Lecture_16-Membrane_Separation process[1].pdf

Separation Processes-I
(ChE-206)
Lecture No. 16
Membrane Separations

Membrane separations
(a) Membrane materials
(b) Membrane modules
(c) Membrane cascades
(d) Transport through porous membranes
(e) Liquid and gas diffusion through porous membranes
(f) Transport through nonporous membranes

What is a membrane?
• Membranes are materials which have voids in them letting some
molecules pass more conveniently than some other molecules.
• In a membrane-separation process, a feed consisting of a mixture of
two or more components is partially separated by means of a
semipermeable barrier (the membrane) through which some species
move faster than others.

Membrane separation process
(that part of the feed that does not pass through the
membrane)
Permeate(that part that does pass through the membrane)

What is a membrane?
Membrane provides an interphase separating two phases
and selectively controlling the transport of materials
between those phases.
•interphase not interface

Membrane Types
• Permeable Membrane
• Semipermeable Membrane

Driving Forces
• Pressure difference,
• Concentration difference,
• Voltage difference, etc.
• In membrane separations:
(1) the two products are usually miscible
(2) the separating agent is a semipermeable barrier
(3) a sharp separation is often difficult to achieve

Lecture_16-Membrane_Separation process[1].pdf

Process to disproportionate toluene into
benzene and xylene isomers

Membrane Materials
• Processed from natural polymers (cellulose, rubber)
• Custom-made synthetically
• Synthetic polymers are produced by condensation reactions, or from
monomers by free-radical or ionic-catalyzed addition (chain) reactions.
• The resulting polymer is categorized as having:
(1) a long linear chain, such as linear polyethylene
(2) A branched chain, such as polybutadiene
(3) A three dimensional, highly cross-linked structure, such as a condensation polymer
like phenol–formaldehyde
(4) A moderately cross-linked structure, such as butyl rubber or a partially cross-linked
polyethylene

Common Polymers used in membranes

Polymer membranes classification
• Dense
• For dense, amorphous membranes, pores of microscopic dimensions
may be present, but they are generally less than a few A° in diameter,
such that most, if not all, diffusing species must dissolve into the
polymer and then diffuse through the polymer between the segments
of the macromolecular chains.
• Microporous
• Microporous membranes contain interconnected pores and are
categorized by their use in microfiltration (MF), ultrafiltration (UF),
and nanofiltration (NF).

• Most microfiltration membranes have a symmetric pore structure,
and they can have a porosity as high as 80 per cent.
• Ultrafiltration and reverse osmosis membranes have an asymmetric
structure comprising a 1–2 μm thick top layer of finest pore size
supported by a ∼100 μm thick more openly porous matrix.

Membrane Modules
• In industry the equipment for MF, UF, and RO is applied in form of
modules.
• Membrane area is in the range of 1-20 m2.
• Modules can be connected in parallel or in series.
• Commonly used membrane shapes are:
• Flat sheets
• Tubular
• Hollow fibre
• Monolithic

• Flat sheets have typical dimensions of
1 m x 1 m x 200 mm thick, with a
dense skin or thin, dense layer 500 to
5,000 A in thickenss.
• Tubular membranes are typically 0.5
to 5.0 cm in diameter and up to 6 m
long.
• Very small-diameter hollow fibers, are
typically 42 µmm i.d. x 85 µmm o.d. x
1.2 m long with a 0.1- to 1.0-µmm
thick dense skin. The hollow fibers
provide a large membrane surface
area per unit volume.
• A honeycomb, monolithic element for
inorganic oxide membranes is
included in Figure 14.4d. Elements of
hexagonal and circular cross section
are available for filtration.

• The shapes in Figure 14.4 are incorporated into modules and
cartridges:
• Plate and frame
• Spiral-wound
• Four leaf spiral wound
• Hollow fiber
• Tubular
• Monolithic

Tubular modules
• Used in turbulent flow regime
• Concentration of high solids content feed
• Membrane is cast on the inside of a porous support which is often housed
in a perforated stainless steel pipe
• Individual modules contain a cluster of tubes in series held with in a
stainless steel permeate
• Diameter in 10-25 mm , 1-6 m length
• Feed is pumped through the tubes with Reynold Number greater than
10,000
• Can be easily cleaned
• Main disadvantages are the relatively low membrane surface area and
higher volumeric hold up

Flat sheet modules
• Similar to conventional filter
presses
• Consists of annular
membrane discs of outer
diameter 0.3 m
• Suitable for laminar flow
• A single module contains 19
m2 of membrane area
• Permeate is collected from
each membrane so that
damaged membranes can be
easily identified
• Replacement of membrane
requires dismantling of the
whole stack

Spiral-wound modules
• Consist of several flat membranes
separated by turbulence-promoting
mesh separators and formed into a
Swiss roll
• This produces a cylindrical module
which can be installed within a
pressure tube.
• Feed enters at one end of the
pressure tube and encounters a
number of narrow, parallel feed
channels formed between adjacent
sheets of membrane.
• Permeate spirals towards the
perforated central tube for collection.
• Up to six such modules may be
installed in series in a single pressure
tube.

Hollow-fiber modules
• Consist of bundles of fine
fibres, 0.1–2.0 mm in
diameter, sealed in a tube.
• For desalination
applications, feed flow is
usually around the outside
of the unsupported fibres
with permeation radially
inward.
• Capable of high pressure
operation
• Readily fouled and difficult
to clean

Book
• Seader, J. D.; Henley, E. J.; Roper, D. K., Separation Process Principles:
Chemical and Biochemical Operations. 3rd Ed.; John Wiley & Sons,
Inc.: 2011.
• Chapter 14: Membrane Separations

Lecture_16-Membrane_Separation process[1].pdf

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