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Crystallization is a state change from liquid to solid, where compounds dissolved in a solvent become solid following an ordinate pattern.
Crystallization (equipment)
Crystallization is a unit process through which a chemical compound, dissolved in a given solvent, precipitates in controlled conditions to allow successive separation between the phases.
Crystallization is therefore an aspect of precipitation, obtained through a variation of the solubility conditions of the solute in the solvent, as compared to precipitation due to chemical reaction.
History
Crystallization is one of the pristine unit processes. It may be assumed that our ancestors used sodium chloride found in crevices of the surfacing rocks after drying caused by solar radiation: this process is still in use in modern solar ponds.
Other crystallization processes, for example in sucrose production (this is the crystalline product having the largest word production, followed by sodium chloride), or used in pigment manufacturing, were used in ancient times: thes eproducts were sometime produced crystallizing the solutes of some more or less natural brine.
In more recent times, the fast expansion of chemical industry has required a thorough study of the dynamics of crystallization, and this unit operation is now used in many branches of chemical engineering. Mass products, such as table salt, sugar, sodium sulphate, urea. just to name a few, are produced by crystallization from solutions.
Crystallizer technology has progressed alongside with the new processes. Once simple tanks in which, through cooling, evaporation or maybe through pH variation a crystal was obtained, nowadays continuous machines ensure a remarkable constance in the product characteristics. Among the first models of modern crystallizers were probably the calandria type, being today the standard crystallizer for sucrose, and the Oslo, named after the Norwegian capital, since it was developed to produce salt in a climate not particularly fit for solar ponds, salt being widely used in Norway in stockfish production. The Oslo type was probably the first crystallizer designed specifically for the control of crystal growth.
Crystallization dynamics
As said above, a crystal is formed following a well-defined pattern, or structure, dictated by forces acting at molecular level. As a consequence, during its formation process the crystal, shall be in an environment where the solute concentration reach a certain critical value, before changing status. Solid formation, impossibile below the solubility threshold, at the given temperature and pressure conditions, may then take place at a concentration higher than the theoretical solubility level. The difference between the actual value of the solute concentration at the crystallization limit and the theoretical (static) solubility threshold is called supersaturation and is a fundamental factor in crystallization dynamics.
Once the first, small crystal, called nucleus, this acts as a convergence point if unstable (due to supersaturation) molecules of solute touching - or coming neart to - the crystal so that it increases its own dimension in successive layers, something like an onion, as shown in the picture, where each colour shows the same mass of solute; this mass creates increasingly thin layers due to the increasing surface of the growing crystal. The supersaturated solute mass the original nucleus may capture in a time unit is called growth rate expressed in kg/(m2*h), and is a constant specific to the process: it is influenced by several phisical factors, such as surface tension of solution, pressure, temperature, relative crystal velocity in the solution, Reynolds number, etcetera.
The main values to control are therefore:
- Supersaturation value, as an index of the quantity of solute available for the growth of the crystal;
- Total crystal surface in unit fluid mass, as an index of the capability of the solute to fix onto the crystal;
- Retention time, as an index of the probability of a molecule of solute to come into contact with an existing crystal;
- Flow pattern, again as an index of the probability of a molecule of solute to come into contact with an existing crystal (higher in laminar flow, lower in turbulent flow, but the reverse applies to the probability of contact).
The first value is a consequence of the physical characteristics of the solution, while the other define a difference between a well- and poorly designed crystallizer.
Main crystallization processes
The main factors influencing solubility are, as we saw above:
- Concentration
- Temperature
so we 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. This is not a different process, rather a special application of one (or both) of the above.
Cooling crystallization
Application
Most of chemical compounds, dissolved in most solvents, show the so-called direct solubility that is, the solubility threshold increases with temperature.
So, whenever the conditions are favorable, I will obtain the formation of crystals by cooling the solution. Here cooling is a relative term: austenite crystals in a steel form well above 1000 °C. An example of this crystallization process id the production of Glauber's salt, a crystalline form of sodium sulphate. In the picture, where equilibrium temperature is on x-axis and equilibrium concentration (as mass percent of solute in saturated solution) in y-axis, it is clear that sulphate solubility quickly decreases below 32.5 °C. Assuming I have a saturated solution at 30 °C, by cooling it to 0 °C (note that this is possible thanks to the freezing-point depression), I obtain the precipitation of a mass of sulphate corresponding to the change in solubility from 29 % (equilibrium value at 30°C) to approximately 4.5 % circa (at 0°C) - actuallya larger crystal mass is precipitated, since sulphate entrains hydration water, and this has the side effect of increasing the final concentration.
There are of course limitation in the use of cooling crystallization:
- Many solute precipitate in hydrate form at low temperatures: in our previous example, this is acceptable, and even useful, but it may be detrimental when, for example, the mass of hydration water to reach a stable hydrate crystallization form is more than the available water: a single block of hydrate solute will be formed - this is the case of calcium chloride);
- Maximum supersaturation will take place in coldest points. These may be the heat exchanger tubes, so becoming sensitive to scaling, and heat exchange may be greatly reduced or dicontinued;
- 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 of course not applicable to compounds having reverse solubility, a term to indicate that solubiliti increases with temperature decrease (an example is sodium sulphate in the picture, where solubility is reverse above 32.5 °C).
Cooling crystallizers
The simplest cooling crystallizers are tanks provided with a mixer for internal circulation, where temperature decrease is obtained by heat exchange with an intermediate fluis circulatin 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 product along 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 on the surface 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 T0 temperature is transferred in a chamber at a pressure P1 such that the liquid saturation temperature T1 at P1 is lower than T0, 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, I cool the liquid by evaporating a part of it.
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, I have to increase the solute/solvent mass ratio using the technique of evaporation. This process is of course insensitive to change in temperature (as long as hydration state remains unchanged).
Al consideration on control of crystallization parameters are the same as for the cooling models.
Evaporative crystallizers
Most of 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 mixer keeps the crystal slurry in homogeneous suspension throughout the tank, including the exchange surfaces; by controlling pump 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.
The DTB Crystallizer
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 term of crystal specific surface and the fastest possible growth. This is 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 at the end of the '50s. (rumors say the name derives from the initial of the inventor - Dick Bennett - but mistery still surrounds the letter T). 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 ia 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 granulometry are extracted and eventually destroyed by increasing or decreasing temperature, thus creating additional supersaturation. A quasi-perfect control of all parameters is achieved. This crystallizer, and the derivate models (Krystal, CSC, etc.) could be the ultimate solution if not for a major limitation in the evaporative capacity, due to the limited diametre of the vapour head and the relatively low external circulation not allowing large amounts of energy to be supplied to the system.