Increased speed and reduced steam consumption were some of the benefits realized by freesheet producers
August 2008
By Christopher D. Smith, global marketing manager, Graphic Papers, Nalco Company and Melissa Callejo, industry analyst, Graphic Papers, Nalco Company
Today's papermaker is challenged with efficiently manufacturing and managing multiple grades as a result of the flexibility and customization demanded by the print market. To make the challenge even more difficult, the papermaker is asked to manufacture these grades on papermaking assets designed prior to such grades even being envisioned or on assets designed to produce a completely different type of paper. Although the sales and marketing departments demand the production of a large and ever increasing grade mix with short lead-times and downward pressure on inventories, they do so at the expense of paper machine efficiency.
One notable example of this is on paper machines that are required to run a grade mix of wide-ranging basis weights. In these cases, a trade-off is normally made between fabric selection, chemical retention and drainage programs, machine configuration and production. These trade-offs result in non-optimal production efficiencies as efficiencies are compromised on some grades in order to manufacture others. These compromises result in poor efficiency in some grade runs as machine speed is slowed, product quality is diminished or direct costs, such as energy or chemicals are recognized.
Many times this loss in efficiency can be attributed to the effect that higher basis weight grades have on the overall dewatering of the sheet. As basis weights of production increase with an unchanged machine configuration, the machine can become steam limited and require that the speed of the machine, and subsequently production, to be reduced. If a robust chemical retention and drainage program is being applied, dewatering can be enhanced and some additional machine speed achieved, but this may come at the expense of formation and other product quality attributes.
In these cases there are numerous levers available that are both effective and efficient to the papermaker to increase productivity as they move to these higher basis weights. These options include capital intensive changes such as press rebuilds or vacuum system upgrades; operational changes such as nip loading or showering practices; or consumable options such as fabric design or chemical additives. A chemical dewatering aid is now available for graphic paper producers, which is designed to enhance the performance of the existing chemical retention and drainage program with no negative effects on formation or other sheet properties.
The Fundamental Purpose
The removal of water from the fiber suspension is the fundamental and singular purpose of a paper machine. The cost of water removal increases exponentially as the web moves down the paper machine from the table to the presses and finally to the dryer section. (Figure 1) Also, the rate and method of dewatering affects the final sheet quality and operational efficiency. Too much early drainage will undoubtedly hurt formation, while not enough will increase the cost of dewatering as well as negatively affect wet-web tensile. It is commonly accepted that a one percent increase in dryness after the last press will increase the wet-web tensile of the paperweb by 11-13% and will simultaneously reduce the steam demand in the dryer section by 3-4%. By managing the rate and the location of the dewatering through both mechanical and chemical means, the papermaker is able to optimize the ideal dewatering profile for economic and operational efficiency gains. Solutions such as Nalco's Velox™ Dewatering Aid Technology allow for chemically enhanced dewatering to be expanded from the table to the press section, which reduces the amount of moisture entering the dryer section and allows recognition of reduced steam consumption and speed increases on machines that are dryer limited. Other benefits of this type of dewatering technology are improved operational efficiency as higher press solids translates to increased wet-web tensile as well as opportunities for machine runnability improvement with lightweight grades and for bulk development as the more efficient pressing can afford the opportunity to lower press loads.
Meeting the Demand
In most cases, this dewatering enhancement has been seen on high basis weight grades of >50 lb/3,300 ft² (75 g/m²) of uncoated or coated freesheet. By delivering several points of increased press solids, the papermaker can recognize an immediate reduction in dryer section steam demand, or, if previously dryer limited, increases of machine speed by up to 10%. Furthermore, the additional 1-2 % press solids gained with this technology has shown to improve 2-sigma moisture variation up to 25% in lightweight grades, which further builds confidence that the machine is operating within acceptable moisture levels into the size press. These gains can be achieved without having to change press felt design or sacrifice runnability or product quality allowing the papermaker to efficiently run and providing a larger operating window for multiple grades of relatively high and low basis weights. The papermaker can choose to increase machine speed, recognize energy savings, reduce measurement variability or even take advantage of certain enhancements in sheet attributes such as bulk.
In one coated freesheet application (CFS) a mill was producing a mix of 20-200 lb/1,000 ft² (100-300 g/m²) grades. The mill was pressed to increase incremental production volume to stay competitive within the market. With market fluctuations and grade consolidations, the mill had the need to quickly produce multiple basis weight grades to serve their customers, but needed to reduce variable costs and maintain machine efficiency. The mill initiated a Velox Dewatering Aid Technology project to increase machine efficiency on steam-limited grades (>150 g/m²) with the intention of increasing machine speed on these grades.
After a comprehensive system survey was completed, including cationic demand and zeta potential determinations, the Velox technology program that was designed was a wet end program that combined a reactive polymer with an engineered borosilicate microparticle based retention program. In this particular program, the Velox technology polymer was added between the wet end starch and microparticle injection points. Once the retention aid program was optimized, the Velox technology dose was varied with the intention of affecting dryer section steam consumption.
The technology was first tested on a 300-g/m² grade with various dosage rates. Over a period of several hours, as the dose increased, a 5% reduction in steam consumption was observed. (Figure 2) The reduction in base sheet moisture did not negatively affect sheet specification of machine specification and the moisture in the base sheet decreased from 7.2 to 5.6%. By improving the base sheet moisture profile, the technology was effectively removing water from the fiber suspension while enhancing sheet consolidation and in turn reducing the steam needed for drying and reducing the risk of sheet breaks.
The second part of the trial consisted of running the Velox Dewatering Aid Technology on 150 and 170 g/m² grammage grades to investigate the reactive polymer's effectiveness on speed increase across different basis weight grades. Figure 3 illustrates that the machine speed could be increased above the previously achieved maximum speed for the tested grades. For these two grades, the optimal dose resulted in an average gain of 30 m/min (98 ft/min) demonstrating to the mill that the reactive polymer afforded the opportunity to increase machine speed on dryer limited grades and the flexibility to use the technology only when speed is limited by available steam.
A secondary benefit of increased ash retention was also observed without the negative effect on formation that was typically encountered at higher ash retention on these grades.
Moving to a Higher Basis Weight
Another CFS mill was facing a similar problem, except that due to organizational consolidation, the mill was being asked to produce higher basis weight grade papers than its paper machine was originally engineered for. To produce the higher basis weight grades, more steam was required to dry a heavier than usual fiber suspension while running at a relatively slow production rate. As a result, the paper machine was limited by the mill's steam availability, which exposed the mill to closure as a result of rationalization with other more productive mills within the organization. This was particularly apparent on grades with basis weights of 100 lb/3,300 ft² or greater where the machine was forced to slow to a non-viable production rate.
A machine survey indicated that the ideal addition point of the Velox Dewatering Aid technology would be at the suction side of the headbox fan pump at various dosages. For the trial key, benchmarking parameters were recorded and analyzed with Nalco's TrendGen™ statistical software. Included in the analysis were such variables such as couch vacuum pressure, dryer can speed, and steam consumption to determine the program's effectiveness on press solids, productivity and energy savings.
On the first trial on a 100-lb/3,300-ft² (148-g/m²) basis weight grade the technology showed enhanced dewatering at the forming section as indicated by a statistically significant correlation between the technology and the couch vacuum pressure. (Figure 4) As the vacuum pressure decreased the paper machine speed was increased by 100 ft/min (30 m/min) while allowing for a controlled steam pressure. Upon introduction of the reactive polymer the steam pressure dropped from 62 psi (427 kPa) to 55 psi (379 kPa). The machine speed was then increased by 102 ft/min until the steam reached pre-trial steam pressure of 62 psi.
In another application, a major uncoated freesheet producer needed to increase productivity, specifically for their high basis weight grades. As with most high basis weight grade production, there is a large demand for steam to dry the sheet, which limits the speed of the machine. With the introduction of Velox Dewatering Aid Technology, the machine recognized an immediate drop in steam pressure in the dryer section. The reduced drying demand allowed the machine to increase speed by 5%. The mill personnel not only observed a machine speed increase, but also a reduction in variability in both steam and CD moisture profile 3-sigma measurements. By reducing the variability in both readings, the mill could operate closer to set point centerline. Furthermore, the reduced variability of the CD moisture profile reading translated to a better profiled-sheet for size pick-up and less wrinkles at the reel. Additionally, sheet property analysis concluded that Velox technology caused densification of the sheet on the wire-side, leading to an overall reduction of roughness two-sidedness. (Figure 5) With the speed increase and added benefits such as sheet properties improvements, the mill has considered operating the technology on lower basis weight uncoated freesheet grades.
Conclusion
There are numerous methods to improve the flexibility and operational efficiency of existing paper machines. As the market demands a nearly infinite amount of grades into the foreseeable future, the papermaker will continue to be challenged with balancing profitability with satisfying such demand. Chemical additives can be leveraged to provide such flexibility by offering economically viable solutions to improve machine efficiency while providing sustainable benefits such as reduced energy demand and asset optimization.
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