Juice Extraction Performance in Milling Tandem in sugar cane industry

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

• The optimum cane preparatory Index is 85+
• Long fibrous preparation
• Avoid excess 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/m³ to 400 kg/m³

The minimum value of the preparation index is 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/m3)	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 the task left to the following mills to recover more sugar by the trouble of some step of wet extraction and better 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 obtaining high primary extraction, the first mill should be provided with a 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 the 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, getting 50% is possible with strict supervision on operative parameters 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 to find the fiber % bagasse and mill extraction.
• c) It will be necessary to make fine adjustments in operative parameters 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 equipment, 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	Unefficient 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 the extra depth in the bottom roller grooves. This will affect 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 the equal tip speed of top and bottom rollers
3	Different grooves from the cavity of trapedial cross section. A narrow section of trapezoid forms the wedge action which improves the bagasse pressure improving the coefficient of friction. This arrangement not only improves the juice extraction but also improves bagasse travel through the mill.	Due to equal grooving wide space of rollers. Juice Extraction  is less

 

Mill	Pitch
1st MILL	75
2nd MILL	67
3rd MILL	50
4th MILL	50
5th MILL	37.5
6th 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 has more retention time for effective juice drainage.
• d) Lower speed of the roller improves co-efficient of friction and it will have a better grip on bagasse reducing the slippage.
• e) It not only increases extraction but also allows to adjust mill at a 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 openings 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.)

	1st Mill	2nd mill	3rd mill	4th mill	5th mill
12 Roller	240	220	250	275
15 Roller	240	220	230	250	275

Above values of SHP are in tons/ft²

Nitrogen gas pressure in the 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 the preparatory Index is more than 85, then the specific hydraulic pressure will be reduced by 2 to 4 Ton/dm².

Factors Increase 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)
• The trash plate is properly drawn and positioned
• Setting of feeding devices
• Proper setting of scrapers and 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 the fourth roller opening should be 2.5 for 1st mill to 2 for the 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

1. 1. Speed ratio (A) 1.2 to 1.4
   2. Volumetric ratio (B) 1.3 to 1.5

TRPF SETTING

1. 1. Speed ratio (A) 1.0 to 1.2
   2. Volumetric ratio (B) 2.0 to 3.0

Pressure Chute Settings

• Normal value of the inlet pressure chute angle is 21 degree
• The normal value of outlet angle of the pressure chute is 45 degrees
• The divergence angle of pressure angle varies from 5 to 7 degrees
• The pressure chute should be symmetrical to the axis joining midpoint of the PF opening and mill feed opening.

Imbibition

Imbibition and juice maceration play important roles in achieving better mill extraction. Imbibition water dilutes the juice in bagasse and reduces it viscosity. The characteristics of low viscosity of juice increase the flow of juice through bagasse. The hot water further helps in reducing the viscosity of the juice. However, higher temperature of water sometimes leads to slippage of bagasse in the mill. The higher imbibition water is restricted due to the capacity of boiling house and the availability of steam and therefore it restrains from increasing the crushing rate.

It may please be noted that higher imbibition water do not increase the moisture in the bagasse.

However inadequate juice drainage, low compression, higher speed, and improper cane preparation are the main reasons for the increase in moisture of bagasse.

Advantages of hot water imbibition

• The optimum temperature of hot water 70^oC.
• The power to pressure the fibre will be less (60^o to 65^oC)
• The efficiency of imbibition increases by extraction increases.
• The mixed juice temperature also increases by 10^o to 15^oC these by the exhaust requirement reduce by about 2 to 2.5%.
• Bagasse temperatures also increase to about 45^o to 50^oC.
• As it travels up to the boiler about 1% of moisture will be evaporated.
• These reduce the moisture 1% by 1% and GCV increased by 45 – 70 Kcal/Kg.

Disadvantages of hot water imbibition:

• More wax soluble in juice, which gives problems 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.
• The 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 the 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

• The aim of maintenance is to keep down.
• The aim of maintenance is to keep downtime minimum
• To achieve the time minimum and 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 .

Summary

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 the sugar industry

Publisher – Organization: www.sugarprocesstech.com.

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