ENERGY SAVING

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ENERGY SAVING COST REDUCTION IN STOCK
PREPARATION
Dr. Y. V. Sood Scientist F Head Stock
Preparation Paper Making Conversion Division
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ENERGY SAVING IN THE STOCK PREPARATION

• Ever increasing price of energy has become a
  serious concern for Paper mills and every
  possibility of saving it is their main approach
  for profitability.
• In stock preparation the main step involved is
  refining.
• Despite the small size of papermaking fibres,
  less than 1 cm long and 100 micron diameter at
  the upper limit, equipment with plates exceeding
  one meter in diameter and motor rated in excess
  of 500 kW are used for their modification.
• Despite decades of study, debates are still
  active on the efficiency of the refining process
  and modification by which it transforms fibres.

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EFFECT ON FIBRES ON REFINING

• 1. Fibre delamination
• Forces on the fibre in the refiner cause fibrils
  to move relative to each other, breaking internal
  bonds and thus causing fibre delamination. This
  has been clearly observed with the scanning
  electron microscope. Fibre delamination
  contributes to other changes in the fibre such as
  swelling and increased flexibility.
•  
• Fibre swelling
• Delamination of the fibres allows the entry of
  water into the fibre walls, causing the fibres to
  swell. This water breaks additional hydrogen
  bonds creating further swelling. Water Retention
  Value has been shown to increase with refining,
  proving that there is an increasing amount of
  water bound to the fibre as the level of refining
  increases. 

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EFFECT ON FIBRES ON REFINING

• Increased fibre flexibility
• Breaking of internal bonds allows fibrils to
  move relative to each other, making the fibres
  less resistant to deformation i.e. more flexible.
  This increased flexibility enables surface
  tension forces to bring more fibres into close
  proximity during consolidation of the web. This,
  in turn, increases the level of fibre / fibre
  bonding and thus influences paper strength,
  optical properties and other paper
  characteristics.
• Removal of outer layers
• The thin primary wall (or what remains of it
  following pulping and bleaching ) and part of the
  S1 layer can be removed by refining, exposing a
  new surface and allowing an increase in fibre
  swelling. The removed layers increase the fines
  level in the pulp and thus affect paper
  properties. 

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EFFECT ON FIBRES ON REFINING

• Microcreping of fibres and the introduction of
  other defects
• These both affect the mechanical properties of
  the fibres. Micro creping produces a more
  extensible fibre and defects such as kinks and
  local ballooning of the fibre can cause a
  reduction in the local fibre strength. 
• 6. Curling and Twisting of fibres
• Fibre twisting and curling influence the network
  structure of paper and thus affect its
  properties.
• 7. Increased specific surface
• The production of fine material with the removal
  of outer layers increases the specific surface.
  This is further increased by fibrillation i.e.
  loosening of the surface fibrils.
• 8. Fiber length reduction
• This occurs with harsher beating. It has been
  used to reduce the flocculating tendencies of
  long fibre pulp but at the expense of tearing
  strength.

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GENERAL EFFECTS ON SHEET PROPERTIES ON REFINING

• Drainage resistance (water removal resistance )
  increases.
• Tensile strength, tensile stiffness, burst
  strength, internal bonding strength, and fracture
  toughness increases.
• Tear strength of softwood fibres might slightly
  improve at first, but then decreases, whereas
  that of hardwood fibres at first significantly
  increases but then decreases after prolonged
  refining.
• Air permeability, bulk, absorbency, opacity and
  light scattering decreases .
• Brightness slightly decreases
• Fibre strength is little affected in refining .

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Main types of refiners being used in Paper mills

• Conical refiners- Jordan, wide angle, shallow
  angle
• Disc refiners

Operating parameters for the different refiners
Parameter Type of refiner Type of refiner Type of refiner
Parameter Jordan Wide angle Shallow angle
Motor power (kW) 263 250 200
No-load power (kW) 132 85 32
Rotational speed (rpm) 419 360 740
Efficiency () 50 66 82
Cutting edge (km/s) 30-80 23-44 30-150
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CONFIGURATION OF DIFFERENT DISC REFINERS
Single Disc Refiner
Mono-Flow
Duo-Flow
Double Floating Disc Refiner
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DISC REFINERS

• The energy required to refine pulp with a disc
  refiner is only about 75 of that of a conical
  refiner.
• Disc refiners have a pumping action on pulp as
  the position of intersection of rotor and stator
  bars moves outwards as the rotor rotates.
• Pumping is more as the angle the bar makes with
  the radial direction is increased and the plates
  are new I.e., groove width is grater
• The pumping by refiner is not a problem if
  refiners discharge to atmosphere, but modern
  refiners discharge into a pipe. The pumping thus
  causes an increase in pressure. Pressure on
  outlet side of refiner are typically 300 to 500
  kPa. A maximum 700 kPa is recommended.
• To avoid excessive energy consumption the pumping
  pressure in the refining line should be
  controlled.

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VARIABLES NORMALLY CONSIDERED FOR REFINING

• Net specific energy
• Specific edge load
• Net specific energy is defined as
• Net specific energy (Ne)
• The specific edge load describes the intensity of
  refining which is defined as
• Specific edge load (Bs) Ne/aLn1n2
• Where a refiner speed (rev/s)
• L length of one bar (m)
• n1 number of rotor bars
• n2 number of stator bars
• Specific edge load tends to range from 0 to 4
  Ws/m with a lower specific load providing a
  gentle action.

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RECOMMENDED VALUES OF SPECIFIC EDGE LOAD FOR
VARIOUS PULPS
Pulp type Recommended specific edge load
Unbleached softwood kraft 2.5 to 4.2
Bleached softwood kraft 2.0 to 3.2
Unbleached softwood sulphite 0.8 to 1.8
Bleached softwood sulphite 0.9 to 2.2
Hardwood kraft and sulphite 0.6 to 1.9
Recycled pulp (wood) 1.0 to 2.7
High yield kraft 1.1 to 3.0
High yield sulphite 0.9 to 1.4
NSSC 0.8 to 1.3
Chemi-mechanical pulp 0.2 to 1.2
Mechanical pulp 0.2 to 1.0
Bamboo 2.0 to 2.5
Straws 0.5
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REFINER PLATE DESIGN

• Refiner plates configuration plays an important
  role in refining efficiency. Plate variables
  include bar pattern, bar width, groove width,
  groove depth, bar angle, sharpness of the bar
  edge and plate/bar material.
• For strength development narrow bars are required
  for short fibred pulp than softwood pulp.
• Grove width influences plate life, refining
  treatment and pumping effect of the refiner.
• As the bar angle increases the bar length
  increases, increasing the time fibres spend in
  the refining zone hence more refining. On the
  other hand pumping effect of refiner increase as
  the bar angle increase.
• While treating short fibres like eucalypt pulp to
  produce high strength products like kraft paper
  considerable refining power is needed. The
  combination of refining power and eucalypt pulp
  produce plate clashing which wears the plate.
  Proper choice of plate material is needed in such
  case.

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SELECTING THE REFINERS
Refining actions required Type of refiners needed
Fibre shortening, cutting High specific edge load, narrow bar angle
Fibrillation, opening of fiber structure Low specific edge load, wide bar angle
Fiberizing, disintegration of fiber bundles or lumps No load, high speed, high fibre to fibre friction, turbulence. Deflakers work on this principle
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COMMONLY USED DEWATERING AND DRAINAGE AID
CHEMICALS IN PAPERMAKING
Charge Neutralizing Salt Alum, PAC Cationic
Polyelectrolytes Cationic polyacrylamide,
Polyethylemine, Cationic starch, Cationic guar
gum, Polyamidoamine Enzymes
Cellulases, Hemicellulases Anionic
Microparticles Colloidal silica, Sodium
montmorillonite or Bentonite Another
possibility of reducing cost and energy saving is
increasing the filler content in the paper.
Fillers are normally 2 to 3 times cheaper than
cellulose fibres. Proper paper machine lay out ,
its configuration and automation are also the
factors that control energy consumption.
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• CONCLUDING REMARKS
• In designing a refining system the range of
  specific energy and flow rate that will be
  required for the paper machine should be
  determined carefully.
• Refiners should be selected to provide the
  required refining action and throughput. In
  choosing refiners their energy efficiency should
  be taken into account. On the basis of energy
  efficiency the conical refiners grading is
  Jordanlt wide anglelt shallow angle.
• The double disc refiners offer greater efficiency
  than standard conical refiners. These refiners
  also offer considerable degree of flexibility.
  Blocking or unblocking the passage through the
  rotor a disc can change refiner from duo flow to
  mono flow and vice versa. This can be necessary
  where type of fibre usage changes.

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CONCLUDING REMARKS

• A large refiner will provide a large maximum
  throughput but also a high idling load if the
  throughput requirements are not very high. Two
  smaller refiners in parallel may have high idling
  load.
• In refiners pressure should not be allowed to
  build up excessively. Sometimes it is necessary
  to place a chest in the refiner line where number
  refiners are in series. Pressure on outlet side
  of refiner is typically 300 to 500 kPa. A maximum
  700 kPa is recommended.
• Wet end of paper machine should be carefully
  optimized to get better retention and drainage of
  the furnish. Micro particle retention system
  mostly helps in achieving this. Better drainage
  helps in saving energy during drying on the paper
  machine.

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Thanks