17 Mechanical separation-2 Filtration
Dibyakanta Seth
28.1 Introduction
Filtration is a unit operation where separation of insoluble solids from a solid-liquid suspension is done with the application of mechanical or gravity force through a porous membrane. The solids are retained in the porous medium and form a layer, called filter cake. The liquid that passes through the porous medium which is free from any solid particles is called as filtrate. The porous medium is known as filter medium. The desired phase can be either cake or filtrate. In filtration of fruit juice, the filtrate is the clear juice which is the required phase. The driving force for the separation of the two phases may be gravity force or mechanical force. Pressure is created at the upstream or vacuum at the downstream to cause the flow of filtrate through the medium.
Clarification is a term used in food industries which is synonymous to filtration; the only difference is that, the suspension contains very few solid particles. Microfiltration is the separation of very tiny particles, which are impossible to separate by normal filtration. The limiting size of the solid particle for microfiltration is 0.1 mm.
28.2 Principle of Filtration
The driving force for filtration is most often the pressure difference. In the beginning of the filtration process, filtrate flows easily through the medium with least resistance. The rate of filtration which is the ratio of filtrate volume and time of filtration is high in the beginning. But, as the filtration progresses, the layer of cake deposition upstream gradually increases. So, now the filtrate not only ought to pass through the medium, but also it should cross the layer of cake. Hence, there is a constant pressure drop across the medium and it increases with time. After certain time, the filtration virtually stops. Two approaches can be made in filtration process. One can either follow filter process at constant pressure drop or at constant flow rate. If constant pressure is adopted, the rate of filtration gradually decreases. If later case is taken, one needs to increase the pressure with time to maintain a constant flow rate. The pressure drop depends on the two phases through which filtrate passes. These are filter cake and filter medium. So, the pressure drop is a function of cake characteristics like specific surface area and porosity and medium characteristics.
The total pressure drop (–ΔP) is the summation of pressure drop across the cake (–ΔPc) and pressure drop across the medium (–ΔPm).
(−∆?) = (−∆??) + (−∆??) … (28.1)
The negative sign indicates the pressure drop from high to low values.
Filtration capacity is the ratio of filtrate volume and the time of filtration cycle. The total filtration time is the addition of filtration time, washing time and transition time for assembling, adjustment of filter etc.
Where, t, tw and ttr are filtration time, washing time and transition time respectively.
28.3 Resistances during filtration
28.3.1 Filter cake resistance
The cake formed upstream contains pore spaces which form tiny channels through which filtrate can able to pass. Therefore, cake acts as a packed bed. Assuming the flow of filtrate inside the cake is in laminar region, we can express the flow by Poiseuille’s equation.
Fig.28.1 shows a section of filter cake and medium with direction of filtrate flow. Let, the thickness of cake is L m, the filter cross sectional area is A m2, the velocity of filtrate is v m/s.
The pressure drop per unit length of cake is given by Poiseuille’s equation as shown below;
As we see here specific cake resistance is a function of void fraction of cake and the specific surface area. It is also affected by the pressure, since pressure can affect void fraction. From the equation (28.8), we can see that, flow rate of filtrate is directly proportional to the pressure drop and inversely proportional to the volume of filtrate. If pressure drop is kept constant, the flow rate decreases with time, since volume of filtrate increases with time.
28.3.2 Filter medium resistance
The resistance offered to the filtrate flow by the medium is called filter medium resistance. The same filtrate which passes through the cake has to pass through the medium. So, an analogy can be built with cake resistance and application of Carmann-Kozeny equation proves to be valid.
The filtrate flow through medium wou ld be;
28.3.4 Compressibility of cake
If the specific cake resistance α does not change with pressure−∆ and with thickness, the cake is said to be incompressible. Rigid solid particles keep the integrity of the cake and do not allow the porosity and specific surface to change by compression applied to the bed. In general, the solid particles are flexible and deformable. So, the cake characteristics changes with pressure. Such cake is called compressible cake. Many empirical formula are used to calculate the specific cake resistance of compressible cakes. Almy and Lewis empirical formula is mostly used.
Here, 0 and n are empirical constants. n is called compressibility constant whose value is zero for incompressible cake. The value of n ranges between 0.1 and 0.8. The values can be found experimentally by plotting a graph between α and ∆ . The slope in logarithmic plot is the compressibility constant and the intercept is the constant a0.
Another formula given by Ruth as follows;
28.4 Constant pressure filtration
The filtration process carried out at constant pressure drop is called constant pressure filtration. In constant pressure filtration the volumetric flow rate decreases with time. So, we need to derive the equation for volumetric flow rate from equation (28.11). By rearranging equation (28.11) we get,
The empirical constants K3 and K4 can be found out analogous to K1 and K2. If pressure drop is plotted against time, a straight line is obtained. The slope of the line will be the constant K3 and the intercept will be K4. The values of α and can be found out from these constants.
28.6 Washing of cake
Washing of cake is an important step in filtration process. Unless it is washed, the porosity of cake decreases which affect the rate of filtration. Washing is carried out by a washing liquid passed through the cake either in the same direction as the filtrate flow or in opposite direction. The solid concentration of wash water initially is high and this concentration decreases with time as
28.7 Filtration Equipments
28.7.1 Depth filters
Depth filter is one where the solids take the space of the pores instead of forming cakes on the surface. Sand filter is a common type of depth filter used to purify water (Fig.28.4). It is suitable for filtration of suspension with less solid concentration. The sand is filled from the bottom in pattern of decreasing size. Flocculants are usually added to water before filtration. The filtration continues till the solid particles fill the sand pores. Back washing is possible only when the suspended particles are non-adhesive to the sand particles.
28.7.2 Surface filters
Cartridge filters are a series of filtering elements attached to a suction pipe. The set of filter is placed on the suspension flowing stream. The perforated element sucks in the filtrate and the suspended solids are left in the stream.
The use is limited to a small solid removal like in separation of solids from water before spraying through nozzle. Continuous rotary vacuum filter Continuous rotary vacuum filter consists of a cylindrical drum rotated horizontally. It contains hollow tubes inside the drum that lead to a central concentric pipe where vacuum is maintained. The filtering medium is wrapped over the drum which is separated by intermittent guide. The filtrate is sucked to the radial channels via the medium. The cake collected on the surface of the medium is scrapped away by a blade mounted at the end of the cycle.
In vacuum filters, the pressure drop is limited to less than 100 kPa. For this reason the vacuum filters are not in the following Fig.28.6: Continuous rotary vacuum filter cases.
- High viscous slurry
- Slurry containing small particles
- Rapid filtration process
Centrifugal filter
Centrifugal filter is basically a basket with perforated wall rotated by a shaft connected centrally. The inner wall is wrapped with filtering medium. When basket is rotated, slurry is forced to move towards the wall. The
28.8 Filter medium and aidThe requirements of filter medium are,
i. It should efficiently remove the suspended solids giving a clear filtrate,
ii. There should not be any clogging of pores during filtration,
iii. The washing of cake from the medium should be easy, and
iv. It should have sufficient strength and chemically inactive to the suspension.
The examples of some industrially used filter media are woollen cloth, glass cloth, paper, felted pads of cellulose, metal cloth and nylon cloth. The ragged fibers of natural materials are used to separate very fine particles.
Filter aids are chemical substances used to increase the porosity of cake. These are primarily composed of silica gel. In some cases cellulose, asbestos and other inert porous solids are used. The filter aids can be used either by mixing it to the suspension before filtration or by putting a layer of these materials over the medium as precoat. The limitation of the application of the aid is in the filtration process where cake is discarded.
References
- Unit Operations in Food Engineering (1st Edition), Albert Ibarz and Gustavo V. Barbosa-Canovas, CRC Press, NY, Publ., 2003.
- Food Process Engineering and technology, Zeki Berk, Elsevier Inc. New York., Publ., 2009.
- Fundamentals of Food Process Engineering (3rd Edition), Romeo T. Toledo, Springer Science, 2007.