Engineering R&D: Shear waves in filtration
November 7, 2012
From fractionating milk solids to waste treatment aimed at zero liquid discharge, filtration is playing a growing role in the food industry, provided manufacturers can keep membranes clean.
Fouling is the enemy of filtration technology. Cross-flow filtration utilizing high-velocity fluid flow helps reduce fouling, but viscous fluids still pose a problem, limiting applications for cross flow. Instead of relying on the velocity of the flow to reduce membrane buildup, engineers at New Logic Research rely on the torsion oscillation of filter stacks to create high shear that keeps membrane pores open and minimizes the likelihood of fouling, regardless of viscosity.
The firm calls its technology vibratory shear enhanced processing (VSEP). It was originally developed in the 1980s by Company Founder Joseph Bradley Culkin. He holds 14 US patents, most of them involving refinements and advancements in the original design. A chemical engineer by education and training, Culkin attended the University of Pennsylvania and Johns Hopkins before completing doctoral work at Northwestern University.
FE: How does VSEP differ from conventional cross-flow filtration?
Culkin: Shear forces of more than 10-15 thousand inverse seconds are not economic in typical cross-flow designs, limiting their use to watery fluids. They also suffer from significant pressure drop from the inlet to the outlet of the device when velocities increase, which results in premature membrane fouling.
With VSEP, shear rate at the membrane surface is approximately 150,000 inverse seconds, sweeping away solids and making rapid filtration possible. Filter packs consist of leaf elements arrayed as parallel disks and separated by gaskets. The disk stack is oscillated 2.22 cm (7/8 in.) above a torsion spring at extremely high speed, producing the equivalent of over 200 G’s of force. A 10-20hp motor drives an eccentric weight. Almost 99 percent of the energy input to the resonating drive system is converted to shear at the membrane surface, about nine times the efficiency of a typical cross-flow system.
Another distinction is VSEP’s batch mode. Instead of high-velocity flow, feed slurry moves in a leisurely, meandering flow between parallel membrane leafs. The retentate is extruded in a single pass between the disk elements and exits once it reaches the desired concentration. A PLC controls the opening and closing of infeed and retentate valves at programmed intervals. Residence time can be altered by manipulating vibration amplitude, pressure and slurry temperature.
FE: When did you begin augmenting filtration with vibratory force?
Culkin: We were pitching a medical instrument to Bill Freytag, then-head of DuPont’s medical division. Bill said, “I just bought a company that addresses that particular problem; what I really need is a way to separate whole blood from plasma on a microscale.” They were looking for an alternative to centrifuging small samples of blood, and I said, “I’ll build a machine to do that in two weeks.”
The prototype was built from Radio Shack parts and involved a capillary tube with a loudspeaker to produce vibration. They went nuts when we demonstrated it and funded the patent filing. Then, all of a sudden, the parent corporation pulled the plug on the blood testing business, and we were back on the street.
Back in California, we had money, a facility and patents. We weren’t going to be a miniature blood-culture company, so I decided to scale up the technology to deal with high-solids streams. We were addressing Leonhard Euler’s second problem, which is finding a steady state solution to an infinite flat plate immersed in a Newtonian fluid and oscillating parallel to itself in a sinusoidal motion. The solution is a velocity field describing a propagating shear wave with maximum velocity amplitude at the plate and decaying in a direction normal to the plate. The decay envelope is exponential, and the scale is a function of the fluid properties and the frequency of oscillation. That was the secret: Spend your money moving the membrane, where the fouling occurs.
Of course, scaling up from processing blood samples of 5 milliliters or less required some changes. A loudspeaker that could produce vibration amplitudes of up to one inch, peak to peak, would be pretty silly looking, so instead we built a resonator to create large amplitudes of motion. The motor was at one end of the structure, and at the other end was the filter pack, which you intended to shake the heck with large amplitudes of motion.
FE: Is brute force part of the anti-fouling, or do acoustical waves do all the work?
Culkin: Anti-fouling results from pure shear wave, which begins at the membrane surface and moves at a speed well below sound waves. It’s analogous to earthquake energy, which is both a shear wave and a compression wave, each with its own arrival time and phase speed. Activity is more pronounced with Newtonian fluids than rock, and there is negligible acoustic, or compressive, wave transport in VSEP.
Young engineers should pursue designs based on this concept. We’ve focused on wave transport through membranes, but it can be applied to heat transport and other things. In the post-WWII years, there were many books like Harry F. Olson’s Dynamical Analogies that stressed the power of nodal network analysis, which combines Kirchoff’s voltage and current laws with Thevenin equivalent circuit methods and “Z” transforms for analytic complexity reduction. These methods can be applied to mechanical and hydraulic systems. These theorists were right, and 60 years later, the opportunity remains.
FE: Mechanically induced vibration adds cost. Where does VSEP fit in relation to commercial options?
Culkin: We compete in two areas. In applications involving separation of high suspended solids, we compete with one-inch tubular membranes that struggle with fouling. The other area is zero liquid discharge (ZLD), and we’re in those markets big time.
Ten years ago, we realized if we could increase the pressure at which our system operates, we could replace evaporators and crystallizers in high-volume applications. Thermal brine concentrators and evaporators entail outrageous energy costs. VSEP eliminates the need for those units entirely, which gives our system a significant cost advantage.
China is going ZLD, and they are quite rational and clever about adopting new technology. They invite everyone to show them what they’ve got, and in that kind of environment, we can win. The government of Inner Mongolia recently adopted our system for wastewater treatment and told a competitor, “You’re out,” because [it] had a multi-step separation system that included thermal processes, and we had basically one step that produced clean water and concentrated goo.
FE: Are you making inroads in the US?
Culkin: It’s frustrating in the US because there are a lot of extra cooks in the kitchen. Engineering firms are highly opinionated and conservative, and they’ve held our technology at arm’s length. The domestic market is very risk averse. But if we can show US firms large-scale overseas applications, mostly outside of the food area, we’re hopeful we can replace some of the brine concentrators and thermal crystallizers being installed by electric power plants and mining operations that are being pushed toward ZLD.
FE: Is there potential in food applications, such as fractionation of dairy products?
Culkin: We think so, particularly in energy-intense processes such as whey protein isolates. We can meet the engineering requirements of 3A sanitary standards, but the requirement to be able to disassemble the system by removing no more than 12 bolts is neither realistic nor rational. The Japanese are as pathologic as anyone when it comes to sanitary design, and we’re well down the road on some applications with Japanese food companies.
FE: Are specialty membranes required with VSEP?
Culkin: We only use polymeric membranes, sourcing more than 200 different types. California is considered to be the epicenter of the new membrane technology, including nanofiltration membranes that can withstand extreme temperatures and pH ranges. High temperatures and pressures impact membranes, and down the road we may have to fabricate membranes with higher tolerances. But so far, the commercially available membranes have met our needs, and we’re filtering crankcase oil in the 130°-150°C range. For 20 years, our systems were rated at 500psi, and we doubled it to 1,000psi in a two-year period.