Latent heat of crystallization

The congeal point, also called the setting point (O Brien, 2003), is a measure of the solidification point of a molten fat under the conditions of the test. The molten fat, contained in a beaker, is cooled until the cloud point is observed, and then cooled further until a certain subjectively assessed degree of turbidity (caused by the presence of fat crystals) is reached. The beaker is then kept at 20°C and the temperature of the sample recorded over time. The temperature rises initially owing to the release of latent heat of crystallization and then drops. The maximum temperature reached is recorded as the congeal point. 

The following properties belong to the calorimetric category (1) specific and molar heat capacities, (2) latent heats of crystallization or fusion. It will be shown that both groups of properties can be calculated as additive molar quantities. Furthermore, starting from these properties the molar entropy and enthalpy of polymers can be estimated. 

The supercooled stream from the A unit flows directly to the worker unit. There is normally a 5-8°C (10-15°F) temperature rise across the B unit most of which results from latent heat of crystallization mechanical power does not add significantly to the total heat input. The plasticized fat from the B unit is forced through a special extrusion valve that also maintains an internal pressure of 17-20 bar... 

The heat of fusion normally increases with bigger chain lengths and decreasing unsaturation in the triglycerides. Blends of triglycerides have less latent heat of crystallization than the similar nonblended triglycerides (19). 

In the literature, one can hnd theoretical and practical studies relating to heat transfer conditions in scraped-surface heat exchangers (19, 54), which cover factors such as specific weight, specific heat, latent heat of crystallization, dry matter content, retention time, and overall heat transfer conditions. 

The worker unit is normally mounted with a heating jacket for tempered water on the cylinder and often also equipped with its own built-in water heater and circulation pump for the tempered water. This is advantageous in preventing product buildup on the cylinder wall and allows better product temperature control during the passage through the worker unit. Product temperature increases of 2°C or more due to release of latent heat of crystallization and mechanical work can be observed in the worker imit (3). 

However, Equation (5.b) neglects the heat generated by the latent heat of crystallization. An analytic solution of equation (5.b) with the boundary conditions given by equations (6) is provided in some texts (Luikov, 1980), i.e. 

Figure 1. When a liquid polymer is cooled to a temperature below its melting temperature then crystallizing particles (spherulites) appear at random in the liquid and grow with time. Notice impinging (overlapping) particles and the gradual exclusion of liquid volume for the appearance of new nuclei. The release rate of latent heat of crystallization must be less than the rate of heat removal from the polymer for the growth to continue.

The reversing heat capacity and the total heat-flow rate of an initially amorphous poly(3-hydroxybutyrate), PHB, are illustrated in Fig. 6.18 [21]. The quasi-isothermal study of the development of the crystallinity was made at 296 K, within the cold-crystallization range. The reversing specific heat capacity gives a measure of the crystallization kinetics by showing the drop of the heat capacity from the supercooled melt to the value of the solid as a function of time, while the total heat-Uow rate is a direct measure of the evolution of the latent heat of crystallization. From the heat of fusion, one expects a crystallinity of 64%, the total amount of solid material, however, when estimated from the specific heat capacity of PHB using the ATHAS Data Bank of Appendix 1, is 88%, an indication of a rigid-amorphous fraction of 24%. 

The transition temperature is given by the intersection of the vertical segment and the baseline. In the case of a rather big sample and a rather high cooling rate (t> 5 K/min) crystallization is not instantaneous. The important release of heat during the transformation induces the solidification of some parts of the sample at higher temperature. Thus, the enthalpies of solidification measured are overestimated. Bulk water represents the most unfavorable case Its latent heat of crystallization varies strongly with temperature. 

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