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Clifton E. Meloan Chemical separations A
Wiley-Interscience Publication,1999
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Contents

• Introduction
• Distillation
• Solvent Extraction
• Solid Phase Extraction
• Solid Phase micro Extraction
• Supercritical Fluid Extraction
• Supercritical Fluid Chromatography
• Field Flow Fractionation
• Electrophoresis
• Membrane separations

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Introduction
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Introduction

• In order to affect a separation, separating
  agents are needed in the form of either
• Energy input (heat, pressure, electricity,
  magnetism, kinetic or potential energy)

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Introduction

• Withdrawal of energy ( cooling, freezing)
• Matter (filter, membrane, chemicals)

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• A separation process is an operation carried out
  in a special separation device which transforms a
  mixture into at least two product streams which
  are different in composition.

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• In the separation device, separation takes place
  due to an imposed gradient such as temperature,
  concentration, pressure or electrical field.

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Two important elements of separation are

• Separating agent used (heat, pressure, solvent,
  matter such as resins, filters, adsorbents etc.)

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• Principle of separation used, separation gradient
  applied (temperature, concentration, chemical
  potential, magnetic field etc.)

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Distillation theoryand practice
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Clausius- Clapeyron equation

• This relationship can be used to determine the
  Hvap from the p0 of a liquid at two
  temperatures.

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Clausius- Clapeyron equation

• An estimate of P0 can be made of any
  temperature provided the Hvap and the boiling
  point at atmospheric pressure is known.

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Field Flow Fractionation
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Flow FFF

• Two crossed flow streams are superimposed on the
  same channel.
• Channel walls are permeable and the pore size
  determines the lower size limit for separation.

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Field flow fractionation

• The driving force is the viscous force
  exerted on the particle by the cross stream
  based on sample diameter.

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Solid Phase Extraction
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Solid Phase Micro Extraction
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Supercritical Fluids
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Supercritical FluidExtraction
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Supercritical Fluid Chromatography
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    Electrophoresis
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Capillary Electrochromatography
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Membrane Separations
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• Definition A membrane is a thin barrier which
  allows selective passage of different species
  through it.
• This selectivity is utilized for separation.

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• The selectivity is due to
• Size
• Shape
• Electrostatic charge
• Diffusivity
• Physicochemical interactions
• Volatility
• Polarity/solubility

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Membrane separation processes Applications

• Product concentration, i.e. removal of solvent
  from solute/s
• Clarification, i.e. removal of particles from
  fluids, a special case being sterilization which
  refers to removal of microorganisms from fluids

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• Removal of solute from solvent, e.g. desalting,
  desalination, demineralization, dialysis
• Fractionation, i.e. separation of one solute from
  another

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• Gas separation, i.e. separation of one gas from
  another
• Pervaporation, i.e. removal of volatiles from non
  volatiles (usually solvents)

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Membrane material

• Organic polymers
• Polysulfone (PS)
• Polyethersulfone (PES)
• Cellulose acetate (CA)
• Regenerated cellulose
• Polyamides (PA)
• Polyvinylidedefluoride (PVDF)
• Polyacrylonitrile (PAN)

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Membrane material

• Inorganics
• ?-alumina
• ?-alumina
• Borosilicate glass
• Pyrolyzed carbon
• Zirconia/stainless steel
• Zirconia carbon

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Membrane preparation

• Polymer casting
• Precipitation from vapour phase
• Precipitation by evaporation
• Immersion precipitation
• Thermal precipitation

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Membrane preparation

• Other methods
• Stretching
• Sintering
• Slip casting
• Leaching
• Track etching

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Driving force in membrane processes

• Transmembrane pressure (TMP)
• Concentration gradient
• Chemical potential
• Osmotic pressure
• Electric field
• Magnetic field
• Partial pressure
• pH gradient

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Membrane processes primarily based on species
size

• Microfiltration (MF)
• Micron sized pores
• Mainly used for particle-fluid separation
• TMP 1 to 50 psig

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Membrane processes primarily based on species size

• Ultra filtration (UF)
• Pores 10 1000 angstroms
• Used for Concentration, desalting, clarification
  and fractionation
• TMP 10 100 psig

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Membrane processes primarily based on species size

• Nanofiltration (NF)
• TMP 40 200 psig
• Reverse osmosis (RO)
• TMP 200 300 psig
• Dialysis
• Concentration gradient driven
• Selectivity based indirectly on size

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Membrane processes based on principles other
than species size

• Pervaporation (PV)
• Driven by partial pressure
• Selectivity depends on volatility and solubility
  of species in membrane

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• Gas separation
• Driven by partial pressure
• Selectivity depends on solubility of species in
  membrane
• Electrodialysis (ED)
• Driven by electric field
• Selectivity depends of charge exclusion

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Membrane Separations

• In membrane separations a mixture is separated by
  using a semi permeable membrane

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Membrane Separations

• which allows one component to move through faster
  than others resulting in differential transport

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Membrane Separations

• The mixture is separated into a retentate,
  enriched in the less mobile species and a
  permeate,
• enriched in the components which move through the
  membrane fastest.

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Membrane Separations
Retentate
Feed mixture
Membrane
Purge (optional)
Permeate
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Transport Mechanisms Through Membranes

• Transport Through Membranes
• Bulk flow through pores (membrane is microporous
  with pores larger than the mean free path).
• Diffusion through pores (pores are large enough
  for diffusion, but small relative to the MFP).

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Transport Mechanisms Through Membranes

• Restricted diffusion through pores (if pores are
  large enough for some species, but not others).
• Solution-diffusion (Diffusion through dense
  membranes with diffusant dissolved in polymer
  matrix).

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Transport Mechanisms Through Membranes
Diffusion through pores
Bulk flow through pores
Solution-diffusion
Restricted diffusion
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Microfiltration and Ultrafiltration

• Microfiltration is based on the restricted
  diffusion of species through pores Larger
  speciesor particles are restricted from entering
  pores of 0.1 to 1 micron in size.

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Microfiltration and Ultrafiltration
Restricted diffusion
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• Ultrafiltration is similar, except the pore size
  is even smaller (on the order of
• the molecule size) and the number of pores small.
  This allows for separation

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• of smaller components, for example separating a
  small molecule from solvent.

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Bulk Flow Through Membranes
D
L
Bulk flow through pores (if membrane is
microporous with pores larger than the mean free
path).
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If flow is in the laminar regime then the
Reynolds Number NRe (which is related to the pore
and fluid properties) is less than 2,100
Similar to Darcys Law
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Bulk Flow Through Membranes
Combining
Density
Porosity
Velocity
Flux (molar or mass)
Note that the the porosity gives the total
cross-sectional area of the flow perpendicular to
the flow direction
A
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• If the pores are not straight or cylindrical then
  we must modify this equation by factors that
  describe
• the tortuosity and specific surface area.

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Electrodialysis
Feed solution
Electrode rinse solution
Electrode rinse solution
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Cathode

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Anode
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Anion selective membranes
Cation selective membranes
Concentrate (brine)
Diluate (less salts)
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Osmosis and Reverse Osmosis
Membrane (only permeable to solvent)
A, B, CP1
C
C
A, B, CP1
P2
P2
P2
Equilibrium Condition (pressure
difference maintained by osmotic pressure)
Initial Condition (equal pressures)
Reverse Osmosis (Transport againstconcentration
gradient if pressure above osmotic pressure)