Composite Curing Oven and RTO for Adhesives and Sealants
Composite Curing Oven and RTO for Adhesives and Sealants
In the curing process of epoxy resins, polyurethane, phenol-formaldehyde resins, and other organic compounds, a significant portion of toxic VOCs are produced. The issue with VOCs rich flue gas treatment is complex with a wide range of compounds with different properties. Consequently, VOCs’ complete absorption from the flue gas is hardly feasible and extremely expensive since multiple liquid absorbents are need. Another issue with VOCs’ absorption is that those gases need to be desorbed from the liquid in the second treatment stage and then disposed of as toxic waste. Therefore, the most efficient method of VOCs treatment is oxidation under high temperatures – up to 1500°F to 1600° F for 99%+ VOCs removal, causing a significant increase in the plant’s variable costs and significantly increase energy demands.
Adhesives and sealants solutions must be heated to a particular temperature and uniformly mixed before being applied on the material’s surface. After their application, the bonding process occurs in curing ovens where temperatures are often greater than 200° F with retention times in the range of 15 minutes to 24 hours. For optimal and uniform bonds, it is essential to control the following process parameters accurately: temperature, atmospheric pressure, and retention time of each process stage, which requires real-time control with custom made software due to multiple transitions of process temperatures that should be executed in the shortest periods.
The Engineering team at Epcon Industrial Systems developed a patented solution for the composites industry to address these key operating challenges; The system is composed of the Regenerative Thermal Oxidizer (RTO), Secondary and Tertiary Heat Exchangers. The RTO is usually assembled of three chambers where flue gas enters alternately, designed for 1.0 second or more residence time. By using this airflow reversal method through the ceramic beds, a minimal amount of thermal energy is required to keep the incoming exhaust stream at the system’s minimum operating temperature. The ceramic media beds’ shape and sizing ensure a 95%+ heat recovery efficiency.
Based on the process parameters, such as volume flow of flue gas, VOCs’ concentration, and lower explosive limit, Epcon’s engineers custom design the Regenerative Thermal Oxidizer specifically to meet all requirements. This system is supplied with burners with more than a 20:1 turndown ratio and combustion air fans controlled via variable frequency drive (VFD). These features are essential for manufacturers with multiple production lines with different working parameters or one production line whose parameters may vary significantly, which is always the case with curing processes. Regardless of the flue gas stream temperature and volume flow changes, the combustion chamber’s set temperature will be achieved by automated combustion air and natural gas flow adjustment, meeting the desired excess air ratio and combustion efficiency at all times. The combustion chambers’ temperatures depend on the nature of used adhesive and sealants. They often may be up to 1500°F -1600° F to enable complete oxidation of VOCs, leaving a tremendous latitude to use exhaust stream as the heat source for plant processes.
The oxidized process exhaust air stream then exits the ceramic bed and enters an outlet manifold that supplies treated air stream to the indirect Secondary Heat Exchanger. This Secondary Heat Exchanger then transfers heat indirectly from the “clean” RTO outlet exhaust to a secondary process stream consisting of fresh air. The fresh air’s volume flow is controlled by variable frequency drive in real-time according to needs in the curing oven. Since there are two requirements for air entering the curing oven: the pressure and temperature, it is important to include another manipulated variable that will enable temperature control. Regulation of the exhaust stream volume flow allows the maintenance of the desired secondary process stream temperature, while the mixing of a hot by-pass stream with the RTO inlet stream significantly reduces fuel and combustion air requirements to maintain combustion chamber operating temperature. A booster burner is placed at the exchanger outlet to provide additional heat regulation.
The last step of the oxidized air treatment is entering the Tertiary Heat Exchanger fluidly connected to a water source conduit, a coil, and a heated water outlet conduit. The air-to-water heat exchanger recovers a significant portion of the remaining waste heat in the exhaust air stream by preheating the circulating water in the economizer coil. The heat absorbed by water may be sufficient in phenol-formaldehyde adhesive production, where phenol, formaldehyde, water, and catalyst should be heated to around 160°F. The heated water is often used to heat the adhesives or sealants batches indirectly.
Epcon Industrial Systems has innovated a custom-made control system enabling users to achieve a wide range of controlled variables’ set values while maintaining maximum heat efficiency at all times. The system’s dynamic state between steps is minimal due to specially tuned programmable logic controllers (PLC) that enable short settling time and fast achievement of a set point in a particular range that the customer specifies. The connected software enables users to define the set values in every process step a priori instead of manually entering the values during the process.
This unique system provides an optimal solution for the 99%+ removal of VOCs from the flue gas, while simultaneously, generating thermal energy for curing ovens and raising water temperature for heating adhesives or sealants chemical solutions. By integrating the process heating and gas treatment units into one process with remote control capabilities, allow for accurate control of the curing process. Using this system significantly reduces the plant’s operating costs in terms of natural gas and electricity demands due to highly efficient process integration and the optimal control system. Most importantly, this integrated system is designed to secure that gases discharged into the atmosphere meet stringent air quality standards at all times.