Thin film composite membrane

Thin film composite membranes (TFC or TFM) are semipermeable membranes manufactured principally for use in water purification or desalination systems. They also have use in chemical applications such as batteries and fuel cells.

Essentially, a TFC material is a molecular sieve constructed in the form of a film from two or more layered materials.

Membranes used in reverse osmosis are typically made out of polyimide, chosen primarily for its permeability to water and relative impermeability to various dissolved impurities including salt ions and other small, unfilterable molecules.

History

The first viable reverse osmosis membrane was made from cellulose acetate as an integrally skinned asymmetric semi-permeable membrane. This membrane was made by Loeb and Sourirajan at UCLA in 1959 and patented in 1960. The current generation of reverse osmosis (RO) membrane materials are based on a composite material patented by FilmTec Corporation in 1970. FilmTec's FT30 membrane is known as a polyamide thin film composite membrane.

Structure and Materials

As is suggested by the name, TFC membranes are composed of multiple layers. Membranes designed for desalination use an active thin-film layer of polyimide layered with polysulfone as a porous support layer.

Other materials, usually zeolites, are also used in the manufacture of TFC membranes.

Element construction

membranes are thin film composite membranes packed in a spiral wound configuration. Spiral wound designs offer many advantages compared to other module designs, such as tubular, plate and frame and hollow fiber module design for most of the reverse osmosis applications in water treatment. Typically, a spiral wound configuration offers significantly lower replacement costs, simpler plumbing systems, easier maintenance and greater design freedom than other configurations, making it the industry standard for reverse osmosis and nanofiltration membranes in water treatment.

The construction of a spiral wound membrane element as well as its installation in a pressure vessel is schematically element contains from one, to more than 30 membrane leafs, depending on the element diameter and element type. Using unique automated manufacturing process, each leaf is made of two membrane sheets glued together back-to-back with a permeate spacer in-between them. Automated process produces consistent glue lines about 1.5 in (4 cm) wide that seal the inner (permeate) side of the leaf against the outer (feed/concentrate) side. There is a side glue line at the feed end and at the concentrate end of the element, and a closing glue line at the outer diameter of the element. The open side of the leaf is connected to and sealed against the perforated central part of the product water tube, which collects the permeate from all leaves. The leaves are rolled up with a sheet of feed spacer between each of them, which provides the channel for the feed and concentrate flow. In operation, the feed water enters the face of the element through the feed spacer channels and exits on the opposite end as concentrate. A part of the feed water – typically 10-20 % – permeates through the membrane into the leaves and exits the permeate water tube.

When elements are used for high permeate production rates, the pressure drop of the permeate flow inside the leaves reduces the efficiency of the element. Therefore elements have been optimized with a higher number of shorter membrane leaves and thin and consistent glue lines. The element construction also optimizes the actual active membrane area (the area inside the glue lines) and the thickness of the feed spacer. Element productivity is enhanced by high active area while a thick feed spacer reduces fouling and increases cleaning success. Such precision in element manufacture can only be achieved by using advanced automated precision manufacturing equipment

In membrane systems the elements are placed in series inside of a pressure vessel. The concentrate of the first element becomes the feed to the second element and so on. The permeate tubes are connected with interconnectors (also called couplers), and the combined total permeate exits the pressure vessel at one side (sometimes at both sides) of the vessel.

Semipermeable Membranes are at the Heart of RO Systems

The process of reverse osmosis (RO) represents the finest level of liquid filtration available today. While ordinary liquid filters use a screen to separate particles from water streams, an RO system employs a semipermeable membrane that separates an extremely high percentage of un wanted molecules.

For example, the membrane may be permeable to water molecules, but not to molecules of dissolved salt. If this membrane is placed between two compartments in a container as shown in Figure 1, and a salt solution is placed in one half of the container and pure water in the other, water passes through the membrane while the salt cannot.

Pressure is Applied to Reverse Natural Osmotic Flow

Now a fundamental scientific principle comes into play. That is, dissimilar liquid systems will try to reach the same concentration of materials on both sides of the membrane. The only way for this to happen in our example is for pure water to pass through the membrane to the salt water side in an attempt to dilute the salt solution. This attempt to reach equilibrium is called osmosis.

But if the goal in our example water purification system is to remove the salt from water, it is necessary to reverse the natural osmotic flow by forcing the salt water through the membrane in the reverse direction. This can be accomplished by applying pressure to the salt water as it’s fed into the system, creating a condition know as “reverse osmosis

Cross-flow Filtration Permits Long-term Performance

While the principals of reverse osmosis are simple, in practical terms, the RO process cannot go on indefinitely unless steps are taken to ensure that the membrane doesn’t become clogged by precipitated salts and other impurities forced against it by the pressurized stream of feed water. To significantly reduce the rate of membrane fouling, RO

systems employ cross-flow filtration which allows water to pass through the membrane while the separate flow of concentrate sweeps rejected salts away from the membrane surface

Elements Maximize the Performance of RO Water Purification Systems

The membrane element is the heart of any RO water purification system. To make sure you’re getting the most effective, efficient system available, make sure it’s built around a element..

• Home drinking water membrane elements are constructed using advanced automated manufacturing technology. This means elements are built to optimum physical tolerances, and element-to-element differences are minimized. What’s more, critical fastening points are sonic-welded for maximum strength and durability, and extensive quality tests are performed to ensure that high standards for fabrication of elements are met. Some other manufacturers handroll and assemble their elements on primitive manufacturing lines.
• Elements have been installed as part of more systems...in more applications...than any other thin-film composite membrane. Besides offering superior performance, their consistency and reliability in service are well documented. elements have been the market leader in both consumer and commercial RO systems for more than a decade. Hundreds of thousands of drinking water systems based on elements are in successful operation today.
• The membrane inside screens out a higher percentage of dissolved solids than cellulosic membranes. Salt passage, for instance, is often 50% lower than that of cellulose acetate membranes operated at the same water recovery rate. The membrane has also been found to reject a higher percentage of undesirable dissolved solids- such as chloride, lead and nitrates- than other thin-film composite membranes.
• Elements produce more high-quality water per day- two to three times more- than elements containing cellulosic membranes. They also last longer than cellulose based membranes under typical operating conditions due to superior resistance to compaction, chemical degradation and microbiological attack.

Applications

Thim film composite membranes are used in

• Water purification;
• as a chemical reaction buffer (batteries and fuel cells);
• and in industrial gas separations.

Limitations

Thin film composites membranes typically suffer from compaction effects under pressure. As the water pressure increases, the polymers are slightly reorganized into a tighter fitting structure that results in a lower porosity, ultimately limiting the efficiency of the system designed to use them. In general, the higher the pressure, the greater the compaction.

Surface fouling: Colloidal particulates, bacteria infestation (biofouling).

Chemical decomposition and oxidation.

Performance

A filtration membrane's performance is rated by selectivity, chemical resistance, operational pressure differential and the pure water flow rate per unit area.

Due to the emphasis on flow rate, a membrane is manufactured as thinly as possible. These thin layers introduce defects that may affect selectivity, so system design usually trades off the desired flow rate against both selectivity and operational pressure.

In applications other than filtration, parameters such as mechanical strength, temperature stability, and electrical conductivity may dominate.