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by J. Peter Clark

Membranes, Microfiltration, Microsieves, and More

embranes generally refer to desired product. For example, in side of the membrane is an aqueous materials with controlled abil- water treatment, the permeate is the solution while on the other side is Mity to pass or retain molecules purified water, while in concentra- either a sweep gas or a vacuum. The or particles of specific sizes. Familiar tion of fruit juices, the retentate is the small molecules vaporize and move applications include microfiltration, concentrate. through the membrane while water ultrafiltration, and reverse osmosis, Concentration of fruit and vege- and larger molecules are rejected. differing in the scale of substances table juices is limited by osmotic Such a process could remove flavors that are retained. Reverse osmosis pressure, as the soluble solids from a juice, if desired, and even membranes, for example, can retain become higher, and also by viscosity, recover a concentrated essence by ions such as sodium and chloride, which impacts mass transfer and condensing the permeate stream. permitting production of pure water from sea water. A typical membrane separation process may consist of Bacteria and yeast, on the other several stages of filtration, perhaps using different materials hand, are retained by microfiltration membranes, permitting sterilization and configurations. of liquids such as draft beer and water. The driving force in membrane pressure drop through the equip- Electrodialysis separations is often pressure, but it ment. Because membrane Simple dialysis relies on the also can be a concentration differ- concentration does not involve heat, concentration difference between a ence or an electric field, as in as does evaporation, there is less dilute solution on one side of a semi- electrodialysis. loss of flavor. In most cases, this is permeable membrane and a more Depending on the application, the desirable, but in some situations the concentrated solution on the other retentate or the permeate may be the flavor may be undesirable. Examples side. An example is purification of SieveCorp offers a dead stop (100% retention) for bacterial spores such as the are the use of apple juice concen- human blood by removal of small Bacillus pumulis spores shown in this SEM photo. Photo courtesy of SieveCorp trate, grape juice concentrate, and molecules such as urea while sweet potato concentrate as sweet- proteins are retained. In eners in beverages. The natural electrodialysis, positively and sweeteners are attractive on the negatively charged membranes are label and can provide some of the alternated in a stack between recommended fruit and vegetable electrodes. In an electric field, servings, but a bland taste may be positively charged ions attempt to required as well. In such cases the migrate towards the negative concentrate may be deliberately fin- electrode and negatively charged ished by thermal evaporation to ions migrate towards the positively remove some volatile flavors. charged electrode. The positively charged ions pass through the Pervaporation positively charged membrane but are Pervaporation is a membrane sepa- retained by the negatively charged ration process in which small volatile membrane and vice versa. Ions components are selectively removed accumulate in the spaces between from a mixture by diffusing through a the membranes leading to a semi-permeable sheet of polymer, separation. Flux rates are relatively such as polydimethylsiloxane. On one low, and some membrane materials pg 98 07.13 • www.ift.org [PROCESSING]

Membranes, Microfiltration, Microsieves, and More continued...

can foul after a period of operation. To SieveCorp’s rates of 16 m3/m2/hr (MMH) reduce fouling, the direction of the (for an array of 90 disks) are equivalent to electric field can be reversed. 9,428 GFD, a very high rate. The major application promoted for SieveCorp is Microsieves filter sterilization of milk, whey, beer, One of the more intriguing variants of clear juices, and parenteral nutrition membranes is offered by SieveCorp of fluids (solutions that are injected into the Netherlands, according to Jorine patients). The etched pores retain Zandhuis (jorine.zandhuis@sievecorp. bacteria and spores while passing com). SieveCorp fabricates sieves from proteins, fats, salts, and sugars. 6-in disks of silicon using the An interesting engineering issue is semiconductor manufacturing process of how to uniformly distribute flow among microlithography and etching. The disks so many modules. The reason for a size are only 1 µm thick, so pressure drop limitation on the disks is the availability through the very precise pores is small. of silicon wafers in a standard size of 6 in. Pores are etched to have an exact size, Other types of membranes experience which may lie between 0.25 µm and some economy of scale as area and 0.8 µm. On a single disk, there may be capacity are increased. billions of pores. Flux rates are said to be SieveCorp has developed a high 180 L/hr per disk. Ninety disks provide frequency pulsing technique to back The SieveCorp microfiltration systemis applied for the 1 m2 of membrane area. Pressures are flush the disks and remove debris that cold sterilization of fluid foods and medicines. said to be 0.5 bar or about 7 psi. might otherwise clog the pores. Flow is Photo courtesy of SieveCorp In U.S. units, fluxes are typically across the face of the disk to also assist expressed in gal per sq ft per day (GFD). mass transfer.

pg 100 07.13 • www.ift.org Other Membrane Configurations In addition to cross-flow across flat surfaces, other membranes are fabricated in tubes that may be 0.5 in, hollow fibers about the size of human hairs, and spiral wound modules. Wide tubes are used where there may be sus- pended solids that would otherwise clog flow passages. Typically the membrane may be deposited on the outside or inside of a porous fiberglass tube that provides mechanical support. Many polymer membranes have a thin active layer that is very tight and a larger, porous support layer that may even be a dif- ferent polymer. Dynamic membranes are formed in place by deposit of a layer of solids on a porous support. Still other membranes are fabricated from sintered powder metal or ceramics. These are especially useful in cleaning up corrosive solutions such as caus- tic and acids used in cleaning in place (CIP) of food and dairy equipment. Metal and ceramic filters can also withstand higher tempera- tures than can polymer membranes. Hollow fiber membranes are typically self- supporting because of their small diameter. Very large areas can be fabricated in a small volume using hollow fibers. Flow may be from outside in or the reverse, depending on pres- sures required and viscosity of the feed. Spiral wound modules also have a high area per unit volume. Flat sheets of polymer are separated by flexible porous layers and then wound around a hollow feed tube. The assembly is held inside a pressure-resistant shell. Especially with hollow fibers, but also for some other forms of membranes, pre-filtra- tion is often a good idea. In general, it is difficult to make too great a change in any process, whether it be size reduction, mixing, or filtration. Thus, it is usually good practice to pre-break feed material that is being ground, and grind it in several steps. In mixing, it is often helpful to pre-mix minor and micro ingredients before dispersing in bulk or major ingredients. In filtration and separation, it is often helpful to remove large particles using a relatively coarse filter, or some other technique, before applying a rela- tively tight membrane that might easily clog. Thus a typical membrane separation process may consist of several stages of filtration, perhaps using different materials and configurations. Back flushing can help, but membranes

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Membranes, Microfiltration, Microsieves, and More continued... still can foul. When flux rates begin water treatment plants. The same greatly improved as has selectivity. to decline to unacceptable levels, the fine pores that enable separations of Processes based on membranes are membranes must be cleaned or particles and solutes can produce used for sterilizing, concentration, replaced. Accordingly, membrane very fine bubbles that enhance mass and purification of water, milk, and materials must be resistant to effec- transfer. GE, among other firms, juices. Concentration of whey, a tive cleaning procedures. Typically, offers membrane bioreactor waste waste from cheese making, is a major hot water and mild detergents are water treatment systems that have a application, as is concentration of adequate for cleaning most small footprint and a high capacity raw milk intended for cheese as a membranes. for handling industrial wastes, such way of reducing transportation costs In operation, it is often tempting as food plants produce, according to and the production of whey at the to increase feed pressure as mem- Yuvbir Singh, general manager, engi- cheese plant. With the popularity of brane flux begins to decline, but this neered systems—water and process Greek yogurt, a new source of whey may be counterproductive, as technologies for GE Power & Water. has been created, and thus a new increased pressure may cause mem- In some cases, existing treatment opportunity for membrane concen- branes to compact, decreasing flow systems can be retrofitted within the tration. Membrane separations are still further. Back flushing or cleaning same space and increase capacity. usually energy efficient because they is a better approach, which may pro- The first selective membranes do not involve a phase change, and long membrane life and maintain a were made of cellulose acetate and so they are attractive to firms seek- higher net production rate than care- were applied to desalination of ing sustainable processes. FT lessly increasing pressure. water. Since that work in the 1960s, there have been great strides made J. Peter Clark, Ph.D., CFS, Contributing Editor, Membranes as Diffusers in membrane materials, including the Consultant to the Process Polymer membranes can be used to various polymers, ceramics, metals, Industries, Oak Park, Ill. disperse air into bioreactors, such as and silicon. Flux rates have been • [email protected]

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