PULP & PAPER MAGAZINE: New mixing technology for wet end improves environmental performance

A new wet end optimization technology allows mills to replace fresh water with white water or clear filtrate, saving water and energy without deposition buildup

July 2007
By Casimiro da Silva Santos

As with other intensive resources industries, the paper industry has been under increasing pressure to contribute to climate change policy and resource conservation. The paper industry recognizes that it has a key role to play in the climate change mitigation process. On average, one tonne of paper contains some 1.4 tonnes of CO₂ equivalent. The annual stock increase of carbon stored in paper products represents approximately 10% of the annual fossil CO₂ or green house gas emissions from the industry.¹

The industry has been working diligently over the last seven decades to change its market perception of being non-environmentally friendly. Significant progress has been made as the paper industry is a leader in recycling. For example, 62.6% (54 million tonnes) of paper consumed today in Europe was collected in 2005 for recycling.¹ More recently, the same type of commitments have been made for energy and water, which are intimately related.

Depending on the grade produced, a modern paper machine uses an average of 6-8 m³ of fresh water per tonne of paper produced, representing a decrease in water consumption of more than 60% when compared with the mid-1980s. Papermakers continue to pursue the closure of their water cycles; however, some water must be reintroduced in the process to overcome the losses due to evaporation during the drying process. Nevertheless, most of the fresh water used today in the paper industry is used for the preparation and post-dilution of retention aids and other wet end chemicals, which is required to be heated to process temperature. This avoids temperature shocks on the short-loop that can lead to on-machine efficiency (OME) losses due to inorganic and pitch deposition buildup, retention losses, instability of the wet end chemistry and performance losses of wet end additives.

Papermakers do not realize that the reduction of relatively small additions of cold fresh water to the process can lead to significant reductions in terms of energy consumption. For example, if a 379 L/min reduction of cold fresh water for post-dilution of retention aids is achieved, annual energy savings in the excess of 73 GWh can be immediately recognized.² These savings occur because energy is not being consumed to heat additional fresh water to process temperature or previously heated process water is not being sewered.

To achieve these savings, fresh water must be replaced by process water (whiter water or clear filtrate water) to post-dilute retention aids and other speciality wet end chemistry additives. There are a number of commercially available technologies, such as Nalco's trademarked Pareto wet end optimization technology, that have been developed to accomplish just that.

This article will focus on the introduction of Pareto technology as a viable method to reduce energy and water costs. As is the case with any other industry, the paper industry has been under pressure due to globalization and demand volatility to improve its profitability by reducing manufacturing costs and increasing paper machine efficiency. This article, with several mill examples, will show how Pareto technology can help improve paper machine efficiency and paper quality, as well as the principal benefits recognized from the water and energy savings.

The Technology

Papermaking involves the continuous mixing of fiber, water and chemicals. Most of the chemicals are mixed into a main stream of stock. The injection and mixing of wet end specialty chemicals is extremely important as it directly affects the efficacy of such chemical aids, the quality of the paper produced, the paper machine runnability and the overall efficiency of the process.

There are several traditional ways of mixing chemicals into the stock stream, from the simple T-mixer to the most sophisticated quill. Except for a few cases, most mixing achieved is non-optimal. In all of these, the utilization of process water is limited, and plugging of the feed lines, losses in chemical efficacy and operational efficiencies are common. So in most cases, traditional feeding arrangements are used with significant amounts of fresh water that must be heated to process temperature to avoid efficiency and process stability issues.

The T-mixer is the most common and simple way to inject a chemical into a papermaking furnish. Its efficiency is strongly dependent on the penetration of the injection jet to the main stream, which depends on the diameter of the approach pipe and the diameter of the T-port, as well as the velocity of the main stream and jet.³ When using a T-mixer for the injection of retention aids, the jet velocity is relatively small when compared with the stock flow, which can lead to poor mixing and homogeneous distribution of the chemical across the diameter of the pipeline in the approach system. This can lead to retention losses of fine material, such as fines and fillers (calcium carbonate, kaolin clay, titanium dioxide, etc), and to localized areas of over-flocculation, causing formation issues due to the presence of higher floc sizes.

To overcome these problems while replacing cold fresh water for the post-dilution of retention chemicals, Nalco developed the Pareto wet end optimization technology. It is an environmentally friendly, new proprietary (patent pending) feeding arrangement for Nalco's wet end chemistry applications that makes the use of process water for the post-dilution of retention aids possible, ensuring their optimal mixing and distribution within the stock stream while providing for water and energy savings without deposition buildup in the retention-aid feeding and injection lines.

The Pareto technology was developed using sophisticated computational fluid dynamics (CFD) techniques and algorithms to simulate the mixing of retention aids with process water, and the injection and their optimal distribution into the stock flow across the approach system pipeline diameter.

The objective was to integrate wet end specialty chemicals (initially, retention aids, sizing and wet strength resin) into the approach system for optimal performance by paying particular attention to the three interrelated variables of:

• Wet end chemistry: Chemical used (molecular weight, charge, architecture etc), residence time for reaction and program type (single, dual, microparticle system)
• Hydrodynamics: Local stoichiometrics, residence times of the components and shear (power, energy and type)
• Water management: Post-dilution water usage, replacement of fresh water with process water for energy and water conservation efforts and environmental issues

The CFD simulations (Figures 1-2) illustrate the comparison between a single jet and the Pareto feeding arrangement with the same mass and energy input.

Figure 1 - CFD normalized mass concentrations on the simulated retention aid injection using a T-mixer (MD profiles)

Figure 2 - CFD normalized mass concentrations of simulated retention aid injection using Pareto technology (MD profiles)

It can be seen that at a distance of 10 m (typically the distance from a post-screen injection to the headbox of paper machine) the single jet allows for the presence of localized, highly concentrated areas of chemistries, which result in over-flocculation and/or reduced chemical efficacy due to rapid reaction rates. For the same distance, the new technology allows for the optimal mixing and distribution of the retention aids. For this reason, the new technology delivers benefits associated with improved mixing as an add on to the fresh water and energy reduction (Table 1).

Real Life Examples

Functionality has been verified with laboratory tests, pilot machine trials, mill scale tests and commercial applications across all paper grades. The following examples outline efficiency issues that were faced at mill sites and how Pareto technology was a successful solution.

Case Study 1: The installation of Pareto technology provided water and energy savings for a North American graphic papers mill. The mill, which produces 124,000 tonnes/yr of lightweight coated grades (50-75 g/m²) from groundwood and kraft pulp, was using 252 m³/day of fresh water for post-dilution of its retention aid chemicals. The top former paper machine running at 930 m/min was using a flocculant/ microparticle based retention program, which was fed post-screen at 15 m before the headbox.

The value proposition to install the new wet end optimization technology was to achieve the complete replacement of the fresh water by process water (white water or clear filtrate), resulting in the desired water and energy savings, without causing runnability problems attributed to deposition buildup or loss of efficiency of the retention program.

After a straightforward design and easy installation of four Pareto optimizers and injection systems, as well as a trouble free startup, the mill recognized immediately 113,560 m³/yr and 4.9 GWh of water and energy savings, respectively, the equivalent to 1,400 tons of CO₂ emissions -- an extremely positive environmental impact. The new wet end optimization technology was installed 9 m from the headbox, allowing for improved mixing, thus resulting in better retention aid distribution in the stock flow, which improved sheet quality. Reduced retention aid use was also recognized. These were secondary but all important benefits recognized by the mill.

Case Study 2: This example illustrates how optimal feeding and distribution of retention aid chemicals into the thin stock flow can improve paper machine runnability. A European mill was producing 250,000 tonnes/yr of newsprint grades (45-48 g/m²) from thermomechanical, kraft and deinked furnish on a gap former machine. After the installation of Pareto technology by replacing two injection quills, the machine realized a speed increase of 3.2% from 1,540 to 1,590 m/min and a 20% reduction in wet end web breaks (Figure 3). By replacing the fresh water used for retention chemicals post-dilution with already heated process water, the mill recognized 70,000 m³/yr and 2.9 GWh of water and energy savings, respectively, the equivalent of 828 tons of CO₂ emissions.

Figure 3 - Improved OME on a newsprint paper machine due to a 20% reduction in web breaks

Case Study 3: This example shows how the use of Pareto technology can decouple retention and formation, resulting in improved sheet quality at higher retention levels at a packaging mill. A European coated board mill producing 150,000 tonnes/yr of food packaging grades (170-270 g/m²) from groundwood and kraft pulp, on a multi-fourdrinier machine running at 420 m/min, saw improvements in CD profiles and on formation for higher retention levels.

Analyzing Figure 4, it can be seen that improved retention at equal or better formation, eg for a formation index of 70, first pass retention improved by 7 points from a minimum of 88.5% with jet injection to a maximum of 96% with Pareto technology for the same grade. This confirms the partial decoupling of retention and formation. Figure 5 depicts the reduction of white water consistency due to improved retention and the reduction of 2-sigma value on the CD profiles of the basis weight for the same grade. This was possible due to improved mixing and optimal distribution of retention aids.

The new wet end optimization technology allowed savings of 200,000 m³/yr and 7.6 GWh of water and energy, respectively, the equivalent of 2,200 tons of CO₂ emissions, by replacing fresh water use for process water on the post-dilution of retention aids.

Figure 4 - Improved retention at equal or better formation for the same grade

Figure 5 - Improved CD basis weight profiles for the same grade

Achieving Significant Environmental Gains

The paper industry is under increasing pressure to contribute to climate change policies while increasing its profitability through operational efficiency gains. The industry can be classified as resource intensive in terms of water and energy, which together with wood are at the core of the papermaking process. The use of cold fresh water for wet end additives preparation and post-dilution is common in the industry. Previous attempts to replace it with process water have proven in some cases to be ineffective, causing on-machine efficiency losses due to wet end instability and deposition buildup on the feeding lines of such chemical aids.

However, papermakers do not realize that reductions of relatively small amounts of cold fresh water can lead to significant reductions in terms of energy. Pareto wet end optimization technology is a novel concept for optimal mixing and feeding of retention aid chemicals using process water for their post-dilution, allowing papermakers to achieve significant water and energy savings, offering a positive environmental impact. Installed Pareto technology applications can help these mills save more than 2,250,000 m³ of fresh water and 65 GWh of energy this year and prevent approximately 15,000 tons of CO₂ gases from being emitted to the environment. In addition to the significant water and energy savings, customers have recognized improvements in chemical efficacy that have resulted in improved operational efficiencies and certain improved paper quality attributes.

Casimiro da Silva Santos is global program manager, Expertise Center RDF & Functionals, Paper Services Div, Nalco Europe.

1. Confederation of European Paper Industry. 2007. A Contribution To Climate Change Policy. http://www.cepi.org/docshare/docs/1/ PLLCJJEDKBCDLLCJMBBCMGGC5D1651QPBOHTT9U6HNQO/ CEPI/docs/DLS/CepiClimateChangesiteST-20070323-00013-01-E.pdf
2. Smith, C. D. 2006. Energy Savings Delivered Through Water Removal. Pulp & Paper, August: 25
3. Perttu, L., Koponen, A., Houni, J., Leino, T., Laakonen, K. 2005. Investigation Of A New Type Of Retention Aid Mixer. Paperi ja Puu - Paper and Timber, 87 (8): 512-516