Pulp Mill Uses Absorption Chiller to Improve Process and Environment
Some experts have claimed that with computers, society is on the verge of a paperless culture. But looking around today's world, it is clear it has a long way to go. From postage stamps to currency, from sales brochures to invoices and packaging, society still revolves around the use of paper. World demand for paper is growing, not declining. Paper is a feedstock of civilization.
In British Columbia, manufacture of pulp for papermaking is an important industry. Pulp is an intermediate step between trees in the forest and finished paper. It is the source for the wood fiber used in papermaking and is created in a combined mechanical-thermal-chemical process that removes the lignin, or "wood glue," that holds the fibers together in wood.
Commonly in British Columbia, pulp is produced from forest products at a pulp mill and then shipped in bundles of dry sheets to paper mills nearer the paper markets in Canada, the United States and around the world. Such is the case with the pulp mill of Northwood Inc., Prince George, BC. The mill employs 625 and ships pulp by rail throughout North America and via ship from the ports at Vancouver, Squamish and Prince Rupert, BC, to overseas markets. Northwood is a joint venture of Canada's Noranda Forest Inc. and the Mead Corp., Dayton, OH. Northwood operates a range of forest product ventures, including mills for kiln-dried dimension lumber and plywood, wood-preserving operations and chipping plants.
Pulp Chips a Byproducts of Lumber OperationsPrince George is located about 400 miles north of Vancouver on the Fraser River. The Northwood pulp mill receives chipped wood from nearby sawmills by truck and from distant locations by rail. The chips are a byproduct of dimension lumber and plywood operations. In addition, reject logs are chipped and used for pulping. More than 60% of the feedstock for the mill is from Northwood operations at other sites. The sprawling pulp plant uses the kraft process for cooking and whitening the pulp as it is transformed from chips to sheets of finished dry pulp.
The wood processed at this plant is so-called "softwood" chips from coniferous trees, mostly white spruce and lodgepole pine. Softwood pulp is particularly desirable because its long fibers add strength to paper. It can be bleached to a brilliant white, making it attractive for print applications. The pulp from the plant goes for many high quality paper products, including currency, stamps, bond paper, tissue, magazines and business papers.
The chip feedstock is screened, digested (cooked), washed, screened and bleached before being formed into sheets of pulp and dried. It is within the chemical preparations area that the absorption chiller is used. Like many other pulp mills, this plant originally used elemental chlorine as a bleaching agent. Because of environmental concerns regarding the effluent, the Northwood mill, like many others, has converted to an elemental chlorine-free (ECF) bleaching operation. The process uses chlorine dioxide (ClO2) as a bleach agent because of its environmental compatibility and bleaching efficiency. The mill also uses other bleaching agents such as oxygen and hydrogen peroxide.
Because of its chemically unstable nature, chlorine dioxide for the bleach solution is generated at the site in a chemical process involving a mixture of sulfuric acid, sodium chlorate and methanol. A total of 55 tons per day of ClO2 is produced. The chlorine dioxide is mixed with water drawn from the Fraser River to create the bleach solution.
Normally, the Fraser River water goes through three annual stages. In late fall and winter, the water runs cold and clear and requires little treatment. Some filtration before mixing in the bleaching process can take place. In spring, the river water becomes turbid and requires more pretreatment to remove discoloration. In late spring through early fall, the water clears and warms up. During this period of the year, the challenge is to hold the chlorine dioxide in solution.
At temperatures above 54°F (12°C), chlorine dioxide gas does not readily absorb into solution, dramatically reducing the effectiveness of the chlorine dioxide production process. Thus, as the water temperatures increase in the spring, steps need to be taken to hold solution temperatures below 54°F (12°C). In fact, for maximum absorption efficiency, the target temperature is 40°F (4.5°C).
Absorption Chiller Refrigerates Bleach SolutionThe mill uses a 900-ton single-stage absorption chiller manufactured by Trane, LaCrosse, WI. The chiller is operated by process steam to refrigerate river water for the chlorine dioxide solution, keeping it at the target temperature. The process steam required to operate the chiller originates on-site from boilers fueled by byproduct wood from the nearby mills and from the chip screen rejects at the mill. In addition to the chiller, the steam is used in many other processes, with peak usage in the winter months.
In this way, the Northwood complex fully utilizes the forest resources remaining after production of veneer, sawed dimension lumber and chips for pulping. Because other process requirements peak in the winter, the use of the steam for chiller operation in the warm weather months does not require increased steam production capacity.
The Trane absorption chiller uses saturated steam at 60 psi to chill the filtered river water to the target temperature before it absorbs the chlorine dioxide gas. The chiller can be independently controlled through its own unit control, but in normal operation it is interfaced with the central bleach plant process control system.
According to John King from the Vancouver Trane office, the use of absorption chillers for this particular application is growing. "Most pulp mills have surplus process steam in the warm months, so this application is a natural. Absorption chillers normally require relatively little attention. Here at Northwood, they check tube cleanliness regularly and clean the tubes as required. They've learned to equip the chiller with cupronickel tubes to resist pitting and corrosion from the somewhat acid river water standing in the tubes when the chiller is not in operation. Very little attention is required after the unit is started up in the spring."
King noted that operator training is important. "When we started out, the mill operators didn't understand how you can get chilled water from steam. We have training modules that teach the basics of the absorption process. They learn what they have to know, what to watch for. But the unit usually runs with minimal attention."
At Northwood, the river temperatures in the spring are monitored and typically chiller startup is scheduled in late April. In a typical year, the mill refrigerates the bleach water for about six months. The plant operators dial in their setpoints and the unit follows the demand for chilled water. One of the things most liked about the absorber is that it just runs.
For more information on absorption chillers for industrial process applications from Trane, call (608) 787-4810 or visit www.trane.com.