Membrane Separation Processes
UNIT II
MEMBRANE SEPARATIONS
8
Types and choice of membranes, Plate and frame, tubular, spiral wound and hollow fibre
membrane reactors and their relative merits, Commerical, pilot plant and laboratory membrane
permeators involving dialysis, reverse osmosis, Nanofiltration, ultrafiltration, Microfiltration and
Donnan dialysis, Economics of membrane operations, Ceramic membranes
Introduction:
Equilibrium and Rate Governed Separations:
Separation processes can be classified into two categories: Equilibrium separation and
rate governed separation. Conventional separation operations such as heat transfer and
mass transfer operations, performed on large scale in industry, are based on equilibrium
distribution. Two immiscible phases come into contact with each other, separation occurs
until equilibrium is attains and then the phases are separated. In rate governed operations,
separation is based on differences in transport rate of various species through some
medium under the influence of a driving force resulting from a gradient in pressure,
concentration, temperature, or electrical field. Membrane is one such medium separating
one phase from the other.. Hence membrane separation operations are rate governed
separations.
Membrane:
The Latin word membrana means skin. The membrane is a barrier which separates two
phases and restricts transport of various chemicals in a selective manner. It is a structure
having lateral dimensions much greater than its thickness. The thickness may vary from
few microns to several millimeters. The membrane can be a selective or contacting
barrier. In the aforementioned case, it is selective; it controls the exchange of the species
between the two regions adjacent to it in a specific manner. If it is a contacting barrier, its
function is mainly to contact the two regions between which the transport occurs.
A membrane can be homogeneous or heterogeneous, symmetric or asymmetric in
structure, a solid or liquid, can carry a positive or negative charge, or can be neutral or
bipolar.
Picture of membrane separation process:
The membrane is placed on a rigid, highly porous support plate held between two
compartments. The feed, composed of A and B, enters into the upper chamber and flows
across the membrane (cross flow). Species which can permeate through the membrane
transfers into the bottom chamber. If A is the permeable species and if the membrane is
an ideal one, only A can pass into the bottom chamber, while species B is retained in the
upper chamber and comes out as Retentate. A is discharged as permeate.
In reality, some amount of B also passes through the membrane and not all A permeates
through the membrane.. Hence the permeate is a mixture of A and B rich in A and
retentate is rich in B.
Differences between equilibrium conventional separation and Rate governed separation
processes:
No Equilibrium conventional
Rate governed separation processes
separation processes
(Membrane separation processes)
1
Two different (immiscible phases) are Same phases are involved (except in
brought into direct contact.
pervaporation) and they are contacted
indirectly; separated by a barrier.
2
Resistances to transfer are two, one on Resistance to transfer are three: one on
either side of the interface.
either side of the membrane and one across
the membrane.
3
Separation factor is determined by
Separation factor is determined by
thermodynamic equilibrium.
thermodynamic equilibrium as well as
Knowledge of equilibrium data about transport properties of the system.
the system is sufficient to compute
Permeability of the membrane is one of the
separation factor.
governing factors.
4
Mechanism of separation is mainly by Both diffusion and siecing mechanism.
diffusion.
5
Flux is governed by concentration
Flux is governed by both concentration
difference or pressure difference.
difference (or pressure difference) and
concentration polarization.
Advantages of membrane separation processes over conventional processes:
1. Appreciable energy savings:
Conventional processes like distillation and dewatering operations such as evaporation
and freeze drying are coming under ESA and hence highly energy consuming. As
membrane separations do not require a change in state (except for pervaporation), they
save considerable amounts of energy.
2. Clean technology with operational ease:
No complicated heat exchange or heat generating procedures are needed No thermal
pollution.
3. Replaces conventional processes:
The shortcomings related to conventional processes such as emulsification, foaming,
weeping, entrainment etc, lack of flexibility, requirement of large space are not existing
in membrane separations.
4. Recovery of high value product:
In conventional processes, separation efficiency drops as the concentration of the
desired component falls. It is the contrary in membrane separation processes.
5. Greater flexibility in designing systems:
With the development of new membranes with improved transport properties and better
chemical and thermal stability in recent years, a large number of potential applications
are identified.
Hybrid process development:
In conventional process design, either a conventional unit operation or membrane
process is selected. In modern design, both can be conveniently and effectively combined
to get better results.
Disadvantages:
1. Membrane fouling.
2. Upper solid limits:
Membrane processes are quite limited in their upper solid limit. It is decided by
various factors. In RO, , it is the osmotic pressure of the concentrated solutes; in UF and
MF, it is the low mass transfer rate and high viscosity that makes the pumping of the
retentate difficult.
3. Expensive:
Membrane processes are expensive due to the fabrication method, occasional
replacement, fouling and poor cleanability of the membrane.
Types of membrane separation processes:
(1) Osmosis
(7) Gas permeation or
(2) Reverse Osmosis:
RO
Gas separation
(3) Dialysis
(8) Liquid membrane
(4) Microfiltration
MF
(9) Nanofiltration :
(5) Ultrafiltration
UF
(10) Electrodialysis
(6) Pervaporation
PV
(11) Electrodeionization
GS
NF
ED
EDI
Industrial applications of membrane separation processes
Industrial sector
Membrane process
Drinking water
RO,NF,UF
Demineralized water
RO, ED. EDI
Waste water treatment
Direct
MF,UF,RO,ED
Membrane bioreactor
MF,NF
Food industry
Dairy
UF,RO,ED
Meat
UF,RO
Frit and vegetables
RO
Drain milling
UF
Sugar
UF,RO, MF, NF, ED
Beverages
Fruit juice
MF, UF, RO
Wine and brewery
MF, UF, RO,PV
Tea factory
MF, UF,NF
Biotechnology
Enzyme purification
UF
Protein separation
UF
Concentration of fermentation broth
MF
SCP harvesting
MF,UF
Biomedical
Control release
Facilitated transport
Haemodialysis
Dialysis
Chemical industry
Gas separation
GS
H2 recovery
GS
CO2 separation
GS
Vapor-Liquid Separation
Ethanol dehydration
PV
Organic recovery
PV
Chlor-alkali process
Membrane electrolysis
Hydrometallurgical processing
UF
Energy
Fuel cell
Proton exchange membrane
Types and Choice of membranes:
Types of membranes:
Membrane classification according to the morphology.
Detailed Classification:
Dense- phase symmetric membranes: These are relatively thick (1/1000 of an inch or
greater) in order to avoid tiny holes, which reduced the separation by allowing viscous or
Knudsen flow of the feed. Hence they are nonporous membranes. Development of
silicone polymers increased permeability by a factor of 10-20.
A dense or non porous membrane is a thin film of a suitable material that allows selective
passage of one or more components of a mixture. The permeating component ‘dissolves’
at the surface of the membrane, diffuses through the intermolecular spaces or free volume
within the membrane material and leaves at the opposite surface as the permeate or the
product. Dense membranes are used for reverse osmosis, most gas separation process
and pervaporation.
Asymmetric (or composite) membranes: If a membrane is thick (20 µm or greater), the
solvent flow or flux becomes too low to be acceptable. If the thickness is less than 20 µm,
the membrane loses its mechanical strength and its fabrication also is difficult. This
formidable problem is solved by fabrication of the asymmetric membrane. An
asymmetric membrane has a thin (0.1 – 1.0 µm) permselective layer on a porous
substructure. The ultrathin layer allows selective permeation (permselective). This layer
may be non- porous (for use as an RO membrane) or may have very fine pores (for use as
UF membrane). The porous substructure offers the necessary mechanical strength to the
membrane without offering any appreciable resistance to permeation. The entire
membrane is an integral piece of the same material. Such a membrane predictably allows
a high permeate flux as the flux is inversely proportional to the membrane thickness.
Symmetric or Isotropic Microporous membranes: These are functionally similar to
conventional filters but differs in pore size and thickness. These porous membranes
separate very fine particles and colloidal or even dissolved solutes. Membranes are much
thinner than filters. Microporous symmetric membranes have interconnected pores and a
high porosity, and are used for microfiltration. Isotropic membranes have a pore size
distribution of 0.1 -10 µm. The pore size distribution and the size and shape of the
particles determine the effectiveness of separation. Microporous membranes are
designed to retain all particles above its rating. For example, a 0.45 μm membrane will
not allow particles larger than 0.45 μm
Composite membranes: These are functionally similar to asymmetric membranes.
A composite membrane consists of a porous or dense, thin permselective upper layer
cast on a thick, mechanically strong backing or support. Both are often made of different
materials. Usually a polymer is coated on a polysulfone or PTFE film.
Electrically charged membranes: These have ionic groups that give the membrane fixed
charged sites. A perfluoro ion exchange membrane (Nafion) has SO3 – groups on a PTFE
backbone. This group gives negatively charged fixed sites. The cation (H+ in the acid
form, or a metal ion like Na+ in the salt form of the ion exchange polymer) can move
freely within the membrane matrix and hence the membrane is called cation exchange
membrane. Since the fixed charged sites repel opposite charges (here –ve charges), the
membrane does not allow anions to pass through it . This rejection of ions because of
electrical repulsion is called Donnon exclusion. But cations can permeate through a
cation exchange membrane without electrical repulsion. If the fixed charges are positive,
the membrane is called anion-exchange membrane.
Inorganic and ceramic membranes: Inorganic membranes have 2 broad classes:
metallic and ceramic.
Metallic membrane: Example: Palladium membrane in the form of a thin film or thin
walled tube that can separate hydrogen from a gas mixture.
Ceramic membranes: These are developed to compete with polymeric membranes.
They are more expensive. Most of them are made by sol-gel technique. Alumina and
zirconia membranes are very common. A suitable ceramic substrate is coated with a sol
and the sol gets converted to a gel by losing water. The coated substance is dried under
controlled conditions and then vey slowly heated and sintered to 5000 C. A thin
permselective layer of alumina or zirconia forms on the substrate. Ceramic membranes
on tubular substrate are more useful because of the convenience of the fabrication.
Liquid membranes: A liquid may also act as a separation barrier between two phases or
mediums where the transport occurs by the solution-diffusion mechanism. Since a liquid
film l;acks mechanical strength as such, an appropriate technique has to be applied to
stabilize the liquid membrane so that it does not get ruptured during use. There are two
common types of liquid membranes:
Immobilized liquid membrane (ILM): It is also called supported liquid membrane. It is
made by impregnating a thin porous film of a suitable solid substance with a liquid. The
porous film only supports a liquid within the pores. It does not have any role in
transporting of the solute. If a solute has to be separated from an aqueous solution, the
feed flows on one side of the liquid membrane and water on the other. The solute
dissolves in the membrane liquid,diffused through it and then is taken up by the water on
the other side. Also, if the feed is a gas mixture, the desired compound passes through the
membrane, by solution-diffusion mechanism, and is received by a carrier gas on the other
side of the membrane.
Emulsion liquid membrane (ELM): It is a liquid droplet which contains a no. of very
small globules of another liquid within it, suspended in a liquid medium. Transport
through such a membrane separating two phases is shown. ELM is prepared by the
following method:
Two immiscible phases, an oil and water, for example, and a small quantity of a
surfactant are mixed vigorously to form an emulsion. A water in oil emulsion, in which
oil is the continuous phase, is more common. It is formed when the oil is in excess. If
such an emulsion is poured into a large quantity of water or an aqueous solution, droplets
of oil with fine globules of water in them are formed. This type of ELM is more common
and is called water-oil-water emulsion liquid membrane.
Membrane modules (configurations):
Three membrane configurations are being used in membrane separation processes:
They are: (i) Plate and Frame (Flat fibres) (ii) Hollow fibres and (iii) Spiral- wound
cartridges and (iv) Tubular modules.
(i) Plate and Frame (Flat –Plate) configurations are easy to construct and allow easy
membrane replacement. They are similar to plate and frame filter presses with the
difference being membranes replace filter cloth. The channel width can be altered to
reduce plugging problems. Membrane support must be added to allow operation at high
pressures. Flat-plate systems have a low membrane surface- to- volume ratio.
Plate modules are mainly used for dehydration applications; with permeate channels as
open as applicable. A rectangular support plate is provided on both sides with gaskets,
which partially cover slots in the plate, acting as distribution channels. A membrane is
placed on each gasket, its feed side facing the plate.
The permeate side of each membrane is supported by a perforated plate, a grid or spacer
is placed between two perforated plates. A membrane, one side of the support plate, and a
gasket form a feed chamber, two perforated plates and the space between them a
permeate chamber. Each feed chamber is thus adjacent to a permeate chamber, each
permeate chamber has a feed chamber at each side. Alternating feed and permeate
chambers are arranged in a module.
The module package is held together by means of flanges and bolts. The thickness and
weight of bolts and flanges limits the maximum internal pressure for such modules to 6 to
10 bar. In order to keep weight and handability of the modules within a reasonable range,
the maximum size of these modules does not exceed 30 to 50 m 2 of membrane area or
less than 130 support plates.
Major Advantages:
Sheets can be quite close to one another to reduce concentration polarization.
They can be taken apart for cleaning purposes.
(ii) Hollow fibre membranes:
Hollow fibre membranes for Water Purification:
The membrane module consists of fibres with an OD of 25 to 250 µm and a wall
thickness of 5 to 50 µm. The fibre wall has a structure of asymmetric membrane, the
active skin layer being placed close the feed side. A bundle of hollow fibres are mounted
in a pressure vessel, the open end of U-shaped fibres are potted into a head plate. Input
water under high pressure flows over the outside surface of the fibres. The permeated
water then flows out through the base of the fibres and is collected as product.
In a hollow fibre module the permeate flow per unit area membrane is low, and
therefore, the concentration polarization at the membrane surface is not high. The hollow
fibre units work under a laminar flow regime.
Advantages:
A huge number of fibres (4.5 x 10 6) can be packed inside a 25.4 cm diameter cartridge,
thereby rendering a high area / volume ratio and low cost.
Hollow-fibre is a flexible membrane; it can carry out the filtration by two ways: either
inside-out or outside-in.
Tangential flow outside membrane surface limits membrane fouling.
Hollow fibre membranes can be designed for circulation, dead-end and single-pass
operations.
Hollow fibre separators: The membranes are in the shape of very small dia hollow
fibres with inner dia: 100 – 500 µm and outer dia : 200 – 1000 µm; length upto 3 – 5 m,
The fibre module resembles shell and tube heat exchanger. Thousands of fibre tubes are
bound together at each end into a tube sheet that is surrounded by a metal shell of dia 0.1
to 0.2 m so that the membrane area per unit volume is upto 10000 m2 / m3.
High pressure feed enters shell side at one end, leaves at other end. Hollow fibres are
closed at one end of the tube bundles. The permeate gas inside the filter flows
countercurrent to the shell side flow and is collected in a chamber where the open ends of
the fibres terminate. Permeate exits the device.
Hollow fibre Assembly:
(iii) Spiral-wound cartridge: These cartridges provide a large surface-to-volume ratio.
But they plug more easily than the other two configurations. Spiralcartridges contain
rolled membranes and are essentially flat plate systems configured to increase the surface
–to-volume ratio. Most UF membranes operate with ∆P < 5 to 7 bars and the MF
membranes at a slightly lower ∆P. (UF: ultrafiltration; MF: microfiltration.)
The more common design employing the flat-sheet is the spiral-wound membrane
design shown in Fig. 2.3.
In this design, a spacer containing permeate flow channels is enclosed in a bag or
envelope made of two pieces of flat membrane sheet that are sealed on three edges. The
open edge attached and sealed to a slotted mandrel that acts as the permeate conduit and
the support for the membrane assembly. Other spacers are placed between a number of
leaves attached to the mandrel. The leaf and spacer assembly is then rolled around the
mandrel to form the spiral arrangement from which it derives it name. The assembly is
completed by applying a covering and insertion in a pressure vessel. Feed gases pass
axially through flow channels in the spacer-filled gaps between the spiral leaves. Gases
that permeate through the membrane pass along the spiral permeate channels to the
central collection pipe. Manufacturers utilizing the spiral-wound design have some
flexibility in the number and the length of the leaves they employ, spacer thickness and
design, and the axial length and package diameter. Particular care is taken in the spacer
design to ensure that loading arising from high flows and corresponding high pressure
differentials between the axial ends do not cause deformation and damage to the spiral
assembly. Some manufacturers add a perforated backing plate mounted on the
downstream side of the spiral assembly to minimize distortion and damage to the spiral
membrane assembly.
Advantages:
High pressure spiral elements can withstand pressures in excess of 1000 psi.
They are compact; high membrane packing density results in more efficient utilization of
floor space.
Low permeate pressure drop and membrane contamination.
Minimum concentration polarization.
Minimum pressure drop at the permeate channel.
(iv)Tubular Modules:
Modules with membranes in the form of tubes were the first to be used in
membrane applications. The obvious arrangement is a tube bundle fixed and
sealed at both ends, similar to a tubular heat exchanger. With the active separating
layer on the inside surface of the tube, and the feed flowing through the inner
lumen an even distribution and high velocities of the feed can be reached
and thus polarization effects minimized. Depending on the inner diameter of
the tube the ratio of feed volume to membrane surface is rather high, and the
feed stream cannot be heated inside the module. At high linear velocities this
may require partial recirculation of the feed or very small modules in series
with the respective large number of intermediate heat exchangers. The burst
pressure of the tubes has to be sufficiently high; otherwise additional porous
support structures around the outside of the tubes are necessary. Ceramic tubes
or tubes made from polymers with small diameters (capillaries or hollow fibers)
are sufficiently stable to be employed without an additional support. The removal
of the permeate from the outside surface of the tubes is not obstructed, the permeate
vapor can be condensed inside the module shell that has to be kept under vacuum.
Composite polymeric membranes can be formed into tubes or hoses of relatively large
diameter (12 to 22 mm) by winding a strip of a flatsheet membrane spirally around a
mandrel and welding (by heat or ultrasound) or gluing the edges together [42]. These
hoses need to be supported in a perforated tube, and sealing each tube individually on
both sides is complex and expensive. On the other hand, a module with such tubes could
be advantageous and economical for large-scale vapor permeation applications.
Advantages:
These can be operated with simple pretreatment of feed.
These can be mechanically cleaned by forcing a sponge ball through the tubes.
Membrane area / module space ratio is small.
Membrane contamination can be minimized by high feed flow rate.
Apart from the 4 modules, there are several other modules:
(a)Thin channel; (b)parallel leaf type; (c) transversal flow capillary module;
(d) monolithic; (e) cassette or flat spiral module; (f) rotating disc module etc.