Markel has long recognized the need for a robust potting system with properties that match those of the PTFE hollow fibers they contain. Having high performance PTFE hollow fibers with an inferior potting system greatly limits the potential of the membrane. Markel has developed a family of modules using two proprietary mounting technologies that are as robust as the PTFE fibers themselves.
The Markel potting system for hollow fiber, with an ID <=3mm, consists of a family of potting options with one thing in common: the use of a fluoropolymer potting compound as the binder. With a fluoropolymer potting system, the fibers are bound in a system with chemical and thermal properties matching that of PTFE.
For our tubular membranes, with an ID >= 3 mm, Markel has developed a mounting system that allows the tubular membranes to be secured in a fluoropolymer tubesheet in a durable, robust manner, that offers among other advantages, the ability to remove and replace individual tubular membranes without loss of surface area or capacity.
Key to the effective design and operation of contactors and membrane filters employing the porous PTFE hollow fiber is a potting system that is fully chemically and thermally compatible with the PTFE fiber itself. Along with the chemical and thermal stability, hallmarks of a robust potting system are the ability to obtain high fiber packing density, compatible thermal expansion properties with the fiber, and similar hardness and elasticity to the fiber.
Markel has developed patented potting technologies based on fluoropolymer chemistry that retain the chemical resistance, high service temperature, and purity of the PTFE fiber itself. The potting solutions are tailored to fit the full range of applications and are chosen based on the diameter of the fiber, the total number of fibers, the size of the contactor, and the chemistry of the application.
The Markel fiber assembly and potting technologies are adaptable to systems ranging from a few fibers in a small tube up through commercial scale filter elements containing thousands of fibers and 200 mm or more in diameter. The continuous lengths of fiber available through the Markel process coupled with the Markel potting technology and the natural robustness of the PTFE fiber itself enable the creation of modules from only a few centimeters long to bundled and potted assemblies over 100 meters long.
Typical potting system modes of failure such as fiber pull-out, potting compound embrittlement, cracking, and separation from the housing are all avoided with the Markel fluoropolymer based systems.
Markel potting and assembly techniques allow the fiber to be configured in either a cross-flow mode (potted at opposite ends of the contactor bundle, with the lumens on both ends exposed) or in a dead-end configuration, where the fibers are looped so that all the lumens are potted in a single end assembly. In either case, the same high packing efficiency and robust seal design is obtained.
Each potted end may be banded with an additional fluoropolymer ring for added strength and surface for sealing. Additionally, because of Markel’s core business in fluoropolymer tubing, we have developed the ability supply bundled fiber modules sealed into fluoropolymer tubes. These designs are useful in the creation of in-line filtration cartridges that may be installed with commercially available fluoropolymer or HDPE compression fittings.
This same design may be used as an isolation system for aqueous process streams, relying on the hydrophobic nature of the PTFE to prevent the flow of aqueous streams while allowing gases to move freely through the fiber.
Markel porous PTFE membranes are available in a range of sizes from 700 µm to over 16 mm. With that range in sizes, we recognize that the potting or mounting system must necessarily change as one moves from the smaller hollow fibers to the larger diameter tubular membranes.
For the larger tubular membranes, Markel has developed a patented, proprietary mounting system employing fluoropolymer tubesheet designs that result in efficient packing of the tubular membranes and provide a long lasting, leak proof, robust seal. The nature of the seal design makes the tubesheet impervious to pressure and temperature cycles, important for the frequent cleaning cycles so many of these modules experience.
Most importantly, these designs afford the opportunity to remove and replace a clogged or damaged tubular membrane without destroying the unit or suffering significant down time.