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Improvement of melt crystallization's efficiency for industrial applications


To improve the melt crystallization efficiency for ultra-purification and separation of organic substances, by elucidating the mechanisms of mass transfer between crystal and liquid phases. More specifically the phenomena of sweating will be studied both for crystal layers and suspended crystals.

Separation and ultra-purification of organic chemicals is very difficult and energy consuming when conventional methods are used (e.g. distillation). Melt crystallization appears to be an energy and cost efficient separation method for these applications (crystallization heat is generally two to four times lower than evaporation heat used for distillation: the large separation effect results in a very limited need for reflux). Melt crystallization is a new technology which has still much scope for improvement, but the bottleneck for further development is lack of basic knowledge. This project will focus on the investigation of the basic mechanisms of mass transfer between crystal and liquid.

The following topics will be studied :

The mechanisms by which, and the process conditions under which, molecules of impurities are incorporated or included into crystal lattices.

Whether and under which conditions molecules of impurities can be transferred from the crystal phase to the melt. These items will be investigated in the case of :

crystals in suspension like in a crystallizer;
crystal layers grown on the wall of a heat exchanger, and
crystals e.g. in a packed bed in crystal-liquid separation columns.

Throughout the project two test materials with different physical properties will be used.

Improved design procedures and/or devices resulting from this study will lead to a higher product purity, a higher yield and lower production costs (decrease of investment costs and energy savings).

A comparison between processes based on the growth of crystals as layers or in suspension will be made and guidelines will be drafted for the selection of equipment. It is also expected that, due to increased knowledge, it will be possible to make better estimates on the feasibility of melt crystallization for new compounds.

An extension of the project focusses on separation from low concentration organic mixtures, where the feed contains a higher amount of impurities (10% instead of 1%). Naphthalene and caprolactame are considered as feed materials. The same research tasks and the same equipment as before are largely used. Particularly, sweating must show whether the crystal purity can be increased. A specific objective is to show that satisfactory recoveries can be obtained in a single crystallization step.


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Business Unit of TNO Built Environment and Geosciences
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Van Mourik Broekmanweg 6

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