5.membrane based bioseparation-purification

7/31/2019 5.Membrane Based Bioseparation-purification

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MEMBRANEBASEDBIOSEPARATION

Recovery & Isolation


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DEFINITION

A membrane is a thin semi-permeable barrier which

can be used for the following types of separation1. Particle-liquid separation

2. Particle-solute separation

3. Solute-solvent separation

4. Solute-solute separation


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FACTORSUTILIZEDINMEMBRANEBASEDSEPARATION,

1. Solute size

2. Electrostatic charge

3. Diffusivity

4. Solute shape


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Purification


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MODEOF MEMBRANE SEPARATION

Spiral Wound

Hollow Fibre
http://en.wikipedia.org/wiki/File:Spiral_flow_membrane_module-en.svg


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FILTRATION MODE


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MICROFILTRATION


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MICROFILTRATION

Microfiltration membranes are microporous and retainparticles by a purely sieving mechanism.

Typical permeate flux values are higher than inultrafiltration processes even though microfiltration isoperated at much lower TMP.

A microfiltration process can be operated either in a

dead-end (normal flow) mode or cross-flow mode


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APPLICATIONSOF MICROFILTRATIONINDOWNSTREAMPROCESSING

1. Cell harvesting from bioreactors

2. Virus removal for pharmaceutical products

3. Clarification of fruit juice and beverages

4. Water purification

5. Air filtration

6. Sterilization of products


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TRANSPORT EQUATION

cMV RRP

J

c

M

sc r

A

VrR

c = Cake layer

For micron sized particles

23

11180

sd

r


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EXAMPLE

Bacterial cells having 0.8 micron average diameter arebeing microfiltered in the cross-flow mode using amembrane having an area of 100 cm2. The steady statecake layer formed on the membrane has a thickness of10 microns and a porosity of 0.35. If the viscosity of the

filtrate obtained is 1.4 centipoise, predict the volumetricpermeate flux at a transmembrane pressure of 50 kPa.When pure water (viscosity = 1 centipoise) was filteredthrough the same membrane at the same

transmembrane pressure, the permeate flux obtainedwas 10-4 m/s.


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SOLUTION


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BATCH FILTRATION (INCOMPRESSIBLE CAKE)

dt

dV

A

Jv1

cM RR

P

dt

dV

A

1

A

VR

oc

o = mass of cake solids per volume of filtrate

MAVo RP

dtdV

A

1

PR

AV

PVAt Mo

2

PA

VR

A

V

PtMo

2

2

y = mx+c

t=0, V=0

V= total volume of filtrate


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EXAMPLE2

A broth containing the yeast was filtered using microfilter and its timeneeded to collect the volume of filtrate is as below

Filtration Time (sec) Volume of filtrate (ml)

2 20

10 56

25 90

50 175

The microfilter has a total area of 9 x 10-3 m2 and the filtrate has aviscosity of 1.15 cP. The pressure drop is 68,000 Pa and the feedcontains 0.01 kg dry cake per liter

Determine the specific cake and membrane resistance


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ULTRAFILTRATION


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ULTRAFILTRATION

UF membranes can retain macromolecular solutes.

Solute retention is mainly determined by solutesize.

Other factors such as solute-solute and solute-membrane interactions can affect solute retention.

Ultrafiltration is used for:

Concentration of solutes

Purification of solvents

Fractionation of solutes

Clarification


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ULTRAFILTRATIONINDOWNSTREAMPROCESSING

UF is widely used for processing:

therapeutic drugs,

enzymes,

hormones,

vaccines,

blood products

antibodies.


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ULTRAFILTRATION

The major areas of application are listed below:

Purification of proteins and nucleic acids

Concentration of macromolecules

Desalting, i.e. removal or salts and other low molecular

weight compounds from solution of macromolecules

Virus removal from therapeutic products


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ULTRAFILTRATION

The ability of an ultrafiltration membrane to retainmacromolecules is traditionally specified in terms ofits molecular cut-off (MWCO).

A MWCO value of 10 kDa means that themembrane can retain from a feed solution 90% ofthe molecules having molecular weight of 10 kDa.

Ultrafiltration separates solutes in the molecularweight range of 5 kDa to 500 kDa. UF membranes

have pores ranging from 1 to 20 nm in diameter.


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TRANSPORT EQUATIONS

The solvent flow is proportional to the applied force

(solvent velocity) (force on solvent)

PLj pv Lp is the solvent permeability

Include the osmotic pressure

PLj pv

PLj pv

For solute leak through the membrane

is the reflection coefficient, 0 <


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is osmotic pressure which can be determined from its solutionconcentration

RTC


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GOVERNINGLAWSDARCYLAW

Darcy's law is a simple proportional relationship

between the instantaneous discharge ratethrough a porous medium, the viscosity of thefluid and the pressure drop over a given distance.

The total discharge, Q(units of volume per time, e.g., m3/s) isequal to the product of the permeability of the medium, k(m2),the cross-sectional area to flow, A (units of area, e.g., m2), andthe pressure drop (Pa), all divided by the viscosity, (Pa.s)

and the length the pressure drop is taking place over.


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MODEL-POISEUILLEFLOW

The flow of a solvent through ultrafiltrationmembranes can be described in terms of a poreflow model which assumes ideal cylindrical poresaligned normal to the membrane surface:


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TRANSMEMBRANE PRESSURE

The transmembrane pressure in cross-flow UF isgiven by:


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CONCENTRATIONPOLARIZATION

the retained macromolecules accumulate near themembrane surface caused concentrationpolarization.

At steady state, a stable concentration gradient

exists near the membrane owing to back diffusionof solute from the membrane surface.

offers extra hydraulic resistance to the flow ofsolvent

development of osmotic pressure which actsagainst the applied transmembrane pressure


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MODEL

As most ultrafiltration membranes can not bevisualized as having parallel cylindrical pores, aparameter, the membrane hydraulic resistance isused for calculating permeate flux:


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If the solute build-up is extensive, a gel layer maybe formed on top of the membrane


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LIMITINGFLUX

At lower values of transmembrane pressure, thepermeate flux increases linearly with increase inpressure

However, as the pressure is further increased, there

is deviation from the solvent profile, this being dueto concentration polarization.

At very high transmembrane pressures, thepermeate flux usually plateaus off, clearly

suggesting the formation of a gel layer.


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LIMITING FLUX

Beyond this point, increasing the transmembranepressure has a negligible effect on the permeateflux

This value of permeate flux being referred to as thelimiting flux (Jlim).


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CROSSFLOWFILTRATION

The fluid in crossflow filtration flows parallel to themembrane surface, resulting in constant permeateflux at steady state


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MODEL


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CONCENTRATIONPOLARIZATIONMODEL

At steady state, a material balance of solutemolecules in a control volume within theconcentration polarization layer yields the followingdifferential equation:

Integrating this with boundary conditions (C= Cwatx= 0; C= Cbat x= b), we get

k(= mass transfer coefficient) = D/b

0dx

dCDCJCJ pvv

pb

pw

vCC

CCkJ ln


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CONCENTRATIONPOLARIZATIONMODEL

For total solute rejection, i.e., when Cp = 0, theequation reduces to

b

w

vCCkJ ln


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ULTRAFILTRATION-EXAMPLE 1

A protein solution (concentration = 4.4 g/1) is being

ultrafiltered using a spiral wound membrane module,which totally retains the protein. At a certaintransmembrane pressure the permeate flux is 1.3 x10-5 m/s. The diffusivity of the protein is 9.5 x 10-11

m2

/s while the wall concentration at this operatingcondition is estimated to be 10 g/1. Predict thethickness of the boundary layer. If the permeate fluxis increased to 2.6 x 10-5 m/s while maintaining thesame hydrodynamic conditions within the membrane

module, what is the new wall concentration?


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ULTRAFILTRATION-MODEL-SOLUTEMASSTRANSFERCOEFFICIENT

The solute mass transfer coefficient (k) is a

measure of thehydrodynamic conditions within amembrane module.

The mass transfer coefficient can be estimated from

correlations involving the: Sherwood number (Sh),

Reynolds number (Re), and

Schmidt number (Sc):


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These correlations are based on heat and mass

transfer analogy. In the case of fully developed laminar flow, the

Graetz-Leveque correlation can be used:

For turbulent flow (i.e. Re > 2000), the Dittus-Boelter correlation can be used:


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MEMBRANEPERFORMANCE - RETENTION

If a solute is not totally retained (or rejected), theamount of solute going through the membrane canbe quantified in terms of the membrane intrinsicrejection coefficient (R,) or intrinsic sieving

coefficient (S,):

More practical parameters such as the apparent

rejection coefficient (Ra) or the apparentsievingcoefficient (Sa) are frequently preferred:


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A correlation between the intrinsic sieving coefficient andthe apparent sieving coefficient can be obtained as:

If the intrinsic sieving coefficient could be considered aconstant, above equation provides a way by which themass transfer coefficient and the intrinsic sievingcoefficient could be determined by plotting experimentaldata


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EXAMPLE

The intrinsic and apparent rejection coefficients fora solute in an ultrafiltration process were found tobe 0.95 and 0.63 respectively at a permeate fluxvalue of 6 x 10-3 cm/s. What is the solute mass

transfer coefficient?


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MEMBRANE PERFORMANCE - FLUX

The permeate flux in an ultrafiltration processdetermines its productivity

The permeate flux depends on:

properties of the membrane

Properties of the feed solution.

transmembrane pressure

the solute mass transfer coefficient (which affects theconcentration polarization).

membrane fouling


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ENHANCINGPERMEATEFLUX

By increasing the cross-flow rate

By creating pulsatile or oscillatory flow on the feedside

By back flushing the membrane

By creating turbulence on the feed side usinginserts and baffles

By sparging gas bubbles into the feed


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MEMBRANEPERFORMANCE - RETENTION

The retention of a solute by a membrane primarilydepends of on

the solute diameter to pore diameter ratio.

the solute shape,

solute charge,

solute compressibility,

solute-membrane interactions (which depend on thesolution conditions)

operating conditions (such as cross-flow velocity andtransmembrane pressure).


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NANOFILTRATION


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NANOFILTRATION

Nanofiltration (NF) membranes allow salts andother small molecules to pass through but retainlarger molecules such as peptides, hormones andsugars.

The transmembrane pressure in NF ranges from 40to 200 psig.

ST


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GOVERNINGLAWS - FICKS 1STLAW

Fick's first law relates the diffusive flux to the

concentration field, by postulating that the flux goesfrom regions of high concentration to regions of lowconcentration,

where

J is the diffusion flux in dimensions of [(amount of substance)length2 time1]

D is the diffusion coefficient or diffusivity in dimensions of[length2 time1] (for ideal mixtures)

is the concentration in dimensions of [(amount of substance)length3],

x is the position [length],


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GOVERNINGLAWS - FICKS 2NDLAW

Fick's second law predicts how diffusion causes

the concentration field to change with time (derivedfrom Fick's First law and the mass balance):

Where

is the concentration in dimensions of [(amount of substance)length3],

t is time [s]

D is the diffusion coefficient in dimensions of [length2 time1],

x is the position [length]


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The extended Nernst-Planck equation proposed bySchlogl and Dresner forms the basis of thedescription of ion transport through the membranes.

The equation can be expressed as


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REVERSE OSMOSIS

Reverse osmosis (RO) membranes allow water togo through but retain all dissolved species presentin the feed.

In osmosis water travels from the lower solute

concentration side to the higher soluteconcentration side of the membrane.

In RO the reverse takes place due to theapplication of transmembrane pressure.

The normal transmembrane pressure range in ROis from 200 to 300 psig.


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ASSIGNMENT 3

A protein solution at a concentration of 0.5 g/liter andvolume of 1000 liters must be concentrated on acrossflow ultrafilter to a concentration of 10 g/L. Theultrafiltration has an area of 100 m2 and operates at 5oC

with an inlet pressure of 14.0 bar, and back pressure onthe permeate of 1.4 bar. The protein has a molecularweight of 20,000 Da and will not pass through theultrafiltration membranes. The feed has a viscosity of1.2 cp. The membrane resistance (Rm) of the ultrafilter

has been determined to be 2 x 1013

cm-1

. The Reynoldsnumber for flow within the ultrafilter is large enough torender concentration polarization negligible. Determinethe time required to perform the ultrafiltration.


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SOLUTION

cpMV

RR

PJ

RTC

5.membrane based bioseparation-purification

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