuous filters.
• Membrane filters.

Plate And Frame Filter Press

Plate And Frame Filter Press Principle:

The mechanism 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 volume of slurry can be processed simultaneously with or without washing.

Plate And Frame Filter Press Construction:

• The filter press is made of two types of units, plates and frames.
• Frame: Maintains the slurry reservoir, inlet (eye) for slurry.
• Filter medium.
• The plate along with supporting the filter medium, receiving the filtrate and outlet (eye).
• Assembly of plate and frame filter press.

These are usually made of aluminum alloy. Sometimes these are also lacquered 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.
• It is indicated by two dots in the description. The plate has a studded or grooved surface to support the filter cloth and an outlet. It is indicated by one dot in the description.
• The filter medium (usually cloth) is interposed between the plate and frame.
• Frames of different thicknesses are available. It is selected based on the thickness of the cake formed during filtration.
• The optimum thickness of the frame should be chosen.
• The plate, filter medium, frame, filter medium, and plate are arranged in the sequence and clamped to a supporting structure. It is normally described by dots as 1.2.1.2.1 so on.
• A number of plates and frames are employed so that the filtration area is as large as necessary. In other words, a number of filtration units are operated in parallel.
• Channels for the slurry inlet and filtrate outlet can be arranged by fitting eyes to the plates and frames, these join together to form a channel.
• In some types, only one inlet channel is formed, while each plate has individual outlets controlled by valves.

Plate And Frame Filter Press Working:

The working of the frame and plate process can be described in two steps, namely filtration and washing of the cake (if desirable).

1. Filtration operation:

• Slurry enters the frame from the feed channel and passes through the filter medium on to the surface of the plate.
• The solids form a filter cake and remain in the frame.
• The thickness of the cake is half of the frame thickness because, on each side of the frame, filtration occurs.
• Thus, two filter cakes are formed, which meet eventually in the center of the frame.
• In general, there will be an optimum thickness of filter cake for any slurry, depending 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.
• As filtration proceeds, the resistance of the cake increases,s and the filtration rate decreases.
• At a certain point, it is preferable to stop the process rather than continue at very low flow rates.
• The press is emptied and the cycle is restarted.

2. Washing operation:

• If it is necessary to wash the filter cake, the ordinary plate and frame press is unsatisfactory. Two cakes are built up in the frame meeting eventually in the middle.
• This means that flow is brought virtually to a standstill.
• Hence, water washing using the channels of the filtrate is very inefficient, if not impossible. A modification of the plate and frame press is used.
• For this purpose, an additional channel is included. These wash plates are identified by three dots.
• In half the wash plate there is a connection from the wash water channel to the surface of the plate

The sequence of arrangement of plates and frames can be represented by dots as 1.2.3.2.1.2.3.2.1.23.2.1 and so on (between 1 and 1,23.2 must be arranged). Such an arrangement for the operations of filtration and water washing, respectively.

The steps are as follows:

1. Filtration proceeds in the ordinary way until the frames are filled with cake.
2. To wash the filter cake, the outlets of the washing plates (three dots) are closed.
3. Wash water is pumped into the washing channel. The water enters through the inlets on to the surface of the washing (three dots) plates.
4. Water passes through the filter cloth and enters the frame (two dots) which contains the cake. Then water washes the cake, passes through the filter cloth, and enters the plate (one dot) down the surface.
5. Finally, washed water escapes through the outlet of that plate.

Thus with the help of special washing plates, the wash-water can flow over the entire surface of the washing (three dots) plate, so that the flow resistance of the cake is equal to all points. Hence, the entire cake is washed with equal efficiency.

It should be noted that water washing is efficient only if the frames are full of filter cake. If the solids do not fill the frame, the wash water causes the cake to break (on the washing plate side of the frame) then washing will be less effective. Hence, it is essential to allow the frames to become filled with the cake. This helps not only in emptying the frames but also helps in washing the cake correctly.

3. Special provisions:

• Any possible contamination can be observed by passing the filtrate through a glass tube or sight glass from 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 event of a broken cloth, the faulty plate can be isolated and filtration can be continued with one plate less.

Plate And Frame Filter Press Uses:

• Filter sheets composed of asbestos and cellulose are capable of retaining bacteria so that sterile filtrate can be obtained, provided that the whole filter press and filter medium have been previously sterilized.
• Usually, steam is passed through the assembled unit for sterilization.
• Examples include collection of precipitated antitoxin, removal of precipitated proteins from insulin liquors, and removal of cell broth from the fermentation medium.
• Heating/cooling coils are incorporated in the press to make it suitable for the filtration of viscous liquids.

Plate And Frame Filter Press Advantages:

1. Construction of filter press is very simple and a variety of materials can be used.
   • Cast iron for handling common substances.
   • Bronze for smaller units
   • Stainless steel is used thereby contamination can be avoided.
   • Hard rubber or plastics where metal must be avoided.
   • Wood for lightness though it must be kept wet.
2. It provides a large filtering area in a relatively small floor space. It is versatile, the capacity being variable according to the thickness of frames and the number used. Surface area can be increased by employing chambers up to 60.
3. The sturdy construction permits the use of considerable pressure difference. About 2000 kilopascals can’ be normally used.
4. Efficient washing of the cake is possible.
5. Operation and maintenance are straightforward because there are no moving parts, and filter cloths are easily renewable. Since all joints are external, a plate can be disconnected if any leaks are visible. Thus contamination of the filtrate can be avoided.
6. It produces dry cake in the form of a slab.

Plate And Frame Filter Press Disadvantages:

• It is a batch filter so there is a good deal of ‘down-time’, which is non-productive.
• The filter press is an expensive filter. The emptying time, the labor involved, and the wear and tear of the cloth result in high costs.
• The cake is difficult to remove.
• 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.
• Examples include the collection of precipitated antitoxin and the removal of precipitated proteins from insulin liquors.

Filter Leaf

Filter leaf is the device that is used in the filtration of the solids containing the suspensions and is involved in the separation of the solids from the liquids. They are applied for polishing slurries with a very low solids content of 1 – 5% or for cake filtration with a solid 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.

Selection criteria:

• 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.

Filter leaf Principle:

The principle involved in this type of filtration is the surface mechanism which acts as a sieve or strainer. A vacuum or pressure is applied to increase the rate of filtration.

Filter leaf Construction:

The Leaves:

• 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.

The Vessels:

1. There are two types of vessel configuration:
2. Vertical vessels
3. Horizontal vessels
4. 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.
5. 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.
6. The vessels are fitted with highly secured cake discharge openings to ensure the 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.
7. The head cover of vertical vessels is often pivoted so that it is swung away to allow the upward removal of the leaves in the stack. It is good practice to design a special ring that will support a leaf that is removed from the vessel.
8. 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.

Filter leaf Working:

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.

1. Precoating:
   • 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.
2.  Filtration:
   • 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 to enhance cake permeability.
   • However, it should be kept in mind that the addition of body aid. increases the solid 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 filtered may not be used since they mix and discharge together with the cake.
3.  Heel Removal:
   • 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 the slurry.
4.  Cake Drying: 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.
5. Cake Discharge:
   • 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 backwashed.with water after cake discharge to dislodge and remove any cake residue that adhered to the medium.

Filter leaf Uses:

The leaf filter is satisfactory, if the solids content of the slurry is not too high about 5% that is dilute suspension.

Filter leaf Advantages:

1. 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.
2. This allows thorough washing of the medium with high-impact jets manually in case that the cake bridges between the leaves.
3. On vertical tanks, the head cover must be unbolted and removed to access the leaf stack.
4. Mechanically simple since there are no complex sealing glands or bearings.

Filter leaf Disadvantages:

• High headroom is required for dismantling the leaves on vertical vessels.
• Large floor space is required for discharging the cake on horizontal vessels.
• Maintenance
• The leaf. filter requires attention regularly 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.

Filter leaf Precautions:

• 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 leaf’s neck 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 is monitored 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.

Rotary Drum Filter In Pharmaceutical Engineering In Pharmaceutical Engineering

A rotary vacuum filter drum consists of a drum rotating in a tub of liquid to be filtered. The technique is well suited to slurries, and liquids with a high solid content, which could clog other forms of filter. The drum is pre-coated with a filter aid, typically of diatomaceous earth (DE) or Perlite.

 Rotary Drum Filter Principle:

Rotary drum filters work on the principle or function of filtering the slurry through sieve-like mechanism on a rotating drum surface under the condition of the vacuum. In addition compression drying (using hot air) and removing the filter cake (using a knife) are possible.

 Rotary Drum Filter Construction:

The construction of a rotary drum filter consists of a metal cylinder mounted horizontally. The drum may be up to 3 meters in diameter and 3.5 meters in length and give a surface area, of the 20-meter square. The curved surface is a perforated plate, which supports a filter cloth.

The drum is radially partitioned dividing the annular space into separate compartments. Each of it is connected by an internal pipe to the center of the drum through a rotating valve. Various designs available are belt discharge, scraper discharge, roll discharge, string discharge, and precoat discharge.

 Rotary Drum Filter Working:

After pre-coat has been applied, the liquid to be filtered is sent to the tub below the drum. The drum rotates through the liquid and the vacuum sucks liquid and solids onto the drum pre-coat surface, the liquid portion is “sucked” by the vacuum through the filter media to the internal portion of the drum, and the filtrate is pumped away. The solids adhere to the outside of the drum, which then passes a knife, cutting off the solids and a small portion of the filter media to reveal a fresh media surface that will enter the liquid as the drum rotates. The knife advances automatically as the surface is removed.

Rotary Drum Filter Uses:

• It is a continuous operation and is utilized to filter slurries containing a high proportion of solids up to 15 to 30 percent.
• It is used to extract the penicillin from the mycelium or cell mass by the drum filters.
• These are used for collecting calcium carbonate, starch, and magnesium carbonate.
• A drum filter is a large and typically used in industrial applications to filter liquids carrying high concentrations of suspended solids.
• Perforated drum filters are often used in water treatment plants that remove large amounts of fine sediment from water.
• They can also be used to remove wastewater
  from suspended slurry products.

Rotary Drum Filter Advantages:

• The rotary vacuum drum filter is a continuous and automatic operation, so the operating cost is low.
• The variation of the drum speed rotating can be used to control the cake thickness.
• The process can be easily modified (pre-coating filter process).
• Can produce relatively clean product by adding a showering device.

 Rotary Drum Filter Disadvantages:

• Due to the structure, the pressure difference is limited up to 1 bar.
• Besides the drum, other accessories, for example, agitators and vacuum pump are required.
• The discharge cake contains residual moisture.
• High energy consumption by vacuum

Meta Filters In Pharmaceutical Engineering

Meta filters are the filters that are used to separate very fine-sized particles from the liquid or solid suspensions. It consists of a series of metal rings which are made of stainless steel and drainage grooves.

Meta Filters Principle:

Meta filter functions as the strainer (surface filtration) for the separation of the particles. In this method metal rings contain semicircular projections which are arranged as a nest to form channels on the edge. These channels offer resistance to the flow of solids. The clear liquid is collected into a receiver from the top.

Meta Filters Construction:

It consists of a large number of metal rings packed on a fluted rod. The groove on the surface of the rod provides a channel for the discharge of the filtrate. The rings are made of stainless steel having an internal diameter of about 15 mm and outside diameter of about 22 mm, thickness is about 0.8 mm.

The plate contains several semicircular projections. When the rings are packed on the rod channels are formed in between the plates that are tapered from about 250 mm down to 25 mm. One or more of these packs are mounted in a vessel. The slurry to be filtered is pumped under pressure or a vacuum may also be used. The cake formed can be removed from the outside edge by back flushing of water or by a scraping blade.

Meta Filters Working:

In meta filters, filters are placed in a vessel and may be operated by pumping the slurry under pressure or occasionally by the applications of reduced pressure to the outlet side. The slurry passes through the channels formed on the edge between the rings.

The clear liquid rises and is collected from the outlet into the receiver. Meta filter functions as a strainer. For the separation of the fine particles, a bed of suitable materials such as kieselguhr is first built up. The pack of rings serves essentially as a base on which the true filter medium is supported.

Meta Filters Uses:

• Meta filters are used for the clarification of syrups
• Filtration of the injection solutions.
• Clarification of the insulin liquors.
• Filtration of the viscous liquids can be achieved by applying pressure.

Meta Filters Advantages:

• Very strong, so high pressure can be used, with no danger of bursting the filter medium.
• No filter medium is required,: so running cost is low.
• Meta filter can be made of corrosion-resistant material.
• It is useful for filtering coarse particles. If a filter bed is prepared and then filtration is carried out finer particles can also be filtered.
• Removal of the cake is effectively carried out by back-flushing with water. In automatic cleaning devices, a scrapping bale cleans the outer edge.

Meta Filters Disadvantage:

It is used for low solid content.

Membrane Filters In Pharmaceutical Engineering

A membrane is a thin layer of semi-permeable material that separates substances when a driving force is applied across the membrane. Membrane processes are increasingly used for the removal of bacteria, micro-organisms, particulates, and natural organic material, which can impart color, taste, and odors to water and react with disinfectants to form disinfection by-products.

Membrane Filters Principle:

Membrane filters act just like a sieve and retain the particulate matter along with micro-organisms according to their sizes.

Membrane Filters Construction:

These are plastic membranes based on cellulose acetate, cellulose nitrate, or mixed cellulose esters with pore sizes in the micron or submicron range.  They are very thin (about 120 microns thick) and must be handled carefully. They act like a sieves trapping particulate matter on their surface.

• Several grades of filters are available with pore sizes ranging from 0.010 ± 0.002 microns to 5.0 ± 1.2 microns.
• Type codes VF and SM are given by Millipore Filter Corp. For these two extreme ranges respectively.
• Filters with pore sizes from 0.010 to 0.10 microns can remove virus particles from water or air. Filters with pore sizes from 0.30 to 0.65 microns are employed for removing bacteria.
• Filters with the larger pore sizes, viz. 0.8, 1.2, and 3.0 to 5.0 microns are employed, for example, in aerosol, radioactivity, and particle sizing applications.
• During use membrane filters are supported on a rigid base of perforated metal, plastic or coarse sintered glass as in the case of fibrous pad filters.
• If the solution to be filtered contains a considerable quantity of suspended matter, preliminary filtration through a suitable depth filter avoids clogging of the membrane filter during sterile filtration.
• They are brittle when dry and can be stored indefinitely in the dry state but are fairly tough when wet.

Membrane Filters Uses:

• It is used for sterilization of solutions containing heat-sensitive materials.
• Membrane filters fitted in disc-containing growth media can be used to grow micro¬ organisms.

Membrane Filters Advantages:

• No bacterial growth through the filter takes place during prolonged filtration.
• They are disposable and hence no cross-contamination takes place.
• Adsorption is negligible they yield no fibres or alkali into the filtrate. The filtration rate is rapid.

Membrane Filters Disadvantages:

• They may clog though rarely.
• Ordinary types are less resistant to solvents like chloroform.

Cartridge Filter In Pharmaceutical Engineering

Cartridge filters are defined as fabric or polymer-based filters designed primarily to remove particulate material from fluids. Cartridge filters use a variety of media to remove contaminants, depending on your application.

• The filter media in our cartridge filters encompass a wide range from sand, anthracite, and quartz to conditioned media for iron and manganese removal, and activated carbon.
• Cartridge filters range in style from particulate and high-purity water cartridge filters, to activated carbon filters, vent cartridge filters, and replacement cartridge filters for laboratory usage.
• Cartridge filters have a filtration range from 0.1 up to 500 microns.
• They are manufactured by affixing the fabric or polymer to a central core and they are usually rigid or semi-rigid. Cartridge filters are disposable and easily replaceable.

Cartridge Filter Principle:

A cartridge filter is a thin porous membrane in which the pre-filter and membrane filter are combined into a single unit. The filtration action is mainly sieve-like and the particles are retained on the surface.

The cartridge filter systems are basically of two styles:

1. Smaller systems usually use a single-wound cartridge.
2. Larger systems usually consist of multiple cartridge filters.

Smaller cartridge filter systems such as those used in a home filtering system typically are constructed from some type of plastic or stainless steel. The body of the vessel usually is made of clear plastic (or stainless steel).

The lid usually contains the outlet ports, inlet ports, and pressure relief valve. Taps or ports for pressure gauges may or may not be contained in a smaller cartridge filter system. Larger cartridge filter systems can use either pleated or wound filters and usually use multiple filters in a single housing.

Cartridge Filter System:

• Single Filter System: A single filter system would likely be somewhat rare in a water treatment application. A single filter system would only be applicable for extremely small systems with an extremely high-quality source of water. Home water filter systems are usually single filter systems.
• Prefilter-Post Filter System: It is configured so that the feed water initially passes through a filter with a relatively large pore size and then is filtered through the finer post or final filter.
• Multiple Filter System: A pre-filter-post filter configuration is an extension of the multiple filtration system. Rather than it is having a pre-filter and a post-filter, a multiple filtration system would consist of progressively finer filters plumbed in series.

Cartridge Filter Working:

The slurry is pumped into the cartridge holder. It passes through catridge filter unit by the mechanism of straining. The clear liquid passes to the center and moves up to collect through the outlet.

Cartridge Filter Uses:

• These are used in the filtration of sterile solutions.
• Filtration of beverages.
• Liquid filtration: Bulk chemicals, petrochemicals, water purification, hydraulic fluids, cosmetics/pharmaceuticals, reagent grade chemicals, paints, varnishes, semiconductors, sugars, electric utilities, paints/varnishes often used as final filtration after other filters
• Gaseous filtration: Gas dust removal in industrial, atmosphere, compressed air filtering, atmospheric dust, smoke, fumes, solid contaminants in the system.

Cartridge Filter Advantages:

• Stainless steel construction permits autoclaving for sterile operations.
• Cartridges with self-cleaning devices are advantageous.
• Rapid disassembling as well as reusing of the filter media is possible.
• They are used in line continuous filtration which reduces handling of solutions.
• It minimizes the chance of contamination.

Cartridge Filter Disadvantages:

• The cost of disposable elements offsets the labor-saving in terms of assembly and cleaning of cartridge clarifier.
• Several manufacturers provide components that are generally not interchangeable between suppliers.

Seitz Filter In Pharmaceutical Engineering

These are developed in Germany. Seitz filter consists of a pad of compressed asbestos as a filtering medium. Typical Seitz filter pads are about 2 mm thick and offer a wide range of sizes, they are effective in removing particles of size even less than one micrometer, down to well below one micrometer diameter. The finest pore size gives almost perfect filtration and retains small viruses. When these filters are used for air filtration, the effects of surface charging and electrostatic attraction have a significant influence in the removal of particles

Seitz filter Uses:

• Seitz filters are being used for filtration sterilization.
• The finest pads give almost perfect filtration of small volumes.
• Viscous solutions can also be filtered.
• These filters can be used for air filtration

Seitz filter Advantages:

• Filtration is rapid with fewer tendencies to clog.
• These are better than ceramic and Sintered glass filters for viscous solutions.
• The apparatus is very simple to handle.

Seitz filter Disadvantages:

• Seitz filters are pliable and fragile when wet they must be supported on metal discs.
• A new pad must be used for each filtration to avoid residues of previous filtration.
• Asbestos may shed loose fibers.
• Pad may absorb a sufficient amount of solution or drug

Filtration In Pharmaceutical Engineering Multiple-Choice Questions

Question 1. Which one of the following contains both filters as well as prefilter?

1. Meta filter
2. Rotary drum filter
3. Seitz filter
4. Cartridge filter
5. Answer: 1. Meta filter

Question 2. Which mechanism is involved in meta filter?

1. Cake filtration
2. Depth filtration
3. Surface filtration
4. Zig-Zag filtration

Answer:  3.  Surface filtration

Question 3. Which one of the following is not the property of filter aid?

1. Porous
2. Chemically active
3. Recoverable
4. Removes colour

Answer: 2.  Chemically active

Question 4. Which one of the filters is used for sterile filtration?

1. Meta filter
2. Rotary drum filter
3. Seitz filter
4. Cartridge filter

Answer:  3. Seitz filter

Question 5. Which one of the following gives the dry cake after filtration?

1. Membrane filter
2. Rotary drum filter
3. Seitz filter
4. Cartridge filter

Answer: 2. Rotary drum filter

Question 6. The purpose of using a filter, aid Is ______________

1. To prevent blockage of medium
2. During the filtration of viscous liquids
3. When the particle size is much smaller
4. To hasten the speed of filtration

Answer: 1. To prevent blockage of medium

Question 7. Which filter is used for clarification of, syrups?

1. Drum filter
2. Meta filter
3. Filter leaf
4. Plate and frame

Answer: 2. Meta filter

Question 8. The pores in the stainless steel plates act as channels in one of the following filters?

1. Meta filter
2. Rotary drum filter
3. Seitz filter
4. Cartridge filter

Answer: 1. Meta filter

Question 9. Which mechanism is involved in the plate and frame filter press?

1. Cake filtration
2. Depth filtration
3. Electrostatic filtration
4. Surface filtration

Answer: 4. Surface filtration

Question 10. Which one of the following is not a mechanism of filtration?

1. Entanglement
2. Impact
3. Straining
4. Impingement

Answer: 2. Impact