Membrane based bioseparations

1
Membrane based bioseparations

• A membrane is a thin semi-permeable barrier
• It may be used for
• Particle-liquid separation
• Particle-solute separation
• Solute-solvent separation
• Solute-solute separation
• Basis of separation
• Size
• Electrostatic charge
• Diffusivity
• Shape
• Membrane transport
• Convective, i.e. pressure driven
• Diffusive, i.e. concentration gradient driven
• A combination of the two

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Membrane based separation
Membrane module
Retentate
Feed
Permeate
Membrane
Pressure driven separation
Membrane module
Retentate
Feed
Permeate
Membrane
Sweep
Concentration gradient driven separation
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Membrane material
Polymeric (organic) Cellulose Cellulose
acetate Polysulfone (PS) Polyethersulfone
(PES) Polyamides (PA) Polyvinylidedefluoride
(PVDF) Polyacrylonitrile (PAN) Inorganic ?-alumi
na ?-alumina Borosilicate glass Pyrolyzed
carbon Zirconia/stainless steel Zirconia carbon
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Polymers
Natural polymer Cellulose
Synthetic polymers
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Membrane structure and morphology

• Morphological classification
• Symmetric or isotropic
• Asymmetric or anisotropic

• Structural classification
• Porous
• Non-porous or dense

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Membrane element

• There are three basic forms
• Flat sheet
• Tubular membrane
• Hollow fibre membrane

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Membrane properties

• Mechanical strength e.g. tensile strength,
  bursting pressure
• Chemical resistance e.g. pH range, compatibility
  with solvents
• Permeability to different species e.g. pure
  water, solutes
• Average porosity and pore size distribution

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Membrane fouling

• Deposition or adsorption of material on the
  membrane
• Undesirable
• Permeate flow declines due to fouling
• Two mechanisms
• pH and salt concentration affect fouling
• Fouling may be reversed to an extent by membrane
  cleaning
• Often fouling is irreversible

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Classification of membrane processes

• Pressure driven
• Reverse osmosis
• Nanofiltration
• Ultrafiltration
• Microfiltration

• Diffusion driven
• Dialysis

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Reverse osmosis

• Used to remove small solutes such as ions and
  salts from solvents
• Solvent is forced through RO membrane towards the
  lower solute concentration i.e. opposite to
  osmosis
• Normal transmembrane pressure range is 200 to 300
  psi
• Extreme cases require transmembrane pressure up
  to 600 psi
• Recently developed membranes allow as low as 125
  psi in some applications

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Nanofiltration

• Separates at the low-molecular level
• Low molecular weight substances can be separated
  ions and salts
• Transmembrane pressures range from 40 to 200 psi
• Most NF membranes are composites
• Overlap with ultrafiltration

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Ultrafiltration

• Separates large molecules from solvents
• Separate large molecules from small molecules
• Separate large molecules from one another
• The primary mechanism is size exclusion
• Chemical interactions between solute and membrane
  can affect separation
• Normal transmembrane pressure ranges from 10 to
  100 psi
• Membranes are anisotropic

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Microfiltration

• Separates fine particles from solutions
• Transmembrane pressures range from 1 to 50 psi
• Separation is purely size based
• Surface filtration
• Depth filtration
• Microfiltration is commonly used for
  clarification, sterilization and slurry
  concentration
• Membranes are isotropic

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Dialysis

• The mode of transport is diffusive
• Separation is size based
• Separates small molecules from macromolecules
• Small molecules diffuse more rapidly than larger
  ones
• Dense or porous membranes are used
• Membrane are isotropic