K-Patents SeedMaster 2 - Manual (Page 6)

2 SUGAR CRYSTALLIZATION IN BRIEF 6
6
2 SUGAR CRYSTALLIZATION IN BRIEF
2.1 The last step: crystallization
The main steps of sugar manufacturing are:
• sugar extraction (from cane or beet),
• juice purification and concentration, and
• crystallization.
SeedMaster 2 was designed to provide all the vital information required to control the process of sugar
crystallization on an unprecedented level. It is therefore logical to concentrate further discussions on the
process of crystallization.
Two methods to produce sugar crystals can be distinguished:
• cooling crystallization and
• evaporative crystallization.
Most of the sugar crystallized is produced in evaporative crystallization in batch or lately in continuous
evaporative crystallizers, but the basic principles of operation are the same in both types of crystallization
methods. Traditionally the vacuum pan operating in batch mode became the major piece of machinery and
enjoys wide use all over the world.
Crystallization is a very important part of sugar manufacturing. This is a process which has a large influence
on product quality and on the cost of production, both of which are very important when competitiveness is
at stake.
During most of its long history the control of sugar crystallization in batch pans was the undisputed authority
of the pan-men, who acquired their skill during a large number of years in practice. Working mostly without
any real instruments, they used only their eyes and fingers to keep the process under control. No wonder,
they still are being regarded as “artisans” of their profession. However, artisans can be quite different, and
quite often tend to behave rather unpredictably, which is far from being compatible with the quality- and cost-
conscious requirements of industrial mass production, so representative of our times.
Modern control of crystallization must rely on the reliable on-line measurement of the parameters which are
vital in the control of the process performed by a local operator (manual control), or by an advanced
automatic process control system (PCS).
2.2 Supersaturation: the driving force of crystallization
It is well known that crystallization can take place only in solutions which contain more solids in solution than
required to saturation. In case of sugar solutions the same mass of water can dissolve different amounts of
sugar depending on its temperature: the amount of sugar is larger, if temperature is higher. A saturated
sugar solution therefore can be supersaturated either by decreasing its temperature (cooling), or by
decreasing the mass of water (evaporation).
Supersaturation is defined as the amount of sugar dissolved divided by the amount of sugar required for
saturation in the same amount of water at the same temperature. We have real supersaturation only if this
ratio is larger than 1,0 (saturation).
Supersaturation is the driving force of crystallization: crystal growth (speed of crystallization) depends very
much on this parameter. High supersaturation means faster crystal growth and vice-versa. There is no
crystal growth if supersaturation is less than 1,0, in which case instead of growing, already existing crystals
begin to dissolve. It is important to emphasise that supersaturation is a complex multivariable function of the
liquid phase (mother liquor) parameters and should be calculated taking into account all of its governing
parameters:
Supersaturation = f(C, Q, T, m, b, c) (Eq. 1.)
where:
C : syrup / mother liquor concentration (%)
Q : syrup / mother liquor purity (%)
T : temperature (
˚ C)
m, b, c : syrup quality parameters (-)
Syrup quality parameters are discussed in detail in Ch. 12 APPENDIX.
It follows from its definition that, among others, concentration of the mother liquor should be measured on-
line, undisturbed by the presence of crystals in the massecuite in order to be able to calculate it.
2.3 Seeding methods
Seeding is a very important step in the process of crystallization, which has a large influence on the quality
of the product. When completed, the crystals begin to grow in size, if supersaturation is larger than 1,0.