Continuous crystallisation from solution is the major separation technology for the isolation, purification and solidification of crystalline products in the fertiliser, the food and the chemical industry. Gunter Hofmann reports on the latest developments,

The design of crystallisers is understood very well and there are numerous established crystallisation processes to produce mass products like sugar, salt, potash, adipic acid, etc. These processes are based on decades of operation experience and established design rules. The plants in these fields are usually tailor-made. Their process design must be based on laboratory studies regardless a redesign, a modification or a taking over of the experiences form the established processes. Plant constructors in this field are used to operate a development centre with lab and test facilities.
Driven by the worldwide need to save our world's nature for a more and more growing world population there is a remarkable increase rate in the number of sustainable processes based on renewable resources. Fermentation processes fed with starch or sugar or similar carbohydrates are replacing many petrol-based syntheses for numerous organic products, eg in the field of amino acids or vitamins or even targeting the plastic market with biodegradable plastics like PLA or PHB/V. This is the field of the so-called life science products. Fermentation broths are typically contaminated with remaining proteins and carbohydrates, impured with by-products and typically strongly coloured. On the contrary, the products have to be colourless or white, should be of high purity and recovered at a high yield.
Again crystallisation is the most important separation technology among the unit operations required. The development of these downstream processes is characterised by the same systematology and is using the same set of tools like filtration and membrane filtration, ion exchange and active carbon treatment as they are required for the development of a processes in the classical field of industrial crystallisation.
The Messo Technology Group as technology based plant constructor in the field of mass crystallisation is a driving force for the setup of sustainable technologies, being active in the field of downstream design. New downstream processes have been established for the Vitamin C process line, the vinegar and alcohol industry, the amino acid and the sugar industry.
Development of downstream processes has become a major activity during the past decade. Fig. 1 shows the crystalliser heads in a sugar mill using a pure and an after crystallisation in the downstream line of a speciality sugar.
A typical example and a latest development in this field is the CaSO4 - reduced downstream process for a new citric acid route (Fig. 2).
The broth from the sugar-based fermentation is separated from the organic impurities by a NF unit, followed by an ion exchange for the separation of foreign ionic compounds and finally crystallised in a vacuum evaporation crystalliser. The product purity is controlled by the withdrawal of a liquid purge from this crystalliser. This liquid purge is given to a second crystalliser, which is only responsible for the process yield not for purity. The recovered crystals from this second crystalliser are re-dissolved in the feed solution to the product crystalliser. The product crystals are finally dried.
This new process can replace the classical gypsum precipitation up to 75percent, ie the by-product gypsum is reduced at least to one quarter. Even without counting the costs for land filling the gypsum there is a cost advantage against the existent citric acid process.

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Gunter Hofmann is with Messo-Chemietechnik GmbH, Duisburg, Germany. www.messo.com