Method for controlling a pulping process

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.

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 banding to monitor the performance 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. In bin 2, the chips are med 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 25a 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 8. 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 U.S. Pat. No. 6,606,405, U.S. Pat. No. 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 according 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 packing 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 horizontal 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 channeling of the flow. In channeling, 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, channeling 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.