Historically, membrane manufacturers have utilized tangential or cross flow filtration to reduce solids loading or fouling of the membrane. In this method, the feed material is pumped at high velocity into the system, which creates shear forces at the surface of the membrane. However, the forces were economically limited to between 10,000 and 15,000 s-1. This has restricted cross flow type filtration to low viscosity, watery materials. In addition, a stagnant boundary layer remained on the surface, in which solids could accumulate and eventually impede the flow.
In order to overcome these limitations, a vibrating membrane system called VSEP (Vibratory Shear Enhanced Process) was created by New Logic Research of Emeryville, California. Rather than simply reducing solids loading by pumping at high velocity, VSEP eliminates membrane blinding by vibrating the membrane surface at extremely high frequency. This vibration produces shear waves that propagate sinusoidally from the surface of the membrane. This increase in the shear energy produces 5-15 times higher rates of filtration and makes membrane separations possible for very concentrated or viscous materials.
The industrial VSEP unit contains hundreds of sheets of membranes, which are arrayed as parallel disks separated by gaskets. The disk stack is contained within a fiberglass reinforced plastic cylinder (FRP). This entire assembly is vibrated in torsional oscillation, similar in principle to the agitation of a washing machine.
VSEP can produce extremely high shear energy at the surface of the membrane. The membrane module is attached to a spring assembly and moves at an amplitude of 7/8” peak-to-peak displacement, and oscillates between 50 and 55 Hz. The fluid is gently pumped through the module while a highly focused shear zone at the surface of the membrane is created by the resonating oscillation. Rejected solids at the membrane surface are repelled by the shear waves and flow to the bot- tom of the module becoming more and more concentrated until exiting the system.
VSEP has made it possible to dewater or separate high solids applications previously not possible with conventional membranes. This has created opportunity in the chemical processing industry for a technologically advanced separation device that can efficiently and economically process specialty chemical products. The following are just a few examples of successful installed VSEP membrane systems used today.