MICHAEL FREEMANTLE, C&EN LONDON
The chemical process industries mostly rely on conventional techniques, such as distillation and solvent extraction, to carry out separations. However, the need to reduce or eliminate by-product waste streams and environmental contaminants, while at the same time improving the purity of chemical and biological products, is driving the development of novel separation technologies such as liquid chromatography, electrophoresis, supercritical fluid extraction, ionic liquid acid scavenging (C&EN, March 31, page 9), and membrane filtration.
Membrane systems, in particular, are expected to play an increasingly important role in the future for concentrating, fractionating, and purifying liquid streams in industry. A variety of membrane filtration systems were on display at the ACHEMA 2003 exhibition.
||EXTRACTION Christopoulou (left) and MET process engineer Dinesh Nair exhibit cell used for reverse-osmosis and nanofiltration separations. PHOTO BY MICHAEL FREEMANTLE
"Membranes are often the separation technology of choice because they can be customized in many ways and they offer a cost-effective way to achieve the selectivity that is required by the process," notes an ACHEMA 2003 trend report on separation technology. The report points out that the total demand in the U.S. for membrane systems, including associated pumps, pipes, and vessels, is projected to approach $6 billion in 2006.
Four membrane filtration processes--microfiltration, ultrafiltration, nanofiltration, and reverse osmosis--are employed in industry, depending on what needs to be separated.
Microfiltration, which can be used for clarification of fermentation broth and biomass, is a low-pressure, cross-flow membrane process for separating colloidal and suspended micrometer-size particles. "Almost all membrane filtration is carried out as cross-flow filtration to avoid the formation of a filter cake and a high concentration of solutes or solids on the membrane surface," explained Johan Persson at the exhibition. Persson is a public relations coordinator at the Swedish multinational engineering group Alfa Laval, based in Lund.
"The liquid flows parallel to the membrane at high velocity and under pressure, thereby splitting the feed stream into two streams, one of which passes through the membrane," Persson said. "The continuous flow of liquid across the membrane performs a cleaning action, whereby fouling is reduced and the concentration on the surface is decreased to ease passage through the membrane."
Ultrafiltration, typically used for concentration and purification, employs membranes with smaller pore sizes than those used for microfiltration. The process, which uses pressures up to 145 psi, concentrates suspended solids and solutes with molecular weights greater than 1,000. Salts, sugars, organic acids, and small peptides permeate the membranes, leaving behind proteins, fats, and polysaccharides.
Nanofiltration uses membranes to separate lower molecular weight organic solutes and small inorganic ions. It is applied for color removal and demineralization, for example. And reverse osmosis, which employs membranes with the smallest pore sizes, is a high-pressure water-removing process that is widely used to purify wastewater and desalinate seawater.
IN ALL FOUR TYPES of membrane filtration, the liquid that passes through the membrane is known as the permeate, and the material that does not pass through is called the retentate. The membranes are made of a variety of materials, including polymers, ceramics, and metals. They are usually manufactured as flat sheets mounted on supports or as spiral-wound or tubular modules.
"The pores of the membrane material are so small that considerable pressure is required to drive the liquid through them," Persson told C&EN at ACHEMA. "A pump supplies hydraulic pressure on one side of the membrane, while the other is at atmospheric pressure. The pressure difference across the membrane is the driving force that allows the separation to take place. The pressure required varies inversely with the size of the pores in the membrane and also depends on the product that is filtered." He pointed out that, because the separation of molecules that occurs when a feed stream flows across a membrane is only partial, all membrane systems involve recycling of the feed material in various ways.
Alfa Laval has a range of membrane filtration products that cover all four processes and are designed for use in the biotech, pharmaceutical, and food industries. The company used ACHEMA to exhibit its plate-and-frame cross-flow membrane filtration module, M39. It consists essentially of a frame containing a series of parallel flat-sheet membranes mounted on plates.
"The modules are fitted with a range of Alfa Laval membranes made of different polymers--cellulose acetate, triacetate, polysulfone, polyethersulfone, or fluoropolymers," explained Gary Lloyd, an account manager at the firm's office in Copenhagen, Denmark. "The membranes are based on a unique construction of polypropylene support material."
Thomas Zachrisson, a general manager at Alfa Laval in Tumba, Sweden, pointed out that the modules are suitable for ultrafiltration and microfiltration of highly viscous products and fermentation broths and were developed for duties in the processing of antibiotics, enzymes, blood products, and yeast extracts.
The membrane filtration unit is available in membrane areas up to 645 sq ft and can handle operating temperatures up to 80 °C and pH values between 1 and 14. It can be built into systems suitable for either continuous or batch production.
Koch Membrane Systems, a company headquartered in Wilmington, Mass., highlighted what it claims to be the world's largest reverse-osmosis element at ACHEMA. Named the MegaMagnum, the spiral element contains more than 2,400 sq ft of membrane surface area. The membrane is a thin-film composite material.
"With this element, customers can install reverse-osmosis and nanofiltration systems that achieve space savings of up to 15%, require fewer manifolds and pressure vessels, and cut capital and civil-works costs by more than 20%," said Tobias Haarburger, a sales manager at Koch's Membrane Systems Division in Düsseldorf, Germany.
The MegaMagnum element, which is currently in field tests in the U.S. and Europe, is designed for seawater desalination and brackish water purification. The company intends to offer it commercially later this summer. "The economies of scale offered by an element of this size make it an affordable option in large- and medium-scale municipal plant projects," Haarburger said.
Membrane Extraction Technology (MET), a company based at Imperial College, London, is the sole European distributor of a range of organic solvent nanofiltration (OSN) membranes known as STARMEM, manufactured by Grace Davison Membranes, a division of Columbia, Md.-based W.R. Grace.
THE MEMBRANES are made of polyimide and are stable in solvents such as toluene, xylene, ethyl acetate, and hexane at temperatures up to 60 °C. They are available with molecular-weight cut-offs between 200 and 400 daltons. A molecular-weight cut-off is defined as the molecular weight at which the membrane rejects 90% of solute molecules.
|STAINLESS STEEL MembraLine pressure vessel produced by German company Sommer + Strassburger houses membranes. STETTIN/DECHEMA PHOTO
"STARMEM has the lowest molecular-weight cut-offs available," observed MET Commercial Development Manager Lina Christopoulou. "This means that the membranes can be used to filter tiny molecules."
MET has investigated several potential applications for OSN, including separating antibiotics from organic solvents, exchanging a high-boiling-point solvent such as toluene for a low-boiling-point one like methanol, and separating catalysts from products in processes such as phase-transfer catalysis and organometallic catalysis.
"OSN technology is available from lab and pilot-plant scale to production-plant scale," Christopoulou said. "The commercialization of OSN took place in December last year. ACHEMA is the first time that these products have been exhibited in public. Indeed, module-sized versions of the membranes, for use in large-scale operations, only became available at the beginning of March.
"OSN is a new technology which, among other things, offers a separation alternative to distillation and chromatography, both of which are unsuitable for sensitive products or large volumes," she continued. "Imagine that you are producing an antibiotic, and part of the reaction chain leaves you with a temperature-sensitive product in an organic solvent. You could evaporate the solvent off, but heat would destroy your product of interest. OSN does not require heat, and moreover, the solvent is retained and can be reused."
MET also used ACHEMA to launch METcell, a new piece of equipment for benchtop reverse-osmosis and nanofiltration separations. "METcell is a stainless steel, high-pressure stirred cell that is capable of performing a wide range of membrane separations," Christopoulou said. "It is suitable for small-scale testing, using both aqueous and nonaqueous solvents. It is the only cell of its size that can be used in continuous operation, which means that you do not need to reopen the cell to charge it with solution."
Puron was another company that exhibited membranes for water treatment at ACHEMA. The company is located in Aachen, Germany. Puron membrane filters are used for municipal wastewater treatment, drinking water production, and industrial applications. They consist of several 2-meter-long bundles of membranes mounted in rows in a frame. Each membrane is a hollow fiber, 2 mm in diameter, made of a membrane-active film of polyethylsulfone supported on a polyester.
"The top ends of the fibers are sealed," said Puron Project Leader Olaf Kiepke. "The bundles are fixed in a resin collector at the bottom of the frame and immersed in the raw water like seaweed. The filtrate is drawn through the membranes using slight suction. A central air nozzle integrated into the collector ensures that the fibers are kept in motion to prevent clogging. The membrane pores are so small they even form a barrier to germs like bacteria and viruses."
Rhodia Orelis, based in Saint Maurice de Beynost, France, exhibited a range of organic and inorganic membranes and modules with plate-and-frame, spiral, and tubular construction. The products are used in industry for treating effluents, recycling wastewater, and other applications.
THE PRODUCTS on display included KERASEP membrane tubular modules, which the company launched in March of this year. The modules are ceramic tubes with up to 27 channels, each lined with a film of a ZrO2 or TiO2 membrane.
"The membranes can be used for microfiltration, ultrafiltration, or nanofiltration," Communications Manager Nathalie Garassino told C&EN. "They have a wide range of cut-offs for the separation of organic molecules, water-soluble polymers, emulsions, and specific inorganic products."
Applications of KERASEP include clarification of soft drinks, fruit juice, and fermentation liquids, microfiltration of milk, recycling of cleaning water in the automotive industry, and treatment of biodegradable wastewater.
Ceramic membranes offer a number of advantages over their organic counterparts. They have greater mechanical strength; are durable and resistant to acids, alkalis, and organic solvents; and can be washed at high temperatures and even sterilized by steam.
At ACHEMA, General Electric Osmonics, which has its corporate headquarters in Minnetonka, Minn., displayed its UltraFilic M-Series membrane elements for separating oil and water. The membranes are made of a polyacrylonitrile polymer that is both hydrophilic and oleophobic.
"Oil and water separation by ultrafiltration is a well-proven technology," noted District Sales Manager Pierre-Yves Melchior, who is based in Le Mée sur Seine, France. "It is used in the petroleum and gas industries and also for removing free oils from waste streams in industrial plants. However, fouling by free oil is a common problem. The pore structure of our membranes creates a surface filter with its smallest opening at the outer skin of the membrane. As a result, oil and dirt molecules are rejected at the surface rather than being irreversibly entrapped in the depths of the pore."
Industry uses various technologies to minimize freshwater purchases and wastewater disposal, but few achieve the greater objectives of economically extracting valuable materials from waste streams, according to Osmonics literature. In addition, the company has developed membrane systems for several industries that routinely separate and concentrate acid, caustic, dye, glycol, lignin, lubricant, metal, oil, protein, solvent, sugar, starch, TiO2, and other materials from product and waste streams.
"Developed for fluid treatment in residential, commercial, and industrial applications, the use of membranes to selectively remove or separate extremely small substances from water and process streams has become a technological success story," the company concludes.
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