K-Patents SeedMaster 2 - Manual (Page 11)

2 SUGAR CRYSTALLIZATION IN BRIEF 11
11
2.7 Common instruments in use for crystallization control
There are two major types of data needed for good control of crystallization:
1. supersaturation, and
2. information related to crystal content.
The basic requirements concerning the sources of these data are:
accuracy:
supersaturation : fairly high,
crystal content : modest;
in - and on-line instruments:
should provide reliable data in real time during the full course of crystallization;
long-term stability:
it is a basic requirement.
Besides instruments providing data on vacuum, massecuite temperature, level and vapour pressure in the
calandria, common instruments in use for boiling control measure
•
electrical parameters of the massecuite (conductivity, conductivity and capacitance (RF sensors),
•
viscosity / consistency,
•
stirrer motor consumption,
•
density / solids content (nuclear),
•
density / solids content (microwave),
•
boiling point rise and
•
refractive index.
None of these instruments provide reliable and quantitative data neither on supersaturation, nor on
crystal content, the real massecuite parameters that matter.
Conductivity is due to the presence of different types of ions in the syrup or mother liquor. It depends on
several parameters like concentration, composition and amount of non-sugars present, crystal content and
temperature. It certainly has (through syrup / mother liquor concentration) some correlation to
supersaturation (see Eq. 1.), but due to changes in other parameters governing it, it is far from being able to
provide reliable information on supersaturation.
Capacitance (RF sensors) depends mostly on the water content of syrup / mother liquor in a unit volume of
massecuite, therefore, besides concentration it shows strong dependence on crystal content.
Viscosity of the syrup / mother liquor depends on their concentration and temperature, but instruments in
use for boiling control do not have temperature compensation feature.
Consistency is the property of the liquid / solids (mother liquor / crystals) mixture. Viscosity / consistency are
measured in crystallization control practice by the same instrument. Consistency, however, sharply
increases with increasing crystal content. At the end of a strike consistency can be 20-30 times larger than
the viscosity of the mother syrup alone.
Stirrer motor current or power consumption provide data on viscosity / consistency in a different form.
Density of the massecuite comes from two sources: density of the mother liquor and that of the solid
crystals. Besides these the crystal to mother liquor ratio in the massecuite will determine its value.
Solids content of the massecuite is similarly determined by the solids content of the mother liquor, that of the
crystals and by the crystal to mother liquor ratio.
The common features of all instrument types discussed above are:
1. None of them is able to provide selective and accurate enough information on mother liquor
concentration in the presence of crystals, therefore they are unfit to give even approximate, indirect
information on supersaturation all over a strike (see Eq. 1).
2. Having completed seeding all instruments discussed above provide data which are more and more
governed by the increasing crystal content. It is logical therefore that these instruments can be put to good
use only to provide – though indirect and only approximate - information on crystal content.
There are only two principles of measurement which can be used to measure syrup concentration
selectively even in the presence of sugar crystals.
Boling point rise of a solution depends – among others – on its concentration, therefore it is correlated to
supersaturation. Supersaturation, however, is a multivariable function of several parameters. On the other
hand: boiling point rise depends on syrup / mother liquor purity and non-sugar composition, too. Boiling point