Crystallization solid-liquid equilibria

Whether a task can be performed concurrently with other tasks depends on two factors. One is whether the input information for the activity under consideration depends on the output from other activities. The other is the availability of manpower and equipment. Consider a team that has only one chemical engineer to design both the reactor and the crystallizer. Even though reaction kinetics, solid-liquid equilibrium data and crystallization kinetics can be measured in parallel, the total time for these activities is determined by what the single individual can achieve. 

It is proposed that in mixed organic base-alkali systems, the presence of the organic base changes the solid-liquid equilibrium and stabilizes larger sol-like aluminosilicate species ( 25 m/ ). The alkali ion affects agglomeration of the sol particles to larger amorphous precipitate particles from 100 to 500 min size which subsequently crystallize to zeolite. 

Of interest in crystallization calculations is solid-liquid equilibrium. When the solid phase is a pure component, the following thermodynamic relationship holds ... 

Slaughter, D. W., and Doherty, M. E, Calculation of solid-liquid equilibrium and crystallization paths for melt crystallization processes. Submitted for publication (1994). 

The pharmaceutical and life science industries often deal with large, complex molecules, and separation via crystallization is an important practice. Robust flash algorithms for solid-liquid equilibrium, particularly systems with multiple polymorphs, are highly desirable. 

Solid-liquid equilibrium phase diagrams play an important role in the design of industrial crystallization processes. The calculation of phase diagrams can be used to validate the activity coefficient model used for process simulation. 

In the presence of polymorphs, developing a proper crystaUization process to control the desired crystal form is a real challenge. A good understanding of solid-liquid equilibrium behaviors under different conditions—for example, the temperature or solvent mixmre— is a must. Seeding and control of supersaUiration are two critical, if not indispensable, requirements. Example 7-5 shows an example of developing a crystallization process for a polymorphic compound. 

Issues freeze drying, freeze crystallization, solid-liquid (S-L) equilibrium, amorphism, and crystallinity... 

Partial Miscibility in the Solid State So far, we have described (solid + liquid) phase equilibrium systems in which the solid phase that crystallizes is a pure compound, either as one of the original components or as a molecular addition compound. Sometimes solid solutions crystallize from solution instead of pure substances, and, depending on the system, the solubility can vary from small to complete miscibility over the entire range of concentration. Figure 14.26 shows the phase diagram for the (silver + copper) system.22 It is one in which limited solubility occurs in the solid state. Line AE is the (solid + liquid) equilibrium line for Ag, but the solid that crystallizes from solution is not pure Ag. Instead it is a solid solution with composition given by line AC. If a liquid with composition and temperature given by point a is... 

Equilibrium intensive thermodynamic properties of pure compounds that exist as a single phase, e.g. crystal (solid), liquid or gas, are functions of two independent observables. Temperature and pressure are usually the selected variables, although other pairs may be used. 

Accurate solid-liquid equilibrium data mast be obtained to evaluate the process design options for crystallization processes. These data are required in the earliest stages of the conceptual de9iga phase and are necessaty for the following reasons ... 

The crystallization temperature (Tc) commonly called the wax appearance temperature (WAT) is determined from the intersection of the baseline mid the extrapolation of the peak in cooling experiments. Routine use of DSC apparatus shows a thermal noise of 7 pw. Therefore, the crystallization temperature is defined as the temperature at which tiie thermal power developed by the heat of crystallization of paraffins is 15 pw. In order to have a reasonable sensitivity, a smaller cooling rate ( l-2 C/min) must be used. This rate of cooling allows a better approach to file solid-liquid equilibrium temperature. Determination of... 

The freezing point of a solution is the temperature at which the first crystals of pure solvent form in equilibrium with the solution. Recall from Section 11.6 that the line representing the solid-liquid equilibrium rises nearly vertically from the triple point. It is easy to see in Figure 13.21 that the triple-point temperature of the solution is lower than that of the pure liquid, but it is also true for all points along the solid-Uquid equilibrium curve the freezing point of the solution is lower than that of the pure liquid. 

For the synthesis and design of crystallization processes, the reliable knowledge of the solid-liquid equilibrium (SLE) behavior, respectively, the solubility of solids in a solvent or solvent mixture is of special importance. Additionally, reliable kinetic information about metastable zone nucleation and crystal growth is required. Information about the SLE behavior, respectively, the driving forces is also required to avoid blocking of, for example, in pipelines or other process units. 

P8.10 Determine the solid-liquid equilibrium temperature of the ideal ternary system m-xylene (l)-o-xylene (2)-p-xylene (3) for a composition ofxi = 0.1 and X2 =0.1 with the help of the melting temperatures and enthalpies of fusion given in Example 8.4 in the textbook. Which component will crystallize ... 

In Section 11.4, it was shown how suitable solvents can be selected with the help of powerful predictive thermodynamic models or direct access to the DDB using a sophisticated software package. A similar procedure for the selection of suitable solvents was also realized for other separation processes, such as physical absorption, extraction, solution crystallization, supercritical extraction, and so on. In the case of absorption processes or supercritical extraction instead of a g -model, for example, modified UNIFAC, of course an equation of state such as PSRK or VTPR has to be used. For the separation processes mentioned above instead of azeotropic data or activity coefficients at infinite dilution, now gas solubility data, liquid-liquid equilibrium data, distribution coefficients, solid-liquid equilibrium data or VLE data with supercritical compounds are required and can be accessed from the DDB. 

Phase behavior involving solid-liquid equilibrium is the basis for crystallization in chemical and materials engineering. Binary mixture systems can have up to three degrees of freedom according to the Gibbs phase rule. 

This is not the case for FI2O. Because of the crystal structure of solid H2O, the solid phase of H2O has a larger volume than the equivalent amount of liquid-phase H2O. This is reflected in the negative slope of the solid-liquid equilibrium line in the phase diagram of H2O, Figure 6.3. When the pressure is increased (at certain temperatures), the liquid phase is the stable phase, not the solid phase. H2O is the exception, not the rule. It s just that water is so common, and its behavior so accepted by us, that we tend to forget the thermodynamic implications. 

In the 60-80 ps section of Fig. 13.1 the benzene molecules are literally pushing the crystal lattice apart under the effect of the suddenly available kinetic energy. Diffusional and rotational freedom results and the crystal collapses to the liquid, which is then normally simulated at 300 K in the 80-120 ps section of the run. Since the part of the simulation where melting occurs is a non-equilibrium simulation, one cannot draw any conclusions from averages, nor can one claim to have simulated the actual solid-liquid equilibrium or to have predicted the melting temperature. Nevertheless, such dynamic runs offer a window over the evolution of the internal structure of the system as it goes from crystal to liquid an example taken from a study of acetic acid is shown in Fig. 13.2. One cannot say for sure that this picture is a representation of the tfue structural changes that occur when the acetic acid crystal melts molecular... 

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