Melt solid layer crystallization

The economy of melt crystallization processes depends on the product purity, which is normally increased by an additional cleaning step. The application of gases under pressure is investigated to show possibilities of product quality improvement. Experimental devices for the determination of the freezing curve under gas pressure and for a solid layer crystallization process are shown. The influence of gas and pressure in respect to the freezing curve are explained on the basis of two binary mixtures (trioxane/water and para-/meta-dichlorobenzene) under CO2- and N2- pressure are presented. Furthermore the results of solid layer crystallization experiments with naphthalene/biphenyl and para-/meta-dichlorobenzene mixtures are shown. 

Solid layer crystallization is a process in which the growth of a crystal layer takes place perpendicular to a cooled surface into the bulk of a melt (sophase change is used as the basis for the separation of the feed mixture. Such a phase separation is possible due to different equilibrium concentrations of the solid and liquid phases of the mixture (see Chapter 3). The driving force for the crystal growth is the temperature difference between the equilibrium temperature of the melt (the bulk) in front of the soUd layer and the temperature of the cooled surface (see Figure 15.2). 

The dynamic operating mode of the solid layer crystallization (see Chapter 16) can be tested quite realistically with the shown cold finger equipment, too. However, several modifications of the setup are required, for instance, a circulating loop of the melt has to be installed in order to create a falling film from the top of the cold finger (see, for example. Refs [4,14]). 

The basics of melt crystallization are provided in Chapter 15. Here, the concepts of plants and/or existing and commercially available equipment are shown. As mentioned in Chapter 15, the concepts of plants can be divided into two lines of technology solid layer crystallization and suspension crystallization. Furthermore, in industrial applications these two techniques are split into continuous and batchwise as well as into static and dynamic (stagnant or flowing melt) operating modes. A detailed overview of the different techniques and apparatuses in solid layer as well as suspension crystallization is provided in the Sections 17.1.1 and 17.1.2. 

The structures of the solid-melt interface and the melt confined within a narrow gap are of great significance in diverse areas of research such as lubrication, adhesion, or in future nanometer science. It is well recognized that the melt of n-alkanes, and other simple molecules show anomalous oscillations in density, viscosity, etc. vs. depth from the surface showing the presence of marked layer structures in the melt [40]. Even in polymer melts similar layering phenomena were suggested near the solid surface [41], but no pronounced ordering or the onset of crystallization were reported. 

A mixture of ethyl 5-(2-formyl-3-benzyloxyphenoxy)pentanoate (3.61 g, 0.01 M), potassium hydroxide (1.19 g, 0.021 M) and ethanol (40 ml) were stirred at 50°-60°C for 5 hours. The ethanol was then removed in vacuum, the residue dissolved in water (50 ml) and the solution extracted with ether (2 x 80 ml). The aqueous layer was then acidified by the addition of 2 N aqueous hydrochloric acid and the product extracted with ether, and the combined extracts washed with water to neutrality, dried, and concentrated in vacuum to give 5-(2-formyl-3-benzyloxyphenoxy)pentanoic acid, 3.0 g, 91% as a yellow oil which crystallized on standing. The crude solid was crystallized from benzene/petroleum ether to give pale cream crystals, melting point 110°C. 

Peters-Erjawetz, S., Ulrich, J., Tiedke, M., Hartel, R.W. 1999. Milkfat fractionation by solid-layer melt crystallization. J. Am. Oil. Chem. Soc. 76, 579-584. 

Iodoform can be recognized by its odor and yellow color and, more securely, from the melting point (119-123°C). The substance can be isolated by suction filtration of the test suspension or by adding 0.5 mL of dichloromethane, shaking the stoppered test tube to extract the iodoform into the small lower layer, withdrawing the clear part of this layer with a Pasteur pipette, and evaporating it in a small tube on the steam bath. The crude solid is crystallized from methanol-water. 

Crystal growth rates within crystallization processes from solutions in most cases are in the range 10 -10 m/s. Growth rates in melt crystallization are quite often in the range of about 10 m/s and in extreme cases in some solid layer processes as high as 10 m/s. 

Among the first ones to present a complete theoretical approach are Burton, Prim, and Slitcher (1953). The authors developed a mathematical formulation of the problem for metallic systems with partial solid solubility crystallized from the melt in a suspension process. Their theory is based on a boundary layer model and does not account for the mutual dependence of heat and mass transfer but regards the influence of heat transfer as negligible. 

A solid layer type of crystallization from the melt is often called progressive freezing (see, e.g., Jancic 1989), or directed crystallization (Ulrich 1988), and directed solidification (Smith 1988). All expressions describe a crystal layer growing perpendicular to a cooled wall and use the phase change as the basis for the separation of the feed mixture. This is possible due to different equilibrium concentrations of the solid and liquid phase of a mixture (see Section 7.3.). 

Finally, in some cases it can also be a disadvantage for the product to leave the apparatus in liquid form and to be solidified again. The last point as well as the third, could well be avoided someday if it becomes possible to build continuously operating processes in one plant for solid layer melt crystallization processes. 

The second group of the batch type of solid layer techniques are those with moving melts. Here again, three processes must be named the MWB-Sulzer, nowadays called Sulzer falling film (CH-PS 1967 U.S. 1985), the ICI-process (GB-PS 1964), and the BASF-process (DE-PS 1976), which is now distributed by the Kvaerner company. In all processes, the crystallization takes place on the inside of tubes, which are cooled from the outside. The melt coming from a feed tank is continuously circulated through the tubes until the crystal coat at the walls is thick enough, i.e., until... 

The crystals have to be separated from the remaining melt (mother liquor) to achieve the intended purification. In case of layer crystallization, this is done by draining the remaining melt, collecting it separately, and melting down the crystal layer afterward (Fig. 8.2-11). In case of suspension crystallization the solid-liquid separation is done either by conventional filtration or by a sedimentation apparatus, with or without support of centrifugal forces. Another device repeatedly discussed in the context of solid-liquid separation is the wash column (Arkenbout 1995). 

Suspension melt crystallization crystals and melt same temperature, design on degree of supersaturation, separation of crystals from melt depends on density difference in countercurrent operation. Scraped surface crystallizer. Section 4.6. The suspension methods have slower rates of crystal growth compared with the solid layer processes. 

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