Ciystal size-dependent

Selective Crystal Removal or Size-Dependent Ciystal Growth, Chem. Eng. Common.. 6, 293 (1980). 

Ciystallization from solution is an important separation and purification process in a wide variety of industries. These range from basic materials such as sucrose, sodium chloride and fertilizer chemicals to pharmaceuticals, catalysts and specialty chemicals. The major purpose of crystallization processes is the production of a pure product. In practice however, a number of additional product specifications are often made. They may include such properties as the ciystd size distribution (or average size), bulk density, filterability, slurry viscosity, and dry solids flow properties. These properties depend on the crystal size distribution and crystal shape. The goal of crystallization research therefore, is to develop theories and techniques to allow control of purity, size distribution and shape of crystals. 

Whether a vessel is called an evaporator or a ciystallizer depends primarily on the criteria used in arriving at its sizing. In an evaporator of the salting-out type, sizing is done on the basis of vapor release. In a crystallizer, sizing is normally done on the basis of the volume required for crystallization or for special features required to obtain the proper product size. In external appearance, the vessels could be identical. Evaporators are discussed in Sec. 11. 

Ctystaffianrion is a commonly used industrial separation and purification technique- If the desired product is an evaporate (or filtrate) rather than a crystalliue phase, then the process emphasis is primarily that of separation rather than purification. In either case there is a strong irteraction and dependence between both degree of separation and purification and the particulate nature of the solid phase produced. Fundamental research on crystallization has focused mainly on understanding the variables that influence the structure and size of the ctystaHine phase, secogntzing that better knowledge and control of this aspect would permit improvement of the unit operation of ciystallization, both as a separation and purification technique. 

The supennolecule approach is used to study the linear and second-order nonlinear susceptibilities of the 2-methyl-4-nitroaniline ciystal. The packing effects on these properties, evaluated at the time-dependent Hartree-Fock level with the AMI Hamiltonian, are assessed as a function of the size and shape of the clusters. A simple multiplicative scheme is demonstrated to be often reliable for estimating the properties of two- and three-dimensional clusters from the properties of their constitutive one-dimensional arrays. The electronic absorption spectra are simulated at the ZINDO level and used to rationalize the linear and nonlinear responses of the 2-methyl-4-nitroaniline clusters. Comparisons with experiment are also provided as well as a discussion about the reliability of the global approach. 

During isothermal crystallization the actual ciystallization temperature (for example 122.2°C) was not exactly equal to the set temperature (125°C). This is due to the fact that the lamellar size can have only defined values, which depend not only on the conditions of crystallization, but also on the macromolecule structure. 

The synthesized LDHs showed well-ciystallized hydrotalcite structure. The thermal treatment converted the precursors into mixed metal oxides (V, Mo, W, Nb). X-ray diffraction analysis indicates the formation of an orthorhombic Mo7,gVi,2Nbo28,9 and M0O3 phase, the peaks are observed at 20 = 22.1, 28.2,6.2,45.2,50.0° (3). The textnral properties of the systems depend on the eonditions of preparation of LDHs, in particular on the particle size of the sample. For example BET surfaee area increased simultaneously with the partiole size (Table 2). 

---