Heat of crystallization

Heating value Heating values Heat insulation Heat lamps Heat of crystallization Heat picture... 

Melting and recrystallization behavior of virgin PTEE has been studied by dsc (64). A quantitative relationship was found between and the heat of crystallization (A/T) in the molecular weight range of 5.2 x 10 to 4.5 X 1 0, where is heat of crystallization in J/g, which is independent of cooling... 

Thermodynamic Properties. The thermodynamic melting point for pure crystalline isotactic polypropylene obtained by the extrapolation of melting data for isothermally crystallized polymer is 185°C (35). Under normal thermal analysis conditions, commercial homopolymers have melting points in the range of 160—165°C. The heat of fusion of isotactic polypropylene has been reported as 88 J/g (21 cal/g) (36). The value of 165 18 J/g has been reported for a 100% crystalline sample (37). Heats of crystallization have been determined to be in the range of 87—92 J/g (38). 

If appropriate enthalpy data are unavailable, estimates can be obtained by first defining reference states for both solute and solvent. Often the most convenient reference states are crystalline solute and pure solvent at an arbitrarily chosen reference temperature. The reference temperature selected usually corresponds to that at which the heat of crystallization A/ of the solute is known. The heat of crystallization is approximately equal to the negative of the heat of solution. For example, if the heat of crystallization is known at then reasonable reference conditions would be the solute as a soUd and the solvent as a Uquid, both at The specific enthalpies then could be evaluated as... 

Cooling Ciystalli rs use a heat sink to remove both sensible heat from the feed stream and the heat of crystallization released as crystals are formed. The heat sink may be no more than the ambient surroundings of a batch crystallizer, or it may be cooling water or another process stream. 

S. G. Mueller, R. Eckstein, D. Hofmann, L. Kadinski, P. Kaufmann, M. Koelbl, E. Schmitt. Modelling of the PVT-SiC bulk growth process taking into account global heat transfer, mass transport and heat of crystal-Uzation and results on its experimental verification. Mater Sci Eorum 0 51, 1998. 

Thermal transitions can be studied by DSC. The crystallization transition is usually sharp with a good baseline. The melting transition is more complex and often not a single transition (Fig. 3.19)48 as it depends on the thermal history of the sample and the structural changes that can take place upon heating. In warming, solid-state transitions can take place in the unit cell, the lamellae can thicken, and secondary crystallization can also take place. The heats of crystallization and... 

For AHss, A2, D4, and q we take the values given previously D6 = 80 kcal/mole and I4 = 200 kcal/mole. Thus AHd = 65 - AHS7. A value of 65 kcal/mole for AHS7, required to make AHd zero, is not excessively high, especially as it includes terms for the complexing of TiCl3 with one of the components of the reaction mixture, or its heat of crystallization, and for the destruction of the radical function. This type of reaction therefore appears feasible, at least for reducible metal halides. 

Andrews, E J., Eynn, G., and Johnston, J. The heat capacities and heat of crystallization of some isomeric aromatic compounds, J. Am. Chem. Soc, 48(5) 1274-1287, 1926. 

For crystallization by acid addition, the effects of acid addition rate, agitation, and initial solution composition were examined. The experiments had constant initial and final temperatures, although there were some variations in temperature during a run because of the heat of crystallization of L-isoleucine hydrochloride (L-Ile HCl H20) and heat of solution of acid. The acid used was in the form of 37% HCl. The initial conditions of the batch corresponded to an HCl concentration giving the maximum solubility of L-Ile (1). A schematic of the batch crystallizer is shown in Figure 3. 

Notice especially that of the total heat released in this example, only 13-9% comes from lowering the temperature. Most of the heat comes from the two transformations of state—condensation and crystallization. For Fi20, the fact that the heat of condensation is almost 7 times greater than the heat of crystallization may be interpreted as meaning that the molecular description of the liquid state is much more like the solid than the gas. 

The degree of crystallinity is also reflected in the density of the polymer so that the determination of density provides at least a relative measure for crystallinity. Differential scanning calorimetry (DSC) is frequently applied to determine the crystallinity from the heat of crystallization or melting (see Sect. 2.3.4.8). 

Heat of Crystallization. The number of calories liberated or absorbed per mol, or gram when a substance passes into the crystalline state. In cases of solidification of crystalline compounds, the heat liberated on freezing may be considered as the heat of crystallization... 

The heat of crystallization (freezing) of nitroglycerine has been determined by many authors (Nauckhoff [18], Beckerhinn [44]). The most complete investigations were those performed by Hibbert and Fuller [46] who have established the following data ... 

Several times when benzoin was being prepared, the condensation took place and was complete after boiling five to ten minutes or even less, the product separating out in the hot solution with evolution of heat. The yield under these conditions was always good and the color better than when the reaction took a longer time. The cause of this occasional rapid reaction was not discovered, but it was probably due to inoculation with a little benzoin in certain of the experiments. When it takes place as just described, the flask and efficient condenser used will be sufficient to condense the sudden, rapid evolution of alcohol vapors caused by the heat of crystallization of the benzoin. 

Molecular properties can be classified according to their end-poinl observables, such as chemical I reactivity. solubility, acid-basel. physical (a function of physical state gas. liquid, solid thermodynamic), or biological (ligand or enzyme agonist or antagonist). These properties reflect macroscopic, or bulk, properties, which exist only for the bulk material, e.g.. heat of crystallization, ur microscopic properties, which exist for an ensemble of the molecule. As use of CAMM methods... 

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