1. Department of Chemical Engineering Separation Processes – 1 Module -1 Membrane Separation Processes
Prof. Mohammad Asif
Room 2B45, Building 3
Tel:

2. Module -1 Books Main Topic Text Book Reference Book
Separation Process Principles,
3rd Edition
ISBN:
J. Seader, E. Henley, D. Roper
Text Book
Transport Processes and Separation Process Principles,
4 edition Prentice Hall
ISBN:
C. J. Geankoplis
Reference Book

3. Lecture Schedule Till Semester Break
Event
Topics
Dates
Week
Introduction 1
Module 1 2
Surprise Quiz 3 4 5 6 7
Test 1: Monday
Module 2 8

4. Lecture Sequence: Module 1
General introduction
Industrial applications
Membrane material
Membrane modules
Module flow patterns
Module cascades
Transport in membranes
Concentration polarization
Reverse osmosis
Gas permeation
Dialysis

5. Membrane based separation processes
Reverse osmosis:
Transport of solvent in the opposite direction, against the concentration gradient, is affected by imposing a pressure, higher than the osmotic pressure, on the feed side using a nonporous membrane.
Dialysis:
Transport by a concentration gradient of small solute molecules through a micro-porous membrane. Smaller molecules are able to pass through the membrane.
Microfiltration:
A microporous membrane selectively allow passage of small solute molecules/solvent. This prevents passage of large dissolved molecules and suspension. Microfiltration retains to 10 µm while Ultrafiltration retains 1 to 20 nm.
Gas permeation:
This involves separation of gas mixtures. The pressure on feed side is much higher than permeate side.
Pervaporation
The phase on one side of the pervaporation membrane is different from that on the other. Feed to the membrane module is a liquid mixture.

6. Reverse Osmosis
Osmosis and reverse osmosis phenomena: (a) Initial condition, (b) At equilibrium after osmosis, (c) Reverse osmosis ( is called osmotic pressure)
Osmosis refers to passage of a solvent, such as water, through a dense membrane.
Membrane is permeable to solvent, but not to solutes.
For reverse osmosis (RO) to occur, the pressure difference must be greater than the osmotic pressure (), which is a function of solute concentration and the temperature, and can be predicted by, π=1.12 𝑇 𝑚 𝑖 where 𝜋 is in psia, T is in K, and 𝑚 𝑖 is the molarity in mol/L.

7. Applications of reverse osmosis:
Desalination of sea water, brackish water
Purification of waste water
treatment of industrial wastewater to remove heavy-metal ions, non biodegradable substances, and other components of possible commercial value
Treatment of rinse water from electroplating processes to obtain a metal-ion concentrate and a permeate that can be reused as a rinse
Separation of sulfites and bisulfites from effluents in pulp and paper processes
Treatment of wastewater in dyeing processes
Recovery of constituents having food value from wastewaters in food processing plants (e.g., lactose, lactic acid, sugars, and starches)
Treatment of municipal water to remove inorganic salts, low-molecular-weight organic compounds, viruses, and bacteria
Dewatering of certain food products such as coffee, soups, tea, milk, orange juice, and tomato juice
Concentration of amino acids and alkaloids.

8. For a general case of reverse osmosis when solutes are present on both sides of the membrane, the trans-membrane flux of water is given by,
𝑁 𝑤𝑎𝑡𝑒𝑟 = 𝑃 𝑀 𝑤𝑎𝑡𝑒𝑟 𝑙 𝑀 ∆𝑃−∆𝜋
∆𝑃= 𝑃 𝑓𝑒𝑒𝑑 − 𝑃 𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒 ; ∆𝜋= 𝜋 𝑓𝑒𝑒𝑑 − 𝜋 𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒
For pure solvent, 𝜋 𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒 =0
At 250C, typical values of osmotic pressures are,
𝐶 𝑁𝑎𝐶𝑙 =1.5 𝑔/𝐿 => 𝜋 1 ≈17.1 𝑝𝑠𝑖𝑎
𝐶 𝑁𝑎𝐶𝑙 =35 𝑔/𝐿 => 𝜋 1 ≈385 𝑝𝑠𝑖𝑎
For reverse osmosis; ∆𝑃>∆𝜋
Generally ∆ 𝑃 1 =400−600 𝑝𝑠𝑖𝑎
∆ 𝑃 2 =800−1000 𝑝𝑠𝑖𝑎
Salt passage, 𝑆𝑃= 𝐶 𝑠𝑎𝑙𝑡 𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒 𝐶 𝑠𝑎𝑙𝑡 𝑓𝑒𝑒𝑑 ; Salt rejection, 𝑆𝑅= 1−𝑆𝑃
Generally, 𝑆𝑃 ↑ => ∆𝑃↓
The trans-membrane flux of salt is given by,
𝑁 𝑠𝑎𝑙𝑡 = 𝑃 𝑀 𝑠𝑎𝑙𝑡 𝑙 𝑀 𝐶 𝐹 𝑠𝑎𝑙𝑡 − 𝐶 𝑃 𝑠𝑎𝑙𝑡

9. EXAMPLE: Reverse Osmosis
At a certain point in a spiral-wound membrane, the bulk conditions on the feed side are 1.8 wt% NaCl, 250C, and 1,000 psia, while bulk conditions on permeate side are 0.05 wt% NaCl, 250C, and 50 psia. For this membrane the permeance values are 1.1×10-5 g/cm2-s-atm for H2O and 16×10-6 cm/s for the salt. If mass-transfer resistances are negligible, calculate the flux of water and salt.
Step 1: Molar concentration of solute
Step 2: Osmotic pressure (NaCl molecule = 2 ions)
Step 3: Water flux, 𝑁 𝑤𝑎𝑡𝑒𝑟 = 𝑃 𝑀 𝑤𝑎𝑡𝑒𝑟 𝑙 𝑀 ∆𝑃−∆𝜋
Step 4: Salt flux, 𝑁 𝑠𝑎𝑙𝑡 = 𝑃 𝑀 𝑠𝑎𝑙𝑡 𝑙 𝑀 𝐶 𝐹 𝑠𝑎𝑙𝑡 − 𝐶 𝑃 𝑠𝑎𝑙𝑡
𝑁 𝑠𝑎𝑙𝑡 =16× 10 − − = 4.86× 10 −9 𝑚𝑜𝑙 𝑐𝑚 2 𝑠

10. Concentration polarization effects in Reverse Osmosis
The flux of water to the membrane carries with it salt by bulk flow. Since the salt cannot pass through the membrane, its concentration (of the salt) in the liquid adjacent to the membrane surface starts to build up. Therefore, 𝐶 𝑠 𝑖 = 𝐶 𝑠 𝐹 Since 𝐶 𝑠 𝑖 − 𝐶 𝑠 𝐹 >0, this causes back diffusion of salt from the membrane surface back to the bulk feed. The back rate of salt diffusion depends upon the film mass transfer coefficient. For low 𝑘 𝑆 , the difference 𝐶 𝑠 𝑖 − 𝐶 𝑠 𝐹 is more. The value of 𝐶 𝑠 𝑖 is important since 𝜋( 𝐶 𝑠 𝑖 ) which affects the driving force for water transport.

11. Reverse Osmosis Process