Membrane Contacting

Membrane contactors have routinely been used for moving gases into and out of aqueous solutions. The mechanism of the process is essentially the same as it is with a conventional contacting column. However, the membrane provides both the contacting area and a barrier to direct contact of the two phases. Although the membrane is a barrier to direct contact of the two phases, it imparts little additional resistance to transport, the overall transport rate being dominated by the liquid film coefficient. The membrane contactor offers some advantages over the conventional gas/liquid contacting towers.

Membrane contactors, devices in which hydrophobic membranes promote contact between phases, make important contributions to several useful process intensification methods: membrane reactors, absorbers, and degassers. They also find application in liquid/liquid extractions, scrubbing, stripping and other operations (listed below).

In a membrane absorber or degasser, the membrane serves as a gas or vapor permeable barrier between the gas and liquid phases. With respect to traditional systems, they allow the independent variation of flow rates without flooding, foaming, entrainment or channeling, do not require separation after the process, and present higher surface area/volume ratios. CMS is actively developing applications for our membrane contactors since they offer improved performance and value over current unit operations, mainly absorption and gas stripping processes as performed by spray, tray bubble and packed towers, venturi injectors and wet scrubbers, distillation columns, and falling film/wetted wall reactors.

Table 1 lists desirable design and operation characteristics of membrane contactors that incorporate polymeric separation membranes.

Table 1. Operating Characteristics of Membrane Contactors
(Compared to Conventional Gas-Liquid Contactors)

Membrane contactors provide up to two orders of magnitude more surface area per volume than conventional contactors.  The ‘specific’ surface area (per unit volume) of various contactors range from ~0.033-0.33 [cm²/cm³] for free dispersion columns, ~0.33-3.3 [cm²/cm³] for packed and tray columns, ~1.6-5 [cm²/cm³] for mechanically agitated columns and ~16-66 [cm²/cm³] for membrane contactors [1] Equipment volume reductions of up to 20x for gas absorption and up to 500x for liquid extraction have been reported [2]

Table 2 shows that mass transfer coefficients for CMS membrane contactors are superior to bubble contactors and packed columns, and have the widest range of operational flexibility of any gas/liquid contactor.

Table 2. Operating Characteristics of Different Gas-Liquid Contactors

Most commercial separations involve aqueous solutions, although separation of organic fluids has increased with the development of improved, oleophobic membranes. Compact Membrane Systems' product focus is on applications/unit operations that cannot be performed by other existing commercial membrane contactors. 

Some membrane contactors utilize a microporous polypropylene membrane. The microporous polypropylene is hydrophobic and resists membrane wet-out (seepage of liquid into and through the membrane) when used with water. However, when the liquid phase is a low surface tension solution such as organic mixtures, water/alcohol mixtures, or water/surfactant mixtures these microporous polypropylenes wet-out easily after a few hours. Wet-out floods the membrane structure with liquid and is detrimental to successful operation of membrane contactors as it significantly reduces gas transport rates by creating a stagnant liquid film. A solution to this problem is to use a non-porous membrane with high gas permeability.

CMS Gas/Liquid Contactors use a non-porous highly fluorinated polymer membrane supported on a microporous material. The microporous material provides mechanical strength allowing the non-porous membrane to be extremely thin, minimizing its gas transport resistance. In use, the fluoropolymer membrane is in direct contact with the liquid phase while the support material is in contact with the gas phase. The fluoropolymer is extremely resistant to wet-out, is highly permeable to gases, resistant to fouling, and is chemically compatible with a wide range of chemicals.

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