ChE 427 NOVEL TOPICS in SEPARATION PROCESSES Chp 4: MEMBRANE PROCESSES

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ChE 427NOVEL TOPICS in SEPARATION
PROCESSESChp 4 MEMBRANE PROCESSES
Instructor Prof. Dr. Hayrettin YücelAssistant
Ms. Hale Ay
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DEFINITIONS

• MEMBRANE A permeable or semi-permeable phase
  which selectively passes one or more components
  of a stream while restricting the motion of
  other species.
• PERMEATE Stream passing the membrane
• RETENTATE Stream retained by the membrane
• MODULE The vessel in which the membranes are
  contained

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Simple Membrane Separation
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FLOW PATTERNS
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() plusses compared to other separation
processes

• More compact, less capital intensive
• Energy requirements are low
• Easily operated, controlled and maintained
• Membrane processes present a very simple
  flowsheet
• Very large number of separation needs might
  actually be met by membrane processes.

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()plusses

• membranes can be produced which can produce
  extremely high selectivities for components to be
  separated.
• A very large number of polymers and inorganic
  media can be used as membranes

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(-)minuses

• Membrane modules with polymeric materials can not
  operate at much above room temperature
• Membrane process do not often scale up very well
  to accept massive size streams. Many paralell
  units may be required
• Membrane processes can be saddled with major
  problems of fouling of the membranes

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• Membrane processes are classified by the size of
  the particles

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Membrane Characteristics
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Mechanisms

• Size exclusion By hole or pores which are of
  such a size that certain species can pass through
  and others can not.
• By selective retardation by the pores when pore
  diameters are close to molecular sizes.
• By dissolution into the membrane , migration by
  molecular diffusion across the membrane,
  solution-diffusion.

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Figure . Mechanisms for permeation of gases
through porous and dense gas-separation membranes.
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CROSSFLOW DEAD-END FILTRATION
Feed
Permeate
CROSSFLOW FILTRATION
DEADEND FILTRATION
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Membranes have been inexistence at least as long
as life has existed on this planet. Living cells
nearly always have membranes which allow for the
selective passage of nutrients into the cells and
the passage of wastes out of the cells.
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MEMBRANE MATERIALS

• POLYMERS
• Polyisoprene
• Aromatic polyamide
• Polycarbonate
• Polyimides
• Polystyrene
• Polysulfones
• Polytetrafluoroethylene
• CERAMICS
• Alumina
• Zirconia
• METALS
• Palladium and palladium alloys

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POLYMERS

• They are high molecular weight components built
  up from a number of basic units, called monomers.
• Structural units linked together to form long
  chain molecule is defined as degree of
  polymerization.
• Molecular weight of polymer depends on degree of
  polymerization and the molecular weight of
  monomer.

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Polyethylene

• During polymerization double bond is opened
  and large number of C2H4 molecules are coupled
  together.
• n CH2 CH2 ? -CH2 -CH2-n
• Segment
• With increasing segments physical, chemical and
  mechanical properties of polymer changes.

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Character of a sample polymer in relation to
molecular weight
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• Homopolymer
• The repeating unit is same throughout the
  polymer. It is not necessary that a single
  monomer is used
• Copolymer
• Repeating units are different.
• Random Copolymers
• The sequence of the structural units
  is completely irregular
• (...AABABBABBBAABBAABB....)
• Examples
• NBR (nitrile-butadiene-rubber)
• SBR (styrene-butadiene-rubber)
• EPDM (ethene-propene-diene rubber)
• EVA (ethylene-vinyl acetate copolymer)
• ABS (acrylonitrile-butadiene-styrene rubber)
• EVAL (ethylene-vinyl alcohol copolymer)

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• Block Copolymers (...AAAAABBBBBBBBBAAAAAAA....)
• The chain is built up by linking blocks of each
  of the monomers
• Example
• SIS (styrene-isoprene-styrene)
• Graft Copolymers (...AAAAABBBBBBBBBAAAAAAA....)
• B B
• B B
• B B
• B B
• B
• B
• The irregularities occur in the side chains
  rather than
• in the main chain

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Polymers

• Linear Polymer, Ex. polyethylene
• Branched Polymer, Ex. polybutadiene
• Crosslinked Polymer, ex. Phenol-formaldehyde

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Linear chain molecules soften with an increase in
temperature , are often soluble in organic
solvents and are referred as thermoplastics.Cross
linking has an enermous effect on pysical
,mechanical and thermal properties of the
resulting polymer. Thermosettting polymers are
insoluble in most organic solvents, do not soften
with an increase in temperature and do not melt.
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At low temperatures(Tlt100 oC) polymers can be
classified as glassy or crystalline

• Glassy polymers
• Brittle
• Glassy in appearance
• Lacks any crystalline structure(amorphous)
• Crystalline polymers
• Brittle, hard and stiff
• crystalline

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Glass-transition temperature(Tg ) and melting
temperature(Tm)

• If the temperature of a glassy polymer is
  increased , a point , called the glass-transition
  temperature may be reached where the polymer
  becomes rubbery
• If the temperature of a crystalline poymer is
  increases , a point ,called the melting
  temperature, Tm, is reached where the polymer
  becomes a melt.
• Most polymers have both amorphous and crystalline
  regionsmaking it possible for some polymers to
  have both Tg and Tm
• Membranes made of glassy polymers can operate
  below or above Tg membranes of crystalline
  polymers must operate below Tm.

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MEMBRANE MATERIALS

• Polyisoprene (natural rubber), is hard and rigid
  when cold, but soft, can be easily deformed and
  sticky when hot.
• Aromatic polyamides, are high-melting crystaline
  polymers that have better long-term thermal
  stability and higher resistance to solvents than
  do aliphatic polyamides such as nylon. Some
  aromatic polyamides are easily fabricated into
  fibers, sheets and films.
• Polycarbonates, are mainly amorphous in
  structure. It has an aromatic and aliphatic form.
  Due to its amorphous structure, they possess
  ductility and toughness below Tg and can be
  fabricated into a wide variety forms, including
  fibers, sheets and films.

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MEMBRANE MATERIALS

• Polyimides, are tough, amorphous polymers with
  high resistance to heat and excellent wear
  resistance. They can be fabricated into a wide
  variety forms, including fibers, sheets and
  films.
• Polystyrene, is a linear,amorphous, highly pure
  polymer and can be easily fabricated. It can be
  annealed (heated and cooled) to convert it to a
  crystalline polymer.
• Polysulfones, are relatively new synthetic
  polymers. Because of SO2, it has high strength.
  They are easily spun into hollow fibers.
• Polytetrafluoroethylene, is a straight-chain,
  highly crystalline polymer with a considerable
  strength. It possess exceptional thermal
  stability and can be formed into sheets, films
  and tubing.

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Types of Membranes

•  1. Microporous(porous)membranes
• 2. Dense(nonporous) membranes
• 3. Electrically charged barriers
• 4. Liquid membranes.
• Asymetric vs Symetric Membranes

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Figure 1. Schematic diagrams of the principal
types of membrane
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MORPHOLOGY

• Porous membrane
• Macropores, gt50 nm (microfiltration)
• Mesopores, 2ltpore sizelt50 nm (ultrafiltration)
• Micropores, lt2 nm (nanofiltration reverse
  osmosis)
• Non-porous membrane (gas separation
  pervaporation)

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HOMOGENEOUS MEMBRANES

• A homogeneous membrane is a dense film through
  which a mixture of molecules is transported by a
  pressure, concentration, or electrical potential
  gradient.
• Separation of the components of a mixture is
  directly related to their transport rates within
  the membrane phase.
• An important property of homogeneous membranes is
  that chemical species of similar size, and hence
  similar diffusivity, can be separated efficiently
  when their concentrations differ significantly.
• Homogeneous membranes are prepared from
  polymers, metals, or metal alloys by film-forming
  techniques

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ASYMMETRIC MEMBRANES

• Consists of a very thin (0.1 1 µm) "skin" layer
  on a highly porous, 100 200-µm-thick
  substructure.
• Its separation characteristics are determined by
  the nature of the membrane material or the pore
  size, whereas the mass transport rate is
  determined mainly by the skin thickness.
• Used in pressure-driven membrane processes such
  as reverse osmosis, ultrafiltration, or gas
  separation

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Early synthetic membranes consisted of dense
,thin polymer films. Permeation fluxes were too
low to be of commercial interest on any scale. If
a greater pressure difference were used , then
the film had to be made much thicker to resist
being torn or deformed.In 1960s, Loeb and
Sourirajan discovered how to make a cellulose
acetate membrane with an asymmetric density. This
discovery permitted reverse osmosis to become the
practical processes as it is today, and all
commercial membranes now use the asymmetric
structure in one form and another.
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Asymetric membranes
Asymetric pore structure with dense separating
layer and nonseparating defect -filling layer
Asymetric pore structure with dense separating
layer
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ION-EXCHANGE MEMBRANES

• In cation-exchange membranes, negatively charged
  groups are fixed to the polymer matrix.
• In anion-exchange membranes, positively charged
  groups are fixed to the polymer matrix.

a) Polymer matrix with negative fixed charges
b) Positive counterions c) Negative co-ions
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Asymmetric Loeb-Sourirajan Membrane
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Silicone Rubber Composite Membrane
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Three-layered Alumina Membrane/Support
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Asymmetric Hollow-Fiber Membrane
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MODULES

• In the membrane design, the basic problem is how
  to pack the most area of membranes into the least
  volume, in order to minimize the cost of the
  containment vessel.
• The earliest membrane designs were based on
  simple filtration technology and consisted of
  flat sheets of membrane held in a type of filter
  press these are called plate-and-frame modules.

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PLATE AND FRAME MODULES

• Plate-and-frame modules were among the earliest
  types of membrane system the design originates
  from the conventional filter-press.

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PLATE AND FRAME MODULES

• An envelope of two membranes, feed spacers, is
  formed with a spacer in between two end plates.
• This envelope is roughly circular with a circular
  hole in the center.
• These envelopes are packed onto a porous tube.

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PLATE AND FRAME MODULES

• By the use of baffles to control the direction of
  the flow across these envelopes, it is possible
  to maintain the feed/retentate flow velocity
  nearly constant throughout the entire model by
  varying the number of envelopes between baffles
• The flow-setting ability is essential in
  minimizing the buildup of permeating species near
  the membrane surface and achieved in a single
  model.

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PLATE AND FRAME MODULES

• A number of plate-and-frame units have been
  developed for small-scale applications, but these
  units are expensive compared to the alternatives,
  and leaks caused by the many gasket seals are a
  serious problem.
• These modules are generally limited to
  electrodialysis and pervaporation systems and a
  limited number of highly fouling reverse osmosis
  and ultrafiltration applications.

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PLATE AND FRAME MODULES

• Plate-and-frame modules contain less membrane
  surface area than the other modules.
• A major advantage of plate-and-frame
  configurations is that they can apparently be
  designed and successfully operated at much higher
  trans-membrane pressure drops than can other
  configurations.
• ?Plt67 bar for other modules
• ?P can be 2-3 67 for plate and frame type

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TUBULAR MODULES

• Typically, the tubes consist of a porous paper or
  fiber glass support with the membrane formed on
  the inside of the tubes.

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TUBULAR MODULES

• Tubular modules are generally limited to
  ultrafiltration applications, for which the
  benefit of resistance to membrane fouling because
  of good fluid hydrodynamics overcomes the problem
  of their high capital cost.

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MODULES

• Systems containing a number of membrane tubes
  were developed because of their relatively high
  cost, they have been largely displaced by two
  other designs
• the spiral-wound module
• the hollow-fiber module.

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SPIRAL WOUND MODULES

• Spiral-wound modules were used originally for
  artificial kidneys, but were fully developed for
  reverse osmosis systems.
• Such modules create high amounts of surface area
  per unit volume.
• They consist of two rectangular sheets of
  membrane material, with the dense layers facing
  away from each other, are sealed together on
  three sides.

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SPIRAL WOUND MODULES

• Such modules consist of membrane envelope wound
  around a perforated central collection tube.
• The wound module is placed inside a tubular
  pressure vessel, and the feed gas is circulated
  axially down the module across the membrane
  envelope.
• A portion of the feed permeates into the membrane
  envelope, where it spirals toward the center and
  exits through the collection tube.

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SPIRAL WOUND MODULES
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SPIRAL WOUND MODULES

• It is possible to have more than one envelope
  wrapped around the central tube.
• Such modules are used for reverse osmosis,
  nanofiltration, ultrafiltration and gas
  separation.
• Commercial spiral-wound modules are typically
  100150 cm long and have diameters of 10, 15, 20,
  and 30 cm.
• These modules consist of a number of membrane
  envelopes, each with an area of approximately 2
  m2, wrapped around the central collection pipe.

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MULTI-LEAF SPIRAL WOUND MODULE

• Multileaf spiral-wound module, used to avoid
  excessive pressure drops on the permeate side of
  the membrane.
• Large, 30-cm diameter module may have as many as
  30 membrane envelopes, each with a membrane area
  of about 2 m2.
• Such designs are used to minimize the pressure
  drop encountered by the permeate fluid traveling
  toward the central pipe.

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HOLLOW-FIBER MEMBRANES

• Hollow-fiber membranes were invented in the
  1960s. They are used in many fields such as
  desalination of water, waste-water reclamation,
  medicine, agriculture, gas separation, and
  pervaporation.
• A hollow-fiber membrane is a capillary having an
  inside diameter of gt0.25  mm and an outside
  diameter lt1  mm and whose wall functions as a
  semi permeable membrane.
• The fibers can be employed singly or grouped
  into a bundle which may contain tens of thousands
  of fibers and up to several million fibers as in
  reverse osmosis.

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HOLLOW-FIBER MEMBRANES

• The feed gas contacts the outside of the fibers,
  part of the gas permeates to the bores of the
  fibers and passes out of the module.
• The high-pressure feed side is separeted from the
  low-pressure permeate side by a tube sheet.

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HOLLOW-FIBER MEMBRANES

• In most cases, hollow fibers are used as
  cylindrical membranes that permit selective
  exchange of materials across their walls.
• However, they can also be used as containers to
  effect the controlled release of a specific
  material, or as reactors to chemically modify a
  permeate as it diffuses through a chemically
  activated hollow-fiber wall, e.g. loaded with
  immobilized enzyme.

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HOLLOW-FIBER MEMBRANES

• Shell-side feed modules are generally used for
  high pressure applications up to about 7  MPa
  (1000 psig). Fouling on the feed side of the
  membrane can be a problem with this design, and
  pretreatment of the feed stream to remove
  particulates is required.
• Bore-side feed modules are generally used for
  medium pressure feed streams up to about 1  MPa
  (150 psig), where good flow control to minimize
  fouling and concentration polarization on the
  feed side of the membrane is desired.

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SHELL-SIDE FEED MODULES

• Such models were design by Monsanto for hydrogen
  recovery or air separation systems or by DuPont
  for reverse osmosis fiber systems.
• Such models are loops of fiber or a closed bundle
  contained in a pressurized vessel permeate passes
  through the fiber wall and exits through the open
  fiber ends.

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SHELL-SIDE FEED MODULES

• Gas or liquid passes through the small diameter
  fiber wall and exits via the open fiber ends.
• Shell-side feed modules are generally used for
  high pressure applications up to 7  MPa (1000
  psig).
• Fouling on the feed side of the membrane can be a
  problem with this design, and pretreatment of the
  feed stream to remove particulates is required

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BORE-SIDE FEED MODULES

• The fibers in this type of unit are open at both
  ends, and the feed fluid is usually circulated
  through the bore.
• To minimize pressure drops inside the fibers, the
  fibers often have larger diameters than the very
  fine fibers used in the shell-side feed system.

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BORE-SIDE FEED MODULES

• These so-called capillary fibers are used in
  ultrafiltration, pervaporation, and in some low
  to medium pressure gas applications.
• Feed pressures are usually limited to less than 1
  MPa (150 psig) in this type of module, where good
  flow control to minimize fouling.

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MONOLITHS

• Feed flow is normal to the drawings and through
  the holes.
• The membrane is a thin layer on the surfaces of
  the holes.
• The permeate flows into the porous, solid
  structure, out the edges and into the shell
  surrounding the monolith.

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MONOLITHS

• This material has no seperating ability and is
  porous enough that the permeate, which flows
  through it, will encounter very little pressure
  drop.
• On the surface of each hole is deposited a very
  thin layer of much smaller particles than exist
  in the monolith, an this layer performs the
  separation.
• The size of the pores is controlled by the size
  of the particles on the surface.
• These membranes are not capable of performing
  reverse osmosis, nanofiltrations and gas
  separations used in ultrafiltrations

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MODULE DESIGN CHARACTERISTICS
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Permeation Cascade. The separation obtained in a
single permeation stage can be increased by
connecting an appropriate number of stages in
series to form a countercurrent permeation
cascade with or without reflux of the retentate .
 In the simple cascade without reflux of the
retentate (Fig), the permeate from stage n
becomes the feed for the next higher stage n 1,
and the retentate is disposed of. This cascade is
of use only when the retentate has virtually no
value and a large enrichment factor for the
product in the permeate is required. In Figure B
the retentate is refluxed, i.e., the retentate of
stage n is mixed with the next lower stage n 1
and so on. All permeate streams must be
recompressed before entering a higher stage.
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A cascade with reflux of the retentate consists
of the enrichment section, where the product is
enriched in the permeate, and the stripping
section, where the product is enriched in the
retentate.
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Flow diagram of permeation cascades without (A)
and with (B) reflux of the retentate  
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IDEALIZED FLOW PATTERNS A)Complete mixing of
feed and permeate B) Cocurrent plug flow of feed
and permeate C) Countercurrent plug flow of feed
and permeate