WO2006072653A1

WO2006072653A1 - Method for controlling a pulping process

Info

Publication number: WO2006072653A1
Application number: PCT/FI2005/050479
Authority: WO
Inventors: Erkki KIVEKÄS; Ari Havu; Lasse Hernesniemi; Seppo Hiljanen
Original assignee: Metso Paper, Inc.
Priority date: 2005-01-05
Filing date: 2005-12-22
Publication date: 2006-07-13
Also published as: CA2592887C; FI123011B; CA2592887A1; US20090250180A1; EP1846614A1; EP1846614A4; FI20055011A; FI20055011A0

Abstract

The quality of the raw material in a pulping process, wood chips, varies due to variations in its origin. The variation in chip quality will result in variations in the cooked pulp as well as problems in the operation of a digester. The control of the digester takes place by feed-back control so that the process values in the digester are adjusted by measuring the quality of the produced pulp. The residence time of the pulp in the digester is several hours so that a corrective action affects the pulp quality with considerable delay. Conditions for a successful and economic cook are a correct cooking chemical dosage, correct concentrations of impregnation and cooking liquor, accurate control of the residence time and the temperature of the cooking process as well as accurate control of the flows in a chip column in relation to the flow properties of the chip column. 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 particles are calculated, and the process values for the digester are anticipatorily controlled by means of a mathematical model, which model includes calculating the degree of packing in the digester and the dependency of the flow resistance of the liquid flowing through the chip column on the size and shape of the chip pieces. In the model, the Ergun equation among others is used, which equation expresses the pressure loss during the flow of a liquid through a volume filled with solid bodies.

Description

METHOD FOR CONTROLLING A PULPING PROCESS

TECHNICAL FIELD

The present invention relates to controlling a pulping process. Particularly, the invention 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 raw 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 knives 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 especially 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 classifier 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 1 are transported by a conveyor to a chip bin 2. hi bin 2, the chips are steamed 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 compartment 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 digester. From the chip meter the chips are led to a chip chute 4. From the chip chute 4 the chips 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 communicating with the chip chute 4 and the low pressure liquor circulation 6. In its horizontal position, the compartment 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 digester 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 8.

In the upper part of the digester 10, an impregnation zone 11 is arranged wherein a cooking chemical is impregnated into the chips. Below the impregnation 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 digester.

White liquor required for the cook is added to the chips in the high pressure circulation 8. At the beginning of the impregnation zone 11, the chips charged to the digester form a chip column which moves downwards in the digester. The impregnation zone 11 comprises an impregnation circulation 15. The liquid circulating in the impregnation circulation 15 is discharged from the digester through a screen 16 and returned to the top of the impregnation zone 11. In the impregnation zone, as shown by arrows 17, free liquid flows downwards 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 column 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 steam 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. 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 column as shown by arrows 23. The mixture 24 of the liquid from the cooking 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 breaking circulation 24a is arranged. In the breaking circulation 24a, the liquid is discharged from the digester through a screen 25 a and is returned via a pipe 26. The liquid flowing upwards in the washing zone 13 exerts an upward force on the chip column, which force impairs the downward movement of the chip column.

In continuous digesters, the wood chips form a column flowing continuously from top to bottom. The mechanical properties of the chips will change during the progress of the process as the chips pass through the digester. As lignin and carbohydrates dissolve, the structure of the chips weakens. 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 packing. Impregnation liquor and cooking 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 through a heat exchanger. 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 column. In batch cooking, the chips maintain their shape during the whole cooking 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 sink.

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. 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 amount of dry chips in a unit volume. The bulk density depends on the wood species used, its properties and the size and the shape of the chip particles. The density of the chip column in the digester is measured by means of its porosity ε. 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 chips fed into the digester is controlled by changing the rotation speed of a chip meter. The chip meter is a rotatable compartment feeder in which the volume of the compartments is known. 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 takes 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 corrective 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 FI 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 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 downstream zone constant in order to keep constant also the dynamic forces affecting the pack- ing of the chip column. Because these dynamic forces depend on the porosity of the chip column, the chip quality, which is assumed to be constant, very rarely achieves an optimal situation in the downstream sections of a digester, especially when using heavy wood species (such as birch), which have a tendency to get excessively packed by mere gravity effects.

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 consistency of the digester blowoff may be adjusted within a certain range by means of the rotational 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 adjusted, the wash factor has to be reduced to allow the chip column to descend. This control is generally carried out by slow feedback, wherefore the action taken 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 chemicals, 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 column in relation to the flow properties of the chip column. In addition to the impregnation duration, also chip size, and especially chip thickness, influences the optimal concentration of the impregnation liquor, because impregnation proceeds considerably 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 (assuming constant cooking time and cooking temperature). Too high a flow and a high pressure loss result in channelling of the flow. In channelling, the flow breaches the chip column, forming one or more passages. Consequently, a chemical 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 cooking of the displacement type, channelling during displacement leads to mixing of the displaced 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 column. In the impregnation 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 improved by anticipating the effect of the aforesaid properties of the chips when controlling the cooking process.

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 degree of packing in the digester and the dependency of the flow resistance of the liquid flowing through the chip column on the size and the shape of the chip particles. 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 structure 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 measured in the fiber direction. The thickness and the width are dimensions perpendicular to the fiber direction. The length of a chip piece is normally 10 to 30 mm, the thickness 3 to 10 mm and the width 10 to 50 mm. The aforesaid geometric properties may be measured during the process, for instance by means of an optical metering device of a type commercially available for example under the name VisiChips. The chip analysis may be performed 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 D_p is the diameter of a sphere, whose volume is the same as the volume of the chip piece. The sphericity factor ψ is the ratio of the area of a sphere having diameter D_pto 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:

wherein Δp = pressure loss

L = chip column thickness in the flow direction Vo = liquid surface velocity ε = column porosity μ = liquid viscosity p = liquid density ψ = particle sphericity factor D_p = equivalent diameter

Harkδnen, TAPPI J. 79(12):122 (1986) has presented a simplified version of the Ergun equation

^_=i?i ⁽lz£⁾l_{Vo +i?2} Jl-)_{Vo2 (formula 2)}

wherein R₁ and R₂ are chip and liquid specific constants. The constants R₁ and R₂ can be determined experimentally for different chip size distributions. The constants R₁ 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 conditions for mass and heat transfer for the chips present in the digester at any given moment.

The change of the porosity ε of a chip column in the digester as the cook proceeds can be calculated using the formula presented by Harkonen

ε = a +p^b (- c + dlnK) (formula 3)

wherein a = 1- basic bulk density / wood density p = chip column pressure b, c, d = raw material-specific factors 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 column and the pressure loss of the liquid flowing through the column.

The progress of the cooldng reaction and the obtained result of the cook are monitored using the kappa number. The kappa number reflects the amount 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 temperature 1 H-factor unit is formed in one hour. On page A292 of the publication "Chemical PuIp- ing" by Gullichsen and Fogelholm, the formula

H = Jexp(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. 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.

The present invention relates to control of the operation of a digester by feedforward control using a mathematical model formed from the above formulas.

In a known manner, the dimensions (chip size and chip shape) of the chip raw material fed into a continuous digester are measured, and from these dimensions the sphericity factor and the equivalent diameter 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 dosage 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 particles, 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 obtain 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 optimal 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: - the rotational speed of the chip meter is controlled in accordance with the calculated output

- in each cooking zone, the set value for the alkali dosage and the temperature of said cooking 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 accordance 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 volume 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 cooking time and the temperature required to obtain a desired kappa number are calculated by means of the H-factor, correspondingly to continuous cooking. Furthermore, in each cooking 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 cooking 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 cooking 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 Finnish 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 from the device to further processing, or directly to the control system of the pulp mill. The measuring device 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.

Claims

1. A method for controlling the process values of a pulp digester, characterized in the steps of

- measuring the geometrical properties of the chips fed into the digester

- calculating, based on the measured results, the factors indicating the shape and size of a chip particle

- anticipatorily controlling the process values of the digester based on a mathematical mo- del and the measured chip properties.

2. A method according to claim 1 , characterized in that the porosity of a chip column is calculated at different stages of the cook using a mathematical model.

3. A method according to claim 1, characterized in that the pressure loss of the liquid flowing through the chip column is calculated using a mathematical model.

4. A method according to claim 1, characterized in that the H-factor of the cook is calculated using a mathematical model.

5. The use of a method according to claim 1 in a continuous digester, characterized in the steps of a) measuring the particle size and particle shape of the chips fed into the digester b) calculating, based on the measured results, the sphericity factor and the equivalent diameter of the chips c) determining the chip bulk density from the measured values d) determining the output of the digester based on the size and the rotation speed of a chip meter and on the bulk density e) determining a target kappa number as well as target values for alkali dosage and H-factor required therefore f) calculating the porosity of a chip column in the digester in each coo- king zone, g) calculating the residence time of a chip column in each cooking zone h) calculating the H-factor in each cooking zone i) determining the kappa number in each cooking zone j) iterating by returning to item f) if required k) calculating the pressure loss of the liquid flowing through the chip column in each cooking zone, and

1) adjusting the set value for the chip meter in accordance with the output calculated according to d) m) adjusting the set value for the alkali dosage and the temperature in each cooking zone in accordance with the calculated H-factor and kappa number n) adjusting the set values for the circulation flow rates in the digester in each cooking zone based on the calculated pressure losses.

6. Use of a method according to claim 1 in a batch digester, characterized in the steps of a) measuring the particle size and particle shape of the chips fed into the digester b) calculating, based on the measured results, the sphericity factor and the equivalent diameter of the chips c) determining the bulk density of the chips from the measured results d) calculating the amount of chips fed into the digester on the basis of the bulk density and the volume of the digester e) determining the target kappa number f) determining the target values for alkali dosage and the H-factor g) calculating the H-factor in each cooking stage h) calculating the kappa number in each cooking stage i) calculating the porosity of a chip column in the digester in each cooking stage j) calculating the pressure loss of the liquid flowing through the chip column in each cooking stage, and k) adjusting the set value for the temperature and the residence time in accor- dance with the H-factor and the kappa number

1) adjusting the set values for the liquid circulation flow rates in the digester in each cooking stage.