NEW ANALYSIS METHODS FOR BETTER DIGESTER MANAGEMENT

ABSTRACT

In order to attain stable cooking reactions, we must ensure that all of the raw material fibers undergo the same treatment in terms of chemicals and energy. Unstable cooking may result in a number of problems including poor pulp quality and bleachability, increased reject, and low yield.

Alkali/wood ratio is one of the most important control variables and a powerful means of stabilizing cooked pulp quality. Disturbances in alkali/wood ratio may be caused by many factors, such as changes in white liquor concentration. However, traditional laboratory measurements cannot determine the alkali concentrations frequently enough for accurate control. Using an alkali analyzer, variations in white liquor concentration can be measured quickly and accurately and integrated in the alkali/wood ratio control, thus stabilizing the entire cooking process.

The article presents experiences and control results from mills where the cooking process control was enhanced by using the on-line alkali analyzer for alkali/wood ratio control and alkali concentration profile control. In the case where alkali profile cannot be controlled, the H-factor target is corrected on the basis of measured alkali concentration deviations.

1 INTRODUCTION

Pulp delignification rate is dependent on numerous process variables: temperature, time, sulphidity, chip quality and dimensions, liquor-to-wood ratio, and alkali dosage and concentration. Temperature, time, alkali dosage and alkali concentrations also have a major effect on carbohydrate dissolution and pulp yield ¹. One goal of new modified cooking processes has been to cook the pulp to a low kappa number, at the same time achieving high yield and strength. To achieve this goal it is essential to maintain a more stable alkali profile throughout the process and especially to avoid very high initial alkali concentrations that are detrimental to pulp yield ². Adding white liquor in several stages/points to the cooking helps to keep the alkali concentration at a sufficient level, preventing recondensation of lignin polymers into the fibers ¹.

The strength of white liquor may vary by about 5%. Unless taken into account in alkali/wood ratio control, this variation may cause considerable swings in alkali concentrations and blowline kappa number. Another potential source of process variations, particularly in batch cooking, is the variation of alkali concentration in black liquor. Accurate liquor strength measurement is necessary to combat this problem.

Many alternatives are nowadays available for the measurement of cooking liquor strength and residual alkali. The traditional methods – laboratory analyses and conductivity-based sensors – are not without problems with regard to automatic control: laboratory measurement is slow, and the accuracy of conductivity sensors is sensitive to changes in process conditions (e.g. sulphidity). One possible measurement technique is Near Infrared Reflectance spectroscopy ³. With this technology both alkali and lignin concentrations can be measured at the same time, but high price and complicated calibration limit its applicability.

Nowadays there is also another possibility: residual alkali and white liquor strength can be measured with an automatic on-line alkali analyzer, which utilizes a standardized laboratory titration method. Its quick and accurate measurements are suited for cooking control applications, and it has already given promising results both in continuous and batch cooking processes.

2 ALKALI ANALYZER FOR COOKING

The kajaaniALKALI analyzer, introduced in the 1990's, is an automatic on-line titrator suited for both continuous and batch cooking processes. Its calibration-free measurement is based on the SCAN-N 33:94 laboratory standard, and it is able to measure residual alkali levels in cooking circulations together with Effective Alkali (EA) of white liquor and oxidized white liquor. As option, white liquor can also be measured according to the SCAN 30:85 standard to determine Effective Alkali, Active Alkali, Total Titratable Alkali, Causticizing Degree, Sulphidity, NaOH, Na₂S and Na₂CO₃.

The analyzer takes samples automatically from up to 8 different sampling points, in the desired order, and backflushes the lines after each sampling with high-pressure water to prevent plugging. With a wide measuring range (2–180 g/l EA as Na₂O) and excellent repeatability (std. dev. < 0.3), it measures accurately even at very low alkali concentrations. Accuracy is not affected by scale buildup or sulphidity changes and thus the analyzer does not require sensor cleaning or removal.

Figure 2.1 Units and samplers of ALKALI analyzer

The reliable sampling system uses large-diameter sample lines (10 mm) and a proven sampler (SD-711). A special sampler (SD-712) with a built-in screen can be used if the liquor samples contain chips and pins, as in transfer circulations (Fig. 2.1). Effective alkali measurement takes about 5 minutes per sample with one titration module, and with two titration modules the measurement frequency is doubled. Examples of sampling points in batch and continuous cooking are shown in figures 2.2 and 2.3.

Figure 2.2 Example of ALKALI ANALYZER'S sampling points in SuperBatch cooking

Figure 2.3 Example of ALKALI ANALYZER'S sampling points in continuous cooking

3 ANALYZER BASED COOKING APPLICATIONS IN CONTINUOUS COOKING

3.1 Improving alkali/wood ratio control with alkali analyzer in continuous cooking
Studies have shown that the strength of white liquor, the principal cooking chemical, may vary by about 5% or even more after production breaks. The alkali/wood ratio control must pay attention to this variation, otherwise it will have significant effects on cooking conditions and pulp quality.

The alkali analyzer was installed to a continuous digester to measure incoming white liquor quality. This mill uses a two-vessel vapor/liquid phase digester with a daily production of about 1,100 ADt of softwood kraft and about 1,500 ADt of hardwood kraft. Softwood cooking takes on an average two days, hardwood cooking about one day. Blowline Kappa target after cooking is 32 for softwood, 18 for hardwood. All white liquor is added to the cooking before impregnation tower. Effective alkali in the white liquor flow to cooking is normally in the range 105 to 120 g/l (as NaOH), sulphidity around 40–45%. An example of normal Effective Alkali variations in the white liquor can be seen in Fig. 3.1; during this one-week period EA variation was about 5%.

Figure 3.1 EXAMPLE OF Effective Alkali VARIATIONS in white liquor (as NaOH)

The mill previously controlled the alkali/wood ratio according to laboratory measurements of white liquor, but these measurements were subsequently replaced by the automatic analyzer. This provides more accurate control, as the analyzer measures Effective Alkali in white liquor 4 times per hour – more than 8 times the earlier frequency. If needed, the analyzer could also be put to measure only white liquor samples, giving a new result every 5 minutes.

Figure 3.2 shows blowline Kappa variations and averages before and after the analyzer was put to alkali/wood ratio control: with the analyzer in control, kappa variations for both softwood and hardwood have been about 20% lower.

Figure 3.2 Digester's blowline Kappa variations before and after laboratory's white liquor measurement (ea) was replaced by analyzer in alkali/wood ratio control

3.2 Residual alkali measurement
Alkali consumption and the required dosage are affected by many process variables. Especially chip thickness is a crucial factor for chemical penetration during impregnation and cook. Chip characteristics (quality, size), pre-steaming etc. also play a role in chip compaction and may thus cause inaccuracy in the production rate measurement, as well as variations in alkali to wood ratio and digester blowline kappa number. The list of typical factors affecting residual alkali measurements, shown in Table 3.1, illustrates that this measurement is a good indicator of raw material variations.

Table 3.1 Disturbance sources affecting residual alkali measurement

Disturbance source Effect on Chip source Impregnation rate Chip size Alkali impregnation Chip moisture Alkali/Wood & Liquor/Wood Bark content Alkali consumption Chip decay Alkali consumption Wood species Reaction rate White liquor strength Alkali/Wood White liquor sulphidity Reaction rate Liquor/wood ratio Reaction rate & Movement & Packing Residual alkali EA + Lignin condensation Inaccuracy of production control Alkali/Wood & Liquor/Wood

A sufficient residual alkali concentration – 5 to 15 g/l is considered normal – is crucial for pulp quality and bleachability ^{1, 2}. If alkali concentration at some stage of cook drops too low, lignin begins to precipitate and the pulp will be very hard to bleach. This will in turn result in yield and strength losses and high chemical consumption in the subsequent oxygen delignification and bleaching stages. But high residual alkali concentration is harmful as well, particularly in the beginning of cook: it promotes cellulose degradation, i.e. reduces pulp yield, and increases chemical consumption ². Reliable residual alkali measurement is therefore much needed, particularly in modern cooking processes that aim to lower the initial alkali levels and to maintain a more stable alkali profile throughout the cook.

3.3 Residual alkali control in transfer circulation
All factors affecting alkali consumption and the required alkali dosage cannot be measured, which means that the alkali/wood ratio must be frequently corrected in order to keep the alkali profile stable. In two-vessel continuous cooking this correction can be made by applying alkali measurements from transfer circulation after impregnation to adjust the white liquor addition to transfer circulation. Measurements from transfer circulation and neural networks have been utilized to develop models that reliably predict blowline kappa number ⁴. In this way the alkali/wood ratio can be corrected already before the cook, counteracting the effect of disturbing factors on produced pulp quality.

In recent years on-line analyzers have made headway in process control. These analyzers are able to measure variables alternately from several process stages and are often more accurate than in-line sensors. However, in-line measurements have the advantage of being continuous, which in some cases may be essential for control. It may therefore prove necessary to use two different measurements to control certain crucial process variables: one a relative, continuous in-line measurement, the other a highly accurate on-line analyzer or laboratory measurement. Such measurement combinations are used in many stages of the pulping processes: in causticizing the total titratable alkali of green liquor is controlled by a combination of density and alkali analyzer measurements, while bleaching control often uses a continuous brightness measurement together with an on-line kappa/brightness analyzer. Residual alkali control, too, can be constructed upon a combination of conductivity sensors and alkali analyzer; the resulting control loop is quicker and more reliable than methods using either one of these measurements alone.

Figure 3.3 shows residual alkali results from the transfer circulation, measured by an in-line conductivity sensor and on-line analyzer. In this mill white liquor is applied to the digester before impregnation, to the transfer circulation after impregnation, and to cooking circulation. Measurement from transfer circulation indicates the effect of disturbing factors (e.g. inaccuracy of production rate control) on alkali consumption.

Figure 3.3 Residual alkali in transfer circulation as measured by conductivity sensor and alkali analyzer, and a combination of these. INSET: CLOSE-UP VIEW OF ONE PROCESS CHANGE SITUATION

Depending on the sampling sequence, the analyzer gives a new measurement result every 5 to 20 minutes. Conductivity sensor follows the changes in residual alkali continuously, but its measurement accuracy is undermined by other factors, e.g. cooking liquor sulphidity variations. The graph shows that both measurements are from time to time at the same level, while at some points there is a considerable difference between them. The mill decided to combine these two measurements, using the analyzer's measurement to calibrate the conductivity sensor (Fig. 3.3). The application program automatically updates the parameters used and thus the combined measurement is reliable even when changes occur in the process conditions.

The measurement combination provided an accurate, continuous signal that was easy to integrate in residual alkali control to correct the alkali/wood ratio in the transfer circulation (Fig. 3.4). This control system clearly stabilized alkali concentrations in the digester, and with the control on the blowline kappa variation was about 5% lower than during a reference period (Fig. 3.5). In the long run, a more stable alkali profile also enables more stable pulp quality, bleachability and yield, and makes the task of evaporation plant easier.

Figure 3.4 PRINCIPLE OF RESIDUAL ALKALI CONTROL IN TRANSFER CIRCULATION

Figure 3.5 Residual alkali variation (EA as Na₂O) and blowline Kappa variation during reference period and with the transfer circulation alkali control

4 ANALYZER BASED COOKING APPLICATIONS IN BATCH COOKING

4.1 Using alkali analyzer for better batch digester management
The alkali analyzer was installed in a Swedish pulp mill which produces approximately 280,000 tons of bleached pulp annually, mostly softwood. Kappa target after cooking is 24 for softwood, 19 for hardwood. All white liquor is added to the cooking process with hot black liquor during the liquor filling sequence. Effective alkali in white liquor is normally in the range 105–120 g/l (as NaOH).

In this process the alkali/wood ratio was previously controlled according to laboratory measurements and an old analyzer. The problem of this control was that laboratory analyses were made too seldom, and the old analyzer was not reliable enough. As a consequence, the white liquor charge was sometimes changed incorrectly due to the faulty analysis results and this resulted in kappa variations.

In March 2002 the old methods were replaced by white liquor measurement with the new alkali analyzer. This considerably improved the accuracy of alkali/wood ratio control, as the analyzer measures Effective Alkali in white liquor every 10–15 minutes and, most important, very reliably. This solution also enables measuring the concentration of the white liquor / hot black liquor mixture added to the digester in the liquor filling step.

The new analyzer also measures residual alkali in the digester circulation line during the heat-up (2 samples) and at the end of cooking (2 samples); Fig. 4.1. Sampling is scheduled by a special application based on the current H -factor of cook. Alkali concentrations are typically 16–21 g/l at the first sampling point, 6–10 g/l at the second sampling point.

Figure 4.1 Example of ALKALI ANALYZER sampling points in batch cooking

4.1.1 Alkali analyzer in residual alkali monitoring and fine-tuning of cook end point
Residual alkali variations may be caused by a large number of process variables such as white liquor strength and chip characteristic. Residual alkali measurement can therefore be used to indicate the effect of raw material variations on the cook.

The basic idea in control is to identify two distinct types of changes in the cooking conditions:

1) Alkali/wood ratio errors, caused by inaccurate measurement of wood amount in the digester, faulty white liquor flow or concentration measurement, and

2) Changed reaction rate, caused by chip age, changes in chip size fractions, changes in white liquor sulphidity, or inaccurate temperature measurement.

These changes in cooking conditions will eventually be seen in the residual alkali measurement (Figure 4.2).

Figure 4.2 Residual alkali PREDICTS kappa deviations

The deviation between measured alkali profile and a typical residual alkali profile during the cook is measured to correct the end point of cooking.  In practise this is a feed-forward control that corrects the H-factor target of the cook to compensate for estimated kappa deviations (Figure 4.3).

Figure 4.3 UTILIZING Residual alkali measurement for control

The benefits of using analyzer's measurements to control the cooking end point can be seen in the short term variation of pulp kappa number: after the feed-forward H-factor correction was introduced, short-term kappa variation has been reduced by more than 30% (Fig. 4.4). Kappa number feedback control takes care of long-term corrections and keeps the pulp kappa number at the desired level all the time. This project also included an optimization control package – production and scheduling controls, sophisticated quality controls, and tank farm management – and the total reduction in kappa deviation was over 50%.

Figure 4.4 ALKALI ANALYZER IMPROVES SHORT-TERM Kappa deviation

4.2 Alkali/wood ratio control with alkali analyzer in batch cooking
Variations in white liquor strength have a considerable effect on cooking conditions and produced pulp quality. Especially in batch cooking, the effect of black liquor strength variation on the total alkali charge is also significant. When we measure both white liquor and black liquor and integrate their strength variations in the calculation of alkali charge, it is possible to apply exactly the required amount of chemical. Table 4.1 shows an example from a batch digester: even though white liquor makes up about 70% of the total alkali charge, variations in black liquor strength may in fact cause considerable swings in the total alkali concentration. This mill is planning to change the alkali dosage control in such a way that the analyzer's measurements from impregnation liquor, hot black liquor and white liquor will be included to calculate the correct alkali dosage.

Table 4.1 EFFECT OF WHITE LIQUOR, IMPREGNATION LIQUOR AND HOT BLACK LIQUOR STRENGTH VARIATIONS ON TOTAL COOKING ALKALI DOSAGE IN A BATCH DIGESTER

~250 cooks   White liquor Impregnation liquor Hot black liquor Volume m3 129 287 293 EA, average g NaOH/l 116.3 8.4 14.6 EA, st. dev. g NaOH/l 1.4 0.4 1.1 Dosage, average kg NaOH 15005 2399 4268 proportion of total dosage % 69 11 20 Variation of dosage, EA std kg NaOH 175 113 317

The effect of white and black liquor concentration variations on digester's residual alkali levels is further illustrated by Table 4.2. The table shows how the calculated residual alkali levels after the alkali charge will behave if liquor strength variations are not properly taken into account in alkali/wood ratio calculation. According to the table, an error of 2.2% in white liquor strength will change initial alkali concentration by about 1 g NaOH/l, a change of 7% (about 1 g/l) in black liquor strength by some 0.4 g NaOH/l. Moreover, residual alkali variations also have an effect on pulp quality and blowline kappa number: studies indicate that a change of 1 g NaOH/l in effective residual alkali may change the Kappa number by 2–4 points without changes in the H-factor ⁵.

Table 4.2 effect of white and black liquor concentration errors on alkali/wood ratio and calculated initial alkali concentration

			WL EA variation +2.2 %		BL EA variation +7 %
Variable		assumed value	actual value	absolute error	actual value	absolute error
Chip moisture	%	50.000	50.000	0	50.000	0
Liquid-to-wood ratio	m³/t	4.000	4.000	0	4.000	0
White liquor charge per dry wood	m³/t	1.4906	1.4906	0	1.4906	0
Black liquor charge per dry wood	m³/t	1.5094	1.5094	0	1.5094	0
Effective alkali concentration of white liquor as NaOH	t/m³	0.1200	0.1226	0.003	0.1200	0
Effective alkali concentration of black liquor as NaOH	t/m³	0.0140	0.0140	0	0.0150	0.0010
Effective alkali-to-wood ratio as NaOH	%	20.0000	20.3935	0.394	20.1479	0.148
Calculated initial effective alkali concentration as NaOH	kg/m³	50.000	50.984	0.984	50.370	0.370

5 CONCLUSIONS

Cooking alkali consumption is influenced by a host of disturbing factors (e.g. chip quality), and this makes it difficult to calculate exactly the required alkali charge. Accurate alkali/wood ratio control requires data on white liquor strength. In order to attain a stable alkali profile and cook conditions it is necessary to monitor alkali concentrations throughout the cook and change these if needed. When residual alkali levels in the digester circulations are under control, we can avoid too high or low residuals that are detrimental to yield or bleachability. Less Effective Alkali variation means more uniform pulp quality and helps to optimize the alkali dosage, thus reducing the load on chemical recovery. A new tool for measuring white liquor and residual alkali concentrations is now available: kajaaniALKALi for cooking.

This analyzer is able to measure residual alkali concentrations from cooking circulations, and Effective Alkali of the white liquor and oxidized white liquor. When the optional titration method is used, it also measures sodium hydroxide, sodium sulphide, sodium carbonate, active alkali, total titratable alkali, causticizing degree, and sulphidity. The analyzer takes samples from up to 8 sampling points, provides quick and accurate measurements over a wide measuring range, and is insensitive to scale buildup or sulphidity changes.

The analyzer's measurements have been successfully used for cooking control in both batch and continuous cooking processes. At a Swedish batch cooking plant, the new alkali analyzer replaced the white liquor measurement of an older analyzer in alkali/wood ratio control, and the new analyzer was also put to monitor alkali profile and to fine-tune the cooking end point. As a result, short-term variation of blowline kappa number was reduced by more than 30%.

In a Finnish mill with a continuous digester, laboratory's white liquor measurement was replaced by the analyzer in alkali/wood ratio control. As a result, blowline kappa deviations for both HW and SW were reduced by some 20%. In another continuous cook the analyzer was applied to calibrate in-line conductivity measurement from the transfer circulation line. When this continuous and accurate measurement combination was integrated in alkali/wood ratio control, it clearly stabilized the residual alkali levels in all cooking circulations, and also kappa  deviation during control period was slightly lower.

One future control application of the analyzer is in the oxygen stage: an accurate measurement of oxidized white liquor strength can be incorporated in alkali dosage calculation to give better oxygen stage control.

6 REFERENCES

1. Gullichsen, J. Fiber line operations. In: Gullichsen, J. & Fogelholm, C.-J. (editor.). Papermaking Science and Technology, Chemical Pulping. Helsinki 1999, Fapet Oy, pp. 44–52.

2. Kettunen, A., Råmark, H., Harsia, K. and Henricson, K. Effect of cooking stage EA concentration profiles on softwood kraft pulping. Paperi ja puu 79(1997)4, pp. 232–239.

3. Hodges, R. and Krishnagopalan, G.A. Near infrared spectroscopy for online white and black liquor composition analysis. Tappi Pulping Conference, 31.Oct.–4.Nov.1999, Orlando, Florida. TAPPI PRESS, Atlanta 1999, pp. 1097–1109.

4. Haataja, K., Leiviskä, K., and Sutinen R. Kappa number estimation with neural networks. XIV IMEKO World Congress Proceedings, Finnish Society of Automation, vol. XA, 1–6 Jun.1997, Tampere. pp. 1–5

5. Wallin, G. and Wesslen, T. Kappa-Batch. Pulp & Paper International 16(1974)6, pp. 48–52.

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