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Degassing and Debubbling with ECLIPSE MEMBRANES™

ECLIPSE : Contacting and Separation : Degassing and Debubbling

Degassing and Debubbling with Porous PTFE Membranes

Membrane degassing and de-bubbling applications fall into the technical class of gas/liquid separation. The breadth and scope of degassing and de-bubbling applications make it a potentially large membrane application and a very promising opportunity for porous PTFE for aqueous systems due to the hydrophobicity of the fiber. Degassing and de-bubbling are processes in which dissolved gases (degassing) or bubbles are pulled out of solution, either from aqueous or organic liquids. The dissolved gas or bubbles may be oxygen, nitrogen, carbon dioxide, i.e. gases in the conventional sense or they may be volatile organic compounds (VOCs) which are present in trace quantities in aqueous systems and are pulled out as a vapor.

 

Various liquid streams require degassing and/or debubbling, generally to protect some downstream process or piece of equipment. Analytical equipment and drug delivery systems require debubbling as the bubbles interfere with the sensitive flow or analytical detection systems downstream. Aqueous systems may require purification by removal of volatile organic compounds or removal of dissolved gases either to generate potable water or to purify the water for subsequent processing use. The semi-conductor industry employs significant numbers of degassing systems for their ultra pure water systems. Bubbles present during chip and wafer manufacture result in costly defects. Beverages are degassed to pull out CO2 and replace it with N2, ink jet printers would benefit from degassing by allowing higher print speeds, and boiler systems require degassing the water to avoid pitting and corrosive action from heated dissolved gases, especially CO2.

Designing Porous PTFE Membranes for Degassing and Debubbling

Typical construction of a membrane degasser or de-bubbler would be a fiber or an arrangement of fibers contained within a shell wherein the process stream or the stream requiring degassing is passed down the lumen of the fiber while a vacuum is pulled on the outside of the fiber. The only contact between the degassing liquid and the vacuum is through the fiber wall. The fibers must be designed to not allow passage of the liquid stream through the pores, nor have the pores blocked by any contaminant in the liquid stream.

 

The capacity of the unit is determined by the total area of flow on the liquid side (through the lumen) and the flow rate thus defining the residence time in the contactor for degassing and the surface area through which the liquid may be degassed, determined by the number of fibers and the outside diameter. Another rate-limiting step is the mixing that occurs (or does not occur) on the lumen side as the dissolved gases or bubbles must find their way to the proximity of the fiber wall while within the contactor in order to be liberated. If one relies on diffusion, this may well become the rate limiting step.

 

Key to the design of degassers and de-bubblers is balancing the contact time (the time the fluid to be degassed or de-bubbled) is in contact with a vacuum source via the membrane, the total surface area available for degassing, and preventing the liquid being degassed from escaping through the pores or penetrating and blocking the pores in the membrane wall. In some critical degassing applications, a thin, non-porous, permeable membrane is applied over top of a porous sub-structure to ensure no process liquid leaves through the membrane wall. The total volume of the device, the pressure drop across the device, its propensity to block or clog with any solid or gel present in the stream being treated are all factors in the design.

Applications for Porous PTFE Membrane Degassing and Debubbling

De-bubbling and degassing applications range from low volume systems to very high volume systems. Low volume operations such as those employed to remove bubbles from analytical equipment such as HPLC units, medical systems such as automated syringe injection systems where the presence of bubbles would cause erroneous dosages. While the volumes of liquid being degassed are quite small, the amount of gas being pulled out is typically quite low, requiring high vacuum levels and very careful control over the pore size distribution to avoid bleed through.

 

Higher volume systems would be employed in the food and beverage industries and various industrial applications such as degassing ultrapure water for the semi conductor industry or degassing boiler feed streams. Commercial installations for degassing ultrapure water for semi-conductor production are in the range of 800 to 1000 gallons per hour per line or higher.

 

For many of the larger volume applications, the sensitivity to low levels of dissolved gases is not key, but throughput and efficiency, frequency of cleaning, installed cost, and longevity of the unit tend to dominate system requirements.

 

For food and beverage applications, the ability to clean and sterilize becomes a very important component for the design. Beers and wine often require alcohol content to be adjusted (lowered) and a membrane process is ideal as it does not rely on thermal driving forces to reduce the alcohol. Heat tends to ruin the flavor and bouquet of wine or beer. Beer will often require its CO2 content to be reduced to reduce the bite of the beer. Often times it may be replaced with N2 to provide the fizz but without the acidity of the CO2. The beer and wine market is an excellent target for porous PTFE hollow fiber in that it is a large market, they often will employ ceramic membranes as filters which are prime targets for porous PTFE hollow fiber, and coupled with degassing or re-gassing applications may prove to be an outlet for significant volume of PTFE hollow fiber.

Advantages offered by porous PTFE hollow fibers and tubes: "more degrees of freedom…"

Conventional polymeric hollow fiber membranes are widely employed in degassing operations. While they produce acceptable results and even attractive results when compared to alternative degassing methods, the nature of the fibers do not lend themselves readily to the full scope of degassing and de-bubbling applications. Aside from the obvious material limitations in the presence of high temperatures or aggressive fluids, there is a significant limitation on the outer diameter and wall thickness of solvent-cast polymeric hollow fibers. One cannot adequately adjust the design of the fiber to accommodate the volumes and conditions of the full spectrum of degassing operations. This leaves staging multiple contactors in parallel as the only viable alternative.

 

It is clear that beyond the overarching advantages offered by PTFE hollow fibers (temperature, chemical resistance, inertness, purity, etc.) there are design advantages for degassing and de-bubbling operations using PTFE hollow fibers because of the range of diameters and wall thicknesses at a given pore size.

 

By being able to accommodate larger flow rates by using larger diameter fibers, porous PTFE hollow fibers offer additional design degrees of freedom over many other polymeric hollow fibers. While it is important to remember that the limitations will ultimately come down to residence time of the liquid to be degassed, porous PTFE hollow fibers offer lower resistance to flow and alternative designs to the traditional contactor that are more favorable to higher flow rates.

 

By being able to increase wall thicknesses at a constant porosity, porous PTFE hollow fibers offer stronger physical properties (burst, tensile, crush, tear, etc) for a given porosity, and as a consequence, can be used in modes where the thinner walled, smaller diameter fibers would not be strong enough.

 

Because of advantages with diameter and wall thickness, porous PTFE hollow fibers are not limited to being used in a conventional contactor configuration. The physical strength and large area for throughput allows the fiber or tube to be run to very long lengths, thus allowing a “tube within a tube” type design, moving away from the issues associated with potting a large number of small fibers. While the “tube within a tube” configuration would not be limited to degassing and debubbling but could be used for filtration and other separation mechanisms, there are specific advantages for degassing and debubbling.

 

Newer degassing and debubbling designs are emerging every day (Thielen et al. US7144443 “Axial Transfer Line Degassing”, Gerner et al. US Application 2007/0012190 “Integrated Degassing and Debubbling Apparatus”, both of which employ a tube within a tube configuration) which shift the paradigm from a contactor with all the construction issues associated with potting a multitude of small diameter fibers, complex housing design, managing flow patterns through the bundle of fibers, etc. to a simpler, in-line device. Perhaps most importantly, while less compact than a contactor, the installed cost for such a device would be considerably smaller than that for a standard contactor with the associated valves, fittings, connections, etc. For each of these devices, assuming that the chemical resistance or inertness of the fiber is important, the same would have to hold true for the associated housings, fittings, etc. The in-line design minimizes the associated ancillary plumbing.

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degassing and debubbling with porous ptfe membranes