Solvent Recovery Systems

Transcription

1 Solvent Recovery Systems Use Steam Recycling A new technique that recycles the heat normally lost during solvent recovery can cut fuel bills and reduce plant exhaust emissions. by Stanley J. Macek Compliance with U.S. Environmental Protection Agency (EPA) air quality requirements regarding the release of solvent laden air into the atmosphere can mean costly capital equipment outlays. But, as long as there s no acceptable alternative, why not make compliance pay for itself! This possibility exists through the use of a high efficiency solvent recovery system equipped for steam recycling. Exact solvent-reduction requirements can vary by solvent, by state, by industry and by direct negotiation between company and EPA officials. Sometimes they are made even more stringent by local or muqicipal authorities. Local regulations have been known to require recovery of 85 to 95 percent of solvent used, or to restrict plant exhaust emissions to as little as 10 parts per million (ppm) concentration of solvent to ambient air. Compliance at these levels is expensive because it requires the use of sophisticated solvent recovery plants with a cost of $20 to $50 per cfm capacity, plus monthly operating costs that can pile up mammoth fuel and water bills. However, in solvent-intensive processes where fresh solvent must be continually bought and expended through evaporation, an efficient solvent recovery plant can offset operating costs by reducing or eliminating the need to buy fresh solvent. In some operations, where fresh solvent is routinely mixed with other purchased materials that arrive already relatively high in solvent content, recovery might offset costs even further by returning more solvent than would otherwise be bought. Recovered surplus often can be sold back to the material supplier - much the same way in-plant cogeneration can make excess electricity available for resale back to the plant s utility company. There is another way a steam-regenerated solvent recovery plant can provide a return on investment. In some cases, the return is sufficient to not only pay all of the recovery plant s fuel and fresh water costs, but also to gradually pay back its initial purchase cost and subsequently start producing a profit. It s one of the few ways a business can cut costs without any impact on the quality of its product or service. Such savings are possible through the application of a newly developed technique that recycles the heat normally lost in the solvent recovery process. That heat is then used to reduce the energy cost of producing the steam needed to run the recovery plant s regeneration cycles. This technique is especially well suited to the recovery of aromatic and saturated (straight) hydrocarbons, acetates, ketones and alcohols. Such commonly used solvents cover a wide range of businesses that produce or apply inks, paints, coatings and adhesives, plus those using solvents for large-scale cleaning or processing operations. In some of these industries, adding steam recycling to solvent recovery plants has already demonsffated the capability to cut fuel bills by thousands of dollars, even tens of thousands per month, while reducing the cost of boiler feed water. Carbon, steam extract solvent A review of a state-of-the-art solvent recovery process shows how steam recycling works. Solvent-laden air (SLA) is drawn away from its source by high-capacity fans through specially constructed ductwork, filtered and cooled if necessary. It then is distributed by manifold to a bank of adsorption tanks, where it passes through beds of pelletized activated carbon. The solvent adsorbs on the carbon, and the air vents to the atmosphere nearly solvent-free. Although the high level of solvent removal satisfies EPA emission standards, it s only the first step in the solvent recovery process. When any individual adsorption tank s carbon reaches its solvent ad- The steam recycling process involves the bot water tank with its vacuum steam flash tank on top and tbermocompressor mounted alongside. (Source: Dedett Corp.) i f 84 POLLUTION ENGINEERING JANUARY 1991

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3 hot water tank at center. Leftover solvent vapor exits at the top of the stripper tank through pipes leading back to the main SLA duct at top right. GINEERING 85

4 c n, The steam recycling technique saves about $10,000 a month in natural gas bills. Four variable speed fans maintain even vacuum pressure on the main SLA duct, drawing SLA through filters, then blowing it through coolers into the five desorption tanks in the background. sorption limit, a sensor in the exhaust stack detects solvent breakthrough. Immediately, the tank is automatically valved out of the system and a regeneration cycle begins. Low pressure steam, at around 220"F, flushes backward through the carbon bed, desorbing the solvent and carrying it off as a vapor called desorbate. Moving through a condensing system, the desorbate emerges as a liquid at around 90 F and passes into a decanter tank. Having a lower specific gravity than water, the solvent stratifies above the water, overflows the decanter's baffle plate and is piped to solvent storage. The remaining decantate water - now containing only trace amounts of solvent - is pumped to the top of a vertical stripper, where it descends through a packing medium by gravity. Meanwhile, ambient air is drawn up through the medium by ductwork vacuum. The rising air evaporates the leftover solvent out of the water and is fed back to the recovery plant's main intake duct, where it mixes with incoming SLA for another pass through the recovery plant. 86 POLLUTION ENGINEERING JANUARY 1991 In systems working with water soluble solvents such as acetates, ketones and alcohols, the decanter stage is replaced by alternative equipment that separates the solvent from the water through a combination of distillation, stripping and drying. Reclamation At this point in the process, this type of solvent recovery system typically has reclaimed 95 percent to 99 percent of the airborne solvent from its incoming SLA, and has removed the solvent from the condensate making it clean enough for sewer discharge, or more often, for recycling as boiler feed water. A gravure publications printer recovering toluene reported that because the desorptionlstripper recovery system recovers nearly all of the airborne solvent, the system provides the printer with about 30 percent more fresh solvent than needed to thin toluene-based inks. As a result, the excess toluene is sold back to the ink supplier, in effect cutting ink costs. But the biggest plus of the system is said to be the new steam recycling technique that saves about $10,000 a month in natural gas bills. System taps condenser heat The design cuts steam requirements dramatically by reusing the latent heat given by the desorbate as it passes through the first-stage condenser, then using that heat to generate fresh steam. More specifically, this is accomplished by recirculating water from the firststage condenser, through a large insulated thermal water storage tank. The water is pumped through a flash-evaporator, where low-pressure saturated steam is produced under vacuum (8 psia) generated by a thermocompressor. The low-pressure steam passes through the thermocompressor where it combines with high-pressure live steam. Combining the two steam sources produces the low-pressure process steam used for carbon-bed regeneration. This system cuts steam consumption to 50 percent of the usual three to five pounds of fresh steam to recover one pound of solvent. This reduced steam requirement results in a proportional reduction in boiler fuel use. With boiler makeup coming from the hot water storage tank, already partially heated, and with lower condensers and precoolers running through a cooling

5 tower, fresh water intake is minimized across the solvent recovery system. Other tips for peak efficiency Regardless of whether steam recycling is used, other provisions can be made for maximizing the efficiency of the solvent recovery system itself. For example, dryers, ovens, vats and other equipment from which SLA arises can be fully enclosed with hoods designed to prevent SLA from going anywhere but into recovery plant ductwork. These hoods can be equipped with automated vent valves that hold back the SLA until it builds up to a higher concentration. At a prescribed ppm, usually set conservatively below the solvent s lower explosion limit (LEL), a sensor signal opens the valves and allows the SLA to be drawn into the recovery system. Although 95 percent or more of the operation s SLA will exit via these hooded enclosures, extra vents tapping the surrounding room can be used to maintain a negative pressure throughout the area to add more assurance that stray fumes don t escape recovery. By using welded steel-plate ductwork construction rather than conventional sheet metal, the system can be made strong enough to prevent the possibility of ductwork collapse due to the negative pressures available when fans are operating and intake valves are closed. The extra strength of this construction eliminates the need for external relief devices, thereby avoiding potential points of leakage and SLA losses. Ductwork is usually kept at a prescribed static vacuum pressure by fans at the far end of the duct. If variable speed fans are used, fan speed can be regulated by a pressure control loop that responds automatically to the number of SLA induction vents opened throughout the shop at any given time. This allows more efficient use of fan power and helps reduce operating costs. Fans should be equipped with airtight shaft seals, gasketed access covers, pressure-balanced wheels and other features designed to eliminate any SLA loss from the fan, which can be as much as 5 percent of total SLA when standard industrial fan designs are used in solvent recovery systems. Adsorber regeneration cycles can be initiated manually or by automatic timers. However, operating costs can be trimmed further by equipping the regenerating system to operate unattended on programmable controllers linked to the desorption tank stack sensors. The type of steam recycling system described here adds about 10 percent to the initial cost of a carbon-desorption solvent recovery plant. Stanley J. Macek is manager of the Evaporator Division of Dedert Corp. Reader Interest Review Please circle the appropriate number on the Reader Service Card to indicate your level of interest in this article. High 456 Medium 457 Low 458 The condensers that cool the desorbate/steam vapor into a liquid are kept shaded for more efficient operation. t JANUARY 1991 POLLUTION ENGINEERING 87