K-Patents SeedMaster 2 - Manual (Page 7)

2 SUGAR CRYSTALLIZATION IN BRIEF 7
7
Shock seeding is the traditional way of seeding. It relies on the building up of high supersaturation in the
solution, when a small amount of seed crystals entered into the pan results in the formation of new crystals
(nucleation). The number of these crystals keeps growing as long as the value of supersaturation is above a
“safe” (nucleation-free) limit. The final number depends very much
• on the actual value of supersaturation, and
• on the time (time of nucleation) spent in the region of high supersaturation above the “safe” limit.
There are at least 3 important parameters in this method of seeding:
1. the actual value of supersaturation maintained in the “seeding point” and during nucleation;
2. the length of nucleation;
3. the limit value of supersaturation above which nucleation begins, if there are already crystals in the
solution.
Point 1. calls for a reliable measurement of supersaturation.
Point 2. has some difficulties of its own, too. How to determine the correct length of nucleation? The method
of trial and error can only be used, if supersaturation was the same all over the trials, which again calls for its
measurement. Even if it is known, in case of manual control of crystallization is it possible to ensure exactly
the same time for nucleation from strike to strike, even during the night shift?
Point 3. requires the knowledge of the critical supersaturation, above which nucleation begins in the
presence of already existing (seed) crystals. There are not many reliable data on this limit. A recent
publication (see below) provides data depending somewhat on syrup purity and temperature, too (the
dependence on temperature is more pronounced with low-purity syrups). In case of high syrup purity (larger
than 94 %) this supersaturation limit is about 1,12…1,13. An important point is that the formation of new
(often unwanted) crystals can begin any time supersaturation exceeds the critical supersaturation limit.
CRITICAL SUPERSATURATION TO START NUCLEATION
1,1
1,11
1,12
1,13
1,14
1,15
1,16
1,17
1,18
1,19
1,2
74 76 78 80 82 84 86 88 90 92 94 96 98 100 102
SYRUP / MOTHER LIQUOR PURITY (%)
S
UP
E
RS
T
URAT
IO
N.
TEMP.
60 C
TEMP.
70 C
TEMP.
75 C
TEMP.
80 C
Fig. 2.1
(This figure is based on the equation published in:
M.Saska: Boiling point elevation of technical sugar cane solutions and its use in automatic pan boiling.
International Sugar Journal 2002, VOL. 104., No.1247., 500-507).
Fig.2.2 shows some of the trends typical of shock seeding. It is evident that shock seeding has quite a few
uncertainties and consequences, which make its use in modern practice undesirable.