Digester Additive Controls Mill's Pitch Problems, Boosts Production Efficiency
By Ken Patrick
Four trials have been run at a 1,000 ton/day northern bleached kraft mill in the U.S. using a surfactant-type digester additive to help control aspen pitch problems on its paper machines during the winter months. Results from the first three trials conducted in February – March of 1999 were supported by more in-depth studies conducted during the current winter period.
The additive, Basperse 71DR10, manufactured by Bastech, Jacksonville, FL, was used during the trials to dissolve dichloromethane (DCM) extractives/pitch from the fiber during the cook, allowing them to go with the black liquor to the recovery boiler rather than carry over with the fiber to the paper machines.
As a result, the mill is experiencing longer wire life on the paper machine, but also reports other significant operating advantages, including a reduction in kappa number, alkali, and cooking time in the digester, about a 50% reduction in pulp rejects, and reduced bleach plant D-stage dirt count and lower associated bleaching chemical costs, as well as improved screening, runnability, and overall pitch deposits. Recovery boiler steam capacity also improved during the trials due to the increase in black liquor solids, and the evaporators ran cleaner due to the surfactant effects of Basperse in black liquor delivered to the recovery cycle.
According to Raymond Basso, president and CEO of Bastech, the standard preventive measure for pitch control is to add talc in the pulp mill and bleach plant and diatomaceous earth (Diasource) near the paper machines to absorb free pitch. Talc does and excellent job of tying up the free pitch and keeping the pulp mill and bleach plant areas deposit-free, Basso explains. However, he adds, encapsulated pitch that is released at shear points throughout the process (such as stock preparation refiners), continues to deposit on the paper machines.
This is a new application for Basperse, and we are very excited about the results we are seeing in these trials. — Raymond Basso, president and CEO of Bastech
Basso explains that this particular northern U.S. mill uses 100% aspen, which usually results in a pitch problem as the winter months proceed. He points out that, compared with softwoods such as pine, hardwoods are generally low in fatty acid soaps, which have some positive effects on liquor penetration into the chips during the cooking process, thus helping to dissolve the extractives/pitch.
In the summer months, bacteria and enzymatic action tend to deplete the mill's hardwood of organic waxes and pitches as the wood is aged for 30-60 days. But when the outside temperature drops to 30oF, or below, the enzymes and bacteria no longer function in this regard, and this natural "cleaning" of the aspen ceases in the winter months, Basso says.
Figure 1. Chip DCM extractives in the mill's aspen chips increase during the winter months, and then decrease in the summer due to bacteria and enzymatic action on organic waxes and pitches, as this chart of the past five years shows.
First three trials
The primary objective of the first three trials conducted at this mill was to determine is Basperse could reduce the quantity of DCM extractives in the brownstock and bleached hardwood kraft pulp. Compared with pre-trail and post-trial data, the DCM extractives were reduced 24%, 12%, and 31% for the first, second, and third trials, respectively. Basperse addition for the first trial was 4.5 lb/ton, of pulp, followed by 2.0 lb/ton and 4.0 lb/ton for the second and third trials.
Pulp properties were not seriously affected during these trials, indicating that Basperse would not cause any major pulp strength changes for the paper machines if it is used as a pitch control agent during part of the year.
In the bleach plant, D-stage dirt counts were reduced by 23%, 9%, and 17%, respectively, for the first three trials, compared with pre- and post-trial data. The Basperse did not cause any complications to the bleach plant. In fact, bleaching costs were actually lower during the trial periods.
As mentioned above, Basperse was added to the mill's digesters with white liquor at 4.5, 2.0, and 4.0 lb/air dry ton of pulp. According to Basso, Basperse's effect on extractives is additive, i.e., the more added the better the reduction in DCM extractives. The dosage rate was controlled via the mill's Bailey control system by allowing two different fill times to accommodate both the big and small digesters.
DCM extractives of the chips, brownstock, and the final bleached stock were measured frequently. Because the DCM extractives of softwood pulp are much lower than that those of hardwood, all DCM extractive data that came from softwood or softwood contamination were discarded. Table 1 below shows the averages of a few selected process variables of the pulp mill and the bleach plant for pre-trial, trial, and post-trial periods.
As shown in Table 1, prewasher conductivity was higher during all of the trial periods, indicating more black liquor carryover (soda loss) into the bleach plant. However, with the final brightness relatively constant, the average bleaching costs during all of the trial periods were lower. This is shown graphically in Figure 2 below.
Figure 2. Impact of Basperse on the bleach plant
Basso explains that D-stage dirt counts were definitely lower during the trial periods, and this was easily detectable by the bleach plant operators. D-stage hardwood freeness appeared to be relatively constant throughout the entire trial period. Furthermore, he points out, the differences before, during, and after the trials were not statistically significant (95% confidence interval). Fiber strength varied throughout the entire trial period but, again, this was not statistically significant.
Table 2 below shows the averages of chip belt, brownstock decker, and D-stage to storage DCM extractives from before the first trial through the end of the third trial period. The percent reduction from the chip belt to brownstock is also shown in this table.
As shown in Table 2, the percent reduction in DCM extractives from chips to brownstock was lower during all of the trials, indicating that the Basperse had a positive effect on reducing extractives. At 4 lb/ton or more (trials 1 and 3), the Basperse appears to be much more effective in reducing the brownstock DCM extractives. Although pitch deposition on the paper machines was not measured quantitatively during these trials, there was no noticeable increase in deposition during these trial periods.
Table 3 below is an evaluation of the brownstock hardwood pulp properties. Samples of hardwood pulp were obtained from the brownstock decker MC chute during and after the first trial period.
Basso points out that the pulp properties from the trial period did not appear to be significantly different from the post trial properties. Therefore, he concludes, the Basperse digester additive can reduce the DCM extractives of pulp without changing the pulp properties.
The fourth trial, which began last November and is still continuing, is exploring a "broader picture" of the Basperse's effect, Baso explains. While the first three trials were confined mainly to DCM extractives reduction, this trial examined effects on kappa reduction, rejects, alkali usage, wire life on the paper machines, recovery island efficiency, steam production in the recovery boiler, etc.
DCM extractive reductions between the chips on the chip belt and the brownstock decker pulp were similar to those experienced in the first three trials, as graphically shown in Figure 3 below. As the figure illustrates, when chip DCM extractives levels increased during the early November to late December trial period, the mill was able to maintain a steady extractives level in the decker pulp and pulp from the bleach plant D-stage. As in the first three trials, dirt count also decreased in the D-stage pulp.
Figure 3. Basperse 71DR10 Trial, 11/18 through 12/27/99
Basso points out that during the trial, the kappa number was reduced and the mill was able to cut its cooking time in the digester by 2 min (in a 70-min. normal cooking cycle). "Since this mill cooks to a constant H factor, it was able to reduce the cook by 2 min overall, which translates into increased production," he explains.
Rejects were actually reduced by 50% during the trial, at about 3.5 lb of Basperse/ton of air dry pulp, resulting in increased yield. Talc was reduced from 23 lb/ton to 13 lb/ton. At approximately $0.30/lb, this reduction in talc translated into about a $3/ton savings in production costs. So, as Basso calculates, the additional cost for the Basperse agent at $0.79 - $0.80/lb, using 3.5 lb/ton, was more or less a break-even situation cost wise, but with the extraction reduction and production/operating advantages that Basperse provided.
Alkali was also reduced from 12.1 lb to 11.9 lb/ton. "We started out at 12.1 lb of alkali and then went to 12 lb, and finally to 11.9 lb for the remainder of the trial with no problems, operating at the same H factor," Basso says.
The paper mill is also experiencing longer wire life on the paper machine. Basso emphasizes that this "follows…if you reduce extractives and talc going to the paper machine, then you reduce the junk that can accumulate on the wires and plug up press felts, and the operating life of the clothing should increase, " he says. The mill has now gone beyond 100 days of wire life, which it has not previously accomplished.
In the recovery boiler, the mill also achieved increased steam capacity/lb of black liquor solids, as illustrated in Figure 4 below. In addition, bleaching costs improved and brightness was increased slightly, as was experienced in the earlier trials.
Being a surfactant-dispersant, Basperse "keeps things in solution," Basso points out. "It helps to keep the evaporators and the entire liquor recovery system clean, improving overall heat transfer. Evaporator fouling can be reduced significantly with this type of additive."
Figure 4. Steam production (lb of steam/lb of black liquor solids) steadily increased during the Nov. 15 – Dec. 13, 1999 portion of the fourth trial period, which is still continuing.
Basso says that Basperse is similar in cost to anthraquinone (AQ), but without the "negatives" often associated with AQ. He explains that some negative side effects of AQ include coating of the evaporators and a resultant loss in heat transfer, refiner problems, problems in the tall oil recovery process, etc., in addition to possible environmental problems.
Basperse, Basso explains, was developed as a deresinating agent for use with dissolving pulp. But the trials at this northern hardwood mill represent an altogether new application. "We are using this agent for a different purpose here, and what we are finding is very promising and exciting."
The mill conducting the Basperse trials uses batch digesters. However, Basso says that the agent also has significant potential with continuous digesters. Possible advantages include better washing in the bottom of the continuous digester, better chip penetration, and a decrease in rejects.