Pressure Crystallization

In this technique, crystallization is induced by compression rather than by cooling. The solid phase of most (organic) materials has a higher density than the liquid phase and consequently the melting point rises with increasing pressure. Benzene, for example, has an atmospheric melting point of 5.5°C but solidifies at room temperature when the pressure is about 2 MPa. Not only the melting points but also solid-liquid phase equilibria shift upward upon a pressure increase, which is illustrated for a mixture of cresols in Figure 15.

An additional advantage of pressure crystallization is found in the transmission of pressure being much faster than the transfer of heat in both liquid and solid phases. This means that a uniform pressure can be established quickly throughout the system. Therefore local variations in supercooling of the mother liquor can easily be avoided.

The separation of cresol isomers by conventional (atmospheric) cooling crystallization is rather difficult due to a high viscosity of the melt. Kobe Steel

Figure 13 Pressure-temperature-composition diagram of s-caprolactam-water. Tti = triple point component i; Tq = quadruple point; Tcrys = crystallizer temperature; Tcon = condenser temperature; Si, Li, Vi = solid, liquid, vapour phase component i; (LV)i, (SL)i, (SV)i liquid-vapour, solid-liquid, solid-vapour equilibrium curves pure component i; x, y = liquid, vapour concentration (wt%) at three-phase equilibrium; SiLV = three-phase equilibrium line with solid phase of component i. (Reproduced with permission from Diepen PJ (1998) Cooling Crystallization ofOrganic Compounds. PhD thesis, Technical University Delft.)

Figure 14 Part of pressure-temperature diagram of £-caprolac-tam-water. (Reproduced with permission from: Diepen PJ (1998) Cooling Crystallization ofOrganic Compounds. PhD thesis, Technical University Delft.)

Figure 13 Pressure-temperature-composition diagram of s-caprolactam-water. Tti = triple point component i; Tq = quadruple point; Tcrys = crystallizer temperature; Tcon = condenser temperature; Si, Li, Vi = solid, liquid, vapour phase component i; (LV)i, (SL)i, (SV)i liquid-vapour, solid-liquid, solid-vapour equilibrium curves pure component i; x, y = liquid, vapour concentration (wt%) at three-phase equilibrium; SiLV = three-phase equilibrium line with solid phase of component i. (Reproduced with permission from Diepen PJ (1998) Cooling Crystallization ofOrganic Compounds. PhD thesis, Technical University Delft.)

(Japan) developed in the 1970s a pressure-crystallization process, which was applied for the separation of p-cresol from its isomers on a commercial scale. A given amount of slurry from a chiller is fed to a cylindrical pressure vessel, compressed to crystallize and drained to remove residual impure melt (Figure 16). During the subsequent depressuring, purification by sweating can occur. Finally, a cylindrical block of purified crystals is taken out as the product. The operating temperature is 62.5°C at 200 MPa (versus 12.5°C at atmospheric pressure) which effectively reduces the viscosity of the mother liquor. Under these conditions, the selectivity and the rate of crystal growth rate are enhanced while solid-liquid separation is facilitated. Purification by sweating during depressuring is analogous to (thermal) sweating at elevated temperatures.

The features of pressure crystallization can be summarized as follows:

• batch-wise operation;

• product purity up to 99.5%, feasible in one operating cycle from 70 to 80% in feed;

Figure 14 Part of pressure-temperature diagram of £-caprolac-tam-water. (Reproduced with permission from: Diepen PJ (1998) Cooling Crystallization ofOrganic Compounds. PhD thesis, Technical University Delft.)

high yield, limited only by the solid-liquid equilibrium of a given system;

short cycle times, only a few minutes per cycle, due to rapid penetration of pressure; low energy consumption - only mechanical energy used for compression;

Figure 15 Phase diagram of mixture of p-cresol and m-cresol. (Reproduced with permission from Yasuda M, Sato Y and Suematsu H (1991) p-Cresol with high pressure crystallization. Kagaku Kogaku 55(4): 290-291.)
Figure 16 Operating cycle of pressure crystallization process. (Reproduced with permission from Yasuda M, Sato Y and Suematsu H (1991) p-Cresol with high pressure crystallization. Kagaku Kogaku 55(4): 290-291.)

• simple process design - crystallization, separation and sweating are carried out in a single vessel;

• complex mechanical design - materials selection and construction of mechanical parts are critical factors for success.

The scale-up of pressure-crystallization equipment is not easy due to mechanical complexity, and it becomes increasingly difficult at larger vessel diameters to remove residual melt from the centre of the cake by compression. The production capacity of a single plant is in the order of 500 tons per year.

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