Recrystallisation by pressure reduction

When a gas at high pressure containing a condensable solute expands, the gas is cooled and the solute condenses to form crystalline particles. The production of particles upon decompression of supercritical solutions was first noted over a century ago by Hannay and Hogarth [85]. In recent studies, expansion now typically occurs from a reservoir or flow system through a subsonic or supersonic nozzle. One of the main objectives has been the production of microcrystals with a narrow particle-size distribution. To this end, a number of studies on model systems have been made on the effect of experimental parameters on particle morphology. 

In 1972 Hagena and Obert [86] studied the effect of pressure, temperature, nozzle size as well as the nature of the supercritical-fluid solvent, on the size 

The relative importance of the nucleation and growth phases in the formation of crystals from supercritical fluids has received attention. Mohamed et al. [96] recently studied the size distribution of naphthlene crystals from a supercritical mixture. The supercritical solution was first completed by contact of carbon dioxide with solid naphthalene at a high pressure in an extraction cell, then expanded through a nozzle in a crystalliser. Apparently both the pre- 

It has been mentioned previously that some studies showed that slow depressurisation favoured the growth of larger crystals. In this context, Shlichta [101] published a patent for the production of crystals of high morphological quality by a slow, time-controlled reduction of pressure. During the pressure reduction the interchange of heat between the solution and the environment is controlled to be under either isothermal or adiabatic conditions. 

As mentioned earlier, the control of properties in a supercritical fluid can allow separation to be incorporated into a recrystallisation process. This uses the so-called crossover effect, which arises when the isotherms of solubility versus pressure for two compounds at the same temperature can cross, i.e. the solubility of one compound is greater at a lower temperature, whereas that of the other compound is higher at a higher temperature, and they are equal at an intermediate temperature. Chimowitz et al. [103], by expansion of the solution from an extraction vessel down to 1 bar, measured solubility of 1 10 w/w decanediol-benzoic acid mixture in supercritical carbon dioxide. They experimentally defined the crossover behaviour for the two compounds, and subsequently achieved the separation of almost pure benzoic acid [104] from the 1 10 w/w decanediol-benzoic acid mixture. In a subsequent study, Sako et al separated phenanthrene and naphthalene to 97 wt% purity from a 1 1 w/w mixture using a similar technique [105]. 

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