In 1985, the Pewaukee, Wis.-based printing company purchased two 96-inch web, eight-color, high-speed presses and installed them at its new printing plant in Lomira, Wis., near Milwaukee. Conventional wisdom at the time was that four production presses were needed for a printing company to be economically viable. However, Quad/Graphics planned to operate eight or more presses. This year, the company commissioned its twelfth press, making the Lomira facility the world's largest gravure printing plant.
Print production. Gravure printing is used to produce multicolored magazines, catalogs and other high-volume, high-quality printing. Text and each multicolored photograph are color-separated by computer into yellow, magenta, cyan and black images that are engraved onto chrome-plated copper cylinders. The cylinders rotate in ink baths, where the ink adheres only to the engraved area of each cylinder. The paper web then is pressed against the cylinder, transferring the ink to the web. The four colors are applied sequentially to the web, and the layered colors produce additional colors -- for example, yellow on cyan creates green, and so on. In this way, the original, multicolored photographs are reproduced accurately. The web is 96 inches wide and travels 3,000 feet per minute, producing 40,000 copies of 48- to 96- page magazines or other documents per hour, using about seven tons of paper.
Each gravure ink color is dried before the next color is applied. Thus, each color ink station contains a web dryer about 30 feet long. To achieve rapid drying, ink solids are dissolved in volatile solvents that quickly evaporate from the print web in the dryer. A hydrocarbon, such as toluene, is the universal solvent for publication gravure inks. Typical, press-ready gravure inks are 20 percent ink solids and 80 percent solvent. Ink laydown on the web varies with color intensity. A 96-inch, eight-unit press uses between 500 and 1,500 pounds of solvent per hour. A production press consists of eight print stations, eight web dryers, ink sumps, a paper-reel station and product folder. The equipment requires a space 15 feet wide by 100 feet long by 30 feet high.
The Environmental Protection Agency classifies solvents used in publication gravure as volatile organic compounds. Some of these, such as toluene, an aromatic hydrocarbon, also are listed as hazardous air pollutants. To control VOC emissions, EPA since the late 1970s has required use of recovery systems (such as activated carbon), oxidation systems or material substitution.
The 1980 new-source performance standard for publication gravure required reduction of VOC emissions by at least 84 percent. EPA and many states since that time have imposed increasingly stringent reduction requirements. The latest regulation requires print facilities to implement maximum achievable control technology, or MACT, by 1998. Achieving MACT likely will require publication gravure facilities to attain about 94 percent overall reduction through press- and building-capture methods, and increased control system efficiency.
Like all U.S. publication gravure facilities, Quad/Graphics uses activated-carbon systems to recover solvents for reuse in the printing process. Water-based inks, despite 30 years in development, still do not achieve the print quality of solvent-based ink, even at half the web speed.
Solvent recovery. Quad/Graphics selected Amcec, a highly experienced systems supplier, to implement a solvent recovery project for its new gravure facility. Establishing the solvent-laden air volume from each press was a key issue; 42,000 standard cubic feet per minute per press was considered sufficient for ventilation, fire and explosion hazard purposes. (One thousand five hundred pounds per hour toluene in 42,000 scfm is about 20 percent of the lower explosive limit). However, EPA requirements, as well as anticipated future regulations, mean fugitive emissions from the print stations and dryers have to be minimal. For these reasons, an exhaust rate of 50,000 scfm per production was selected.
To transport the solvent-laden air from the first six presses, a 9-foot diameter exhaust duct was installed over the roof of the press hall and the solvent recovery area. As more presses were added, a second duct was installed parallel to the first, and the two were inter-connected. Thus, for two presses, 100,000 scfm was exhausted to the solvent recovery system. This has increased to 600,000 scfm for the 12 presses now operating.
The solvent-laden air is passed through activated-carbon beds to capture the solvent. The carbon has an immense internal surface area (1,100 square meters per gram, or the equivalent of about 100 football fields per pound). As the solvent-laden air passes through the carbon bed, the solvents are retained by the activated-carbon mass. Working capacity is about 6 pounds of solvent per 100 pounds of carbon. When a carbon bed has been charged with solvent, it is taken off-line for in situ regeneration.
During regeneration, the carbon is heated by steam, which releases the solvent, and the solvent-vapor and steam mixture passes to a condenser and subsequent separation into solvent and water phases. The recovered solvent is clear, free of water and acids, and suitable for direct reuse in the printing process. Most of the recovered solvent is used to dilute the inks to a suitable printing consistency. The remainder is returned to the ink factory to dissolve the next batch of ink solids used to produce printing inks.
Amcec sized each carbon adsorber for 50,000 scfm, the equivalent of solvent-laden air flow from one press. The initial system consisted of three adsorbers, two of which adsorbed in a parallel but staggered cycle, while the third was used off-line for regeneration. To draw solvent-laden air from the presses, the system used multiple 50,000 scfm fans with associated filters and cooling coils. As more presses were added, the system was expanded. For six presses, the system included seven adsorbers (six adsorbing, one regenerating), six fans with filters and coolers, and one condensing train with a solvent-water separator, cooling towers and related features.
When press capacity was expanded to a dozen 96-inch presses, the solvent-recovery system was enhanced to include 14 adsorbers (two groups of seven), 12 fans with filters and coolers, and two condensing trains. The system contains more than 600,000 pounds of activated carbon. To date, the carbon in the original adsorbers has been regenerated more than 10,000 times with little sign of performance loss. The installation is the largest air-flow-capacity solvent recovery system in the world. On a typical day, the system returns more than 25,000 gallons of solvent to the production process.
A programmable logic controller sequences the recovery system's adsorbers, and monitors valve actions, pressures, flows, levels and other process variables, including VOC emissions. A color monitor graphically displays the operational sequence and data, and produces on-screen alarm logs and daily reports.
The company recently installed two Amcec ECOVAP energy recovery systems to capture heat from the desorption vapor. The recaptured heat is used to produce fresh steam to continue desorption of the carbon bed. This innovation reduced steam usage more than 40 percent and will return the capital invested in less than three years. It also eliminates the need to install additional boilers and natural gas mains as the site is expanded.
The solvent recovery system consistently returns more than 95 percent of the solvents used by the gravure presses. As system efficiency exceeds 99 percent, the amount of solvent retained on the web or lost as fugitive emissions is less than 4 percent of total usage, confirming that the initial solvent-laden air volume selected effectively brings the solvent fume to the recovery system. The system operates 24 hours a day more than 360 days a year.