In general, the hollow fibers are of little use by themselves, and are usually manufactured into some sort of contacting device wherein a few to several hundred or several thousand fibers are joined in such a way that a process stream may flow across the surface of the fibers and discharge through the collected lumens, or flow in through the lumens and discharge out through the outer fiber wall. Such a device, whether for filtration or membrane separation process, is referred to as a contactor.
The contactor housing completes the third leg of the design (along with the fiber and the potting technology).
While the focus of Markel’s porous PTFE hollow fiber membrane business has been on the first two elements, we have developed technology on housing design that takes best advantage of our potting technology to yield a robust, durable seal within the housing and, in certain cases, offers the ability to retain the housing and replace the fiber bundle, a useful feature for pilot or developmental scale programs.
Often times the weakest link in a membrane contactor is not the hollow fiber membrane itself but either the potting system or the means by which the potted bundle is secured within the module housing, shortening what should be a long lifetime for the module as repairs for this sort of failure are almost impossible to implement.
For commercial membrane contactors and filter systems Markel porous PTFE hollow fiber bundles are mounted in stainless steel, PFA or CPVC housings via an extension of our fluoropolymer potting technology. In this manner, we are assured of a chemical resistant, leak proof, durable seal between the bundle and the wall of the housing.
While each design is unique, the contactors all share the common characteristic of a fluoropolymer potting system tightly bound to the housing. This system eliminates the possibility of leaks and the bypassing or accumulation of debris.
For lab systems and those situations when the fiber design is not firm or locked in, there are advantages to having a system which allows the bundle to be removed and replaced with a new replacement bundle and or a different design . In this way, one can reduce the cost of testing a variety of fiber membrane designs by employing a common housing system to test the fiber bundles.
Like the commercial design, the lab design takes advantage of the fluoropolymer potting technology to provide a robust seal between the lumens of the hollow fibers and the outer surfaces, but unlike the low solids elements, the hollow fiber bundle can be removed easily and safely by removing end clamps from either end of the housing and simply sliding the bundle out. An outer perforated tube protects the PTFE hollow fibers from damage.
Reusable seals allow the next bundle to be inserted, the housing to be closed, and the unit put back in service. Replacement of a test element on a 50 mm housing can be accomplished in a few minutes (excluding time for cleaning, etc.)
This technology can be employed on very small elements having <1sq. meter surface area up to 14 sq. meter surface area and containing over 3,000 fibers in the bundle.
There are applications for membrane contactors where an ideal design would not follow conventional contactor thinking. One of the advantages of porous PTFE hollow fibers is the ability to produce long, continuous lengths of a strong, flexible tube or hollow fiber that will not kink, snap, or tear on handling creates opportunities where the functionality can fit the geometry or accommodate the physical constraints of the application.
A case in point might be employing a singular porous fiber or tubular membrane as a transfer device, allowing one to degas or to distribute a gas into a liquid while the fluid is en-route to its destination. Such designs can capitalize on pre-existent piping runs, minimize the number of fittings, connections, and other sources of leaks, cost, or contamination.
One such example is shown in the accompanying figure. A 7.5 mm tubular membrane is suspended within a fluoropolymer (FEP) outer transfer jacket, allowing annular flow of a fluid between the porous tube and the outer jacket. This design permits the fluid in question to be continually degassed or de-bubbled while flowing in the annular region, or for a gas or liquid to be continually distributed into the fluid moving in the annular channel. The tough and flexible porous PTFE membrane enables such a design to be produced.
Alternate designs may include employing the porous fiber as large coils around the perimeters of reactors for infusion of gases or liquids, using small diameter porous hollow fibers as loops inside devices requiring venting, etc.
A wide variety of contacting configurations are possible, ranging from conventional contactors through concentric tubes, and various series-parallel configurations.