ERT 313 : BIOSEPARATION ENGINEERING Mechanical - Physical Separation Process “1. FILTRATION” By; Mrs Haf iza Bint i Shu kor ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011)

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) Students should be able to; APPLY and CALCULATE based on filtration principles; ANALYZE cake filtration, Constant Pressure Filtration, Continuous Filtration and Constant Rate Filtration.

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) Introduction Filtration is a solid-liquid separation where the liquid passes through a porous medium to remove fine suspended solids according to the size by flowing under a pressure differential. The main objective of filtration is to produce high- quality drinking water (surface water) or high- quality effluent (wastewater)

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) 2 categories of filtration, which differ according to the direction of the fluid feed in relation to the filter medium. Results in a cake of solids depositing on the filter medium Minimize buildup of solids on the filter medium

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) Application of Filtration in Bio-industry  Recovery of crystalline solids  Recovery of cells from fermentation medium  Clarification of liquid and gasses  Sterilization of liquid for heat sensitive compound

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) Filtration Equipment Filtration for biological materials is generally completed using batch filtration, rotary drum filtration, or ultrafiltration methods. 1. Batch Filtration Usually performed under constant pressure with a pump that moves the broth or liquor through the filter Filter cake will build-up as filtration proceeds and resistance to broth flow will increase The filter press is the typical industrial version of a batch vacuum filter, using a plate and frame arrangement Can be used to remove cells, but does not work particularly well for animal cell debris or plant seed debris

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) Cont….Filtration Equipment 2. Rotary Drum Filtration Rotary vacuum filters can be used to efficiently remove mycelia, cells, proteins, and enzymes, though a filter aid or precoat of the septum may be necessary 3. Ultrafiltration Utilizes a membrane to separate particles that are much larger than the solvent used Successful removal occurs in the partical size range of 10 solvent molecular diameters to 0.5 μ

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) Filter Media To act as an impermeable barrier for particulate matter. Filtration media for cross-flow filtration are generally referred as “MEMBRANE” First and foremost, it must remove the solids to be filtered from the slurry and give a clear filtrate Also, the pores should not become plugged so that the rate of filtration becomes too slow The filter medium must allow the filter cake to be removed easily and cleanly Some widely used filter media (for conventional filtration) like filter paper, ceramics, synthetic membrane, sinterd & perforated glass, woven materials (woven polymer fiber).

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) Filter Aids Substance (solid powder)that are mixed with the feed for creating very porous cakes ( increase filtration rate very significantly) Can be added to the cake during filtration to increases the porosity of the cake and reduces resistance of the cake during filtration Can also be added directly to the feed to: i) maintain the pores in the filter cake open ii) Make the cake less compressible iii)Provide faster filtration Common types of filter aid is diatomite (types of algae) and perlite. The structure of diatomite particles gives them a high intrinsic permeability

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) Filtration Principles When a slurry containing suspended solids flow against a filter medium by the application of a pressure gradient across the medium, solids begin to build up on the filter medium The buildup of solids on the filter medium is called a cake This type of filtration is sometimes referred to as “dead-end” filtration Darcy’s law describes the flow of liquid through a porous bed of solids and can be written as follows: where V is the volume of filtrate, t is time, A is the cross-sectional area of exposed lilter medium, Δp is the pressure drop through the bed of solids (medium plus cake), µ 0 is the viscosity of the filtrate, and R is the resistance of the porous bed. In this case, R is a combination of the resistance R m of the filter medium and the resistance R c of the cake solids: It is convenient to write the cake resistance R c in terms of specific cake resistance α as follows: where ρ c is the mass of dry cake solids per volume of filtrate. Thus, the resistance increases with the volume filtered Combining Eq. (1), (2) and (3), we obtain (1) (2) (3) (4)

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) For the case of zero filtrate at time zero (before start an exp), integration of this equation yields where and (can determine specific cake resistance,α and medium resistance, Rm by plotting the graph) In a cake filtration process where a significant amount of cake is allowed to accumulate, the medium resistance, Rm become neglegible compare witn the cake resistance. (Rm=0). So, Incompressible Cake (5)

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) Example 1 Batch Filtration A Buchner funnel 8 cm in diameter is available for testing the filtration of a cell culture suspension, which has a viscosity of 3.0 cp. The data in Table E1 were obtained with a vacuum pressure of 600 mm Hg applied to the Buchner funnel. The cell solids on the filter at the end of filtration were dried and found to weigh 14.0 g. Determine the specific cake resistance α and the medium resistance R m. Then estimate how long it would take to obtain 10,000 liters of filtrate from this cell broth on a filter with a surface area of 10 m 2 and vacuum pressure of 500 mm Hg. TABLE E1

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) Solution According to Equation (5), we can plot t/(V/A) versus V/A and obtain α from the slope and R m from the intercept. We see that the data are reasonably close to a straight line. A linear regression of the data in this plot gives the following results (Figure E1): Example 1 Figure E1 Plot of batch filtration data for the determination of α and R m. (5)

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) From these values, we can calculate α and R m : This is a typical value of R m for a large-pore (micrometer-sized) filter. To determine the time required to obtain 10,000 liters of filtrate using a filter with an area of 10 m 2, we must make the assumption that α does not change at the new pressure drop of 500 mm Hg. We use Equation (5) and solve for time: Example 1 (5)

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) We calculate the two components of this equation as follows: and finally Thus, this filter is probably undersized for the volume to be filtered. In addition, from this calculation we see that at the end of the filtration, Therefore, the filter medium is contributing very little of the resistance to filtration, a typical situation in a lengthy dead-end filtration. Example 1 (5)

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) Almost all cakes formed for biological material are compressible. As these cake compressed, filtration rate drop (flow become relatively more difficult as pressure increase) The pressure drop is influence by α, the specific cake resistance α can be increased if the cake is compressed The specific resistance of the cake is directly affected by Δp c, the pressure drops across the cake Studies have shown that the relationship between specific resistance and pressure drop commonly takes the form: where α’ and s are empirical constants. The power s has been called the “cake compressibility factor”. (for incompressible cake, s=0 and for compressible cake, s= ) Compressible Cake (6)

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) Cake Washing After filtration, the cake contains a significant amount of solute-rich liquid broth that usually removed by washing the cake 2 function of washing: A) displaces the solute-rich broth trapped in pores in the cake B) allows diffusion of solute out of the biomass in the cake(enhance recovery if the desired product is in the biomass)

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) It is often necessary to wash the filter cake with water or a salt solution to maximize the removal of dissolved product from the cake. Frequently, the wash must be done with more than the volume of the liquid in the cake because some of the product is in stagnant zones of the cake, and transfer into the wash liquid from these zones occurs by diffusion, which takes place at a slower rate than the convective flow of wash through the cake Data for the washing of the filter cakes has been correlated by Choudhary and Dahlstrom using the following equation: where R’ is the weight fraction of solute remaining in the cake after washing (on the basis that R’ = 1.0 prior to washing), E is the percentage wash efficiency, and n is the volume of wash liquid per volume of liquid in the unwashed cake. Assuming that the liquid viscosity and the pressure drop through the bed solids are the same during the filtration of the solids, the washing rate per cross-sectional area can be found from the filtrate flow rate per unit area given in Equation (4) at the end of the filtration Thus, for negligible filter medium resistance for filtrate volume V f at the end of time t f to form the cake, this yields (7) (8)

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) If V w is the volume of wash liquid applied in time t w, then Using the definition of (dv/dt) V=V f from Eq. (8), we obtain At the end of filtration, the integrated form of the filtration equation (Eq. 5), with R m neglected, can be written Substituting this expression for V f /A in Eq. (10) and simplifying gives Filtration Principles (9) (10) (11) (12)

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) From Eq. (11) and (12), the ration of t w to t f is It is helpful to write t w /t f in terms of n, the ration of the volume V w of wash liquid to the volume V r of residual liquid in the cake: where f is the ratio of V r to the volume V f of filtrate at the end of filtration. The ratio f can be determined by a material balance Thus, for a given cake formation time t f, a plot of wash time t w versus wash ratio n will be a straight line Filtration Principles (13) (14)

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) Example 2 Rotary Vacuum Filtration It is desired to filter a cell broth at a rate of 2000 liters/h on a rotary vacuum filter at a vacuum pressure of 70 kPa. The cycle time for the drum will be 60 s, and the cake formation time (filtering time) will be 15 s. The broth to be filtered has a viscosity of 2.0 cp and a cake solids (dry basis) per volume of filtrate of 10 g/liter. From laboratory tests, the specific cake resistance has been determined to be 9 x 10 cm/g. Determine the area of the filter that is required. The resistance of the filter medium can be neglected. Solution: We can use the integrated form of the filtration equation, Equation (5), with R m = 0: We solve for A 2 to obtain In applying this equation, it is helpful to focus on the area of the drum, which is where the cake is being formed and where filtrate is being obtained.

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) We use this volume of filtrate with t = 15 s in the equation for A 2 to obtain The area A’ of the entire rotary vacuum filter can be calculated from the cake formation time (15s) and the total cycle time (60s) as This is a medium-sized rotary vacuum filter, with possible dimensions of 1.0 m diameter by 1.0 m long. Thus, A is the area of that part of the drum. We can calculate the volume of filtrate that needs to be collected during the cake formation time of 15 s:

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) Example 3 Washing of a Rotary Vacuum Filter Cake For the filtration in Example 2, it is desired to wash a product antibiotic out of the cake so that only 5% of the antibiotic in the cake is left after washing. We expect the washing efficiency to be 50%. Estimate the washing time per cycle that would be required. Solution; From Equation (7) for the washing efficiency of a filter cake where R’ is the weight fraction of solute remaining in the cake after washing (on the basis of R’ = 1.0 before washing), E is the percentage wash efficiency, and n is the volume of wash liquid per volume of liquid in the unwashed cake. Substituting R’ = 0.05 and E = 50% into this equation gives From Equation (14), the relationship between the washing time t w, and the cake formation time t f is given by

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) where f is the ratio of volume V r of residual liquid in the cake to the volume of filtrate V f after time t f. Thus, we need to estimate the volume of residual liquid in the filter cake to determine t w. At the end of the 15 s cake formation time, Assuming the cake is 70 wt% water, which is typical for filter cakes, we find Thus, Cake solids per volume of filtrate Volume of filtrate need to be collected during the cake formation time of 15s

ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011) The End