FI123011B - Method for regulating a cellulose cooking process - Google Patents

Description

METHOD FOR CONTROLLING THE BOILING PROCESS OF PULP PULP 5 TECHNICAL FIELD

The present invention relates to controlling the cooking process of cellulosic pulp. In particular, the invention relates to a method in which the size and shape of chips are measured before cooking and the shape factors calculated from the measured results are used to calculate the packing density of the chips and to adjust process variables such as fluid flows and chemical doses.

TECHNICAL BACKGROUND

The raw material for the pulp cooking process is wood chips. The quality of the chips varies due to the 15 origin of the chips. Factors that influence the quality of wood chips include the size and age of the wood, the structure of the wood and the condition of the wood blades, and the structure and location of wood chips in the process chain. Especially in factories that do not themselves produce chips but buy it from a variety of sources, the variation is particularly large. In some mills, wind conditions during external storage of chips can cause fluctuations in the size of chips fed into the digester. Wind chips of different sizes are transported to different places during wind chipping and during storage, due to the wind. This phenomenon is known as wind sorting.

Chip quality is monitored at pulp mills on a random basis. In SCAN or TAPPI screening tests, a batch of wood chips is screened using a multiple screen sieve sorter and the chips remaining on each screen are weighed. The cm test can be performed separately in wood treatment to monitor the operation of the chips and to control the quality of incoming chips.

One embodiment of the continuous cooking process is shown in reduced form in section 30 of Figures 30. 1. Chip 1 is introduced by conveyor into a chip silo 2. In silo 2, the chips are steamed to heat and remove air from the chips. The steamed chips are fed from the chips silo o 2 to the chips 3. My chips are a rotary tray feeder whose speed is controlled by controlling the amount of chips entering the digester. From the chips meter, the chips are led to the feeding neck 4. From the feeding neck 4, the chips are led along with the lye cycle 6 to the 2 high pressure feeders 5. The high pressure feeder has a rotating rotor and one or more trays 7 is connected to the high pressure cycle 8. With the high pressure cycle 5 8, the chips are fed to the separator 9 at the top end of the digester 10, the separator 9 ha being separated from the liquid carrying it back to the tray 5 via the return pipe 8.

At the top of the digester 10 there is an impregnation zone 11 where the cooking chemical is absorbed by the chips. Below the impregnation zone 11 is the cooking zone 12, where the actual cooking reaction takes place. In boiler wash zone 13, the cooked pulp is washed. The cooked mass 14 leaves the bottom of the digester.

The white liquor needed for cooking is added to the chips in a high pressure cycle 8. At the beginning of the absorption-zone 15, the chips fed to the digester form a chips statue which moves downward in the digester. The impregnation zone 11 has a soaking cycle 15. The fluid circulating in the soaking cycle 15 is removed from the digester through a screen 16 and returned to the upper end of the soaking zone 11. In the impregnation zone, the free liquid flows downwardly in the chipping column as indicated by arrows 17 at a higher rate than the chipping column itself moves downward. The downward flow through the Hake-20 statue creates a downward force on the wood chips.

At the bottom of the impregnation zone 11 is a heating circuit 18 which raises the temperature of the chips and the liquid therein to the temperature of the cooking zone. The fluid circulating in the cooking circuit 25 is removed from the digester via a screen 19 on its periphery and brought back to the center of the digester via a center tube 20. The circulating liquid is heated by steam in a heat exchanger o 21. Cooking In zone 12, the heated chips and liquid flow downward for the time required by the cooking reactions o.

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t-30 Washing liquid 22 is introduced into the bottom of the digester and flows upwards through the chipping column in section 13 of the digester washing zone as indicated by arrows 23. The mixture 24 of liquid and washing liquid 22 coming from the cooking zone exits the screen 25 from the strainer. The cooked pulp 14 leaves the bottom of the digester. At the bottom of the washing zone 13, there is a cut-off circuit 24. In the cut-off cycle, the fluid exits the strainer 25 and is returned through the tube 26. The upwardly flowing fluid in the wash zone 3 exerts an upward force on the chipping column, which impedes downward movement of the chipping column.

In continuous digesters, wood chips form a top-to-bottom pat-5. The mechanical properties of the chips change as the cooking progresses as the chips pass through the digester. As lignin and carbohydrates dissolve, the structure of the chips breaks down. However, the hawk pieces retain their shape until the end of the soup. As the soup progresses, the chips statue becomes somewhat more compact.

10 In batch cooking, the empty machine is initially filled with chips. During filling, steam is introduced into the wood chips to warm it and improve its packaging. A soak liquid or cooking liquid is introduced into the cooker filled with chips. The temperature of the digester is raised to the cooking temperature by circulating the liquid in the digester via a heat exchanger. As it circulates through the chip, the liquid raises the temperature of the entire chip and conveys the cooking chemical 15 evenly throughout the chip. In batch cooking, the chips retain their shape throughout the cooking process and do not disintegrate into fibers until the cooked mass is discharged from the digester. As the soup progresses, the wood chips become denser and their surface decreases

In displacement-type batch cooking, the chips are treated in several steps with different liquids.

The fluid exchange is accomplished by feeding a new fluid into the digester at one end in a steady stream so as to displace the former fluid from the strainers at the opposite end of the digester.

In woodworking and before cooking, the measure of chips is bulk density. c \ j 25 It represents the weight of the dry amount of wood in the unit volume. The bulk density depends on the species of wood used, its properties and the size and shape of the chips. In a digester, the density of a chip is measured by its porosity ε. The porosity is the amount of free space between the chips and the volume of the entire mattress.

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30 Variation in the quality of chips causes variations in the quality of the cooked pulp and problems in the operation of the digester. In continuous cooking, the amount of log supplied to the digester is adjusted by varying the rotation speed of the Applicator. The chip meter is a rotary tray feeder in which the volume of the trays is known. The bulk density of the chips, i.e. the weight of dry wood in the chips per unit volume, varies according to the quality of the chips. This causes inaccuracy in measuring the amount of wood to be fed into the digester.

Continuous digester control is performed by feedback so that process values of the digester are adjusted by measuring the quality of the pulp produced. The pulp has a turnaround time of several hours, so the effect of the corrective action on pulp quality is delayed.

WO 94/20671 discloses a method for measuring the bulk density of chips fed to a digester from samples taken from a stream of chips entering the digester. The bulk density is measured by measuring the size of each particle size of the sample and calculating the bulk density of the sample.

Methods and apparatus for measuring chip size by various optical methods are disclosed in US 6606405, US 5818594, WO 91/05983, WO 91/05984 and FI84761.

15 The number of radial lye cycles of a continuous digester is controlled according to the production speed of the digester, i.e. the number of rotations / production ratios is kept constant. Deterioration of the chip quality results in clogging of the circulating strainer due to too much flow through the target into the chip column. wood chip quality. Clogging of the rotary sieve results in quality losses and loss of production. Likewise, the fluid / wood ratio of the digester is kept constant, which aims to keep the relative flow rates of the downstream flow zone of the digester and the free liquid constant, so that the dynamic forces packing the chips are also constant. Because these dynamic forces are dependent on the porosity of the chipping column, it is very rare for a presumed chip quality to be optimal for downstream portions of the digester, especially heavy wood species (such as birch), which tend to over-pack by gravity alone.

δ cv i cp The washing liquid to be added to the bottom of the digester, flowing against the descending chips column, o should be adjusted according to the digestion coefficient of the digester. The consistency of the digester butt can be adjusted within certain limits by the rotation speed of the bottom scraper and the washing liquid through the vertical and horizontal nozzles of the bottom of the digester. If the bottom consistency is not adjustable enough, the washing factor will have to be lowered to lower the chips column, oo This adjustment is usually done through slow feedback, which means that the action taken is up to hours late for optimum results because the chips column and its flow resistance and also cumulative 5, that is, the delayed correction must be too large compared to the timely correction.

The prerequisites for successful and economical cooking are the correct dose of cooking chemical, the correct concentrations of absorption-5 and cooking liquor, the precise control of the dwell time and temperature of the cooking process, and the precise control of the flow rates of the chip column. In addition to the absorption time, the optimum concentration of the absorption liquor is also influenced by the size of the chips and especially by the thickness of the chips, since small and thin chips are absorbed much faster than large and thick chips. If, for example, there is a large 10 particle size distribution in the chips stream, successful absorption of thick chips requires an increased dose of alkali (increased absorption liquor concentration) so that the rejection content of the cooked pulp does not increase excessively (at constant cooking time and cooking temperature).

Excessive flow and high pressure drop cause the flow to duct. In ducting, the flow pierces one or more passageways through the chipping column. In this way, the chemical or heat supplied to the chipping column by the flow is not evenly distributed in the chipping column and the result is an uneven boiling of the mass. In sputter-type batch cooking, ducting by displacement causes a mixing of displaced and displaced liquid and results in a deterioration of the entire cooking process.

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The force that causes the movement of the chip column in continuous cooking is created by the difference in density between the chip and the free liquid. In addition, the magnitude and direction of the pressure drop in the fluid flowing through the chips will affect the movement of the chips. In the absorption zone of Fig. 1, the flow 15 of the suction cycle causes a downward force on the chipping column and the flow 23 of the washing cycle 13 of the digester 25 the upward force.

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SUMMARY OF THE INVENTION

c The invention is based on the observation that the size of the chips and the shape of the chips to be fed into the digester affect the operation of the cooking process and the quality of the wood pulp produced as a result of the digestion. The invention improves the operation and pulp quality of both continuous and batch cooking processes by proactively taking into account the effect of the above-mentioned properties of chips in the control of the cooking process.

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In the method of the invention, the chip size and shape of chips entering the cooker are measured, factors determining the size and shape of the chips are calculated and the process values of the digester are proactively adjusted by a mathematical model including the compression ratio of the digester.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to the accompanying drawings, in which: Figure 1 illustrates one embodiment of continuous cooking shown in the background section,

Figure 2 shows the structure and dimensions of the chips, and

Figure 3 illustrates the concepts of equivalent chip diameter and sphericity.

DETAILED DESCRIPTION OF THE INVENTION

Figure 2 shows the structure and dimensions of the chip. The log is fed in the direction of its longitudinal axis and the log cuts the log obliquely with respect to its direction of travel. The chip length is the dimension of the chip measured in the direction of the fiber. Thickness and width are dimensions perpendicular to the direction of the fiber. The chips are normally 10-30 mm long, 20 3-10 mm thick and 10-50 mm wide.

The aforementioned geometrical properties can be measured during the process, for example, by an optical measuring device such as those commercially available, for example, under the name VisiChips. Chip analysis can be done according to, for example, SCAN or TAPPI standards. The chip aggregator shape can be represented by two mathematically calculated coefficients, an equivalent diameter and a sphericity coefficient. Figure 3 shows the calculation of the equivalent cm diameter and spherical coefficient. The equivalent diameter Dp is the diameter of an i 00 cp sphere having the same volume as the chip. The sphericity coefficient ψ 0 is the ratio of the diameter of the sphere Dp to the area of the chip.

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<- 30 The pressure drop as the fluid flows through the volume filled with solid bodies is expressed by the Er-§ Gun equation:

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Ap_150 // vo (1-f) 2 + 1.75 pv; (1- g) L f 2 D; ε3 ψΰν ε3 7 where Δρ = pressure drop L = chip thickness in the flow direction vo = surface velocity ε = porosity 5 μ = viscosity p = density ψ = spherical particle coefficient.

Dp = equivalent diameter.

10 Härkönen, TAPPI J. 79 (12): 122 (1986) has presented a simplified Ergun equation of the form g = * i (1f v0 + R2 (1 ~ £) v02 (Formula 2) L ε ε, where Ri and R2 is a chips and fluid specific constants R 1 and R 2 can be experimentally determined for different chip size distributions, and constants R 1 and R 2 contain the variables of the original Ergun equation.

It is important to be able to predict the flow resistance of the liquid as it flows through the chipping column when adjusting the liquor cycle of the soup. In this case, the control of the liquor circulation by controlling the valves can proactively control the flow rate of the liquid flowing through the chipping column, thereby optimizing the conditions of material and heat transfer for the respective chips present in the digester.

The change in the porosity ε of the wood chips in the digester as the cooking progresses can be calculated using the formula given by Härkönen C C \ J h / \ pk 25 ε = α + ρ {-c + dln K) (formula 3) o rl where oxa = 1 basic density / wood density cn p = chip pressure 5 b, c, d = raw material specific factors o 30 K = kappa number o C \ l

The basic density is the bulk density of the chips fed into the digester and can be calculated, for example, as disclosed in WO 94/20671.

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The pressure p on the chip pillar is due to the hydrostatic pressure of the pillar and the pressure loss of the fluid flowing through the pillar.

The progress of the cooking reaction and the result of the cooking is monitored by the number of kappa. It measures the amount of lignin remaining in the pulp. Vroom's H-factor model is commonly used to calculate a paragraph number. Here, the kappa number decrease is calculated by a factor H, which is a time integral of the relative reaction rate. The reaction rate depends on the absolute temperature. The reference point is a temperature of 373 K, in which a 1 H-factor unit is formed per hour. Gullichsen and Fogelholm "Chemical Pulping" has the formula shown in si-10 on A292: H = Jexp (43,2-T / 16115) dt (formula 4)

Additional kinetic models on page A294 of the same publication, or similar models, may also be used to calculate the paragraph number.

The residence time of the chips in each zone of the continuous digester is calculated by knowing the production of the digester (tonnes of wood per hour), the porosity of the chips in that zone and the volume of the zone.

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In a batch digester, the chip statue is immobile and has a certain flow resistance at the beginning of the cooking process, depending on the porosity and shape of the chips. The resistance changes during cooking as the porosity changes as the chips soften.

The amount of continuous digester production is controlled by changing the rotational speed of the logger. The chip meter is a rotary tray feeder with standard compartments. The amount of chips fed into the digester, measured in tonnes per hour, is calculated from the speed of rotation when the chopping density of the chips is calculated or measured, x

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It is an object of the present invention to control the operation of a digester by means of a feedback control i- a mathematical model of a digester made from the above formulas.

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g Measure πύ ο tat (particle size and particle shape) of the raw material fed into the continuous digester, from which the spherical coefficient and the equivalent diameter can be calculated.

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From the measured values, the bulk density of the chips is determined, for example, by summing the volume of the chips and comparing it with the volume of the sample lot. The production of the digester can be calculated based on the volume and rotational speed of the logger tray when the chip density is known.

5 Once the target kappa number has been determined, target values for alkali dose and factor H can be determined. The relationship between H-factor, kappa number and alkali dose is known for different wood grades (see e.g. Gullichsen and Fogelholm "Chemical Pulping", 6A.) Thus, in continuous cooking, the chips volume required for a given production is calculated using the above formulas. , the amount of chemicals to be fed to the digester, the delay time for the soup and the cooking temperature target to achieve the desired cupboard level.

Further, the porosity of the chip formed in the digester at various points in the digester and the optimum production flows at these points are calculated. The porosity is also utilized to calculate the soup dwell time mentioned above. Correspondingly, the optimum amount of counter-current washing typical of the production through the chips column is calculated.

The following feedbacks are used to control the process: 20 - Chip meter rotation speed is controlled by calculated output - Each cooking zone controls the target kappa number for the alkaline dose and the corresponding cooking zone temperature.

cm 25 cm The amount of chips introduced into the batch digester is calculated on the basis of the volume of the digester and the density of the chip i o. For control, batch cooking also measures the particle size and shape of the chips that are fed into the digester and determine the sphericity and equivalent diameters. Calculate the amount of chemicals needed to reach a given kappa number, as well as the cooking time T-30 and the temperature using the H-factor, similar to continuous cooking. Further, calculate the porosity of the chipping column at each cooking step, the corresponding pressure losses of the flowing fluid, and the optimum flow rates.

In displacement-type batch cooking, flow rates of displacement liquid are calculated to achieve optimal displacement.

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The following feedbacks are used to control the batch process: - The setpoints for temperature and dwell times are adjusted according to the H-factor and kappa number calculated for each cooking step. - The setpoints for the liquid flow rates are adjusted according to the calculated pressure drop.

The effect of chip size and chip shape on the operation of the digester was studied at a Finnish pulp mill. For the analysis of the chips, a measuring device was constructed which measures, in a 10-liter batch of samples, the three-dimensional shape of each chip. In addition, the device calculates from the measurement results various coefficients and statistical factors describing the shape of the chip chopper. The measurement results can be transferred from the device for further processing or directly to the pulp mill control system. The measuring device can be equipped with an automatic sampler, allowing it to analyze 4 samples per hour unmanned and to forward the results of analysis and calculations.

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Claims (4)

A method for adjusting process values of a pulp digester comprising: 5) a) measuring the particle size and shape of the chips fed to the digester, b) calculating the chipping sphere coefficient and the equivalent diameter, c) determining the chip density from the measured values; pressure loss of the fluid flowing through the chips in various stages. Method according to Claim 1, characterized in that the mathematical model calculates the H-factor of the soup. Use of the method according to claim 1 in a continuous digester, characterized in that d) determining the production speed of the digester based on the size and rotational speed of the digester and the bulk density e) determining the desired kappa number and target values for alkaline dose and H-factor f) in the cooking zone g) calculating the residence time of the chips in each cooking zone h) calculating the H-factor in each cooking zone c i) determining the kappa number in each cooking zone cm j) if necessary, iterating through the flow of liquid into the cooking liquid o) and a. adjusting the dial setpoint according to the production calculated in (d) T-30 1) adjusting the temperature setpoint for each cooking zone § according to the calculated H-factor and kappa number o (m) adjusting the set-points for the flow rates of the digesters on the basis of the pressure losses calculated for each cooking zone. Use of the method according to claim 1 in a batch digester characterized in that d) calculating the amount of chips fed to the digester based on the bulk density and volume of the digester 5 e) determining the target number f) determining the target alkali dose and factor H g) calculating the factor H in each cooking step h) (i) calculating the porosity of the chips in the digester at each cooking step; 15 tus values. 20 c \ j δ {M co o h- · o X IX CL δ m m o o {M