Process Technology: An Introduction - Haan A.B. 2015
13 Crystallization and precipitation
13.3 Crystal characteristics
In crystallization processes product requirements are a key issue in determining the ultimate success in fulfilling the function of the operation. Key parameters of product quality are the size distribution (including mean and spread), the morphology (including habit or shape and form), and purity. Crystal size distribution (CSD) determines several important processing and product properties. It is often important to control the CSD. Most favored is monodisperse where all crystals are of the same size and dissolve at a known and reproducible rate. Critical phenomena that influence the CSD outside nucleation and growth are breakage and agglomeration. Breakage of crystals is almost always undesirable, because it is detrimental to crystal appearance and it can lead to excessive fines and have a deleterious effect on crystal purity. Agglomeration is the formation of a larger particle through two or more smaller particles sticking together.
13.3.1 Morphology
Every chemical compound has a unique crystal shape that depends enormously on the conditions in the crystallizer. Some substances illustrate polymorphism, meaning that the substance can crystallize into two or more unique forms. A good example is carbon, which can crystallize into graphite and diamonds. The general shape of a crystal is referred to as its habit. A characteristic crystal shape results from the regular internal structure of the solid with crystal surfaces forming parallel to planes formed by the constituent units. The unique aspect of the crystal is that the angles between adjacent faces are constant. The surfaces (faces) of a crystal may exhibit varying degrees of development but not angles. With constant angles but different sizes of the faces the shape of a crystal can vary enormously. This is illustrated in Fig. 13.6 for three hexagonal crystals. The final crystal shape is determined by the relative growth rates of the crystal faces. Faster-growing faces become smaller than slower-growing faces and may disappear from the crystal altogether in the extreme. The relative growth process of faces and thereby final crystal shape are affected by many variables, such as rate of crystallization, impurities, agitation, solvent used, degree of supersaturation, and so forth. The appearance of the crystalline product, purity and its processing characteristics (such as washing and filtration) are affected by crystal habit. The effect on purity becomes even more important when crystallization is employed as a purification technique. Mechanisms by which impurities can be incorporated into crystalline products include adsorption on the crystal surface, solvent entrapment in cracks, crevices and agglomerates, and inclusion of pockets of liquid. An impurity having a structure sufficiently similar to the material being crystallized can also be incorporated into the crystal lattice by substitution or entrapment.
Fig. 13.6: Various shapes of a hexagonal crystal.
13.3.2 Crystal size distribution
Particles produced by crystallization have a distribution of sizes that varies in a definite way over a specific size range (Fig. 13.7). A crystal size distribution (CSD) is most commonly expressed as a population (number) distribution relating the number of crystals at each size to the size or as a mass (weight) distribution expressing how mass is distributed over the size range. The two distributions are related and affect many aspects of crystal processing and properties. An average crystal size can be used to characterize a crystal size distribution. However, the average can be determined on any of several bases such as number, volume, weight, length. The dominant crystal size LD is most often used as a representation of the product size. The coefficient of variation (cv) of a distribution is a measure of the spread of the distribution around the characteristic size. It is often used in conjunction with dominant size to characterize crystal populations through the equation
(13.11)
where σ is the standard deviation of the distribution.
Fig. 13.7: Crystal size distribution.
13.3.3 Size control
Crystal size distributions produced in a perfectly mixed continuous crystallizer are highly constrained. The form of the CSD in such systems is entirely determined by the residence-time distribution of a perfectly mixed crystallizer. Greater flexibility can be obtained through introduction of selective removal devices that alter the residence time distribution of materials flowing from the crystallizer. Clear-liquor advance is simply the removal of mother liquor from the crystallizer without simultaneous removal of crystals. The primary objective of classified-fines removal is preferential withdrawal of crystals whose size is below some specified value. A simple method for implementation of classified-fines removal is to remove slurry from a settling zone in the crystallizer. Constructing a baffle that separates the zone from the well-mixed region of the vessel can create the settling zone. The separation of crystals in the settling zone is based on the dependence of the settling velocity on crystal size. Such crystals may be redissolved and the resulting solution returned to the crystallizer. Classified-product removal is carried out to remove preferentially those crystals whose size is larger than some specified value.