By G. Srikanth
This article addresses different membrane products and processes in terms of their state of development and technical and economic relevance.
The development of ion exchange membranes some 40 years ago paved the way for membrane separation technology. Since then, due to a whole lot of technological innovations, especially in the area of new materials, membrane technologies have been established as very effective and commercially attractive options for separation and purification processes.
What is a membrane?
A membrane can be defined essentially as a semi-permeable barrier, which separates a fluid and restricts transport of various chemicals in a selective manner. A membrane can be homogenous or heterogeneous, symmetric or asymmetric in structure, solid or liquid, can carry a positive or negative charge or be neutral or bipolar. Transport through a membrane can be affected by convection or by diffusion of individual molecules, induced by an electric field or concentration, pressure or temperature gradient. The membrane thickness may vary from as small as 100 microns to several millimeters (mm).
Newer membrane technologies
Recently developed membrane based solvent extraction devices appear to eliminate high capital, operating and maintenance costs of centrifugal devices and additionally provide very high volumetric transfer rates. Systems have been designed using microporous membranes (hydrophobic or hydrophilic) for nondispersive solvent extraction.
Hollow-fiber-contained-liquidmembranes (HFCLM) have been used for gas separation through a nonporous polymeric membrane. Microporous polypropylene hollow fibers have been used as the membrane material. Gas separations such as N2-CO2, CH4-CO2, SO2-CO2-N2 and others have been studied by HFCLM technique.
A new class of intelligent ion-exchange membranes
Developed by the membrane research group at McMaster University, Canada, a new type of ion-exchange membrane is based on the simple concept of taking a cheap, chemically and physically rugged microporous membrane and filling the pores with a flexible, ionic ‘jelly’. This gel fill is anchored into place and confined by the microporous substrate.
These membranes, which in fact contain 70 percent water, show remarkable separation properties including very large chemical value effects. One feature of this new class of membranes is that they show ‘intelligence’: they are able to sense the nature of the solution in contact with the membranes and modify their properties.
These membranes find extensive application in electrodialysis, diffusion dialysis, nanofiltration, membrane solvent extraction and facilitated transport applications. The prototype membranes have excited the interest of a series of companies in different parts of the globe.
An interfacial polymerization/coating technique has recently been developed at McMaster University. It evenly coats all the interior surfaces of a microfiltration membrane with polymers such as polyamides, polyesters and polysulphonamides. The coatings, which cannot be removed, modify the surface chemistry and provide different types of properties for the membranes. Thus, the membrane remains microporous, but with different surface chemistry. Photochemically active groups have also been incorporated into the coated layer to modify their chemical properties.
Pervaporation membranes are being developed for the removal of trace organics from water using coatings derived from silicone based oligomers. Research in this area is also being carried out in collaboration with National University of Singapore and Industrial Membrane Research Institute, University of Ottawa. The membrane pervaporation performance test is also carried out using a completely automated testing apparatus that analyzes feed and permeate streams online for 24 hours continuously.
Other emerging membrane technologies
The other emerging membrane technologies with excellent application potential are membrane reactors, hydrogen generation, purification and degassing, hydrogen sorption in metals, membrane based transport devices, electrostatic pseudo-liquid membrane (ESPLIM), membrane distillation, controlled release, etc.
One breakthrough in the application of ionic-conducting polymer membranes is the proton exchange membrane fuel cell, a device that converts chemical energy directly into electrical energy without burning. Research in the area of design and preparation of encapsulating media for environmentally sensitive materials addresses phospholipid liposomes modified with synthetic polymers anchored on the membrane outer surface and non-phospholipid liposomes (NPL) prepared from commercially available surfactants.
Supported Liquid Membrane (SliM) process is the first membrane technology that is capable, in a single process, of selectively extracting multiple elements or compounds from a mixed process stream. The conventional methods for separating metals and/or removal from solubilized process streams presently include ion exchange, RO, UF, nanofiltration, precipitation and chromatography. Most of these methods have certain drawbacks, including lack of selectivity in the separation process, inability to handle certain metals in the process streams and (often) the creation of sludges and other harmful byproducts, which require further post-treatment prior to disposal.
SliM is a superior and cost-effective alternative to the existing forms of membrane filtration technologies. It involves passing a contaminated aqueous or gaseous feedstream through a hollow, porous fiber membrane. This membrane is previously loaded with chemicals whose composition varies depending on the targeted substance in the feedstream. As the feedstream enters the module, the metal or other substance to be extracted reacts with the proprietary chemical combination in the module and the metallic or other ions are extracted through the membrane into a strip solution which is concentrated and collected in a separate storage container. The balance of the feedstream is either recycled or simply discharged as normal effluent. This is an ideal system for finishers, electroplating shops, PCB manufacturers, remediation sites, landfills and mines.
Membrane separation processes can be treated as a good alternative to traditional filtration, ion-exchange and chemical treatment systems. Although the basic scientific principles behind membrane technology were developed in the 1950s, it was not until the 1970s that crossflow membrane technology (in the form of UF and RO) began to be recognized as an efficient, economical and reliable separation process. Purification systems utilizing crossflow membrane filtration, such as RO, NF or UF can be a good alternative to conventional systems.
A variety of industries are finding that it makes sense to re-evaluate the way they treat industrial processes, both to improve the quality of their products and increase the efficiency of their processes. The ongoing evolution of membrane technology allows greater flexibility in designing systems that function under a variety of operating conditions. The development of new membranes continues to expand both the range of chemical compatibilities and physical operating conditions (including pressure, temperature and pH) in membrane systems.
About the author
G. Srikantha is a scientist with Department of Science & Technology (Government of India). He can be reached at Department of Science & Technology, Technology Bhavan, New Mehrauli Road, New Delhi – 110016; Telephone: +91-11-26567373, 26962819; Fax: +91-11-26864570, 26862418 or via email to email@example.com. With the specific focus on business opportunities in such knowledge-based membrane technology, his work is the result of a detailed study in active collaboration with the National Chemical Laboratory, Pune, India. NCL is a research, development and consulting organization with a focus on chemistry and chemical engineering. It has a successful record of research partnership with industry.