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Pulp producers throughout the world are committing to environmentally friendly products and processes. To meet the expectations of various regulatory organizations, significant capital expenditures are often required to bring processes up to the "best available technology" referred to in regulations like the United States' Cluster Rules. If the best available technology equipped with the right quality controls can also produce a more consistent quality product, that can create a win-win situation for the pulp producer. A case in point is the recent conversion to elemental chlorine free (ECF) bleaching at the Somerset mill of Sappi Fine Papers, North America.
The Somerset mill, an integrated bleached kraft pulp and paper mill located in Skowhegan, Maine, produces 450,000 tpy of hardwood and softwood pulp. The single vessel Kamyr continuous digester produces brownstock at a Kappa number of about 26 for softwood and 17 for hardwood. Hardwood pulp, comprised of mostly aspen, maple, and birch, is produced at a rate of 1,500 tpd. Softwood pulp, comprised mostly of spruce, fir, and hemlock is produced at a rate of 1,200 tpd. The 89% to 90% ISO brightness pulp is used as furnish for three paper machines at the mill that produce 680,000 tpy of lightweight coated freesheet papers.
There is no oxygen delignification stage in this mill. Before the ECF conversion, the existing five-stage CdEoDED bleach plant was using 10% chlorine dioxide (Cl02) substitution in the Cd tower.
Sappi committed to a $20 million ECF conversion project, scheduled for startup in the summer of 1997. The best available technology would comply with the evolving Cluster Rules, Maine's 1998 and 1999 environmental standards for dioxins and furans, and Maine's 2001 final effluent color standard. The project included a new 46 tpd R-10 generator to meet the increased demand for Cl02 in the D100 stage in the new ECF sequence. A hydrogen peroxide addition system was added to the extraction stage. The new ECF bleach plant now operates a DEopDED sequence.
Changed Chemistry, Control Dynamics. Mill engineers and consultants were aware that the change from a traditional chlorine-based first bleaching stage to one with 100% Cl02 chemistry could have an impact on both the process design and control of the pulp quality controls. The reaction dynamics of chlorine and chlorine dioxide with lignin have proven to be significantly different in other ECF conversion projects.
The existing single upflow Cd stage was converted to two towers in series-one with seven minutes retention time, the other with 13 minutes retention time. There was a blend chest between the two towers. If the 20-minute retention time was not enough to produce the required delignifcation, an additional tower with 20-minute retention time was considered as a contingency. As it turned out, the extra tower was not required to meet process requirements. The mill found that Cl02 reacted quickly with lignin, leaving very low or no residual chemical. The brownstock temperature, at 120ºF entering the D100 stage, did not have to be changed to accommodate any insufficient residence time. The brownstock had a low consistency, ranging from 3.5% to 4.0%.
While the fast reaction of Cl02 with lignin was good news for the process designers, the accelerated reaction rate presented a new challenge for controlling pulp quality. Traditional C stage or Cd stage control using compensated brightness has been the industry standard for many years. The measurements of both brightness and chemical residual are weighted and combined to produce a compensated brightness measurement. This single set point control strategy is very simple for the operator and has proven to be very effective for chlorine bleaching.
However, the situation when bleaching with Cl02 is quite different. Valmet Automation's product manager Teuvo Peltomaki explains that, "the reaction rate is so fast that there is little or no chemical residual left. The compensated brightness control is therefore not based on a solid feedback of residual chemical, and the traditional compensated brightness control can be unstable." Also, unlike chlorine-based bleaching, the pH of the mixed pulp entering the D100 stage can be higher and less stable, thereby affecting the reaction rate.
Anne Gould, Sappi's process systems manager, puts the bleach plant control issues in perspective as she reports that, "The control of the first (D100) stage is so important in bleach plant control." With so much emphasis on establishing the right chemical reaction conditions in the D100 stage, the mill decided to implement a new Compensated Kappa Factor Control supplied by Valmet Automation.
Automated Online Kappa Measurements. Delignification control is based on measurements of the Kappa numbers of the brown stock, and the stock after the first extraction tower (CEK). A Kajaani Kappa Analyzer plus the traditional Cormec brightness and Polarox chemical residual sensors are used. For this project the existing Kajaani sensors were updated with new electronics, and new brightness and chemical residual sensors were added.
The Kappa Analyzer automatically samples, screens, and thoroughly washes the pulp samples before measuring the lignin content in the fibers using UV light. The sampling system also deposits samples in removable bags so that mill technicians can occasionally cross-check the instrument readings to the laboratory's findings. The Kappa Analyzer has proven to be very reliable, with uptime better than 98%, according to Gould.
The instrument also provides a measurement of the long-fiber and short-fiber fractions of the pulp. This measurement is useful in a mill like Somerset to indicate when fiber species "swings" are coming through the process. The fiber length measurements are taken on the brown stock sample, the CEK sample, and the D2 stage sample.
Figure 1: Online instrumentation for the Skowhegan mill's compensated Kappa Factor controls.
The instrumentation for Compensated Kappa Factor control at the Sappi mill is shown in Figure 1. Because of the fast reaction time, chemical residual is measured only five seconds after mixing in the new static mixer before the first (seven minute) tower. Brightness is measured after 25 seconds reaction time. Both brightness and residual are measured again after the second tower of the D100 stage.
Shift in Emphasis. For D100 stage control there has been a shift in emphasis from traditional compensated brightness control to the control of delignification. Delignification is indicated by the difference of the Kappa numbers of the pulp before the D100 stage and after the first extraction tower (CEK). The input of lignin in the fiber to the first stage is indicated by the Kappa measurement combined with brownstock flow and consistency. In the feed forward part of the control strategy, the input of lignin to the D100 stage and the Kappa factor target determine the chemical dosage to be applied to the pulp. The Kappa factor target-percent active chlorine divided by the Kappa number-determines the chemical dosage based on lignin content. With this control strategy, Anne Gould says that "every pound of lignin gets the same amount of chemical." The total Cl02 dosage is split between the two stages, with 65% going to the first mixer and 35% to the second mixer.
During normal operation, without fiber species changes, the Kappa analyzer is programmed to sample the brownstock and the extracted pulp sequentially, screen and wash each sample, and provide an absolute Kappa number measurement. The D2 sampling sequence is run only during fiber species changes when fiber length swings are expected. The total time for each sampling, washing, and measurement sequence is about seven minutes. The all-important washing cycle is set to ensure that any traces of liquor containing dissolved lignin are removed from the sample and therefore do not interfere with the measurement of lignin remaining in the fibers. Mr. Peltomaki reports that the total time required for the sampling, washing, and measurement sequence could be as low as five minutes.
For this mill, the time between updates of the brownstock or the CEK measurements is two sampling sequences, less than 15 minutes. If a shorter turnaround time is needed, an extra washing unit can be added to prepare each sample in parallel.
But process upsets can occur faster than the update time of the sampled Kappa measurement. To address this important issue, the new control strategy uses the online brightness measurement after chemical mixing to provide a fast, real-time indication of short term fluctuations in lignin content. The Kappa analyzer provides the brightness measurement with a calibration factor to ensure that the readings are absolute. This proven relationship between lignin content and brightness is an important principle that enables the control strategy to reduce short-term variations at the front end of the bleach plant. The CEK number provides a feedback trim control to ensure stability through the remaining bleach plant stages.
Peltomaki says, "Many people say that the Eop Kappa number is the most important parameter in the ECF bleach plant, allowing chemical savings and stable final brightness. It is the main tool for measuring the total bleaching effectiveness of the D100 and Eop stages combined; it shows the combined effects of chemical oxygen demand (COD), soda loss, pH, chemical dosage and the operation of the Eop stage."
The D1 stage control is very similar to the D100 stage control, but the focus shifts from delignification to brightness increase. The chemical dosage is calculated by using the Eop Kappa number and the post-D1 brightness target. The set point is then trimmed by using relative brightness and residual chemical sensors.
Figure 2: The optimum Kappa Factor target must be chosen to get the maximum delignification after the Eop stage without consuming excess chemicals.
Optimum Chemical Dosage. Selecting the best Kappa Factor for each fiber species optimizes the delignification reaction with Cl02. There is a point where any marginal gain in delignification is balanced against extra chemical usage (Figure 2). If too much chemical is applied, the reaction will saturate. Extra chemical will be added with no increase in delignification; that extra chemical is wasted. Peltomaki theorizes that the lignin, which is oxidized by ClO2, acts as a protective layer on the fibers, thereby preventing additional reaction. Beyond the optimum operating point, which is specific to the fiber species and the mill, the chemical residual sensors detect the wasted chemical. It acts as a "watch dog" for chemical consumption or for excess chemical carryover from the brownstock washers. The response of the Compensated Kappa Factor Control to changes in hardwood brownstock Kappa number is shown in Figure 3.
Figure 3: Chemical dosage, determined by the Kappa Factor target, changes according to incoming Kappa number.
The D100 stage inlet pH, before Cl02 addition, is quite low and stable-around 1.8 to 2.0.The D100 stage filtrate is used for stock dilution. Sulfuric acid is used to adjust pH, if required. The bleached pulp quality measurements and controls are completed with brightness before chemical mixing and chemical residual after chemical mixing on the final two D stages.
Smooth Transition. The Somerset mill went elemental chlorine-free on July 15, 1997. According to Gould, "The transition to ECF was a non-event and better controls made it better." Based on trial results before and after the ECF conversion, chemical costs are about what was projected for ECF hardwood pulp. Softwood chemical costs are lower than projected.
As reported in a technical paper at the 1998 Tappi Pulping Conference, the mill now complies with the Cluster Rules and the State of Maine environmental regulations. Sensitive environmental emissions are at a non-detectable level. The ECF conversion places the mill "three years ahead of schedule for the best available technology requirement," according to Gould.
Figure 4: Frequency distribution of D2 stage brightness before and after Kappa Factor control. The off-control data is skewed because of drifting of the data.
The most significant bottom-line benefits can be seen in the final product produced by the pulp mill. "Our brightness variability has been reduced by 52% to 53%," says Gould, "and that quality to the paper machines pays off."
Mr. Williamson is a freelance writer based in Thornhill, Ontario.
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