Refining Soft Models

METHOD TO VISUALIZE & OPTIMIZE THE PULP

REFINING MECHANISM

Why do we need Soft Models?

Mathematical models are so complex with so many assumptions needed that practical application to operating refiners is limited.

Actual refining zone measurements of pressure, temperature, plate bar forces along with high speed photography of steam & fiber flows in plates & pulp samples extracted from multiple radial locations in the plates give added insight into the refining mechanisms, but math models based upon this data are still extremely complex which limits practical use.

A less complex and more practical modelling method is needed to allow a broader range of individuals like refiner operators and non-mathematically inclined persons to visualize what is happening in the refining zone.

My High Consistency

Single Disc Soft Model

Three Distinct Regions of the Single

Disc Conceptualization Model

Rotor

Stator

Plate Gap

Rotor Region

Fiber movement dominated by centrifugal force due to

rotation.

Rotor plate less filled & plate bars less covered with fiber

than stator where no rotation & centrifugal force exists.

Can result in more rapid rotor plate wear as bar tops more

exposed to fibers sliding across metal.

Wear differences more obvious at lower plate gaps associated

with lower energy plate concepts and refining processes.

Plate gap changes due to refining consistency changes

dominated by changes in rotor plate fiber filling, as fiber

mass variation changes centrifugal forces, varying fiber

retention time.

Pulp Displacement

Rotor

Stator

Rotor feeds forward due to centrifugal force.

The steam flowing back in the stator grooves will

carry the pulp, and make it move backwards

towards the plate inlet. This results in a large

accumulation of pulp in the inner part of the plate

(thick pulp pad).

In the outer zone, the forward flow of steam carries the pulp forward and out of the refining zone.

Stator Region

Without rotation, fiber flow dominated by steam flow conveying

forces.

Fiber surface area affects ease of conveying.

Fiber mass(consistency) also affects ease of conveying but much

less than rotor centrifugal forces.

Temperature & pressure measurements indicate a steam pressure

peak exists at some radial location in the refining zone.

Outboard of peak, steam & fiber flow forward

Inboard of peak steam & fiber flow back to the inlet

Rotor must feed production rate + fiber returning in stator

Results in higher inlet zone filling; can completely fill inlet and feeding screw.

Temperature and pressure profile

110

120

130

140

150

160

170

180

190

Primary

Secondary

Tem

pe

ratu

re °

C

Note : More coarse chips & fiber bundles absorb energy even at larger gaps @ plate inlet

Stator Region, cont’d

Publications have shown that the differential between

refiner inlet and outlet steam pressures impacts the

retention time of fiber in the refining zone and thus the

plate gap.

Higher outlet than inlet pressure(pressure boost) increase

fiber retention & plate gap.

Lower outlet than inlet pressures decrease fiber retention &

gap.

I would suggest that stator plate fill rate(fiber retention)

changes dominate the differential pressure gap changes, as

stator filling is dominated by steam flow conveying effects.

Plate Gap Region – Fiber trapped

between rotor & stator plate bar tops.

Conveying influenced by rotor and stator bar crossing

angle(sliding direction) & stator frictional forces.

Bi-directional plates evenly alternate inward and outward

sliding direction.

Directional feeding plates increase the portion of outward

vs inward sliding forces.

Directional holding plates do the opposite.

Typically need feeding inboard of pressure peak to reduce

inlet filling, and holding outboard of peak to retain fiber &

increase filling.

Directional Refiner Plate

Feed

Hold

Plate Gap Region, cont’d

Higher stator bar frictional forces tend to hold fiber,

promoting sliding of fiber across the rotor bar tops,

increasing rotor bar edge rounding & wear.

Fiber tends to slide on bar surface with the lowest frictional

forces.

Bar angle and bar edge condition influence holding forces on

fibers.

Newly manufactured plates exhibit break-in period with

higher plate gaps and higher energy consumption.

I would suggest effect caused by the grinding burr on bar edge

giving increased roughness and holding = larger plate gap

Once burr worn away reach normal holding level until

excessive bar edge rounding begins to reduce plate gap.

My Counter-rotating

Refiner Soft Model

Counter-rotating vs Single Disc

2 Rotors & no Stator

Must feed through holes in 1 disc(Feed End)

Feeding challenge w/chips, even more issues with fiber

Use added dilution water to help feed(lower refining consistency)

Use steam to help feed(discharge pressure lower than inlet)

Add steam to inlet to heat chips(not self pressurized)

Steam flow effects upon fiber flow more limited, centrifugal forces dominate feeding

At 60 hz(1200 rpm) should be lower intensity than single disc as higher retention time & more bar crossings(2400 vs 1800)

Lower refining consistencies & plate designs without surface dams, both feeding requirements, actually make intensity higher

Non-feeding disc adjusts axially to control gap & load(Control End)

Counter-rotating vs Single Disc

Although a pressure peak & steam seal may still be formed

between the plates, backflowing steam must be limited & the

seal ring behind feed disc allows steam flow around disc.

Limited steam pressure boost possible for retention time

increase

Pressure boost would cause more steam flowing back to the inlet

across seal ring

Plate Gap Fiber in the plate gap stops as it is not rotating with either

disc & dragging forces cancel out

Limits how large gap can be before feeding stops(Pinch-off)

Reason for sub-surface not surface dams @1200 rpm?

Keep fiber in Feed End disc in the inlet area to assure feeding.

No bars on Control End opposite feed openings to prevent stoppage

of Feed Disc rotational motion to maintain feeding

Cavitation(high pressure collapse) on any Control End bars

opposite the feed openings in the Feed disc

My Low

Consistency

Refining Soft Model

Model Similarities – Lo-Co vs Hi-Co

3 Zones – Rotor, Stator & Gap

Shear & Compression forces on fibers

Plate Gap impact upon energy transfer &

efficiency

Fiber retention time impacts probability of

treatment

Stator fiber recirculation

Model Differences – Lo-co vs Hi-co

Fiber retention function of flowrate & internal

recirculation, not centrifugal force

Fiber holding affect of surface dam only at dam, not

region before dam

Dams increase groove pressure drop significantly thus

reducing flow capacity

Inlet & outlet pressures function of flow, feed pump

operation, plate geometry, refiner rotational speed

and plate diameter, not independently adjustable

Lo-co Fundamentals

Rotor plate increases pressure from inlet to outlet similar to a pump impeller

Grooves in rotor and stator cause pressure drop as a series of small pipes in

parallel

Pressure increase from inlet to outlet is the net result of rotor plate pumping

less groove pressure drop

As plate gap closes, initially pumping efficiency increases as pump impeller

clearance decreases thus more pressure increase

Further gap closing can decrease pressure increase as less gap & more

groove flow increasing pressure drop

Larger refining gaps give a higher proportion of shear forces, promoting

more external fibrillation – potentially higher energy to a given pulp quality

Smaller refining gaps give a higher proportion of compressive forces,

promoting more internal fibrillation(fiber splitting) – potentially lower energy

Limit in reducing gap is fiber cutting(length reduction)

Lo-co Fundamentals, cont’d

Lower plate bar height with wear reduces rotor pressure

increase & increases groove pressure drop thus reducing

pressure increase across refiner & potentially causes a

pressure drop from inlet to outlet

As long as discharge flow control valve is < 100% open,

flow control is still possible

If valve 100% open, refiner plates are the flow control

valve

Rotor Region

Strongest groove vortex to load fibers onto bar edge

Potential for forward flow regime below vortex

Influenced by fiber type, consistency, groove geometry

Refiner plate geometry dominates pumping

characteristics:

Bar angle, & height

Plate diameter

Presence of dams, surface(full bar height) or sub-

surface(< full bar height)

Tracing of fluid particle at the top and

bottom of the rotor groove.

Reference: Numerical simulation of the flow in a disc refiner,

Gohar.M.Khokhar, Master’s Thesis 2011

Outlet

Stator Region

Weaker vortex rotationally but less vortex pitch radially vs rotor

Internal recirculation due to higher discharge pressure than inlet displaces main flow volume

Unrefined fiber velocity increases/retention time decreases

Groove flow below vortex, if present, radially inward if typical higher discharge than inlet pressure

Groove volume influences pressure drop not pumping

Fluid particle tracing at the top and bottom of

stator groove going in the negative Y-direction

Reference: Numerical simulation of the flow in a disc refiner,

Gohar.M.Khokhar, Master’s Thesis 2011

Outlet

Plate Gap Region

Fiber flocs compressed & locally de-watered

Compressive and shear forces applied to flocs

Groove vortices bring flocs to gap region for possible

trapping

Flocs trapped then released

Plate bar geometry(width and crossing angle) impact

treatment probability & severity as well as closing force

Bar surface sliding on fiber wears while floc holding

surface is protected from wear

Lo-Co Refining Evaluation & Optimization

Energy consumption comparison must include feed pump energy

Less refiner no-load(pumping) but more pump energy reduces savings

Feed pump VFD needs to be part of any energy reduction plan

Replacing multiple refiners with a single refiner reduces no-load energy

even with the same refiner concept

Therefore the true energy reduction of any new refiner concept must be

single refiner vs single refiner

Refiner filling design determines actual forces on fibers, therefore filling

selection for optimization will impact long term peak performance.

Single source of fillings can limit long term optimization

Any increased filling cost per ton produced needs to be deducted from

energy cost savings

Filling supply competition can speed optimization & reduce filling cost