Amine Thermal Degradation
April 8, 2008
Process engineers often express concern about amine reboiler temperatures being high enough to cause thermal degradation of the amine. However, thermal degradation is generally not a concern in amine reboilers heated with steam or heat transfer fluids.
In “DEA degradation mechanism,” Hydrocarbon Processing, October 1982, A. Meisen and M. L. Kennard discuss the fact that DEA and MDEA thermal degradation is minimal up to 400°F. Although the degradation of DEA is caused by reaction with CO2 and not temperature alone, temperature does affect the rate of degradation caused by reaction with CO2. This reference states: “Degradation increases strongly with temperature. This is not due to thermal breakdown of DEA, but it requires the presence of carbon dioxide. Design and operation of DEA units must avoid creation of elevated temperature throughout the plants. Heat transfer surfaces of DEA stripper-reboilers (especially when gas fired) are particularly prone to formation of localized hot spots. To prevent such hot spots in operating plants, DEA circulation through the stripper-reboiler should be kept high and steam (or gas) temperature kept low. In many DEA units only the bulk solution temperatures are measured. It must be remembered that the skin temperatures of heat transfer surfaces can be very much higher, particularly during process upsets. Reliance upon bulk temperatures is therefore inadequate.”
In the paper “Reduce amine plant solvent losses, Part 2” from Hydrocarbon Processing, June 1994, E. J. Stewart and R. A. Lanning mention that the thermal degradation of amines accelerates above 350°F, so the skin temperature of direct fired reboilers should be kept below 350°F. They recommend a reboiler operation with an amine bulk temperature below 260°F. This reference goes on to say: “With hot oil and steam heating systems, risk of thermal degradation is low since the heat media is usually not operated at high temperature. However, in fired-reboiler operation, the temperature of amine on the tube’s surface can easily exceed 350°F. In fired reboilers, forced circulation is often used to maintain low skin temperatures. The rule of thumb is to maintain amine skin temperatures between 300°F and 325°F, and not exceed 350°F. for these temperatures a conservative design heat flux of less than 8000 Btu/ft2 of tube area is recommended.”
Other references indicate 400°F as the thermal degradation temperature of MEA. Keep in mind that the reboiler temperature is set by the stripper operating pressure. To reduce the reboiler temperature, the stripper pressure can be reduced.
Authored by Lili Lyddon (BR&E Technical Support and Help Author)