JP5684655B2 - Multivariable control method and apparatus for bleaching process - Google Patents

Description

  The present invention relates to a multivariable control method and apparatus for a bleaching process, and more particularly to a multivariable control method and apparatus for a bleaching process that takes into account the content of a hexeneuronic acid component in a pulp raw material.

Chlorine water (Cl ₂ ) was used as a bleaching chemical in “chlorine bleaching”, which was once widely used as a bleaching method for pulp raw materials. However, in recent years, it has been found that this bleaching method may generate chloroform components and dioxins that are harmful to living organisms. Therefore, it is now possible to avoid the generation of chloroform components and dioxins for environmental protection. (Elemen- tary Chlorine Free bleaching) has been widely adopted in the bleaching process of pulp raw materials. In ECF bleaching, chlorine dioxide water (ClO ₂ ) is used as the main bleaching chemical instead of chlorine water (Cl ₂ ) used in chlorine bleaching. The whiteness of a certain pulp is adjusted mainly by changing the addition rate of this chlorine dioxide water.

However, chlorine dioxide is an expensive chemical and reduces the amount of bleaching chemical used, so the bleaching ability is slightly inferior. However, a part of this chlorine dioxide addition is hydrogen peroxide, a bleaching chemical for cheap pulp raw materials. The replacement with water (H ₂ O ₂ ) is widely performed.

  On the other hand, in the ECF bleaching of hardwood pulp such as eucalyptus wood, which has recently been widely used as a chip material used as a raw material for paper, a part of bleaching chemical is converted into hydrogen peroxide solution for the above reasons. If replaced, the whiteness value that is inspected from the appearance of the pulp raw material immediately after production is not known only, but when it becomes a paper product, it will cause the paper whiteness to deteriorate. ) Is particularly important as regards the content of the hexeneuronic acid component contained in the pulp raw material.

The reason for this is that pulp materials made from hardwood chip materials such as eucalyptus wood that are planted trees have a higher content of hexeneuronic acid component than other chip materials as a characteristic of the tree species. In that case, if the addition rate of chlorine dioxide water having the ability to decompose hexeneuronic acid component is also increased, the amount of hexeneuronic acid remaining in the pulp raw material can be reduced, and it becomes possible to avoid paper discoloration trouble. Thus, if the addition rate of chlorine dioxide water (ClO ₂ ) is reduced by cost measures and replaced with the addition of hydrogen peroxide water (H ₂ O ₂ ) as an alternative bleaching chemical, Since there is no chemical effect to decompose the hexeneuronic acid component in the pulp raw material, there is no problem with the final value of whiteness, but in some cases the content of the hexeneuronic acid component in the pulp raw material cannot be reduced sufficiently In addition, originally, the expected pulp quality cannot be ensured, and the risk of paper fading trouble is further increased.

  That is, based on the appearance inspection of the pulp material only for the whiteness, it is assumed that there is no problem with the whiteness quality. If a return trouble occurs at the customer, it becomes a big complaint problem.

In addition, as a supplementary explanation, “chlorine bleaching” of pulp raw material using chlorine water (Cl ₂ ), which was widely used before “ECF bleaching” became widespread, is a hexeneuronic acid component that is abundant in hardwood chips. Was strongly decomposed by the bleaching reaction using chlorinated water, and the fading trouble in the paper product caused by hexeneuronic acid was not recognized as a problem in the bleaching process.

  In addition, the whiteness value of pulp raw materials is generally measured online in a manufacturing plant using an optical whiteness sensor, but the content of hexeneuronic acid components in pulp raw materials is originally Since it is a very small amount (a content of the order of millimol / kg) and is a special substance, its detection is extremely difficult. When the problem of paper discoloration has become apparent, it has been used in the laboratory (laboratory) as a raw material for pulp. After processing, there is only a means for detecting the content using a large and precise spectroscopic device, and until recently there has not been a sensor that can be measured online in a manufacturing plant.

  However, using the optical "kappa valence meter" that was already installed during the manufacturing process and used online to measure the kappa number as an index of the content of the lignin component in the pulp raw material using the spectroscopic principle, This “Kappa valence meter” is equipped with a spectroscopic device with a light emitting device of a specific wavelength band inside the sensor, and by measuring the absorption of light in this wavelength band by the hexeneuronic acid component, It was found that the content of the hexeneuronic acid component can be accurately measured, and the on-line measurement has become possible.

  That is, light of a specific wavelength band of 230 to 240 nm in the ultraviolet light region is applied to a diluted pulp raw material to be measured, and the amount of hexeneuronic acid component contained in the pulp raw material is measured from the ultraviolet light absorbance. This has become possible (see Patent Document 2).

This measurement method is an effective measurement method for pulp materials in the high whiteness range with an ISO whiteness value of 80% or more. Also, ECF (Elementary Chlorine Free) using chlorine dioxide water (ClO ₂ ) as a bleaching chemical. ) This is a method for measuring the amount of hexeneuronic acid component that is effective not only in the bleaching step but also in a TCF (Total Chlorine Free) bleaching step in which no chlorine element component is used as a bleaching chemical. The value of ISO whiteness, which is the management value of pulp whiteness, is set to 100% as the maximum value, and as the bleaching of the pulp proceeds and the whiteness of the pulp raw material increases, the value increases.

  This sensor development has made it possible to analyze the content of hexeneuronic acid components in pulp raw materials, but using an on-line spectroscopic sensor using this measurement principle, the value of the content of hexeneuronic acid components in pulp raw materials It has become possible to apply to the bleaching control of the pulp raw material intended in this specification by continuously measuring.

  On the other hand, in order to manage the bleaching reaction of complex pulp raw materials so that it is always in an appropriate state overall, when incorporating the function of automatic bleaching control into the bleaching process, it is necessary to use multiple variables. Although application of control methods is indispensable, as a method used for multivariable control, a control function called “model predictive control”, which is frequently used as a control method suitable for chemical plants, is widely known in recent years. (See Patent Document 1, Non-Patent Documents 1 and 2.)

  In addition, the ECF bleaching process in the pulp manufacturing process is realized with equipment consisting of multiple multi-stage bleaching towers, and the pulp raw material is gradually bleached while moving slowly through multiple bleaching towers. The reaction proceeds. And although the residence time of a pulp raw material changes with a production rate, it becomes a long reaction time of 3 to 6 hours as a whole.

JP 2010-255138 A JP 2008-266798 A

Yoshiteru Mori, Masahiro Kawakita, Atsushi Nishimura, Kazuo Sueyoshi, “Improvement of bleaching efficiency in pulp bleaching process by pH control”, 52nd Automatic Control Alliance Lecture, Osaka University, November 2009 Masahiro Kawakita, Yoshinori Mori, Atsushi Nishimura, “Multivariable Control of Bleaching Whiteness Using Model Predictive Control”, Paper-Paper Technical Journal, Vol. 61, no. 3, 2007 Yoshiteru Mori, Masahiro Kawakita, Satoshi Nishimura, Kazuo Sueyoshi, “Application and Effect of Model Predictive Control in Paper Pulp Bleaching Process”, Instrumentation, Vol. 50, no. 6, 2007 Rick Van Fleet, “Spreading the Bleaching Load”, Honeywell, MPC in the Bleach Plant through the use of RMPCT, PAPTAC Control Systems 2000 / Workshop document, 2000 Teuvo Peltomaki, “Latest Technology for ECF Bleaching Control”, Tappi 1998 Pulping Conference, Montreal.

  However, the bleaching reaction of pulp raw materials proceeding with bleaching chemicals inside each bleaching tower is very complicated, and the pH value during the bleaching process, the residual chlorine of bleaching chemicals remaining in the pulp raw materials and washing filtrate Many of the response variables such as the concentration and the operation variables and the response variables interfere with each other, and the operation of the ECF bleaching process is not easy.

  Therefore, it is possible to automate the operation of adding multiple types of bleaching chemicals, such as bleaching chemicals that are effective in increasing pulp whiteness and bleaching chemicals that are effective in degrading hexeneuronic acid, so that they can always be performed in an appropriate manner. There was a problem of wanting to make it.

  In addition, management operation by a skilled operator is necessary for the operation of the bleaching process. However, in the DCS device used by the operator for the operation of the plant, the control performance is naturally required for the operation performed only by the PID control function of the DCS device. There is a limit in terms.

  Therefore, for such a bleaching step, it is expected to perform an effective control operation in such a manner that the accuracy of reaching the target quality of the pulp raw material is increased as much as possible and the manufacturing cost is reduced.

  The present invention has been made for such problems, and its purpose is to control the whiteness control for bleaching pulp raw materials, and to measure the content of hexeneuronic acid components in pulp raw materials installed in the bleaching process. It is an object of the present invention to provide a multivariable control method and apparatus for a bleaching process capable of controlling the content of hexeneuronic acid together with whiteness using a sensor and further suppressing the use of chlorine dioxide and other bleaching chemicals.

In order to solve the above-mentioned problems, the invention according to claim 1 is a multivariable control method of a bleaching step of ECF bleaching a pulp raw material, and chlorine water is added to the pulp in a first bleaching tower In the first step of measuring the whiteness and pH of the pulp , and in the second bleaching tower, caustic soda and hydrogen peroxide solution are added to the pulp that has flowed from the first bleaching tower, a second step of measuring pH, whiteness, and hexeneuronic acid component content; and in a third bleaching tower, chlorine dioxide is added to the pulp that has flowed from the second bleaching tower, and the pH of the pulp, A third step of measuring residual salt concentration, whiteness, and content of hexeneuronic acid component, and the chlorine dioxide addition rate in the first step, which is an operation variable, is added to the first step, which is a control variable. Oh Wherein the whiteness and the pH of the pulp that the content of whiteness and HexA components of the pulp in the second step, as the whiteness of the pulp in the third step is each a predetermined value The caustic soda addition rate in the second step that is set and manipulated variable is the pH, whiteness, and hexeneuronic acid component content in the second step that is the control variable, and the content in the third step. The pH of the pulp is set to be a predetermined value, and the hydrogen peroxide solution addition rate in the second step which is an operation variable is the pH and whiteness of the pulp in the second step which is a control variable. Are set to predetermined values, and the chlorine dioxide addition rate in the third step, which is an operation variable, is controlled by the control variable. Wherein the 3 pH of the pulp in step, residual chlorine concentration, the content of whiteness and HexA component and sets so that each becomes a predetermined value is.
The invention according to claim 2 is the multivariable control method of the bleaching step according to claim 1, wherein the pulp raw material is hardwood pulp.

Invention of Claim 3 is the multivariable control method of the bleaching process of Claim 1 or 2, The content of the hexeneuronic acid component in the said pulp raw material is 230-240 nm ultraviolet light absorptivity of a measurement sample or JIS P 8206: characterized in that it is obtained from a K value defined in 1994.

The invention according to claim 4 is the multivariable control method of the bleaching process according to any one of claims 1 to 3 , wherein the second step measures the liquid level of the second bleaching tower. And adjusting the feed flow rate so as to maintain the liquid level of the second bleaching tower at a predetermined level .

The invention according to claim 5 is the multivariable control method of the bleaching process according to any one of claims 1 to 4 , wherein a transfer function is used for the prediction of the control variable .

The invention according to claim 6 is the multivariable control method of the bleaching process according to any one of claims 1 to 5 , wherein the deviation amount of the control variable is used as the evaluation function of the first to third steps. Each control variable is controlled so that the evaluation function is minimized, using the total value of the control variables, the total operation amount of the operation variables, and the total cost of the added chlorine dioxide, caustic soda and hydrogen peroxide water. And

The invention according to claim 7, multivariable control method odors Te, hexenuronic acid content prior Symbol pulp in the raw material of the bleaching process according to any one of claims 1 to 6, measurements and online sensors The estimation value by the function of the soft sensor is used.

The invention according to claim 8 is a multi-variable control device for bleaching process, which is a multi-variable control device for bleaching process for ECF bleaching pulp raw material, and in the first bleaching tower , chlorine dioxide water is added to the pulp. A first chlorine dioxide control unit to which the first bleaching tower is added; a first whiteness meter for measuring the whiteness of the pulp flowing into the first bleaching tower; and the pH of the pulp in the first bleaching tower. A first pH meter to be measured; a caustic soda controller for adding caustic soda to the pulp flowing from the first bleaching tower in the second bleaching tower; and the first bleaching tower in the second bleaching tower. A hydrogen peroxide controller for adding hydrogen peroxide to the pulp flowing in from the tower, a second pH meter for measuring the pH of the pulp in the second bleaching tower, and the second bleaching tower Hex to measure the content of hexeneuronic acid component of the pulp in In the uronic acid analyzer, the second whiteness meter for measuring the whiteness of the pulp in the second bleaching tower, and the third bleaching tower, the pulp flowing from the second bleaching tower into the A second chlorine dioxide controller for adding chlorine, a third pH meter for measuring the pH of the pulp in the third bleaching tower, and a residual salt concentration of the pulp in the third bleaching tower. A residual salt thermometer to be measured, a third whiteness meter to measure the whiteness of the pulp in the third bleaching tower, and a content of the hexeneuronic acid component of the pulp in the third bleaching tower A hexeneuronic acid analyzer for measuring, and the chlorine dioxide addition rate operated in the first chlorine dioxide control unit is the whiteness and pH of the pulp in the first bleaching tower, and the second bleaching tower The whiteness of the pulp and the content of the hexeneuronic acid component, The whiteness of the pulp in the third bleaching tower is set to be a predetermined value, and the caustic soda addition rate operated in the caustic soda control unit is the pH, whiteness of the pulp in the second bleaching tower And the content of the hexeneuronic acid component, the pH of the pulp in the third bleaching tower is set to a predetermined value, and the hydrogen peroxide solution addition rate operated in the hydrogen peroxide solution control unit is The pH and whiteness of the pulp in the second bleaching tower are set to predetermined values, respectively, and the chlorine dioxide addition rate operated in the second chlorine dioxide control unit is set to the third bleaching tower. It is characterized in that the pH, residual chlorine concentration, whiteness and hexeneuronic acid component content in the tower are set to predetermined values .

  The present invention uses a model predictive control function to simultaneously control the quality of both the whiteness of the pulp raw material and the content of the hexeneuronic acid component in the pulp raw material. The mutual balance of the distribution of additive amounts can be automatically performed in a more appropriate manner, thereby reducing the total bleaching chemical cost used for pulp bleaching to the limit and realizing economic effects during pulp production. It is possible to continue.

  The determination of the optimum value of each added amount of multiple types of bleaching chemicals, which is always required when this driving operation is continued for 24 hours continuously, is a very difficult operation judgment by human driving, According to the automatic operation using the model predictive control, it is possible to accurately perform the optimization calculation inside the control function, and it is possible to simultaneously reduce the operation load on the operator.

It is a figure which shows the flow of the multistage bleaching process of the kraft pulp which concerns on the Example of this invention. It is the table | surface which showed the example of installation of the installation sensor used for the pulp bleaching control of the Example of this invention. It is a figure shown about the structural example of the pulp bleaching control using the multivariable control of this invention. It is a figure which shows the specific example of the structure of the pulp bleaching control which applied model prediction control based on the structure of FIG. (A) is a figure which shows distribution of the amount of hexeneuronic acid components in a pulp raw material, and "absorbance" when using the ultraviolet light of a wavelength band of 232 nm by a precision spectrometer, (b) is a kappa number. The distribution of the "absorbance" measured by applying light having a wavelength of 232 nm to the pulp raw material using a meter and the "K value" of the pulp raw material is shown.

  Hereinafter, embodiments of the present invention will be described in detail.

  The bleaching process to be controlled by the present invention is a process for ECF bleaching of hardwood pulp raw material, and without using molecular chlorine as a bleaching chemical, for example, chlorine dioxide, ozone, hydrogen peroxide, peracid, etc. Is a bleaching process.

  ECF bleaching is bleached in multiple stages, for example, chlorine dioxide stage (sometimes referred to as D stage or D), ozone stage (sometimes referred to as Z stage or Z), alkali extraction stage (E stage or E and Oxygen-added alkali extraction stage (sometimes called Eo stage or Eo), hydrogen peroxide-added alkali extraction stage (sometimes called Ep stage or Ep), oxygen, hydrogen peroxide-added alkali extraction Stage (sometimes referred to as Eop stage or Eop), peracid stage such as persulfuric acid or peracetic acid (sometimes referred to as Pa stage or Pa), acid treatment stage (sometimes referred to as A stage or A), etc. A bleaching sequence combining the respective bleaching stages can be applied. The number of stages to be combined may be appropriately selected according to the purpose, and the number of bleaching stages may be increased to increase the bleaching effect.

  As specific bleaching sequences in ECF bleaching, D-E-D, Z-E-D, Z / D-E-D, D-Eo-D, Z-Eo-D, Z / D-Eo- D, D-Ep-D, Z-Ep-D, Z / D-Ep-D, D-Eop-D, Z-Eop-D, Z / D-Eop-D, Pa-D-Ep-D, Examples include D-Ep-D-Pa, D-Eop-D-Pa, AD-Eop-D, AZ-Eop-D, and AZ / D-Eop-D.

  And bleaching conditions in chlorine dioxide bleaching, bleaching conditions in ozone bleaching, bleaching conditions in ozone / chlorine dioxide bleaching, extraction conditions in alkali extraction, extraction conditions in oxygenated alkali extraction, alkali extraction with oxygen hydrogen peroxide The extraction conditions in and the bleaching conditions in peracid bleaching are not particularly limited, and can be appropriately changed according to the purpose.

In the present invention, in the ECF bleaching as described above, a sensor for measuring the content of the hexeneuronic acid component in the bleached pulp raw material on-line is installed in the bleaching process to be controlled, and this measurement value is multivariable control. A multi-variable control method and apparatus for a bleaching process, which are incorporated into control variables of “model predictive control” and comprehensively execute bleaching control together with other control variables. As a result, when the pulp raw material produced in the bleaching process becomes a paper product, it has the function of decomposing hexeneuronic acid within the range that does not cause discoloration trouble, but the addition rate of expensive “chlorine dioxide (ClO ₂ )” It is possible to increase the rate of addition of “hydrogen peroxide solution (H ₂ O ₂ )” which is inexpensive but does not function to decompose hexeneuronic acid.

In kraft pulp, cooking chemicals made of caustic soda (NaOH), sodium sulfide (Na ₂ S), etc. are added to wood chips that are used as raw materials for paper. The resulting kraft pulp is dark brown. For this reason, in a pulp bleaching process called an ECF (Elementary Chlorine Free) bleaching process, ECF bleaching is performed so that the pulp material has a predetermined whiteness.

  In the control in the bleaching process in the conventional pulp manufacturing process, only the pulp whiteness control for managing the whiteness of the pulp raw material has been focused. However, in the control of the ECF bleaching process of the present invention, this pulp whiteness is controlled. In addition to the value management, the content value of the hexeneuronic acid component in the pulp raw material is also managed and controlled by multivariable control.

  In other words, as a quality control item other than pulp whiteness, the content of hexeneuronic acid component is further added, and the amount of hexeneuronic acid contained in the pulp raw material is also managed at the same time. It is possible to prevent a paper fading trouble that may occur after the pulp raw material becomes a paper product.

  In addition, as a control method, “Model Predictive Control (MPC)” widely used in multivariable control is applied. In the present invention, the overall configuration of the bleaching control system using this control method is applied. Provide a method.

  At present, for example, by using a spectroscopic sensor manufactured by Metso Automation Co., Ltd. as an online hexeneuronic acid component content measurement sensor, the content of the hexeneuronic acid component in the pulp raw material can be measured (patent) Reference 2).

  The “absorbance” by the hexeneuronic acid component with respect to light in a specific wavelength band of 230 to 240 nm in the ultraviolet region measured by this spectroscopic sensor is an index of the degree of progress of the digestion reaction of the pulp raw material at the operation site of the pulp manufacturing process. There is a good proportional relationship with the value of “K value (depending on JIS P 8206: 1994)” which is a manual analysis measurement value by titration operation that has been used in the quality control method for determining the lignin component content. I understand.

  Therefore, in multivariable bleaching control, which will be described later, the value of “absorbance” by the spectroscopic sensor is used, or the “K value” by the manual analysis measurement value that has been used as an index of lignin component content at the operation site. You may substitute. In addition, it is known that the measured value by “K value” corresponds to an index value in which the contents of both the lignin component and the hexeneuronic acid component in the pulp raw material are added as a mixed component.

  For reference, FIG. 5 (a) shows the relationship between the amount of hexeneuronic acid component in the pulp raw material and “absorbance” when ultraviolet light having a wavelength band of 232 nm by a precision spectroscopic device is used. Further, in FIG. 5B, the “absorbance” measured by applying the same 232 nm wavelength light to the pulp raw material and the “K value” of the pulp raw material using the above-described online sensor kappa valence meter. The relationship between the hand analysis values is shown, and they all show a good proportional relationship.

  Note that the content of the hexeneuronic acid component remaining in the pulp raw material decreases as the “absorbance” value and the “K value” value decrease. On the contrary, when these values increase, the content of the hexeneuronic acid component in the pulp raw material increases, and when the pulp raw material becomes a paper product, the risk of causing a problem of fading trouble increases. However, when the content of this hexene uronic acid component is managed by the value of “K value”, it is possible to manage the discoloration trouble by managing the allowable value of this value to be 1.1 or less, for example. Can be avoided.

  Instead of using an on-line sensor for measuring the content of the hexeneuronic acid component, a hexeneuronic acid is provided by providing a function of a soft sensor that can estimate the content of the component in the pulp raw material from other operating variables and control variables. You may substitute by using the estimated value of content of a component.

  In order to achieve automation of the pulp bleaching process using model predictive control, the final pulp must be carefully paid attention to the mutual balance of the addition ratios of both chlorine dioxide water and hydrogen peroxide solution. It is important to manage so that both the whiteness and the content of the hexeneuronic acid component in the pulp raw material fall within the specified range. In some cases, the bleaching of pulp raw materials may be carried out using only a single bleaching chemical with chlorine dioxide water instead of using hydrogen peroxide water during bleaching. It is equally important to pay attention to both the contents of the hexeneuronic acid component in the hexeneuronic acid component and manage them so that both are within the specified range.

In order to realize the bleaching control by the model predictive control, for example, as shown in FIG. 3, “control variable (CV value: Controlled Variable)”, “disturbance variable (DV value: Disturbance Variable)”, and “operation variable” A control function comprising a plurality of transfer function models composed of (MV value: Manifold Variable) is incorporated, but in the present invention, at least one hexeneuronic acid component is contained in the manipulated variable (MV value). It includes either an “on-line sensor” that measures the amount or a “soft sensor” function that is an alternative to the measuring sensor, and the control variable (CV value) is, for example, “chlorine dioxide (ClO ₂ )” It is characterized by including an addition rate such as “hydrogen peroxide solution (H ₂ O ₂ )”.

  In addition, in the function of model predictive control, which is multivariable control, the “evaluation function” used when determining a plurality of control operation amounts is the sum of the control deviation amounts of the variables at the control end, as is normally done. In addition to the sum of the operating variables of the operating variables at the operating end, for example, the total chemical cost of the main chemicals added in the bleaching process (chlorine dioxide, hydrogen peroxide, caustic soda chemicals, oxygen gas, etc.) It is sufficient to incorporate it so that the total sum is minimized.

  Next, the embodiment of the present invention will be described in detail with respect to an example in which multivariable control is performed on a D-Eop-D bleaching sequence of ECF bleaching. The present invention is not limited to this example, and even in the case of other ECF bleaching sequences, it is possible to control based on the technical idea of the present invention.

  The flow of the multistage bleaching process of the kraft pulp which concerns on FIG. 1 at embodiment of this invention is shown.

Kraft pulp made from wood chips as a raw material and cooked in a continuous kraft pulp digester has a dark brown color. Although not shown in FIG. 1, after this digested pulp raw material is subjected to a washing process and a fine selection process, the pulp raw material is sent to an oxygen exposure tower shown on the left side of FIG. 1. In the oxygen bleaching tower, oxygen gas (O ₂ ) is injected into the pulp raw material, and an oxygen bleaching process is performed as a pretreatment for ECF bleaching at a high temperature and a high pressure.

Next, after the pulp raw material is washed and dehydrated with a press device, the pulp raw material is again washed with a bleaching filter and dropped onto a stand pipe. After the addition of strong acidic chlorine dioxide water (ClO ₂ ), which is a bleaching chemical, to the pulp raw material that exits the standpipe, it enters the D0 tower (D stage) installed at the beginning of the ECF multistage bleaching process, The first stage bleaching reaction proceeds in such a manner that the lignin component, which is a wood-derived colored chemical substance remaining in the digested pulp, is oxidatively decomposed with chlorine dioxide water. In addition, the residence time spent for the bleaching reaction of the pulp raw material in D0 tower is about 1 hour in the case of normal production.

  Further, the whiteness of the pulp raw material when entering the D0 tower is a light brown color of about 40% in terms of ISO whiteness, but the whiteness of the pulp raw material that has gone out of the D0 stage is increased to about 73% and becomes quite white. come. The whiteness value is 100% as the maximum value, and as the pulp bleaching progresses and the whiteness of the pulp raw material increases, the value increases.

  The pulp raw material exiting the D0 tower is washed again with the D0 filter, and then caustic soda (NaOH) chemical solution is added to the stand pipe.

Then, after exiting the standpipe, hydrogen peroxide (H ₂ O ₂ ) and oxygen gas (O ₂ ) are further added to the pulp material, and then the pulp material is placed in the second stage Eop. Enter (Eop stage).

  In the Eop tower, the pulp raw material takes about 1 hour in a downflow state to extract and remove the lignin component oxidatively decomposed in the D0 stage by an alkali extraction reaction, and the bleaching reaction is further advanced by oxygen gas. In addition, the whiteness of the pulp raw material after the alkali extraction reaction leaving the Eop tower is generally slightly reduced to about 72% from the time of leaving the D0 tower.

  In this Eop stage, the liquid level of the pulp raw material in the Eop tower is measured, and the Eop filter feed amount is adjusted so that the liquid level is maintained at a predetermined level. Thereby, the residence time of the raw material is kept constant, and the bleaching reaction time of the pulp raw material in the Eop tower is not changed.

The pulp material that has exited the Eop tower is again washed with an Eop filter, dropped onto a standpipe, and again added with chlorine dioxide water (ClO ₂ ), which is the third and final stage of the bleaching process. Enter the D1 level of the replacement tower. Here, the entire bleaching step of the multi-stage bleaching is completed after receiving the final bleaching reaction for about 1 hour. Note that the final pulp whiteness of the D1 stage is controlled at about 84% here.

  Although the raw material residence time in each bleaching tower varies greatly depending on the production amount, the reaction in the entire bleaching process proceeds over a long reaction time of about 3 hours even in the case of a normal production amount. The finished pulp raw material is stored in a large-capacity cushion chest and finally sent to a paper machine process for producing paper.

  Next, the sensor installed in this bleaching facility will be described.

  On the entrance side of D0 tower, install one optical brightness meter (101-1) that continuously measures the whiteness of pulp raw material on-line, and one on the exit side (101-2). Yes. Further, a whiteness meter (101-3) for measuring the whiteness of the pulp raw material online is installed on the exit side of the Eop tower, and the last online whiteness meter (101-4) is provided on the exit side of the replacement bleaching tower. Is installed so that the whiteness value of the pulp raw material that is bleached on the entry side and the exit side of each bleaching tower can be monitored.

  In addition, a spectroscopic kappa valence meter (102) is installed on the entry side of the D0 tower to measure the kappa number, which is an index of the lignin content in the pulp raw material flowing into the bleaching process, and Eop. On the exit side of the tower, there is one spectroscopic measuring sensor (103-1) that measures the content of the hexeneuronic acid component remaining in the pulp raw material online. Install another sensor (103-2) that measures the content of the hexeneuronic acid component remaining in the raw material online, so that the problem of paper discoloration will not occur in the first place. The content value of the acid component can be monitored.

  In addition, pH meters, thermometers, flow meters, and on-line residual chlorine concentration meters that measure chlorine concentration components derived from bleaching chemicals remaining in pulp raw materials are installed throughout the bleaching process. Detailed description of the sensor configuration is omitted. FIG. 2 shows a table of installation examples of sensors used for bleaching control in the bleaching process of the present invention.

  In addition, it is also possible to use the function of a soft sensor instead of actually installing these sensors. The soft sensor is a process value of multiple manipulated variables and control variables in the time zone related to the measured values of the "whiteness" and "content of hexeneuronic acid components" of the pulp raw material and the time zone related to the result values. Can be realized in a form of estimation using a multiple regression equation obtained from statistical analysis or the like.

FIG. 3 shows an outline of a configuration example of bleaching control using multivariable control of the present invention. When applying model predictive control to each bleaching stage of multi-stage bleaching, “manipulated variable (MV value)”, “control variable (CV value: Controlled Variable)”, and “disturbance variable (DV) (Value: Disturbance Variable) ”.
FIG. 4 shows a specific example of the configuration of the bleaching control to which the model predictive control is applied based on the configuration of FIG. “Control variable (CV value: Controlled Variable)” in each bleaching stage is shown in each top row, “Disturbance variable (DV value: Disturbance Variable)” in each bleaching stage is shown in each row on the left side, and “Operation” Variable (MV value: Manipulated Variable) ”.

  Here, out of a total of 131 cells in a matrix of 12 rows × 11 columns, a total of 27 cells required by the system identification analysis using the process data obtained through the step response test in the actual process was obtained. Here, the transfer function is mainly used for each control response, which is represented by “dead time + first order lag model”.

  The numerical values “Aij”, “Lij”, and “Tij” appearing in the equation of the transfer function were statistically obtained from the system identification analysis using process data obtained from the step response test in the actual process. Value. “Aij” indicates the value of “steady gain”, which is the ratio of the “j variable” responding when the “i variable” to be operated is changed, and similarly, “Lij” is the “dead time” at that time. “Tij” represents the value of the “time constant” at that time.

  In FIG. 4, the variable values of “content of hexeneuronic acid component” in the pulp raw material, and the first and sixth columns are shown in the fourth and eleventh columns of the lateral control variable (CV value). The variable value of “pulp whiteness” is incorporated in the 10th column. In addition, in the 4th and 12th lines of the vertical operation variable (MV value), the addition rate of “chlorine water” is incorporated, and in the 9th line, the variable value of the addition rate of “hydrogen peroxide solution” is incorporated. It is out.

  In addition, as an “evaluation function” used for determining the control operation amount, here, as is generally performed in model predictive control, the sum of the control deviation amounts at the control end and the operation variables of the operation variables at the operation end The sum total and the total value of the total chemical costs of the main chemicals added in the bleaching step (in this embodiment, chlorine dioxide water, hydrogen peroxide water, and caustic soda chemical solution) are used. To be precise, the sum of the control deviation amounts at the control end in the evaluation function of model predictive control is obtained by squaring each deviation amount, multiplying the value of each term by the appropriate weight value, and the operation end As the total value of the operating variables of the operating variables of, calculate the value obtained by multiplying the operating change amount at each operating end by the square, multiply the value of each term by the appropriate weight value, and add all these values. Values are used. In addition, the total chemical cost of the main chemicals is calculated by adding a value obtained by multiplying the addition flow rate of each chemical by the unit price of each chemical and adding them together. In the present invention, the manipulated variable is controlled so as to minimize the value of such an evaluation function.

  The bleaching control model described above is incorporated into an actual bleaching plant as a model predictive control system. Based on the values measured by various sensors, the operation amount of each operating variable at the operation end is determined by the model predictive control function. Then, we performed bleach control with the actual machine.

  The control effect is shown in the following table (Table 1). Here, in order to compare the control effect during manual operation and bleaching control by the model predictive control of the present invention, the average value and standard deviation of the final whiteness, and the content of the hexeneuronic acid component in the final pulp material The mean and standard deviation statistics are shown. The numerical value of “content of hexeneuronic acid component in pulp raw material after completion of bleaching (in the case of management by K value)” in Table 1 is obtained by measuring the absorbance with an on-line sensor, as shown in FIG. ) Is a numerical value converted into a K value by the correlation shown in the table.

  According to Table 1, the value of the standard deviation (1σ) of the final whiteness of the pulp material was slightly improved, with a fluctuation of 0.290 at the time of automatic control, compared to 0.308 at the time of manual operation.

  Further, the value of the content of the final hexeneuronic acid component in the pulp raw material is controlled so that the above-mentioned K value is, for example, 1.1 or less in order to manage the fading trouble in the paper product. There is a need.

  In this case, at the time of automatic control in the present invention using model predictive control, a part of chlorine dioxide water, which is a main bleaching chemical, has the effect of increasing the whiteness of the pulp raw material, but the hexeneuronic acid component in the pulp raw material This is a control action that automatically transfers to hydrogen peroxide solution that does not have the effect of lowering the content of the hexene uronic acid component in the pulp raw material that has sufficient margin during manual operation. In contrast to the average value of 0.99 (in the case of management at the K value), the content of the hexeneuronic acid component in the pulp raw material at the time of model predictive control (in the case of management by the K value) is set to 1.03. Although it increased by 04, a sufficient margin is still secured for the condition of 1.1 or less which was the upper limit constraint value of the K value.

  Moreover, in the total bleaching chemical cost, a cost reduction of about 0.07 yen / kg of pulp was recognized by the transfer operation between the bleaching chemicals by the automatic operation using the model predictive control as described above. This seems to be a small decrease, but when calculated in consideration of the production volume of the target plant, the cost will be reduced by about 15 million yen per year.

  In manual operation, to avoid the occurrence of paper product discoloration troubles, the content of the hexeneuronic acid component in the pulp raw material may be made safer and the target value may be lowered more than necessary.Therefore, there are many expensive bleaching chemicals. This tends to increase the total bleaching chemical cost.

On the other hand, in the bleaching control using the model predictive control of the present invention, the economical operation is further performed after satisfying the quality conditions of both the whiteness of the final pulp raw material and the content of the hexeneuronic acid component in the pulp raw material. In other words, it has the function of decomposing hexeneuronic acid as long as it does not cause paper discoloration trouble, but it has no function of decomposing hexeneuronic acid by suppressing the addition rate of expensive “chlorine dioxide (ClO ₂ )”. However, it is possible to safely perform an extreme control operation with an increased rate of adding “hydrogen peroxide solution (H ₂ O ₂ )” by automatic operation safely.

101 Whiteness meter 102 Copper number meter 103 Hexeneuronic acid analyzer

Claims (8)

A multivariable control method of a bleaching process for ECF bleaching a pulp raw material,
A first step of adding chlorine dioxide water to the pulp in the first bleaching tower, and measuring the whiteness and pH of the pulp ;
Second step of adding caustic soda and hydrogen peroxide solution to the pulp flowing in from the first bleaching tower and measuring the pH, whiteness, and hexeneuronic acid component content of the pulp in the second bleaching tower When,
Third step of adding chlorine dioxide to the pulp flowing from the second bleaching tower and measuring the pH, residual salt concentration, whiteness and content of hexeneuronic acid component in the third bleaching tower And
The chlorine dioxide addition rate in the first step which is an operation variable is the whiteness and pH of the pulp in the first step which is a control variable, and the whiteness and hexeneuronic acid component of the pulp in the second step. And the whiteness of the pulp in the third step is set to a predetermined value ,
The caustic soda addition rate in the second step, which is an operation variable , is the pH, whiteness, and hexeneuronic acid component content in the second step, which is a control variable, and the pH of the pulp in the third step. Is set to a predetermined value ,
The hydrogen peroxide solution addition rate in the second step, which is an operation variable, is set so that the pH and whiteness of the pulp in the second step, which is a control variable, become predetermined values, respectively.
The chlorine dioxide addition rate in the third step which is an operation variable is such that the pH, residual chlorine concentration, whiteness and hexeneuronic acid component content of the pulp in the third step which is a control variable are predetermined values, respectively. A multivariable control method for a bleaching process, characterized by being set to be   The multivariable control method of the bleaching process according to claim 1, wherein the pulp raw material is hardwood pulp. The content of hexenuronic acid component of the pulp in the raw material, ultraviolet light absorbance of 230~240nm the measurement sample, or JIS P 8206: it obtained from the K value defined in 1994 to claim 1 or 2, characterized in Multivariable control method of the described bleaching process. The second step includes a step of measuring a liquid level of the second bleaching tower and adjusting a feed flow rate so as to maintain a liquid level of the second bleaching tower at a predetermined level. The multivariable control method of the bleaching process according to any one of claims 1 to 3 . 5. The multivariable control method for a bleaching process according to any one of claims 1 to 4 , wherein a transfer function is used for predicting the control variable . As the evaluation function of the first to third steps, the total sum of the deviation amounts of the control variables, the sum of the manipulated variables of the manipulated variables, the total cost of added chlorine dioxide, caustic soda and hydrogen peroxide water is used. , multivariable control method for bleaching process was claimed in any one of 5, characterized in that said evaluation function to control the control variable so as to minimize. The content of hexeneuronic acid in the pulp raw material is a value measured by an on-line sensor and / or an estimated value by a function of a soft sensor, and the amount of bleaching step according to any one of claims 1 to 6 , Variable control method. A multi-variable control device for a bleaching process for ECF bleaching a pulp raw material ,
In the first bleaching tower , a first chlorine dioxide control unit for adding chlorine dioxide water to the pulp ;
A first whiteness meter for measuring the whiteness of the pulp flowing into the first bleaching tower;
A first pH meter for measuring the pH of the pulp in the first bleaching tower;
A caustic soda control unit for adding caustic soda to the pulp flowing from the first bleaching tower in the second bleaching tower ;
In the second bleaching tower, a hydrogen peroxide controller that adds hydrogen peroxide to the pulp that has flowed from the first bleaching tower;
A second pH meter for measuring the pH of the pulp in the second bleaching tower;
A hexeneuronic acid analyzer for measuring the content of the hexeneuronic acid component of the pulp in the second bleaching tower;
A second whiteness meter for measuring the whiteness of the pulp in the second bleaching tower;
A third chlorine dioxide control unit for adding chlorine dioxide to the pulp flowing from the second bleaching tower in the third bleaching tower;
A third pH meter for measuring the pH of the pulp in the third bleaching tower;
A residual salt thermometer for measuring a residual salt concentration of the pulp in the third bleaching tower;
A third whiteness meter for measuring the whiteness of the pulp in the third bleaching tower;
A hexeneuronic acid analyzer for measuring the content of the hexeneuronic acid component of the pulp in the third bleaching tower,
The chlorine dioxide addition rate operated in the first chlorine dioxide control unit includes the whiteness and pH of the pulp in the first bleaching tower, and the whiteness and hexeneuronic acid component of the pulp in the second bleaching tower. And the whiteness of the pulp in the third bleaching tower is set to a predetermined value,
The caustic soda addition rate operated in the caustic soda control unit is such that the pH of the pulp, the whiteness and the content of hexeneuronic acid component in the second bleaching tower, and the pH of the pulp in the third bleaching tower are predetermined. Set to a value ,
The hydrogen peroxide solution addition rate operated in the hydrogen peroxide solution control unit is set so that the pH and whiteness of the pulp in the second bleaching tower are respectively predetermined values,
The chlorine dioxide addition rate operated in the second chlorine dioxide control unit is such that the pulp pH, residual chlorine concentration, whiteness, and hexeneuronic acid component content in the third bleaching tower are predetermined values, respectively. A multi-variable control device for a bleaching process, characterized by being set as follows.

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