Steam savings from nanochemistry.

Why not dry your sheet at the wet end with the help of nanotechnology? Nanoparticle systems have for many years demonstrated significant reductions in steam demand through the use of silica in conjunction with cationic wet end starches and/or cationic polyacrylamides (CPAMs). It has been long established that for each percentage point increase in dryness out of the press section, a 4% to 5% increase in production was possible--confirming the same principles that have long been established for mechanical improvements in dryness out of the press section.

The original microparticle chemistry relied on a true charge neutralization effect and resulted in the benefits associated with increased retention, drainage and drying. The retention and dewatering effects were each strongly linked to one other. Eventually there was a desire to "dial in" retention or drainage more independently of each other, which allowed the user to optimize his/her system for either retention or drainage or both depending on system requirements. With a reduced incentive to increase production over the past few years, the newer nanoparticles have essentially allowed a separation of the drainage and drying effects from the retention effects--thus minimizing wet end chemical costs.

Consequently, today there are many machines already running nanoparticle systems with the potential to increase production and reduce drying costs with relatively minor optimization. As the economy improves and there is renewed interest in maximizing production, the nanoparticle approach provides a viable tool even on machines not fitted with the latest pressing and drying technology.

For example, a small alkaline coating base machine running at speeds up to 2000 feet/min made some woodfree grades while some other grades used up to 40% stone ground wood. The microparticle effects created by using a silica sol in conjunction with the incumbent cationic starch resulted in not only the elimination of the polymeric retention aid, but immediate drainage and drying improvements on both the woodfree grades and the grades with 40% mechanical pulp. Overall retention rates were improved slightly. However, the improvements in drying are very clear. The daily press solids and steam data taken were averaged by the mill and compared at the same machine speeds. These are summarized in Table I.

Newer state of the art paper machines fitted with more efficient pressing and drying technology may have dryer section steam usage values below 5 GJ/t, which with the most expensive component of the energy costing around $5/GJ, a 10% reduction in drying cost alone probably could not be justified without utilizing this potential to speed up the machine or in some other way exploit the nanoparticle system. For example, a gap former making uncoated freesheet at 3300 feet/min realized very significant steam savings with state of the art nanoparticle chemistry. In addition to dropping the specific steam consumption from 2.1 to 1.7 pounds of steam per pound of paper, the improved runnability and speed increase together with a 25% reduction in ASA use more than justify the program.

On high speed, lightweight wood containing machines, improved dryness off the couch and out of the press section provided by the nanoparticle system generates the additional wet web strength needed to speed up the machine. A blade gap former making newsprint at around 4000 feet/min realized a major improvement in couch solids by adding nanoparticles. A three-month trend is shown in Fig. 1, during which time the machine speed was increased by 125 feet/min.

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It has long been recognized that an increase in CPAM dosage in many wood containing furnishes can result in a loss of drainage and increase in steam demand. Anything that can be done to reduce CPAM dosage without sacrificing retention will therefore contribute to reduced steam demand. In the example cited in Fig. 1, the CPAM dosage was actually reduced by the introduction of the nanoparticle while maintaining the same tray solids target on machine. The combination of higher dryness off the couch and improved pressability provided for a stronger sheet with less center roll draw and has allowed higher speeds.

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On board machines, production can be increased by not only direct steam reduction, but also by exploiting caliper increase and strength development generated by nanoparticle systems to reduce basis weight targets and produce more lineal feet at higher speeds. An example to illustrate this phenomenon on SBS boxboard involved a fourdrinier machine fitted with a top wire former using starch as a drainage aid. Despite the headbox freeness already being around 600 csf, the introduction of a nanoparticle to the existing system provided even more drainage potential to allow the slice to be opened and headbox flow increased around 5%. Without making any changes to the table arrangement or vacuum, the improvement in drying was very evident and allowed the machine speed up shown in Fig. 2.

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In this example the basis weight and caliper of the board were unchanged and the steam saving was around 6% compared to the pretrial reference and around 10% comparing it with the long term average. However, it is possible to exploit the table drainage provided by the nanoparticle still further by a combination of increased slice flow, increased early drainage, and vacuum adjustment to increase the caliper of the board and allow basis weight reduction. Using these papermaking skills on 18 point, a 6-pound reduction in basis weight was possible and is illustrated in Fig 3. In analyzing the data after basis weight reduction, it is no longer pounds of steam/pounds of board that is of interest--it is pounds of steam per lineal foot produced.

To further illustrate the effect on energy savings, one of the more obscure applications for nanoparticles was in tracing paper where considerable steam energy was put into the wet end to raise the stock temperature to 81[degrees]C to facilitate drainage. It was learned that the wet end temperature could be reduced to 72[degrees]C with a 40% reduction in wet end steam flow to maintain the same machine speed.

Much has been learned over the years about optimizing steam savings with nanoparticles. A fully optimized system will generally provide not only a significant reduction in specific steam consumption but important secondary benefits that will invariably justify the program.

                Sheet Moisture          Drying # of steam  Power House
                After Press Section %   per # of paper     GJ/T

CPAM Reference  56.9                    2.50               7.1
Nanoparticle    54.2                    2.24               6.5

Table I: Monthly average of daily press solids and steam data on an
alkaline coating base machine

WHAT YOU WILL LEARN

* Why many machines are already running nanoparticle systems with the potential to increase production and reduce drying costs.

* Examples of machines using nanoparticle technology to reduce drying costs.

* How a fully optimized system will provide a significant reduction in specific steam consumption as well as important secondary benefits.

ADDITIONAL RESOURCES

* Eka Chemicals web site: www.ekachemicals.com

DUNCAN CARR, EKA CHEMICALS

ABOUT THE AUTHOR

Duncan Carr is senior papermaking specialist for Eka Chemicals. He has been involved in the pulp and paper industry for 30 years, nearly 20 years of which has been spent working with Eka's paper chemicals in both Europe and North America. Contact him by email at [e-mail].

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