EP1969176B1

EP1969176B1 - Method for controlling a process for the production of short-fibred cellulose pulp

Info

Publication number: EP1969176B1
Application number: EP06835895A
Authority: EP
Inventors: Sture Erik Noreus; Per Stefan Svensson
Original assignee: More Research Ornskoldsvik AB
Current assignee: More Research Ornskoldsvik AB
Priority date: 2005-12-23
Filing date: 2006-12-21
Publication date: 2011-02-16
Anticipated expiration: 2026-12-21
Also published as: CA2632124A1; BRPI0620303A2; SE0502899L; ES2359163T3; WO2007073312A1; ATE498733T1; DE602006020182D1; SE529420C2; PT1969176E; EP1969176A1; EP1969176A4

Abstract

Special demands are placed on the properties of short-fibred cellulose pulp when this type of pulp is used for the manufacture of paper, and a paper manufacturer primarily desires a pulp with predictable and constant properties. The present invention makes possible the production of such pulp and consists of a method for the production of chemical pulp by the digestion of short-fibred lignocellulose material with the use of a certain ramping profile, a certain maximum temperature, a certain time at this maximum temperature, and certain content of cooking reagents, whereby the pulp in association with the end of the digestion is analysed with respect to at least two properties of the pulp fibres, one of which is constituted by the lignin content of these, and in that also the content of readily soluble carbohydrates in the pulp fibres is determined, which content has an influence on several properties of the pulp and of paper manufactured from the pulp and that control of the specified cooking parameters is based in the first hand, primarily, on the content of readily soluble carbohydrates and in the second hand, secondarily, on the content of lignin, and in this case, preferably, in a manner that is approximate while, however, always taking both contents into consideration.

Description

Technical Field

The present invention concerns a method at the production of chemical pulp by the digestion (cooking) of short-fibred lignocellulose material, in particular it concerns a method for controlling the digestion process such that paper manufactured from the pulp consistently demonstrates even quality with respect to one or several properties that are important for different types of paper.
The concept of chemical pulp comprises the digestion of the lignocellulose material either under acid or under alkaline conditions. One example of a pulp of the first type is sulphite pulp, while examples of the second type are sulphate pulp, polysulphide pulp, and soda pulp, in which lignocellulose material is digested solely with sodium hydroxide, or that this chemical is supplemented by an additive, such as, for example, anthraquinone.
There are many lignocellulose materials and they can be classified as long-fibred and short-fibred materials. It is during the digestion of short-fibred lignocellulose material that the present invention can be used. The lignocellulose material that dominates as raw material when producing pulp is wood from trees. Examples of tree types that demonstrate short-fibred wood are deciduous trees, in particular birch, beech, aspen, oak, eucalyptus, etc.

Prior Art

Currently known control methods for the production of, for example, sulphate pulp, focus on the pulp after digestion having a certain lignin content, often measured as a kappa value. At the time when these control methods were developed, concentration was focussed on the production of sulphate pulp from conifer wood, such as wood from pine and spruce. The dominating property of the pulp was, and still is, its strength. The producer is satisfied if it is possible to produce a final pulp, including pulps that have a high brightness, with a relatively high strength. The extent to which the digestion and delignification progress, i.e. the kappa value that is reached, varies. It depends on such factors as the type of sulphate pulp that is being produced, i.e. the use to which the pulp is to be put, and the wood raw material. Furthermore, a balance is reached with respect to how much of the original lignin of the wood is to be removed during the digestion, and how much is to be removed during the delignification stages in the bleaching plant. Environmental aspects make it preferable to allow the digestion to proceed to an advanced stage, since it is in this case easy to manage the dissolved lignin (and fragments of lignin) in the spent cooking liquor, which is burnt in a soda boiler after evaporation. There is, however, the constant overriding requirement that the strength of the pulp is to be maintained as far as possible.
With respect to the control of the production of sulphate pulp from deciduous wood, such as birch, the ideas developed for the control of the production of sulphate pulp from coniferous wood, such as pine, have been, quite simply, fully adopted.
Even if the strength of short-fibred sulphate pulp is significant for the paper manufacturer, this property is not as dominating as it is when considering long-fibred sulphate pulp. Properties of short-fibred sulphate pulp that are just as important are its beatability, porosity, etc., and its chemical properties, such as, for example, the content of moieties that demonstrate charge, etc. Paper has several different strengths, examples of such are tensile strength and tear strength. Tensile strength is measured as tensile index. A related property for which also the stiffness of the paper has an influence is measured as its tensile stiffness index. Tear strength is measured as tear index. Z-strength demonstrates the resistance of the paper against delamination, i.e. the strength of the paper in a vertical direction. Further, there is bursting strength, which is measured by the bursting index. This describes how the paper resists a force that is directed at the paper from below. It may be the tear strength that is important for certain types of paper, and the value of the tear index is of major significance in these cases, while for certain other types of paper it may be the tensile strength that is of greatest significance. Strength is of secondary importance for a large group of paper types, as has previously been specified, and it is other physical properties or the chemical properties, or both, that are most important for these types of paper.
Parameters of great significance for the pulp producer are the cooking yield and the brightness of the pulp after the digestion. The term cooking yield is used to denote how much of the original lignocellulose material that remains in the form of pulp after the digestion, calculated as a percentage by weight. The brightness of the pulp depends partly on the lignin content of the pulp, which is specified by a kappa value. Also the viscosity of the pulp has a certain significance for the pulp producer, and also this parameter depends, similarly, partly on the lignin content of the pulp. The principal rule is that the viscosity decreases with the kappa value. There is, in certain conditions, a correlation between the viscosity of the pulp and the strength of paper that is manufactured from the pulp. Methods for controlling disgestion of lignocellulosic material are e.g. know from EP 1528148 A2 , US 20030178164 A1 , US 4853084 A and DE 1965530 C1 .

Description of the Invention

Technical Problem

For currently know control methods for the digestion of short-fibred lignocellulose material, the focus has been on the pulp fibres after the digestion are to contain a certain amount of lignin, measured by, for example, the kappa value. It has proved to be the case that two pulps with the same kappa value, i.e. with the same lignin content, after digestion and in their final condition and used for the manufacture of paper often give rise to two papers with different properties. One or several of the properties is or are of greatest significance for each type of paper, and it is a major problem for the paper manufacturer if pulps delivered on different occasions give rise to variations, which may be large, in the particular property or properties of the paper produced.

The Solution

This problem is solved by use of the present invention, which concerns a method for production of chemical pulp through digestion (cooking) of short-fibred lingo-cellulose material during use of a certain ramping profile, a certain maximum temperature and a certain time at this temperature, and a certain content of cooking reagents, whereby the pulp in association with the completed digestion is analysed with respect to at least two properties of the pulp fibres, one of which is constituted by the lignin content of these, characterised in that also the content of readily soluble carbohydrates in the pulp fibres is determined, which content has significance for various properties of the pulp and of paper manufactured from the pulp, and in that control of the cooking parameters specified above in the first hand, primarily, is based on the content of readily soluble carbohydrates, and in the second hand, secondarily, is based on the content of lignin, and in this case, preferably, in an approximate manner, while always retaining consideration of the two levels of content.
The control method in question is principally useful for the digestion of lignocellulose material under alkaline conditions and in particular during the production of pulp according to the sulphate method.
Central to the invention is the determination of the content of readily soluble carbohydrates in the pulp immediately after the digestion. The analysis may take place on the digested chips in the bottom of the digester before they leave the digester, or it may take place on the digested pulp after the digester. There are many methods to determine the content of readily soluble carbohydrates in the pulp. One method is to bring a certain amount of pulp into contact with a sodium hydroxide solution of low concentration, for example somewhere within the interval 1-10% NaOH. A suitable concentration is 5%. It is possible to analyse the R5 value of the pulp and this can take place according to the SCAN-C2:61 method. The term R5 value is used to denote the percentage fraction of the pulp that is not dissolved in a sodium hydroxide solution with a concentration of 5%, i.e. the percentage fraction of the pulp that is resistant to this solution, which is normally known as lye. The R5 value for birch sulphate pulp usually lies within the interval 70-85%. There is a correlation between the different percentage R-values of the pulp, and an individual pulp producer is thus free to choose an own R-value, such as, for example, R3 or R7 instead of, for example, R5. One and the same R-value, however, should be determined during analysis of the pulp after each cook when using any particular control relationship, i.e. any particular formula for the control of the cook. The measurement methods described above are principally manual, i.e. a sample is removed from the pulp and transferred to a laboratory where the analysis takes place. To the extent to which more automated analysis methods of the type described, possibly in the form of online testing, are available, these are to be preferred from the point of view of time and efficiency.
The analysis methods described above are examples of wet-chemistry methods. Other methods in addition to those mentioned above are R18, S5, S10 and S18. Other such methods that may be used are colorimetric methods and titration. Other useful methods of analysis are those that fall within the group of chromatographic methods, such as ion chromatography, fluid chromatography, gas chromatography, size-division chromatography, and capillary electrophoresis. Further, flow injection analysis and sequential injection analysis may be used. A further group of analysis methods includes spectroscopic methods such as FTIR, NIR, NMR, Raman, and UV/VIS.
One method of analysis that may advantageously be used for online testing is NIR (Near InfraRed) spectroscopy. The method is carried out in that the pulp is illuminated with electromagnetic waves that have wavelengths within the NIR region. Certain wavelengths are absorbed by the readily soluble carbohydrates that are present in the pulp, and it is then possible to measure either the absorption or the transmission to obtain what may be described as a fingerprint, which is a result of the particular readily soluble carbohydrates present. This is an indirect measurement method, since the fingerprint must be calibrated against a value of readily soluble carbohydrates, such as, for example, the R5 value, that is known to be correct. A large number of experiments have been carried out, and these show an even and stable relationship between the measurement values obtained according to this measurement method and the R5 value. An online test analysis method is to be preferred, as has been specified previously.
Also the lignin content of the pulp, preferably expressed as a kappa value, is to be determined. This is to take place either for the digested wood chips at the bottom of the digester or for the pulp after the digester. The kappa value is a measure of how large a volume of potassium permanganate (KMnO₄) solution, at a concentration of 20 mmol/l, is consumed by 1 g of dry pulp. This measurement method has been regulated through the years by international standards such as SCAN-C1:00, SCAN-C1:77, ISO 302 1981 and TAPPI T236 cm -85. The most recent standard is ISO 302-2004. All of these standards are fundamentally similar to each other and give very similar results. The measurement is carried out by taking a sample of the pulp and transferring this sample to the laboratory, where it is dried and weighed before being analysed in the manner described above.
Also online test methods are available that measure the kappa value in an indirect manner. These methods are based on NIR spectroscopy. One example of such a measurement system is that manufactured by ABB AB, know as "Smart Pulp Platform". Another kappa value measuring system that is based on NIR spectroscopy is the KNA-5200 system of BTG AB.
As has been stated previously, paper manufacturers are not fully satisfied with, for example, the birch sulphate pulps that are digested according to the control strategy that the lignin content of the pulp that has just been cooked is to be held constant. The personnel in the pulp mill have nothing against such a control strategy, since bleaching of the pulp is facilitated if an essentially constant kappa value is consistently achieved for the pulp that enters the bleaching plant. It is the paper manufacturer who experiences problems, since the pulp that has been digested according to the said control strategy varies with respect to one or more properties that are important for the paper manufacturer. This situation has led to innovative ideas that there may be some other object or property, or both, of the newly digested pulp (other than the lignin content) onto which the control is to be focused and on which it is to be based. It has, surprisingly, turned out to be the case that this object is constituted by a calculated cooking yield = Y (an abbreviation for "Yield"). This is caused by, principally, the amount of the readily soluble carbohydrates in the pulp after the cook, which is relatively simple to determine by analysis, having been proved to have a certain relationship to Y. The reason that a calculated cooking yield is used as the basis or as a dominant factor in the control is that it is not possible to determine this parameter or property in an efficient and reliable manner directly in the digester plant or in the pulp mill. An equation has been produced to calculate Y, based on a large number of laboratory cooks, as it has the following form: $Y = 76.25 - 0.56 \cdot EA - 0.064 \cdot T - 0.0002 \cdot t - 0.006 H,$

where EA = the charge of effective alkali measured as a percentage, T = the maximum temperature in degrees Celsius, t = the time in minutes for which the maximum temperature is held, and H = the H factor, as given by a definition elsewhere in this document. It can be understood from the above that the constants in the equation have been determined empirically. The calculated yield must be coupled with one or several of the cooking parameters, such as the maximum temperature T or the charge of effective alkali EA, in order to be able to control the cook.
A number of equations are given below that will be useful when carrying out the control.
The equations and the way in which the equations are used depend on whether a change in production takes place during the cooking, or if it is desired to maintain a certain set of operating conditions. The charges of effective alkali (EA) given in the equations can be supplemented in practical use by the content of effective alkali (EA) in the cooking liquor during the cook.
In the first case, the following equations are valid for the relevant interval of temperature (it has proved to be necessary to adjust the formulas to two intervals of maximum temperature, such as 150-160 °C respectively 160-170 °C): $Y_{T 1 \leq T \leq T 2} = - \frac{b 1 - R 5}{a 1} - \frac{T - T 1}{T 2 - T 1} \cdot (\frac{R 5 - b 1}{a 1} - \frac{R 5 - b 2}{a 2})$
$Y_{T 2 \leq T \leq T 3} = - \frac{b 2 - R 5}{a 2} - \frac{T - T 2}{T 3 - T 2} \cdot (\frac{R 5 - b 2}{a 2} - \frac{R 5 - b 3}{a 3})$
The experienced EA charge of the wood chips at a yield of Y is described by: ${EA}_{T 1 \leq T \leq T 2} = \frac{Y - d 1}{c 1} - \frac{T - T 1}{T 2 - T 1} \cdot (\frac{Y - d 1}{c 1} - \frac{Y - d 2}{c 2})$
${EA}_{T 2 \leq T \leq T 3} = \frac{Y - d 2}{c 2} - \frac{T - T 2}{T 3 - T 2} \cdot (\frac{Y - d 2}{c 2} - \frac{Y - d 3}{c 3})$
At a change of the operating conditions at the current EA charge, the new cooking yield will be described by: $Y_{T 1 \leq T \leq T 2} = c 1 \cdot EA + d 1 - \frac{T - T 1}{T 2 - T 1} \cdot (c 1 \cdot EA + d 1 - (c 2 \cdot EA + d 2))$
$Y_{T 2 \leq T \leq T 3} = c 2 \cdot EA + d 2 - \frac{T - T 2}{T 3 - T 2} \cdot (c 2 \cdot EA + d 2 - (c 3 \cdot EA + d 3))$
In order to return to the original cooking yield, the charge of effective alkali = EA is changed as specified by: ${EA}_{T 1 \leq T \leq T 2} = \frac{Y - d 1}{c 1} - \frac{T - T 1}{T 2 - T 1} \cdot (\frac{Y - d 1}{c 1} - \frac{Y - d 2}{c 2})$
${EA}_{T 2 \leq T \leq T 3} = \frac{Y - d 2}{c 2} - \frac{T - T 2}{T 3 - T 2} \cdot (\frac{Y - d 1}{c 2} - \frac{Y - d 3}{c 3})$
The constants a1, a2, a3, b1, b2, b3, c1, c2, c3, d1, d2 and d3 are determined empirically.
In the second case, the following equations are valid for the relevant interval of temperature.
In this case, a target value is initially used with respect to the content of readily soluble carbohydrates, R5, in the pulp, and the yield at this target value is determined by: $Y_{T 1 \leq T \leq T 2} = \frac{b 1 - R 5}{a 1} - \frac{T - T 1}{T 2 - T 1} \cdot (\frac{R 5 - b 1}{a 1} - \frac{R 5 - b 2}{a 2})$
$Y_{T 2 \leq T \leq T 3} = \frac{b 2 - R 5}{a 2} - \frac{T - T 2}{T 3 - T 2} \cdot (\frac{R 5 - b 2}{a 2} - \frac{R 5 - b 3}{a 3})$
and where the EA charge is described by: ${EA}_{T 1 \leq T \leq T 2} = \frac{Y - d 1}{c 1} - \frac{T - T 1}{T 2 - T 1} \cdot (\frac{Y - d 2}{c 1} - \frac{Y - d 2}{c 2})$
${EA}_{T 2 \leq T \leq T 3} = \frac{Y - d 2}{c 2} - \frac{T - T 2}{T 3 - T 2} \cdot (\frac{Y - d 1}{c 2} - \frac{Y - d 3}{c 3})$
When the measured R5 value shows that the cooking yield has been changed from Y1 to Y2, the charge of effective alkali EA is compensated by calculating by means of the immediately preceding equation as specified by: $Δ E A = {E A}_{Y 1} - {E A}_{Y 2}$
The constants a1, a2, a3, b1, b2, b3, c1, c2, c3, d1, d2 and d3 are determined empirically.
As has been made clear by the above, the measured content of readily soluble carbohydrates, for example the R5 value, has a direct coupling with the control of the cook. Also the lignin content of the pulp immediately after the cook, preferable given as a kappa value, is measured routinely. The numerical values measured do not contribute directly to the control algorithms, and the lignin content of the pulp is, indeed, consciously allowed to vary between different cooks. What is often done from the point of view of control is to study these values and ensure that the kappa value is not allowed to fall outside of certain determined limiting values. It is also possible to control the kappa value to an optimal value by means of what is known as the Kappa-Batch method described in the Swedish patent 367 451 (6795/70) .

Advantages

There are two major advantages of the control method according to the invention. One is that the pulp is tailored for a certain paper manufacturer. If this paper manufacturer gives highest priority to a particular property or some particular properties of the paper, the pulp producer can produce with the aid of the invention a pulp with the correct properties.
The second (and possibly most important) advantage is that the paper manufacturer is sent in a consistent manner a pulp of even quality with respect to the various pulp and paper properties, and this is achieved independently of the fact that the quality of the wood that the pulp producer uses varies and whether various problems (such as failure of monitor equipment and failure of key equipment) arise during the production of pulp. By measuring frequently the level of readily soluble carbohydrates in the digested material, i.e. in the pulp, knowledge about the condition, i.e. the properties, of the pulp is obtained.
The invention gives the pulp producer also the opportunity to optimise important conditions that are important for him or her. There is a possibility, for example, to optimise the yield of pulp when producing pulp with certain paper properties. A higher yield gives more pulp, and, since payment for pulp is given per tonne of pulp, a greater amount of pulp produced means that the income will be higher.

Description of the Drawings

Figure 1 shows a pulp mill in a very simplified schematic form, in which the control of the cooking process according to the invention will be used.
Figure 2 shows in a polarity diagram how different pulp and paper properties vary for three pulps, which have been produced according to the prior art.
Figure 3 shows the relationship between cooking yield as a percentage and the charge of effective alkali as a percentage for various cooks of birch sulphate pulp.
Figure 4 shows the relationship between the R5 value measured as a percentage and the yield of the cook measured as a percentage in the said cooks of birch sulphate pulp.

Best Embodiment

One preferred embodiment of the method according to the invention will now be described with reference to Figure 1, and finally two example embodiments will be given.
Figure 1 shows a continuous digester 1. Lignocellulose material, normally wood in the form of chips, is fed at a certain speed into the top of the digester 1. Also a certain amount of cooking liquor is fed in, such that the desired wood/liquor ratio is achieved. Furthermore, the concentration of the cooking liquor is controlled such that the desired amount of, for example, effective alkali = EA, is obtained. The wood chips are digested (cooked) during their passage down through the digester to the extent that the pulp producer desires. The digestion of the wood chips takes place at elevated pressure and elevated temperature. There is often a washing stage at the bottom of the digester in which the digested wood chips are freed from the principal part of the used cooking liquor, i.e. the spent cooking liquor. In that the digested wood chips are fed out from the digester, a severe reduction of pressure down to atmospheric pressure takes place, and this means that the wood chips in their softened and modified form are split into principally free fibres of pulp, i.e. a release of the fibres is achieved and pulp has been formed. This pulp is transferred through the line 2 to the remaining part 3 of the pulp mill. Further washing of the pulp is carried out in this part, and, furthermore, screening of the pulp in several stages. The pulp is subsequently bleached and the initial bleaching step or steps is or are normally know as the delignification step or steps, since the pulp is freed in this step or these steps from the principal part of the amount of lignin that remains in the pulp after the digestion. Thereafter, there generally follow further bleaching steps in which the brightness of the pulp is increased to a great degree, and the pulp is, furthermore, freed of chromophore groups. Further cleaning steps may follow after this, before the pulp is transferred through the line 4 either to a pulp receiving machine for conversion of the pulp to pulp for sale, or to a paper machine 5.
Central to the invention is the determination of the level of readily soluble carbohydrates in the pulp that has been produced by the digestion. The R5 value measured in percent can be chosen as a measure of the content of readily soluble carbohydrates, as has been described above. It is preferred that the R5 value is determined by means of NIR spectroscopy (according to the earlier description) of the pulp at some position along the line 2, at position 6, for example. This may be carried out in practice by a defined, washed sample of the pulp being measured with electromagnetic waves within the NIR region. It is possible in certain digesters to remove samples of the digested wood chips that are present at the bottom of the digester. Since this sample is removed from the digester, a fibre-release process is achieved such that pulp is formed, a process that is similar to the process of the release of the fibres described above. The measured signal or value is transferred via the line 7 to a control unit 8.
The content of lignin in the digested wood, i.e. in the pulp, is measured in the same manner at position 9 and the measured signal or value is transferred via the line 10 to the control unit 8. As has been specified previously, existing commercial equipment that can be used is available. Such measurement is routinely carried out in many digester plants.
It is normal during the process for the control of cooking according to the invention that the lignin content of the digested wood chips, i.e. of the pulp, is allowed to vary with time and, if batchwise digestion is considered (which has not been illustrated here), from one cook to the next. It is primarily the delignifying bleaching that must be adapted to the fact that the lignin content, in the form of kappa value, of the input pulp varies from one time to another during continuous digestion, and from one cook to another when batchwise cooking is used. Various control systems are available also for this, and the variations in the lignin content of the pulp are routinely managed by the bleacher. This means that the pulp that leaves the production process has, for example, the pre-determined value of brightness.
In order to check that the pulp at various positions, and principally the final produced pulp, has the properties that are expected not only by the pulp producer but also by the paper manufacturer, the information about these properties should be collected from several positions. The brightness of the pulp, for example, can be determined at one or several positions in the pulp mill 3 and the information is transmitted via the line 11 to the control unit 8. Samples can be taken from the final pulp in the line 4 and various properties, including paper properties, can be determined and the information transmitted via the line 12 to the control unit 8. Furthermore, samples can be taken from the sheeted pulp at position 5, if it is pulp for sale that is produced, for various analyses, and information about these is transmitted via the line 13 to the control unit 8. If the pulp is transported in slurry form to a paper machine 5, the final paper can be analysed and information about this transmitted via the line 13 to the control unit 8. It has been specified here that samples of the pulp, and possibly also the paper, are to be picked out and various analyses carried out. It is, of course, possible also to carry out the measurements directly on the pulp as it is fed out by means of what are know as non-destructive testing methods, for example, those of the type mentioned earlier in this document.
During continuous production of cellulose pulp, and in this case with respect to the digestion or the cooking, both the ramping profile and the time (t) at the maximum temperature (T) are often pre-determined, since a certain amount of wood chips is fed in at a certain speed at the top of the digester 1. Furthermore, the volume of the digester 1 is pre-determined, and this means that also the time that the wood chips are held at the maximum temperature is pre-determined. The cooking parameters that are then available to vary, i.e. that can be used for control, are the maximum temperature (T) and the charge of effective alkali (EA). It is appropriate that a computer and software are included in the control unit 8, and that this software is based primarily on the equations that are reproduced elsewhere in this document. Based on the measured analysis values, and thus principally on the content of readily soluble carbohydrates in the most recently digested wood chips, i.e. in the newly formed pulp, the software provides information not only concerning whether it is necessary to carry out a change, but also about what the change is to consist of. The change often consists of a change in the charge of effective alkali (EA), or in the content of effective alkali (EA) during the cooking procedure, or both.
If it is desired to change the production of pulp at a certain time, for example, to increase the production, the time that it takes for the wood chips to pass through the digester is changed, i.e. the time will be shorter. This means that the time at the maximum temperature will be shorter than it was previously. In order for the digester to have sufficient time for the desired digestion and delignification, it is generally necessary to increase the maximum temperature, and a further means is available that may be used, namely to increase the charge of effective alkali (EA). The details of how this is to be carried out are made clear by the relationships and equations that are given elsewhere in this document.
The control method according to the invention described above is to be applied, as has been made clear, for continuous cooking. It will not be a problem for one skilled in the art to transfer the instructions to batch-wise cooking.

Example 1

Three experiments were carried out at a laboratory with the digestion (cooking) of birch wood according to the sulphate method. The experiments involved the control of the digestion according to prior art technology, i.e. such that one kappa value was achieved for the pulps despite variation in certain cooking parameters.

Birch logs were extracted from a lumber yard in a pulp mill and transported to the laboratory. The logs were debarked by hand and chopped to chips in a chute-fed chip mill. The obtained chips were characterised with respect to geometry, density and chemical composition. The analysis methods used are given in Table 1 and the analysis results in Table 2.

Table 1

Dry matter content	= SCAN-CM 39:94
Basic density	= SCAN-CM 43:95
Bulk density	= SCAN-CM 46:92
Carbohydrates	= In-house method, KA 10.202
Acetone soluble matter	= SCAN-CM 49:93
Content of metals	= SCAN-CM 38:96
Lignin	= CCA 5, somewhat modified
Fibre length	= KCL 225:89

Table 2

Xylose, g/kg	= 169
Mannose, g/kg	= 11.3
Galactose, g/kg	= 10.1
Arabinose, g/kg	= 3.1
Glucose, g/kg	= 313
Fibre length, mm	= 1.04
Dry matter content, %	= 64.1
Bulk density, kg/m³	= 170
Basic density, kg/m³ fixed without bark	= 500
Lignin content, %	= 20.6
Acetone soluble matter, %	= 1.62
Iron, mg/kg	= 49.7
Calcium, mg/kg	= 639
Copper, mg/kg	= 1.6
Magnesium, mg/kg	= 177
Manganese, mg/kg	= 110

The wood material in the form of the chips described was cooked in a conventional laboratory circulation digester. Each cooking consisted of a batch of 2 kg chips. Furthermore, industrial white liquor and deionised water were added to give the desired liquor/wood ratio in order to obtain the cooking liquor. The different cooks then followed the ramping profile or temperature profile given below:

20°C to 120 °C in 5 minutes
120 °C to 145 °C in 120 minutes
145 °C to 151°C in 60 minutes.

The following H factor was used = $\int_{0}^{t} e^{(\frac{38.2 - 14250}{273 + T})}$
Other conditions for each experiment (each cook) are given below.

Cook 1

Alkali charge = 25% effective alkali (EA), i.e. NaOH + ½Na₂S, calculated on the basis of the wood
Liquor/wood ratio =3.5 l/kg
H factor = 270

Cook 2

Alkali charge = 20% EA
Liquor/wood ratio =3.5 l/kg
H factor = 350

Cook 3

Alkali charge = 17% EA
Liquor/wood ratio =3.5 l/kg
H factor = 500.

Samples of the cooking liquor were taken at the end of the digestion (the cook) in order to check the content of residual alkali.

After washing of the pulp with deionised water in the digester, the pulp was screened. The screened pulp was analysed using the analysis methods that are specified in Table 3 below.

Table 3

Kappa value	= SCAN-C 1:00
Viscosity	= SCAN-CM 15:88
Brightness	= SCAN-P 3:93
Total yield	= In-house standard
Level of rejects	= In-house standard
Residual alkali	= SCAN-N 33:94
Carbohydrates	= KA 10.202
Beating revolutions	= EN 25264-2:1994
Degree of beatability	= ISO 5267-1:1999
Surface weight	= ISO 536:1995
Young's modulus, tensile strength, tensile stiffness	= SCAN-P 67:93
Tear strength	= EN 21974:1994
Bursting strength	= SCAN-P 24:99
Z-strength	= ISO 8791-4

The analysis results for each pulp are given in Table 4 below.

Table 4

Property	Cook
Property	1	2	3
Kappa value	16.0	16.2	16,3
Viscosity, ml/g	1193	1264	1386
Brightness, %ISO	44.0	41.3	38.1
Residual alkali, g/l	17.9	8.0	2.3
Yield, %	50.5	52.3	54.2
Lignin-free yield, yield - 0.15·Kappa	48.1	49.8	51.6
Rejects, %	0.02	0.03	0.04
Hemicellulose	26.4	28.2	28.6
Tensile index, Nm/g, mean value at beating revolutions 1000, 2000, 3000	83.8	97.5	105.8
Tensile stiffness index, MNm/kg, mean value at beating revolutions 1000, 2000, 3000	9.7	10.3	11.0
Degree of beatability, °SR, mean value at beating revolutions 1000, 2000,3000	20.7	22.3	24.0
Tear index, mNm²/kg, mean value at tensile index 70, 80, 90	7.8	7.2	7.0
Bursting index, kPa m²/g, mean value at beating revolutions 1000, 2000, 3000	4.8	5.4	6.2
Z-strength, kPa, mean value at beating revolutions 1000, 2000, 3000	651	731	758

Certain of these measured values of properties of the three pulps have been entered on the polar diagram that is reproduced as Figure 2. The diagram demonstrates nine axes, where each axis demonstrates the property values as specified below:

14 =: Kappa value, 12-20
15 =: Cooking yield, 50-55
16 =: Tensile index, 80-110
17 =: Tensile stiffness index, 9-11
18 =: Beatability, 20-25
19 =: Tear index, 6-8
20 =: Bursting index, 4-7
21 =: Z-strength, 600-800
22 =: Brightness, 35-45

With respect to the numerical values that are specified after each property, the lower value corresponds to the centre of the diagram (the origin), i.e. where the axis starts, while the higher value corresponds to the end of the axis. The axis 15 can be studied as an example, giving the cook yield as a percentage. A yield of 50% is valid for the origin, while the termination, or end, of the axis corresponds to a yield of 55%.
The various cooks have been given the following symbols:

- ◆ - = Cook 1
- ■ - = Cook 2
- ▲ - = Cook 3

The parameters that differ between the various cooks with respect to the cooking parameters are the levels of cooking chemicals, i.e. the alkali charge in the form of effective alkali, and the time at the maximum temperature, here regulated by, and reproduced as, H factor. It will be clear to one skilled in the art that if the level of cooking chemicals is reduced, the time for the cook must be increased, and in that case, normally the time at the maximum temperature, in order to achieve the same delignification of the wood, i.e. in order to achieve a given, constant, value of kappa.
As the polar diagram makes clear (see Axis 14), all three cooks have resulted in a pulp with the same kappa value. This is the only property that demonstrates equality. The differences between the different pulps with respect to all of the other properties are striking. The diagram shows in an illustrative manner that it is not possible to produce and deliver a pulp with properties that are given a high priority by the paper manufacturer, i.e. properties of a pre-determined value during the production of sulphate pulp from birch wood when the control of the cooking is based on consistently obtaining a certain kappa value, and worst of all that it is not possible to guarantee pulp with the properties given priority from one delivery to another. This is the case for all paper manufacturers, independently of whether the manufacturer receives pulp in a dried form (and sheeted or flaked), or in a non-dry form, as a slurry, from a closely lying pulp mill.

Example 2

A very large number of experiments were carried out in a laboratory involving digestion (cooking) of birch wood according to the sulphate method. The aim of the experiments was not only to develop a control model according to the invention for the digestion of short-fibred lignocellulose material based on the measurement of the content of readily soluble carbohydrates in the pulp that is produced but also to see whether the control model gave the result intended.

Birch logs were extracted from a lumber yard at a pulp mill and transported to the laboratory. The logs were debarked by hand and chopped to chips in a chute-fed chip mill. The obtained chips were characterised with respect to geometry, density and chemical composition. The analysis methods used is clear from Table 1 in Example 1. The analysis results are presented in Table 5 below.

Table 5

Glucose, g/kg	= 320
Xylose, g/kg	= 173
Mannose, g/kg	= 12.0
Galactose, g/kg	= 11.1
Arabinose, g/kg	= 2.8
Dry matter content, %	= 63.1
Bulk density, kg/m³	= 172
Basic density, kg/m³	= 498
fixed without bark

The wood material in the form of the chips described was cooked in a conventional laboratory circulation digester. Each cook consisted of a batch of 2 kg chips. Furthermore, industrial white liquor and deionised water were added to give the desired liquor/wood ratio in order to obtain the cooking liquor. The different cooks then followed the ramping profile or temperature profile given below:

20 °C to 80 °C in 5 minutes
80 °C to the maximum temperature in 60 minutes

The following H factor was used = ${\int_{0}^{t} e}^{(\frac{38.2 - 14250}{273 + T})}$
Other conditions for each cook are given in Table 6. The pulp was washed after the cook with deionised water in the digester. The pulp was then screened in order to be finally analysed using a number of the analysis methods that are specified in Table 3 in Example 1.

Other promised conditions and raw data from 25 cooks are presented in Table 6 below with respect to different properties of the pulp.

Table 6

Cook	Alkali charge %	Max temp °C	Time at max temp	H factor	Total yield %	Lignin -free yield* %	Rejects %	Kappa value	Visc. ml/g	ISO brightness % ISO	R5 %
4	17	160	120	429	53.8	52.5	0.60	16.6	1340	39.3	76.3
5	19	160	120	429	52.8	51.7	0.10	14.4	1198	42.9	76.7
6	21	160	120	429	51.8	50.6	0.11	14.9	1126	42.1	78.2
7	23	160	100	363	51.7	50.6	0.10	14.7	1064	42.6	78.7
8	25	160	100	363	50.7	49.7	0.07	13.1	1026	43.3	80.1
9	27	160	80	297	49.4	48.4	0.12	13.4	995	45.0	81.5
10	29	160	80	297	48.3	47.4	0.23	12.7	939	44.7	83.4
11	17	150	220	350	54.9	53.4	0.91	18.5	1431	37.7	74.6
12	19	150	220	350	53.9	52.6	0.05	16.4	1309	40.3	75.6
13	21	150	220	350	53.2	51.6	0.03	16.4	1256	42.1	76.5
14	23	150	200	320	50.4	49.3	0.01	14.9	1206	42.8	78.2
15	25	150	200	320	51.1	50.0	0.01	14.2	1143	43.5	79.2
16	27	150	160	290	49.4	48.3	0.05	14.5	1127	44.2	80.7
17	29	150	160	290	49.2	48.2	0.06	13.4	1068	44.9	82.6
18	17	170	80	627	52.1	50.6	1.10	17.5	1291	37.8	76.1
19	19	170	80	627	50.8	49.7	0.15	14.6	1098	41.5	77.3
20	21	170	80	627	50.0	48.9	0.15	14.5	1027	40.8	78.5
21	23	170	60	488	49.1	48.0	0.50	15.2	900	41.7	79.5
22	25	170	60	488	48.0	47.0	0.25	13.9	886	42.6	81.5
23	27	170	40	349	47.6	46.7	0.38	13.0	892	43.7	82.5
24	29	170	40	349	45.9	45.1	0.57	12.3	791	44.6	84.3
25	18	150	220	350	54.5	53.0	0.24	17.8	1384	39.3	77.0
26	18	160	120	429	53.1	51.8	0.14	16.9	1291	39.4	74.8
27	22	160	100	363	51.2	50.0	0.05	15.2	1108	42.2	78.1
28	22	170	60	488	49.8	48.7	0.12	14.5	1019	41.9	78.8

* Lignin-free yield = Total yield (1- 0.15· Kappa/100)

Study of the information given in the table makes it clear that the selected levels of charge of effective alkali (given as alkali charge in the table and in Figures 3 and 4), the maximum temperature and the time at the maximum temperature gave large differences in the properties of the pulp. The cooking yield, for example, varied between 46 and 55%, i.e. a difference of 9 percentage points. The cooking yield decreases with increased charge of effective alkali, increased temperature and increased time. There is a marked reduction in yield when the temperature is raised from 160 to 170°C, The difference in yield between the pulps cooked at 150 °C and 160 °C is not as large. It is probable that the cellulose in the pulp is degraded to a higher degree at high cooking temperatures.
Figures 3 and 4 make it clear that if the correlation for different operating conditions - in this case the maximum temperature - is calculated, the yield can be indirectly measured by means of analysis of the content of readily soluble carbohydrates, for example R5, in the pulp.
This relationship and this insight form the basis of the revealed control strategy for, for example, the production of sulphate pulp according to the invention.
A model for the relevant reply, or response - yield, kappa value, viscosity and R5 - has been developed using PLS ("Particle Least Square") calculations on the results of the 25 cooks. The models obtained are linear models and they can be exemplified by the equation for calculating the yield, given on Page 6 of this document.

Table 7 below shows control models that are based on the PLS treatment of the laboratory cooks carried out.

Table 7

Control variables		Constants
Control variables		Yield % 76.25	Visc. ml/g 2509.001	Kappa value 27.46	R5 % 62.661
Charge of EA, %	21	-0.556	-30.72	-0.395	0.5791
Max. temperature, °C	160	-0.064	-4.689	-0.023	0.0261
Time at max temperature, min	120	-0.0002	0.789	0.003	-0.007
H factor	429	51.6	1161	15.6	77.9

The physical properties, such as tensile index, tear index, beatability, etc., of the pulps at different yields have been approximated using linear regression for the physical properties obtained at different yields in the experiments that are presented in Example 1.
Finally, three further experiments were carried out in the form of cooks 29, 30 and 31.
The same chips were used in these experiments as those used in Example 1, and, furthermore, the same ramping profile or temperature profile were used as those used in that example.
The cooks were carried out in the laboratory digester according to the batch method, but continuous cooking at large scale was simulated in the following manner. In a first operating condition 29, which simulates a certain production of pulp in a continuous digester, the cooking parameters were as follows: charge of effective alkali =21%, maximum temperature = 160 °C, time at maximum temperature =120 minutes, and H factor = 429. In the second operating condition 30 it is simulated that the production of pulp is increased to a certain higher level. This means that the wood chips flow down through the digester at a higher speed than previously, which leads to the time at the maximum temperature also being reduced, in this case to 100 minutes. One skilled in the art will know that in this case the maximum temperature must be raised in order to obtain approximately the same degree of digestion of the treated wood chips that leave the digester as previously. The maximum temperature was raised in this case to 165 °C, which meant that the H factor was raised to 532. The charge of effective alkali was the same as previously, i.e. 21%. In the third operating condition, it is attempted to return to the same pulp and paper properties as those demonstrated by the pulp according to the original condition, i.e. cook 29. It is in this condition that the control method according to the invention will be used. The time at the maximum temperature is pre-determined and cannot be changed, and it is, as it was in cook 30, one hundred (100) minutes. Using the measured R5 value and based on this calculated cooking yield, the previously described control relationship (see the equations given for the higher range of temperatures, i.e. 160-170°C) showed that the maximum temperature = 165 °C and the H factor 532 could be retained from operating condition 2, i.e. cook 30, and that the charge of effective alkali should be reduced to 19%.

The measured pulp and paper properties are given in Table 8.

Table 8

	Cook
	29	30	31
Yield, %	51.6	50.6	51.8
Kappa value	15.6	15.3	16.1
Viscosity, ml/g	1161	1110	1172
Brightness, %ISO	42.4	44.0	42.0
R5, %	77.9	78.1	77.0
Tensile index, Nm/g	92.2	83.8	93.7
Tensile stiffness index, MNm/kg,	10.1	9.7	10.1
Degree of beatability, °SR	21.7	20.9	21.9
Tear index, mNm²/g,	7.4	7.8	7.4
Bursting index, kPam²/g	5.2	4.8	5.2
Z-strength, kPa	700	651	709

If the paper properties of the pulps that were produced in cooks 29, 30 and 31 are studied, it can be seen that all of these properties are nearly identical for cooks 29 and 31, while these properties for cook 30 are markedly different. This shows that it is possible with the control method according to the invention to return in a successful manner to producing a pulp with the desired pulp and paper properties after changes in the operating conditions and also after unplanned interruptions in operation. This can be compared with the pulps that during production in respect of the digestion have been controlled according to the prior art and whose pulp and paper properties are represented in the polar diagram in Figure 2. The diagram makes it clear that, for example, the paper properties of these pulps vary in a very uncontrolled manner.

Claims

A method for production of chemical pulp by digestion (cooking) of short-fibred lignocellulose material with use of a certain ramping profile, a certain maximum temperature, a certain time at this maximum temperature, and certain content of cooking reagents, whereby the pulp in association with the end of the digestion is analysed with respect to at least two properties of the pulp fibres, one of which is constituted by the lignin content of these,
characterised in that also the content of readily soluble carbohydrates in the pulp fibres is determined, which content has an influence on several properties of the pulp and of paper manufactured from the pulp, and in that control of the cooking parameters specified above is based in the first hand, primarily, on the content of readily soluble carbohydrates and in the second hand, secondarily, on the content of lignin, and in this case, preferably, in a manner that is approximate while, however, always taking both contents into consideration.
The method according to claim 1,
characterised in that the digestion takes place in alkaline conditions.
The method according to claims 1 and 2,
characterised in that the digestion takes place according to the sulphate method, i.e. such that sulphate pulp is produced.
The method according to claims 1-3,
characterised in that the content of readily soluble carbohydrates in the fibres is determined through the pulp fibres being placed into a sodium hydroxide solution with a concentration of 1 - 10%, preferably 5%.
The method according to claim 1-3,
characterised in that the content of readily soluble carbohydrates in the fibres is determined in an indirect manner by means of NIR (Near InfraRed) spectroscopy.
The method according to claims 1-3,
characterised in that the content of lignin in the fibres is determined as a kappa value in an indirect manner by means of NIR (Near InfraRed) spectroscopy.
The method according to claim 1,
characterised in that the cooking yield (Y), which is related to the measured amount of readily soluble carbohydrates, is selected as dominant in the control of the cook together with the cooking parameters: maximum temperature (T) in °C, time (t) in minutes at the maximum temperature, H factor, and charged cooking chemicals in the form of effective alkali (EA) in percentage by weight (%), and the content of effective alkali (EA) during the digestion.
The method according to claims 1 and 7,
characterised in that the control of the cook is based on a maintenance of the calculated cooking yield Y and takes place according to the following description in the event of a change in production, the cooking yield for the current operation conditions is determined from the content of readily soluble carbohydrates in the pulp in the form of analysis of R5 according to (for the relevant interval of temperature): $Y_{T 1 \leq T \leq T 2} = \frac{b 1 - R 5}{a 1} - \frac{T - T 1}{T 2 - T 1} \cdot (\frac{R 5 - b 1}{a 1} - \frac{R 5 - b 2}{a 2})$
$Y_{T 2 \leq T \leq T 3} = \frac{b 2 - R 5}{a 2} - \frac{T - T 2}{T 3 - T 2} \cdot (\frac{R 5 - b 2}{a 2} - \frac{R 5 - b 3}{a 3})$

and where the experienced EA charge of the wood chips at a yield of Y is described by: ${EA}_{T 1 \leq T \leq T 2} = \frac{Y - d 1}{c 1} - \frac{T - T 1}{T 2 - T 1} \cdot (\frac{Y - d 1}{c 1} - \frac{Y - d 2}{c 2})$
${EA}_{T 2 \leq T \leq T 3} = \frac{Y - d 2}{c 2} - \frac{T - T 2}{T 3 - T 2} \cdot (\frac{Y - d 1}{c 2} - \frac{Y - d 3}{c 3})$

and the new cooking yield in the event of a change in the operating condition at the relevant EA charge is described by: $Y_{T 1 \leq T \leq T 2} = c 1 \cdot EA + d 1 - \frac{T - T 1}{T 2 - T 1} \cdot (c 1 \cdot EA + d 1 - (c 2 \cdot EA + d 2))$
$Y_{T 2 \leq T \leq T 3} = c 2 \cdot EA + d 2 - \frac{T - T 1}{T 2 - T 1} \cdot (c 2 \cdot EA + d 2 - (c 3 \cdot EA + d 3))$

and in order to return to the original cooking yield the charge of effective alkali EA is changed according to: ${EA}_{T 1 \leq T \leq T 2} = \frac{Y - d 1}{c 1} - \frac{T - T 1}{T 2 - T 1} \cdot (\frac{Y - d 1}{c 1} - \frac{Y - d 2}{c 2})$
${EA}_{T 2 \leq T \leq T 3} = \frac{Y - d 2}{c 2} - \frac{T - T 2}{T 3 - T 2} \cdot (\frac{Y - d 1}{c 2} - \frac{Y - d 3}{c 3})$

and where the constants a1, a2, a3, b1, b2, b3, c1, c2, c3, d1, d2, and d3 are determined empirically.
The method according to claims 1 and 7,
characterised in that the control of the cook is based on maintaining constant the calculated cooking yield Y and when a particular operating condition is to be maintained the following takes place, a target value is initially used with respect to the content of readily soluble carbohydrates R5, and the cooking yield at this target value is determined by (for the relevant temperature interval): $Y_{T 1 \leq T \leq T 2} = \frac{b 1 - R 5}{a 1} - \frac{T - T 1}{T 2 - T 1} \cdot (\frac{R 5 - b 1}{a 1} - \frac{R 5 - b 2}{a 2})$
$Y_{T 2 \leq T \leq T 3} = \frac{b 2 - R 5}{a 2} - \frac{T - T 2}{T 3 - T 2} \cdot (\frac{R 5 - b 2}{a 2} - \frac{R 5 - b 3}{a 3})$

and where the EA charge is described by: ${EA}_{T 1 \leq T \leq T 2} = \frac{Y - d 1}{c 1} - \frac{T - T 1}{T 2 - T 1} \cdot (\frac{Y - d 1}{c 1} - \frac{Y - d 2}{c 2})$
${EA}_{T 2 \leq T \leq T 3} = \frac{Y - d 2}{c 2} - \frac{T - T 2}{T 3 - T 2} \cdot (\frac{Y - d 1}{c 2} - \frac{Y - d 3}{c 3})$

and when the measured R5 value shows that the cooking yield has been changed from Y1 to Y2 then the charge of effective alkali EA is compensated by calculation by means of the immediately preceding equation according to: $Δ EA = {EA}_{Y 1} - {EA}_{Y 2}$

and where the constants a1, a2, a3, b1, b2, b3, c1, c2, c3, d1, d2, and d3 are determined empirically.