Crystals :: essays research papers

Melt Suspension Crystallization (MSC) is a technique, that can be used for large scale separation and ultrapurification of organic compounds. Because of the high selectivity of crystallization, the moderate operating conditions, and the relatively low latent heat of fusion, the energy consumption of these kind of processes is low. Furthermore, the use and regeneration of toxic or explosive solvents can be avoided. Thus, MSC can be regarded as an environmentally and economically promising technique for wide industrial application. In MSC crystals are grown from a melt, that contains impurities of up to 30 wt%. The purity of the product is often higher than 99.9 wt%. To obtain this purity an efficient solid-liquid separation is of major importance, because the crystals themselves are almost pure. To optimize the separation efficiency large crystals with a small size distribution have to be formed. The operation of crystallization is exacting. Large—capacity evaporators are not suited to the careful control and flexible manipulation required for a successful ‘strike’, or actual crystallization, of solid sugar crystals. For this purpose, the concentrated liquor is transferred from the evaporators to the ‘pan.’ The pan is essentially a simple vacuum evaporator in which the removal of water continues until the concentration is appropriate for crystallization. As this proceeds, juice continues to be drawn into the pan in order to maintain a supply of crystallizable sugar. The concentration of impurities increases in the syrup as sugar is withdrawn by crystallization, and the entire boiling mass stiffens as the viscosity increases and the quantity of crystals grows. These two factors determine the termination of the strike, since the deposition of sugar on the growing crystals is impeded more and more as the mother liquor becomes more impure and more viscous. The concentration of sucrose probably exceeds the saturation point by a considerable degree before crystallization begins. When this critical condition is reached, significantly large numbers of crystal nuclei of the supersaturated solute are formed suddenly and proceed to grow at their normal pace. Growth would cease as the syrup falls to saturation, were not fresh liquor continually introduced.