Bioseparation Chapter 10 Filtration Dr. Tarek Elbashiti Assoc. Prof. of Biotechnology

Introduction Filtration is a separation process in which a solid-liquid mixture called the feed (or the suspension) is forced through a porous medium on which the solids are deposited or in which they are entrapped. The porous medium which allows the liquid to go through while retaining the solids is called the filter. The retained solid is called "the residue" or "the cake". The clarified liquid is called "the effluent" or the "filtrate". Filtration can be broadly classified into three categories (see Fig. 10.1).

If recovery of solids from high solid content slurry is desired, the process is called cake filtration. The term clarification is applied when the solid content in the feed does not exceed 1 wt %. In a clarification process the filtrate is the primary product. The third type of filtration is called cross-flow filtration in which the liquid flows parallel to the filtration medium. Cross-flow filtration is mainly used for membrane filtration and will be discussed in details in the chapter on membrane based bioseparation processes.

Some of the applications of filtration in the bio-industry are: 1. Recovery of crystalline solids 2. Recovery of cells from fermentation medium 3. Clarification of liquids and gases 4. Sterilisation of liquids Theory of filtration: There are two main mechanisms by which solids are retained by a filter (see Fig. 10.2): Surface filtration: The particles are retained by a screening action and held on the external surface of the filter. The particles are not allowed to enter the filtration medium. Cake filtration and cross-flow filtration are based on surface filtration.

Depth filtration: Particles are allowed to penetrate pores and pore networks present in the filtration medium. They are retained within the filter by three mechanisms: direct interception, inertial impaction and diffusional interception. In direct interception the particles enter the pores or pore networks within the filtration medium and get trapped where the pore diameter becomes equal to the particle diameter. Particles whose diameters are significantly smaller than the pore diameter get trapped where pores are already constricted by collected particles.

Particles being carried by fluids possess momentum on account of their mass and velocity. Pores present in most filtration media are tortuous in nature. The flow of fluids through these pores is usually laminar in nature on account of their small diameters. At tortuous sections of pores the fluid streamlines follow the curves while particles due to their inertia continue to move straight and as a result hit the filter medium, lose their momentum and are retained. This collection mechanism which is referred to as inertial impaction is important for particles larger than 1.0 μm in diameter.

Diffusional interception is more relevant to filtration of gases. The gas molecules, due to their random motion continually bombard suspended particles, particularly those smaller than 0.3 μm in diameter. The suspended particles therefore deviate from their streamlines and impact on the filter medium. Once this happens, the particles lose their momentum and are retained. Most clarification processes rely of depth filtration.

Filter medium: The function of a filter medium is primarily to act as an impermeable barrier for particulate matter. In clarification processes the filtration medium is usually the only barrier present. At the beginning of a cake filtration process, the role of the filter medium is to act as a barrier. However, once the cake formation commences, the cake becomes the main particle-retaining barrier and the role of the filter medium is mainly as a support for the cake. The filter medium should have sufficient mechanical strength, should be resistant to corrosive action of fluids being processed and should offer low resistance to the flow of filtrate.

Commonly used filter media are: 1. Filter paper 2. Woven material (e.g. cheese cloth, woven polymer fiber, woven glass fiber) 3. Non-woven fiber pads 4. Sintered and perforated glass 5. Sintered and perforated metal 6. Ceramics 7. Synthetic membranes

Driving force Filtration is driven by applying a pressure drop across the filter medium. The driving force can be applied by pressurizing the feed side (i.e. positive pressure filtration or simply pressure filtration) or by creating a vacuum in the filtrate side (i.e. negative pressure filtration or vacuum filtration). These two types of filtration are shown in Fig. 10.3. In the industry, both pressure and vacuum filtration are used. Pressure filtration can be driven by pressurizing the feed using compressed air pressurization or by maintaining a hydrostatic liquid head on the feed side.

The feed can also be pressurized with a suitable pump. Vacuum filtration is commonly used in the laboratory since pressure vessels are not required. Vacuum can also be easily generated using a water jet injector or a vacuum pump. Vacuum filtration is preferred from a safety point of view since explosion is more hazardous than implosion. However, with vacuum filtration the maximum pressure drop is restricted to 1 atmosphere. Also substances that form foam cannot be filtered by vacuum filtration.

Constant pressure cake filtration Constant pressure filtration refers to a filtration process where the driving force (i.e. the pressure drop across the filter medium) is kept constant. Constant rate cake filtration Constant rate filtration refers to a filtration process where the filtration rate is kept constant by appropriately adjusting the pressure drop during the process.

Improvement of filtration efficiency The efficiency of cake filtration depends on the achieving high cake accumulation on the filter medium. However, the filtration rate declines with cake accumulation due to the increase in cake resistance. One way to solve this problem is to alter cake properties such that the specific cake resistance is reduced. This can be achieved by: Feed pre-treatment The feed can be pre-treated by physical methods (e.g. heating) or by addition of chemicals (e.g. coagulants, flocculants) to obtain a porous cake with low specific cake resistance.

However, thermolabile substance cannot be heated. Moreover addition of coagulants and flocculants might not be possible in some applications. Filter aids Filter aids are substances that are mixed with the feed for creating very porous cakes. This increases the filtration rate very significantly. The filter aids which are particulate in nature can later be removed from the dried and powdered cake by suitable separation techniques (e.g. sieving). However, in certain cases it might not be possible to completely remove the filter aid and their use is restricted.

Filter aids are rarely used when the cake is the product of interest. Certain deformable substances present in the feed can block the pores within the filtration medium. When such substances are being filtered, it might be a good idea to precoat the medium with a layer of filter aid.

Mode of operation: Filtration can be carried out in different ways: Cake accumulation and removal in batch mode This is the commonest mode for small-scale cake filtration. A batch of feed is pumped into the filter unit and filtration is carried out either at constant rate or at constant pressure. The process is terminated when the filtration rate gets unacceptably low, or when the pressure required gets too high, or when the filtration device is filled with the filter cake. The cake is then removed from the device by scraping it off the filter medium. Often this requires dismantling of the filtration unit.

The filter medium is usually then cleaned and made ready for the next batch. The general scheme for this mode of operation is shown in Fig. 10.6. Examples of filtration devices operated in this mode include: 1. Funnel filter 2. Filter press 3. Leaf pressure filter 4. Vacuum leaf filter

Continuous cake accumulation and removal A batch process may not be suitable for large-scale cake filtration. Continuous filters which allow simultaneous cake accumulation and removal are usually used for large-scale processes. Examples of such devices include: 1. Horizontal continuous filter 2. Rotary drum filter

Slurry concentration by delayed cake filtration When the objective of the filtration process is to thicken the slurry, the build-up of cake on the filter medium is avoided. Slurry thickening can be achieved by controlling the thickness of the cake layer thereby keeping the particulate matter in a suspended form on the feed side. This can be achieved by incorporating mechanical devices such as moving blades, which continuously scrape of the cake from the filter surface. With the moving blade arrangement, the thickness of the cake is limited by the clearance between the filter medium and the blade. This type of filtration can be carried out until the solid content on the feed side reaches a critical level beyond which the slurry does not flow.

Slurry concentration by cross-flow filtration An alternative way by which the build-up of cake on the filter can be discouraged is by using a cross-flow mode of operation. This is achieved by maintaining a very high velocity of feed flow parallel to the surface of the filter medium. Typical cross-flow rates may be 1 0 - 2 0 time the filtration rate. However, cross flow filtration cannot be used for obtaining very thick slurry since the energy required for maintaining the cross-flow velocity becomes prohibitively high.

Cake washing: After its formation, the cake may contain a significant amount of entrapped liquid. When the liquid is the product of interest, this entrapment represents a loss of yield. When the cake is itself the product, the entrapped liquid represents the presence of impurity. The entrapped liquid can be removed by cake washing. The washing liquid should itself not be a "new impurity“. Its presence should either be acceptable in the final product (i.e. either the cake or the filtrate), or it should be easily "removable". If the product is soluble in the washing liquid, as often is the case, the duration of the washing process depends on a trade-off between the amount lost and the purity desired.

Filtration equipment: Filter Press A filter press consists of a series of horizontally arranged vertical filter elements, each consisting of a frame within which the cake can be accumulated sandwiched between filter medium on either side. Each filter medium is supported on a plate which has grooves to allow easy collection of the filtrate. The "press" refers to the external structure which provides the necessary force to seal each filter element and supports the plate and frame assembly. Fig. 10.7 shows the individual plates used in a filter press.

Both faces of each plate are covered with filter medium and together with the frames these form a series of chambers into which the feed is introduced under pressure. The filter medium retains suspended solids and the filter cake builds up within the chamber. When the filtration cycle is complete the pressure holding the system together is released and the filter plates are separated to remove the cake from within the frames. The operation of a plate and frame filter press is summarized in Fig. 10.8.

Plate and Frame Filters

A plate and frame filter press is mainly used for cake filtration and cake washing. It could also be used for an extended clarification process but this is rarely done. It is frequently used for solid-liquid extraction (or leaching). It is a common sight in the chemical, pharmaceutical, food, metallurgical and ceramic industries. The main advantages of this device are its compact design and its high throughput. Disadvantages include high labor cost (due to the need for assembling and subsequent dismantling), high down times, capacity limitation and batch-wise operation.

Rotary Drum Vacuum Filter A rotary drum vacuum filter is a continuous filtration device in which the solids are separated by a porous filter cloth or similar filtration media wrapped around a drum-like structure with a perforated curved surface. The drum is rotated, partially submerged, through a feed solution held in a trough and vacuum applied on the inside. The filtrate flows through the filter media to the inside of the drum and the cake accumulates on the outside. The working principle of a rotary drum filter is shown in Fig. 10.9. At any given location on the filter, the cake layer builds up as it moves through the feed.

Rotary Vacuum Filters

As this emerges from the feed, the vacuum continues to draw the liquid from the cake, dewatering it in the process. If required, the cake can be washed by spraying it with a wash liquid and further dewatered. The semi-dry cake is then removed from the filter medium by using a fixed knife or a cutting wire. A rotary vacuum filter is mainly used for cake filtration, cake washing and de-watering in the chemical pharmaceutical, metallurgical and ceramic industries. These are also used for municipal wastewater treatment. Advantages include continuous operation and the possibility of cake drying. Disadvantages include low driving force (due to being vacuum driven) and complicated design with many moving parts and seals.

Pressure leaf filter A pressure leaf filter consists of a number of rectangular basic filtration units (also called leaves) connected together in parallel by means of flexible hose or a rigid tube manifold (Fig. 10.10). Each leaf is made up of a light metal frame made from wire mesh. These leaves are covered with filter cloth or woven wire cloth. The leaf assembly is housed within a pressure vessel into which the feed is pumped at high pressure. The filtration is driven by pressure and the cake is accumulated within the pressure vessel.

A pressure leaf filter may be used for cake filtration and clarification. Advantages include simplicity of design and flexibility of use. Disadvantages include high labor cost and high equipment footprint.

Pressure leaf filters

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