EP0850333A2

EP0850333A2 - Process and arrangement for monitoring cellulose digestion

Info

Publication number: EP0850333A2
Application number: EP96939808A
Authority: EP
Inventors: Klaus Fischer; Herbert Furumoto; Uwe Lampe; Ingeborg Schmidt
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1995-09-15
Filing date: 1996-09-12
Publication date: 1998-07-01
Also published as: NO981172D0; CA2232171A1; WO1997010384A3; ZA967695B; NO981172L; WO1997010384A2

Abstract

The determination of the end of the digestion time is essential in cellulose digestion for the product quality and/or yield. According to the invention, this is done by measurement of the pH in-situ and possibly the colour of the digestion acid during digestion. The process may be used in both discontinuously and continuously operating cellulose digesters. An arrangement for implementing the process with suitable sensors for the on-line measurement of at least the pH has an evaluator with a computer (70), in which said computer (70) determines the digestion time from the measurement values or their time variation.

Description

description

Method and arrangement for monitoring pulp boiling

The invention relates to a method for monitoring the pulp boiling, in particular for determining the end point of the boiling time, and for controlling the carbohydrate breakdown with measurement of physical and / or chemical parameters on the digestion medium, such as color value, conductivity, pH value, bre- index, IR spectrum, UV absorption, fluorescence and / or concentrations of digestion chemicals used in the cooking. Such a method is used particularly advantageously in the production of chemical pulp, but also of paper pulp. In addition, the invention also relates to the associated arrangement for carrying out the method, with sensors for online measurement of individual characteristic variables and related evaluation.

In the production of cellulose via cellulose boiling, an essential goal of the cooking process is the removal of the lignin from the raw material "wood". However, the cellulose fiber - the desired end product of the cooking - should be attacked or broken down as little as possible, since the latter worsens the yield of the process. An inadequate solution of the lignin conversely leads to a loss of product quality. The manufacture of chemical pulp also involves precise adjustment of the viscosity, i.e. the average polymerization content of cellulose.

In the pulp production in the acidic sulfite process, the wood in the form of debarked wood chips is digested by boiling for several hours in a solution called "cooking acid". This process usually takes place at a temperature between 130 and 140 ° C under a pressure of about 8 bar over a period of 4 to 6 hours. The cooking acid used has an initial pH of between 2 and 3 and usually consists of an aqueous solution of SO2 and calcium bisulfite (Ca (HSC> 3) 2) or magnesium bisulfite ⁽ Mg ⁽ HS0 ₃ ⁾ ₂ ) •

In addition to the latter processes, a whole series of other reactions take place simultaneously during the cooking, which have different reaction speeds due to the different concentration of the reaction components and the sizes of the rate constants. This makes it difficult to determine the optimal time for maximum yield of high quality pulp at which cooking must be stopped.

DD 293 215 A5 describes a method for determining the digestion time for determining the termination of digestion at

Paper pulp production by the acid bisulfite pulping process is known, in which the pulping time required for the production of paper pulp with a specific predetermined residual lignin content L in the unbleached pulp from concrete primary data of the process after the relationship

dl ^

-— = k, - Ü -e ^τ with k, = f (FV, S0 ₇ ) (1) dt ^L

determined and the SO2 concentrations are measured in short successive time intervals. In parallel, the SO2 concentration for the termination time is shown according to the relationship

- ^diS ° - ⁾ = k _sa (S0 ₂ ) ^X 'e ^{~ τ ^} with k _SΑ = f (FV, S0 ₂ ) (2) dt' ^:

calculated in advance and the digestion ended if the calculated and measured SO2 concentration matched. In DD 293 214 A5, the definition of the termination of the digestion for the production of textile pulp is quite the same given viscosity and average degree of polymerization ¬ the cellulose the required closing time on the change in the number of cleavages S according to the relationship

7 _t ^{~ = k} '^{'e't with k} s ⁼ f <- ^FV > ^C so _> ) <3 ⁾

determined, measured in short successive time intervals, the conductivity χ and / or the absorption A of light in the spent liquor and with the corresponding time interval, according to the relationships

dy ^Z ^ ^L

- d ^ t = k _x - e ^τ with k * = f (FV, C _{S (} " _λ .) (4)

e T ^τ with k _λ = f (FV, C _SOι ): 5) dt ^A

compare the previously calculated conductivity χ or light absorption A, the digestion being terminated if the measured values correspond to the calculated values. In equations (1) to (5), the individual L mean the lignin content, t the time, δ an exponential factor, T the temperature in K, FV the liquor ratio, S the number of splits in the cellulose chain, kj _^ the rate constants, Ej_ the activation energies, C _sc , the concentration of SO2 in the

Cooking acid and A the light absorption, where δ = 1 to 3 and χ = 0.5 to 4.

In the latter prior art, it is assumed that the measurement of physical and / or chemical parameters, such as color value, conductivity, pH value, refractive index, IR spectrum, UV absorption, fluorescence and / or concentration, is known to be used to determine the cooking time becomes. However, the linkage with an associated kinetic model is problematic, these problems being taken into account by the iterative methods. Since the measurement of the characteristic values by sampling and similar measures is comparatively imprecise, the latter methods have not found their way into practice.

The object of the invention is to provide a method and an arrangement with which the pulp boiling can be better monitored. With an associated evaluation method, the optimal end point of the cooking time should be kept as accurate as possible.

The object is achieved according to the invention by an inline measurement of the pH and, if appropriate, the color of the cooking acid during the cooking. The pH of the cooking acid is preferably determined "in situ" to an accuracy of 0.1 pH and recorded at least in the final phase of the cooking at a time interval of 30 s. So-called enamel electrodes are used as sensors for the pH value. To record the color values, the absorbance of the cooking acid inline is measured under operating conditions in the range of visible light. Immersion probes are preferably used for "in situ" extinction measurement. With a spectrometer, spectra in the range between 400 and 800 nm can advantageously be recorded at least every 30 s simultaneously with the pH value.

The invention results in an improvement in productivity in cellulose cooking compared to the prior art. For the first time, the sensors for so-called inline measurement, i.e. at approx. 140 ° C and approx. 8 bar. Specifically, this means that neither bypasses, sampling or dilutions are required, but the measured values are available "in situ". No more SO2 can escape, which led to incorrect measurements during conventional sampling. Rather, the measured values are now unadulterated and can thus be used in an optimal manner to monitor the pulp boiling.

The process according to the invention can be used both in the discontinuous and in the continuous pulp cooker realize. In particular, the sensors for the pH value can be used at a suitable point.

The advantage of the enamel sensors for measuring the pH is, in particular, the long service life with virtually undetectable wear, which means that an entire cooking process can now be followed. The H ⁺ concentration is measured, which is actually present on the cellulose molecule and which catalyzes the depolymerization of the carbohydrates. As has been explained in detail above, in contrast to measurements on open samples, no more SO2 can escape, so that in particular no errors can occur due to reaction with the waste liquor.

The same applies analogously to the spectroscopic measurement. By using immersion probes for the first time, an inline measurement at a suitable point under reaction conditions, i.e. with the then available temperature and pressure values, possible. The measurement can be made directly on the undiluted acid at the reaction site, i.e. can be carried out without a bypass, so that the errors already mentioned due to dilutions or SO2 escape are excluded. Simultaneous spectroscopy can be used specifically for optical measurement, with which, for example, a spectrum over the desired wavelength range can be achieved without delay using a diode array. If necessary, measurements can be carried out with a single spectrometer by means of multiplexers at different points using individual immersion probes, which at the same time results in measurement values for a plurality of measurement locations in the area of the cooking zone or the bleaching.

In the associated arrangement for carrying out the method, which has sensors for online measurement of the pH of the cooking acid on the one hand and their color extinction values on the other hand, which are used at a suitable point within the cooker, ie inline, a control device is provided a computer available, the computer the connection between the measured variables or their temporal course and the product quality and / or product yield determined and fed back to the control device. In particular, a correlation can be derived by a multi-component correlation analysis, which describes the results between product quality and / or product yield and the measured variables.

With such an arrangement controlled variables can win ^advertising% to with which the properties of the produced Zellu¬ loose, such as viscosity, kappa number tenacity u. Like., are specifically adjustable. This allows the yield in production to be maximized.

Further details and advantages of the invention result from the following description of the figures of exemplary embodiments with reference to the drawing in conjunction with further subclaims. Show it

Figure 1 shows the relationship between the pH of the cooking acid and

Cooking time, Figure 2 shows the relationship between absorbance values

Cooking acid and cooking time, FIG. 3 shows a basic illustration of a batch-operated cooker,

4 shows the evaluation of the measured variables in a cooker according to

FIG. 3, FIG. 5 the principle of a continuously operating cooker, FIG. 6 the evaluation of the measured variables in a cooker according to FIG. 5 and

FIG. 7 an associated evaluation device.

The figures are partly described below together. Corresponding parts have the same reference numbers.

It is known that the production result in pulp production using the sulfite process is crucially dependent on the pH depends on the cooking acid during the cooking process and the duration of the cooking. It is essentially a matter of removing the lignin from the raw material wood. The lignin is a macromolecule with an aromatic basic structure, in which the individual benzene rings are linked as basic elements via side chains. The side chains are particularly reactive and are split in the cooking acid with the aid of catalytically active hydrogen ions and substituted by sulfone groups.

The water solubility of the lignin is caused by the introduction of the strong hydrophilic sulfone groups. In addition, the existing bonds between the lignin and the cellulose also have to be hydrolytically split in order to achieve complete removal. It follows that the delignification -d [L] / dt is proportional to both the bisulfite concentration and the hydrogen ion concentration. For that applies

-d [L] / dt = kι_ * [H ⁺ ] * [HS0 ₃ -] (6)

The undesired hydrolysis of the carbohydrates -d [Ko] / dt (cellulose and hemicellulose), which causes the fibers to break down, is only proportional to the hydrogen ion concentration:

-d [Ko] / dt = k ₂ * [H ⁺ ] (7)

In addition to the breakdown of the fibers, undesirable side reactions occur which e.g. decompose the cooking acid or let separate lignin units condense again. This happens because the monomers of the carbohydrates react with the bisulfite ion with oxidation, whereby a thiosulfate ion is formed in accordance with the reaction:

Glucose + bisulfite => gluconic acid + thiosulfate On the one hand, the thiosulfate can decompose the bisulfite accordingly

5 S ₂ 0 ₃ - + 4 HSO ₃ ^" => 6 S ₂ 0 ₃ - + 2 S0 ₄ ^2" + 2 H ⁺ + H ₂ 0 (8) and on the other hand condensation of the lignin units via the α-C- Cause atoms. In addition, depending on the pH value, the pentoses (carbohydrates with 5 carbon atoms) which are very common in hardwoods can easily be dehydrated to furfural. The furfural can condense with itself or the lignin, which is equivalent to resinification.

It follows from the above that the pH of the cooking solution is an essential measurement. The results of extensive tests are shown in FIG. 1, the abscissa of the diagram representing the cooking time in hours and the ordinate the pH in absolute terms.

In particular, the absolute values strongly depend on the type of wood and the liquor ratio (FV) of the raw materials used. It can be seen from FIG. 1 that during the boiling the pH value according to Graph 11 rises from an initial value of approx. PH = 2.5 to pH = 3.6, and then slowly to a value within several hours decrease from approx. pH = 3.4. The maximum of the pH value or the rate of change before and after the maximum represent significant measured variables.

Due to the comparatively small changes, it follows that the pH value is on the one hand directly in the cooker, i.e. Inline, and on the other hand with a sufficient repetition rate, i.e. continuously or with small time intervals, must be measured. Interference due to cooling, outgassing of SO2 or other reactions are avoided by the inline measurement.

The diagram according to FIG. 2 shows the results of optical measurements obtained from a simultaneous spectrometer with a diode array. It is essential that the dilute cooking acid _/ ie "in situ" was measured, for which temperature-resistant immersion probes are used, which can be placed anywhere in the cooker. The same location as that of the pH probes is advantageously chosen as the measuring location. By means of multiplexing, different measuring probes can be queried in succession with a single spectrometer.

With known spectrometers, a complete spectrum is acquired at the time of measurement, i.e. the extinction as a function of the wavelength. Appropriate processing of such spectra allows time-dependent graphs 21 to 23 to be derived, the abscissa in FIG. 2 representing the time in hours corresponding to FIG. 1 and the ordinate representing the associated absorbance value in relative units. The wavelengths of the spectral light, for example between 400 and 800 nm, are given as parameters.

The finished pulp can be characterized by the so-called kappa number. From Figure 2 it follows that up to

If a certain kappa number is reached, i.e. the essential value for determining the end of the cooking time for paper pulp, the extinction values and the pH value change significantly. The change in absorbance is still dependent on the wavelength. Since the signal relationships are comparatively complex, a direct functional relationship or a specific algorithm cannot be derived. However, from the large number of empirical values with a computer, certain statements result which can be used in combination with the pH value for controlling and / or regulating the pulp boiling.

FIG. 3 shows a cellulose cooker 30 which works discontinuously. Essential for the operation of such a cellulose cooker 30 in the present context are a filler neck 31 for the wood on the one hand and a feed line 32 for acid on the other hand and the outlet nozzle 33 for emptying. In addition to various input and output lines, a circuit 35 with a circulation pump 36 is essential, with which a circulation via a heat exchanger 37 is realized.

It has been shown that in such a discontinuously operating cooker 30, the pH measurement upstream of the circulation pump 36 is sufficient to record the significant measurement curve shown in FIG. The extinction values for deriving a dependency according to FIG. 2 are measured at the same place.

It can be seen from FIG. 4d) that the pH value according to FIG. 1 rises to a maximum during heating. The pH value can be influenced by various measures independently of the actual cooking process: on the one hand, the cooking liquid can be diluted with wood water, on the other hand, SO2 diffuses into the wood. The pH is further influenced by the so-called peezen, i.e. Pressure drop before the addition of SO2, or by the addition of liquid Sθ2 according to Figure 4a).

The aim is to either keep the pH constant over time after the maximum or to drive graph 41, 43 or 44 according to a predetermined profile. For example, an MgO addition according to FIG. 4b) or a pressure reduction according to FIG. 4c) can be used.

It can be seen from FIG. 1 that the significant evaluation variables Δ1, Δ2, the maximum and the time of the maximum come into question. The first derivative according to time (d (pH) / dt) and the total integral over time (JpHdt) can be used as further significant evaluation variables. By specifically influencing these quantities, the carbohydrate breakdown can be controlled in the desired sense. For this purpose, the following sizes can be changed, for example, in accordance with sub-figures 4a) to d): - During the cooking, MgO is added, which increases the pH;

- SO2 is added during cooking, causing the pH to drop; - The point in time at which the cooking is stopped is specifically determined;

- The temperature for cooking is set.

By influencing the cooking process in this way, the viscosity of the pulp can be specifically adjusted, in particular in the production of viscose pulp. In the production of paper pulp, the tear length can be specified accordingly. Apart from that, a maximum yield in the cooking process can be achieved by the appropriate control and regulation of the above sizes.

FIG. 5 shows a continuously operating cooker 50 with inlet 51 for the wood and feed line 52 for the cooking acid and outlet 53 for pulp. Because of the continuous operation, such a cooker is considerably more complex than a discontinuous cooker. Certain points which are essential for the operation of the cooker are picked out in the hydraulic lines. On the one hand, these are the so-called impregnation zone, the cooking zone and the washing zone. Corresponding points are designated in Figure 5 with C2, C3, C5, C6, C15, C16 and C8.

In particular, C2 denotes the entry of the wood and the cooking acid, C2 to C5 the so-called impregnation zone, C6 to C15 the cooking zone, C16 to C8 the so-called washing zone and C8 the discharge of the pulp.

The course of the concentrations of chemicals rich in cooking on the one hand and the pH value on the other hand results from the graphs in FIG. 6. In this case, in contrast to FIG. 4, the measured variables are above the so-called cooker height, which is along the main streams in the continuous cooker according to FIG. 5, shown: Starting from the cooker head, thus point 41 in FIG. 5, the significant points C2, C3, C6, C15, C16 and C8, which denote the circulations on the stove, are plotted on the ordinate from top to bottom.

In particular, graph 61 in FIG. 6 shows the pH to be controlled above the height of the cooker, while graph 62 shows the MgO concentration and graph 63 the SO2 concentration as control variables. It can be seen that there are coordinated concentration profiles in the continuous cooker. This gives the following options for influencing the pH:

By adding Sθ2 ~, which leads to a lowering of the pH value. In particular, different measures are possible for this, namely: - In the preparation of the acid and thus in the setting of the basic level of the pH using the cooker model;

- on the cooker head (C3 circulation) by adjusting the pH value via the pH value measurement in the C2 / C3 circulation, by specifying the setpoints for the pH value, the temperature T and the circulation quantity in the C2 and C3 Circulation via the cooker model;

- measure in the C5 / C6 circulation at the beginning of the reaction zone

(C5, C6), preferably with the C5 circulation, optionally also in the middle of the cooking zone, specification of the setpoints for the pH value, the temperature T and the circulation quantity in the C5 and C6 circulation via the stove model. By adding MgO in dissolved form as a slurry, which leads to an increase in the pH. Various measures are possible for this, namely: - In the preparation of the acid and thus in the setting of the basic level of the pH using the stove model;

- on the cooker head (C3 circulation) by adjusting the pH value via the pH value measurement in the C2 / C3 circulation, by specifying the setpoints for the pH value, the temperature T and the circulation quantity in the C2 and C3 Circulation via the cooker model; - measure in the C5 / C6 circulation at the beginning of the reaction zone

(C5, C6) preferred for the C5 circulation, optionally also in the middle of the cooking zone, specification of the setpoints for the pH value, the temperature and the circulation quantity in the C5 and C6 circulation via the cooker model;

- In the wash water circulation C5 on the base of the stove and thus adjusting the pH at the entrance to the wash zone. The pH value, the wash water temperature and the amount of wash water are calculated using a laundry model which ^" takes into account both the efficiency of the wash and the after-reaction, in particular the carbohydrate breakdown in the wash zone. The pH value measurement in the C16 is used The carbohydrate breakdown in the washing zone can thus be slowed down to the required extent, and the measurements and the addition of the chemicals can be used alternatively, in different combinations or in total.

The representations as a function of time according to FIG. 4 are therefore omitted in this case, since the circulation always provides a cooking zone in the continuous cooker.

The complexity of the stove model and the laundry model depend on the task, in particular on the quality of the pulp and the required accuracy with which the quality is to be adjusted and the yield is to be increased. For particularly high demands on the pulp quality, color measurements in the C5 and C6 circulation can increase the model accuracy and thus the uniformity of the pulp.

On the basis of an analytical model, in which the raw materials of the raw material, such as the age of the wood, liquor ratio and other sizes, as well as the specified operating conditions, such as, in particular, the temperature, can be considered by those described with reference to FIGS. 1 and 2 Measured variables and the associated evaluation methods based on the figure 4 and 6 such statements are made that shape the end point of the cooking time.

In FIG. 7, a computer 70 is provided with an associated evaluation device, which uses the measured values to determine associated function values for determining the cooking time in accordance with a predetermined algorithm. For example, a function η (t) can be calculated that has a significant progression for the cooking time. The structure of the associated algorithm is:

η (t) = α ₀ + 0 ^ x / (pH) dt + α ₂ / (pH) dt / ^er dt + α ₃ / (S0 ₂ ) dt

+ αjpdt + ...

Corresponding process functions with "measured in situ" measured time profiles and in particular pH measured inline are input to computer 70 via separate inputs. The start values of the process variables at t = 0 and the measured values via individual switches 81 to 84 Certain simulation values can also be entered in assigned modeling units 91 to 94 via the switches 81 to 84. The output of the computer 70 supplies the function η (t) corresponding to a modeling unit 95, which can in particular represent the viscosity In addition to the cooking time, this can also be used to make statements about the

Product quality and / or product yield can be derived.

The process functions can also be fed to a regulating device and specifically regulated in order to optimize the cooking time by means of appropriate means for feedback.

The method described and the associated device can be used particularly advantageously in the acid sulfite method for cellulose cooking.

Claims

claims

1. A method for monitoring the pulp boiling, in particular for determining the end point of the cooking time, and for controlling the carbohydrate breakdown, with measurement of physical and / or chemical variables on the digestion medium, such as color value, conductivity, pH value, refractive index, UV -Absorp¬ tion, fluorescence and / or concentration of used during cooking and pulping chemicals gekennzeic ^v h - net by an in-line measurement of the pH value, and ge optionally the color of the cooking acid during cooking.

2. The method of claim 1, d a d u r c h g e k e n n ¬ z e i c h n e t that the pH of the cooking acid is determined "in situ" to 0.1 pH and recorded at least in the final phase of the cooking at a time interval of 30 s.

3. The method of claim 1, d a d u r c h g e k e n n ¬ z e i c h n e t that so-called enamel electrodes are used as a sensor for the pH.

4. The method of claim 1, d a d u r c h g e k e n n ¬ z e i c h n e t that the absorbance of the cooking acid is measured "in situ" under operating conditions in the range of visible light to detect the color values.

5. The method of claim 4, d a d u r c h g e k e n n ¬ z e i c h n e t that optical immersion probes are used for the absorbance measurement, whose signals are evaluated with a spectrometer.

6. The method according to claim 5, dadurchgekenn ¬ characterized in that the spectrometer spectra in the wavelength range between 400 and 800 nm are detected at least at a time interval of 30 s simultaneously with the pH.

7. The method according to claim 3, wherein a discontinuous pulp cooker is used, so that the pH value is measured after the discontinuous cooker circulation pump.

8. The method according to claim 3, wherein a continuous pulp cooker is used, so that the pH value is measured at least in the pumping lines.

9. The method according to claim 8, d a d u r c h g e k e n n ¬ z e i c h n e t that there are at least three measuring points for the pH value in the impregnation zone, the cooking zone and the washing zone.

10. The method according to claim 7 and 8, so that the spectra are measured at the same point as the pH value by multiplexing with a single spectrometer and several immersion probes.

11. The method of claim 1, d a d u r c h g e ¬ k e n n z e i c h n e t that a correlation model is used for evaluation.

12. The method according to any one of the preceding claims, g e k e n n z e i c h n e t used in the whole sulfite process.

13. Arrangement for performing the method according to claim 1 or one of claims 2 to 12, with sensors for online measurement of the pH of the cooking acid on the one hand and their color extinction values on the other hand, which are attached at a suitable location within the cooker (inline), ¬ characterized in that an evaluation device with a computer (70) is provided, wherein the computer (70) determines the cooking time from the measured variables or their temporal course.

14. Arrangement according to claim 13, so that an algorithm is derived in the computer by means of a multi-component correlation analysis which describes the relationship between product quality and / or product yield and the measured variables with an optimal cooking time.

15. Arrangement according to claim 13, so that means are provided with which the measured values and the product quality or product yield are fed back to a control device for optimizing the cooking time.