Crossflow Filtration for Hostile Environments

1.0 Brief History of Membrane Filtration

Although membrane filtration is regarded understandably as a relatively new technology the first documented industrial installation dates back to 1863 when dialysis was introduced into the sugar industry.

Dialysis is the most simple of the membrane separation techniques not being pressure driven, and generally was based on the use of pigs bladder.

This innovation did not however prove successful and it was not until the 1950s that pressure driven membrane process were considered viable as a means of fractionation.

The first RO membranes with reasonable fluxes and permeabilities were produced at University of California from cellulose acetate in 1960 and the US Government quickly became aware of their potential for use in sea water desalination.

Whilst having excellent separation characteristics cellulose acetate is mechanically weak with poor chemical and thermal stability.

Throughout the 1960 development work went on apace, well documented in the US, although few papers were published in Europe where AEA Harwell was at the forefront of the development.

From 1970 onward crossflow membrane filtration became a commercial reality in the US Dorr Oliver and Abcor and in Europe DDS RO division and PCI being amongst the market pioneers.

Many of the early applications included treatment of brackish and municipal water rather than desalination and applications within the sugar industry which had been the original drivers.

Whilst in 1975 the first polysulphone ultrafiltration membrane cast upon a polypropylene backing material was introduced, which gave greatly enhanced thermal, mechanical and chemical resistance it was not until 1980s that the Filmtec Thin Film Composite RO membrane, a polysulphone sublayer supporting a polyamide active layer became a commercial reality.

In parallel to the ongoing development of the polymeric membranes was the development of inorganic membranes from materials including graphite oxide, various glasses and inorganic oxides.

Spurred on by the needs of the French nuclear industry an alumina based membrane was commercialised used by SCT in the late 80's.

Expectations were high, perhaps to high for a membrane which appeared robust and with such high thermal and chemical resistance, which in combination with a poor understanding of the limitations of such a membrane often resulted in disappointing plant reliability of early installations which possibly set back the adoption of the technology.

2.0 The Situation Today

During the past 15-20 years we have seen a rapid development in the membranes themselves, system design and the broad range of applications to which membrane technology has successfully been applied.

In particular the process has become more robust as a result of:

2.1 Polymeric Membranes

• Increase in membranes availability from chemical and thermally resistant materials including: polysulphone, fluropolymer, teflon, thin film composite technology.
• Technology to allow membranes to be cast on more resistant backing materials e.g. polysulphone instead of polyester.
• Improved glue technology allowing spirally wound elements to perform at elevated temperatures and increased pH range.

2.2 Inorganic Elements

The true capability of the ceramic elements are now being exploited with:

• Improved sealing arrangements to accommodate the effects of thermal expansion.
• Range of materials including Alumina Zirconia mixes and Titanium Dioxide.
• Improved system design to protect against air entrapment.
• Improved control of trans membrane pressure to minimise fouling and formation of concentration polarisation layer.

3.0 The Future

A number of novel membranes have recently been developed designed to give enhanced flux and separation characteristics even under extreme operating conditions, one of the most interesting of these is the Scepter® Membrane from Graver Technologies.

4.0 Scepter® Membrane

The Scepter® Membrane is primarily used for 'cross' or tangential flow filtration and as with such filtration techniques:

• pressure drives permeate through the membrane,
• permeate flow moves particles towards the membrane surface,
• turbulence moves particles away from the membrane,
• at equilibrium particle transport is balanced,
• the resulting gel layer, or concentration of retained material at the membrane surface may effect separation characteristics.

It comprises a sintered Titanium Dioxide active membrane on a porous, tubular AISI 316L support.

Stainless Steel Substrate (500X)

Sintered TiO2 Membrane (20,000X)

4.1 Membrane Options

Membrane pore sizes:		0.1 and 0.02 micron 0.5 µm in development
Membrane support materials:		316L standard C-22 available
Membrane tube diameters:		6, 9.6, 12, 16, 19 and 25mm ID

4.2 Scepter® Module Design and Construction

The fabrication of the module is similar to that of shell and tube heat exchangers, with one, two or four pass module designs and tube lengths of 1.5, 3 and 6 meters.

Single module can be supplied with an area of less than 1m2 to in excess of 800m2.

The modules are constructed by welding the membrane tubes into the module hence avoiding seal issues, and can be fabricated for vertical or horizontal mounting in accordance with ASME PV codes or equivalent European specifications.

Typically the modules have an all stainless steel construction.

4.3 Operating Extremes

The Scepter® Module is uniquely designed to operate in the most hostile environments.

Likewise CIP philosophy is keep it simple, inexpensive, quick and efficient.

Typical cleaning chemicals.

Alkaline:

• Strong caustic pH 13+ for organic foulants
• Typically 1 to 3% NaOH at 80oC
• Can add surfactant or chelator or sanitizer

Acid:

• Strong acid pH 2- for inorganic foulants.
• At 80oC
• Typical Nitric, Citric, Phosphoric

4.4 Lifetime of Scepter® Membranes

Nothing is indestructible. The Scepter® membrane has physical and chemical limitations, resulting from its construction from sintered AISI 316L stainless steel powder and a thin layer of titanium dioxide.

• Hence it is important to avoid chemical and environment conditions that are damaging to the materials of construction for example: Hydrochloric or hydrofluoric, acids; sodium chloride solutions; sodium hydrochorite at low pH.
• Potentially abrasive particles such as diatomaceous earth or sand should be minimised, if necessary by installing adequate pre-filtration devices.
• Solutions which may irreversibly foul the membrane such as silicated and silicone containing anti-foams should be avoided.
• As with any system it is essential to practice good process system design to minimise temperature and pressure shocks and to minimise harmonic vibrations.

Except for the few membranes damaged by the above conditions, all Scepter® membrane systems installed are still in operation using the original membranes.

5.0 Scepter® Membrane Applications

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