Syrup clarification at Blairmont Estate, Guysuco
T. V. Tahal Guyana Sugar Corporation Inc.
Abstract
Syrup clarification was initiated at Blairmont Estate to improve the quality (lower insoluble solids and colour) of its value added sugar Demerara Gold, a direct consumption sugar. Juice clarification can be considered good but there are still “floaters” e.g. bagacillo in the sugar, hence the need for the syrup clarifier. The insoluble solids are normally in the vicinity of 150-300 ppm so our aim was to reduce that further for the direct consumption Demerara Gold. The syrup clarifier was was commissioned in the second crop of 2003 and trials are currently being carried out. Initial results are showing reductions in insoluble solids and colour. Further trials and modifications will will be undertaken to optimize optimize the performance of the clarifier. Introduction
This is the first of of such clarifiers to be installed installed in Guysuco. The clarifier was designed in 2000 based on clarifiers operating in South Africa and Booker-Tate managed sugar factories around the world. The results reported are based on 18 trials carried out to date. The results are indicating 39% reduction in insoluble solids, 7% drop in the colour of sugar and 41% reduction in turbidities. Syrup clarifiers use the floatation method unlike the juice clarifiers, which utilise a settling method. The settling method would not be possible with syrup because of the the high density and viscosity. The mechanism of syrup clarification involves the trapping of air bubbles within the flocs and intermolecular attraction across the air/liquid interface. Syrup clarification occurs by heating the syrup, then aerating it to trap the fine particles, which are finally coagulated by a low molecular weight flocculant. The scum will float to the top of the clarifier where it is scraped off and returned to the mixed juice tank. The factors that directly directly influence the syrup clarifier are temperature, flocculant – mixing and dosage, air and syrup flowrates. Previous trials of syrup clarification
Syrup clarification is an old technology that is being used to good effect especially in South Africa, Australia, Belize, India and Mauritius.
It was first tried in South Africa in 1974 at Noodsberg with the following results;
1. Improved sugar quality 2. Reduction in massecuite viscosities 3. No boiling house improvements This process was discontinued at Noodsberg because the emphasis was not on quality.
At Tongaat – Hulett Sugar Limited, Felixton several trials were undertaken and they have shown good results with respect to; 1. Insoluble solids 2. Colour reduction 3. Better boiling house recoveries
Then trials were undertaken at Belle Vue Sugar Mill in Mauritius with the following results; 1. 2. 3. 4. 5.
Sugar quality was improved Massecuites were found to be less viscous and easier to cure A 67% reduction in the syrup turbidity No improvements on final molasses There was an increase in colour
In 1960 the Sugar Research Institute of Australia commenced its first syrup clarifier project and has since constructed syrup clarifiers in Australia, India and China to give a higher quality raw sugar.
Syrup clarification is being done at Tower Hill Sugar Mill to reduce the insoluble solids for direct consumption sugars.
Guysuco provided drawings and additional information for the syrup clarifier that was built at Tower Hill Mill in 2001. A 73% reduction in turbidity was reported in 2001.
The syrup clarification process
There are a few key parameters, which must be controlled before proper results can be obtained. Those key parameters are; 1. 2. 3. 4.
Air supply Heating of syrup Syrup flow Flocculant dosage and concentration
Air supply Air is applied to the syrup clarifier by air diffusers. The air is used for two reasons in the clarifier; 1. Aerating the syrup – this is done so that the air bubbles cling to the precipitates in the syrup. 2. It floats the floc to the top of the syrup clarifier.
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When starting up the clarifier the air must be added at a higher rate (15-20 p.s.i) than during operations. This is to create bubble formation at the top of the clarifier. It was found that excess air promotes faster scum formation. During operations the air should be regulated so that the minimum air is used to float the floc to the top of the clarifier and produce the lowest possible turbidity. This range is between 5-15 p.s.i. Heating of syrup Heating of the syrup is essential to he coagulation of the flocs. A shell and tube heater, which uses 2nd vapour as the heating agent, heats the syrup. Studies from (Rein and Cox, 1987) have shown that changes in temperature will affect the turbidity removal of the syrup and it peaks at a temperature of about 85 oC, hence no specific trials were undertaken. Syrup flow The flow of syrup is very important to obtain constant turbidity from the clarifier. Excess flow or low syrup brix conditions will cause syrup to overflow into the scum take off and returned to the mixed juice tank, thereby affecting the performance of the juice clarifier. There will be an increase in colour and the juice clarifier will also become disturbed. If the syrup flow is not maintained within a range the ratio of syrup to flocculant will change and hence the turbidity will be affected unless there is an automatic link between the syrup flow and flocculant dosage. If the station is manned then the operator will need to change the flow of flocculant to get the desired turbidity, but it would be best if a flow controller is installed to more accurately control the flocculant dosage as the flow changes. At present a flow controller is not installed but one is being acquired to monitor the flow and ultimately control the dosage. Flocculant The flocculant dosage and concentration affects the clarity of the syrup and this was evident from the scum at the top of the clarifier. The best results were obtained at concentrations of 0.1% and dosage of 15 ppm on brix. The flocculant used was Talodura with other flocculants such as the midlands 9000 to be tested in the coming crops. The sequence of air and flocculant additions affected the operation of the syrup clarifier. It was found that better results were obtained when the syrup was aerated before the addition of flocculant, but syrup clarifiers at other factories operate with the reverse. The flocculant is added into the syrup line by the use of a variable speed mono pump just before a specially designed mixer situated about 6 feet before the clarifier. The mixing system for the flocculant is very important because if there are lumps in the flocculant, it will affect clarification. It must be noted strict guidelines should be followed before the flocculant can work effectively. These include the mixing temperature and the aging of the flocculant before usage. The flocculant is mixed by air with a funnel used to distribute the flocculant at small quantities into the mixing tank, but this inevitably creates lumps. A new flocculant mixing system equipped with a venturi is being installed to improve flocculant mixing.
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The original mixing system can be seen in Figure 1, while Figure 2 shows the modification to the mixing system. Monitoring of clarifier
An SMRI turbidity meter is installed to monitor the turbidity as it changes. This is displayed on a chart recorder, which also records the temperature of the syrup going into the clarifier. These results can be downloaded to a computer for analysis. This is done on daily basis. The laboratory does a turbidity analysis on the unclarified and clarified syrup on alternate hours so that the reductions can be monitored. Also an insoluble solids analysis is done on a shift basis for the composite sugar samples. Syrup clarifier station equipment
Figure 1 shows a schematic diagram of the major components of the syrup clarifier station, which includes; 1. 2. 3. 4. 5. 6. 7. 8.
Clarifier Syrup heater Air diffusers Syrup pump Flocculant Tank Flocculant pumps Inline Turbidity meter and Recorders Inline mixer
Detailed specifications of the equipment are shown in the appendix. Cost analysis
Cost of project: Equipment Instrumentation Installation Total
G$ 14 M G$ 8 M G$ 5 M G$27 M
Modification Cost: Operating Cost (Materials & Labour) Maintenance Cost
G$ 0.75 M G$1M/Yr G$ 0.3M
The cost of the clarifier seems high, but Guysuco stands to benefit tremendously from this project once all the problems are eliminated. From a quality point of view there will be a sustained quality product, which will ensure Guysuco remain competitive on the markets. In addition, improvements in the final molasses purities will result in increased sugar production, for example it is estimated that if the final molasses purity drops two unit this factory should recover a further 150 tonnes of sugar per year, which is equivalent to $8 M per year increased revenue.
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Future modifications
Also in Figure 2 are other modifications that will be done in the near future which include adding phosphate and lime. Trials of this will be undertaken in the second crop of 2004. With the addition of lime and phosphates to the syrup better clarification will be achieved because of the formation of more agglomerates, which will further trap the insoluble particles and hence a greater reduction in colour and insoluble solids will result. Experiences with the syrup clarifier
During the commissioning and after, there were several changes that were required so better operation of the clarifier could be achieved. 1. The turbidity meter was removed and repositioned so that it could work continuously. In its previous position it could only work when the remelt tank was working. 2. On the first day of commissioning the syrup temperature was not changing despite vapour was being passed through the heater. This was because the condensate removal line was not sufficient to remove the condensate and as such the heating of the syrup was affected. 3. The rotary vane syrup pump that was supplied was undersized so most of the syrup was bypassed and hence not clarified. To counter this problem two centrifugal pumps were installed but they had contrasting problems. The smaller of the centrifugal pumps could only handle the throughput when the desired syrup brix was being achieved but at other times (syrup brixes lower than 55) syrup was bypassed. The other centrifugal pump was too large and the motor could not be controlled. Currently the smaller centrifugal pump is being used but due to the turbulent flow it affects the settling in the clarifier. A larger magmaflo rotary vane pump is on order, this will provide a more stable flow to the clarifier. 4. Temperature gauges were installed so that the persons mixing the flocculant could be guided and not mix the flocculant at a high temperature since this would damage the flocculant. 5. Since there was no flow meter installed the flocculant dosage depended on a guide from the syrup brix being achieved. Also since the station was not manned by an operator and was only overlooked by a process supervisor, sometimes the changes in flow went unnoticed and the turbidity fluctuated. 6. The mixing of the flocculant was done by a pan floor assistant with the use of a funnel to distribute the flocculant in the mixing tank. At times this was not adhered to and as such the flocculant was not mixed properly on several occasions. 7. The inline mixer for the flocculant was changed since it was restricting the flow of syrup, hence a fabricated mixer with a larger diameter was installed and is working to good effect. 8. The flocculant pump stators were damaged twice because; 1. the pumps ran dry; 2. pieces of metal (slag) passed through pump;
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3. poor mixing of flocculant. Results from trials at BCF
Initially the clarifier was commissioned with air added after the flocculant and the results were very moderate with turbidities greater than 50. From Figure 3 below it can be seen that from run 10 when the air and flocculant points were switched the results improved. The reason for the change was that with the previous arrangements the syrup was not fully aerated and hence the scum formation was minimal. On changing around the flocculant and air, better results were consistently achieved. The highest turbidity removal recorded for the trials was 70 %, with an average turbidity removal for the trials of 41% being achieved.
Raw and Clarified syrup
100.00 90.00 80.00
Raw Syrup
70.00
Clarified Syrup
60.00
% y t i d i 50.00 b r u T 40.00
30.00 20.00 10.00 0.00 0
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Run
Figure 3: Reductions in turbidities from raw syrup to clarified syrup
Colour reduction from unclarified to clarified
2500.00
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1500.00 r u o l o C
1000.00
Raw Syrup Clarified Syrup
500.00
0.00 0
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Run
Figure 4: Colour reduction from raw syrup to clarified syrup Of the trials there were significant changes in the colour with an increase of 1% for one run and a general decrease in the colour for the other 17 runs. The reduction ranged from 1 % to 24% and an average of 7% over the eighteen trials done.
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Turbidity and Syrup Temperature 60.00
50.00
40.00
% y t i d i 30.00 b r u T
20.00
10.00
0.00 76.00
78.00
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Syrup Temperature ( oC)
Figure 5: Effects of syrup temperature on turbidity The range of temperature the clarifier operated under for the trails did not have a significant effect on the turbidity, however as seen in Figure 5 at about 85 oC the turbidity is slightly lower than at the lower temperature used in the trials. Turbidity vs Insoluble Solids
700.00
600.00
500.00
) m p p ( s 400.00 d l i o s e l b 300.00 u l o s n I
200.00
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0.00 0.00
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Clarified Turbidity (Lab)
Figure 6: The effect of turbidity on insoluble solids The general trend indicates in Figure 6 a lower turbidity will result in syrup and ultimately sugar of lower insoluble solids. The lowest possible turbidity that can be achieved from the clarifier is about 8 ppm. The average insoluble percent removal for the trails done was 39% and largest percent removal was 63%. Conclusions
It must be noted that the clarifier operated well for only two months and during those periods trials were carried out. During the remaining time over the last eight months we were faced with several challenges to get the clarifier back into operation. Despite the numerous challenges there were in getting the clarifier running, and the design problems, we have come a long way in
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understanding what is needed for the clarifier to operate to its optimum. Some of the things that will be done to counter the problems and improve the performances of the clarifier are; 1. 2. 3. 4.
A new syrup pump will be installed. An operator will be placed at the station so that better controls can be achieved. The phosphate and lime dosing will be tried in the second crop of 2004. The level controllers on the evaporators will be put into operation so that consistent syrup brix can be achieved. 5. The flocculant mixing will be modified so that better mixing is achieved. 6. A flow controller will be installed so that a consistent syrup flow will be achieved. Though the clarifier has not performed to its expectation as yet because of the many challenges faced, during the course of the coming crops it is believed that it will show its worth. It is expected, once the clarifier performs as it should and from the limited results collected so far, definitely there will be colour and insoluble reductions and possibly reductions in final molasses purity.
References P.W. Rein and M.S.G Cox (1987), “Syrup Clarification in Raw Sugar Mills”. Proceedings of The south African Technologists’ Association- June 1987 R. B Gray (1994), “ Syrup Clarification Operating Manual-Draft”, Booker Tate Limited, October 1994. A.M. Katryan and S. Ramsuchit (2001), “Report on visit to the Tower Hill Factory in Belize.
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Appendix – equipment specs (a) Clarifier specifications
Working volume Clarifier Dia. Aeration chamber Dia Inclined take off depth Total Depth Raw syrup tank
11.5 m3 3050 mm 500 mm 1250 mm 1888 mm 8.09 m3
(b) Air diffuser
Porvair air Diffuser Specification: Flow rate Micron size Air pressure
250 m, 316 SS, ½” BSP inlet 1 m3 /hr 60 micron 690 kPa (g)
(c) Syrup heater specifications
Horizontal shell and tube raw syrup heater Nominal Flow 23 m3 /hr Juice Velocity > 2.0 m/sec Inlet temp 400C Outlet temp 85oC Heating surface 45 m2 Tube material SS 304 Tube length 3910 mm Tube thickness 1.65 mm Tube OD 38.1 mm Tube ID 34.8 mm NO. of tubes 96 Tubes/pass 3 (d) Syrup pump
Currently a centrifugal pump is being used which could only handle 20 tonnes/hr. A new vane pump is on order and this would Be capable of pumping 28 tonnes/hr. (e) Flocculant tanks
Working Volume No. of tanks Parallel belt diameter Parallel belt height Conical angle
3.4 m3 2 1520 mm 1875 mm 15o
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Conical height
200 mm
(f) Flocculant pump
Description
mono pump
(g) In-line turbidity meter
Description SMRI Turbidity Meter (h) Paperless chart recorders
Description
Records turbidities Syrup Temperature
(i) In-line flocculant mixer
Description Locally made. A 4” x 2’ pipe with spirals to mix flocculant with syrup.
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Figure 1: Initial syrup clarifier layout
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Figure 2: Proposed Modifications
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