Process Engineer

Canfor – Specialty Pulp and Paper Mill

P.O. Box 6000, Prince George, BC, V2N 2K3

Production rates were restricted at the Canfor Specialty Pulp and Paper Mill due to unacceptable brownstock washer performance. Acidifying the brownstock washer shower water with carbon dioxide was investigated as a means of enhancing the washer operation.

A number of trials were performed between November 1997 and August 1998, investigating different aspects of the washer, screen room, and bleach plant performance. The trial results showed:

• A reduction in screen room white water resin acids of 20 to 50%,
• Soda loss reduction from the washers of 25 to 30%, and
• Bleach plant feed COD reduction of 10 – 30%.

These results were achieved with a CO₂ application between 2.0 and 4.0 kg CO₂ / ADUt pulp.

The trials showed such a significant improvement in the operation that the CO₂ system has been in continuous use since January 1998. The use has been justified based on increased screen room effluent cleanliness and improved digester rates during periods of typically high resin acids.

Injecting gaseous carbon dioxide (CO₂) into the shower water produces carbonic acid, a weak acid. When applied to the pulp, the acidified shower water causes less fibre swelling than with the untreated showers. The reduced fibre swelling leads to better drainage on the washers, with a number of potential benefits. The trials evaluated the impact of CO₂ addition on the brownstock washers against the following criteria:

• Soda loss of the discharged pulp
• COD of the discharged brownstock pulp
• Bleach plant chemical demand
• Production rate changes
• BOD and AOX of pulp mill effluent
• Screen room sulphuric acid addition

Canfor’s Specialty Pulp and Paper Mill, circa 1965, is located in the centre of British Columbia, at the confluence of the Fraser and Nechako Rivers. The mill produces 300 t/d of sack kraft paper and 500 t/d of market kraft pulp. There are bleached and unbleached grades of both products. Figure 1 shows the basic process layout.

There are two kraft pulp production lines, each with one Kamyr continuous digester followed by two vacuum washers. The pulp machine process line has a screen room with five Cowan-type screens and a decker discharging into a high-density storage ahead of the bleach plant. The paper machine process line has a screen room with a primary pressure screen, secondary and tertiary Cowan-type screens, and a decker to thicken the stock prior to the paper machine. The brown white water system is common for both screening lines.

The bleach plant has five bleaching stages to produce ECF pulp and/or paper, depending on the sequence being used. The sequence for producing full bleached pulp is D-Eop-D-Ep-D. Finishing is done on either a 500 t/d pulp machine or a 300 t/d paper machine.

The brownstock washing and screening areas were the focus of work on this project. Figure 2 shows a generalized process layout for the brownstock washing area, including the CO₂ addition system.

In the brownstock washing area, there are two identical process lines, one for each digester. Each line has a single primary pressure knotter and a single secondary knotter. Accepts from both knotters feed to the #1 washer vat at a consistency of about 1% B.D. The #1 washer discharges through an intermediate repulper into the vat of #2 washer. The #2 washer discharges through a repulper conveyor into a thick stock pump and into either a 300t or 90t high-density storage.

Figure 2 – Washing Area Process Layout

Combined condensate from the steam plant is used on the showers for both #2 washers. This water is heated to about 75° C ahead of the showers, and is applied through four LaValley^TM shower bars. The flow, temperature, and conductivity of the shower water are measured and recorded in the DCS control system.

The filtrate flows counter-current to the stock, from #2 filtrate tank to #1 showers to the digesters as cold blow.

The brown white water system in the screen room is primarily made up of the filtrate from the pulp decker and paper decker. Fresh water may be used as level control make-up, however it is rarely required and should be avoided as cold fresh water has been shown to cause pitch deposition problems.

Over the years since the Specialty Pulp and Paper Mill began operation, the production rate of the mill has increased by about 50%, without substantial changes in the major process equipment for washing and screening. The latest change was the installation of pressure knotters in the late 1980’s, reducing the knots load and improving the stock quality entering the washers. One major impact of the increased production has been a higher soda loss level from the brownstock washers. This in turn has led to higher levels of contaminants in the brown white water leaving the screen room to the effluent treatment system.

The resin acid level in the brown white water is monitored as a measure of the cleanliness of the wash. On a number of occasions, the digesters have been slowed down to allow the washers to clean the stock better and reduce the resin acid level in the brown white water. As the screen room is not closed, a resin acid level above 3 ppm in the brown white water discharge (or white water conductivity above 1200 m S) can potentially cause toxicity problems in the effluent treatment system. Since any failure to meet the environmental permits is unacceptable to Canfor and any production losses caused by slowing the digester are not recoverable, it became necessary to investigate any means of cleaning the stock and reducing the resin acid level so that digester slowbacks would not be required.

One relatively simple means of cleaning the stock, that did not require major capital investment or process changes, was the acidification of the washer showers by dissolving carbon dioxide gas in the shower water. Increasing the shower water acidity leads to a cleaner wash and a number of other benefits as outlined below.

According to a number of studies, carbon dioxide has been used on brownstock washers showers for a number of years, with varying degrees of success [1 – 3]. Benefits identified include reduced chemical demand in the bleach plant, increased solids to the recovery cycle, cleaner effluent, improved production capacity, and reduced or eliminated sulphuric acid addition to the screen room.

There are two different effects working together when the CO₂ is added to the shower water, a physiochemical effect and a chemical effect.

When CO₂ is injected into the shower water, it dissolves and forms carbonic acid, as shown in the reaction below:

The carbonic acid reduces the pH of the shower water, from the typical 9.0 – 10.5 to about 5.5 – 6.0 pH. This in turn reduces the pH of the stock discharging from the washer. The lower pH leads to a reduction in fibre swelling on the washer mat. This allows better drainage through the mat and results in cleaner stock leaving the washer.

Chemically, H⁺ ions in the carbonic acid displace Na⁺ ions from the calcium soaps in the liquor. The resultant sodium salts dissolve in the liquor and are more readily carried away with the filtrate.

The pH control of the shower water is critical to maintaining the reactions. The optimum pH to achieve both the chemical and physiochemical benefits is between 5.5 and 6.0 pH.

For the Specialty Pulp and Paper Mill, the primary goal was to maintain the digesters at maximum operating rate during periods of potentially high resin acids. This is primarily a problem in winter when chips are frozen, and have not undergone normal aging. As chips age, the resinous components undergo degradation reactions that reduce resin acid content after digestion. However, high resin acids can occur at any time during the year as a result of process upsets or changes in the condition of the chips.

A number of secondary benefits were also assessed, including bleach plant chemical demand and brownstock washer discharge COD (chemical oxygen demand).

A number of potential complications were also considered prior to the trial. These included determining any effects of the reduced stock pH on the paper machine operation, watching for pitch outbreaks as a result of higher brown white water pH, and safety concerns with the handling of liquid and gaseous CO₂.

The CO₂ is supplied from a 50t pressurized storage tank. The pressure is maintained at about 275 psi for distribution to the control manifold. The control manifold (see Figure 3) has a flowmeter with accumulator, a low pressure switch to alarm if the tank pressure drops too low, and control valves for the individual application points. Originally, there were two control valves, intended to control the CO₂ to the showers on two washer lines and to the screen room for brown white water pH control. The manifold is now set up to control CO₂ independently to each washer line, with none to the screen room.

The CO₂ is injected through a sintered metal sparger into the shower water line about 100 m upstream of the washers, ahead of the shower water heat exchangers. The long piping run is required to allow the CO₂ to fully dissolve in the shower water. Any undissolved CO₂ will gas off in the shower header, reducing the effective shower flow.

The brown white water pH control was installed at the supplier’s recommendation. In other CO₂ installations with this supplier, foaming problems had occurred in the screen room when a strong acid, such as sulphuric acid, contacted stock that had been treated with CO₂, due to interactions with the carbonates in the stock. In our mill, the H₂SO₄ is added to the white water chest, maintaining a pH target of 6.3. The brown white water is then used for stock dilution from the 300T storage. At the start of the trial period, CO₂ was set to control the white water pH at 6.3. However, the CO₂ flow needed to maintain this pH was higher than the control manifold could handle. No foaming occurred in a brief trial using H₂SO₄ for white water pH control at the same time as CO₂ was being applied to the washers. Following the successful simultaneous use of H₂SO₄ in the brown white water and CO₂ on the washers, the control manifold was altered to control the CO₂ flow to each washer line independently (see Figure 3). This trial report focuses on the CO₂ application on the washers.

Figure 3 – CO₂ Control Manifold

The CO₂ flow is controlled manually based on tests of the shower water pH. The pH of the combined condensate ranges from 9.0 to 10.5. With the CO₂ on, the pH range for the shower water was set at 5.3 to 5.7. The minimum pH in our shower water was around pH 5.0 to 5.3. The CO₂ flow needed to reduce the pH below 5.3 was about 3 to 5 times greater than that needed to maintain a pH of 5.5, due to an apparent buffering action of the salts in the shower water.

The application rate started at about 5.5 kg CO₂ per ADUt pulp. Once the process had stabilized, sampling and testing was done to determine the effectiveness of the CO₂ application.

The initial trial was scheduled for a two month run in November and December 1997, to allow enough time to properly evaluate any benefits of the CO₂. There was a learning period at the beginning of the trial when the mill personnel tested the shower water pH to determine the required CO₂ application rate. The target pH between 5.3 and 5.5, as determined by the CO₂ supplier based on previous trials, required an application of approximately 5.5 kg CO₂/ADUt. Once the initial CO₂ flow was determined, further testing was done to establish the effectiveness of the CO₂.

For each trial period, samples were taken before and after the CO₂ was turned on. The short process residence time, about 30 seconds from injection point to the washer showers, allowed for multiple sampling periods within a day, to allow comparisons of the operation with and without the acidified shower water.

After a number of short trials to determine the effect on the washed stock, the long-term use of CO₂ was studied. This was evaluated by measuring the resin acid content of the screen room effluent and the soda loss level from the washers.

Two Co-op students performed the bulk of the testing during the CO₂ trial periods. Samples were also sent to the Canfor Research Centre for analysis that was not available at the mill site. In addition, the supplier provided help during the commissioning phase and later in the trial to help evaluate some of the secondary benefits.

Over the first three months of the CO₂ trial, the following variables were tested or measured to evaluate any effect of the CO₂:

• shower water
• pH
• conductivity
• washer discharge mat liquid phase
• pH
• conductivity
• resin acid content
• COD
• dissolved solids
• organic solids
• Kappa number
• washer discharge and decker discharge mats
• COD (as a measure of bleach chemical demand)
• washable soda loss
• washer filtrate solids content

Other process variables were evaluated for the effect of the CO₂ addition. These included:

• bleach plant chemical demand
• digester production rate
• pulp mill effluent BOD and AOX
• brown white water sulphuric acid addition
• brown white water resin acid content

Before the trial began, the paper machine area Process Engineer contacted a number of mills that had similar wet end processes to the PG paper machine. In each case, the references indicated that they had experienced no problems in their paper machine operation that could be attributed to reducing the incoming stock pH.

The concern of pitch outbreaks due to higher brown white water pH was resolved by maintaining the pre-trial pH during the trial and in subsequent months. No pitch outbreaks have been attributed to the CO₂ use since it began.

This trial was the first use of carbon dioxide as an industrial chemical at the Specialty Pulp and Paper Mills. Mill personnel were familiar with CO₂ as a fire extinguishing material, but were concerned with the potential dangers of a 50 tonne pressurized tank of CO₂. The tank stores liquefied CO₂ at 300 psi and –50° F. The main concern was with the possibility of asphyxiation from a leak of CO₂ from the piping or at the control skid. Since CO₂ is denser than air, it could collect in the floor drains in the area. However, the supplier believed, based on past experience, that the area was open enough and had enough air exchange to avoid having CO₂ build up to a dangerous concentration. In addition, the trial used 3000 psi-rated hydraulic hose to carry the CO₂ from the storage tank to the control skid and from the control skid to the process. This resolved the concerns brought forward by the mill personnel.

The initial trials in November and December were very positive. There were definite improvements in the wash quality based on all the measured variables, with an average improvement of 28%, as shown in Tables I, II, and III. The critical performance variables are brown white water conductivity and resin acid level, washer soda loss, and COD.

Canfor R&D Testing - Market Pulp Washer Line

Pulp Mill Testing – Average of Pulp and Sack Kraft Washer Lines

Canfor R&D Testing - Market Pulp & Sack Kraft Paper Washer Lines

It was recognized that the initial trials did not evaluate the long-term effect of the acidified shower water. The trials so far had only evaluated the effect on the second washer on each production line. However, the washer #2 filtrate is used as the shower water for washer #1 on each line. The improved wash on #2 would increase the organic and inorganic content of the showers for #1, possible negating the improved performance on #2 washer. The first sets of trials were relatively short duration, with not enough time to turn over the filtrate tank volume. This shortcoming would be addressed in subsequent trials.

The CO₂ trial was extended beyond the initial two months into 1998. The CO₂ trial had been justified based on reduced resin acids in the brown white water overflowing to the effluent treatment system. The spring period normally has the highest resin acid levels, therefore the greatest potential for improvement with the CO₂ application. This also allowed time to see if the increased solids in washer #2 filtrate would affect the final performance when applied as showers on washer #1 and used as vat dilution for washer #2.

The second set of trials was aimed at determining any long-term effects of CO₂ use, by assessing the pulp cleanliness (soda loss), the brown white water COD, and the bleach chemical demand. These trials were completed over an 8-month period from January to August 1998. The same samples and analysis as in the first trial were assessed for the second trial.

Long term testing took place during the winter months to evaluate the effect of the winter wood and the potentially higher resin acid levels. During this testing, the CO₂ was applied continuously for extended periods (more than 4 weeks), during which time sampling and testing was completed that confirmed the benefits seen during the shorter trials.

While the results were not as dramatic as the shorter trials, there was sufficient improvement in the key variables to continue the CO₂ application (refer to Tables IVa, IVb).

Market Pulp Washing & Screening Line

Sack Kraft Paper Washing & Screening Line

It was anticipated that the COD reduction in washed pulp and in the screen room would result in reduced bleaching chemical demand. However, the weekly bleaching chemical usage reports did not show a correlation between CO₂ use and chlorine dioxide demand. Another trial was run specifically to assess the effect on the D_O chlorine dioxide demand. This trial showed no change in the bleach chemical demand related to the CO₂ use. No further work was done in this area, as sufficient benefits had been seen in other areas.

Bleach Plant Chemical Demand Testing

There was an average 5% reduction in K# and a 21% reduction in the pulp COD with the CO₂ on, as shown in Table V, which should have resulted in significantly lower bleach chemical demand. However, analysis of the bleaching operation in the periods with and without the CO₂ turned on showed only a 2.4% reduction in D_O Kappa factor and a negligible change in the ClO₂ application rate.

The final assessment of the CO₂ operation was to determine the minimum CO₂ application rate that would maintain the resin acid levels seen in the earlier trials. The supplier had recommended a CO₂ application rate of 5.0 – 6.0 kg CO₂/ADt of pulp, to maintain the combined condensate shower pH around 5.3. The application rate of 3.6 kg/ADt used in the April and May trials resulted in the same benefits as at the higher rate. The CO₂ application was further reduced to 2.0 kg/ADt with no significant change in the brown white water resin acid level, as shown in Table VI.

Determination of the Optimum CO₂ Application

The longer trials through the spring and summer of 1998 showed the following results:

• reduced washable soda content and COD content in the pulp
• reduced resin acids in the brown white water
• no measurable bleach chemical savings attributable to the cleaner pulp
• a CO₂ application rate at 1/3 of the initial rate was sufficient to achieve the benefits

The decision was made to run at the lower application rate, as the same benefit was being realized at a lower chemical cost to the mill.

The CO₂ trials all showed sufficient improvements in washing to justify the continued use, in spite of not realizing any bleaching chemical savings.

The CO₂ has been applied on a continuous basis since January 1998. The CO₂ has been shut off only during periods in the summer months when the resin acid levels in the brown white water were very low, and the digesters were running at the maximum rate.

The primary justification for the continued use was that the reduced resin acid level would allow the digesters to run at higher rates during the period of high resin acids. Analysis of the production losses in the six years before the CO₂ use compared with the two years since the CO₂ trial started show that approximately 1250 ADt / year of increased production is attributable to the cleaner pulp. This was enough to justify installing a permanently mounted CO₂ storage tank with its control system in the DCS. The construction should be completed by the end of the summer, pending approval of the funding.

The author thanks N. Stoffelsma and C. Lad, students from the UBC Co-op program who performed most of the sampling and data collection from the trial periods during their work terms. The author also thanks J. Ball, S. Forster, J. Dunn, R. Krzywanski, and R. Parnell of Canfor for their assistance in the reviewing of this manuscript.

1. FERWEDA, G.B., "Washing Improvements through Brownstock Acidification with Carbon Dioxide", presented at Whistler CPPA Conference, (1995)
2. WHITE, B., "Carbon Dioxide on Pulp during Washing in the Minimum Impact Mill", presented at CPPA Pulp Washing Conference, (1996)

 

3. BOKSTROM, M., RASIMUS, R., AGA, "Method for Washing of Alkaline Pulp", Canadian Patent 1,286,454, (issued July 23, 1991).