Crystallization pressure

It has been illustrated that polycrystalline materials can be operated in regenerative electrolytic solar cells yielding substantial fractions of the respectable energy conversion efficiency obtained by using single crystals. Pressure-sintered electrodes of CdSe subsequently doped with Cd vapor have presented solar conversion efficiencies approaching 3/4 of those exhibited by single-crystal CdSe electrodes in alkaline polysulfide PEC [84]. 

In 1955, Aerojet-General Corporation issued a report (Higgins, 1955) that described a few experiments in which molten aJuminum (and other metals) were poured into water. The water vessel was a vertical 30-cm-diameter, 25-cm-long pipe with a steel bottom plate. The water depth was 23 cm. Metal was dropped from an overhead crucible, usually with a free fall of 51 cm before contacting the water. A barium titanate crystal pressure transducer was located in the side of the water vessel about 13 cm below the water surface. 

If ice-cream is warmed or the temperature fluctuates, some ice will melt, and an infinite variety of lactose concentrations will emerge, some of which will be in the labile zone where spontaneous crystallization occurs while others will be in the metastable zone where crystallization can occur if suitable nuclei, e.g. lactose crystals, are present. At the low temperature, crystallization pressure is low and extensive crystallization usually does not occur. However, the nuclei formed act as seed for further crystallization... 

Controlling the crystallization pressure is essential for both purification by crystallization and for efficient operations on scale. By adjusting solution conditions to decrease the solubility of the product within the metastable zone, the desired molecules can be pressured to come out of solution and crystallize (Figure 11.3) [18]. Gradual cooling without seeding leads to one nucleation event and the... 

The importance of avoiding excessive crystallization pressure is shown with the crystallization of the phosphinyl acetic acid 6 (Figure 11.4). In the initial process 6 was crystallized from an aqueous solution, which led to the formation of an oil dispersion, then a crystalline suspension [19].This process provided high-... 

Crystallization pressure may be applied by diluting a solution of the product with an antisolvent. Antisolvents are generally selected from those solvents that are miscible with the solvent. Water may be a good antisolvent for compounds readily soluble in EtOH heptane may be a good solvent for compounds readily soluble in EtOAc. Supersaturation can be quickly reached when an antisolvent is added to a solution, leading to the formation of oils and poor crystallization. Oil formation may be noted at the entry point of the antisolvent to the solution and can be minimized by adding antisolvents slowly. 

Solution develop process using gentler crystallization pressure. 

FIGURE 6.3 Quadruple effect evaporator-crystallizer. Pressure and boiling temperature decrease from left to right, temperature decreases about 20-30°C across the four effects. Feed brines to each effect may be directly from brine wells, or may be fed cascade fashion from the filtrate to the salt brine slurry of the previous effect, or a combination of these methods. (Adapted from Kirk-Othmer [10], with permission.)... 

Crystallization occurs for many common fluids, such as carbon tetrachloride, benzene, and cyclohexane, at pressures less than 200 MPa, thus their entire fluid range is rather limited. The supercritical noble gases significantly extend this range, achieving a maximum crystallization pressure for helium of P = 11.6 GPa. The viscosity increase prior to all such transitions is modest. The viscosity for a typical dense fluid is 1-100 mPa and this will increase by, at most, about three orders of magnitude. Experimentally, this viscosity and pressure regime is covered by many of the viscometers discussed below, and hard-sphere theory can explain most of the viscosity increase. 

Glass formation, the other common fate of a compressed fluid, however, gives a much larger viscosity increase. For mixtures and fluids with nonspherical molecules, it is common to produce a metastable glass at some pressure in excess of the equilibrium crystallization point. Methanol is one example. It can be easily superpressed past its crystallization pressure of 3.5 GPa at T = 25°C towards its glass-transition pressure of 11.4 GPa and beyond. Being a... 

Sulphate waters. The influence of these waters, mostly groundwaters, on concrete is the most intensive. The main reason for the destruction of concrete by sulphate waters is the reaction of sulphate anions with tricaJcium aluminate to form the already mentioned strongly hydrated compound. This compound crystallizes in needles with a considerable increase in volume. Due to the considerable crystallization pressure the concrete cracks, becomes porous and the destructive effects of aggressive water penetrate deeper through the cracks formed. 

The hypothesis related to the crystallization pressure is proved by very well known high susceptibility to crystallization of portlandite and ettringite, which are very easy detected and qitickly formed crystalline phases of cement paste. The results reported by Chatteqee and Jeffery [67] seem to comply well with this hypothesis and show that the CafOHJj crystallization causes the repulsion of grains the growth of pores and the formation of the new one is thus promoted. 

Dron and Brivot [80] prepared the tablets of groimd ettringite pressed imder the pressure of 50 MPa, with low porosity, of about 17%. The tablets treated with water do not practically swell—swelling is on the level 0.4%, while in situ ettringite formation from calcium sirlphoaliuninate gives the volume increase of 40% and the destraction of samples. The authors [80] state that the Mehta hypothesis is not valid and the crystallization pressure is the cause of expansion. Simultaneously Dron and Brivot [80] calculated the approximate crystalhzation pressure based upon the change of free enthalpy of reaction ... 

The crystal pressure (Eq. 1) and hydration pressure (Eq. 2) are calculated from the following equations ... 

Applying equation 1, Winkler and Singer (19) calculated the crystallization pressures for several salts at various C/C and T values. Table 1 is condensed from their Table I (p. 3512). 

---