Mechanical separations (Types of filter membranes, ultra filtration, micro filtration, Nanofiltration, reverse osmosis)

Submitted to:

Madam Sidra Jabeen

Submitted by:

Muhammad Usman (08-chem-02-B)

Date: 14October, 2009

Department of Chemical Engineering

University of Engineering and Technology, Lahore

1. Microfiltration:Process of removing contaminants from a fluid by passing it through a micro porous membrane

Characteristics:1. Typical pore size ranges are 0.1-10 µm2. Excessively used in the filed of microbiology, safe drinking water.3. Microfiltration is used where sterile (a state of being free from biological contaminants) product

is required.4. Cake filtration is also sometimes considered as microfiltration but in this case filtration

rate is very low if particles are of 1µm or greater than it (due to high resistance of filter cake).

5. Driving force in this process is pressure differential across membrane.

2. Nanofiltration:1. It is a pressure driven separation process2. Filtration process takes place on a selective separation layer formed by an organic semi

permeable membrane.3. Because of the selectivity, one of several compounds of a dissolved mixture are retained by

membrane despite driving force4. Water and substances with a molecular weight less than 200 amu are able to permeate through

the membrane5. Charges of dissolved compounds also affect the selectivity of membrane. E.g. monovalent ions

will pass the membrane & divalent & multivalent ions will be rejected6. A demineralization of 75% is possible with Nanofiltration

3. Ultra filtration:In this process hydrostatic pressure force a liquid against a semi permeable membrane. Suspended solids, solutes of high molecular are retained, while water and low molecular weight solutes passes through.

Characteristics:1. Particle size ranges are (0.02-0.001 µm)2. Molecular weight cutoff ranges from (300- 300,000 amu)3. Pressure differential across the membrane provides suction. Transmembrane pressure is

generated by a pump on feed side and atmospheric pressure exist on permeate side.4. Sometimes is used as a pretreatment method for reverse osmosis.

5. Separation process is used for purifying & concentrating macro molecules (103-108amu) solutions, especially proteins.

4. Reverse osmosis:

Diffusion

Movement of molecules from a region of higher concentration to a region of lower concentration

Osmosis

A special case of diffusion in which the molecules are water and the concentration gradient occurs across a semi permeable membrane. The semi permeable 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.

Reverse osmosis:

Reverse osmosis occurs when the water is moved across the membrane against the concentration gradient, from lower concentration to higher concentration.

Example:

Semi permeable 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

Characteristics:

1. Particle size ranges from 0.0001-0.001 µm.2. Is done at high pressure differential (3000-1000 lb/in2)3. Used when molecular weight of particles to be retained is less than 300 4. Osmotic pressure of retained molecules is very high that is why this process is named as

reverse osmosis.5. Often used in commercial & residential water filtration.6. One of the methods used to desalinate sea water.

Types of membranes on the basis of their construction materials:Membranes are classified into following types…

1. Sintered metal membrane:Ultra filtration and microfiltration membranes are made from sintered stainless steel. With usual methods a large pore distribution of 1-100µm can be achieved. Narrow size distribution can e achieved by partially filling the surface layer with zirconium oxide or other inorganic material.

Inorganic membranes can operate at higher temperature than polymer membranes and can better stand with chemical cleaning and sterilization treatment.

2. Polymer membranes:Membranes are made up of many polymers that are in the form of thin selective skin supported on a thick layer that supports thin layer and make it withstand with hydraulic resistance.

In industrial applications these membranes are supported by porous metal or ceramic tubes.

Common Polymers used are …

a. Cellulose acetateb. Polyacrylonitrilec. Poly sulfoned. Polyamidee. Polyimide

3. Miscellaneous membranes:Porous metals, porous carbon or alumina are also used in the preparation of membranes.

Silicon and some other ceramic materials are also used as membrane constituent elements.