Biology Reference
In-Depth Information
The number of crystals determines the product mean size . . .
Equal mass
L
27 particles L=1
1 particle L= 3
filtration 9 times faster
and hence operate to control the number generation
Fig. 4. Control of product crystal mean size. The product crystal mean size from a crys-
tallization process is determined by the number of crystals that shares the total mass of
crystalline material. The number of crystals is mainly controlled by the nucleation in the
process. The figure compares how many particles that are required to carry the same mass
if one particle with a relative diameter of three is replaced by particles with a relative diam-
eter of unity. Filtration rate is proportional to size raised to power two and accordingly fil-
tration is much faster when the particles are larger.
the end-use, and is definitely of importance for the processing down-
stream of the crystallization, e.g. filtration, washing and drying. The pos-
sibility of separating the particles from the filtrate and washing liquid is
of importance for the purity. The size of the crystals from a crystallization
process is controlled by the number of particles sharing the total crystal
mass produced (Fig. 4).
The total number of crystals in turn depends on the number added in
the form of seeds and the number formed during the process as a result of
nucleation and crystal breakage. The formation of new crystals is gov-
erned by kinetic factors. In principle then, it is possible to obtain any sub-
stance at any particle size. In practice, it is of course easier or cheaper to
obtain large particles for certain substances.
Crystals are not spherical, and the measured particle size will depend
on the method used for determination: different techniques measure dif-
ferent dimensions of the particle. In addition, particles are usually not of
a uniform size, but are found in a more or less wide distribution of sizes. The
crystal size distribution (CSD) can be given as a cumulative distribution
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