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Separation Processes

10.3 Crystallizer types and design

3 min readLast Updated on July 24, 2024

Crystallizers are essential in separation processes, transforming solutions into solid crystals. From tank to tubular designs, each type serves specific applications, balancing factors like mixing, heat transfer, and residence time to achieve desired crystal properties.

Batch and continuous crystallizers offer distinct advantages for different production scales. Understanding design considerations and performance factors is crucial for optimizing crystallizer operation, ensuring product quality, and meeting production goals in various industries.

Crystallizer Types and Design

Types of crystallizers and applications

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  • Tank crystallizers facilitate controlled crystallization in vessels
    • Batch crystallizers operate cyclically for small-scale or specialized production
    • Continuous stirred tank crystallizers (CSTR) maintain steady-state for large-scale operations
  • Tubular crystallizers promote uniform growth in elongated chambers
    • Forced circulation crystallizers use pumps to enhance mixing and heat transfer
    • Draft tube baffle (DTB) crystallizers improve suspension and classification
  • Scraped surface crystallizers prevent scaling on heat transfer surfaces (ice cream production)
  • Fluidized bed crystallizers suspend growing crystals in upward-flowing solution (sugar refining)
  • Vacuum crystallizers operate at reduced pressure for temperature-sensitive materials (pharmaceuticals)
  • Melt crystallizers separate components based on freezing point differences (purification of organics)
  • Cooling crystallizers induce supersaturation by temperature reduction (salt production)
  • Evaporative crystallizers remove solvent to increase concentration (table salt)
  • Reaction crystallizers form crystals through chemical reactions (precipitated calcium carbonate)

Batch vs continuous crystallizer principles

  • Batch crystallizers operate in cycles
    1. Charging the crystallizer with feed solution
    2. Creating supersaturation (cooling, evaporation, or reaction)
    3. Inducing nucleation and crystal growth
    4. Harvesting final product
    5. Cleaning and preparing for next batch
    • Suitable for small-scale production and specialized products (pharmaceuticals)
  • Continuous crystallizers maintain steady-state operation
    • Constant feed input and product withdrawal
    • Classified by suspension characteristics
      • Mixed suspension, mixed product removal (MSMPR) uniform crystal distribution
      • Mixed suspension, classified product removal (MSCP) selectively removes larger crystals
    • Ideal for large-scale production and consistent quality (commodity chemicals)

Crystallizer design and sizing considerations

  • Product specifications drive design choices
    • Crystal size distribution determines equipment selection
    • Purity requirements influence crystallization method and post-processing
  • Feed characteristics impact crystallizer configuration
    • Concentration affects supersaturation generation method
    • Impurities may require pretreatment or special materials of construction
  • Production capacity dictates equipment size and number of units
  • Heat transfer requirements determine jacket or coil design
  • Mixing intensity influences impeller selection and power input
  • Materials of construction chosen based on corrosion resistance and product contamination risk
  • Energy efficiency considerations optimize heating/cooling systems
  • Operational flexibility allows for product grade changes or capacity variations
  • Maintenance and cleaning requirements affect equipment accessibility and CIP systems
  • Capital and operating costs balanced against product value and market demand

Factors affecting crystallizer performance

  • Mixing effects crucial for product quality
    • Suspension uniformity prevents settling and ensures consistent growth
    • Mass transfer enhancement accelerates crystal growth rates
    • Secondary nucleation control by adjusting shear forces
    • Agglomeration prevention through proper particle dispersion
  • Heat transfer considerations impact supersaturation control
    • Cooling or heating rate determines nucleation and growth kinetics
    • Temperature uniformity prevents localized supersaturation variations
    • Supersaturation management balances nucleation and growth rates
  • Residence time influence on crystal properties
    • Crystal growth duration affects final size distribution
    • Size distribution control through residence time manipulation
    • Yield optimization by allowing sufficient time for solute depletion
    • Product quality consistency maintained by steady-state operation
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© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.