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Short notes on $-$ a) reverse osmosis b) electro dialysis c) ultra-filtration

Mumbai University > First Year Engineering > Sem1 > Applied Chemistry 1

Marks: 6M

Year: June 2014

1 Answer
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a) Reverse Osmosis:

  • When 2 solutions of different concentrations are separated by a semi-permeable membrane, solvent flows from a low region concentration to higher one until concentration is equal on both sides.

  • This process is called osmosis and is used for the removal of dissolved salts from sea water called desalting of water.

  • Principle of reverse osmosis-in this process dissolved salts are separated from water by using semi-permeable membrane placed between water containing dissolved salts and pure water.

  • The pure water now flows through this membrane into salty water due to osmotic pressure.

  • This natural tendency of water is reversed by applying a higher pressure on the salty water part which is forced to flow to pure water having lower concentration. This is called reverse osmosis.

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b) Electro dialysis:

  • Electro Dialysis (ED) is a membrane process, during which ions are transported through semi permeable membrane, under the influence of an electric potential.

  • It is an electrochemical process in which ions migrate through ion-selective semipermeable membranes as a result of their attraction to two electrically charged electrodes.

  • ED is able to remove most charged dissolved ions.

  • The membranes are cation- or anion-selective, which basically means that either positive ions or negative ions will flow through.

  • Cation-selective membranes are polyelectrolytes with negatively charged matter, which rejects negatively charged ions and allows positively charged ions to flow through.

  • By placing multiple membranes in a row, which alternately allow positively or negatively charged ions to flow through, the ions can be removed from wastewater.

  • Benefits: ED can operate with minimal fouling or scaling or chemical addition, low pressure requirements, long membrane life expectancy, low chemical usage for pretreatment.

c) Ultrafilteration - It is a cross flow separation process.

  • Principle Here the liquid is to be treated, flows tangentially along the membrane surface, thereby producing two streams. The stress of liquid that comes through the membrane is called permeate.

  • Ultrafilteration is the process using membrane with pore size of the range of 0.1 to 0.001 micron.

  • The other liquid stream is called concentrate which gets progressively concentrated in those species removed by the membrane.

  • In cross flow separation, therefore, the membrane itself does not act as a collector of ions, molecules, or colloids but merely as a barrier to these species.

  • Ultrafilteration is used in industry to separate suspended solids from solution.

  • The particles that are removed vary in size, and their removal is a function of the pore size.

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