Systematic approach to mill closure
As referenced earlier, water optimization can involve a large amount of investment in time, resources, and money. It takes time to learn the system and its parameters, and to observe the results of changes made during the project before subsequent actions are taken. Financial investment will eventually be required, with the amount dependent on the project depth. Equipment options are often the first consideration in a water optimization project. Before major capital is expended, a mill can perform some very basic actions to progress towards improved water resource management. Some are considered best practices while others are slightly more complicated. Once these actions are taken, at that point equipment options can be evaluated. Proceeding through a step-wise process will help the mill realize its water optimization goals while minimizing capital expenses, operating costs and downtime, but still maintain productivity and finished product quality.
Mill closure, as it is known today, is now widely being investigated so there is more comparison data becoming available. Any data that is available is subjective, however, based on who is providing it.
Some mills calculate closure based on the influent and effluent flows of the mill. The average effluent flows can look very different in varying regions of the world even when producing similar grades of wood-grade paper. This data can also look very different for mills producing virgin linerboard, recycled paperboard and printing and writing grades using bleached virgin fiber.
Table 1 provides background information on the amount of fresh water used per ton of paper made. Included for reference in the table are figures for North American linerboard mills surveyed by Nalco. Additionally, Tables 2-3 provide information on the North American water consumption for unbleached board and bag mills, and paper mills producing mechanical pulps, as provided by the Lockwood-Post's Directory of the Pulp, Paper and Allied Trades.
Table 1 - Parameter Changes in Open Versus Closed North American Recycle Linerboard Mills
Mills in North America currently are considering more source point reduction and in-process treatments to reduce or minimize the amount of wastewater treatment. If water optimization can be done within the process, it will reduce the overall cost of the project because smaller and less expensive types of equipment can be used both in the process and wastewater treatment plant. Figure 1 shows the total water consumption per ton of paper produced for 63 North American linerboard mills using FisherSolve.
Figure 1 - Water Consumption for North American Linerboard Mills
Implication of closure on chemical programs
Closing a water system often leads to an increase in dissolved and suspended solids, an increase in temperature and a reduction in dissolved oxygen. If these conditions are not addressed, the mill will suffer a loss in productivity and product quality.
Water chemistry plays a major role in retention aid effectiveness. In general, high salt concentrations interfere with the adsorption of polymeric additives onto the fiber and fines surfaces. This can reduce the effectiveness of retention and drainage programs as well as the adsorption of strength aids such as dry strength additives and cationic starch. An increase in water conductivity from 1,000 to 10,000 µohms due to system closure can reduce absorption of cationic starch by as much as 80%.
It is also the case that the level of dissolved oxygen in water decreases as the temperature rises. When a mill increases water reuse, an increase in temperature of 10°F or more is common. This change in environment causes a shift in the microbial population from aerobic bacteria to facultative or obligate anaerobes. Problems associated with anaerobes in paper systems include corrosion, formation of explosive or toxic gases and odors in the finished sheet.
The shift from an oxidizing to reducing environment also affects the efficacy of certain biocide chemistries. This, coupled with the change in microbial population, results in the need to review and change the microbiological control program.
Obtaining and maintaining acceptable levels of small particle retention becomes critical for mills as systems become more and more closed. One example of small particle management is preventfing or minimizing pitch outbreaks. Whether in the paper mill or the pulp mill, the ultimate point of exit for contaminants from the process is the finished sheet. Recent studies have shown that successful management of small particles, like pitch, in closed systems may require unique combinations of treatment programs and feed points to prevent the contaminants from aggregating and depositing throughout and later in the process.
Table 2 - Water consumption for North American unbleached board/bag mills
New PARETO mixing technology
This technology from Nalco optimizes chemical injection into a process pipe and allows mills to optimize fresh water use. The design parameters are based on computational fluid dynamics modeling and Nalco's best application practices. This patented technology has been confirmed in hundreds of commercial installations globally. It is applicable to most chemical injections and is especially applicable to high molecular weight polymers as is encountered in water treatment applications and/or products being fed in the top of the chest (dripped in open tanks) such as defoamers, sizing agents and coagulants (organic and inorganic).
PARETO Mixing Technology can help reduce chemical consumption through more effective mixing in the process stream by as much as (10%-40%) and by the potential to move the chemical addition points to a lower shear area (where applicable). This technology also allows the use of process water to replace fresh water for secondary dilution to reduce energy costs and conserve water. Mill water reuse targets can be achieved with PARETO installations across multiple applications.
The benefits of the technology in waste water treatment can include improving system performance and operational stability by:
- Increasing cake solids
- Optimizing injection system (device/feedpoint)
- Less variability on system
- Reducing effluent solids
Table 3 - Water consumption for North American mills producing mechanical pulps
Case study: Linerboard & medium
Grade:Linerboard and corrugated medium
Basis Weight Produced:23 to 45 lb/1,000ft2
Machine Type:Two-ply fourdrinier (70:30)
Production Rate:72 tons/hr
Machine Speed:2,000-3,000 ft/min
Business situation: A recycled linerboard and corrugated medium mill wanted to capture the benefits of running RDF chemistry post screen and reducing fresh water usage. These benefits include chemical efficiency, drainage, production and process reliability. In the past, post screen RDF chemistry created variability in the process due to poor mixing. The end result was paper machine breaks.
In an effort to assist the mill with their key business drivers, Nalco proposed installing PARETO Mixing Technology post screen on the base ply for both the flocculant and microparticle retention aids. In addition to the aforementioned benefits of running RDF chemistry post screen, the technology also allows use of filtered white water for secondary chemical dilution. The net result is improved drainage, reduced freshwater usage and energy savings from not heating cold water. Additionally, Nalco reports water and energy savings environmental return on investment (eROI) values to customers to account for contributions in delivering both environmental performance and financial payback. The eROI results for this mill application are:
1. 24% reduction in flocculant dosage without loss in tray solids
2. Production increase of 1.4% or 1.0 ton/hr
3. 8% reduction in Base ply ASA usage with a 26% improvement in variation
4. 21% reduction in anti-skid chemical usage
5. Fresh water reduction of 28,000,000 gal/yr
6. Energy reduction associated with heating fresh water, >15,000 MM Btu/yr
7. Water and energy savings >$100,000/yr.
3D TRASAR® technology for cooling water
Fresh water is commonly utilized as a make-up water source for open recirculating cooling towers to compensate for water losses due to evaporation and cooling tower blowdown. As mills close up, alternative sources of cooling tower make-up have begun to be utilized; reclaimed process water, as a result the stress being placed on the cooling water systems can dramatically increase. When system stress is too high, scale, corrosion and fouling can occur, resulting in a loss of heat transfer at the process interface and subsequent process inefficiency.
Nalco's 3D TRASAR Technology optimizes system performance and prevents operational problems by constantly measuring key parameters related to system stress, identifying changes in system conditions and taking appropriate corrective actions to address the varying process demands thereby maintaining clean heat transfer surfaces throughout the cooling water system, which minimizes operational costs.
3D TRASAR Technology for Cooling Water is designed to deliver superiorperformance, system efficiency, and asset protection, and can address the following operational concerns:
- Maximize cooling tower scale and corrosion protection via real time TRASAR® chemical control technology.
- Monitor scale and corrosion stress conditions via the use of "tagged" chemistry and Nalco Corrosion Monitor (NCM) and adjust treatment programs to address changes in those conditions.
- Improve cooling tower operation/control, water quality and heat transfer surface cleanliness and efficiency.
- Improve water and energy savings by allowing for maximum control of cooling tower cycles.
The best way to minimize capital investment during water optimization is to proceed logically and slowly through a step-wise planning process. People throughout the mill need time to observe the results of any changes. These changes can be as simple as the removal of a fresh water source, movement of water to other locations, or the addition of a mechanical or chemical application. By making too many changes at once, positive results can easily be obscured by the negative impact of another change. Furthermore, a mill may be able to realize its water optimization goals before the main recycle project is tackled, thus minimizing capital investment.
Along with closure comes the need to re-evaluate many of the chemical additives and treatment programs used throughout the mill. Changes in water chemistry and solids loadings also can impact the performance of most papermaking additives.
1. ROBERTSON, L. R., 1995 TAPPI Papermakers Conference, Impact of Water Reuse on Microbial Colonization of Paper Machines, TAPPI Press, Chicago, IL
2. FOSTER, C. and RENDE, D., How Recycling, Water Reuse Impact Chemistry, PIMA's North American Papermaer, p. 48-53 (January 1997).
3. HARRY DYER, ed, 2004 Lockward-Post's Directory of the Pulp, Paper and Allied Trades. (Miller Freeman,
4. Fisher Data Base FisherSolve 2010.