Crystallization technology

DTB Crystallizers

A crystallizer designed to produce coarse crystals with a narrow size distribution – limited secondary nucleation and an efficient fines destruction promoting crystal growth.

DTB Crystallizers have been studied widely in crystallization theory and can be modelled with accuracy. Its distinct zones of growth and clarified mother liquor make it possible to define in terms of kinetic parameters and thus, growth and nucleation rates can be determined. 

These features make the DTB crystallizer very suitable to mathematical description and subject to good operating control.

Particular features:

  • Low secondary nucleation due to low mechanical energy input (internal circulation)
  • Removing of fines (d < dcritical) from the internal baffle
  • Efficient fines destruction due to heating and/or dilution
  • Production of large size crystals (up to 3mm)
  • No or little heat make-up (fine dissolution)
  • Controlled residence time for optimal growth
  • Long operating cycle with limited encrustation
  • Appropriate for vacuum cooling (no external loop) and evaporative crystallization (moderate capacities)
  • Compact arrangement

Working Principle

Working Principle of DTB Crystallizers

The Draft Tube Baffle Crystallizer (DTB) consists of five basic elements:

  • The crystallizer vessel. Provides most of the active volume dictated by the residence time requirements and enables a proper disengagement of process vapors. 
  • The bottom-flanged impeller pump. Provides sufficient internal circulation rate along the draft-tube to operate the crystallizer under optimal supersaturation conditions. Reduced mechanical energy input leads to minimized secondary nucleation by crystal attrition.
  • The baffle. Controls the crystal population by separating fine crystals (to be dissolved by heating or dilution) from coarse crystals (to further growth). 
  • The circulation pump. Provides sufficient external circulation rate to operate the crystallizer under optimal superheating conditions for fines redissolution. Typically, axial-flow propeller pumps are used.
  • The heat exchanger. Supplies the required thermal energy to the crystallizer for the desired evaporation rate. Designed to minimize encrustations.

It is specially designed to produce coarse crystals with narrow size distribution. The DTB can produce large size particles (up to 3mm) due to its gentle agitation of growing crystals with very limited breakage. It has a controlled residence time for optimal growth and a long operating cycle with limited encrustations.

Slurry of a desired solid density is circulated within the draft tube crystallizer impulsed by a bottom-flanged impeller pump. A DTB Crystallizer´s mechanical energy input is lower than that of a FC crystallizer thanks to a reduced pressure drop that limits attrition and therefore, secondary nucleation.

Crystals of a smaller and specific size are withdrawn from the baffle zone and are dissolved in the external circulation loop because of the clarified solution´s superheating.

Said superheating is relieved through evaporation and the evolving supersaturation leads to the growth of the suspended crystals. The evaporated solvent is either conducted to the subsequent process steps or reused by applying a recompression system of choice.

dtb-crystallizer-diagram
evaporation-type-01

Heating options for thermal separation plants

Traditionally, an evaporator or crystallizer is heated by live steam, but waste heat can be used as energy source as well, as long as the amount of energy required for the thermal separation process is given.

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