Porous PTFE Membranes

porous ptfe membranes

ECLIPSE Membranes®

Markel : ECLIPSE Membranes®

Introduction to ECLIPSE™ Membranes - Porous PTFE Membranes

Markel Corporation has commercialized a proprietary technology for manufacturing porous Polytetrafluoroethylene (PTFE) membranes in a variety of useful geometries. Whether as a hollow fiber, tube, or porous rod, the porous PTFE structure imparts to the finished device unique and exciting properties previously unavailable in the market.

 

Markel Porous PTFE hollow fibers and tubing may be employed either singularly or in bundled arrays for micro- and ultra filtration, gas/liquid separation, gas/liquid contacting, liquid/liquid contacting, pervaporation, membrane and osmotic distillation, or as a sub-structure for reactive and catalytic systems.

 

To enable the effective commercial deployment of porous PTFE hollow fibers and tubes, Markel has developed a family of proprietary potting technologies which ensure the fiber customer can not only produce a robust and efficient module, but gain design latitude heretofore unavailable with hollow fiber membranes.

 

Ancillary coating and surface modifications presently under development will further tailor the properties of the fiber for specialized filtration and membrane applications.

 

Markel porous PTFE hollow fiber products are finding applications in the semi-conductor market, food and beverage processing, environmental systems, analytical and process instrumentation, ozonation of water, the chemical process industry in general and the petrochemical industry in particular.

 

Porous PTFE rod and solid fiber comes in the same wide range of diameters and porosities, and typically sees applications in wicking, venting, pressure control, and VOC (volatile organic compound) isolation and containment. The porosity of the rod or solid fiber can be modified to regulate the pressure drop or flow rate through the porous media.

Why Use PTFE?

PTFE is one of the most chemically inert polymers commercially available and its excellent chemical resistance, purity, thermal stability, and strength combine to make the hollow fibers suitable for the most severe operating environments. PTFE is typically stable to service temperatures in excess of 250 °C. Markel porous PTFE hollow fibers can be repeatedly chemically washed, autoclaved, or treated with acid or caustic systems for cleaning without harm to the fiber.

 

The low surface energy of PTFE makes these products ideal for separation, contacting or filtration applications for organic systems, especially where the chemical purity and inertness of the membrane are important to the end user. Such attributes are key to the electronic chemicals market, the semiconductor industry, food and beverage, pharmaceuticals and others.

 

The natural hydrophobicity of PTFE also makes it the ideal membrane for degassing and debubbling aqueous systems, membrane and osmotic distillation, separation of aqueous and organic streams, separation or isolation of organic liquids from gases, and distribution and dissolution of ozone into water.

Porous PTFE is of particular value in the distribution of ozone for water treatment, sanitization of fruits and vegetables, and for cleaning systems in the semi-conductor industry due to its combination of corrosion resistance, purity, and chemical resistance.

Why Choose Fiber Over Film?

While porous PTFE has been available as sheet and film for some time, the geometry of the hollow fiber or tube offers important design advantages for filtration and contactor modules. A filter or contactor module designed with PTFE hollow fiber or tubes requires no support structure for the membrane due to the strength of the fibers themselves. The hollow fiber geometry allows far greater surface area to be contained within a given volume compared with sheet or film. This combination results in a lower cost, higher surface area per unit volume contactor or filter.

 

The design latitude afforded by hollow fibers allows clearer flow paths and results in a unit that is far easier to clean. The geometry of a hollow fiber coupled with a suitable potting system lends itself to a more robust sealing and assembly system when compared to the spiral wound designs for flat sheet.

 

Most importantly, the valuable internal space in a filter housing is filled with active membrane surface rather than bulky support structures that add cost, create flow obstructions, and are possible sources of contamination.

Why Work with Markel?

To create a working filter or membrane contactor, two key technologies are required: one for creating the fiber itself, and a potting technology for assembling the fiber into bundles and ultimately into a housing.

 

markel porous ptfe membranes

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Markel’s unique process can generate porous hollow fibers to suit a wide spectrum of membrane applications. Outer diameters range from less than 800 μm to over 15 mm. The inner or lumen diameter can be brought to within 90% of the outer diameter, resulting in low resistance to transport across the membrane wall. In contrast to solvent-cast polymeric membranes, the Markel porous PTFE process also allows one to generate thick membrane walls, offering control over permeability, as well as providing control over depth of filtration for micro and ultra filtration applications.

 

The Markel porous PTFE hollow fiber can be produced with mean pore sizes from 0.005 µm up to 20-40 µm, making these products suitable for applications ranging from the most stringent filtration applications through venting and moisture removal and catalyst support.

 

The ability to adjust the pore structure from the core to the outer surface, especially in instances of a tight pore structure on the inner or outer fiber surface, allows the fiber to be used in a variety of modes including through flow and cross flow configurations, with the process stream flowing from inside the lumen to the outer surface or from the outer surface, discharging through the lumen.

 

Establishing an asymmetric pore structure allows the pressure drop across the fiber wall to be minimized while maintaining a small pore size at the outer wall, preventing intrusion of process streams into the fiber and generating very fine dispersions in the case of gas/liquid contacting systems. The ability to tailor the pore size along with the fiber diameter allows the fiber to be designed for a specific flow rate – pressure drop relationship while maintaining a desired external surface average pore size.

 

The performance properties of the membrane may be further modified beyond changes to the pore structure itself by providing surface treatments to the membrane. Through coatings or surface modifications of the PTFE, an even greater range of applications become possible.

ECLIPSE Membranes™...More Design Freedom

An important additional benefit of the unique nature of PTFE membranes is the ability to move away from the conventional small diameter fiber bundled into a cylindrical module. The capacity to create large diameter membranes, or to vary the wall thickness of the membrane have both been reviewed, but designs with Markel porous PTFE hollow fiber can move beyond those two variables.

 

There are times when the conventional membrane contactor design is simply not practical for the application at hand. 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 the porous fiber or tube as a transfer device, allowing one to degas or to distribute a gas in 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.

 

housing design

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One such example is shown in the accompanying figure. A large diameter (0.25 inch) porous tube 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.

 

A wide variety of contacting configurations are possible, ranging from conventional contactors through concentric tubes, and various series-parallel configurations.

housing design
markel porous PTFE membranes