Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C3/00—Pulping cellulose-containing materials
  • D21C3/22—Other features of pulping processes
  • D21C3/228—Automation of the pulping processes

Definitions

• the present invention relates to controlling a pulping process.
• 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.
• 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.
• the variation is especially strong.
• 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.
• the quality of the chips is controlled by random sampling.
• 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.
• an impregnation zone 11 is arranged wherein a cooking chemical is impregnated into the chips.
• a cooking zone 12 wherein the actual cooking reaction takes place.
• 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.
• 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.
• 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.
• 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.
• 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.
• a breaking circulation 24a is arranged at the bottom of the washing zone 13.
• 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.
• the wood chips 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.
• a digester 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.
• 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.
• 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 ⁇ .
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the magnitude of the pressure loss and the direction of the liquid flowing through the chip column influence the movement of the chip column.
• the flow 15 of the impregnation circulation exerts a downward force on the chip column
• the flow 23 of the washing circulation of the digester washing zone 13 exerts an upward force.
• 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.
• 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.
• 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.
• 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
• Fig. 3 shows the terms of equivalent diameter and sphericity of a chip particle.
• 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 ⁇ is the ratio of the area of a sphere having diameter D p to the area of the chip piece.
• R 1 and R 2 are chip and liquid specific constants.
• the constants R 1 and R 2 can be determined experimentally for different chip size distributions.
• the constants R 1 and R 2 include variables of the original Ergun equation.
• 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.
• 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.
• 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.
• the chip column 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.
• 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.
• 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.
• 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).
• 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.
• the optimal flow rate of the counter-current washing through the chip column typical for the rele- vant output is calculated.
• 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.
• 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
• 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.
• 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.

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• 2005
  • 2005-01-05 FI FI20055011A patent/FI123011B/fi not_active IP Right Cessation
  • 2005-12-22 CA CA2592887A patent/CA2592887C/en not_active Expired - Fee Related
  • 2005-12-22 US US11/794,605 patent/US20090250180A1/en not_active Abandoned
  • 2005-12-22 EP EP05818865A patent/EP1846614A4/en not_active Withdrawn
  • 2005-12-22 WO PCT/FI2005/050479 patent/WO2006072653A1/en not_active Ceased

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