Isothermal evaporative crystallization

The maximum yield Y of an isothermal evaporative crystallization is given by the starting volume Vo and the final volume Vo, (Equation 10.2). It is apparent that... 

The scale of operation often has an overriding importance on the selection of the equipment because of the means used for heat transfer. For very small-scale crystallization work it is common to use radiation. The capacity of such equipment varies from a few liters up to several hundreds of liters per day (of solution cooled). For operation on scales up to several thousand liters per day, it is possible to use tanks with water-cooled coils and an agitator. For large-scale applications where the quantity of solution is thousands of liters per day, it is almost universal practice to use vacuum evaporation to remove the solvent this is true whether the solution is cooled by adiabatic evaporation or in equipment where crystallization occurs because of isothermal evaporation. 

For example, a solution S, isothermally evaporated to a condition represented by point M would yield a mixture of crystals (solid solution) of overall composition C at one end of the tie line suspended in a solution of composition represented by point L at the other end. If evaporation were to be continued to dryness, the overall composition of the solid solution would be represented by point F on line AB. The deposited crystals would not be homogeneous, however, since they would have successively grown from a whole range of solution compositions and would tend to reflect these conditions by their outer layers being of slightly different composition from their insides. 

Figure 4.33. Representation of water content a) isothermal evaporation , b) crystallization by cooling...

The liquidus curve was determined by the solubility method. The Cd-clathrate composition at different temperatures was determined by the analysis of the crystals, formed during the isothermal evaporation of the saturated solution after drying them by filter paper (at the same temperature). The most accurate data on the clathrates composition were obtained at room temperature by the saturation of the. initial complexes by 4-methylpyridine through the gas phase. 

Example. Consider an evaporative, isothermal. Class II crystallizer with fines removal. It is assumed that the growth rate is size-independent and that there is no growth rate dispersion. Because we assume fines removal, the method of moments can not be applied. 

Germanium single crystals of the four n-type samples, with 0.14-, 1.8- to 2.1-, 5.5- to 10-, and 20- to 30-ohm-cm. resistivities, were crushed in an agate mortar and stored in air for 15 days before measurements were begun. Both argon surface areas and propanol isotherms were determined after activation for 15 hours at better than 10-5 mm. of Hg at six different temperatures ranging from 25° to 500° C. Even after outgassing at 500° where oxide film is lost by reduction and subsequent evaporation, additional oxide forms instantaneously at the ordinary vacuum employed and over the relatively small surface areas in the sample tube totally about 1 sq. meter. 

Simultaneous heat and mass transfer plays an important role in various physical, chemical, and biological processes hence, a vast amount of published research is available in the literature. Heat and mass transfer occurs in absorption, distillation extraction, drying, melting and crystallization, evaporation, and condensation. Mass flow due to the temperature gradient is known as the thermal diffusion or Soret effect. Heat flow due to the isothermal chemical potential gradient is known as the diffusion thermoeffect or the Dufour effect. The Dufour effect is characterized by the heat of transport, which represents the heat flow due to the diffusion of component / under isothermal conditions. Soret effect and Dufour effect represent the coupled phenomena between the vectorial flows of heat and mass. Since many chemical reactions within a biological cell produce or consume heat, local temperature gradients may contribute in the transport of materials across biomembranes. 

A supercooled liquid solidifies when inoculated with a small crystal of the solid, while the latent heat is evolved in the process. The solidification can be performed reversibly and made to yield work by evaporating the liquid at its vapour pressure allowing the vapour to expand isothermally until the pressure is equal to the vapour pressure of the solid, and then condensing to the solid state. 

An accurate measurement of the concentration of the solute in the continuous phase can be obtained by sampling and evaporating to dryness, but this is, of course, prohibitively time-consuming for control purposes. There are several methods for the rapid measurement of continuous-phase concentration. The measurement of refractive index has been shown to provide quick and accurate concentration determination (Kelt and Larson 1977 Mullin and Leci 1972 Sikdar and Randolph 1976). Garside and Mullin (1966) suggested the use of an on-line densitometer to determine solution concentration, and Witkowski (1990) successfully used densitometry for concentration measurements to estimate the kinetic parameters of an isothermal crystallizer. It should... 

PLLA is a crystallizable polymer with melting temperature at around 175°C. Semicrystalline PLLA films can be prepared by various techniques, such as solution casting with a slow evaporation rate of solvent, isothermal crystallization from the melt, and annealing of amorphous samples. Since PLLA has a glass transition temperature (Tg) of 60 "C, which is higher than room temperature, a completely amorphous film can be obtained by melt quenching. 

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