ERT 313 :
BIOSEPARATION ENGINEERING
Mechanical - Physical Separation Process
“1. FILTRATION”
By;MrsHafizaBintiShukor
By; Mrs Hafiza Binti Shukor
ERT 313/4 BIOSEPARATION ENGINEERING
SEM 2 (2010/2011)
ERT 313/4 BIOSEPARATION ENGINEERING
SEM 2 (2010/2011)
Students should be able to;Students should be able to;
APPLY and CALCULATE based onfiltration principles; ANALYZE cakefiltration, Constant Pressure Filtration,Continuous Filtration and Constant RateFiltration.
ERT 313/4 BIOSEPARATION ENGINEERING
SEM 2 (2010/2011)
IntroductionIntroduction
•Filtration is a solid-liquid separation where theliquid passes through a porous medium to removefine suspended solids according to the size byflowing 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
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2 categories of filtration, which differ according to thedirection of the fluid feed in relation to the filtermedium.
Results in a cake of solidsdepositing on the filter medium
Minimize buildup of solids on thefilter medium
ERT 313/4 BIOSEPARATION ENGINEERING
SEM 2 (2010/2011)
Application of Filtration inBio-industryApplication of Filtration inBio-industry
Recovery of crystalline solids
Recovery of cells from fermentationmedium
Clarification of liquid and gasses
Sterilization of liquid for heat sensitivecompound
ERT 313/4 BIOSEPARATION ENGINEERING
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Filtration EquipmentFiltration Equipment
Filtration for biological materials is generally completed using batchfiltration, rotary drum filtration, or ultrafiltration methods.
1. Batch Filtration
•Usually performed under constant pressure with a pump thatmoves the broth or liquor through the filter
•Filter cake will build-up as filtration proceeds and resistanceto broth flow will increase
•The filter press is the typical industrial version of a batchvacuum filter, using a plate and frame arrangement
•Can be used to remove cells, but does not work particularlywell for animal cell debris or plant seed debris
ERT 313/4 BIOSEPARATION ENGINEERING
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Cont….Filtration EquipmentCont….Filtration Equipment
2. Rotary Drum Filtration
•Rotary vacuum filters can be used to efficiently removemycelia, cells, proteins, and enzymes, though a filter aid orprecoat of the septum may be necessary
3. Ultrafiltration
•Utilizes a membrane to separate particles that are much largerthan the solvent used
•Successful removal occurs in the partical size range of 10solvent molecular diameters to 0.5 μ
ERT 313/4 BIOSEPARATION ENGINEERING
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Filter MediaFilter Media
•To act as an impermeable barrier for particulate matter.
•Filtration media for cross-flow filtration are generally referredas “MEMBRANE”
•First and foremost, it must remove the solids to be filteredfrom the slurry and give a clear filtrate
•Also, the pores should not become plugged so that the rate offiltration becomes too slow
•The filter medium must allow the filter cake to be removedeasily and cleanly
•Some widely used filter media (for conventional filtration) likefilter paper, ceramics, synthetic membrane, sinterd &perforated glass, woven materials (woven polymer fiber).
ERT 313/4 BIOSEPARATION ENGINEERING
SEM 2 (2010/2011)
Filter AidsFilter Aids
•Substance (solid powder)that are mixed with the feed forcreating very porous cakes ( increase filtration rate verysignificantly)
•Can be added to the cake during filtration to increasesthe porosity of the cake and reduces resistance of thecake 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 diatomiteparticles gives them a highintrinsic permeability
ERT 313/4 BIOSEPARATION ENGINEERING
SEM 2 (2010/2011)
Filtration PrinciplesFiltration Principles
•When a slurry containing suspended solids flow against a filter medium by the application of apressure 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 isthe pressure drop through the bed of solids (medium plus cake), µ0 is the viscosity of the filtrate, and R isthe resistance of the porous bed. In this case, R is a combination of the resistance Rm of the filtermedium and the resistance Rc of the cake solids:
•It is convenient to write the cake resistance Rc in terms of specific cake resistance α as follows:
•where ρc 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
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•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 andmedium resistance, Rm by plotting the graph)
In a cake filtration process where a significant amount of cake is allowed toaccumulate, the medium resistance, Rm become neglegible compare witn the cakeresistance. (Rm=0). So,
Incompressible CakeIncompressible Cake
(5)
ERT 313/4 BIOSEPARATION ENGINEERING
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Example 1Example 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
Rm. 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 m2 and vacuum pressure of
500 mm Hg.
TABLE E1
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Solution
According to Equation (5), we can plot t/(V/A) versus V/A and obtain α from the slope
and Rm 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 1Example 1
Figure E1
Plot of batchfiltration datafor thedeterminationof α and Rm.
(5)
ERT 313/4 BIOSEPARATION ENGINEERING
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From these values, we can calculate α and Rm:
This is a typical value of Rm 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 m2, 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 1Example 1
(5)
ERT 313/4 BIOSEPARATION ENGINEERING
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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 1Example 1
(5)
ERT 313/4 BIOSEPARATION ENGINEERING
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•Almost all cakes formed for biological material are compressible.
•As these cake compressed, filtration rate drop (flow become relatively moredifficult 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 Δpc, the pressure dropsacross the cake
•Studies have shown that the relationship between specific resistance and pressuredrop commonly takes the form:
•where α’ and s are empirical constants.
•The power s has been called the “cake compressibility factor”. (for incompressiblecake, s=0 and for compressible cake, s=0.1-0.8)
Compressible CakeCompressible Cake
(6)
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Cake WashingCake Washing
•After filtration, the cake contains a significant amount ofsolute-rich liquid broth that usually removed by washingthe cake
•2 function of washing:
A) displaces the solute-rich broth trapped in pores inthe cake
B) allows diffusion of solute out of the biomass inthe cake(enhance recovery if the desiredproduct is in the biomass)
ERT 313/4 BIOSEPARATION ENGINEERING
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•It is often necessary to wash the filter cake with water or a salt solution to maximize theremoval of dissolved product from the cake.
•Frequently, the wash must be done with more than the volume of the liquid in the cakebecause some of the product is in stagnant zones of the cake, and transfer into the washliquid from these zones occurs by diffusion, which takes place at a slower rate than theconvective flow of wash through the cake
•Data for the washing of the filter cakes has been correlated by Choudhary and Dahlstromusing the following equation:
•where R’ is the weight fraction of solute remaining in the cake after washing (on the basisthat R’ = 1.0 prior to washing), E is the percentage wash efficiency, and n is the volume ofwash liquid per volume of liquid in the unwashed cake.
•Assuming that the liquid viscosity and the pressure drop through the bed solids are the sameduring the filtration of the solids, the washing rate per cross-sectional area can be foundfrom 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 Vf at the end of time tf toform the cake, this yields
(7)
(8)
ERT 313/4 BIOSEPARATION ENGINEERING
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•If Vw is the volume of wash liquid applied in time tw, then
•Using the definition of (dv/dt)V=Vf from Eq. (8), we obtain
•At the end of filtration, the integrated form of the filtration equation (Eq. 5), with Rmneglected, can be written
•Substituting this expression for Vf/A in Eq. (10) and simplifying gives
Filtration PrinciplesFiltration Principles
(9)
(10)
(11)
(12)
ERT 313/4 BIOSEPARATION ENGINEERING
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•From Eq. (11) and (12), the ration of tw to tf is
•It is helpful to write tw/tf in terms of n, the ration of the volume Vw of wash liquid to thevolume Vr of residual liquid in the cake:
•where f is the ratio of Vr to the volume Vf of filtrate at the end of filtration.
•The ratio f can be determined by a material balance
•Thus, for a given cake formation time tf, a plot of wash time tw versus wash ratio n will be astraight line
Filtration PrinciplesFiltration Principles
(13)
(14)
ERT 313/4 BIOSEPARATION ENGINEERING
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Example 2Example 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 beneglected.
Solution:
We can use the integrated form of the filtration equation, Equation (5), with Rm = 0:
We solve for A2 to obtain
In applying this equation, it is helpful to focus on the area of the drum, which is where the cakeis being formed and where filtrate is being obtained.
ERT 313/4 BIOSEPARATION ENGINEERING
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We use this volume of filtrate with t = 15 s in the equation for A2 toobtain
The area A’ of the entire rotary vacuum filter can be calculated fromthe cake formation time (15s) and the total cycle time (60s) as
This is a medium-sized rotary vacuum filter, with possible dimensions of1.0 m diameter by 1.0 m long.
Thus, A is the area of that part of the drum. We can calculate thevolume of filtrate that needs to be collected during the cakeformation time of 15 s:
ERT 313/4 BIOSEPARATION ENGINEERING
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Example 3Example 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 tw, and the cake
formation time tf is given by
ERT 313/4 BIOSEPARATION ENGINEERING
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where f is the ratio of volume Vr of residual liquid in the cake to thevolume of filtrate Vf
after time tf. Thus, we need to estimate the volume of residual liquid inthe filter cake to determine tw. At the end of the 15 s cake formationtime,
Assuming the cake is 70 wt% water, which is typical for filter cakes, wefind
Thus,
Cake solids per volumeof filtrate
Volume of filtrate need to becollected during the cake formationtime of 15s
ERT 313/4 BIOSEPARATION ENGINEERING
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The End