When UCAR Emulsion Systems, Cary, NC, a major supplier of latex to the paint industry, and a business unit within Union Carbide Corp., looked for a way to reduce the emissions of vinyl acetate and other VOCs during its manufacturing process, it faced a particular problem.
Because Union Carbide uses a proprietary batch process for latex production, the VOC concentrations in the process exhaust stream vary from almost 0% lower explosive limit (LEL) to a flammable mixture. A large variation in concentration — combined with the 95% destruction efficiency currently mandated by U.S. regulations — poses a problem for the most commonly used technologies.
Catalytic oxidation is often used to promote oxidation of VOCs at a temperature that is lower than thermally possible. By operating at these lower temperatures, catalytic oxidizers use less fuel, thereby saving money. The chief drawback to this method, however, is that catalytic units are normally limited to an LEL concentration of less than 25%. Concentrations greater than 25% LEL result in excess heat release in the catalyst bed, which can damage the catalyst. Air dilution can be used to reduce concentrations, but then extra fuel is needed to heat the increased volume.
A second method used to control VOC emissions is thermal oxidation using either a regenerative or recuperative heat exchanger. Because of variations in LEL concentrations and heat releases, however, a heat exchanger cannot be properly sized to handle the high heat release while still being efficient enough to make the unit economical to operate with lower LEL concentrations.
A final option is direct flame oxidation, in which the process gas is burned in a burner. This is also problematic. Although direct flame oxidation works well when VOC concentrations are high, the method requires large amounts of natural gas to be added to the process stream when the process LEL is very low. Otherwise, the mixture will not reach combustibility. This makes the method uneconomical.
Union Carbide solved the dilemma with the use of a specialized oxidation system, the Series WFR Econ-Abator®. Based on the standard catalytic oxidizer, the system features a fluidized bed of inexpensive, non-noble metal catalyst. This fluidization, along with the catalyst’s rugged characteristics, prevents the catalyst from fouling, and allows the catalyst to be used in streams with both chlorinated and sulfur compounds. These features, along with the system’s castable refractory lining and strong carbon steel shell, provided a good base unit for Union Carbide.
However, the system still needed to be able to operate economically while handling the high VOC concentrations. For this, a second burner was added to the system through which the process gas is introduced into the oxidizer. This burner provides direct flame incineration when concentrations reach a flammable level. At this point, the gas is burned as a fuel to provide heat for the system. As a result, the pre-heat burner running on natural gas turns down to a low-fire state to conserve fuel. Since fuel isn’t burned at 100% efficiency, some of the VOCs make it past the burner. The catalyst oxidizes VOCs that are not oxidized in the burner for a destruction efficiency in excess of 95% (higher destruction efficiencies can be reached by adding more catalyst).
At low VOC concentrations, the unit operates as a normal catalytic oxidizer, with the vinyl acetate-laden air being oxidized in the catalyst bed. The catalyst allows for high destruction efficiencies at lower operating temperatures than thermal oxidation. For this application, the operating temperature is set at 700ºF.
The use of the Series WFR Econ-Abator allowed Union Carbide to solve a potentially cost-prohibitive, as well as technologically difficult, problem with a cost-effective and relatively simple solution.
For more information on thermal/catalytic oxidizers, contact John Berger, P.E., Marketing Manager, Huntington Environmental Systems Inc., 707C West Algonquin Road, Arlington Heights, IL 60005; phone 847/545.4451; fax 847/545.1947; e-mail email@example.com.