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Crystallization

Crystallization is a process step in a vast range of applications and industries such as chemicals, API production, food manufacturing and precipitation of ores.

Depending on the process requirements, cooling, evaporation or precipitation is selected to form crystals. Accordingly, the variation of applied machines is numerous.

Loop-Crystallizers are often called Forced Circulation (FC) Crystallizers due to the fact that the solution is forced to flow by a pump while the crystallization in layers is a static process.

In agitated services there are two main mixing systems. Either an open system with a hydrofoil impeller or a draft tube is utilized. EKATO provides highly efficient solutions, which do not end at the impeller itself.

Example for crystallization products: 

  • Potash
  • Sugar
  • APIs

EKATO is glad to assist you during the whole project starting from a first estimation until detail engineering, manufacturing and operation. EKATO has excellent experience in designing not only the agitators, but also can assist with expertise in vessel design and process know-how. Furthermore we can support with CFD simulations and  trials in our laboratory or on site.

Batch Operation

It is common to run crystallizations in batch units using a hydrofoil impeller. In general, cooling is the method to reach supersaturation in batch mode. In rare cases evaporation is utilized, too.

Batch operation is highly flexible, i.e. changes in raw material can be accommodated easily. Even multiple products often can be processed without major changes of the agitator. Usually the only parameter to adjust is the shaft speed.

To achieve an efficient and high quality process, there are some important parameters to be observed.

First of all the agitator has to be designed to suit the process best. Beside the basic mixing tasks blending and suspending of the solids, heat transfer is an important parameter. For shear sensitive crystals, a low shear system has to be selected.

The design of the vessel and its internals bear further potential for improvement. The vessel geometry as well as the use of optimized vessel internals can influence the product quality significantly.

An adapted mode of operation finalizes the design. The cooling strategy with cooling ramps and hold points can increase the size of the crystals and narrow the particle size distribution at the same time. Seeding with selected crystals sizes can offer further improvements. A narrow particle size distribution with a small amount of fines increases the yield and is beneficial for downstream filtration and drying steps.

Continuous Operation

Continuously operated crystallizers are often equipped with a draft tube. Those units are called Draft Tube Baffled Crystallizer or DTBs. The impeller is located inside the draft tube and applies a directed axial flow with minimized turbulences.

The advantages of continuous operation are mainly for high capacity productions of a single product. As an example, potash is commonly processed in a DTB. Aside a high flow rate, a constant product quality and a graded separation of the crystals is required. Smaller particles should remain in the process until a certain size is reached.

EKATO has developed the Torusjet, a three-bladed draft tube circulator, in order to significantly increase the efficiency. By observing the whole system including draft tube geometry and specially designed straightening vanes the ratio of axial flow to turbulent flow can be increased considerably. Hence less energy is required to achieve a given flow rate. This effects both investment and operational cost. As a side effect the crystals are preserved as less turbulence also means less collision and hence less chipping.

To ensure safe and long term operation it is worthwhile to check the stresses by a finite element analysis (FEA) and frequencies by a modal analysis during the design phase as the  DTBs are exposed to high mechanical loads and vibrations. 

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