Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/1026—Other features in bleaching processes
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/147—Bleaching ; Apparatus therefor with oxygen or its allotropic modifications
  • D21C9/153—Bleaching ; Apparatus therefor with oxygen or its allotropic modifications with ozone

Definitions

• the present invention relates to the pulping industry and, more particularly, to process for controlling the degree of ozone bleaching in the pulping delignification bleaching process for achieving a desired brightness.
• Wood is comprised of two main components - a fibrous carbohydrate, i.e., cellulosi portion, and a non-fibrous component.
• the polymeric chains forming the fibrous cellulos portion of the wood are aligned with one another and form strong associated bonds wit adjacent chains.
• the non-fibrous portion comprises a three-dimensional polymeric materi formed primarily of phenylypropane, known as lignin. Part of the lignin is between th cellulosic fibers, bonding them into a solid mass, although a substantial portion of the ligni is also distributed within the fibers themselves.
• Pulp For use in paper-making processes, wood must first be reduced to pulp. Pulp may b defined as wood fibers capable of being slurried or suspended and then deposited upon a scree to form a sheet, i.e. , of paper. Pulping of the wood by the well known kraft or modified kra pulping processes results in the formation of a dark colored slurry of cellulose fibers know as "brownstock.” See, for example, Rydholm, Pulping Processes. Interscience Publisher 1965 and TAPPI Monograph No. 27, The Bleaching of Pulp. Rapson, Ed., The Technic Association of Pulp and Paper Industry (1963).
• the dark color of the brownstock is attributable to the fact that not all of the lignin ha been removed during digestion and has been chemically modified in pulping to for chromophoric groups.
• ozone will readily react with lignin to effectively reduce the amount of lignin in the pulp. Under many conditions, it will, however, deleteriously attack the carbohydrate which comprises the cellulosic fibers of the wood to substantially reduce the strength of the resultant pulp. Ozone, moreover, is extremely sensitive to process conditions with respect to its oxidative and chemical stability, and such changes can significantly alter the reactivity of the ozone with respect to lignocellulosic materials.
• a gaseous ozone bleaching agent is used to provide a highly selective removal and bleaching of the lignin with minimal degradation of cellulose.
• Process parameters that must be controlled include pulp feed rate, pulp particle size, pulp consistency, pulp Ph, pulp temperature, degree of pulp delignification, ozone feed concentration, ozone feed rate, ozone application on pulp which is a function of ozone concentration and gas flow rate, and the residence time of both the gas and the pulp in the ozone reactor.
• the pulp is advanced in a dispersed plug flow manner that subjects substantially all of the pulp particles to the ozone in a uniform fashion to obtain a substantially uniformly increased brightness pulp.
• a process for controlling the bleaching of pulp is realized by utilizing feedforward control, feedback control or a combination thereof.
• the feedforward control adjusts the amount of bleaching agent to a reactor based on on-line measurements of a characteristic property of the ' pulp entering the reactor, and ingoing amounts of pulp and bleaching agent. More particularly, using a predictive equation based on the operating characteristics of the reactor, the bleaching agent application required to achieve a desired characteristic pulp property or degree of bleaching is calculated and the amount of bleaching agent entering the reactor adjusted accordingly.
• Sensors in a feedback loop configuration with the reactor monitor an outgoing characteristic pulp property for generating an error control signal representing a corrective term that compensates for any inaccuracies in the predictive equation and/or measured operating parameters.
• the process control in accordance with the present invention affords a method for controlling the amount of ozone application to the pulp to most rapidly achieve a desired pulp property or degree of bleaching despite these varying operating conditions.
• the process control is employed for controlling the outgoing pulp brightness while minimizing and thus optimizing ozone use in an ozone bleaching reactor.
• a feedforward control strategy is utilized to adjust the ozone gas flow rate.
• On-line sensors monitor an outgoing pulp property such as brightness and provide information for generating a feedback signal which compensates for any errors in the feedforward control.
• the control process is composed of five or more steps, with a number of possible variations within and between the steps. Among, but not limited to, the effective embodiments of the present invention are the following steps:
• the ozone application is base loaded to the reactor. Once having reached a steady state operation, the ozone application is adjusted solely under feedback control. Preferably, however, the error corrective signal from the feedback control is summed with a component from the feedforward control.
• Fig. 1 is a block flow diagram of the preferred process control of the present invention
• FIG. 2 is schematic drawing of an ozone reactor system utilizing the process control o the present invention
• Fig. 3 is a graph of the entering pulp K Number versus entering pulp brightness
• Fig. 4 is a graph of the ozone consumption (%) to achieve an exiting pulp brightnes of 53% GEB versus entering pulp brightness;
• Fig. 5 is a graph of the ozone consumption (%) to achieve an existing pulp K Numbe of 4.5, versus entering pulp K Number;
• Fig. 6 is a graph of the ozone consumption (%) to achieve an existing pulp brightnes of 53 % GEB versus pH during ozone bleaching;
• Fig. 7 is a graph of the ozone consumption (%) to achieve an existing pulp brightnes of 53 % GEB versus temperature during ozone bleaching;
• Fig. 8 is a graph of the existing pulp brightness versus ozone consumption (%) for tw different brightness levels of the entering pulp
• Fig. 9 is a schematic diagram of the control logic for a feedforward control wit feedback proportional integral (PI) control.
• Fig. 10 is a graph of the ozone/oxygen gas flow rate and ingoing/outgoing pulp brightness as a function of time, useful in explaining the performance of the feedforwar control with proportional-plus-integral (PI) feedback control when there is a change in ingoin pulp brightness.
• PI proportional-plus-integral
• Fig. 11 is a graph of the ozone/oxygen gas flow rate, outgoing pulp brightness an reactor throughput (air dried tons per day, ADTPD) as a function of time useful in explainin the performance of the feedforward control with PI feedback control when there is a chang in reactor throughput.
• the present invention relates to a novel process for controlling a characteristic pulp property such as the brightness or kappa number of pulp during bleaching, particularly ozone bleaching, while minimizing the amount of ozone required.
• the method for controlling an ozone bleaching process to a desired degree of bleaching comprises:
• the "degree of delignification” is normally used in connection with the pulping process and the early bleaching stages. It tends to be less precise when only small amounts of lignin are present in the pulp, i.e., in the later bleaching stages.
• the "brightness” factor is normally used in connection with the bleaching process because it tends to be more precise when the pulp is lightly colored and its reflectivity is high.
• the normal permanganate test provides a kappa number or a permanganate number ("kappa number or K Number") which is the number of cubi centimeters of tenth normal potassium permanganate solution consumed by one gram of ove dried pulp under specified conditions.
• K Number kappa number or K Number
• This parameter i usually a measure of reflectivity and its value is expressed as a percent of some scale.
• standard method is GE Brightness (GEB) which is expressed as a percentage of a maximu GE Brightness as determined by TAPPI Standard Method TPD-103.
• Frill factor refers to the amount of pulp in the open space of the reactor by volume. For example, a fill factor of 25% indicates that 25% of the open space of the reactor is filled with pulp, based on the bulk density of the pulp when it is at rest, the amount of pulp in th reactor, and the reactor volume. For a particular reactor design and size, pulp feed rate and shaft rpm, a particular fill factor is obtained. Further, by varying the rpm at a constant pulp feed rate, the fill factor can be changed. For instance, increasing the rpm correspondingly reduces the fill factor.
• plug flow In an ideal plug flow reactor, all of the material flowing through the reactor has the same residence time, i.e., it spends substantially the same amount of time in the reactor before emerging at the other end. Accordingly, a reference to "plug flow” is to be understood to mean that the residence time distribution of the pulp particles with the gaseous bleaching agent is as narrow as possible such that most of the pulp particles spend substantially the same amount of time in the reactor.
• the dispersion index (DI) is defined as follows:
• ⁇ 2 is the variance of the residence time distribution and t jVg is the average pulp residence time.
• t jVg the average pulp residence time.
• Control of the outgoing pulp property such as brightness with variability in one or more process parameters, e.g., ingoing pulp brightness or production rate, is obtained by use of the process of the present invention.
• the process is utilized to control a bleaching reactor that employs ozone.
• bleaching with ozone under properly defined conditions minimizes the degree of attack upon the cellulosic portion of the wood, thereby forming a product having acceptable strength and brightness properties for the manufacture of papers and various paper products.
• Fig. 1 sets forth broadly in schematic form the various steps utilized in the process control in accordance with the principles of the invention.
• the invention comprises various measuring steps and corrective algorithms, including, but not limited to, the following steps:
• FIG. 2 Shown in Fig. 2 is an exemplary block diagram of an ozone reactor system 20 utilizing the steps set forth above for the present process control. It is to be understood however, that the reactor system depicted in Fig. 2 is for the purpose of illustration only an not for the purpose of limitation.
• the pulp Prior to entering bleaching reactor 205, the pulp is directed to a feed tank 210 wher it is conditioned by treatment with acid and a chelating agent.
• the acidified, chelated low- o medium-consistency pulp is introduced into a thickening unit for removing the excess liqui from the pulp, such as a de watering press 215 where the consistency of the pulp is raised t the desired level. At least a portion of this excess liquid may be recycled to the feed tank.
• the resulting high consistency pulp is then passed through a feeder/fluffer 220, whic acts as a gas seal for the ozone gas at one end of ozone reactor 205 and comminutes th incoming high consistency pulp to pulp fiber particles which then fall into reactor 205.
• Ozone gas 230 is introduced into reactor 205 in a manner such that it preferably flow countercurrent to the flow of the pulp.
• the pulp fiber particles are bleached by the ozone i reactor 205 to subsequently remove a substantial portion, but not all, of the lignin therefrom
• the pulp fiber particles are intimately contacted and mixed with the ozone by use of a paddl conveyor, which is the preferred embodiment and is disclosed in U.S. Patent No. 5,181,989
• the paddle conveyor advances the pulp fiber particles in a plug flow-lik manner at a controlled pulp residence time.
• the ozone gas residence time is also controlled.
• a controlling factor for the bleaching of the pulp is the relative amount of the ozon used to bleach a given amount of the pulp. This amount is determined, at least in part, by th amount of lignin which is to be removed during the ozone bleaching process, balanced agains the relative amount of degradation of the cellulose that can be tolerated during ozon bleaching. Preferably, an amount of ozone is used which will react with about 50% to 70 of the lignin present in the pulp.
• the amount of lignin to be removed during ozone bleaching is an important factor i control of the ozone bleaching stage. This amount of lignin is related to the degree o brightening desired across the stage, i.e., the difference between the ingoing and outgoing pulp brightness levels.
• Fig. 8 graphically shows the exiting pulp brightness as a function of the ozone consumption.
• the data shown in Fig. 8 were laboratory ozone bleaching data on pine pulp taken at two different levels of entering pulp brightness; 34% an 37.5% GEB as indicated by
• the ozone gas which is used in the bleaching process may be employed as a mixture of ozone with oxygen and/or an inert gas, or as a mixture of ozone with air.
• the amount of ozone which can satisfactorily be incorporated into the treatment gases is limited by the capability of the ozone generation to economically generate ozone, as well as by the stability of the ozone in the gas mixture.
• Ozone gas mixtures which typically, but not necessarily, contain about 1-8% by weight of ozone/oxygen mixture, or 1-4% by weight of ozone/air mixture, are suitable for use in the bleaching process.
• a preferred mixture is 6% ozone with the balance predominantly oxygen.
• a higher concentration of ozone in the ozone/oxygen mixture allows for the use of relatively smaller size reactors, and a shorter reaction time to treat equivalent amounts of pulp.
• the reaction temperature at which the ozone bleaching is conducted is likewise an important controlling factor in the optimal operation of the ozone bleaching reactor.
• the ozone bleaching can be effectively conducted at temperatures up to a certain level. Above this level, the reaction commences to cause excessive degradation of the cellulose and increased ozone requirement to reach a target brightness level, due to factors such as increased ozon decomposition at higher temperatures.
• the maximum temperature of the pulp fiber at whic the reaction should be conducted should not exceed the temperature at which excessiv degradation of the ozone occurs, which with southern U.S. softwood is a maximum of abou
• Ozone bleaching likewise, is extremely sensitive to the pH with respect to its oxidativ and chemical stability. For example, changes in the pH conditions can significantly alter th reactivity of the ozone with respect to the lignocellulosic materials.
• the duration of the reaction used for the ozone bleaching step is determined by th desired degree of brightening required in this stage. It is important that this ozone bleachin reaction be accomplished with complete or substantially complete consumption of the ozone, i.e., ozone conversion.
• the reaction time will vary depending upon factors such as th efficiency of mixing of the pulp with the ozone, the concentration of the ozone in the ozon gas mixture, with relatively more concentrated ozone mixtures reacting more quickly, and the relative amount of lignin which it is desired to remove.
• An important feature of the invention is maintaining a desired level of brightness or Number despite varying production rate and operating parameters. This feature is obtained, in part, by monitoring the above controlling factors, among others, to adjust the amount o ozone application for the given operating conditions in order to achieve the desired brightness, change in brightness or desired K Number or change in K Number. Importantly, consisten brightness in this bleaching process facilitates subsequent control of the final bleaching stage, such as with chlorine dioxide or a peroxide.
• the first step in a preferred embodiment of the present invention is to measure th ingoing pulp brightness and the ingoing amounts of pulp and ozone supplied to the bleaching reactor.
• ozone reactor 205 utilizes an on-line brightness sensor 235, such as Model BT-5000 sold by BTG Inc., to measure the ingoing pulp brightness.
• the pulp is exposed to ultra-violet radiation at a wavelength of approximately 457 ⁇ m.
• Brightness sensor 235 detects the intensity of the scattered light from the pulp, which intensity is proportional to the brightness of the pulp.
• the ingoing brightness is measured after acidification, that is in the 6% consistency stock line after feed tank 210 and before acid dewatering press 215.
• sensor 235 is dedicated to sampling a specific location and accordingly continuously samples the brightness of the ingoing pulp with an accuracy of ⁇ 0.5 GEB units.
• the kappa number of the incoming pulp is measured just prior to acidification, that is before feed tank 210.
• Model KNA-5000 sold by BTG having a relative accuracy of ⁇ 0.5 for kappa numbers between 10-100 and a relative accuracy of ⁇ 0.2 for kappa numbers between 2-10 may be employed as sensor 240 for measuring the kappa number of the ingoing pulp.
• a characteristic relationship like Fig. 3 which depicts graphically the relationship between the K Number and pulp brightness, is used to calculate the expected pulp brightness based on the K Number measured by sensor 240.
• Instrument calibration check 241 compares the expected pulp brightness sensor 240 with the measured pulp brightoess from sensor 235. Differences which are more that a pre-specified level are used to signal an alarm so that sensors 235 and 240 can be checked for malfunctions.
• the next step in the process control of the present invention is to measure the various operating parameters and/or conditions of the ozone reactor.
• thermosensor 245, 250, 255, 260 and 261 are use to measure the temperature (°F), pH, inlet gas flow rate, pulp tonnage (ADTPD) and reacto rpm, respectively. These sensors are well known in the art and accordingly are not discusse here.
• ultra-violet analyzers 270 and 275 are located in the ozone feed line to th reactor and in the exit gas line therefrom, respectively. Suitable ozone analyzers are availabl as model HI, series AFX from IN USA, Inc. Each analyzer measures the O 3 concentratio (%) within the respective feed line to about 2-3% relative to the measured ozone concentration.
• the current ozone feed rate (lbs O 3 per hour) is calculated using well known conversio factors.
• the next step of the process is to determine the amount of ozone to be applied to th pulp (ozone application in lb. ozone per 100 lb. OD pulp) to achieve the desired exiting pul brightness or kappa number.
• a regression fit is performed on, for example, pilo plant data in order to yield a predictive equation setting forth an empirical relationship betwee the percentage of O 3 consumed to achieve the desired exiting brightness or kappa number an the ingoing pulp brightoess or kappa number.
• the ozone application to achiev a desired pulp brightoess is modeled as being proportional to the sum of, first, the differenc between the ingoing pulp brightoess and the target pulp brightoess, and second, this differenc squared, i.e., a quadratic model.
• Fig. 4 shows a graphical representation of the functional relation that defines th empirical relationship between the ozone consumed to reach a specific exiting pulp brightoes level and the ingoing pulp brightness for a specific set of reactor operating parameters.
• Thi functional relationship is given by the following equation:
• Fig. 5 shows a graphical representation of the functional relation tha defines the empirical relationship between the ozone consumed to reach a specific exiting pul K Number and the ingoing pulp K Number for a specific set of reactor operating parameters.
• a linear regression was used to determine the "best fit" line.
• dj is the outgoing target pulp K Number at which th data was taken, and d 2 is a constant for fitting the equation to the experimental data.
• the ozone fractional conversion X 0 is defined as one minus the ration of the ozon concentration in the gas stream leaving the reactor 0 3out divided by the ozone concentration i the gas stream entering the reactor, 0 3in ; i.e.,
• Typical running conditions that affect ozone conversion are ozone fee rate, gas flow rate, ozone concentration, ingoing pulp properties, and reactor conditions such as pulp feed rate, pulp residence time, pH and temperature.
• a modified predictive equation including the effects of pH and temperature, and target
• modified predictive equation (2) or (2a) Utilizing modified predictive equation (2) or (2a), the required ozone consumption for the measured operating parameters in the preceding step is determined.
• the ozone application, O 3 app is then calculated, assuming a 90% ozone conversion (i.e., a 0.9 ozone fractional conversion or "ozone consumption factor"). That is, the calculated ozone consumption from equation (2) or (2a) is divided by 0.9 to obtain the required ozone application.
• f is one divided by the ozone consumption factor, or f -1/0.9.
• the amount of ozone addition can then be determined.
• flow controller 280 accordingly adjusts the ozone supply to the reactor via flow valve 285.
• the ozone application to reach an exiting pulp K Number, K ⁇ g is calculate as follows:
• the brightoess and kappa number ar sampled after exiting reactor receiver tank 265 by sensors 290 and 295, respectively. Eac of these sensors can be dedicated to continuously monitoring the exiting pulp.
• sensor 290 has an accuracy of ⁇ 0.2 units for kappa numbers less than seven, whereas GE sensor 290 has an accuracy of ⁇ 0.5 GEB units.
• the general strategy for controlling the outgoing pulp brightness is to dynamicall control the amount of ozone application.
• a process control strategy is based o feedforward control with feedback proportional integral (PI) control.
• PI proportional integral
• the ozone application to the reactor is appropriately adjusted to match the calculate ozone application derived from the empirical relationship set forth above in equation (2) t achieve the desired pulp brightoess.
• the feedback control wil further provide a corrective component to the feedforward calculation.
• feedforward contro and/or feedback control there are various strategies for utilizing the feedforward contro and/or feedback control.
• the ozone application is base loaded to the reactor. Once having reached a steady stat operation, the ozone application is adjusted solely under feedback control. An error correctiv signal from the feedback control adjusts the ozone application to the reactor.
• feedforward and feedback controls will be used in combination, each may be used individually to control the ozone application.
• corrective feedback to the feedforward control is required due to a non-perfect process model or, more likely, due to inaccuracies or changes in the measured variables.
• on-line sensors and flow meters assess the current status of the reactor by providing feedforward controller 310 with various parameters, including ingoing pulp brightoess (GEB), temperature (°F), pH, pulp feed flow rate (ADTPD), inlet ozone concentration (%), and inlet gas flow rate.
• the gas flow rate can be expressed in terms of standard cubic feet per minute or SCFM.
• the desired ozone application to achieve a desired brightoess is determined. That is, the pounds of ozone per 100 lb. OD pulp required to achieve the desired brightness is calculated according to the empirical formula set forth above in equation 2b.
• the ozone feed rate in pounds of ozone per hour is calculated. Note that this entails, first, calculating the ozone application required as a percentage of ozone on oven dry (OD) pulp and then, second, based on the density of inlet gas stream and the ozone concentration in the inlet gas stream calculating the required SCFM and lbs of ozone per hour. A comparison between the calculated amount of ozone needed to achieve the targe brightoess and the current ozone application is made.
• Flow controller 280 then accordingl adjusts flow control valve 285 based on the difference between the calculated and actual ozon application in accordance with well known flow control theory.
• Predictive feedback may be used to regulate the ozone application to the reactor base on first measuring the difference between the desired and measured outgoing pulp brightoes
• Flow controller 280 accordingly adjusts flow valve 285 in order to provide an ozon gas flow rate corresponding to the new level of ozone required.
• Predictive feedback control works as follows. First, from the current ozon consumption and the desired and measured outlet pulp brightoess, the ozone consumptio needed to achieve the target brightness is calculated in accordance with the following equation
• GEB ⁇ g , 03 is the current ozone consumption
• GEB ⁇ g is the target GE Brightness
• GEB 0Ut is the ingoin pulp brightoess.
• 0 3 req is the required ozone consumption, where what is needed is the require ozone application (amount actually added to the reactor).
• the amount of ozon consumed by the pulp is less than the amount of ozone applied to the pulp. Therefore, modified, linearized plug flow performance equation is used to calculate how much ozone ha to be applied to the pulp, i.e. , added to the reactor, according to the following equation:
• Equation (lb) Equation (lb).
• SCFM req is the gas flow rate required to achieve the desired brightoess
• VT is the reactor volume
• Cj are constants determined empirically from previously collected reactor data.
• C 2 is 60
• c is 0.0566. This equation identifies the precise level of gas flow rate required, SCFM ⁇ ,, to obtain the desired brightoess (GEB ⁇ g ).
• the feedback control strategy includes assessing the current status of an outgoing pulp property and measuring the difference between this measured value and the desired (target) value of that pulp property, e.g., pulp brightness of K Number. Once the difference between the desired pulp property and outgoing pulp property has been measured, the ozone application, or more exactly the gas flow rate, is accordingly increased or decreased until the desired target property level is detected by on-line sensors located at the exiting feed line of the pulp.
• the desired (target) value of that pulp property e.g., pulp brightness of K Number.
• PI feedback control 320 provides a corrective term to the ozon application level based on the difference or error between the outgoing and desired pul property of interest, in this case pulp brightoess.
• the mode of control proportional integra or PI, calculates a correction factor to ozone application based on the sum of a ter proportional to the error plus a term related to the integration of the error over time.
• feedforward control in accordance with equation (2b) is solely used during the start-up phase to base load the amount of ozone application to the reactor. Once a steady state operation is reached, feedback control is then used with the feedforward control to adjust the ozone application. Such an adjustment is based on the exiting pulp brightness and ozone conversion within the reactor.
• the feedback control strategy includes assessing the current status, such as the operating parameters of the reactor, and measuring the difference between the measured and desired pulp brightness.
• the ozone application level is calculated using plug flow reactor performance equation (5) based on a model of the ozone conversion within the reactor t identify the correct ozone application needed to achieve the desired brightoess.
• on-line brightoess measurements will be employed t control the ozone application in order to achieve a desired brightness level.
• the presen strategy is based on employing feedforward control with proportional integral (PI) feedbac control.
• feedforward control strategy uses a process model from, for example, pilot plant data to calculate the ozone application needed to achieve a desired brightoess level in accordance with feedforward predictive equation (2).
• PI feedback control 320 provides a corrective term to the feedforward calculation and is largely based on the error between the outgoing and desired pulp brightness.
• the ozone application or more exactly the gas flow rate, is accordingly increased or decreased until the desired target brightoess level is detected by on-line brightoess sensors located at the exiting feed line of the pulp.
• PI feedback control 320 produces a relative gas flow rate value which is indicative of the amount of ozone needed to be subtracted or added to the feedforward gas flow rate.
• the sum of the gas flow rates from the feedforward and feedback controls are continuously summed at summer 330. This summed quantity controls the ultimate ozone gas flow rate to the reactor vis-a-vis flow controller 280 and flow valve 285.
• control system hardware is implemented by distributed microprocessors, each of which is dedicated to performing a specific function.
• the processing elements are linked to one another to form an integrated process control system, typically with a highly parallel distributed architecture.
• the operation of the process control system utilizing a feedforward/predictive feedbac strategy is illustrated in Fig. 9 using a SAMA diagram.
• the process control comprises a feedforward control loop 510, a feedback control loo 520, and a control loop 530 to adjust the gas flow rate to the reactor.
• Each of the function blocks in Fig. 9 stimulates an arithmetic operation performed o the inputs thereto in a accordance with well known control theory. Moreover, the function blocks interconnected in a specific arrangement perform a desired calculation according to a desired equation.
• the function blocks in feedforward control loop 510 simulate equation (2b), while the function blocks in feedback control loop 520 utilize standard PI control.
• the control hardware architecture for the ozone reactor may comprise several stand alone computing controllers. These controllers ensure that data is exchanged between other controllers to facilitate and coordinate the implementation of the block diagram of Fig. 9.
• Figure 10 shows the ozone gas flow rate response versus time superimposed with outgoing pulp brightness for a variable ingoing pulp brightness.
• the feedforward control start at time zero, resulting in an exiting pulp brightness of 54 GEB by about 400 seconds.
• feedback PI control is turned on resulting in a downward correction in gas flow rate and attainment of (target) 53 GEB by about 800 seconds.
• Starting at 1200 seconds there is a change in ingoing brightness which is efficiently compensated for by the PI feedback control, again allowing target 53 GEB to be maintained.
• Fig. 11 shoes the ozone gas flow rate response versus time superimposed with the outgoing pulp response versus time superimposed with the outgoing pulp brightoess for a pulp tonnage turndown.
• the exiting pulp brightoess there is very little swing in the exiting pulp brightoess, indicating the stability in the process control of the present invention.
• the combination of the feedforward and feedback control strategies results in the outgoing pulp brightoess being held to better than 1 GEB unit of the desired brightoess.
• the feedback control may be used to adaptively alter the empirical relationship that defines the amount of initial ozone supplied to the reactor. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description set forth herein, but rather that the claims be construed as encompassing all the features of patentable novelty that reside in the present invention, including all features that would be treated as equivalents thereof by those skilled in the art to which this invention pertains.

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• 1993
  • 1993-11-08 AU AU56662/94A patent/AU5666294A/en not_active Abandoned
  • 1993-11-08 WO PCT/US1993/010687 patent/WO1994017238A1/en active Application Filing
• 1994
  • 1994-01-17 ZA ZA94311A patent/ZA94311B/xx unknown
  • 1994-01-24 CN CN94100571A patent/CN1099445A/zh active Pending
  • 1994-01-24 CO CO94001843A patent/CO4370785A1/es unknown

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