Eric Ouderkirk, Global Center of Expertise-Fiber, Nalco Company
April 28, 2009
Scale formation is a common phenomenon in most pulping operations. Depending on the mill and its process conditions, different types of scales can form in a variety of areas throughout the pulp mill. These resultant scales can have a negative impact on the runnability and total cost of operation. This article will explore the mechanisms of scale formation, its effect on the process, and finally present some novel approaches that can help combat this problem.
Types, causes and effects
Scale occurs when inorganic salts, such as calcium carbonate, calcium oxalate, calcium sulfate and barium sulfate, precipitate from pulping process streams to form a deposit on system surfaces.
The types of scales that form vary, depending on the process stream itself and its corresponding pH. At alkaline pH, calcium carbonate (CaCO3) is the most common type of scale formed. Calcium oxalate (CaC2O4) is frequently encountered in oxidative bleaching stages.
At acid pH, barium sulfate (BaSO4) is easily formed in systems that employ alum or sulfuric acid. Calcium sulfate (CaSO4) scale forms in the evaporators of sulfite mills. A "soft" scale, burkeite (2Na2SO4•Na2CO3), frequently forms in black liquor evaporators, along with calcium carbonate. Pirssonite scale (CaCO3•Na2CO3) often occurs during green liquor clarification. Silicate scales (as various forms of sodium aluminum silicate) can also form in black liquor evaporators.
Three conditions are necessary in order to form scale: supersaturation, accelerated kinetics, and optimum substrates (surfaces). Supersaturation occurs when dissolved ions such as Ca+², CO3-², C2O4-², Na+¹, Ba+², and SO4-² increase in concentration to levels that exceed the normal solubility limits of the process stream. For illustration purposes, supersaturation is what happens if table salt is added to a glass of hot water. Eventually, the salt will no longer dissolve, ending up as a pile of salt crystals at the bottom of the glass.
Accelerated kinetics is a phrase used to describe conditions that are conducive to scale formation. These can include temperature shocks, intense mechanical and hydrodynamic shear forces, optimum pH conditions, and sudden changes in pressure.
Optimum substrates refer to the various non-uniform surfaces in the pulping process that are conducive to scale formation. These non-uniform surfaces include heat exchanger tubes made rough by corrosion or erosion, washer face wires, and screen plates, to name a few. These surfaces provide a mechanical "foothold" for scale microcrystals to begin growing.
It should also be mentioned that some scale species exhibit what is known as inverse solubility. Some common species in a pulp mill that exhibit inverse solubility include calcium carbonate (CaCO3) and calcium sulfate (CaSO4). Simply put, as temperatures rise, these species are more prone to come out of solution and deposit. This indeed presents an added challenge in hot systems such as digesters and evaporators.
The effect of scaling on a pulp mill manifests itself in both subtle and profound ways. The following table summarizes some of the detrimental effects that scaling has on various unit operations in a pulp mill.
Some common themes include increases in steam consumption and lost production. Both of these negative effects can significantly affect the bottom line: energy and tons. Not referenced in the table is the effect on quality and uniformity. All pulp producers strive to produce the required quality level and do it consistently at the lowest cost position possible. Formation of scale detracts from the uniformity with which the process
can be run. As an example, in a bleach plant, to compensate for poorer washing due to scale formation on the washer wire, ever increasing quantities of bleaching chemicals must be applied downstream from this stage. This is not only a direct cost, but it is also a detriment to uniformity; the process must be manipulated to overcome a deficiency.
|Unit operation ||Effect of scale |
|Continuous digester ||• Loss of heat transfer – higher steam demand |
• Increased alkali demand – in lieu of temperature
• More frequent heater cleaning – increased frequency (includes cost & risk) of hazardous cleaning process
• Shortened run times – increased frequency of digester acid cleanings
• Lost production
|Black liquor evaporators ||• Loss of heat transfer – higher steam demand |
• More frequent boilout
• Lost production
|Bleach plant ||• More frequent downtime for chemical and mechanical cleaning (hydroblasting) |
• Loss of washing efficiency
• Increase in bleaching chemical consumption
• Lost production
Determining root cause
At Nalco, customer problems are solved through thorough discovery processes. A systematic, engineering-based approach is taken to understand both the root cause of the problem as well as the nuances of each mill’s operation. This begins by conducting a Mechanical, Operational, and Chemical (MOC) audit, led by members of Nalco’s Global Fiber Expertise Center.
In conducting the MOC audit, each aspect of the process is examined. Examples covered in the Mechanical portion of the audit could include: Review of each unit operation, configuration of process flows, equipment condition assessment, piping configurations, and face wire selection.
Under the Operations portion of the audit, some typical items that would be reviewed might include an assessment of how liquor flows are split/managed in and around the digester, tower management practices in the bleach plant, current boilout practices and frequency in the evaporators, and maintenance and cleaning procedures for the bleach plant washers.
Finally, the Chemical portion of the audit would include a review of current chemistries (both commodity and specialty) and how they are applied. The MOC audit is a standard operating procedure for Nalco. The examples are taken from various areas in the pulp mill for illustrative purposes. In practice, if a customer’s scaling problem were in the bleach plant, the audit efforts would be directed in that area and not diluted across all pulp mill processes.
In addition to the MOC audit, additional analytical work is typically conducted. If samples of deposited scales are available, they are analyzed. That analysis yields both the constituency of the scale as well as its ionic make-up. It may yield information that is critical to solving the problem; it may be determined that the scale is not simply one type, but a combination of several different scales. Beyond examining the scale itself, the ionic content of the process stream is often characterized. This analytical characterization can yield valuable information about treatment levels that may be required to combat the problem.
At times, this ionic characterization work has also pointed Nalco and the customer down other avenues to solve a problem. An example of this was determining that the Ca+2 level in a mill’s white liquor going to the digester appeared abnormally high when compared to similar mills. Upon further investigation, it was determined that the mill’s white liquor clarification process was performing poorly. Remedying the true "root cause" of the problem can dramatically alter the final approach to treating the problem.
Once the causes are clearly defined, a successful, sustainable solution can be designed. Nalco has a "toolbox" for solving problems in the pulp mill, including a family of products known as SCALE-GUARD PLUS® Technology for effective treatment of all scale forms found in the pulp mill.
In addition to treatment products, Nalco also utilizes a proprietary instrument known as a Scale Rate Monitor (SRM). The SRM can be used to test various treatments on a process stream. Using the SRM, the scaling rate of the process can be determined and the effect of the various Scale-Guard Plus Technology treatment products can be evaluated. The SRM has the advantage of being able to artificially "stress" the process stream being tested, in effect being able to accelerate the scaling process just as though the analysis was conducted over a much longer period of time. The tool allows for much more of an engineering-based program to be designed as compared to conventional trial and error based approaches.
Bleach Hardwood Kraft Pulp Mill - A 1,050-tonne/day bleached hardwood kraft pulp mill had been running a competitive anti-scalant program for four years. The program was performing poorly, with the bleach plant shutting down every six weeks for 10 hours to hydroblast the EP stage tower.
Nalco initiated the discovery process by testing the bleach plant process filtrates utilizing the SRM to determine the nature of the calcium carbonate and calcium oxalate scaling potential in each filtrate stream. In addition, a complete bleach plant audit using the MOC approach was conducted that detailed operating parameters and the specific caustic and anti-scalant addition application points.
SRM data revealed that paper machine process water, D100 washing efficiency, and caustic and anti-scalant addition points all had a major effect on scale control in the bleach plant. The plot shown in Figure 1 illustrates product screening with one Scale-Guard Plus Technology product using the SRM. The SRM data indicated that it would be possible to more than double the length of the bleach plant runs. After detailing the audit findings to the customer, Nalco proposed to integrate Scale-Guard Plus Technology to help correct the situation in the bleach plant and allow the mill to meet its challenges. As a part of the recommendation, an effective cleaning procedure for the D100 washer was also incorporated.
After implementing Nalco’s recommendations, the customer was able to realize the following benefits:
• Bleach plant runs increased from six to 12 weeks. Eliminating half of the bleach plant outages a year yielded $1.2 million in savings in production and cleaning/maintenance costs.
• Inspection of the scale quantity at the 12-week mark with Nalco indicated less than one half of the scale amount present compared with the former treatment at the 6-week mark.
• Bleaching chemicals were reduced as a result of improved washing. Annual savings of $1.5 million were realized.
• Although not assigned a financial value, temperature probes were found to be clean. Better process control leads to a more uniform process and improved quality.
Each pulp mill is a unique operation with unique challenges. Many of these challenges are associated with scale formation in the digester, evaporators or bleach plant. These scaling issues can cut into the vitality of a pulping operation in the areas of safety, quality, production, energy and chemical costs. With fiber and energy costs typically accounting for 40 to 60% of a mill’s annual spend, solving a detrimental scaling problem can have a significant financial effect. As mills continue to close up their processes and reuse process streams more intensely, it is likely that scaling problems will increase in both frequency and severity.
Eric Ouderkirk leads the Global Center of Expertise–Fiber, Nalco Company
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