Eighty percent energy saving by using membranes

More and more industries are becoming aware of the opportunities for saving energy and sparing of the environment by using advanced separation techniques. Ceramic membranes for dewatering organic process streams by means of pervaporation have now been developed up to the stage where they are being demonstrated in the process industry and launched on the commercial market. Pervaporation -allowing only one component of a particular mixture to pass through a membrane- enables the chemical process industry to save up to 80 percent on energy, compared with distillation.

Researchers at the Energy research Centre of the Netherlands (ECN) have developed ceramic membranes using a specially modified silica material. These membranes have been tested with an active area of one square metre for their ability to separate methanol from organic solutions. They have other applications too as they are able to remove water from mixtures of organic components. This process is particularly attractive compared with distillation in the case of azeotropic mixtures, which are mixtures of liquids where the vapour has the same composition as the liquid, making any further separation by means of one-stage distillation impossible.

Schematic overview of ceramic membranes in a hybrid distillation/pervaporation.

Pervaporation is a membrane process that involves evaporating the volatile components in a mixture and passing them through a selective membrane. The pervaporation process is much more energy-efficient than the currently widely used distillation process, as phase separation (the transition from liquid to vapour) takes place selectively for only one component in the mixture. By replacing the entire process with pervaporation, energy savings of up to 80% can be achieved, compared with distillation.

Companies can use pervaporation to remove relatively small quantities of water and/or methanol from organic process streams. Most of the applications of pervaporation are to be found in the chemical, petrochemical, food and pharmaceutical industries. Large-scale applications, with their associated energy savings, are to be found in the production of chemicals, in chemical reactions - to control or influence the process - and recycling and waste processing. Pervaporation also has less environmental impact.

Henk van Veen, project manager at ECN explains: "With our partners, Akzo Nobel, Cytec, Purac, Quest, Unichema and Lyondell, we have assembled a group of major companies that are able to use the silica membranes for methanol separation. Together with them - and Delft University of Technology as the knowledge developer and Sulzer Chemtech - we studied the quality of the membrane, the investment required and the payback time. The membrane is not yet able to cope with extreme conditions: temperatures of over 250°C and acidity of pH=1 is still out of the question. The investment required is in general larger than for a distillation system, but because of the energy savings the payback time of a pervaporation system is between one and two years. We expect a modular membrane system to be launched on the market within five years."

Companies are already using pervaporation based on polymer membranes on an industrial scale under relatively mild conditions, for dewatering organic liquids (hydrophilic pervaporation) and separating azeotropic mixtures that cannot be processed any further using conventional distillation. So far the ceramic membranes have only been tested on laboratory scale and pilot scale. The membrane development is now at such a stage that it is possible to apply them on an industrial scale: information is available, for example, on membrane properties such as flow and selectivity as a function of process parameters such as process time, vacuum pressure and temperature.

The project is co-funded by the Economy, Ecology and Technology (EET) programme, a joint venture of the Ministries of Economic Affairs, Education, Culture & Science, and Housing, Spatial Planning & the Environment.

Paul Pex
ECN Energy Efficiency in Industry
Tel. +31 (0)224 - 56 46 40