Crystallization

Khalid Nawaz cell no:+923007024407institute of chemical engineering and technology Punjab university Lahore Pakistan

Super saturationThe term super saturation refers to

a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances. It can also refer to a vapor of a compound that has a higher (partial) pressure than the vapor pressure of that compound.

Super saturation can be achieved by adding more of a substance (to a solution) than can normally be dissolved. This is a thermodynamically unstable state

nucleationNucleation is the process of forming a nucleus.

It is the initial process in crystallization. It is the process in which ions, atoms, or molecules

arrange themselves in a pattern characteristic of a crystalline solid, forming a site in which additional particles deposits as the crystal grows. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation

Examples: Dust and pollutents provide nucleation sites for water vapor in the atmosphere to form clouds.

Homogeneous and hydrogenous nucleationNucleation normally occurs at nucleation

sites on surfaces contacting the liquid or vapor. Suspended particles or minute bubbles also provide nucleation sites. This is called heterogeneous nucleation. Nucleation without preferential nucleation sites is homogeneous nucleation. Homogeneous nucleation occurs spontaneously and randomly, but it requires superheating or supercooling of the medium

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Crystallization Kinetics: GrowthTypically follows an initial stage of either

homogeneous or heterogeneous nucleation, unless a "seed" crystal, purposely added to start the growth, was already present.

Addition of solute to faces of crystalFor controlled growth operate

crystallization under low supersaturation levels

Growth & nucleation are competing processes!

CRYSTALLIZATION UNIT

Mechanism of CrystallizationCrystal nucleation and amorphous

precipitates are in competition during supersaturation conditions

Nucleation favored by slowly exceeding the equilibrium point of saturation permits time for the protein structure to orient in a crystalline lattice


Continuous or Batch DesignBenefits of Continuous

Can maintain solution in supersaturated stateLarge fluidized bed for crystallization Minimizes operation costsMinimize down time (startup and shutdown)

Benefits of BatchGood when have low concentration of product,

high viscosity or many impuritiesCan produce high quality crystal


Methods of CrystallizationSupersaturation: liquid (solvent)

contains more dissolved solids (solute) than can ordinarily be accommodated at that temperature

Can be achieved by several methods:CoolingEvaporationSolvent additionPrecipitant Addition

Growth of crystals from nucleiGrowth processes then enlarge existing nucleiSmallest nuclei often rediscoverLarger nuclei can get larger Thermodynamics favors the formation of larger nuclei

Ostwald RipeningLarger crystals are more stable than smaller crystals – the energy of a system will naturally trend towards the formation of larger crystals at the expense of smaller ones

In a sense, the smaller crystals are ‘feeding’ the larger ones through a series of dissolution and precipitation reactions

Definition of Ostwald ripening

Many small crystals form in a system initially but slowly disappear except for a few that grow larger, at the expense of the small crystals. The smaller crystals act as "nutrients" for the bigger crystals. As the larger crystals grow, the area around them is depleted of smaller crystals.

Explanation for the occurrence of Ostwald ripening

This is a spontaneous process that occurs because larger crystals are more energetically favored than smaller crystals. (This might be hard to believe seeing as how it seems far more common to get many small crystals than a few large ones, but there is a believable explanation.) . While the formation of many small crystals is kinetically favored, (i.e. they nucleate more easily) large crystals are thermodynamically favored. Thus, from a standpoint of kinetics, it is easier to nucleate many small crystals. However, small crystals have a larger surface area to volume ratio than large crystals. Molecules on the surface are energetically less stable than the ones already well ordered and packed in the interior. (Think of packing your vacation clothes in a suitcase. Which ones are more energetic? The ones in the middle or the ones you are packing in on top, trying to get them to fit?) Large crystals, with their greater volume to surface area ratio, represent a lower energy state. Thus, many small crystals will attain a lower energy state if transformed into large crystals and this is what we see in Ostwald ripening.

So why doesn't Ostwald ripening happen all the time? One reason is that the nucleation of many small crystals reduces the amount of super saturation and thus, the thermodynamically favored large crystals

never get a chance to appear.


Cooling MethodConcentrated

solution gradually cooled below saturation temperature (50-60°C) to generate a supersaturated state

Yields well defined micron-sized crystals

Shell and tube heat exchanger is used to cool solution


Cooling MethodAdvantages:

High purity downstreamDisadvantages:

Temperature change does not always have a positive effect on supersaturation in proteins

Protein stability may be at riskSolubility can be relatively insensitive to

temperature at high salt concentrationsCooling will only help reach supersaturation in

systems where solubility and temperature are directly related


Evaporation MethodSolute dissolves in solution when heated

to a certain temperature (75°C) Slowly cooled until crystals precipitateShell and tube heat exchanger is used to

heat and cool solution


Evaporation MethodAdvantages:

high purity levels downstream Disadvantages:

Vaporization chamber requires high pressuresProtein viability very sensitive to high

temperatures


Solvent MethodSolvents are generally good protein

precipitants Their low dielectric constants lower

the solvating power of their aqueous solutions

Requires acidic solventFor crystallization, an insulin protein falls

out of solution at isoelectric point pH 5.4-5.7


Solvent MethodAdvantages:

Proteins viability not at risk due to temperature change

Disadvantages:Possible protein contamination due to

insufficient downstream solvent recovery


Seeding TechniquesPrimary nucleation is the first step in

crystallization - growth of a new crystal Can bypass primary nucleation (creation of

new crystals) by "seeding" the solution

Secondary nucleation is crystal growth initiated by contactAccelerated by "seeding" adding existing

insulin crystals to perpetuate crystal growth

Education

Crystallization