Crystallization system design

Melt crystallization is carried out either with a suspension of crystals or an advanciag front (layer) of soHds, although a more complete categorization of melt crystallization is available (71). FoUowiag is a brief review of processes ia which melt crystallization is used a more complete review, including a worked out case study for system design, is available (69). 

The reactor effluent is rapidly quenched with aqueous mother Hquor in specially designed equipment operating at pressures essentially equal to the reactor pressure. This operation yields an off-gas consisting of ammonia and carbon dioxide vapor and a crystalline melamine slurry saturated with ammonia and carbon dioxide. The slurry is concentrated in a cyclone mill. The mother Hquor overflow is returned to the quenching system. The concentrated slurry is redissolved in the mother Hquor of the crystallization system, and the dissolved ammonia is stripped simultaneously. 

Figure 9.1 Typical crystallization process systems design procedure...

Crystallization proeess systems design and operation is a eomplex matter requiring extensive data for systematie evaluation. Whilst simplified design methods and heuristies are available, the simple faet remains that the more and better the data input, the better the final design and reliability of the plant. Ideally, amongst the data required are the following ... 

Jones, A.G., 1991. Design and perfonuance of crystallization systems. In Advances in Industrial Crystallization. Eds. I. Garside, R.J. Davey and A.G. Jones, Oxford Butterworth-Heinemann, pp. 213-228. 

Computer simulations therefore have several inter-related objectives. In the long term one would hope that molecular level simulations of structure and bonding in liquid crystal systems would become sufficiently predictive so as to remove the need for costly and time-consuming synthesis of many compounds in order to optimise certain properties. In this way, predictive simulations would become a routine tool in the design of new materials. Predictive, in this sense, refers to calculations without reference to experimental results. Such calculations are said to be from first principles or ab initio. As a step toward this goal, simulations of properties at the molecular level can be used to parametrise interaction potentials for use in the study of phase behaviour and condensed phase properties such as elastic constants, viscosities, molecular diffusion and reorientational motion with maximum specificity to real systems. Another role of ab initio computer simulation lies in its interaction... 

More systematic (but not always unambiguous) is the designation by Pearson symbols their use is recommended by IUPAC (International Union of Pure and Applied Chemistry). A Pearson symbol consists of a lower case letter for the crystal system (cf. the abbreviations in Table 3.1, p. 24), an upper case letter for the kind of centering of the lattice (cf. Fig. 2.6, p. 8) and the number of atoms in the unit cell. Example sulfur-< F128 is orthorhombic, face centered and has 128 atoms per unit cell (a-sulfur). 

AIChESymp. Ser. (a) 65 (1969) no. 95, Crystallization from solutions and melts (b) 67 (1971) no. 110, Factors affecting size distribution (c) 68 (1972) no. 121, Crystallization from solutions Nucleation phenomena in growing crystal systems (d) 72 (1976) no. 153, Analysis and design of crystallisation processes (e) 76 (1980) no. 193, Design, control and analysis of crystallisation processes (f) 78 (1982) no. 215, Nucleation, growth and impurity effects in crystallisation process engineering (g) 80 (1984) no. 240, Advances in crystallisation from solutions. 

For effective control of crystallizers, multivariable controllers are required. In order to design such controllers, a model in state space representation is required. Therefore the population balance has to be transformed into a set of ordinary differential equations. Two transformation methods were reported in the literature. However, the first method is limited to MSNPR crystallizers with simple size dependent growth rate kinetics whereas the other method results in very high orders of the state space model which causes problems in the control system design. Therefore system identification, which can also be applied directly on experimental data without the intermediate step of calculating the kinetic parameters, is proposed. 

In this paper, three methods to transform the population balance into a set of ordinary differential equations will be discussed. Two of these methods were reported earlier in the crystallizer literature. However, these methods have limitations in their applicabilty to crystallizers with fines removal, product classification and size-dependent crystal growth, limitations in the choice of the elements of the process output vector y, t) that is used by the controller or result in high orders of the state space model which causes severe problems in the control system design. Therefore another approach is suggested. This approach is demonstrated and compared with the other methods in an example. 

Ohmura, R. Matsuda, S. Itoh, S. Ebinuma, T. Narita, H. (2005d). Clathrate Hydrate Crystal Growth in Liquid Water Saturated with a Guest Substance Observations in a Methane + Water System. Crystal Growth Design, 5(3), 953-957. 

Any planes that have common factors are parallel. For example, a (222) and a (111) plane are parallel, as are (442) and (221) planes. As with cell directions, a minns sign (in this case, indicating a negative intercept) is designated by an overbar. The (221) plane has intercepts at 1/2, —1/2, and 1 along the x, y, and z axes, respectively. Some important planes in the cubic crystal system are shown in Figure 1.25. 

Triclinic Space Groups. The triclinic crystal system allows no axis of rotation of order higher than one, namely, 1 or 1. Since neither of these can give rise to any additional symmetry, there are just two triclinic space groups. Both are primitive and are designated FI and FI. 

Only fourteen space lattices, called Bravais lattices, are possible for the seven crystal systems (Fig. 328). Designations are P (primitive), / (body-centered), F (face-centered),34 C pace-centered in one set of laces), and R (rhombohedral) Thus our monoclinic structure P2Jc belongs to the monoclinic crystal system and has a primitive Bravais lattice. 

First, the role of system design on the details of convection and solute segregation in industrial-scale crystal growth systems has not been adequately studied. This deficiency is mostly because numerical simulations of the three-dimensional, weakly turbulent convection present in these systems are at the very limit of what is computationally feasible today. New developments in computational power may lift this limitation. Also, the extensive use of applied magnetic fields to control the intensity of the convection actually makes the calculations much more feasible. 

A crystal lattice is an array of points arranged according to the symmetry of the crystal system. Connecting the points produces the lattice that can be divided into identical parallelepipeds. This parallelepiped is the unit cell. The space lattice can be reproduced by repeating the unit cells in three dimensions. The seven basic primitive space lattices (P) correspond to the seven systems. There are variations of the primitive cells produced by lattice points in the center of cells (body-centered cells, I) or in the center of faces (face-centered cells, F). Base-centered orthorhombic and monoclinic lattices are designated by C. Primitive cells contain one lattice point (8 x 1/8). Body-centered cells... 

Kelkar VV, Ng KM. Design of reactive crystallization systems incorporating kinetics and mass-transfer effects. AIChE J 1999 45 69-81. 

Surface effects are negligible in many cases. However, when the surface-to-volume ratio of the system is large, surface effects may become appreciable. Moreover, there are phenomena associated with surfaces that are important in themselves. Only an introduction to the thermodynamics of surfaces can be given here, and the discussion is limited to fluid phases and the surfaces between such phases. Thus, consideration of solid-fluid interfaces are omitted, although the basic equations that are developed are applicable to such interfaces provided that the specific face of the crystal is designated. Also, the thermodynamic properties of films are omitted. 

The basic requirements of a crystallization system are (1) a vessel to provide sufficient residence time for crystals to grow to a desired size, (2) mixing to provide a uniform environment for crystal growth, and (3) a means of generating supersaturation. Crystallization equipment is manufactured and sold by several vendors, but some chemical companies design their own crystallizers based on expertise developed within their organizations. Rather than attempt to describe the variety of special crystallizers that can be found in the marketplace, this section will provide a brief general survey of types of crystallizers that utilize the modes outlined above. 

V. V. Kelkar and K. M. Ng, Design of Reactive Crystallization Systems Incorporating Kinetics and Mass Transfer Effects, AIChE J.,... 

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