Phase Diagrams for Process Design
The knowledge of stable and metastable heterogeneous equilibria is paramount for the rational management of separation/purification processes based on crystallization.
Starting from a collection of binary and ternary phase diagrams the design of crystallization processes is illustrated by careful details on different pathways corresponding to crystallizations induced by: (i) cooling (ii) antisolvent (iii) evaporation. By taking into account the stable and metastable heterogeneous equilibria, the access to different final or transient states are detailed. The fine analyses of the pathways inside the relevant phase diagrams also lead to assess the robustness of the processes including their risks of lack of control. In addition, the thermodynamic optimum yield can be reckoned and thus, the tuning of the kinetic parameters can be framed within sensible limits.
KeywordsPhase diagrams Design of crystallization processes Optimization Risk assessment
List of Symbols and Abbreviations
symbolizes crystals of compound or component P
- Mixed solvate:
at least two different solvent molecules are incorporated in the same crystallographic site.
at least two different solvent molecules are incorporated in two different crystallographic sites.
release of solvent molecules in the gas phase by either: a solvate, a mixed solvate or a heterosolvate.
uptake of solvent from the gas phase up to dissolution of the crystals (often applied to moisture uptake and aqueous solution). This phenomenon can be followed by the recrystallization of a solvate (hydrate in case of moisture uptake).
Undersaturated solution i.e. monophasic domain with a single liquid phase.
- Non-congruent soluble compound:
In a ternary system, the line connecting the point representative of the solid and that of the solvent does not intersect the stable solubility curve of the compound. In order to crystallize that compound in solution without any trace of any other solid, an excess of one component has to be added in the system. A compound can have a non-congruent solubility at one temperature and a congruent solubility at another temperature.
- Binary eutectic:
Three phase invariant which involves the simultaneous and reversible crystallization and melting of two solids from (to give) a liquid. The temperature and the composition of the three phases are fixed.
- Ternary eutectic:
four phase invariant which involves the simultaneous and reversible crystallization and melting of three solids from (to give) a liquid. The temperature and the composition of the four phases are fixed.
- Binary peritectic (non-congruent fusion):
Three phase invariant involving two solid phases and one liquid, all of them of specific compositions. At a defined temperature, it corresponds to the reversible decomposition of an intermediate compound on heating.
- Binary monotectic (stable miscibility gap at high temperature):
Three phase invariant involving two liquids and one solid, all of them of specific compositions. At a defined temperature, it corresponds to the reversible decomposition of a liquid into a solid phase and another liquid on cooling.
relative humidity. This is a ratio (thus non unit) valid at a fixed temperature and defined as: the vapor pressure of water in the system over the saturated vapor pressure.
Dr M-N Delauney is thanked for her important contribution in the illustrations.
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