Crystallisation Flashcards
What are the two main types of binary mixtures?
-Eutectics (easy to separate)
-Solid solutions
What is phase equilibria affected by?
-Pressure
-Temp
-in binary mixtures affected by conc and temp, pressure is negligible usually
What is the eutectic point?
It is the point on the graph which corresponds to the composition of the mixture with the lowest crystallisation temp.
What is an unsaturated/dilute solution?
-It is in a stable state
-No crystals are presents
-No potential for crystals to form
What is a saturated solution?
-In a stable state
-Point where solid matter will start forming
-Solid and liquid phases are in equilibrium with conc. of dissolved material (equilibrium concs.)
What is a supersaturated solution?
-In an unstable state
-System is beyond equilibrium
-Conc. of dissolved species is greater than the equilibrium conc.
-Necessary for crystallisation
What are the 3 general steps for a crystallisation process?
1) Preparation of a solution containing the species to be crystallised
2) Using this solution to prepare a supersaturated solution
3) Precipitation of the crystals from the supersaturated solution
What are the 3 ways supersaturation can be achieved?
-Temp change (cooling)
-Solvent removal
-Addition of a 3rd component
When is a temp change used for supersaturation
-Used for systems where solubility decreases with temp.
-Involves cooling through the solubility curve
When is a solvent removal used for supersaturation?
-Used for systems where solubility is not affected by temp changes
-Involves evaporating part of the solvent
When is the addition of a 3rd component used for supersaturation
-Used if neither cooling or evaporation is desirable
-Added component may form a mixed solvent of reduced solubility, with the original solvent (salting)
-Added component may react chemically with the original solute to form an insoluble substance (precipitation)
-Can lead to very large supersaturations
What does metastable mean in terms of supersaturation
-Spontaneous nucleation (crystal formation) is improbable
What does labile mean in terms of supersaturation
-Spontaneous nucleation (crystal formation) is probable, but not inevitable
What is the equilibrium solubility curve?
Where the transition from the stable to the metastable region occurs
-Can be determined experimentally
What is the super-solubility/ maximum supersaturation curve?
Where the transition from the metastable to labile region occurs
-Can’t be accurately determined as it is affected by factors such as rate of supersaturation, degree of agitation and presence of impurities
How can supersaturation in the labile zone be achieved?
-Cooling without loss of solvent, conc. is constant
Spontaneous nucleation will only occur once in the
labile region
-Removal of solvent (evaporation), temp is constant
Spontaneous nucleation may occur before the labile
region is reached
What is primary nucleation?
-Occurs in the absence of crystals
-Likely, but no inevitably, to occur in labile region
-Unlikely to occur in the metastable region
-Two types, homogeneous and heterogeneous
What is secondary nucleation?
-Only occurs in the presence of crystals of the same species
-Can occur in the metastable region
-Secondary nuclei generated due to damage to growing crystals from shear fields or contact between other crystals or equipment
-Most dominant type of crystallisation in industrial crystallisers
When does homogeneous primary nucleation occur?
-Result of bimolecular collisions and interactions in a supersaturated fluid
-Can cause the spontaneous formation and build-up of lattice-structured bodies (nuclei)
-Formed nuclei may or may not achieve thermodynamic stability
-Nucleation rate increases dramatically when a critical supersaturation ratio is surpassed
When does heterogeneous primary nucleation occur?
-Initiated by the presence of suspended particles of ‘foreign’ substances
-Mechanism not fully understood, likely begins with adsorption of crystalline species
-Crystalline bodies are created which grow into macro-crystals
-In industrial crystallisers, most primary nucleation is heterogeneous
-Nucleation occurs at lower supersaturation ratio than in homogeneous due to foreign particles
What are the two specific process stages within crystallisation
-Diffusional step
Solute transported from bulk fluid through solution
boundary layer to crystal surface
-Deposition step
Adsorbed solute ions/molecules at crystal surface are deposited and integrated into the crystal lattice
At high levels of molecule integration, how is this relationship between crystal growth and mass transfer coefficients effected?
-KG = kd, kr = 0
-Crystallisation is diffusion controlled
At low levels of diffusional resistance, how is this relationship between crystal growth and mass transfer coefficients effected?
KG = kr, kd = 0
-Crystal growth is deposition controlled
What is the mother liquor?
The saturated solution to be separated from the crystallised product
What is the magma?
The slurry of crystals and mother liquor
What assumptions are made in estimating the theoretical yield of crystals?
-Crystallisation only occurs once supersaturation is reached
-Crystallisation proceeds until the saturation limit
-There is no retention of mother liquor by crystals
-No loss of crystals during washing
