This work aims at developing a dynamic layer crystallizer operated batchwise, for freezing desalination of sea water. The experiments were performed with water/NaCl solutions and with samples of sea ...water from Nice, Rabat and Marseille. The pilot crystallizer consists of a cooled tube immersed in a cylindrical double jacketed tank. The solution is poured into the tank and the crystallization takes place on the external surface of the tube, by applying a cooling ramp in the tube. The solution is agitated by air bubbling. The whole process involves the freezing step, leading to the crystallization of the ice layer and the sweating step, which consists of purifying in depth the ice layer by melting the impure zones. A parametric study on the effect of the operating parameters has allowed quantifying the role of the different key parameters of the freezing and sweating steps. Three experiments allowed reaching salinities lower than 0.5g/kg, satisfying the standards of drinking water. The duration of the whole process dropped to only 8h (5h for freezing and 3h for sweating), with a yield of sweating equal to about 50%, provided severe conditions were applied for sweating. Higher yields required longer times. Overall, the results show the feasibility of the technique.
► Initial temperature, ice growth rate and solution salinity are the parameters affecting ice purity. ► The sweating step is able to efficiently complete the purification. ► The duration of the whole process dropped to only 8h (5h for freezing and 3h for sweating).
In the process of desalination by indirect freezing, ice formed on the cooled surface contains impurities due to kinetic effects. Sweating is an efficient method by which ice layer is purified under ...the effect of temperature gradient. The main kinetic parameters influencing sweating of ice are initial concentration of ice, sweating temperature and sweating time. In the present work, the effects of these parameters were studied using an experimental design. A statistical model for ice weight and ice purity was developed. The graphical representation of this model in the space of the variables enabled optimization of the whole desalination process time, which led to ice salinity less than the drinking water standards (0.5g/kg).
►Layer melt crystallization was applied to seawater desalination. ►The process involves two consecutives steps: freezing and sweating. ►Sweating has enabled the obtention of ice with low salt concentration, satisfying standards of potability. ►Optimal operating conditions are proposed by means of an experimental design.
In this contribution, we present an experimental investigation of the growth of four different organic molecules produced at industrial scale with a view to understand the crystallization mechanism ...of acicular or needle-like crystals. For all organic crystals studied in this article, layer-by-layer growth of the lateral faces is very slow and clear, as soon as the supersaturation is high enough, there is competition between growth and surface-activated secondary nucleation. This gives rise to pseudo-twinned crystals composed of several needle individuals aligned along a crystallographic axis; this is explained by regular over- and inter-growths as in the case of twinning. And when supersaturation is even higher, nucleation is fast and random.
In an industrial continuous crystallization, the rapid growth of needle-like crystals is to be avoided as it leads to fragile crystals or needles, which can be partly broken or totally detached from the parent crystals especially along structural anisotropic axis corresponding to weaker chemical bonds, thus leading to slower growing faces. When an activated mechanism is involved such as a secondary surface nucleation, it is no longer possible to obtain a steady state. Therefore, the crystal number, size and habit vary significantly with time, leading to troubles in the downstream processing operations and to modifications of the final solid-specific properties.
These results provide valuable information on the unique crystallization mechanisms of acicular crystals, and show that it is important to know these threshold and critical values when running a crystallizer in order to obtain easy-to-handle crystals.
This work aims in developing a static layer crystallizer for freezing desalination of sea water. The experiments were performed with a simple system of H(2)O-NaCl and with samples of sea water from ...Rabat. The pilot crystallizer consists in a tube cooled by means of a thermostatic bath. The tube is immerged in a cylindrical double jacketed tank cooled by means of a second thermostatic bath. The brine is poured into the tank and the crystallization takes place on the external surfacs of the tube. The global process is divided into 4 steps: (i) crystallization of the ice layer by controlling the cooling rate in the tube (ii) draining off the concentrated brine (iii) purification of the layer by sweating and (iv) melting of the ice to recover the fresh water. A parametric study of the effect of the operating parameters has allowed us to quantify the role of the different key parameters of the crystallization step. Within the studied domain, the purity of the crystalline layer was mainly affected by the initial salinity of the brine. The growth rate of the layer, controlled by the cooling rate in the tube, had also a significant effect. Experiments performed with Rabat sea water showed that a fresh water of salinity close to the drinking water standards could be obtained in one stage within 31 h. Desalination operated in two consecutive stages (10 h+11 h) gave salinity below the standards with a comfortable safety margin. If sufficiently severe operating conditions are applied, sweating is able to purify the interior of the ice layer and to reach the drinking water standards, provided the impurity concentration of the ice produced in the crystallization step is low enough. The mass loss induced by sweating is also high when the impurity concentration is high. These first results are promising and show the feasibility of the process which still requires to be optimized.
A zeolite synthesis procedure with homogeneous stirring of the synthesis medium allows to reduce both the average and local shear rate and to improve the crystallization process. Examples are ...provided for the syntheses of high-silica faujasite, hexagonal faujasite and mazzite. The apparatus is easy to scale up for industrial crystallization.
