Ultra-pure crystallisation

Using proven crystallisation and separation techniques, the Dutch company Niro Process Technology has claimed a breakthrough in the production of ultra-pure paradi-chlorobenzene (p-DCB) - a chemical perhaps better known as the primary constituent of mothballs, but increasingly used in its ultra-pure form in the production of plastics for the electronic components industry.

The first application of the new technology for p-DCB is currently being installed at the Leverkusen, Germany, headquarters of Bayer. It comprises a suspension crystallisation process combined with novel wash columns. After extensive testing, Bayer concluded that this process out-performs conventional layer systems (in which crystal beds are washed in situ), achieving exceptionally high recovery rates of up to 99 per cent.

In principle the process is quite simple. As with any crystallisation, during this part of the process part of the product is converted into crystals that form a slurry with the more concentrated mother liquor. The crystallisation takes place in an industry-proven scraped surface crystalliser; each individual crystal providing a growth surface that can absorb the super-saturation caused by cooling the product at the swept surface. The billions of crystals formed provide near ideal growth conditions for ultra-pure crystals.

But it is the separation part of the process that differentiates it from conventional technologies. The slurry passes from the crystalliser to the Niro wash column or 'purifier', the transfer pump controlling the product output from the system. In the column, a rotating transporter mechanically compresses the slurry to remove the mother liquor and form a packed crystal bed, consisting of the pure product crystals surrounded by some mother liquor.

New crystals entering the column force the bed through the column towards a scraper assembly at the opposite end. This scraper disintegrates the crystal bed and a circulation pump provides melted product to reslurry the crystals. The circulation flow carries the crystals to a heat exchanger where steam provides the heat necessary to melt the crystals. The melted product then leaves through a pressure control valve, forcing the wash liquid through the packed crystal bed. The required pressure is adjusted depending on the level of the washfront, which can be detected by the change in temperature between the washed and unwashed portions of the bed.

Melted product in the recirculation stream countercurrently washes the residual mother liquor from the packed bed as it moves through the column. The wash liquid forms an internal reflux loop and, therefore, does not have to be recovered as would be the case with centrifuge wash liquids.

The system operates as a continuous process, but production levels can be reduced by up to 50 per cent, or even held completely, for up- or down-stream maintenance without the need for a complete start-up.