Page 70 - Subyek Encyclopedia - Encyclopedia of Separation Science

Sepsci*31*TSK*Venkatachala=BG
I / CRYSTALLIZATION 65

separated from the mother liquor, additional de- in this Reld contain an abundance of references. The
mands on Rlterability and washability can be for- authors of these books are Mersmann (1995), Mullin
mulated. (1993), Randolph and Larsen (1987), Myerson
Because of the mostly rigid structure of the solid (1993), NyH vlt (1992), Tavare (1995) and Hurle
phase, the formation of solid particles is a rather slow (1993). SoK hnel and Garside (1992) have written
process, and to reach an acceptable production rate a book on precipitation and Arkenbout (1995)
large vessels are generally needed. This rigid structure a book on melt crystallization.
on the other hand impedes the incorporation of This article reviews industrial evaporation and
foreign substances or solvent molecules, and in only cooling crystallization processes. A basic modelling
one separation step a pure solid product is obtained. approach is presented which enables the analysis and
Crystallization is often used as a generic term for design of industrial crystallization processes, either by
evaporative or cooling crystallization, precipitation analytical calculations or by making use of modern
and melt crystallization. There are, however, con- computational tools.
siderable differences between the three types of
crystallization as far as the processing method and the Crystallization Methods and
corresponding equipment are concerned. In precipita- Supersaturation
tion the drop-out of the solid phase is achieved by
mixing two feed streams that are either two reactants Crystallization only occurs when supersaturation is
or a solvent containing the solute and an antisolvent. created that acts as the driving force for crystalliza-
The hydrodynamics of the process therefore play tion. The crystallization method is mainly chosen on
a predominant role in precipitation with regard to the the basis of the thermodynamic and physical proper-
properties of the obtained product. ties of the compound and the solvent, as well as on
In melt crystallization the potential of crystalliza- the required purity of the product. There are several
tion to produce a pure product is mainly utilized, and ways to represent the phase diagram, depending
the solid phase is remolten to obtain the Rnal product. on the mode of crystallization. For evaporative and
The applications are mainly in the ultrapuriRcation of cooling crystallization a solubility diagram is mostly
organic compounds or to produce pure water as used, in which the solubility of the compound is
a concentration technique. expressed as a function of temperature. In precipita-
An upcoming technique in crystallization is super- tion the solubility is always very low, and the solubil-
critical crystallization, mostly with condensed CO 2 , ity product at the operating temperature is needed. In
because of its benign properties compared to organic antisolvent precipitation, the solubility diagram
solvents. Condensed CO 2 can be used either as a sol- mostly has to be determined for the particular three-
vent or as an antisolvent, and speciRcally adapted phase system. In melt cooling crystallization T-x dia-
processes and equipment have been developed for grams are used, while for melt pressure crystallization
these high pressure crystallization techniques. pT-x diagrams are needed.
Also the crystallization of proteins requires its own A decision scheme for the selection of the appropri-
dedicated approach, because large, sometimes easily ate crystallization method is presented in Figure 1.
degradable molecules require carefully designed For a high purity product or when the use of a sol-
processes. vent poses environmental or safety problems, melt
Because 70% of the products sold by the process crystallization is chosen. A melt temperature lying
industry and the pharmaceutical industry } as bulk between 0 and 1003C is preferred, since at higher
products, intermediates, Rne chemicals, biochemicals, temperatures many organic compounds become un-
food additives and pharmaceutical products } are stable, while at temperatures below 03C operating the
solids, crystallization in its widest deRnition is the process becomes more difRcult, although not im-
largest separation process after distillation. possible. A high viscosity of the melt hampers the
Although this chapter will primarily focus on evap- operation.
orative and cooling crystallization, the energy, mass At solubilities below about 1 mass %, the process is
and population balances treated here as well as the designed as a precipitation process to obtain a reason-
kinetic rate expressions for the physical processes able production rate. The supersaturation that is gen-
such as nucleation, growth and agglomeration and erated by the mixing of reactants often reaches high
the characterization of the particles can equally be values. Also in antisolvent crystallization, low solu-
applied to the other types of crystallization. bility of the solute and high supersaturation are
Several books on the diverse aspects of crystalliza- reached by mixing two solvents.
tion have been published over the last 10 years. These Finally, for the more easily soluble substances the
books that can be recommended for a wide overview choice between cooling and evaporative crystallization