Membrane Processes

1. Membrane Processes CE 370

2. Membrane Processes • Are used to separate substances (solutes) from a solution (solvent) • The main membrane processes are • Dialysis • Electro-dialysis • Reverse osmosis • Driving forces that cause mass transfer of solutes are: • Difference in concentration (dialysis) • Difference in electric potential (electro-dialysis) • Difference in pressure (reverse osmosis)

6. Dialysis • Consists of : • Separating solutes of different ionic or molecular size • Solution • Selectively permeable membrane • The driving force is the difference in the solute concentration across the membrane

9. Batch Dialysis Cell • Solution to be dialyzed is separated from solvent by a semi-permeable membrane • Small ions and molecules pass from solution to solvent • Large ions and molecules do not pass due to relative size of membrane pore • The mass transfer of solute through the membrane is given by • M = mass transferred per unit time (gram/hour) • K = mass transfer coefficient [gram/(hr-cm2)(gram/cm3)] • A = membrane area (cm2) • C = difference in concentration of solute passing through the membrane (gram/cm3)

10. Applications of Dialysis • Sodium hydroxide was recovered from textile wastewater at: • Flowrate = 420 – 475 gal/day • Recovery of 87.3 to 94.6% • Dialysis is limited to small flows due to small mass transfer coefficient (K)

11. Electro-Dialysis • The driving force is an electromotive force • If electromotive force is applied across the permeable membrane: • An increased rate of ion transfer will occur • This results in decrease in the salt concentration of the treated solution • The process demineralizes • Brackish water and seawater to produce fresh water • Tertiary effuents

12. How it Works? • When direct current is applied to electrodes: • All cations (+vely charged) migrate towards cathode • All anions (-vely charged) migrate towards anode • Cations can pass through the cation-permeable membrane (C) but can not pass through (A) • Anions can pass through the anions-permeable membrane (A) but can not pass through (C) • Alternate compartments are formed • Ionic concentration in compartments is less than or greater than that in the feed solution

18. The Membrane • Membranes used in electro-dialysis are: • Porous • Sheet-like • Its structural matrix is made of synthetic ion exchange resin

19. Current Requirement • Can be calculated from Faraday’s laws of electrolysis: • One Faraday (F) of electricity (96,500 ampere-seconds or coulombs) cause one gram equivalent weight of a substance to migrate from one electrode to another • I = current in amperes • F = Faraday’s constant (96,500 ampere-seconds per gram equivalent weight removed) • Q = solution flowrate (liters/second) • N = normality of the solution (gram eq weight per liter) • Er = electrolyte removal as a fraction • Ec = current efficiency as a fraction

20. Current Requirement • If the number of cells in a stack = n, then • Electro-dialysis stack usually have 100 to 250 cells (200 to 500 membranes) • Ec for a electro-dialysis stack and feed water must be determined experimentally • Ec is 0.90 or more • Er is usually 0.25 to 0.50

21. Cell Capacity • The capacity of the cell to pass an electric current depends on: • Current density [ = current / membrane area (ma/cm2)] • Normality of the feed (number of gram equivalent weight per liter of solution) • Current density / normality ratio • This ratio may vary from 400 to 700

22. Power Requirement • The resistance (R) of an electro-dialysis stack treating a particular feed must be determined experimentally • If resistance (R) and current (I) are known: • Required Voltage, E = RI • Required Power, P = RI2 • R = ohms; I = amperes; E = volts; and P = watts

24. Applications • Electrical energy requirement is directly proportional to the amount of salt removed • So, electrical cost is governed by • Dissolved salt content of the feed water • The desired dissolved solids content of the product water • Energy consumption increases with deposition of scale upon the membrane • Consequently, electro-dialysis is not used to deionize seawater

25. Applications • Electro-dialysis is used in demineralization of brackish water • Brackish water having TDS concentration of 500 mg/l can be de-mineralized using electro-dialysis to produce a product water of 500 mg/l TDS • Membrane replacement and power costs are about 40% of total cost • Electro-dialysis have been used to de-mineralize secondary effluents • Scale formation • Organic fouling • 25 to 50% TDS can be removed in single pass • Coagulation, settling, filtration and activated carbon adsorption can used as pre-treatment processes to reduce organic fouling OR by cleaning the membrane using an enzyme detergent solution • Scale formation can be reduced by adding small amount of acid to the feed

26. Electro-Dialysis Installations

31. Reverse Osmosis Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration. Osmosis is a special case of diffusion in which the molecules are water and the concentration gradient occurs across a semipermeable membrane. The semipermeable membrane allows the passage of water, but not ions (e.g., Na+, Ca2+, Cl-) or larger molecules (e.g., glucose, urea, bacteria). Diffusion and osmosis are thermodynamically favorable and will continue until equilibrium is reached. Osmosis can be slowed, stopped, or even reversed if sufficient pressure is applied to the membrane from the 'concentrated' side of the membrane.

37. Reverse Osmosis Reverse osmosis occurs when the water is moved across the membrane against the concentration gradient, from lower concentration to higher concentration. To illustrate, imagine a semipermeable membrane with fresh water on one side and a concentrated aqueous solution on the other side. If normal osmosis takes place, the fresh water will cross the membrane to dilute the concentrated solution. In reverse osmosis, pressure is exerted on the side with the concentrated solution to force the water molecules across the membrane to the fresh water side.

41. Reverse Osmosis Reverse osmosis is often used in commercial and residential water filtration. It is also one of the methods used to desalinate seawater. Sometimes reverse osmosis is used to purify liquids in which water is an undesirable impurity (e.g., ethanol).

42. Reverse Osmosis - Pros and Cons The semi-permeable membrane used in reverse osmosis contains tiny pores through which water can flow. The small pores of this membrane are restrictive to such organic compounds as salt and other natural minerals, which generally have a larger molecular composition than water. These pores are also restrictive to bacteria and disease-causing pathogens. Thus, reverse osmosis is incredibly effective at desalinating water and providing mineral-free water for use in photo or print shops. It is also effective at providing pathogen-free water. In areas not receiving municipally treated water or at particular risk of waterborne diseases, reverse osmosis is an ideal process of contaminant removal.

43. Reverse Osmosis - Pros and Cons The reverse osmosis process contains several downsides which make it an inefficient and ineffective means of purifying drinking water. The small pores in the membrane block particles of large molecular structure like salt, but more dangerous chemicals like pesticides, herbicides, and chlorine are molecularly smaller than water (Binnie et al, 2002). These chemicals can freely pass through the porous membrane. For this reason, a carbon filter must be used as a complimentary measure to provide safe drinking water from the reverse osmosis process. Such chemicals are the major contaminants of drinking water after municipal treatment.

44. Reverse Osmosis - Pros and Cons Another downside to reverse osmosis as a method of purifying drinking water is the removal of healthy, naturally occurring minerals in water. The membrane of a reverse osmosis system is impermeable to natural trace minerals. These minerals not only provide a good taste to water, but they also serve a vital function in the body’s system. Water, when stripped of these trace minerals, can actually be unhealthy for the body.

45. Reverse Osmosis - Pros and Cons Reverse osmosis also wastes a large portion of the water that runs through its system. It generally wastes two to three gallons of water for every gallon of purified water it produces. Reverse osmosis is also an incredibly slow process when compared to other water treatment alternatives.

46. Module Types • Spiral Wound • Hollow Fiber • Tubular