FREEZE CRYSTALLISATION

Crystallisation by freezing, or freeze crystallisation, is a process in which heat is removed from a solution to form crystals of the solvent rather than of the solute. This is followed by separation of crystals from the concentrated solution, washing the crystals with near-pure solvent, and finally melting the crystals to produce virtually pure solvent. The product of freeze crystallisation can be either the melted crystals, as in water desalination, or the concentrated solution, as in the concentration of fruit juice or coffee extracts. Freeze crystallisation is applicable in principle to a variety of solvents and solutions although, because it is most commonly applied to aqueous systems, the following comments refer exclusively to the freezing of water. 

All freeze separation processes depend on the formation of pure solvent crystals from solution, as described for eutectic systems in Section 15.2.1. which allows single-stage operation. Solid-solution systems, requiring multistage-operation, are not usually economic. Several types of freeze crystallisation processes may be designated according to the kind of refrigeration system used as follows . 

Thijssen and Spicer1 1191 has given a general review of freeze concentration as an industrial separation process and Bushnell and Eagen(63) have discussed the status of freeze desalination. The potential of freeze crystallisation in the recycling and re-use of wastewater has been reviewed by Heist 120, and the kinetics of ice crystallisation in aqueous sugar solutions and fruit juice are considered by Omran and King(121). 

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HEIST, J. A. AIChE Symp. Ser. 77 (1984) (209) 259-272. Freeze crystallisation waste water recycling and reuse. 

Continuous freeze-drying equipment has been developed(60), and chopped meat and vegetables may be dried in a rotating steam-jacketed tube enclosed in the vacuum chamber. A model of the freeze-drying process has been presented by Dyer and Sunderland191 , and further details are given in Section 15.8 on freeze crystallisation. 

Type of nozzle body. The body is a mechanical and thermal screen, and its type depends primarily on the plastic structure. In the case of amorphous plastics, which feature slow freezing, intensive heat removal is needed, and thus the use of what are called cold gates. In the case of semi-crystalline plastics (apart from PE and PP), which feature fast freezing (crystallisation), gradual heat removal is needed, and so the use of hot gates (see Chapter 4.1.1 and Table 3.2). In the case of PE and PP, moderate heat removal is needed, with the use of elevated temperature gates. 

Benzene. Pure benzene (free in particular from toluene) must be used, otherwise the freezing-point is too low, and crystallisation may not occur with ice-water cooling alone. On the other hand, this benzene should not be specially dried immediately before use, as it then becomes slightly hygroscopic and does not give a steady freezing-point until it has been exposed to the air for 2-3 hours. Many compounds (particularly the carboxylic acids) associate in benzene, and molecular weights determined in this solvent should therefore be otherwise confirmed. 

It is a well-known fact that substances like water and acetic acid can be cooled below the freezing point in this condition they are said to be supercooled (compare supersaturated solution). Such supercooled substances have vapour pressures which change in a normal manner with temperature the vapour pressure curve is represented by the dotted line ML —a continuation of ML. The curve ML lies above the vapour pressure curve of the solid and it is apparent that the vapour pressure of the supersaturated liquid is greater than that of the solid. The supercooled liquid is in a condition of metastabUity. As soon as crystallisation sets in, the temperature rises to the true freezing or melting point. It will be observed that no dotted continuation of the vapour pressure curve of the solid is shown this would mean a suspended transformation in the change from the solid to the liquid state. Such a change has not been observed nor is it theoretically possible. 

The separation of the solid phase does not occur readily with some liquid mixtures and supercooling is observed. Instead of an arrest in the cooling curve at /, the cooling continues along a continuation of c/ and then rises suddenly to meet the line f g which it subsequently follows (Fig. 1,13, 1, iii). The correct freezing point may be obtained by extrapolation of the two parts of the curve (as shown by the dotted line). To avoid supercooling, a few small crystals of the substance which should separate may be added (the process is called seeding ) these act as nuclei for crystallisation. 

Into a 1-litre beaker, provided with a mechanical stirrer, place 36 - 8 g. (36 ml.) of aniline, 50 g. of sodium bicarbonate and 350 ml. of water cool to 12-15° by the addition of a little crushed ice. Stir the mixture, and introduce 85 g. of powdered, resublimed iodine in portions of 5-6 g, at intervals of 2-3 minutes so that all the iodine is added during 30 minutes. Continue stirring for 20-30 minutes, by which time the colour of the free iodine in the solution has practically disappeared and the reaction is complete. Filter the crude p-iodoaniline with suction on a Buchner funnel, drain as completely as possible, and dry it in the air. Save the filtrate for the recovery of the iodine (1). Place the crude product in a 750 ml. round-bottomed flask fitted with a reflux double surface condenser add 325 ml. of light petroleum, b.p. 60-80°, and heat in a water bath maintained at 75-80°. Shake the flask frequently and after about 15 minutes, slowly decant the clear hot solution into a beaker set in a freezing mixture of ice and salt, and stir constantly. The p-iodoaniline crystallises almost immediately in almost colourless needles filter and dry the crystals in the air. Return the filtrate to the flask for use in a second extraction as before (2). The yield of p-iodoaniline, m.p. 62-63°, is 60 g. 

DMSO - Dimethylsulphoxide is a very common solvent with a freezing point of 20 degrees. When you buy this stuff it will be crystallised in the bottle. To melt, all you need to do is place the bottle in a bowl of hot water for 30 minutes - simple. If you re lucky enough to live somewhere warm it may already be liquid, where I live, no chance. When you open the bottle you will notice that this stuff smells a bit farty, don t worry too much, it doesn t get that bad. 500ml straight into the reaction flask and start the stirrer. 

The density of oleum at 20°C (76) and at 25°C (39) has been reported. The boiling points of oleum are presented in Figure 15 (86). Freezing points are shown in Figure 16 (75,87). An excellent discussion on the crystallisation points of oleum is available (69). The solubiUty of sulfur dioxide in oleum has been reported (68,69). Viscosity of oleum is summarized in Figure 17 (55) additional viscosity data are available (76). 

By fractional crystallisation of the mixture, either by direct freezing or by dissolving in a suitable... 

The technical method for obtaining phenol is by shaking out with caustic soda the middle oil of the coal-tar distill ate,. after some of the naphthalene has crystallised out. The phenol dis-soKes m the alkali, and is then lemoved fiom insoluble oils. The alkaline liquid is acidified, the phenol separated, distilled, and finally purified by freezing. 

Caryophyllene nitrosochloride, (CjgHgJjN OoCL, is obtained when a mixture of the sesquiterpene, alcohol, ethyl acetate, and ethyl nitrite is cooled in a freezing mixture, and then treated with a saturated solution of hydrochloric acid in alcohol. The reaction mass is allowed to stand on ice for an hour and is then exposed to sunlight. Thus prepared it melts at about 158° to 163°, and can be separated into two compounds, one being that of a-caryophyllene and the other that of yS-caryophyllene Deussen s sesquiterpenes of natural caryophyllene from clove oil), a-caryophyllene nitrosochloride melts at 177", and /3-caryophyllene nitrosochloride at 159°. They can be separated by fractional crystallisation. The corresponding a-nitrolbenzylamine melts at 126° to 128°, and the /3-nitrolbenzylamine at 172° to 173°. The bimolecular formula given above is probable but not certain. 

Tammaun has advanced the view that amorphous solids are really liquids which have been cooled far below their freezing-points, and have thereby acquired great viscosity, but have not crystallised. They are supercooled liquids. This hypothesis is supported by the following evidence ... 

In general Q < 0, i.e., heat is evolved when ice crystallises out then T falls with increasing s. In the case of strong solutions of sulphuric acid, Q may be positive, and the freezing-point would rise with increasing concentration. 

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