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MEMBRANE SEPARATION

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MEMBRANE SEPARATION

Presented by : Arvind Singh Heer MSc-II(Sem-IV)Analytical ChemistryPaper-I MITHIBAI COLLEGE

MEMBRANE SEPARATION

CONTENTINTRODUCTIONREVERSE OSMOSISDIALYSISELECTRODIALYSIS

INTRODUCTION What is Membrane ?A membrane is a selective barrier that permits the separation of certain species in a fluid by combination of sieving and diffusion mechanisms.

Membranes can separate particles and molecules and over a wide particle size range and molecular weights.

REVERSE OSMOSIS

Reverse osmosis(RO) is a water purification technology that uses asemi permeable membraneto remove larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcomeosmotic pressure, acolligative property, that is driven bychemical potential, athermodynamicparameter. Reverse osmosis can remove many types of moleculesandionsfrom solutions, including bacteria, and is used in both industrial processes and the production of potable water. The result is that thesoluteis retained on the pressurized side of the membrane and the puresolventis allowed to pass to the other side. To be "selective", this membrane should not allow large molecules or ions through the pores(holes), but should allow smaller components of the solution (such as the solvent) to pass freely.

In the normalosmosisprocess, the solvent naturally moves from an area of low solute concentration (high water potential), through a membrane, to an area of high solute concentration (low water potential). The movement of a pure solvent is driven to reduce the free energy of the system by equalizing solute concentrations on each side of a membrane, generating osmotic pressure. Applying an external pressure to reverse the natural flow of pure solvent, thus, is reverse osmosis. The process is similar to other membrane technology applications. However, key differences are found between reverse osmosis and filtration.

The predominant removal mechanism in membrane filtration is straining, or size exclusion, so the process can theoretically achieve perfect efficiency regardless of parameters such as the solution's pressure and concentration. Reverse osmosis also involves diffusion, making the process dependent on pressure, flow rate, and other conditions.Reverse osmosis is most commonly known for its use in drinkingwater purificationfromseawater, removing thesaltand othereffluentmaterials from the water molecules.

PROCESS Osmosis is a natural process. When two liquids with different concentrations of a solute are separated by a semi permeable membrane, the fluid has a tendency to move from low to high solute concentrations for chemical potential equilibrium.Formally, reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a semi permeable membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. The largest and most important application of reverse osmosis is the separation of pure water from seawater andbrackish waters; seawater or brackish water is pressurized against one surface of the membrane, causing transport of salt-depleted water across the membrane and emergence of potable drinking water from the low-pressure side.

The membranes used for reverse osmosis have a dense layer in the polymer matrixeither the skin of an asymmetric membrane or an interracially polymerized layer within a thin-film-composite membranewhere the separation occurs. In most cases, the membrane is designed to allow only water to pass through this dense layer, while preventing the passage of solutes (such as salt ions). This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 217bar(30250psi) for fresh and brackish water, and 4082 bar (6001200 psi) for seawater, which has around 27 bar (390 psi)natural osmotic pressure that must be overcome. This process is best known for its use in desalination(removing the salt and other minerals fromsea waterto getfresh water), but since the early 1970s, it has also been used to purify fresh water for medical, industrial, and domestic applications.

DIALYSISA process for selectively removing low mol. wt. solutes from solution by allowing them to diffuse into a region of lower concentration through thin porous membranes. There is little or no pressure difference across the membrane and the flux of each solute is proportional to the concentration difference. Solutes of high mol. wt. are mostly retained in the feed solution, because their diffusivity is low and because diffusion in small pores is greatly hindered when the molecules are almost as large as the pores.Uses thin porous membranes

PRINCIPLEDialysis works on the principles of thediffusionof solutes andultra filtrationof fluid across asemi-permeable membrane. Diffusion is a property of substances in water; substances in water tend to move from an area of high concentration to an area of low concentration.Blood flows by one side of a semi-permeable membrane, and a dialysate, or special dialysis fluid, flows by the opposite side. A semi permeable membrane is a thin layer of material that contains holes of various sizes, or pores. Smaller solutes and fluid pass through the membrane, but the membrane blocks the passage of larger substances (for example, red blood cells, large proteins). This replicates the filtering process that takes place in the kidneys, when the blood enters the kidneys and the larger substances are separated from the smaller ones in theglomerulus.

ELECTRODIALYSISElectrodialysis(ED) is used to transportsaltionsfrom onesolutionthroughion-exchangemembranes to another solution under the influence of an appliedelectric potentialdifference. This is done in a configuration called an electrodialysis cell. The cell consists of a feed (dilute) compartment and a concentrate (brine) compartment formed by ananionexchange membrane and acationexchange membrane placed between twoelectrodes. In almost all practical electrodialysis processes, multiple electrodialysis cells are arranged into a configuration called an electrodialysis stack, with alternating anion and cation exchange membranes forming the multiple electrodialysis cells.

Electrodialysis processes are different thandistillationtechniques and other membrane based processes (such asreverse osmosis(RO)) in that dissolved species are moved away from the feed stream rather than the reverse. Because the quantity of dissolved species in the feed stream is far less than that of the fluid, electrodialysis offers the practical advantage of much higher feed recovery in many applications.Electrodialysis reversal(EDR) is an advanced electrodialysis process which utilizes a flow and polarity reversal to de-scale membrane surfaces and enable high concentration operation.EDR is used in higher salinity commercial applications where brine volume and scaling is a concern.

At the anode small amounts of oxygen gas are generated:Electrode reactions: Small amounts of hydrogen gas are generated at the cathode:

REFERENCE Handbook of Industrial Membrane Technology M.C. Porter

-THANK YOU


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