Supersaturation-based control of footing magma crystallization
Supersaturation-based control of the crystallization process has been implemented successfully in several sugar mills around the World. Many references show that this type of control results in superior crystal quality parameters and better product yield as compared to the traditional control methods still used in many mills.
Until now, we always studied the results of such cases where this advanced type of control was implemented on the product pans: crystallization in the product pan itself was controlled based on supersaturation. In this post I present a case where the advanced control is used on a seed pan. This seed pan makes footing magma for the product crystal pans.
We’ve been involved in the implementation of supersaturation-based control of sugar crystallization many times, but this case was special. Our customer uses a regular batch vacuum pan to grow slurry crystals up to 0.2 mm. The final massecuite from this pan is then split and pumped into the five pans that they use for “A” product.
By improving the process of footing magma crystallization, we can improve final product quality. But this is a kind of an indirect improvement: we left the instrumentation and control of the “A” product pans untouched and upgraded the seed pan only.
Crystal size distribution is only measured on the product crystals in this sugar mill, not on the footing magma crystals. With my colleagues we studied if the advanced control of the footing magma crystallization has effects on the size distribution and general quality of the product crystals.
We analyzed the granulometry data of a large number of strikes with the old footing magma crystallization control and with the new one. Then we compared the results and discussed with the customer about their experiences with the new system.
Instrumentation for supersaturation-based control
Our customer already had a microwave probe and a level transmitter installed in the seed pan. A Vaisala K-Patents process refractometer was added to measure liquid phase concentration and massecuite temperature.
To have reliable information on crystal content, it is best to have both microwave probe and process refractometer installed. The microwave probe measures total solids content of the massecuite, while the refractometer measures the dissolved solids only. The difference between these two are the crystals. Before seeding, a process refractometer and a well calibrated microwave sensor will measure the same concentration.
SeedMaster-4 has been installed next to the seed pan. Microwave, refractometer, and level signals were connected via 4-20 mA analog signals, supersaturation output signal was connected to the DCS system.
Analyzing the old control tactics
Typical strikes with the old control were performed like this:
The pan was filled up to seeding level with 78 Brix syrup (purity is 95 %).
The syrup was concentrated to seeding point as fast as possible.
Seeding was performed with slurry at a certain microwave Brix level.
After seeding, they used 74 Brix syrup (also 95 % purity) for feeding.
For a couple strikes, we did not change anything in the process, we just monitored the crystallization with the SeedMaster. We’ve seen that
Seeding happened a bit too late: Supersaturation was already above 1.2 when they performed it. Sometimes above 1.25! That is way too high, spontaneous nucleation starts happening around 1.14 with this purity.
After the peak around seeding point, supersaturation was quite low during the whole time. Somewhere around 1.05. That should be higher for better crystal growth.
Temperature was too high: it was around 78 °C. That holds a risk of caramelization. Also, it is responsible for the low supersaturation in boiling phase.
Technological bottlenecks
To increase supersaturation in boiling phase, our first idea was to increase vacuum. That could give us a couple degrees lower temperature and thus increase supersaturation to the desired range. Unfortunately, it turned out that vacuum capacity was not enough for the mill. Even with the old control, vacuum valve used to be 100 % open all the time. We couldn’t give it more.
Increasing concentration by quicker evaporation could also help boosting supersaturation. But the issue was the same: there was no more steam available.
Changes in the control
Because of the limited availability of vacuum and steam, we did not modify boiling phase control. Instead, we concentrated on the most critical part of the strike: seeding. The low supersaturation in boiling phase results in longer strikes, but the excessive supersaturation around seeding point results in a large number of fines and thus poor product quality and high recirculation.
To get better product crystal size distribution parameters and yield, the control was changed this way:
Seeding is now performed based on supersaturation.
After seeding, there is some time left for the seed crystals to get stronger before feed continues. Length of this phase is also based on supersaturation.
When feed continues, it is done first based on supersaturation setpoint.
In boiling phase, they switch back to old control.
Results
As discussed earlier, this case is special, as we can only analyze the “A” product crystals, not the footing magma crystals. The new control was installed on the seed pan, that creates the footing magma used to seed the “A” product pans. The product pans themselves were not upgraded. Nevertheless, we knew that the upgraded magma crystallization should have an effect on the product crystals.
For the analysis, we received granulometry data of 87 “A” product strikes from the mill. 39 with the old footing magma control and 48 with the new one.
The results of sieving showed this:
Ratio of target sized (0.63 mm sieve) crystals increased from 30.6 % to 34.6 %.
CV under 30 was reached in 10 % of the cases with the old control, 38 % of the cases with the new control.
Our customer reported that sometimes with the old control, it occurred that slurry crystals dissolved after seeding. It never happened after SeedMaster was installed.
Sometimes also “dusty” strikes occurred with the old control: the very fine sugar dust showed up as overload on the product conveyor.
Product crystal size distribution before and after SeedMaster installation on the footing magma crystallizer
Ratio of target-sized crystals increased from 30.6 % to 34.6 %.
These results show that using an advanced, supersaturation-based control for footing magma crystallization leads to significantly better final product crystals. Contact us, if you are interested in advanced crystallization control.