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Steam recovery cuts cost of solvent recovery.

This American printer can recover 6,000 tons of toluene from the 900,000 gallons of ink and extender it uses annually

As with many industries, the commercial printing industry faces the dilemma of conforming to ever-toughening environmental laws while keeping pollution-control costs to a minimum. Printers both large and small are struggling to comply with a growing number of regulations aimed at limiting the emission and disposal of hazardous processing solvents. Despite strict guidelines, one U.S. printer, Brown Printing Company's (a division of Gruner & Jahr AB and Co., Hamburg, Germany) rotogravure printing plant in Franklin, KY, has successfully struck a balance between full compliance and cost-effectiveness.

Shouldering the responsibility for recovering more than 10,000 tons per year of the highly volatile ink solvent toluene, Brown's Franklin Division recently retrofitted its on-site solvent recovery plant with a highly efficient steam recovery system that cut boiler fuel bills by more than 40%. Having recently completed a US $30 million expansion, Brown hopes to see these savings add up as the division continues to grow.

As one of the American industry's premiere gravure plants, Franklin Division rolls through about 20 million pounds per month of paper ... 75% of which is converted into catalogues and flyers; the other 25% leaves as magazines and editorial inserts. Print runs range from 600,000 to 21 million for such high-volume customers as Best Products, Ikea Furniture, K Mart and Woman's World and Parents magazines.

Through all this expansion and increasing success, pollution regulations loom. Kentucky's Lowest Achievable Emissions Rates (LAER) Air Program, administered in accordance with the Environmental Protection Agency's (EPA) and other federal regulations, requires recovery of at least 90% of all volatile organic compounds (VOC). For Franklin Division, this means recovering more than 10,000 tons of toluene from the 1.5 million gallons of ink and extender it uses every year.

Because it has no effect on product quality, one of a printer's best targets for cost cutting efforts is solvent recovery, according to Franklin Division's technical director, Ros Dennison. "Emission control is something that every printer has to deal with anyway, so why not make it pay as many dividends as possible?"

"By recovering nearly all of our solvent," said Dennison, "this closed-loop system provides us with more pure toluene than we need at the presses. As a result, we can sell the excess back to our ink supplier, in effect cutting our ink costs."

At Franklin Division, he explained, gravure inks are piped to the presses in concentrated form, with 50 to 60% toluene content by volume. Varnish and additional toluene each arrive via separate piping. An automated blending system at each press station continuously adds varying amounts of varnish and toluene to the ink, keeping color intensity consistent by adjusting viscosity as the gravure cells wear shallower through the press run. On average, approximately 1.5 gallons of toluene are added for every gallon of ink concentrate and varnish used.

After each press station, the emerging paper web rises to an overhead dryer before descending again to the next station. In a split-second, as the web streaks through the dryer hood at 29 mph, steam-heated air recirculates to flash toluene out of the ink. Vapors are continually drawn from each dryer-hood vent into the recovery system's overhead ductwork. A static pressure of 30 inches from three 300-hp blowers and two 250-hp blowers ensures constant flow of the solvent laden air (SLA) and prevents "fugitive" vapors from drifting through the building.

The solvent recovery system, designed by Dedert Corp., Olympia Fields, IL, incorporates fans equipped with airtight shaft seals, gasketed access covers, pressure-balanced wheels and other features calculated to virtually eliminate SLA losses from the fan, which can be as much as 5% of total SLA with standard industrial fan designs often used in similar systems.

Although as much as 95% of the shops's toluene vapor exits via the dryer-hood vents printing runs, SLA also arises from miscellaneous sources around the plant. Hence, to further lessen atmospheric emissions, any idle presses, a four-unit proof press, parts-washing tank and ink-mixing stations surrender their ambient air to the solvent recovery, system, which maintains a reduced airflow rate on overhead ductwork at all times.

Airborne toluene from working pressrooms and these other areas -- at an average concentration of 1500 ppm, 13% of toluene's Lower Explosion Limit (LEL) -- travels through the ductwork to a 10-ft diameter (O.D.) roof-mounted main SLA header.

In normal operation, SLA leaves the dryer hoods at about 120|degrees~F, and is drawn along the header and into the recovery plant through a particulate filter and a fan cooler. Thus filtered and cooled to around 85|degrees~F, the SLA is distributed by manifold to a bank of adsorption tanks, where it percolates through beds of activated carbon. Toluene accumulates, or adsorbs, in molecular form in the pores of the carbon granules; after surrendering toluene to the carbon, pressroom exhaust vents to the atmosphere nearly solvent-free.

This more than satisfies Kentucky's LAER requirement; because the adsorption takes remove solvent up to 75 ppm breakthrough -- 5% of the 1500 ppm pressroom exhaust -- recovery, is always at least 95% and has been measured at more than 99% at times, at least 5% better than the state's regulatory target. But reducing emissions is only the first step in the system's operation.

When any individual adsorption tank's carbon nears its toluene saturation limit -- typically after 60 to 80 minutes of SLA adsorption from all four presses running "wide-open" -- an exhaust-stack sensor detects toluene breakthrough at 75 ppm and the tank is automatically valved out of the system's operation.

Once a tank is taken off-line, a carbon-bed regeneration cycle begins. Employing the Supersorbon carbon adsorption process developed in West Germany by Lurgi GmbH, the system flushes low pressure steam at 220|degrees~F through the carbon bed to displace the toluene, which is eventually piped to aboveground storage tanks in pure liquid form. With its carbon bed steamed out, the adsorption tank is restored to full effectiveness, a process that occurs about 6400 times a year.

Three natural gas-fire package boilers, with a capacity of 125,000 lb/hr, serve as the primary source of steam needed to flush toluene from the adsorption tanks. Up until Franklin Division's current expansion effort, the boiler house was responsible for churning out 14,400 lb/hr of steam over each 55-minute regeneration cycle. This translated into roughly 4.5 lb of steam for every pound of toluene recovered, a ratio that added a considerable energy cost burden to overall plant operation. With the world's largest commercial press, and its substantial SLA output coming on-line in the near future, that burden was sure to grow heavier.

While planning for the new press, the division sought to increase the capacity of the solvent recovery plant. Franklin Division added three new absorbers (bringing the total to six), which could handle a future fifth press as well. At the same time, the solvent recovery plant was retrofitted with an evaporator-based steam recovery system to boost the cost-efficiency of pollution control.

"From an operation or environmental standpoint, steam recovery wasn't required," said Dennison. "It was strictly a business decision. We wanted to optimize our energy costs associated with the solvent recovery plant. Steam recovery promised a means to that end."

The new system, also designed by Dedert, cuts steam requirements dramatically by tapping the latent heat of the dense, hot mixture of steam and solvent vapor -- desorbate -- flushed from the carbon beds. In using this recycled heat, the steam recovery system altered the carbon-bed regeneration process, according to project engineering manager Kathy Lacy. Now, when one of the adsorption tanks is taken off-line, the boilers generate 22,000 lb/hr of fresh steam over a 12 minute super-heating period to begin desorbate flushing.

Fresh from the adsorbers, Lacy explained, the desorbate is piped to the system's 41-ft recirculated falling-film evaporator, where it disseminates throughout the steam chest; concurrently, make-up water is pumped to the evaporator's upper distribution chamber and gravity-descends through the chest via multiple heat-exchange tubes. The desorbate, heating the tube exteriors at 205|degrees~F psi, easily vaporizes a portion of the recirculating water steam.

The resultant low-pressure saturated steam is drawn through a vapor separator by a thermocompressor, which also brings fresh steam into the system from the boiler house; when recycled steam reaches the thermocompressor, the boilers reduce output to 8000 lb/hr. Combining the two steam sources reduces the fresh steam from its incoming 320|degrees~F/100 psi to the 220|degrees~F/14.97 psia output required for the remaining 23 minutes of the carbon-bed regeneration cycle.

As Lacy points out, whereas the old system used 13,200 lb/cycle of steam (14,400 lb/hr for 55 minutes), the upgraded system needs only 7,466 lb/cycle, or 43% less fresh steam.

"This reduced our steam ration to about 2.7 pounds steam for every 1 pound solvent recovered," said Lacy, down from about 4.5 lb steam/1 lb toluene recovered before the retrofit. "That's a pretty substantial drop when you look at recovering more than 6000 tons of solvent a year. Multiply these numbers out and the economics of the system become very apparent."

After surrendering its useful heat, the desorbate moves through a condenser and aftercooler, emerging as a liquid before passing into a decanter tank, where the toluene is drained off and piped to the storage tanks. The water is continually pumped back to the evaporator and rejoins the cycle. Further, the condenser and aftercooler share a separate closed-loop water cooling system. Although this loop periodically draws fresh water make-up, its primary volume is recirculated through a cooling tower.

All functions of the system are monitored by an automatic control system, which is linked to the plant's main building monitoring system. Although the solvent recovery system typically operates unattended, its tank regeneration cycles can be initiated manually or by automatic timers.

Two Definitions

toluene (tolu and benzene): a colorless liquid hydrocarbon, |C.sub.6~|H.sub.5~C|H.sub.3~, obtained originally from tolu balsam but now generally from coal tar and used in making dyes, explosives, etc. and as a solvent.

toluene: a liquid aromatic hydrocarbon |C.sub.7~|H.sub.8~ that resembles benzene but is less volatile, flammable, and toxic, is produced commercially from light oils from coke-oven gas and coal tar and from petroleum, and is used as a solvent, in organic synthesis, and as an antiknock agent for gasoline.

Toluene Is Printing's Typical Solvent

The Supersorben system is a process based on the adsorption of solvent vapors on activated carbon, with subsequent containment of the solvent in liquid form. The solvent most typically used in gravure printing is toluene. It is present in the ink, is used for dilution of the ink, and is also used for clean-up.

When ink is applied to the paper during printing, it is rapidly dried to prevent smearing by running the paper through a heated section of the press. The solvent in the ink evaporates and is removed from the press by exhaust fans. The exhaust air passes through a tank which contains activated carbon. Activated carbon is a very porous material, and the pores contain enormous surface area to which the molecules or solvent adhere, allowing the cleaned air to pass through and out to the stack.

The solvent is removed from the carbon by heating the carbon with steam. The mixture of solvent and steam (water vapor) is condensed and a gravity separation is usually adequate to extract the liquid solvent from the water.

Toluene is present in printing press exhaust at a concentration of about 2500 ppm. Approximately 99% of this is captured in the Supersorbon system, resulting in about 25 ppm venting to atmosphere. Dispersion from the stack results in much lower ambient levels than this.

A small amount of toluene (about 500 ppm) remains in the separated water. Usually this water is used as boiler feed water or elsewhere internally within the plant. If lower levels of toluene are necessary, air stripping or steam stripping can reduce the toluene concentration to 100 ppm or 10 ppm respectively.

The recovery of 99% of the solvent in a plant's exhaust air has a significant effect on both the plant's operating economy and the environment. This system is one of the few types of pollution control equipment that actually yields a positive payback.
COPYRIGHT 1993 Chemical Institute of Canada
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993 Gale, Cengage Learning. All rights reserved.

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Title Annotation:includes related articles; toulene recovery
Publication:Canadian Chemical News
Date:Feb 1, 1993
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