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Membrane Science and Engineering

(CHE F423)

BITS Pilani Dr. Bhanu Vardhan Reddy

Pilani Campus Assistant Professor in Chemical Engineering

BITS Pilani
Pilani Campus

Introduction to Membrane Separations

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Separation Process
• The separation of substances which mix spontaneously can be accomplished with
some device which consumes energy supplied in the form of heat or mechanical
work.
• The basic principle of any separation process is that a certain amount of energy is
required to accomplish the separation.
• Hence, two substances A and B will mix spontaneously when the free enthalpy of the
product (the mixture) is smaller than the sum of the free enthalpies of the pure
substances.
• The minimum amount of energy (𝑊!"# ) to accomplish complete separation is at
least equal to or larger than the free enthalpy of mixing.
𝑊!"# ≥ ∆𝐺! = ∆𝐻! − 𝑇∆𝑆!
• In practice, the energy requirement will be many times greater than this.

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Separation Process

• The minimum amount of energy necessary for the desalination of sea water
can be obtained by simple thermodynamic calculations.

• When 1 mol of solvent (in this case water) passes through the membrane, the
minimum work done when the process is carried out reversibly is:

𝑊 = 𝜋𝑣$ = 25×10% 𝑁/𝑚& . 18×10'( 𝑚) /𝑚𝑜𝑙

= 45 𝐽/𝑚𝑜𝑙

– Where 𝜋 is the osmotic pressure of sea water ~25 bar and 𝑣! is the molar volume of
water (~0.018 L/mol)
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Separation Process
• In 1861 at about the time that Graham reported his first dialysis experiments using
synthetic membranes

• Maxwell created the ‘sorting demon’: "a being whose faculties are so sharpened
that he can follow every molecule in its course and would be able to what is at
present impossible to us"

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Separation Process Mechanisms

Process Mechanism
Distillation Difference in vapor pressure or difference in
Membrane Distillation partial pressure
Freezing or crystallization Difference in freezing tendencies
Reverse Osmosis Difference in solubility, diffusivity of water, and
salt in the membrane
Electrodialysis Ion transport in charge selective ion-exchange
membranes

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How can a separation process be selected to solve a given problem?

• Since several factors influence the choice of the separation process but
are not generally applicable specific criteria often have to be met.

• However, two general criteria apply to all separation processes:

üthe separation must be feasible technically
üthe separation must be feasible economically

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• Characteristics of
membrane processes

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Membrane-Separation Process

• Most of the membrane separation processes are rate governed.

• Mainly dependent on the rate of transport of species through a barrier

• The driving force is the chemical potential

• Chemical potential can be:
– Concentration gradient
– Pressure gradient
– Temperature gradient
– Electrochemical gradient

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What is a membrane?

• The word “membrane” comes from Latin word for “skin”

• A selective barrier between two phases, the term 'selective'

being inherent to a membrane or a membrane process.

• This is a macroscopic definition while separation should be

considered at the microscopic level. The definition says nothing
about membrane structure nor membrane function.

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History of membrane separations

• 1748 Abbé Nolet coined the term “Osmosis”

• Nineteenth and early twentieth centuries, membranes had no industrial or

commercial uses, but were used as laboratory tools to develop physical/chemical
theories
• 1887 van’t Hoff developed limit law.… leading to van’t Hoff equation.

• Maxwell and others developed concept of perfectly semi-permeable membrane used

in development of kinetic theory of gases

• 1960s, Loeb–Sourirajan process for making defect-free, high-flux, anisotropic reverse

osmosis membranes
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Membrane Process

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Membrane Process
Parameters used to measure the performance:

Ø Retention
𝑐! − 𝑐"
𝑅=
𝑐!

Ø Separation Factor
𝑦# /𝑦%
𝛼#/% =
𝑥# /𝑥%
Ø Flux
𝐷
𝐽# = Δ𝑃 𝑚𝑜𝑙/𝑚! 𝑠
𝑡
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Types of membrane cross sections

Symmetrical Membranes

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Types of membrane cross sections

Anisotropic Membranes

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General Schematic of Membrane Process

• Feed mixture is separated into a
retentate (that part of the feed that does
not pass through the membrane) and a
permeate (that part that does pass
through the membrane).
• Although the feed, retentate, and
permeate are usually liquid or gas, in
bioprocesses, solid particles may also be
present.
• The barrier is most often a thin,
nonporous, polymeric film, but may also
be porous polymer, ceramic, or metal
material, or even a liquid, gel, or gas.

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Membrane Processes

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Membrane Technologies and applications

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Membrane industrial development

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Advantages of Membrane Separations

The benefits of membrane technology can be summarized as follows:

• separation can be carried out continuously;
• energy consumption is generally low;
• membrane processes can easily be combined with other separation
processes (hybrid processing);
• separation can be carried out under mild conditions;
• Up-scaling is easy;
• membrane properties are variable and can be adjusted;
• no additives are required.

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Drawbacks of Membrane Separations

The following drawbacks should be mentioned:

• concentration polarization/membrane fouling;
• low membrane lifetime;
• low selectivity or flux;
• up-scaling factor is more or less linear .

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Textbooks & References

Text Books:
• T1. Nath, K. (2017). Membrane separation processes. PHI Learning Pvt. Ltd.
• T2. Baker, R. W. (2012). Membrane Technology and Applications, Third Edition.

Reference Books:
• R1. Uragami, T., (2017), Science and Technology of Separation Membranes, Wiley.
• R2. Mulder, M (1991), Basic Principles of Membrane Technology, Second Ed., Kluwer Acad.
Pub.
• R3. Ho, W., & Sirkar, K. (2012). Membrane handbook. Springer Science & Business Media.
• R4. Ismail, A. F., Khulbe, K. C., & Matsuura, T. (2015). Gas Separation Membranes.
Springer.
• R5. Matsuura, T. (1993). Synthetic membranes and membrane separation processes. CRC
press.
• R6. Journal of Membrane Science: https://www.journals.elsevier.com/journal-of-
membrane-science

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