In the salt example, cooling will be gradual so we need to provide a "seed" for the crystals to grow on. In continuous crystallization, once primary nucleation has begun, the crystal size distribution begins to take shape.
The second chief mechanism in crystallization is called secondary nucleation. In this phase of crystallization, crystal growth is initiated with contact. The contact can be between the solution and other crystals, a mixer blade, a pipe, a vessel wall, etc. This phase of crystallization occurs at lower supersaturation than primary nucleation where crystal growth is optimal. Secondary nucleation requires "seeds" or existing crystals to perpetuate crystal growth.
In our salt example, we bypassed primary nucleation by "seeding" the solution with a final teaspoon of salt. Secondary nucleation can be thought of as the workhorse of crystallization. Again, no complete theory is available to model secondary nucleation and it's behavior can only be anticipated by experimentation.
Mathematic relationships do exist to correlate experimental data. However, correlating experimental data to model crystallization is time consuming and often considered extreme for batch operations, but can easily be justified for continuous processes where larger capital expenditures are necessary. For batch operations, only preliminary data measurements are truly necessary. Having decided on a method of crystallization, it is important to measure the progress and subsequent success of your crystallization process.
Turbidity probes have been used to monitor crystallizations for decades, due to their ease of use, sensitivity, and affordability. Turbidity probes work by measuring light that is scattered by suspended solids in a liquid. As an example, turbidity information for the crystallization and dissolution of adipic acid is shown below:. FBRM probes work by directing a laser beam down the probe, through rotating optics and focussing it at the probe window and measuring the light backscattered by particles in solution.
As the focussed beam scans the solution, individual particles can backscatter the light to the detector. Video imaging probes have also been used determine crystal shape and crystal size distribution, however, at present, they are hard to implement at a commercial scale due to operating temperature limits and the bulky size of such probes.
Attenuated total reflectance Fourier transform infrared ATR-FTIR has also been used to gather solubility and supersaturation data, by measuring the concentration of solute in the mother liquor. Additional information can also be gleaned, such as confirmation of the presence of additives or impurities. Chemists have a range of tools available to them for performing, monitoring, and controlling crystallization chemistry.
Syrris systems offer a range of solutions to the problem of crystallization monitoring and control. Atlas HD Crystallization is an intelligent and automated jacketed reactor system that offers a turbidity probe for monitoring the crystallization process, and the innovative SonoLab module to perform sonocrystallization or sonomilling techniques.
Using the Syrris Atlas HD Crystallization system for your crystallization studies offers various benefits, including;. Syrris products offer a range of solutions to the problem of crystallization control and monitoring. For more information about crystallization or how you can achieve better results using Syrris products, please contact us.
What is crystallization? How does the crystallization process occur? The crystallization process consists of two major events: Nucleation — Molecules gather together in clusters in a defined manner. It is this point in the crystallization process that defines the crystal structure. Crystal growth is a dynamic process, with atoms precipitating from solution and becoming redissolved. Supersaturation and supercooling are two of the most common driving forces behind crystal formation.
Figure 1: The crystallization process - crystal growth rate vs. Solubility Curves, Supersaturation and the Metastable Zone Width MSZW Traditionally, crystal formation has been achieved by reducing the solubility of the solute in a saturated solution in a variety of ways. Figure 2: A solubility and temperature graph for various solvents. It is useful for separating sand from a mixture of sand and water, or excess reactant from a solution. Filtration works because the filter paper has tiny holes, or pores, in it.
These are large enough to let small molecules and dissolved ions through, but not the much larger particles of undissolved solid. Separating insoluble solids. One beaker contains a mixture of solid and liquid, the other contains a funnel with filter paper. The solid and liquid mixture is poured into the filter funnel.
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