VOC Control for a PTA Plant
Purified terephthalic acid (PTA) is a largescale commodity material used worldwide for fibers, resins and films. primary raw material used to make many consumer and industrial goods including fibers for clothing and furnishings, resins for molded parts and packaging for food and beverages. A potential air emission from PTA production is methyl bromide gas, along with large volumes of other flammable or hazardous compounds.
Treating PTA plant emissions requires a thorough knowledge of the plant's process streams, seamless integration of control devices into the existing process and extensive experience with oxidation technologies. A look at a recent PTA volatile organic compounds (VOC) control project in Asia executed by Anguil Environmental Systems, Inc. (Milwaukee, WI) explains issues in technology selection and design consideration.
Process and technology selection
PTA plants typically generate a high volume of process emissions, in the range of 32,000 to 95,000 scfm (50,000 to 150,000 nominal m3/h). The Asian PTA plant where the VOC control system was installed averages 12 million Btu/h of gas flow. The main emissions are carbon monoxide, methyl bromide and various organic compounds. The high pressure exhausts from the PTA primary absorbers pass through an expander where the pressure is reduced to 4 psig. It is then further reduced to between 1-2 psig through a pressure reduction valve. The CO-rich stream, as well as other minor process streams, must be oxidized before exhausting to atmosphere.
Because of the high loadings of CO and the presence of methyl bromide (about 0.005 vol%), a catalytic recuperative oxidation system was selected, utilizing a specialized catalyst specifically designed by the Johnson Matthey Catalytic Systems Division for PTA plant exhausts. The catalytic recuperative oxidizer operates at 350 °C to oxidize CO and methyl bromide. The lower oxidation temperature of the catalytic recuperative oxidizer means lower fuel consumption which results in reduced operating costs.
Hydrogen Bromide Corrosion
The presence of methyl bromide in the emission stream presented additional design considerations. The catalyst oxidizes the methyl bromide to hydrogen bromide (HBr). If HBr drops below its dew point it becomes corrosive to the equipment, which is fabricated from Type 316L stainless steel. The equipment downstream of the catalyst must be engineered to avoid any "cool spots," where the hydrogen bromide can condense.
The heat exchanger is one potential cool spot. Without proper design, the heat exchanger could lower the temperature of the air exiting the catalytic oxidizer to below the dew point of HBr. To avoid the potential condensation of HBr and subsequent corrosion to the system, Anguil incorporated a steam preheater on the incoming process stream before the catalytic oxidizer.
The exhaust from the PTA plant is heated with a Type 316L stainless steel plate-and-frame steam preheater. The preheated process air then enters the tube side of the shell and tube heat exchanger. The increased temperature of the process air entering the tube side of the heat exchanger prevents the condensation of HBr on the shell side of the heat exchanger, a critical issue in the overall design of the system.
Varied Organic Loadings
The process exhaust from the PTA plant varies in organic loading; therefore, the catalytic oxidizer design must accommodate these varying levels with minimal use of auxiliary fuel. This is accomplished by utilizing a bypass on the 316L stainless steel shell-and-tube heat exchanger. Under low organic loading conditions, the heat exchanger bypass is closed so that the full effectiveness of the heat exchanger is available to preheat the incoming stream. At high organic loadings the outlet temperature of the catalyst is raised, resulting in a high preheat temperature from the heat exchanger. This high preheat temperature could cause overheating and shutdown of the system. The "hot side" heat exchanger bypass supplied by Anguil controls the preheat temperature from the heat exchanger, preventing any high-temperature conditions.
Catalyst Selection
The selection of the proper catalyst that would oxidize both the carbon monoxide and the methyl bromide at low temperatures was critical to the project. Historically, halogenated compounds, which include chlorine, bromine, iodine and fluorine, have had damaging effects on both noble metal and base metal oxidation catalysts. Several significant advances have occurred in recent years in catalyst technology, resulting in the development of catalysts suitable for the airstream under consideration.
A platinum/palladium-based catalyst (brandnamed as Johnson Matthey Halocat) deposited on a ceramic substrate was utilized in this design. Anguil's proprietary catalyst rack design, including specialty gasketing, eliminated the risk of gas bypassing the catalyst and subsequent incomplete
destruction.
Conclusion
This Anguil catalytic recuperative oxidation system went online in early 1999, and is currently operating and achieving the regulatory compliance demanded of the PTA plant. Operating costs are minimized because essentially no auxiliary fuel is needed. With the success of this system, the customer purchased an identical 90,000 SCFM unit and additional systems were installed for PTA manufacturers in the Middle East and Southeastern United States.
Company
Anguil Environmental Systems provides air pollution control solutions worldwide. Specializing in VOC, HAP and NOx control and abatement, Anguil utilizes its engineering expertise and over twenty years of experience to custom-design cost-effective, trouble-free, compliant air pollution control systems.
Edited by Nick Basta
About the author
Anguil Environmental Systems, Inc.s is a project coordinator at Anguil Environmental Systems, Inc.
For more information about this technology, conact:
Chris Anguil
Anguil Environmental Systems, Inc.
8855 North 55th Street, Milwaukee, WI 53223
Tel: 414-365-6400
Fax: 414-365-6410