Capítulo 6 Parte 3

Types of Crystallizers

Precipitation by Cooling

• The first type of crystallizer discussed is precipitation by cooling, which occurs when the solvent is cooled. This method is effective when solubility varies significantly with temperature.

• Cooling reduces solubility considerably, making it a viable option for crystallization. The process can be batch-wise, utilizing a tank with cooling coils to maintain uniform temperature throughout the system.

• Maintaining a consistent temperature is crucial; uneven temperatures can lead to non-uniform crystallization and irregular product quality.

• The equipment used for this method is simple and cost-effective, suitable for small-scale processing due to its low capacity and batch operation.

• Commonly found in sugar production industries, this equipment features cooling water entry points that facilitate heat exchange.

Continuous Crystallization Process

• A continuous crystallizer design includes a semi-cylindrical trough with a helical agitator that ensures uniform mixing and temperature control as the solution flows through.

• The agitator not only maintains uniformity but also scrapes crystals formed in cooler regions to prevent accumulation and promote growth by keeping them in motion.

• Keeping crystals moving allows them to interact with new molecules in solution, facilitating their growth rather than being trapped or hidden from contact.

Precipitation via Evaporation

• Another type of crystallizer operates through evaporation of the solvent. This method contrasts with cooling-based processes previously discussed.

• The most common evaporative crystallizer mentioned is the Oslo crystallizer, which does not rely on mechanical agitation but instead uses fluidized beds for mixing.

• In this setup, the solution enters through specific tubing where it promotes agitation upon entering the chamber, allowing crystal formation as it settles at the bottom.

• Larger crystals are separated from smaller ones using centrifugation; larger crystals settle while smaller ones return to continue growing within the system.

Understanding Crystal Growth and Evaporation Processes

The Role of Crystal Size in Separation

• The process involves discarding small crystals while retaining larger ones, which have had sufficient time to develop in the crystallizer.

• Larger crystals settle at the bottom of a decanter, forming a thick suspension, while smaller crystals remain suspended and are cycled back into the evaporator for further growth.

Mechanisms of Precipitation

• Precipitation occurs through vacuum evaporation and cooling; both processes contribute to solvent removal.

• Solvent evaporation can be achieved by either increasing temperature or reducing pressure, allowing for effective solvent management.

Vacuum Crystallization Process

• A vacuum crystallizer operates under lower than atmospheric pressure, facilitating the evaporation of solvents from a saturated hot solution.

• The product is heated before entering the crystallizer to assist in solvent evaporation; upon reaching low pressure, rapid solvent evaporation occurs.

Heat Exchange Dynamics