Particle crystallization process

Graining, flaking, and spraying have all been used to make soHd ammonium nitrate particles. Most plants have adopted various prilling or granulation processes. Crystallized ammonium nitrate has been produced occasionally in small quantities for use in specialty explosives. The Tennessee Valley Authority developed and operated a vacuum crystallization process (25), but the comparatively small crystals were not well received as a fertilizer. 

Gas AntisolventRecrystallizations. A limitation to the RESS process can be the low solubihty in the supercritical fluid. This is especially evident in polymer—supercritical fluid systems. In a novel process, sometimes termed gas antisolvent (GAS), a compressed fluid such as CO2 can be rapidly added to a solution of a crystalline soHd dissolved in an organic solvent (114). Carbon dioxide and most organic solvents exhibit full miscibility, whereas in this case the soHd solutes had limited solubihty in CO2. Thus, CO2 acts as an antisolvent to precipitate soHd crystals. Using C02 s adjustable solvent strength, the particle size and size distribution of final crystals may be finely controlled. Examples of GAS studies include the formation of monodisperse particles (<1 fiva) of a difficult-to-comminute explosive (114) recrystallization of -carotene and acetaminophen (86) salt nucleation and growth in supercritical water (115) and a study of the molecular thermodynamics of the GAS crystallization process (21). 

Solid particles have a distinct form, which can strongly affect their appearance, product quality and processing behaviour. Thus, in addition to chemical composition, particulate solids have to be additionally characterized by particle size and shape. Furthermore, particles can be generated at any point within the process. For example, nucleation occurs within a crystallization process and large particles are broken down to numerous smaller ones in a comminution process or within a drier. 

A secondary particle formation process, which can increase crystal size dramatically, is crystal agglomeration. This process is particularly prevalent in systems exhibiting high levels of supersaturation, such as from precipitation reactions, and is considered along with its opposite viz. particle disruption in Chapter 6. Such high levels of supersaturation can markedly accentuate the effects of spatial variations due to imperfect mixing within a crystallizer. This aspect is considered further in Chapter 8. 

Secondary nucleation is an important particle formation process in industrial crystallizers. Secondary nucleation occurs because of the presence of existing crystals. In industrial crystallizers, existing crystals in suspension induce the formation of attrition-like smaller particles and effectively enhance the nucleation rate. This process has some similarity with attrition but differs in one important respect it occurs in the presence of a supersaturated solution. 

Several reported chemical systems of gas-liquid precipitation are first reviewed from the viewpoints of both experimental study and industrial application. The characteristic feature of gas-liquid mass transfer in terms of its effects on the crystallization process is then discussed theoretically together with a summary of experimental results. The secondary processes of particle agglomeration and disruption are then modelled and discussed in respect of the effect of reactor fluid dynamics. Finally, different types of gas-liquid contacting reactor and their respective design considerations are overviewed for application to controlled precipitate particle formation. 

For liquid-liquid crystal precipitation systems where the particle formation processes are fast, mixing becomes an important determiner of performance with a subtle interplay of micro- and mesomixing, which changes as scale of... 

In practice, industrial crystallization processes are subject to a number of constraints, which tend to limit equipment selection. For example, since particle size and purity tend to be such important variables, equipment and operating conditions that induce minimum particle breakdown or achieve maximum crystal purity are normally desirable. 

Gertlauer, A., Mitrovic, A., Motz, S. and Gilles, E.-D., 2001. A population balance model for crystallization processes using two independent particles properties. Chemical Engineering Science, 56(7), 2553-2565. 

Pratola, F., Simons, S.J.R. and Jones, A.G., 2000. Micro-Mechanics of Agglomerative Crystallization Processes. Proceeding of Advances in particle formation, American Institute of Chemical Engineers National Meeting, November 2000, Paper 22 g. 

It may finally be pointed out that certain separation processes in addition to packed-bed gas absorption are gas-liquid-particle operations. Examples are flotation and a special type of fluidized crystallization process (Z2). 

A mechanistic model for the kinetics of gas hydrate formation was proposed by Englezos et al. (1987). The model contains one adjustable parameter for each gas hydrate forming substance. The parameters for methane and ethane were determined from experimental data in a semi-batch agitated gas-liquid vessel. During a typical experiment in such a vessel one monitors the rate of methane or ethane gas consumption, the temperature and the pressure. Gas hydrate formation is a crystallization process but the fact that it occurs from a gas-liquid system under pressure makes it difficult to measure and monitor in situ the particle size and particle size distribution as well as the concentration of the methane or ethane in the water phase. 

Figure 2. Relative amounts of various iron species deduced from 57Fe Mossbauer spectra of the Fe-exchanged samples shown in relation to the progress of the hydrothermal crystallization process at 80°C (A), 57Fe Mossbauer spectra of the Fe-exchanged samples after 0 (a), 120 (b), 180 (c) and 240 min (d) of the hydrothermal crystallization process at 80°C (B) and RBS spectra collected on five different particles of the sample crystallized for 240 min (C). The position of surface Fe in Fig. 2C is marked by the vertical arrow. Depth scale (depth into each particle) is increasing toward left (marked with the horizontal arrow). Fit to experimental data with assumed homogeneous depth distribution of Fe is marked with the continuous line.

When the random-walk model is expanded to take into account the real structures of solids, it becomes apparent that diffusion in crystals is dependent upon point defect populations. To give a simple example, imagine a crystal such as that of a metal in which all of the atom sites are occupied. Inherently, diffusion from one normally occupied site to another would be impossible in such a crystal and a random walk cannot occur at all. However, diffusion can occur if a population of defects such as vacancies exists. In this case, atoms can jump from a normal site into a neighboring vacancy and so gradually move through the crystal. Movement of a diffusing atom into a vacant site corresponds to movement of the vacancy in the other direction (Fig. 5.7). In practice, it is often very convenient, in problems where vacancy diffusion occurs, to ignore atom movement and to focus attention upon the diffusion of the vacancies as if they were real particles. This process is therefore frequently referred to as vacancy diffusion... 

In general, zeolites are crystalline aluminosilicates with microporous channels and/or cages in their structures. The first zeolitic minerals were discovered in 1756 by the Swedish mineralogist Cronstedt [3], Upon heating of the minerals, he observed the release of steam from the crystals and called this new class of minerals zeolites (Greek zeos = to boil, lithos = stone). Currently, about 160 different zeolite structure topologies are known [4] and many of them are found in natural zeolites. However, for catalytic applications only a small number of synthetic zeolites are used. Natural zeolites typically have many impurities and are therefore of limited use for catalytic applications. Synthetic zeolites can be obtained with exactly defined compositions, and desired particle sizes and shapes can be obtained by controlling the crystallization process. 

Both experimental and theoretical work has demonstrated that growth rate dispersion exists, and has a measurable effect on the CSD in both batch and continuous crystallization processes. Further understanding of this phenomenon on a fundamental level will be required to develop methods to make use of or control growth rate dispersion and make it a tool in control of particle size and shape. 

There is a parameter necessary to describe the abrasion resistance of each crystalline substance. This abrasion resistance must be correlated with parameters of the power input devices such as pump or stirrer diameter or the stirrer tip speed. The abrasion resistance of the crystalline particles produced by secondary nucleation in industrial crystallization processes is therefore a physical property of the substance. So far there is no physical property known containing all information about this abrasion resis-... 

Crystallization processes are very important in chemical processes whenever there are solid products in a reactor. We saw in Chapter 9 that crystallization and dissolution particle sizes could be handled with the same equations as chemical vapor deposition and reactive etching. We note here that crystallization reactions can be handled with the same equations as polymerization. 

Increasing the size of the crystals as well as their internal ortter by slowing down the crystallization process failed, probably because of deterioration of the particles before they could form aggregates the latter seem to play an essential role in the nucleation process... 

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