Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD FOR CONTROLLING A PULPING PROCESS
TECHNICAL FIELD
The present invention relates to controlling a pulping process. Particularly,
the invention
=5 relates to a method, wherein the size and shape of chip particles are
measured prior to
cooking, and shape factors calculated from the measured results are used for
calculating
the degree of packing and for controlling the process variables, such as
liquid flows and
dosage of chemicals.
TECHNICAL BACKGROUND
Wood chips are used as raiv material in the pulping process. The quality of
chips varies
due to variation in its origin. Factors influencing the chip quality are the
size and the age of
the wood, the structure of the chipper and the condition of the chipper
lcnives as well as the
structure and location of chip screening in the process sequence. Especially
in mills where
chips are not produced internally, but purchased from various sources, the
variation is es-
pecially strong. In some mills, wind conditions during outdoor storage of the
chips may
cause variations in the size of the chip pieces to be fed into a digester.
Chips of various
sizes are carried by the wind to different places during discharge of the
chips to the outdoor
storage and during the storage. This phenomenon is called air classification.
In pulp mills, the quality of the chips is controlled by random sampling. In
screening tests
according to a SCAN or TAPPI standard, a chip sample is screened by means of a
classi-
fier consisting of several screens of different size, and the chips remaining
on each screen
are weighed. The test may be carried out separately in wood handling to
monitor the per-
formance of chipping, and in a cooking plant to control the quality of the
supplied chips.
Fig. 1 shows an embodiment of a continuous pulping process in a simplified
form. Chips I
are transported by a conveyor to a chip bin 2. In bin 2, the chips are
steanled to heat them
and to remove air from the chips. The steamed chips are fed from the chip bin
2 to a chip
meter 3. The chip meter 3 is a rotatable compartiment feeder, the rotational
speed of which
is used to control the amount of chips to be fed into a digester and the
output of the di-
gester. From the chip meter the chips are led to a chip chute 4. From the chip
chute 4 the
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cliips are fed with a liquor circulation 6 into a high pressure feeder 5. The
high pressure
feeder comprises a rotatable rotor and one or more compartments 7 extending
through the
rotor. The compartment 7 is filled with chips when being in a vertical
position and com-
municating with the chip chute 4 and the low pressure liquor circulation 6. In
its horizontal
position, the coinpartment 7 communicates with a high pressure circulation 8.
With the
high pressure circulation 8 the chips are fed to a separator 9 disposed at the
top of the di-
gester 10. In the separator 9 the chips are separated from the transfer
liquid, which returns
to the compartment feeder 5 via a return pipe of the feed circulation S.
In the upper part of the digester 10, an impregnation zone 11 is aiTanged
wherein a cooking
chemical is impregnated into the chips. Below the iinpregnation zone 11 there
is a cooking
zone 12, wherein the actual cooking reaction takes place. In the digester
washing zone 13
the cooked pulp is washed. The cooked pulp 14 is discharged from the bottom of
the di-
gester.
White liquor required for the cook is added to the cllips in the high pressure
circulation 8.
At the beginning of the impregnation zone 11, the chips charged to the
digester form a chip
column wliich moves downwards in the digester. The impregnation zone 11
comprises an
impregnation circulation 15. The liquid circulating in the iinpregnation
circulation 15 is
discharged from the digester through a screen 16 and returned to the top of
the impregna-
tion zone 11. In the impregnation zone, as shown by arrows 17, free liquid
flows down-
wards in the chip column at a higher speed than the chip column itself. The
flow passing
through the chip column applies a force pressing the chip colunui downwards.
At the bottom of the impregnation zone 11, a heating circulation 18 is
arranged, by means
of which the temperature of the chip column and the liquid present therein are
elevated to
the temperature of the cooking zone. The liquid circulating in the cooking
circulation is
discharged from the digester through a screen 19 in the digester periphery,
and is returned
to the centre of the digester via a central pipe 20. The circulating liquid is
heated with
steani in a heat exchanger 21. In the cooking zone 12 the heated chips and the
liquid are
flowing downwards for a time required for the cooking reactions.
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Wash liquid 22 is led to the bottom of the digester and it flows upwards in
the washing
zone 13 of the digester through the chip coluinn as shown by arrows 23. The
mixture 24 of
the liquid from the coolcing zone and the wash liquid 22 is discharged from
the digester
through a screen 25. The cooked pulp 14 is discharged from the bottom of the
digester. At
the bottom of the washing zone 13, a brealcing circulation 24a is arranged. In
the brealcing
circulation 24a, the liquid is discharged from the digester through a screen
25a and is re-
turned via a pipe 26. The liquid flowing upwards in the washing zone 13 exerts
an upward
force on the chip column, which force iinpairs the downward movement of the
chip col-
unm.
In continuous digesters, the wood chips forin a coluunn flowing continuously
from top to
bottom. The mechanical properties of the chips will change during the progress
of the pro-
cess as the chips pass through the digester. As ligniii and carbohydrates
dissolve, the struc-
ture of the chips wealcens. The chips maintain, however, their shape up to the
end of the
cooking. The chip column is slightly compacted as the cook proceeds.
In batch cooking, a digester is first filled with chips. In connection with
the filling, steam is
fed to the chips to heat them and to improve paclcing. Iinpregnation liquor
and cookulg
liquor are fed into the digester filled with chips. The temperature of the
digester is elevated
to the cooking temperature by circulating the liquor in the digester tlirough
a heat ex-
changer. While circulating through the chip column, the liquor elevates the
temperature of
the whole chip column and transports the cooking chemical uniformly throughout
the chip
colutnn. In batch cooking, the chips maintain their shape during the whole
coolcing phase
and decompose to fibers only when the cooked pulp is discharged from the
digester. As the
cook proceeds, the chip column will be compacted and its surface will sinlc.
In batch cooking of the displacement type, chips are treated in several stages
with different
liquids. The liquid changeover is carried out by feeding new liquid into the
digester as a
uniform flow from one end so as to push the previous liquid out of the
digester through
screens disposed at the opposite end of the digester.
RECTtFtE SHEET (RULE 91)
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In wood handling and prior to cooking, the bulk density is used as a measure
for the chips.
The bulk density indicates the weight of the aniount of dry chips in a unit
voluine. The
bulk density depends on the wood species used, its properties and the size and
the shape of
the chip particles. Tlie density of the chip column in the digester is
measured by means of
its porosity s. The porosity indicates the proportion of free space between
the chip pieces
in the volume of the whole chip bed.
The variation in chip quality results in variation in the pulp quality as well
as problems in
the operation of the digester. In continuous cooking, the amount of the cliips
fed into the
digester is controlled by changing the rotation speed of a chip meter. The
chip meter is a
rotatable conipartment feeder in which the voh.une of the compartments is
lcnown. The chip
bulk density, i.e. the weight of dry wood in the chips per unit volume varies
depending on
the chip quality. This results in inaccuracy when measuring the wood dosage.
The control of a continuous digester talces place by feedback control so that
the process
values in the digester are adjusted upon measuring the quality of the pulp
produced. The
residence time of the pulp in the digester is several hours, and thus there is
a delay before a
coiTective control action has an impact on the pulp quality.
In the publication WO 94/20671 is described a method for measuring the bulk
density of
the chips to be fed into a digester from samples taken from the chip flow
supplied to the
digester. The bulk density is determined by measuring the size of each chip
particle of the
sample and calculating the bulk density of the sample from these.
Methods and devices for measuring the chip size by various optical methods
have been
disclosed in patent publications US 6,606,405, US 5,818,594, WO 91/05983, WO
91/05984 and Fl 84761.
The flow rates of radial liquor circulations in a continuous digester are
controlled accord-
ing to the digester output, i.e. the aim is to keep constant the ratio of the
circulation flow
rate to the output. Reduction in chip quality leads to circulation screen
clogging, which is a
result of the target for the flow rate through the chip column being too high
for the chip
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quality in question. The clogging of the circulation screen results in reduced
quality and
yield losses. The liquid-wood-ratio in the digester is also kept constant, the
aim being to
maintain the relative flow rates of the chip column and the free liquid in the
initial down-
stream zone constant in order to keep constant also the dynamic forces
affecting the pack-
5 ing of the chip column. Because these dynamic forces depend on the porosity
of the chip
column, the chip quality, which is assuined to be constant, very rarely
achieves an optimal
situation in the downstream sections of a digester, especially when using
heavy wood spe-
cies (such as birch), which have a tendency to get excessively packed by niere
gravity ef-
fects.
It is desirable to control the wash liquid added to the bottom of the digester
and flowing
against the descending chip column in accordance with the wash factor target.
The consis-
tency of the digester blowoff may be adjusted within a certain range by means
of the rota-
tional speed of the bottom scraper and the wash liquid passing through
vertical and hori-
zontal nozzles at the digester bottom. If the bottom consistency is not
sufficient to be ad-
justed, the wash factor has to be reduced to allow the cliip column to
descend. This control
is generally carried out by slow feedback, wherefore the action talcen may be
even several
hours late to achieve the optimal _result, because changes in the packing of
the chip column
and its flow resistance are slow and also cumulative, i.e. a delayed
correcting action must
be oversized compared to one carried out at the right moment.
Conditions for a successful and economical cook are a correct dosage of
cooking chemi-
cals, correct concentrations of impregnation and cooking liquor, accurate
adjustment of the
residence time and the temperature of the cooking process and accurate
adjustment of the
flows within the chip cohunn in relation to the flow properties of the chip
column. In addi-
tion to the impregnation duration, also chip size, and especially chip
thickness, influences
the optimal concentration of the impregnation liquor, because impregnation
proceeds con-
siderably faster into a small and thin chip than into a large and thick one.
If there is, for
instance, a wide chip size distribution in the chip flow, an increased alkali
dosage (a higher
impregnation liquor concentration) is required to ensure successful
impregnation of thick
chips in order to prevent the reject content from growing too high in the
cooked pulp (as-
suming constant cooking time and cooking temperature).
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Too high a flow and a high pressure loss result in channelling of the flow. In
channelling,
the flow breaches the chip column, forining one or more passages.
Consequently, a chemi-
cal or heat purposed to enter the chip column in the flow will not be
distributed uniformly
throughout the chip column, this resulting in uneven digestion of the pulp. In
batch cook-
ing of the displacement type, channelling during displacement leads to mixing
of the dis-
placed liquid and the displacing liquid, resulting in degradation of the
outcome of the
whole cooking process.
The force causing the movement of the chip column in continuous cooking is
created by
the density difference between the chips and the free liquid. In addition, the
magnitude of
the pressure loss and the direction of the liquid flowing through the chip
column influence
the movement of the chip colunm. In the iinpregnation zone of fig. 1, the flow
15 of the
impregnation circulation exerts a downward force on the chip column, and the
flow 23 of
the washing circulation of the digester washing zone 13 exerts an upward
force.
SUMMARY OF THE INVENTION
The invention is based on the observation that the size and shape of the chip
particles fed
into a digester influence in several ways the operation of a cooking process
and the quality
of the pulp obtained by the process. By means of the invention, the operation
of both a
continuous and a batch cooking process as well as the pulp quality are
iinproved by antici-
pating the effect of the aforesaid properties of the chips when controlling
the coolcing proc-
ess.
In a method according to the invention, the size and shape of the chip pieces
supplied to a
cooking plant are measured; from the measured values, the factors indicating
the size and
the shape of the chip pieces are calculated, and the process values of a
digester are antici-
patorily adjusted using a mathematical model, which model comprises
calculating the de-
gree of packing in the digester and the dependency of the flow resistance of
the liquid
flowing tlirough the chip column on the size and the shape of the chip
particles.
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BRIEF DESCRIPTION OF THE DRAWINGS
In the following the invention is described in more detail by reference to
accompanying
drawings wherein
Fig. 1 shows an embodiment of continuous cooking, described in the section
concerning
technical background,
Fig. 2 shows the structure and the dimensions of a chip particle, and
Fig. 3 shows the terms of equivalent diameter and sphericity of a chip
particle.
DISCLOSURE OF THE INVENTION
Fig. 2 shows the sttucture and the dimensions of a chip piece. A wood log is
fed into a
chipper in the direction of its longitudinal axis, and the chipper cuts the
log at an angle
with respect to the transport direction. The length of a chip piece is the
dimension meas-
ured in the fiber direction. The thiclcness and the width are dimensions
perpendicular to the
fiber direction. The length of a chip piece is normally 10 to 30 mm, the
thiclaless 3 to 10
mm and the width 10 to 50 mm. The aforesaid geometric properties may be
measured dur-
ing the process, for instance by means of an optical metering device of a type
cominer-
cially available for example under the name VisiChips. The chip analysis may
be per-
formed e.g. according to SCAN and TAPPI standards. The size and the shape of a
chip
piece can be expressed using two mathematically calculated factors, equivalent
diameter
and sphericity factor. Fig. 3 shows the calculation of equivalent diameter and
sphericity
factor. The equivalent diameter Dp is the diameter of a sphere, whose volume
is the sanie
as the volume of the chip piece. The sphericity factor T is the ratio of the
area of a sphere
having diameter Dp to the area of the chip piece.
The pressure loss during the flow of a liquid through a volume filled with
solid bodies is
expressed by the Ergun equation:
Ap ' 150,uvo (1- E)2 + 1,75 pvo (1- s) (formula I)
L qjzDp $3 V/Dp 6 3
wherein Ap = pressure loss
L = chip cohttnn thickness in the flow direction
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vo = liquid surface velocity
s = column porosity
= liquid viscosity
p =liquid density
ilr = particle sphericity factor
Dp = equivalent diameter
Harlconen, TAPPI J. 79(12):122 (1986) has presented a simplified version of
the Ergun
equation
~ =R, ~3 )Z vo +RZ (1 3 vo (formula 2)
wherein Rt and R2 are chip and liquid specific constants. The constants Rl and
R2 can be
determined experimentally for different chip size distributions. The constants
Rl and R2
include variables of the original Ergun equation.
For controlling the liquor circulations of a cook, it is important to be able
to anticipate the
flow resistance encountered by the liquid during its flow through a chip
column. The flow
velocity of the liquid flowing through the chip column can thereby be
anticipatorily con-
trolled by adjusting control valves in liquid circulation loops, thus
optimizing the condi-
tions for mass and heat transfer for the chips present in the digester at any
given moment.
The change of the porosity s of a chip column in the digester as the cook
proceeds can be
calculated using the fomiula presented by Harlconen
s=a+pb (-c+d1nK) (formula3)
wherein
a = 1- basic bulk density I wood density
p= chip column pressure
b, c, d = raw material-specific factors
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K = kappa number
The basic bulk density is the bulk density of the chips fed into the digester
and it can be
calculated, for instance, as disclosed in WO 94/20671.
The pressure p acting on the chip column is created by the hydrostatic
pressure of the col-
umn and the pressure loss of the liquid flowing tlirough the column.
The progress of the cooking reaction and the obtained result of the cook are
monitored
using the kappa nunlber. The kappa number reflects the anlount of lignin
remaining in a
pulp. For calculating the kappa number, a model based on Vroom's H-factor is
generally
used. In this model, the decrease of the kappa number is calculated using the
H-factor,
which is the time integral of the relative reaction rate. The reaction rate
depends on the
absolute temperature. As a reference, a temperature of 373 K is used, at which
teinperature
1 H-factor unit is foimed in one hour. On page A292 of the publication
"Chemical Pulp-
ing" by Gullichsen and Fogelholm, the formula
H= f exp(43,2-T/16115) dt (formula 4)
is given.
For calculating the kappa number also more complete kinetic models presented
on page
A294 of the same publication, or other corresponding models, may be used.
The residence time of the chips in each zone of a continuous digester can be
calculated
when the digester output (tons of wood per hour), the chip porosity in the
respective zone
and the volume of the zone are known.
In a batch digester the chip column is stationary, and at the beginning of the
cook it has a
certain flow resistance depending on the porosity and the shape of the chip
pieces. The
resistance will change during the cook, as the porosity changes due to
softening of the
chips.
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The output of a continuous digester is controlled by changing the rotation
speed of the chip
meter. The chip meter is a rotating compartment feeder having compartments of
a constant
size. The amount of the chips fed into the digester measured in tons per hour
is calculated
based on the rotational speed, when the chip bulk density has been calculated
or measured.
5
The present invention relates to control of the operation of a digester by
feedforward con-
trol using a mathematical model formed from the above formulas.
In a lcnown manner, the dimensions (chip size and chip shape) of the chip raw
material fed
10 into a continuous digester are measured, and from these dimensions the
sphericity factor
and the equivalent dianieter can be calculated.
From the measured values, the chip bulk density is determined, for instance by
adding the
volumes of the chip pieces and comparing the result with the volume of the
sample. The
output of the digester can be calculated based on the compartment volume and
the rota-
tional speed of the chip meter when the chip bulk density is known.
When the target kappa number has been determined, the target values for alkali
dosage and
H-factor can be determined. The relation between H-factor, kappa number and
alkali dos-
age for different wood species is known (cf. e.g. Gullichsen and Fogelholm
"Chemical
Pulping" 6A).
Consequently, in continuous cooking, by utilizing the measurement data of the
chip parti-
cles, the chip volume required for a certain output, the amount of the
chemicals to be fed
into the digester, the residence time of the cook and the cooking temperature
target to ob-
tain a desired kappa number level are calculated using the above formulas.
Further, the porosity of the chip column formed in the digester as well as the
optimal flows
typical for the production are calculated at various points of the digester.
The porosity is
utilized also in calculating the aforesaid residence time of the cook.
Analogously, the op-
timal flow rate of the counter-current washing through the chip column typical
for the rele-
vant output is calculated.
For controlling the process, the following feedbacks are used:
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- the rotational speed of the chip meter is controlled in accordance with the
calculated out-
put
- in each cooking zone, the set value for the alkali dosage and the
temperature of said
coolcing zone is controlled in accordance with the target kappa number
- in each cooking zone, the set values for the circulation flow rate are
controlled in accor-
dance with the pressure loss calculated from the porosity of the chip column
(fig. 3).
The chip amount fed into a batch digester is calculated based on the digester
voluine and
the chip bulk density. For control purposes, also in batch cooking the size
and the shape of
the chip pieces to be fed into the digester are measured, from which the
sphericity factor
and the equivalent diameter are calculated. The amount of chemicals, the
coolcing time and
the temperature required to obtain a desired kappa number are calculated by
means of the
H-factor, correspondingly to continuous coolcing. Furthermore, in each
coolcing stage, the
porosity of the chip column, the corresponding pressure losses of the flowing
liquid and
the optimal circulation flow rates are calculated.
In batch cooking of the displacement type, the flow rates of the displacing
liquid for
achieving optimal displacement are calculated.
For controlling a batch process, the following feedbacks are used:
- in each cooldng stage, the set values for the temperatures and the residence
times are
controlled in accordance with the calculated H-factor and kappa number
- in each coolcing stage, the set values for the liquid circulation flow rates
are controlled in
accordance with the calculated pressure loss.
The effect of chip size and chip shape on the operation of a digester was
studied in a Fin-
nish pulp mill. For chip analysis, a measuring device was constructed which
measures the
three-dimensional shape of each chip particle in a ten-litre sample. Further,
based on the
measured results the device calculates various factors indicating the size and
the shape of a
chip particle, and statistic factors. The measured results can be transferred
fiom the device
to further processing, or directly to the control system of the pulp mill. The
measuring de-
vice may be provided with automatic sampling means enabling the unmanned
device to
analyse 4 samples per hour and to forward the analysis and the calculation
results.
