Optimisation of the causticising process
Janne Tolonen (i), Ossi Tolonen (ii) and Jukka Puhakka (iii)
Traditionally the causticising process has been controlled by using a lime/green liquor ratio-controller, dT-controller, or by conductivity sensors. These control methods have not optimized the causticising process in the best possible way. Using a kajaaniALKALi, an on-line autotitrator, it is possible to optimize the whole causticising process when two different controls are built up: Green liquor TTA-control and white liquor CE%-control.
Soon after the implementation of this control strategy, a Finnish pulp mill reported results including over 50 % improvement in white liquor quality, more than 50% faster process start-ups after shutdown, and less frequent acid washings of white liquor filter. Improvements were also seen in the other pulp mill departments (digester, bleach plant, evaporation plant and recovery boiler) due to the reduced variability of white liquor quality and higher white liquor strength.
1. CAUSTICIZING PROCESS
Causticizing plays an important role in the chemical balance of a kraft mill, as this process generates cooking liquor (Figure 1.1).
The object of the causticizing process is to turn inactive sodium carbonate (Na2CO3) into the active cooking chemical, sodium hydroxide, and to make sure that the conversion efficiency of carbonate into hydroxide is as high as possible. The process can be devided into four parts: green liquor clarification/filtration, slaking, causticizing and white liquor clarification/filtration (Figure 1.2).
Figure 1.2 Causticizing Process and the Typical Sampling Points of Alkali Analyzer.
H2O + CaO => Ca(OH)2 + heat (1)
The slaking reaction begins and proceeds only when the green liquor temperature is +60°C (140°F) or higher. However, the slaker temperature should be kept low enough to maintain maximum reaction speed. Normally the temperature of incoming green liquor is between 85-91°C, and the temperature increases in the slaker by 14-15 °C.
Ca(OH)2 + Na2CO3 => 2 NaOH + CaCO3 ...(2)
Degree of causticizing (CE%) describes the completeness of the reaction at equilibrium.
CE% = NaOH * 100 / (NaOH + Na2CO3) .(3)
High conversion efficiency is desired to reduce the load of inert sodium carbonate in the recovery cycle. Typically the causticizing degree is around 70-80 %, depending on alkali concentration and sulfidity level. Goodwin's curve (figure 1.3) describes the highest achievable causticizing degree. In many mills operating without advanced controls, the causticizing degree is far below the theoretical value in order to avoid overliming. Figure 1.4 contains actual data on causticizing degree from European and US mills.
Figure 1.4. Data of Actual Causticizing Degree From About 50 Mills in Europe and North America. Sulfidity-% Varies Between 25 % and 40% in the Mills.
1.3 White Liquor Clarification/Filtration
Good efficiency of the white liquor process (particularly filtering) requires careful control of the lime milk composition . If overliming occurs, the lime mud removal is difficult and filters require more service or may even get blocked.
2. OLD METHODS FOR CAUSTICIZING CONTROLS
2.1 Temperature Difference
However, this control method cannot ensure a stable green liquor or causticizing degree (CE%) and Active Alkali content (AA) of the resulting white liquor. As an example, if lime reactivity goes down (i.e. very hard burned lime), it takes longer to slake and the lime dosage must be increased to achieve the target causticizing degree.
Unreacted excess lime must then be removed from the process through the slaker's classifier part, and again the need for fresh lime increases and easily causes overliming. On the other hand, if lime reactivity goes up, the slaker temperature will also rise and lime feed decreases; this means lower causticizing degree than expected.
As Figure 5.1 illustrates, the quality and concentration of the white liquor varies considerably when only these controls are used. As indicated by Goodwin's curve, at the Sulfidity and TTA-levels of this mill the theoretical CE% maximum of the white liquor is about 85 %. Thus the process has on several occasions run above the overliming limit. Too much lime in the process causes contamination and blockages of the white liquor ecofilter, and it also means higher consumption of fresh lime- due to variations in lime quality (causticizing power, size and surface hardness of lime particles, temperature, fresh/reburned lime ratio), production rate, or green liquor quality (temperature, sulfidity, TTA). Moreover, the causticizing process contains many different particles that cause precipitation and sedimentation, and these complicate the use of various measurement causing inaccuracy and drifting.
All these factors contribute to the fact that temperature difference control cannot handle all process changes in a satisfactory manner. As the results in Figure 5.1 show, in order to achieve adequate results the dT-control setpoint would have to be corrected far more frequently than the manual titration method allows.
2.2 Conductivity Difference
a) Analyzed chemicals have different conductivities.
The goal in causticizing is to convert the Na2CO3 of the green liquor into NaOH with the highest possible causticizing degree, at the same time avoiding overliming. Because the conductivity of Na2CO3 is much lower than that of Na2S, small changes in the concentration of Na2S can cause large inaccuracies in the lime feed and causticizing degree.
b) Conductivity is an inferential measurement that has to be calibrated by means of laboratory titration at regular, frequent intervals due to normal variations in liquor chemistry.
c) Due to scaling problems, conductivity sensors require frequent servicing and cleaning, even several times per week.
3. ON-LINE ALKALI STRENGTH MEASUREMENT WITH ALKALI ANALYZER
The kajaaniALKALi-analyzer is a fully automatic, on-line sampling and titration analyzer for green liquor, causticizing and white liquor. Using a field proven on-line sampling system and autotitrator, it provides outputs for control, based on actual process titration results. The alkali analyzer system consists of sample unit, with 4 or 8 sample points, and a measurement unit with one or two titration modules. New measurement result will be available every 8 minutes.
In a typical application, samples are taken from green liquor, causticizing vessels and white liquor to digesters. The analyzer takes samples and analyzes them automatically. The analysis is based on ABC-titration procedure (Standard SCAN 30:85), the most widely used method in kraft mill laboratories. Figure 3.1 presents the measurement principle and sequence. The analyzer measures the absolute values of Sodium Hydroxide (NaOH), Sodium Sulfide (Na2S), Sodium Carbonate (Na2CO3) and calculates Effective Alkali (EA), Active Alkali (AA), Total Titrable Alkali (TTA), Causticizing Degree (CE%) and Sulfidity (S%). The results can be monitored from the analyzer and sent to mill DCS system as analog or digital signals.
4. ADVANCED CAUSTICIZING CONTROL WITH kajaaniALKALi
In causticizing control there are two different control-loops; Green liquor TTA-control and White liquor CE%-control. Main target of these controls is a high and stable causticizing degree, which in practice means homogenous quality, high strength of white liquor, higher production capacity and reduced operating costs in the causticizing plant and in the rest of the pulp mill.
4.2 Green liquor TTA-control (Figure 4.1)
Green liquor density control is usually carried out in two steps. First the density of recovery boiler dissolving tank is adjusted with weak wash, water, and/or secondary condensate slightly above the target density in the slaker. After this density of incoming green liquor is regulated to the desired level, usually by adding weak wash.
Green liquor density setpoint is adjusted according to the TTA (Total Titrable Alkali) value measured by alkali analyzer. TTA analysis results are available about once per hour, depending on the titration sequence of the alkali analyzer. TTA and density are applied to calculate a conversion factor for converting the TTA values into density, and vice versa. The conversion factor is calculated using results from, the preceding 8 or 24 hours, and it is constantly updated.
The control maintains a stable sodium carbonate feed to the slaker and in combination with the alkali analyzer control it ensures stable quality of produced white liquor.
4.3 White Liquor CE%-Control (Figure 4.2)
The temperature difference between slaker and green liquor indicates the relative progress of the causticizing reaction. Short-term changes in temperature difference indicate changes in either quantity or quality of raw materials entering the slaker.
However, in the long run the temperature difference control alone is not able to maintain the target causticizing result, as changes in lime quality and the temperature level have a significant effect on scaling and causticizing kinetics. For this reason, the absolute measurement results of the composition of green liquor, lime milk, and white liquor, supplied by the kajaaniALKALi, are extremely valuable for slaker controls. These titration results, together with the relative changes in temperature, can be effectively applied to control lime feed to the slaker so that the desired high causticizing degree can be achieved without delay, and also kept sufficiently stable.
Setpoint for the temperature difference control is adjusted according to the causticity of lime milk and white liquor. When the spotwise absolute measurements by the alkali analyzer are combined with the continuous temperature difference measurement, the process can kept within the correct causticity level all the time, and the control is able to react to variations in lime quality before they deteriorate the quality of white liquor. This control also helps to prevent slaker boiling and over-liming situations that may cause problems at the white liquor filter.
4.4 Example: How causticizing controls works
1. Green liquor TTA-control:
This control stabilizes the TTA value of green liquor flowing to the slaker. Alkali analyzer titrates the green liquor TTA at certain intervals, and the system compares the obtained result to the TTA setpoint given by operator. When deviation is observed, a new setpoint is given to the green liquor density controller, which adjusts the weak wash flow to the green liquor between titrations. This stabilizes the green liquor TTA and eliminates one process variable.
2. White Liquor CE%-Control:
Control aims to stabilize the causticizing degree of the produced white liquor. The CE% of lime milk is used as a short feedback signal to the dT-controller: if the lime milk CE% deviates from setpoint, the CE%-controller gives a new setpoint to the dT-controller which in turn controls the lime/green liquor-ratio.
Between titrations the dT-controller changes the lime feed to the slaker to eliminate variation. Titration results from the white liquor from the last causticizing vessel are applied to calculate the CE% difference and to monitor white liquor quality.
The CE% difference between white liquor and lime milk tends to vary because of changes in production rate and lime quality, and the CE% obtained from the white liquor titration is used as a feedback signal regulating the CE% of lime milk.
5. MILL EXPERIENCES OF THE CAUSTICIZING CONTROL
Figures 5.1 and 5.2 shows process data from a Finnish pulp mill. The causticizing controls with alkali analyzer were switched on (Figure 5.2), and the operator has given the white liquor CE% setpoint as 82%. When the data is compared to figure 5.1, or to the situation preceding the control we can see that the control has very clearly stabilized the white liquor quality- both CE% and EA- and harmful over- and underliming situations are avoided. During the control period in the average CE% was 81,95, and standard deviation for CE% 1,12 and EA 1,00%.
Figure 5.1. Results of Temperature Difference Control. White Liquor CE% and EA. One Week Control Period.
Figure 5.2. Results of Advanced Causticizing Controls with kajaaniALKALi. White Liquor CE% and EA. One Week Control Period.
Benefits on the mill:
1. Process start-ups take far less time than before (Figure 5.3).
One example is the causticizing process start-up after the 6- month maintenance downtime: it used to take 2-3 days to reach a stable target green liquor TTA to the slaker. With continuous data from the kajaani-Alkali the operators could manually adjust the process to the right TTA-level- and the target was reached in 17 hours, in other words, 65-75% more quickly than before. Naturally this also means that the quality of the cooked pulp, was stabilized in a much shorter time than earlier.
Figure 5.3 Causticizing Process Start-up.
2. The process is far easier to control even when disturbances occur problem situations take less time and their effects can overcome sooner.
3. The digester now gets better quality white liquor, as a more stable causticizing degree of white liquor also improves the EA and AA concentrations.
This facilitates better digester control, improves yield and reduces Kappa variations from the digester, resulting in lower bleaching chemical costs in the bleach plant.
4. Improvements in white liquor quality;
No more harmful over- or underliming situations; white liquor filters require less frequent cleaning, which means higher causticizing output, plus savings in cleaning chemical costs and reduced fresh lime consumption.
5. Higher output of the causticizing process because of stronger white liquor and less frequent white liquor filter cleanings.
Several benefits will be achieved with on-line alkali strength measurement and slaker control. Fully automatic, on-line sampling and titration of green liquor, caustcized liquor and white liquor helps to stabilize the TTA of green liquor and to automate slaker control, enabling optimization of the causticizing process.
This results in lower emissions, fewer process bottlenecks, higher production capacity and reduced operating expenses in the caustic plant and in the rest of the pulp mill.