Crystallization heat effect

For large deformations or for networks with strong interactions—say, hydrogen bonds instead of London forces—the condition for an ideal elastomer may not be satisfied. There is certainly a heat effect associated with crystallization, so (3H/9L) t. would not apply if stretching induced crystal formation. The compounds and conditions we described in the last section correspond to the kind of system for which ideality is a reasonable approximation. 

In contrast, solid sodium chloride dissolves readily in water at room temperature and without a large heat effect. This can only mean that the water interacts strongly with the ions—so strongly that aqueous ions are about as stable as are ions in the crystal. In fact, water interacts... 

Several parameters have been seen to influence the crystallization of ice crystals in subcooled aqueous solutions. The primary factor is the extent of subcooling of the solution. Other factors include the agitation rate, the types and levels of solutes in solution. Huige (6) has summarized past work on conditions under which dusk-shaped and spherical crystals can be found in suspension crystallizers. The effects of heat and mass transfer phenomena on the morphology of an ice crystal growing in a suspension have not been fully understood. 

Here the TNT sample is compressed at very low pressures from V=1 cc/g to V X).62 cc/g (crystal density). Further compression (increase in pressure) then causes the sample to expand This can only mean that some heat effect is overcoming this compression. Since it can be shown that uniform shock heating at pressures of the order of a few kbars is very small, this heat effect must be produced by exothermic chemical reaction at or very near the shock front. Thus shock Hugoniots for reactive materials can provide information on the presence or absence of chemical reaction at the shock front... 

SYNTHESIS A solution of 33.2 g of veratraldehyde in 15.0 g nitroethane was treated with 0.9 g of n-amylamine and placed in a dark place at room temperature. In a day or so, separated H20 was apparent and, after a couple of weeks, the mixture completely solidified. The addition of 50 mL EtOH and heating effected complete solution and, on cooling, this provided 1 -(3,4-dimethoxyphenyl)-2-nitropropene as yellow crystals, 29.0 g, with nip of 70-71 °C. The more conventional reaction scheme, 6 h heating of a solution of the aldehyde and nitroethane in acetic acid with ammonium acetate as catalyst, gave a much inferior yield of product (33.2 g gave... 

Two typical cases are illustrated in Fig. 2.24 the first scheme (Fig. 2.24 a) is related to high-temperature polymerization, in which newly formed polymer is molten and the processes of polymerization (part Ob of the full curve) and crystallization (part bK of the full curve) are separated in time. The second case (Fig. 2.24 b) illustrates low-temperature polymerization in this situation crystallization starts before the full process of polymerization is completed. This is typical superposition of two kinetic processes, and the shape of the curve in Fig. 2.24 b does not allow the separation of these processes without additional information and assumptions.The net heat effect is the same in... 

A typical example illustrating the separation of the net heat effect AT into parts corresponding to polymerization ATP and crystallization ATC is shown in Fig. 2.25.100 These data were obtained by direct measurement of the quantities of polymeric products formed during the reaction. This gives us the value of ATP calculated from(3(t) and the known value of the maximum temperature increase dur-... 

Figure 2.26. Heat effects, observed during anionic activated polymerization of E-caprolactam at 190°C (a) and 160°C (b). Curves 1 and 2 are components related to crystallization and polymerization, respectively. Curves 3 and 4 are calculated from Eq. (2.36) and from a simple additive rule, respectively.

The choice of the temperature of the initial reactive mass (75 - 90°C) is dictated by two requirements firstly, the reactive mass must be liquid secondly, the reaction rate in this temperature range must be negligible. It was established in preliminary experiments that the temperature of the heater surface needs to be 75 - 125°C higher than the initial temperature of the reactive mass. The necessary operation period for the heater depends on the initial temperature of the reactive mixture and its reactivity (i.e., on its composition). The temperature of the heater does not influence the properties of the final product or the stationary kinetics of the process. The local temperature increase inside the adjoining layer must be supplemented by a heater for 30 - 50 min. This is the time required to set up the reaction front after that, the front exists by itself and propagates due to the exothermal heating effects of chemical reaction and crystallization. 

These observations (non-linear heats of hydration) suggest the following hypothesis In the absence of crystal-field effects the... 

Condensation via Terephthalic Acid. The salt from the acid and the diamine is formed easily in the aqueous phase (8). Crystallization is effected by isopropyl alcohol addition. Recrystallization is unnecessary when pure starting components are used. The aqueous solution of the salt is heated under pressure, the solution water is distilled off, the condensation begins, the temperature is increased, and the pressure is lowered. The reaction is complete when the desired viscosity is reached. 

The actual yield may be obtained from algebraic calculations or trial-and-error calculations when the heat effects in the process and any resultant evaporation are used to correct the initial assumptions on calculated yield. When calculations are made by hand, it is generally preferable to use the trial-and-error system, since it permits easy adjustments for relatively small deviations found in practice, such as the addition of wash water, or instrument and purge water additions. The following calculations are typical of an evaporative crystallizer precipitating a hydrated salt. II SI units are desired, kilograms = pounds X 0.454 K = (°F -I- 459.7)/I.8. 

---