The science behind ultra-thin membranes

Last edited: 10 December 2015

A BP-backed research programme has developed a polymer film 10,000 times thinner than a human hair - with the potential to filter substances 400 times faster than commercial membranes. Could this lead to new ways of cutting energy use in industry and reducing global carbon emissions?

Membranes are widely used in many industries for filtering or separating liquids and gases where the separations happen invisibly, at the microscopic or even molecular level. For example, BP currently uses membranes to produce desalinated water from seawater, both for drinking water supplies and crude oil processing – and membranes will be required to produce the reduced salinity water for its proprietary enhanced oil recovery technology, LoSal® EOR, which will be deployed for the first time in 2017, at Clair Ridge in the UK North Sea.
 
The mechanics of membrane processes are generally well understood but, as with so many technologies, much remains unknown about the fine details of how they work. As such, separations represent one of the four areas of fundamental research targeted by the BP International Centre for Advanced Materials, the BP-ICAM.

Knowns and unknowns

Membranes are usually made up of different layers. Typically a membrane will have a supporting layer that provides structure and strength, and a ‘separating layer’ that determines which particles will pass through the membrane, depending on properties such as size and shape – like the mesh of a sieve, but on a molecular scale. 

“No one has actually been able to work out how the separating layer in a membrane is formed, or how to control the shape and size – the morphology – of that separating layer,” says Professor Andrew Livingston, who heads the BP-ICAM separations programme based at Imperial College, London. 

That is changing, with Livingston and his team publishing a number of linked findings in Science, one of the world’s leading academic journals. “Firstly, we have found a way of making the separating layer of a membrane independently of the supporting layer,” says Livingston. “Secondly, we have found we are able to vary the morphology of the separating layer, to create a film with a crumpled, rather than a smooth surface. And thirdly, by being able to make the separating layer independently, we are able to put this very, very thin film onto a support of our choosing.”

Thin, crumpled and resistant

Each of the steps described by Livingston is highly significant. The ability to make the separating layer independently of the support layer means it can be made as a very thin film, and the thinner the film the more permeable it can be. “It’s a very beautiful process,” says Livingston of the technique his team have developed. Two liquids are brought together, each containing one of the two chemical ingredients – or monomers – needed to make the film. At the interface where the two liquids meet, the two monomers react together to form a polymer film that is just 8 to 10 nanometres thick. A stack of 10,000 would only be about the width of a human hair.
 
The ability to vary the morphology means that a film with a crumpled rather than smooth surface can be created – a result achieved by bringing together liquids containing higher concentrations of the monomers. At higher concentrations the reaction is more vigorous, disrupting the interface between the liquids and creating the crumples as the polymer film forms. The crumples increase the surface area available for molecules penetrate and move through the film – think of it like the difference between the surface area of an egg box compared with a rectangle of card of the same length and width.
 
The third advantage of an independent separating layer is that it means choices can be made about the materials used for the supporting layer. Livingston’s team have been using alumina, which doesn’t get compressed or clogged up during use, something which affects the performance of existing commercial membranes. Alumina has another advantage as well, of being resistant to a wider range of solutions, some of which are known to damage or dissolve conventional membranes. This gives the membranes developed by BP-ICAM the potential for use with organic liquids such as oil, as well as aqueous solutions such as seawater. 

Together these three discoveries add up to a huge potential improvement in membrane performance. “We are seeing our crumpled nanofilm membranes separating substances 400 times faster than commercially available membranes,” says Livingstone. 
“We are seeing our crumpled nanofilm membranes separating substances 400 times faster than commercially available membranes”
Professor Andrew Livingston

Lightening the desalination load

Livingston is concerned to stress that there is a long way to go in terms of how his nanofilm membrane might be scaled up from laboratory experiments to commercial production. But he is confident that the potential is there, starting perhaps with desalination plants using reverse osmosis.
  
“The advantage you might have with nanofilm membranes is the ability to produce lower salinity water at the same rate achievable conventionally, but using fewer membranes,” says Livingston. “That means you could install smaller, lower weight desalination plants on your offshore platforms – and anything that reduces the weight you need to support is going to have significant benefits in an offshore environment.”

Taking the heat out of refining

Looking much further ahead, highly selective membranes have the potential to replace harsher or more energy intensive separation processes. That suggests possible future uses in increasingly difficult wastewater treatment applications, the production of hydrogen and oxygen and improving the efficiency of carbon dioxide removal from natural gas.

Livingston sees the potential for even greater rewards. “Anytime you are doing a concentration process by evaporation – in the chemicals, petrochemicals or pharmaceuticals industries and many others – there is the potential to use nanofilms instead of the more energy intensive demands of raising temperatures or lowering pressure.

“A very long-term goal – a dream – would be to see nanofilm membranes used in refining. In an oil refinery, it is generally estimated that as much as 10 out of every 100 barrels of oil may be used up providing the energy for the refining process. This is some way off, but imagine if you could reduce that energy demand right down using membranes? Or perhaps reduce it by some distance in a hybrid process along with some distillation, using a membrane to achieve a particularly difficult separation.

“About 30 per cent of the world’s energy is currently used by industry across all the sectors, and a substantial fraction of that is used in evaporation and distillation processes. Replacing these processes with membranes would enable major energy savings to be made, with consequent reductions in global carbon dioxide emissions.”
* LoSal® is a registered trade mark of BP p.l.c.
Photo credit: Michael Panagopulos / Imperial College London

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