Semi permeable selective membranes

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Semi permeable (selective) membranes

• membrane
• The membrane allows only the smaller molecules to
  pass through (or, the smaller molecules pass
  through at a faster rate.)
• Either the solvent or the solute may be the
  smaller molecule.
• e.g. in blood dialysis Solvent blood (proteins
  and cells)
• Solute urea and other smaller molecules

Solvent Solute at low concentration
Solvent Solute at high concentration
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Membrane separations

• Similarity with filtration, but different size
  range.
• In filtration, suspended solids are retained on
  one side of the filter medium while the water
  passes through.
• In membrane separations we are trying to separate
  material at the molecular size range.

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Membrane separations

• In absorption and stripping we typically
  transferred one component between the phases by
  direct contact between the phases.
• In adsorption, the transfer was from the fluid
  phase to the surface of a solid phase, again by
  direct contact.
• In membrane separations we are transferring the
  component through a semi-permeable barrier,
  almost as in heat transfer where the fluids did
  not come in direct contact but were in thermal
  contact through the metal separating them.

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Membrane separation applications

• Dialysis
• Electrodialysis
• Water purification (reverse osmosis)
• Wastewater treatment
• Pervaporation (solute goes from liquid on one
  side to gas on the other)

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Membrane materials

• Porous vs non-porous (dense polymer)
• Cellulose acetate and other polymeric materials
• Inorganic membranes
• Thickness ??m, requires structural support
• Membrane typically deposited on a thicker (and
  stronger) support with high porosity and large
  pore size (Fig. 26.3 MSH)

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• Thin, dense membrane layer (on the inside
  surface) supported by a highly porous substructure

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• Blood dialysis urea and other small molecules
  diffuse through membrane, proteins and blood
  cells are retained

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Mass transfer considerations

• The above formulation assumes a porous membrane
  and continuity of the fluid phase.
• In some cases there are distinct phase boundaries
  on either side of the membrane

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Organic-water phase boundary coinciding with
membrane surface
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• Transport of gases through dense (nonporous)
  polymer membranes occurs by a solution-diffusion
  mechanism

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Electrodialysis
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• Seawater
• (35 g/L dissolved solids)
• Osmotic pressure 2740 kPa (27 atm)

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a activity c concentration W water S salt
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