How membranes provide effective material separation
Flat membranes are produced in Geesthacht. Photo: HZG/Christian Schmid
Whether in the chemical industry, in drinking-water purification or the separation of CO2 – substances have to be separated from each other everywhere in the field of industrial process technology. Until now, this has often been achieved by using energy-intensive thermal methods: mixtures are heated and evaporated, for example. The Helmholtz-Zentrum Geesthacht is working on efficient, resource-saving separation technologies – on membranes, for instance, which enrich the methane in bio-gas in an energy-saving way, or filter pharmaceutical residues from water.
To separate different liquids from each other, specialists rely on extremely fine membranes. The nanometre-small pores in these molecular membranes are dimensioned so that essentially only the molecules to be separated out can slip through them – larger molecules or particles cannot pass through.
The membrane-drawing machine is filled with liquid polymer. Photo: HZG/Christian Schmid
However, this can only function if the pores have the greatest possible uniformity in their defined size. The scientists can achieve this through sophisticated production procedures. One option is to use tailor-made macro-molecules: so-called block copolymers. These have a water-repellent end and a fat-repellent end. Under suitable conditions, these molecules can align themselves in a water bath so that they form pure rings of a defined size – the membrane pores. Researchers are working on how to make these systems thermally, mechanically and chemically stable and permanently durable.
They could be used in small, decentralised drinking-water processing plants and for separation tasks in bio-technology fields, amongst other things.
Picture: Jule Clodt, aus Adv. Funct. Mat. No.6 Feb 2013
If a membrane is operated for a long time, there is a risk of blockage by “contaminant molecules”. Here the experts rely on a clever counterstrategy: they make the membrane switchable. The principle: an external impetus – such as changes in temperature or the pH value – stretches and expands the pores so that the contamination can simply be flushed out. The pores then contract and can recommence their filtering effect. Double switchable membranes can also be constructed according to the same principle, and could filter several pharmaceutical residues of different sizes from water.
Sseparation of gases
Thin Film Composite Membrane for Gas Separation, Picture: Sergey Shishatskiy,
Other membranes are required for the separation of gases. Instead of having tiny pores, these are dense. The separating process here works by dissolving the required gas into the membrane. The dissolved gas then diffuses through the membrane. The unwanted gases, on the other hand, do not dissolve that easily and only partially penetrate through the membrane.
The membrane technologies developed in Geesthacht are especially cutting edge in the separation of CO2 or in the enrichment of nitrogen. For example, they could reduce nitrogen-oxide emissions in ship’s engines. Their use is being tested in practice in the processing of bio-gas. A bio-gas plant always generates a mixture of methane and CO2. To feed this into the natural gas network, the majority of the CO2 has to be separated out, which can be carried out effectively using membranes. This has so far been tested in a pilot plant.
The CO2 from exhaust gases from coal-fired or gas-fired power stations can also be separated using membranes. There was a pilot plant on the HZG site, fed by the exhaust gases from a combined heat and power plant. For several months, researchers were investigating the long-term stability and seperation performance of their system. A novel application is the Hamburg “Algae House” prototype, which entered operation in 2013 in the district of Wilhelmsburg. Here, membranes enrich the carbon dioxide from the heating exhaust gases to “feed” the algae integrated in the building’s facade. Once the algae are “ripe”, they can be used, amongst other things, for the production of bio-gas.
The perfect polymer
Mix, stir, heat: The fifty or so members of the Institute of Polymer Research at the Geesthacht site cook and try out new recipes. The common goal is to develop the perfect polymer for various applications in materials separation.