Editing Crystallization (section)

==Main crystallization processes==
[[File:Kristalizacija.jpg|thumb|upright=1.2|Crystallization of [[sodium acetate]]]]

Some of the important factors influencing solubility are:
* Concentration
* Temperature
* Solvent mixture composition
* Polarity
* Ionic strength
So one may identify two main families of crystallization processes:
* Cooling crystallization
* Evaporative crystallization

This division is not really clear-cut, since hybrid systems exist, where cooling is performed through [[evaporation]], thus obtaining at the same time a concentration of the solution.

A crystallization process often referred to in [[chemical engineering]] is the [[Fractional crystallization (chemistry)|fractional crystallization]]. This is not a different process, rather a special application of one (or both) of the above.

===Cooling crystallization===

====Application====
Most [[chemical compound]]s, dissolved in most solvents, show the so-called ''direct'' solubility that is, the solubility threshold increases with temperature.

[[Image:Solubilita Na2SO4.png|upright=1.3|thumb|Solubility of the system Na<sub>2</sub>SO<sub>4</sub> – H<sub>2</sub>O]]

So, whenever the conditions are favorable, crystal formation results from simply cooling the solution. Here ''cooling'' is a relative term: [[austenite]] crystals in a steel form well above 1000&nbsp;°C. An example of this crystallization process is the production of [[Glauber's salt]], a crystalline form of [[sodium sulfate]]. In the diagram, where equilibrium temperature is on the [[Cartesian coordinates|x-axis]] and equilibrium concentration (as mass percent of solute in saturated solution) in [[Cartesian coordinates|y-axis]], it is clear that sulfate solubility quickly decreases below 32.5&nbsp;°C. Assuming a saturated solution at 30&nbsp;°C, by cooling it to 0&nbsp;°C (note that this is possible thanks to the [[freezing-point depression]]), the precipitation of a mass of sulfate occurs corresponding to the change in solubility from 29% (equilibrium value at 30&nbsp;°C) to approximately 4.5% (at 0&nbsp;°C) – actually a larger crystal mass is precipitated, since sulfate entrains [[Mineral hydration|hydration]] water, and this has the side effect of increasing the final concentration.

There are limitations in the use of cooling crystallization:
* Many solutes precipitate in hydrate form at low temperatures: in the previous example this is acceptable, and even useful, but it may be detrimental when, for example, the mass of water of hydration to reach a stable hydrate crystallization form is more than the available water: a single block of hydrate solute will be formed – this occurs in the case of [[calcium chloride]]);
* Maximum supersaturation will take place in the coldest points. These may be the heat exchanger tubes which are sensitive to scaling, and [[heat transfer|heat exchange]] may be greatly reduced or discontinued;
* A decrease in temperature usually implies an increase of the [[viscosity]] of a solution. Too high a viscosity may give hydraulic problems, and the [[laminar flow]] thus created may affect the crystallization dynamics.
* It is not applicable to compounds having ''reverse'' solubility, a term to indicate that solubility increases with temperature decrease (an example occurs with sodium sulfate where solubility is reversed above 32.5&nbsp;°C).

====Cooling crystallizers====
[[File:1-cooling-crystallizer-schladen.JPG|thumb|upright|Vertical cooling crystallizer in a beet sugar factory]]
The simplest cooling crystallizers are tanks provided with a [[Industrial mixer|mixer]] for internal circulation, where temperature decrease is obtained by heat exchange with an intermediate fluid circulating in a jacket. These simple machines are used in batch processes, as in processing of [[pharmaceuticals]] and are prone to scaling. Batch processes normally provide a relatively variable quality of the product along with the batch.

The ''Swenson-Walker'' crystallizer is a model, specifically conceived by Swenson Co. around 1920, having a semicylindric horizontal hollow trough in which a hollow [[screw]] conveyor or some hollow discs, in which a refrigerating fluid is circulated, plunge during rotation on a longitudinal axis. The refrigerating fluid is sometimes also circulated in a jacket around the trough. Crystals precipitate on the cold surfaces of the screw/discs, from which they are removed by scrapers and settle on the bottom of the trough. The screw, if provided, pushes the slurry towards a discharge port.

A common practice is to cool the solutions by flash evaporation: when a liquid at a given T<sub>0</sub> temperature is transferred in a chamber at a pressure P<sub>1</sub> such that the liquid saturation temperature T<sub>1</sub> at P<sub>1</sub> is lower than T<sub>0</sub>, the liquid will release [[heat]] according to the temperature difference and a quantity of solvent, whose total [[latent heat]] of vaporization equals the difference in [[enthalpy]]. In simple words, the liquid is cooled by evaporating a part of it.

In the sugar industry, vertical cooling crystallizers are used to exhaust the [[molasses]] in the last crystallization stage downstream of vacuum pans, prior to centrifugation. The massecuite enters the crystallizers at the top, and cooling water is pumped through pipes in counterflow.

===Evaporative crystallization===
Another option is to obtain, at an approximately constant temperature, the precipitation of the crystals by increasing the solute concentration above the solubility threshold. To obtain this, the solute/solvent mass ratio is increased using the technique of [[evaporation]]. This process is insensitive to change in temperature (as long as hydration state remains unchanged).

All considerations on control of crystallization parameters are the same as for the cooling models.

====Evaporative crystallizers====
Most industrial crystallizers are of the evaporative type, such as the very large [[sodium chloride]] and [[sucrose]] units, whose production accounts for more than 50% of the total world production of crystals. The most common type is the ''forced circulation'' (FC) model (see [[evaporator]]). A pumping device (a [[pump]] or an axial flow [[axial flow pump|mixer]]) keeps the crystal [[slurry]] in homogeneous [[Suspension (chemistry)|suspension]] throughout the tank, including the exchange surfaces; by controlling pump [[Fluid dynamics|flow]], control of the contact time of the crystal mass with the supersaturated solution is achieved, together with reasonable velocities at the exchange surfaces. The Oslo, mentioned above, is a refining of the evaporative forced circulation crystallizer, now equipped with a large crystals settling zone to increase the retention time (usually low in the FC) and to roughly separate heavy slurry zones from clear liquid. Evaporative crystallizers tend to yield larger average crystal size and narrows the crystal size distribution curve.<ref>{{Cite news|url=http://thermalkinetics.net/evaporation-equipment/submerge-circulating-crystallizer|title=Submerge Circulating Crystallizers |newspaper=Thermal Kinetics Engineering, PLLC|language=en-US|access-date=2017-01-03}}</ref>

===DTB crystallizer===
[[Image:DTB Xls.png|frame|left|DTB Crystallizer]]
[[Image:DTB 2.PNG|thumb|Schematic of DTB]]

Whichever the form of the crystallizer, to achieve an effective [[process control]] it is important to control the retention time and the crystal mass, to obtain the optimum conditions in terms of crystal specific surface and the fastest possible growth.<ref name="Seepmaetal2025">{{cite journal |last1=Seepma |first1=Sergěj Y.M.H. |last2=Koskamp |first2=Janou A. |last3=Colin |first3=Michel G. |last4=Chiou |first4=Eleftheria |last5=Sobhan |first5=Rubayat |last6=Bögels |first6=Tim F.J. |last7=Bastiaan |first7=Tom |last8=Zamanian |first8=Hadi |last9=Baars |first9=Eric T. |last10=de Moel |first10=Peter J. |last11=Wolthers |first11=Mariëtte |last12=Kramer |first12=Onno J.I. |title=Mechanistic model advancements for optimal calcium removal in water treatment: Integral operation improvements and reactor design strategies |journal=Water Research |volume=268 |issue=Pt. B |year=2025 |pages=122781 |issn=0043-1354 |doi=10.1016/j.watres.2024.122781|doi-access=free |pmid=39550848 |bibcode=2025WatRe.26822781S }}</ref> This can be achieved by a separation – to put it simply – of the crystals from the liquid mass, in order to manage the two flows in a different way. The practical way is to perform a gravity [[settling]] to be able to extract (and possibly recycle separately) the (almost) clear liquid, while managing the mass flow around the crystallizer to obtain a precise slurry density elsewhere. A typical example is the DTB (''Draft Tube and Baffle'') crystallizer, an idea of Richard Chisum Bennett (a Swenson engineer and later President of Swenson) at the end of the 1950s. The DTB crystallizer (see images) has an internal circulator, typically an axial flow mixer – yellow – pushing upwards in a draft tube while outside the crystallizer there is a settling area in an annulus; in it the exhaust solution moves upwards at a very low velocity, so that large crystals settle – and return to the main circulation – while only the fines, below a given grain size are extracted and eventually destroyed by increasing or decreasing temperature, thus creating additional supersaturation. A quasi-perfect control of all parameters is achieved as DTF crystallizers offer superior control over crystal size and characteristics.<ref>{{Cite news |title=Draft Tube Baffle (DTB) Crystallizer |language=en-US |newspaper=Swenson Technology |url=https://swensontechnology.com/draft-tube-baffle-crystallizers/ |url-status=live |access-date=2023-11-15 |archive-url=https://web.archive.org/web/20160925070515/http://www.swensontechnology.com/equipment/draft-tube-baffle-dtb-crystallizer/ |archive-date=2016-09-25}}</ref> This crystallizer, and the derivative models (Krystal, CSC, etc.) could be the ultimate solution if not for a major limitation in the evaporative capacity, due to the limited diameter of the vapor head and the relatively low external circulation not allowing large amounts of energy to be supplied to the system.