COLOGNE, GERMANY: Lanxess AG is continuously carrying out R&D in the field of water treatment, to ensure that the design process of reverse osmosis systems is even more successful in the future.
Experts from the company presented the latest results – allowing an even better description of the separation behaviour of reverse osmosis (RO) membranes under realistic application conditions.
Understanding complex links
The behaviour of RO membranes is determined under realistic service conditions by a large number of parameters. The composition of the feed, for example, is of major importance. In practice, not only the common salt normally used in tests is dissolved in the feed, but other salts too. The rejection for these different salts or ions varies enormously. The pH and the temperature of the salt solution have, for each salt or ion, an individual influence on the success of separation.
In order to gain a better understanding of all these relationships, experts from Lanxess launched a research project that uses a modern design of experiments (DoE) methods. Only in this way can the number of necessary trials be restricted to a reasonable level even where issues are complex.
The goal of the present tests was to gain a better understanding in particular of the behaviour of highly crosslinked Lewabrane membranes for the treatment of brackish water. The high cross-linkage level leads to a low effective pore size, bringing about the high rejection ability of these membranes. They are also noted for their high stability even in extreme pH and temperature ranges.
Measurements on salt mixtures
The experts were particularly interested in the separation properties of the membranes for different dissolved salts. Use was therefore made of a test mixture containing not only sodium chloride but also nitrate, ammonium, boron and silicate. Such mixtures are found, for example, in industrial and household effluent, with some components also being found in agriculture, groundwater and seawater.
The separation behaviour was examined both on isolated membranes and on complete reverse osmosis elements.
To be able to reliably measure the separation behaviour in the complete parameter range with as small a number of experiments as possible, use was made of the DoE. This was done on the basis of the response surface design method (RSD) in combination with a central composite design (CCD). The evaluation of the experimental results was carried out by adjusting the measuring data to a square model.
The selected target values for the model were total salt rejection, rejection for individually dissolved components and the permeate flow.
Response surfaces for many application scenarios
The response surfaces derived from the test series describe the behaviour of the membrane with regard to individual salts or ions over the entire pH and temperature range. For nitrate, for example, there was a clear pH dependence of rejection, with a maximum at pH 7-8 and with only a low dependence on temperature. The results were completely different, for example, with boron, where, in addition to the pH dependence, there was also a marked dependence of rejection on temperature.
The results allow the parameters for RO elements to be specifically selected so that optimal separation results can be obtained for the respective application.
“So that we can utilise this potential in the best possible way to the benefit of our customers, the dependency functions obtained are immediately incorporated into the integrated design program, LewaPlus,” said Julien Ogier, head of the technical service laboratory for membranes in the liquid purification technologies business, Lanxess.
“With the present version of LewaPlus, we have been working further on the aspect of user friendliness. During the design of a reverse osmosis system, we can see directly how many simulations have been created for a direct comparison. Apart from that, we have improved the design of the pdf report for the condensate polishing module so that, for the progressive regeneration, the regenerated volume and the application concentration can now also be included,” adds Ogier.
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