Concept in Milling Tandem in sugar industry
Objective of milling Tandem
Maximum possible extraction of sucrose
Minimum extraction of non sugars.
Minimum sucrose in final bagasse
Optimum moisture in final bagasse
Optimum power/ energy consumption.
Thrust Areas For Efficient Milling
Cane management
Preparation of cane
Specific fibre loading
Hydraulic load
Proper setting of mills
primary extraction
Secondary extraction.
Imbibition
Mill sanitation.
Cane Management
Quality cane supply
1. Minimum extraneous matters and binding materials
2. Clean and fresh
3. Uniform supply of cane to avoid reduced crushing situation.
Preparation Of cane
Optimum cane preparatory Index as 85+
Long fibrous preparation
Avoid excess of cane preparatory devices
Avoid dusting of cane
Good cane preparation.
Improve Bulk density and higher number of juice open cells
Make easy to extract free juice
Improve mill extraction
Reduction in power consumption.
Improve feed ability to mills
Lower down mill opening
Lower mill speed
Lower hydraulic loading
Effective imbibition
Lower down reabsorption
Preparatory Index
Cane preparation is measured in terms of preparation index (P.I), which is the degree of finesse or percent of open cells, when cane prepared its density increases from 200 kg/m^{3} to 400 kg/m^{3}
The minimum value of preparation index 60% up to 90%,this value depends on the various devices used
Preparatory Index
Before Shredder – 68 to 70
After Shredder – 88 to 90
Bulk Density and preparatory Index
Relationship between bulk density and preparatory index at 15% fiber
B.D. (kg/m^{3}) |
P.I |
400 | 44.5 |
450 | 52.85 |
500 | 61.25 |
550 | 69.65 |
600 | 78.05 |
650 | 86.45 |
700 | 94.85 |
Primary Extraction
a) The greater the extraction of sugar in the first mill the less difficult in the task left to the following mills to recover more sugar by the trouble some step of wet extraction and better is the overall extraction of the tandem.
b) It is proved that a gain of 1% in primary extraction gives a gain in total extraction of 0.12% in 12 roller tandem, 0.10% in 15 roller tandem and 0.09% in 18 roller tandem.
c) It is therefore, necessary to observe first mill extraction carefully and report the figure in weekly chemical control reports.
d) A tandem of 3 roller unit should attain a minimum of 60% primary extraction.
e) It may reach easily to 70% and as high as 75 to 80% according to fibre% cane, preparatory index, feeding, mill setting, hydraulic press, proper juice drainage and optimum mill roller speed.
f) With a view to obtain high primary extraction, the first mill should be provided with Donnelly chute, under feed roller and a lotus roller.
STEPS TO IMPROVE PRIMARY EXTRACTION
1. First and 2^{nd} mill roller pitches are to be changed from 75 mm to 60 mm
2. Low mill setting
3. Mill speed to be reduced from 6.0 rpm to 4.5 rpm
4. Adding of pressure feeder
5. Improving PI.
6. Moisture control unit
**All above parameters impact on crushing rate.
Secondary Extraction
a) The mill extraction of the remaining mill should be in the range of 40 to 45%. However to get 50% is possible with strict supervision on operative parameter and roller lifts.
b) After initial setting of the mills and reached the normal crushing rate, the bagasse analysis of each mill with its discharge roller juice is to be done for finding the fiber % bagasse and mill extraction.
c) It will be necessary to do fine adjustments in operative parameter to get desired each mill extraction and whole plant extraction.
Roller Grooving
The improved cane preparation and increasing crushing, provision of high imbibition water have increased juice quantity in the mill. If this increased juice is not drained effectively, then it affects the mill extraction adversely and will demand extra hydraulic load increasing the power demand in the mill. The pressure feeding equipments, lotus rollers and differential are used in the mill to ideal with this increased juice. Out of these differential grooving is discussed in detail.
Expected functions from grooving are as under
a) To press and squeeze the prepared cane in 1st mill and bagasse in other mills for juice extraction.
b) To drain the juice effectively from mill.
c) To mill the bagasse further to get the uncut juice cells broken to help to increase the extraction in the following mill.
d) To drive the bagasse through the mill effectively.
The following types are used on rollers to get above functions.
- Circumferential “V” grooving.
- Messcheart grooving.
- Chevron grooving
SL.NO | Differential Grooving | Equal grooving |
1 | Efficient juice drainage through low pressure zone at feed roller | Un efficient juice drainage area |
2 | There is an effective shearing action in the bagasse layers in the groove due to different speeds of the tip and bottom of the groove,this is due to extra depth in the bottom roller grooves.This will effect in tearing of unbroken juice cells in the bagasse.This helps to improve the extraction of the following mill | Shearing action is less due to equal tip speed of top and bottom rollers |
3 | Different grooves from the cavity of trapedial cross section. Narrow section of trapezoid forms the wedge action which improve the bagasse pressure improving coefficient of friction. This arrangement not only improves the juice extraction but improves bagasse travel through the mill. | Due to equal grooving wide space of rollers. Juice Extraction is less |
Mill | Pitch |
1^{st} MILL | 75 |
2^{nd} MILL | 67 |
3^{rd} MILL | 50 |
4^{th} MILL | 50 |
5^{th} MILL | 37.5 |
6^{th} MILL | 37.5 |
For better performance and better extraction the mill pitches may be further reduced. But it will effect on crushing rate.
The mill speed can be recognized in two ways
a) As peripheral speed of the rollers i.e. the linear speed of a point at the mean diameter of roller. It is generally expressed in m/min.
b) As speed of rotation of rollers, i.e. the no. of revolutions which they make in unit time. It is generally expressed in R.P.M. Relation between two speeds
V = Pi x D x n
c) Lower the speed of the mill, the bagasse in the mill have more retention time for effective juice drainage.
d) Lower speed of roller improves co-efficient of friction and it will have better grip on bagasse reducing the slippage.
e) It not only increases extraction but also allows to adjust mill at higher fiber index i.e. lower openings without increasing the hydraulic load.
f) This results in improvement in extraction and reduction in reabsorption factor effecting in saving in the power requirement in the mill.
g) The cane preparation of higher cell opening with long fibers and installation of pressure feeders of better designs are essential facilities for the mills to run at lower speeds.
h) Fine cane preparation and effective juice drainage from bagasse in pressure feeder will allow to run the mill at lower speed resulting in reducing the Reabsorption and allows to close the mills. This will shift the neutral plane towards axial plane. This will improve extraction & reduce power in the mill.
Hydraulic Load
To be decided by Specific Hydraulic loading
SHP = F/ 0.1*L*D
Where F = Total load on top roller (Tons)
L = Length of roller (Mt.)
D = Dia. Of roller (Mt.)
1^{st} Mill | 2^{nd} mill | 3^{rd} mill | 4^{th} mill | 5^{th} mill | |
12 Roller | 240 | 220 | 250 | 275 | |
15 Roller | 240 | 220 | 230 | 250 | 275 |
Above values of SHP are in tons/ft^{2}
Nitrogen gas pressure in accumulator should be in the range of 70 to 90% of the hydraulic pressure.
Nitrogen gas pressure should never be higher than the oil pressure.
The s.h.p. of each mill is given as follows:
No.of Rollers | s.h.p. in tons / dm² | |||||
I | II | III | IV | V | VI | |
12 Rollers | 26 | 24 | 27 | 30 | — | — |
15 Rollers | 26 | 23 | 25 | 27 | 29 | — |
18 Rollers | 25 | 22 | 24 | 25 | 26 | 28 |
If preparatory Index is more than 85, then the specific hydraulic pressure will be reduced by 2 to 4 Ton/dm².
Factors Increases Reabsorption in Milling Tandem
Poor cane preparation
Higher surface speed
Higher hydraulic loading than required
Inefficient juice drainage
Roller polishing
Increase in pressure
Setting Of Mills
Relative positioning of three rollers(Top, Feed & Discharge)
Trash plate properly drawn and positioned
Setting of feeding devices
Proper setting of scrapers knives.
Mill settings
Discharge opening (operating)
Dwo=W x 1000d x 3.14 x D x n x L x 60
Where D= Diameter of mill roller
n= Mill rpm
L= Mill roller length
d= Bagasse density
Reabsorption correction as under Dwo / RF
OPENING OF DONNELLY CHUTE SETTING
The ratio of donelly chute to fourth roller opening should be 2.5 for 1st mill to 2 for last mill.
Donelly chute opening =2.5*Fourth roller opening =2.5*160 =400 mm for 1st mill and 320 mm for last
GRPF/TRPF SETTING
GRPF SETTING
- Speed ratio (A) 1.2 to 1.4
- Volumetric ratio (B) 1.3 to 1.5
TRPF SETTING
- Speed ratio (A) 1.0 to 1.2
- Volumetric ratio (B) 2.0 to 3.0
Pressure Chute Settings
- Normal value of inlet pressure chute angle is 21 degree
- Normal value of out let angle of pressure chute is 45 degree
- Divergence angle of pressure angle vary from 5 to 7 degree
- Pressure chute should be symmetrical to the axis joining midpoint of PF opening and mill feed opening.
Imbibition
Imbibition and juice maceration plays important role in achieving better mill extraction. Imbibition water dilutes the juice in bagasse and reduces it viscosity. The characteristics of low viscosity of juice increases the flow of juice through bagasse. The hot water further helps in reducing the viscosity of the juice. However higher temperature of water sometime leads to slippage of bagasse in the mill. The higher imbibition water is restricted due to capacity of boiling house and availability of steam and therefore it restrains to increase crushing rate.
It may please be noted that higher imbibition water do not increase the moisture in the bagasse.
But inadequate juice drainage, low compression, higher speed, improper cane preparation are the main reasons for increase in moisture of bagasse.
Advantages with hot water imbibition
- Optimum temperature of hot water 70^{o}C.
- Power to pressure the fibre will be less (60^{o} to 65^{o}C)
- Efficiency of imbibition increases these by extraction increases.
- The mixed juice temperature also increases by 10^{o} to 15^{o}C these by the exhaust requirement reduce by about 2 to 2.5%.
- Bagasse temperatures also increase to about 45^{o} to 50^{o}C.
- As it travels upto the boiler about 1% of moisture will be evaporated.
- These by reduce the moisture 1% by 1% and GCV increased by 45 – 70 Kcal/Kg.
Disadvantages with hot water imbibition:
- More wax soluble in juice, which gives problem in clarification or filtration.
- Mill capacity slightly reduces because of roller polishing i.e. gripping action is reduced. (if pro-feeders are then this problem can be reduced).
- Last mill bearing temperature increases.
- Range of imbibition efficiency is around 60 to 70% .
NORMAL MILL PRACTICES
- Not to monitor lift of top roller
- It is assumed that top roller lifts only that value considered during mill setting calculations.
- Normally actual lift of top roller during operation is higher considered in mill setting calculation.
- Higher thickness of cane/bagasse blanket passes through mills due to excessive lift of top roller. This leads to poor extraction of sucrose and results in higher sugar loss in final bagasse.
- Hydraulic load applies on top roller simply by assumption/experience.
- It is seldom to consider the lift of top roller while deciding hydraulic pressure.
To compromise with mill performance due constrain
Performance Of Individual Mills
- Plotting of Brix curve for feed and discharge side
- To monitor lift of top roller.
- Analysis of bagasse leaving the mills for free pol and total pol.
- Measurement of temperature of juice on feed and discharge.
- Measurement pol % bagasse leaving a mill and juice from back roller of same mill
All Performance Of Mill
- Pol percent final bagasse
- Brix of last expressed juice
- Primary extraction (PE)
- Reduced mill extraction (RME)
Maintenance For Quality production
- Aim of maintenance is to keep down.
- Aim of maintenance is to keep downtime minimum
- To achieve time minimum and to achieve desired capacity
- utilization with desired capacity utilization with efficiency and quality productivity efficiency and quality productivity
CONCLUSION:
TO ACHIEVE FURTHER BETTER RESULTS.
- To run mills at low speed. for improve mill efficiency like pol% & moisture % bagasse
- To install pressure feeders ,facilitates to run at low speeds.
- To replace transmission gears with planetary gears for reduction of power consumption and increase of mill crushing capacity .
Summery
Author : P.Sita Rama Rao G.M(Engg), KCP SUGAR & INDUSTRIES CORPORATION LIMITED
Short Description : Concepts of milling tandem like Preparatory Index, Primary and secondary extractions, roller grooving, hydraulic load , mill settings, imbibition etc in sugar industry
Publisher – Orgnization : www.sugarprocesstech.com.
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