Sugar Plant Milling vs Diffuser process | Why Milling is favored over cane or bagasse diffuser
There are two methods of cane juice extraction methodologies in Sugar Industry
1. Milling (Grinding of prepared cane )
2. Diffusion (Extraction of juice by leaching of prepared cane with the aid of hot water)
There are two variants of this diffuser technology in sugar industry
1. Cane diffusion wherein prepared cane is made to pass through the diffuser for juice extraction followed by one or two de-watering mills to reduce the moisture in Bagasse suitable for combustion in the boiler
2. Bagasse Diffuser where in the diffuser is preceded by a mill where primary juice extraction takes place and the bagasse coming out of of this pre-extraction mill is made to pass through the diffuser followed by de-watering mills as above
Cane Diffuser technology has the following advantages in sugar factory.
1. Extraction by diffuser is considered to be the efficient technology compared to Milling as the extraction levels possible, theoretically, with the former will be about 98.5% max whereas in a Mill it will not go beyond 98%. With a bagasse % cane of about 32%, the increase in recovery shall be 0.16%
2. Downtime in the conventional milling is said to be higher compared to diffusers.
3. Sanitation is better than mills since the whole diffuser is maintained at 750C, which prevents microbiological growth thereby reducing sugar inversion.
4. The quantity of bagasse is expected to be increased by 1% as vacuum filter will not be in operation due to Mud recycling to diffuser that brings in savings in the form of bagacillo. Also, there will be no maintenance of vacuum filter and its accessories
5. Clarification efficiency will be improved due to percolation of juice through the bagasse blanket.
6. Manpower requirement for maintenance works during the offseason is less, when compared to Milling.
We have studied the operation of diffusers as well as the latest trends in milling technology and concluded that we favor Milling instead of diffuser due to the following reasons:
1. Extraction levels with diffuser will be better if the cane preparatory index is not less than 90% whereas in Mills a Preparatory Index of 85 % plus is sufficient.
2. To achieve these higher extraction levels, imbibition % fiber ( water addition on fiber in cane) should not be less than 300% that makes the evaporator sizes considerably bigger and steam% cane shall be not less than 35-38% whereas Mills could be operated at an imbibition of 240-250% resulting in smaller evaporator heating surfaces and lesser steam requirements (32-34% on cane). This reduced steam requirements in Milling enables us to generate more power and use it in-house or export to the state grid. (Note Steam%cane mentioned here in general conditions)
3. Consistency in maintaining the design parameters in a diffuser is a challenge, continuous and close monitoring of diffuser with respect to bed levels, juice spraying and maintenance of temperatures is required.
4. Flooding of juice in diffusers is a common phenomenon that disturbs the gradient of juice brixes across the diffuser resulting in poor extraction efficiencies.
5. Diffuser is more sensitive to cane feed fluctuations and often the percolation gets affected due to higher powder formation during cane preparation, carryover of soil, trash etc along with cane into the diffuser.
6. In case of any problem in the diffuser, crushing operations will come to a halt till it is set right whereas in a milling tandem, the particular mill experiencing a problem could be bypassed albeit achieving lower extraction levels.
7. Bagasse moisture is unstable; it may require two complete dewatering mills to achieve the same stability obtained with mill tandems. Comparing the cost of two additional mills in a milling tandem with that of a diffuser, the capital cost increase will be about 30-40%
8. Learning curve of diffuser plays an important role in stabilizing the diffuser operations and it takes a longer time to do so. The availability of quality personnel experienced in diffuser operations is difficult compared to the personnel experienced in Mill operations. This makes things easier and quicker for stabilizing the Milling operations.
Other points in favour of milling process in sugar plant are
1. Radiation and convection losses in diffuser, during bagasse de -watering process. For this a Press water heater is to be installed.
2. The extraction of colouring matter in juice will be increased due to higher extraction in diffuser.
3. Ash content in bagasse is comparatively more than that of mills due to Mud recycling to diffuser. This may have effect in the boiler in the form of erosion of boiler tubes.
4. The mixed juice treatment is more difficult when the purpose is to achieve high quality final sugar, because of high level colored contaminants and others.
5. Because of long retention time of cane in the diffuser, start-up and liquidation operations are prolonged.
6. Plant expansion is not possible in a diffuser unless some space is provided for addition in modules with some designs.
Conclusion of Mills and diffusers Technology
Advancements in Milling technology have increased extraction levels on a par with diffuser performance with the installation of lotus rollers for top and discharge, higher heel clearance in trash plates resulting in higher quantity of juice drainage, reduced reabsorption of Juice ultimately improving the extraction numbers.
J Amarnath – DGM Engineering (Nava Bharat Ventures Ltd)