Crystallizers latent heat

At all temperatures the liquid has a higher enthalpy (internal energy) than the sohd. Therefore, upon crystallization latent heat is released (i.e. solidification from a melt is an exothermic process), the amount of which is equal to the heat of melting, but with opposite sign ... 

Several recent patents describe improvements in the basic belt process. In one case a higher soHds polymerization is achieved by cooling the starting monomer until some monomer crystallizes and then introducing the resulting monomer slurry onto the belt as above. The latent heat of fusion of the monomer crystals absorbs some of the heat of polymerization, which otherwise limits the soHds content of the polymerization (87). In another patent a concave belt is described which becomes flat near the end. This change leads to improved release of polymer (88). 

A Hquid crystal compound in more cases than not takes on more than one type of mesomorphic stmcture as the conditions of temperature or solvent are changed. In thermotropic Hquid crystals, transitions between various phases occur at definite temperatures and are usually accompanied by a latent heat. 

The cloudiness of ordinary ice cubes is caused by thousands of tiny air bubbles. Air dissolves in water, and tap water at 10°C can - and usually does - contain 0.0030 wt% of air. In order to follow what this air does when we make an ice cube, we need to look at the phase diagram for the HjO-air system (Fig. 4.9). As we cool our liquid solution of water -i- air the first change takes place at about -0.002°C when the composition line hits the liquidus line. At this temperature ice crystals will begin to form and, as the temperature is lowered still further, they will grow. By the time we reach the eutectic three-phase horizontal at -0.0024°C we will have 20 wt% ice (called primary ice) in our two-phase mixture, leaving 80 wt% liquid (Fig. 4.9). This liquid will contain the maximum possible amount of dissolved air (0.0038 wt%). As latent heat of freezing is removed at -0.0024°C the three-phase eutectic reaction of... 

There are two work terms to consider when a nucleus forms from the liquid. Equations (6.1) and (6.2) show that work of the type AH (T, - T)/T, is available to help the nucleus form. If AH is expressed as the latent heat given out when unit volume of the solid forms, then the total available energy is (4/3)ot AH (T, - T)/T, . But this is offset by the work 4 rr ysL needed to create the solid-liquid interface around the crystal. The net work needed to form the crystal is then... 

Drying is concerned with heat transfer, namely - latent heat of vaporization and sensible heat to the vapor molecules, to the water in the pores, and to the solid and material transfer mainly the transfer of water from the inner portions of the paste or crystal mass to the surface. 

The measured growth rates are illustrated by the circles in Fig. 7. The interface velocity is plotted versus the interface temperature T. The value of T is always greater than Tq because of the release of the latent heat at the interface. Dimensionless units for T and the velocity are used here. The maximum velocity corresponds to 80m /s for argon. The most surprising aspect is the rapid crystallization at low temperatures. Most materials exhibit sharply reduced rates at low temperatures, as expected for an activated growth process. That is, the kinetics can be represented as the product of an Arrhenius factor F(T) and a term that accounts for the net production of crystalline material as a result of the atoms ordering and disordering at the interface,... 

In this description the temperature field has been taken to be linear in the coordinate y and to be independent of the shape of the melt/crystal interface. This is a good assumption for systems with equal thermal conductivities in melt and crystal and negligible convective heat transport and latent heat release. Extensions of the model that include determination of the temperature field are discussed in the original analysis of Mullins and Sekerka (17) and in other papers (18,19). 

The heat balance follows a similar relationship with the rate of latent heat release in proportion with the amount of element crystallized... 

Carefully selected seed crystals are sometimes added to a crystalliser to control the final product crystal size. The rapid cooling of an unseeded solution is shown in Figure 15.20a in which the solution cools at constant concentration until the limit of the metastable zone is reached, where nucleation occurs. The temperature increases slightly due to the release of latent heat of crystallisation, but on cooling more nucleation occurs. The temperature and concentration subsequently fall and, in such a process, nucleation and growth cannot... 

Kistiakowsky-Fishtine equation puys chem An equation to calculate latent heats of vaporization of pure compounds useful when vapor pressure and critical data are not available., kis-te-a k6f-ske fa shtTn i,kwa-zhan kitol oRG CHEM C40H60O2 One of the provitamins of vitamin A derived from whale liver oil crystallizes from methanol solution. ke,tol ... 

Figure 3.3 shows a typical freezing curve which results when heat is removed at a continuous rate from foodstuffs and the temperature decreases. The curve has three zones first, the removal of sensible heat from the food between the initial temperature and the freezing temperature second, the removal of the latent heat of fusion leading to a change of state and the formation of ice crystals and third, further sensible heat removal down to the required storage temperature. A number of features of the freezing curve require explanation. Whilst... 

Give initial conditions, including the melt composition, density, heat diffusivity, viscosity, crystal density, latent heat of fusion, and the initial crystal radius. 

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