K-Patents SeedMaster 2 - Manual (Page 12)

2 SUGAR CRYSTALLIZATION IN BRIEF 12
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rise is small with high-purity syrups and therefore requires very accurate measurement. Due to these
problems its use in the determination of supersaturation with the required accuracy and stability is very
questionable.
Refractive index of a solution has very strong and well-known correlation to its concentration. The
principle of measurement is in use all over the world since over one and a half century in laboratory,
and nowadays in process refractometer types, too. The data provided meet the requirements stated
earlier regarding its use in the calculation of supersaturation.
Summing up:
1. Supersaturation is the most important parameter of crystallization.
2. Supersaturation is a multivariable function of several syrup / mother liquor parameters (see Eq. 1).
3. Common instruments in use measure only a single parameter of the syrup or massecuite, therefore they
are unable to provide real quantitative data on supersaturation.
4. Supersaturation can only be calculated on-line by taking into account all of the parameters that govern
it. It is therefore logical that reliable and accurate on-line data on syrup / mother liquor concentration
are required. The only way to get them is by the use of a reliable process refractometer.
5. Information on supersaturation and on crystal content (even in indirect form) is required for advanced
boiling control.
2.8 Mounting the K-PATENTS refractometer
The K-PATENTS process refractometers have proved their worth in thousands of applications including
crystallization in the sugar industry worldwide. They consist of 2 main parts:
•
the Sensor Head (1 or 2) and
•
the Indicating Transmitter
connected by a 10 to 100 m long cable.
Mounting the Sensor Head.
In crystallizer applications it is recommended to use the long probe version providing longer insertion length.
Selecting the best location to mount the Sensor Head is usually a case of compromise. Crystallizers are far
from being perfect: despite the use of stirrers, circulation of the massecuite becomes more and more
sluggish when the crystal content increases. This means that syrup / mother liquor concentration and
temperature, which have large influence on supersaturation will not be the same in the full massecuite
volume. The following considerations should be taken into account:
1. Select a location, where measured concentration is representative for the largest volume of the syrup or
massecuite. This requirement means that locations close to the feed syrup entry and above the calandria
should be avoided. Too short distance from the entry point results in misleading data valid for a diluted
massecuite in a relatively small volume. A too fast drop of measured concentration after opening the feed
valve is real cause for concern.
2. Supersaturation increases with decreasing temperature if syrup / mother liquor concentration is kept
constant. This means that highest supersaturation is expected where temperature is the lowest.
Temperature is the lowest at the surface of the massecuite, while it is the largest just above the calandria.
Naturally, the rising surface can not be followed with the sensor. In practice temperature in the pan bottom is
fairly close to surface temperature.
In practice, unfortunately enough, feed syrup most often enters the downtake just above the calandria or at
the lower end of the downtake. These are rather questionable designs because of two reasons:
•
Due to the increasing difference between the density of the massecuite and that of the feed syrup due
to increasing crystal content, syrup entering the pan reverses direction and flows upwards in the downtake,
working more and more against the much needed circulation of the massecuite.
•
This design is in complete disregard of the needs of the on-line measurement of any massecuite
parameter (see point 1 above).
A much better solution is when feed syrup enters the pan under the calandria through a circular ring pipe.
Feed should be distributed and directed upwards at several locations around the pan wall. This solution
•
instead of degrading, improves massecuite circulation, and
•
provides good location under the feed pipe for instrument sensors.
Figure 2.7 shows recommended sensor locations for the two cases.