Seperations by continuous column crystallisation
A continuous column crystalliser has been designed and built using a Schildknecht-type Archimedean transportation screw within a stainless steel jacket. Problems concerned with the surface finish on the inner walls of the jacket which affect crystal transportation have been investigated, leading to effective operation of the apparatus. The process of column crystallisation may be considered as a multistage separation and purification technique; it is applicable to both aqueous and organic systems. In the present work the examination has been undertaken by firstly continuously crystallising a flow of feed solution in the column. Thesecrystals are transported along the screw, melted and some of the melt liquid allowed to flow countercurrently to the crystals thus washing them by reflux. The remainder of the melt liquid is removed as pure product, the impure product provided by the now impure reflux liquid being removed from the opposite extremity of the column. Separations in aqueous solutions which have been investigated include the desalination of brine, concentration of metal salts, the extraction of deuteriai.moxide from a heavy water/water mixture and the concentration of ethanol in water. The purification of organic solvents included the removal of cyclohexane from benzene and the extraction of para-xylene from 0-, m-, p-xylene and ethylbenzene. The unit operations involved in column crystallisation fall into the categories of heat transfer, mass transfer and mixing. In order to examine how dependent the efficient operation of the column is on these factors, the system has been optimised for organic, aqueous and mixed organic/aqueous cases. The variables studied including:- (a) the speed of rotation of the transportation screw (b) the length of the column (c) the attitude of the column (ie crystals transported upwards or downwards). (d) the position of the feed into the column (e) the ratio of the products removal rates (f) the rate of production of crystals Since the scale-up of equipment is an ultimate aim of the present investigation it is necessary to study the results mathematically. Where possible, this examination has been undertaken thus establishing the major factors involved in the separation and purification scheme. It should also be possible to determine whether absolute optimisation of operation is attainable - i.e. does separation increase as the crystal rate increases to infinity or is an optimum separation eventually attained?
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