Transcript ChE 316 LMS RO

Department of Chemical Engineering
Separation Processes – 1
Module -1
Membrane Separation Processes
Prof. Mohammad Asif
Room 2B45, Building 3
http://faculty.ksu.edu.sa/masif
Tel: +966 1 467 6849
Module -1
Main Topic
Books
Text Book
Separation Process Principles,
3rd Edition
ISBN: 978-0470481837
J. Seader, E. Henley, D. Roper
Reference Book
Transport Processes and Separation
Process Principles,
4 edition Prentice Hall
ISBN: 978-0131013674
C. J. Geankoplis
Lecture Schedule Till Semester Break
Week
Dates
Topics
1
Introduction
2
Module 1
3
Module 1
4
Module 1
5
Module 1
6
Module 1
7
Module 1
8
Module 2
Event
Surprise Quiz
Test 1: Monday
Lecture Sequence: Module 1
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General introduction
Industrial applications
Membrane material
Membrane modules
Module flow patterns
Module cascades
Transport in membranes
Concentration polarization
Reverse osmosis
Gas permeation
Dialysis
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 microporous 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 0.02 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.
Reverse Osmosis
Osmosis and reverse osmosis phenomena: (a) Initial condition, (b) At equilibrium after osmosis,
(c) Reverse osmosis ( is called osmotic pressure)
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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 𝑚𝑖 where 𝜋 is in psia, T is in K, and 𝑚𝑖 is the molarity in mol/L.
Applications of reverse osmosis:
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Desalination of sea water, brackish water
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Purification of waste water
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treatment of industrial wastewater to remove heavy-metal ions, non
biodegradable substances, and other components of possible commercial value
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Treatment of rinse water from electroplating processes to obtain a metal-ion
concentrate and a permeate that can be reused as a rinse
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Separation of sulfites and bisulfites from effluents in pulp and paper processes
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Treatment of wastewater in dyeing processes
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Recovery of constituents having food value from wastewaters in food processing
plants (e.g., lactose, lactic acid, sugars, and starches)
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Treatment of municipal water to remove inorganic salts, low-molecular-weight
organic compounds, viruses, and bacteria
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Dewatering of certain food products such as coffee, soups, tea, milk, orange
juice, and tomato juice
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Concentration of amino acids and alkaloids.
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,
𝑃𝑀
𝑁𝑤𝑎𝑡𝑒𝑟 = 𝑤𝑎𝑡𝑒𝑟 ∆𝑃 − ∆𝜋
𝑙𝑀
∆𝑃 = 𝑃𝑓𝑒𝑒𝑑 − 𝑃𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒 ;
∆𝜋 = 𝜋𝑓𝑒𝑒𝑑 − 𝜋𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒
For pure solvent, 𝜋𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒 = 0
At 250C, typical values of osmotic pressures are,
𝐶𝑁𝑎𝐶𝑙 = 1.5 𝑔/𝐿
=>
𝜋1 ≈ 17.1 𝑝𝑠𝑖𝑎
𝐶𝑁𝑎𝐶𝑙 = 35 𝑔/𝐿
=>
𝜋1 ≈ 385 𝑝𝑠𝑖𝑎
For reverse osmosis; ∆𝑃 > ∆𝜋
Generally
∆𝑃1 = 400 − 600 𝑝𝑠𝑖𝑎
∆𝑃2 = 800 − 1000 𝑝𝑠𝑖𝑎
Salt passage, 𝑆𝑃 =
𝐶𝑠𝑎𝑙𝑡 𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒
𝐶𝑠𝑎𝑙𝑡 𝑓𝑒𝑒𝑑
Generally, 𝑆𝑃 ↑ =>
;
∆𝑃↓
The trans-membrane flux of salt is given by,
𝑃𝑀𝑠𝑎𝑙𝑡
𝑁𝑠𝑎𝑙𝑡 =
𝐶𝐹𝑠𝑎𝑙𝑡 − 𝐶𝑃𝑠𝑎𝑙𝑡
𝑙𝑀
Salt rejection, 𝑆𝑅 = 1 − 𝑆𝑃
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, 𝑁𝑤𝑎𝑡𝑒𝑟 =
Step 4: Salt flux, 𝑁𝑠𝑎𝑙𝑡 =
𝑃𝑀𝑤𝑎𝑡𝑒𝑟
𝑃𝑀𝑠𝑎𝑙𝑡
𝑙𝑀
𝑙𝑀
∆𝑃 − ∆𝜋
𝐶𝐹𝑠𝑎𝑙𝑡 − 𝐶𝑃𝑠𝑎𝑙𝑡
𝑁𝑠𝑎𝑙𝑡 = 16 × 10−6 0.331 − 0.00855 = 4.86× 10−9 𝑚𝑜𝑙 𝑐𝑚2 𝑠
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,
𝐶𝑠𝑖 = 𝐶𝑠𝐹
Since 𝐶𝑠𝑖 − 𝐶𝑠𝐹 > 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 𝑘𝑆 , the difference 𝐶𝑠𝑖 − 𝐶𝑠𝐹 is more. The
value of 𝐶𝑠𝑖 is important since 𝜋(𝐶𝑠𝑖 ) which affects
the driving force for water transport.
Reverse Osmosis Process