Nucleation occurs when solute molecules assemble in a supersaturated solution and reach a critical size. Primary nucleation occurs directly from solution, while secondary nucleation occurs in the presence of existing solids.

• High nucleation rates can produce excessive fines.

• Unexpected secondary nucleation can create a bimodal particle population.

• Variable nucleation leads to difficult isolation, filtration, and processing.

• Control supersaturation through cooling rate or anti-solvent addition rate.

• Evaluate solvent type, impurities, and mixing during process design.

• Use seeding to initiate nucleation at a consistent point.

• Optimize seed size and seed loading to influence nucleation rate.

Achieve consistent primary nucleation while avoiding uncontrolled secondary nucleation.

• If poorly understood: Inconsistent crystal size distribution and difficult isolation or downstream processing.
• Key resource: Jaroslav Nývlt, “Kinetics of Nucleation in Solutions,” Journal of Crystal Growth, 1968.
• Related content: Visualization of Seeding Mechanisms During Crystallization

Oiling out is liquid-liquid phase separation caused by a miscibility gap in the solute-solvent phase diagram. Oil droplets can act as reservoirs for impurities that later become incorporated into the crystal product.

• Oil droplets can trap impurities and reduce product quality.

• Droplet size can change with shear rate, making scale-up difficult.

• Residue on probes or reactor walls can complicate cleaning and production.

• Map the solute-solvent phase diagram and avoid conditions where oiling out occurs.

• Reduce high supersaturation through slower cooling or effective seeding.

• Improve mixing at feed locations in reactive or anti-solvent crystallization.

• Consider reverse addition when appropriate: add saturated solution to anti-solvent to avoid a liquid-liquid split.
  

Determine the phase diagram and avoid conditions that trigger oiling out.

• If poorly understood: Poor impurity rejection, difficult particle size control at scale, poor isolation, and difficult cleaning.
• Key resource: Kiesow et al., “Experimental Investigation of Oiling Out During Crystallization Process,” Journal of Crystal Growth, 2008.
• Related content: Continuous Crystallization to Circumvent Metastable Liquid-Liquid Separation

Crystal breakage can result from crystal-crystal, crystal-impeller, or crystal-reactor interactions. These effects can create variable or bimodal particle size distributions if not controlled.

• Unwanted breakage creates fines and broad particle size distributions.

• High agitation can improve heat and mass transfer but increase collision rates.

• Wet milling can be useful when fast crystal growth needs particle size control.

• Evaluate breakage or attrition when selecting agitation rate.

• Avoid excessive collision energy when crystals are fragile.

• Use wet-milling parameters that achieve the desired breakage quickly.

• Avoid over-milling, which can cause temperature rise, decomposition, or form changes.
  

• Pair wet milling with temperature cycling when appropriate to refine size and shape distribution.

Avoid unwanted breakage and control desired wet-milling conditions.

• If poorly understood: Bimodal particle size distributions, poor isolation, poor filtration, poor drying, or form conversion.
• Key resource: Fasoli & Conti, “Crystal Breakage in a Mixed Suspension Crystallizer,” 1973.
• Related content: Data-Rich Experimentation Across Scale

Polymorphism is the ability of a solid material to exist in more than one crystal structure. Different forms can have different shapes, stability, solubility, and processing behavior.

• The wrong polymorph can cause failed batches or regulatory issues.

• Form changes often alter crystal shape and downstream filtration or drying performance.

• Scale-up can trigger unexpected forms when conditions are controlled less precisely.

• Use polymorph screening to identify solvent and condition effects.

• Select solvent and supersaturation conditions that favor the desired form.

• In some cases, deliberately crystallize a less stable form before controlled transformation.

• Characterize the design space so scale changes do not create unexpected form changes.
  

Navigate the phase diagram to produce the desired polymorphic form.

• If poorly understood: Failed batches, regulatory issues, and poor isolation properties.
• Key resource: Kitamura, “Strategies for Control of Crystallization of Polymorphs,” CrystEngComm, 2009.
• Related content: Control Polymorph Particle Size and Purity Through Crystal Engineering