Potash, Sugar or APIs are commonly formed by crystallisation. In an optimised crystallisation system the particle size distribution can be narrowed and the impact on the crystals, e.g. breakage or the production of fines, will be reduced.
Crystallisation is a unit operation 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-Crystallisers are often called Forced Circulation (FC) Crystallisers due to the fact that the solution is forced to flow by a pump while the crystallisation 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.
For a proper design of a crystallizer it is important to know the relevant parameters for the application. There are three main routes to achieve supersaturation: cooling, evaporation and precipitation. For cooling and evaporation the goal is to create target particle sizes, mostly in the range of 100 μm up to >2000 μm.
Applying an optimized cooling or evaporation profile in many cases improves the process result in terms of a narrower particle size distribution (PSD). Adequate seeding with approximately one through ten percent of the solid content can improve the crystal quality furthermore. For precipitation processes it is often more desired to create small particles, e.g. in the range of one through ten μm.
In this case the mixing task shifts to high local energy dissipation sometimes even combined with gas dispersion. Typical examples are precipitated calcium carbonate or precipitation of iron from ores. Independent of the process, a dedicated mixing system, including vessel, vessel internals, feed and discharge locations can offer significant improvements in the PSD and power consumption.
Why the Mode of Operation is Decisiive for Particle Size Distribution
For smaller capacities and multipurpose units, it is common to utilize a batch operation. Due to more simple process control, predominantly cooling crystallisation is applied. In a batch operation, the cooling curve can be optimized for maximum growth and yield. Using a variable frequency converter (VFC) for the agitator, the absorbed power can be adjusted to the actual process requirements, i.e. low power for pumping in the beginning and increased power to suspend the solids towards the end of the batch cycle. Hence a significant amount of power can be saved.
For mass products it is more economical to switch to continuous operation as the vessel sizes and power consumption can be decreased compared to batch operated units of the same capacity. For evaporation crystallization there are mainly FC and DTBs in use. Cooling crystallisation can be done e.g. in a cascade of stirred vessels.
Controlled Crystallisation - How it Can be Done
In order to maintain a controlled crystallisation, it is important to apply the correct mode of crystallization. In a first step the saturation curve — i.e. concentration over temperature — needs to be determined. Depending on the influence of the temperature, either cooling (high influence) or evaporation (low influence) shall be selected.
Evaporation can be supported by vacuum. Secondly it is required to define the metastable range. Within this range the kinetics of secondary, heterogeneous and homogeneous nucleation are balanced, i.e. the forming of new crystals and the crystal growth run at comparable speed. Exceeding the metastable range leads to uncontrolled crystallization as the nucleation dominates the growth.
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