The overall objective of sugar pan boiling is to crystallize as much sugar as possible, while still producing crystal of the required size and with a good size distribution. Crystallization work done in the A and B pans has a substantial effect on pan capacity and the quantity of steam or vapor required.
Measurement of the efficiency of pan boiling
In Louisiana crystal content is expressed relative to the dissolved solids in the massecuite, thus excluding water. Then we have:
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Crystal Content % Solids = 100 . McteP - MolP
100 - MolP |
Purity drop is often used to assess the performance of a pan. This is not as useful because it depends too much on the purity of the massecuite itself.
Practical targets for crystal content in massecuites
Typical practical targets which should be achievable with good boiling practice are as follows, for average massecuite purities:
For each unit of massecuite purity change, the crystal content should change by 1 unit, in the range of purities we are interested in. More details are given in the Sugar Bulletin, May 2003, pg 12-17.
These numbers are based on refractometer Brix measurements. Use of spindle Brix inflates the calculated numbers. Most mills have changed or are changing to refractometer Brix, so in time this problem should disappear.
Effect of crystal content on quantities of massecuite to be boiled.
The quantity of A massecuite boiled for typical Louisiana conditions is shown in Figure 1 below, for a three boiling system, with A and B sugar bagged, assuming typical average values.
To assess the potential improvement from average to good performance or from poor to good, the following results are obtained:
The amount of pan capacity needed is directly related to the amount of massecuite to be processed. Batch centrifugal capacity is affected in the same way.
Steam or vapor usage is also directly related to massecuite quantities boiled. For the cases given above, for a factory processing 58 tons Brix in syrup/hour (roughly equivalent to a 10 000 ton/day factory), steam usage can be reduced from 67 to 58 ton/hr, a 15% reduction.
Practical suggestions for improving A and B massecuite measured crystal content values
Achieving high crystal content values in high grade massecuites is not easy. It requires attention to a number of details simultaneously. But the effort is worthwhile in maximizing recovery, making best use of available capacity and reducing costs. Here are some suggestions:
- It is impossible to monitor and/or improve something unless it is measured. Mills have the data with which to calculate A and B crystal content, and these values should be looked at daily by those in charge of factory operation.
- The factory parameter that has the greatest effect on crystal content is massecuite Brix. Ensure that A and B massecuite Brix values are run as high as possible.
- Ensure that your pans circulate well. If they do not, you have to compromise either on massecuite Brix or strike height. Remove any surplus ironmongery from inside the pans which hinders circulation, and ensure that the condensate and incondensable removal systems are adequate.
- If necessary, modify your pans and striking arrangements to handle high Brix massecuites. This may include installing a stirrer or steam-assisted circulation (steam jigger), increasing the size of the massecuite outlet (fitting a butterfly value), modifying striking gutters or making more serious modifications such as converting a flared pan to a straight-sided pan.
- Divert pan steamings. These dilute the massecuites and undo a lot of the good work done in the process. They should go back to syrup, or A molasses, or a remelter, depending on the purity of the massecuite.
- If you have A crystallizers, make sure they are always run full. If the drives are weak, reduce the stirrer element speeds.
- Ensure that the minimum amount of water is used on the centrifugals to give the required sugar purity.
- Ensure that the centrifugal ploughs are properly adjusted, so that they plough the screens clean. This is an often neglected area and has a considerable effect on exhaustion. It is a real waste of effort and money to allow some of the sugar crystallized out of the massecuite to be washed back into the molasses.
- If the centrifugals are old and difficult to adjust to get clean baskets, consider molasses classification, where first runoff goes to molasses and later runoff back to syrup. This is not as beneficial as getting your screens clean in the first place. It helps keep molasses purities down but does not eliminate recycle.