Membrane Separation

Dr.
Syeda Sultana Razia

Professor
Department of Chemical Engineering, BUET
 Types of membrane
 Mechanism of membrane separation
 Calculation of mass flux , permeability, selectivity and yield
 Membrane modules
 Membrane: a selective barrier permits passage of certain
components and retains other components of the mixture
https://nordicmembrane.com/membrane_technology
 Energy savings. The energy consumption is very low as there is no phase change.
 Low-temperature operation. Almost all processes proceed at room temperature, thus
they can deal with compounds that are not resistant at high temperatures.
 Recovery. Both the concentrate and the permeate could be recovered to use.
 Water reuse. When applied to recover water, they avoid the transport of large water
volumes and permit the reduction of the Chemical Oxygen Demand (COD) loading in
sewage plants.
 Compact operation. Which permits saving space
 Easy scale-up. Because usually they are designed in modules, which can be easily
connected
 Automatic operation. Most of the membrane plants are managed by expert systems.
 Tailored systems. In many cases, the membranes and systems can be specifically
designed according to the problem.
 High cost. Membranes (and associated systems) are costly, but for low selective
separations.
 Lack of selectivity. In many cases, the separation factors are still insufficient.
 Low fluxes. The permeate flow rate available is still too low for some applications.
 Sensitive to chemical attack. Many materials can be damaged by acids, oxidants,
or organic solvents.
 Lack of mechanical resistance. Many materials do not withstand abrasion,
vibrations, high temperatures, or pressures.
 Membrane may be classified by
▪ Nature of the membrane: natural vs synthesis
▪ Structure: Porous vs non-porous
▪ Application: gas, gas-liquid, liquid separations
▪ Mechanism of membrane action: adsorptive, diffusive, ion exchange,
osmotic, inert membrane
 Pressure difference across the membrane
▪ Gas separation
▪ Reverse osmosis (RO)
▪ Ultrafiltration (UF)
▪ Microfiltration (MF)
▪ Pervaporation
 Temperature difference: Membrane distillation
 Concentration difference: Dialysis, Membrane extraction
 Electrical potential difference: Electro-dialysis
 Mechanism
 Equipment
▪ Tubular
▪ Filter press
Porous Non-porous/Polymer
Solubility and Selectivity
 Porous and non-porous/thin/dense
membrane/ skin
 Asymmetric membrane: a thin
dense layer on one side supported
by a highly porous substructure
 Support materials are ceramic,
metal or polymer
 Gas separation membranes are 50-
200  m thick with 0.1-1  m skin
UF Asymmetric membrane of polypropylene

 A high selectivity can be obtained from • The similarity between the gas and the polymer
either a favorable diffusivity ratio or a is also important. Polar gases tend to be more
large difference in solubilities. soluble in polymers with a high concentration of
 The diffusivities in the membrane polar groups, and the solubility of water vapor is
depend more strongly on the size and high in materials that can form hydrogen bonds
shape of the molecules than do gas- with water molecules.
phase diffusivities, and large • For most gases, the permeability increases with
differences may exist for molecules of temperature, because the increase in diffusivity
almost the same size. more than offsets any decrease in solubility.
 The gas solubility also varies widely However, an increase in temperature usually
with the gas and the type of polymer. decreases the membrane selectivity, so the
The solubility is low for gases that have operating temperature is determined by
a low boiling point or critical balancing the needs for high flux and high
temperature selectivity.
Laboratory gas separator Membrane filter press
Largest Size: 1 ft diameter and 10-15 ft length
 Application
▪ Reverse osmosis (RO)
▪ Dialysis
▪ Electro-dialysis
▪ Micro/ultra filtration (MF,UF)
 Type
▪ Spiral wound
▪ Tubular
▪ Hollow fiber
 Pure water from dilute aqueous solution, Purifying organic
solvent
 Separation takes place at room temperature
 No phase change
 Energy required comes from the work of pressurizing the
feed which can be recovered partially through the turbine
 Some polymers have high permeability of water and low
permeabilities for dissolved salts i.e. cellulose acetate
 Water and salt diffuse separately by solution diffusion
mechanism
 Driving force for water transport is the pressure difference
 Driving force for salt transport is the concentration difference
20 to 50 atm above the osmotic pressure
(Copyright Excel Water Technologies Inc., Ft. Lauderdale, FL)

Water flux
Salt flux
Dw is the diffusivity in the membrane, Cw is the

average water concentration in g/cm3, and w is
the partial molar volume of water.
 Osmotic and operating pressure: Upstream pressure 20 to 50
atm above osmotic pressure
 Salt rejection : 99.9%
 Flux: The flux of a RO membrane is directly proportional to
temperature and pressure. Unit L/sqm/hr
 a process for selectively removing low-molecular-weight solutes from
a solution by allowing them to diffuse into a region of lower
concentration
 There is little or no pressure difference across the membrane, and the
flux of each solute is proportional to the concentration difference.
 Thin porous membranes are used in dialysis,
 Solutes of high molecular weight are mostly retained in the feed
solution, as the molecules are almost as large as the pores
 The best-known application of dialysis is the use of artificial kidneys to
remove waste products from the blood of persons with kidney disease
 Hollow fiber cellulosic membranes are employed, and blood is passed
through the fibers while saline solution is circulated on the outside
 Urea and other small molecules diffuse through the membrane to the
external solution, while proteins and cells are retained in the blood.
 The dialyzing solution has added salts and glucose to prevent loss of
these materials from the blood
 Recovery of caustic from hemicellulose solutions produced in making rayon by the
viscose process
▪ Flat sheet membranes are placed parallel to each other in a filter-press arrangement and water is
passed countercurrent to the feed solution to produce a dialyzate with up to 6 percent NaOH
 Recovery of salts or sugars from other natural products or other colloidal solutions
could be achieved by dialysis, but ultrafiltration is more likely to be used because of
the higher permeation rates that can be obtained.
 Many large-scale applications of electrodialysis use ion-selective membranes and a
potential gradient to speed migration of ions through the membranes.
 One example is the use of electrodialysis to treat the reject salt solution from a
reverse-osmosis system. The salt concentration is increased eightfold, which
reduces the disposal cost, and the reclaimed water is recycled to the reverse-
osmosis plant.
 In this application, the electrode polarity is reversed at regular intervals to
minimize scale problems at the high salt concentration.
 Pharmaceutical industry
▪ purification of fermented solution, serums and vaccines
 Chemical industry
▪ separation of inorganic and organic solutions/purification of organic substances,
e.g. amino acids/waste water treatment, recycling of chemical substances
 Food industry
▪ demineralization of milk whey/desalination of fruit juices
 Other uses
▪ desalination and conservation of sea and brackish water and industrial waters
 Involves the use of a sieving mechanism to separate components: 0.1 -10
microns/less than 0.1 microns; symmetric porous/asymmetric
membrane
 Wastewater treatment, medical applications, pharmaceutical, food and
dairy industries etc
 Pervaporation
 P2=30 mm Hg
and ∆c
and ∆y
 Industrial membrane plants often require hundreds to
thousands of square metres of membrane to perform the
separation required on a useful scale. There are several
ways to economically and efficiently package membranes
to provide a large surface area for effective separation.
▪ Plate-and-frame
▪ Tubular
▪ Spiral-wound
▪ Hollow fiber
 The plate-and-frame modules were one of
the earliest types of membrane system
 Due to high relative cost they have been
largely replaced in most applications by
spiral-wound modules and hollow-fiber
modules.
 Plate-and-frame modules are now used
only in electrodialysis and pervaporation
systems and in a limited number of reverse
osmosis and ultrafiltration applications
with highly fouling conditions.
 Tubular membranes consist of support tubes with a membrane cast
on the inside. These tubes are often bundled into modules. Inside
diameters typically range from 1/4 inch up to 1 inch.
 The feed solution flows through the interior from one end to the
other. The permeate passes through the membrane and is collected
on the outside of the tube.
 These membranes can be made of several different materials
including ceramic, carbon, stainless steel, and various
( thermoplastics.
Copyright GEA Process Engineering, www.niroinc.com)
 The tubular design is particularly useful for operations involving high solids
concentrations, since plugging is kept to a minimum, and the product
recovery is high
 These modules are now generally limited to ultrafiltration applications, for
which the benefit of resistance to membrane fouling outweighs the high
cost.
 Tubular membranes contains as many as 5 to 7 smaller tubes, each 0.5 to
1.0 cm in diameter, nested inside a single larger tube. In a typical tubular
membrane system, a large number of tubes are manifolded in series.
https://www.youtube.com/watch?v=gmGpPwSfhK0
https://www.youtube.com/watch?v
=gmGpPwSfhK0
https://www.youtube.com/watch?v=6EPDCLgoZpI
https://www.youtube.com/watch?v=6EPDCLgoZpI
❑ Spiral membranes are produced by winding the

membrane around a perforated center tube,
where permeate is collected
❑ Feed water is purified when it passes through
one layer of the membrane and flows into the
permeate tube
❑ Spiral membranes offer the advantages of a wide
range of membrane dimensions, lower energy
costs due to reduced pumping requirements, and
higher packing density and can be operated at
elevated pressure and temperature
 Industrial-scale modules contain several membrane envelopes, each with an area of 1 to 2
m2, wrapped around the central collection pipe. Multi-envelope designs minimise the
pressure drop encountered by the permeate travelling toward the central pipe. The
standard industrial spiral-wound module is 8-inch in diameter and 40-inch long.
 The module is placed inside a tubular pressure vessel. The feed solution passes across the
membrane surface, and a portion of the feed permeates into the membrane envelope,
where it spirals towards the centre and exits through the collection tube
 4 to 6 spiral-wound membrane modules are normally connected in series inside a single
pressure vessel. A typical 8-inch diameter tube containing 6 modules has 100 to 200 m2 of
membrane area
(Copyright Chemical Engineering, Access Intelligence, LLC)
 Hollow fiber membranes allow a high membrane surface area to be

contained in a compact module, providing high capacity.
 These membranes have an overall smaller inner tube diameter than
tubular membranes and consist of unsupported membrane polymers.
Such polymers require a rigid support on each end of the tube.
 Each hollow fiber has a diameter of about 0.5 millimeters.
https://www.youtube.com/watch?v=5iRUYpIoN50
 Hollow-fibre modules are characteristically 4-8 inch (10-20 cm) in diameter and 3-5
(1.0-1.6 m) feet long. Hollow-fibre units are almost always run with the feed stream
on the outside of the fibre. Water passes through the membrane into the inside or
"lumen" of the fibre. A number of hollow-fibres are collected together and "potted"
in an epoxy resin at both ends and installed into an outer shell.
 These so-called capillary fibres are used in ultrafiltration, pervaporation, and some
low to medium pressure gas applications
 The single greatest advantage of hollow-fibre modules is the ability to pack a very
large membrane area into a single module.
The feed flow can go down the interior of the fibers, or around their outside
Hagen–Poiseuille equation gives pressure drop in an incompressible and Newtonian fluid in laminar
flow flowing through a long cylindrical pipe of constant cross-section
 Cellulose (GP, RO, UF),
 Polysulfone (GP, RO, UF)
 Silicone rubber from dimethyl siloxane (GP, pervaporation)

 Example of polyamide (RO, UF)
 polystyrene (matrix for composite resins, pervaporation)

Membrane Separation

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Dr.

Syeda Sultana Razia

UF Asymmetric membrane of polypropylene

(Copyright Excel Water Technologies Inc., Ft. Lauderdale, FL)

Dw is the diffusivity in the membrane, Cw is the

❑ Spiral membranes are produced by winding the

 Hollow fiber membranes allow a high membrane surface area to be

 Polysulfone (GP, RO, UF)

 Silicone rubber from dimethyl siloxane (GP, pervaporation)

 polystyrene (matrix for composite resins, pervaporation)

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