Thermal oxidizers have been used for more than 30 years to destroy VOCs emitted during a variety of surface coating operations. During the past 10 years, VOC concentrators have been increasingly used to concentrate process exhaust and reduce the size and operating costs of thermal oxidizers. Carbon has been the most widely used adsorbent, but recently zeolite has found a niche because of its higher removal efficiency with low-molecular-weight, polar-oxygenated solvents.
For spray booth and flash enclosure exhaust streams that are at ambient temperature, adsorption has been extensively applied. Since most automotive spray booths use Venturi scrubbers for primary removal of particulate matter, it is usually necessary to reduce the relative humidity of the air before the adsorption step. This is easily achieved by raising the temperature of the booth exhaust. Often, a second polishing filtration stage is also needed for final particulate removal. Typically, a multi-stage baghouse is used in newer plants with high-efficiency primary scrubbers. Wet electrostatic precipitators are also used when reduced maintenance and labor costs are important.
Adsorption systems using activated carbon to capture volatiles have been around for decades. Recovery systems using deep static carbon beds to capture solvents from such processes as printing or adhesive application have been common. These systems typically use multiple carbon-filled vessels that are sequentially operated in adsorption and desorption cycles. Desorption can be achieved with steam, hot air or inert gas.
Static deep-bed carbon adsorption systems are ideal for solvent recovery applications, particularly when highly reactive ketones or stabilized chlorinated compounds are not present. However, in those cases where a more complex blend of solvents is present, and recovery is not an economically viable option, the high pressure drop through a deep static carbon bed and the relatively high weight and space requirement of this system are major disadvantages.
For large air streams with relatively low VOC concentrations, rotary adsorbers can be used to concentrate large volumes into smaller air streams that can be handled much more economically.
The VOC-laden air passes through the rotary adsorption unit where the VOCs are adsorbed on zeolite or carbon media. The purified air is exhausted to atmosphere. The solvent adsorbed on the media is then removed by desorption with a small stream of hot air. The desorbed air, containing a high concentration of solvent, is delivered to an oxidation device.
The rotary concentration system is designed to continuously adsorb VOCs from an air stream onto zeolite or carbon and discharge purified air. This is achieved through the use of a rotating wheel that is continuously desorbed one section at a time. This design eliminated the need for dual running and stand-by adsorption beds.
Solvents are adsorbed onto the adsorbent material and the purified air exits through the center of the wheel. A portion of the rotating wheel is simultaneously desorbed with hot air. This desorption section is sealed off from the remaining adsorption section of the rotor by accurate seals. This allows for high efficiency in the system.
Capture efficiencies for this type of system may be as high as 98 pct. Depending on the applications, the concentration ratio (ratio of process exhaust to the desorption air) can be as high as 1:15.
Rotary concentration systems can be designed using either zeolite or activated carbon as adsorption media. In certain applications, a granular activated carbon prefilter is used upstream of the rotor. The important design parameters for this type of control system include the type of VOCs and their concentration, adsorption media choice, face velocity on the adsorption media, desorption time/temperature, concentration ratio and material of construction.
The advantages of rotary concentration systems include low energy consumption, low operating cost, no NOx created, low pressure drop across the system, high reliability and ease of operation and maintenance. As with any adsorption system, particulate filtration is required upstream of the adsorber to prevent blinding the media.
Hybrid systems using rotary concentration adsorbers to concentrate solvents in large dilute air streams and thermal oxidizers to destroy the concentrated VOCs are in wide use. The operating cost benefits of using a concentrator in conjunction with a small oxidizer make this an attractive option.
Painting Automotive Parts. An exhaust stream of 83,000 cfm containing up to 250 ppm of VOCs for spray booths and 6,000 cfm from paint drying ovens required a high level of control. Although the facility was using a waterborne base coat, emission levels were higher than similar plastic painting operations using a solvent-borne base coat and add-on controls. A solvent-borne clear coat also required controls.
The solvent mixture presented a challenge. Methanol, 15 pct of the solvent load, is not effectively removed by carbon. Xylene and ethylene glycol butyl ether acetate molecules are not effectively removed by zeolite having roughly the same pore size. One solution was the use of carbon blocks in the outer row and zeolite blocks in the inner row of a rotor. Pilot tests verified that humid exhaust from wet Venturi spray booths was controlled to 60 pct or less.
Twenty weeks after the pilot test, the novel concentrator and an RTO were installed. Compliance tests demonstrated a 95 pct overall VOC removal/destruction efficiency.
Automotive Assembly Plant. A new assembly plant was built in a state requiring the Lowest Achievable Emission Rate of any previously built assembly plant. The automobile manufacturer chose low-VOC, waterborne base coat. It decided to control VOC emissions from the base coat heated flash and clearcoat spray zones with a rotary carbon concentrator followed by destruction and a regenerative thermal oxidizer (RTO). This was all part of a systems approach to meet the emission rate for the topcoat process. The emission rate was fully achieved by also destroying VOC emissions from the topcoat oven in the same RTO.
Micron-sized paint particles are filtered from clearcoat robot spray zone exhaust before they are cascaded through a second clearcoat spray zone using high efficiency electrostatic bell applicators. Cascading exhaust air from one spray zone to another both reduces the amount of outside air that must be heated or cooled and the size and cost of the VOC control system.
Combined clearcoat spray booth, base coat flash-off and final topcoat exhaust is concentrated 10:1 by two rotary carbon concentrators. Nearly 90,000 scfm of exhaust and the solvent it contains is concentrated 10:1. This further reduces the size and costs of the RTO and increases Btu content to minimize the amount of natural gas required for VOC destruction.
Rotary carbon concentrators remove more than 98 pct of VOCs in the combined topcoat exhaust streams. More than 95 pct of VOC captured from the topcoat is destroyed in the RTO. The VOC destruction removal efficiency of the entire topcoat process is greater than 80 pct.
Aircraft Painting Facility. An aircraft maintenance facility required control of emissions from both paint stripping and painting operations. The major solvents used in these stripping and painting operations were, respectively, methylene chloride and xylene. The removal efficiencies of carbon, zeolite shaving pore sizes of six and eight angstroms, and carbon followed zeolite were determined for methylene chloride and xylene. Based on these initial results, additional tests were performed with mixtures of six and eight angstrom zeolites on a single substrate. Rotary concentrators were installed using two rows of blocks containing a proprietary mixture of zeolites capable of removing more than 70 and 90 pct, respectively, of methylene chloride and xylene emissions.
The 250,000 scfm of booth exhaust was concentrated with six rotary zeolite concentrators prior to destruction in an RTO. Overall concentrator removal efficiency was greater than 95 pct for the complex solvent mixture. Rotary concentrators were fabricated from 304 stainless steel to minimize corrosion from methylene chloride. The RTO was fabricated from 316L stainless steel to minimize corrosion from combustion products of methylene chloride.
Field experience has shown that activated carbon and synthetic zeolite, either alone or in combination, can effectively remove paint solvents used in most surface coating operations. Carbon alone effectively removes xylene and most common paint solvents, except methanol. Zeolite having six angstrom pores is effective at removing methylene chloride, methanol and most low-molecular-weight solvents, though somewhat less effective with xylene and other solvents larger than six angstroms in diameter. Zeolite having eight angstrom pores is more effective in removing higher molecular weight solvents. Carbon and zeolite may be used in series to remove solvent mixtures containing moderate amounts of methanol (15 to 25 pct) at low total VOC loadings (<250ppmv). A physical mixture of carbon and zeolite on a single substrate is more effective than the two in series in removing high concentrations of solvents (>300 ppmv) with lower (<20 pct) alcohol content.
Reprinted with permission from ESD-The Engineering Society, from its Proceedings of the Advanced Coatings Technology Conference, April 7-10, 1997, Detroit, Michigan.