Digital Valve Positioner Helps Stabilize Air Separation Plant

A modern digital electro-pneumatic positioner from ABB minimizes flowrate swings of rich-liquid vapor at an air separation plant in Louisiana. The new positioner replaces one installed on the actuator of a 20-inch butterfly valve. Tighter control of flowrate has helped to stabilize the sensitive cryogenic distillation process, resulting in increased argon production.

The plant employs cryogenic fractional distillation to separate air into its major components: nitrogen (78%), oxygen (21%), argon (0.96%). Distillation takes advantage of the different boiling points of the three elements (N-77.36K, O-90.2K, and Ar-87.15K). According to the plant manager, airflow through the plant is 4.2 million SCFH.

Prior to distillation, the plant first purifies and dries the air to remove contaminants that would become solids at cryogenic temperatures, such as moisture, carbon dioxide, and residual hydrocarbons. A compressor takes the air to about 75 psi, most of which serves as the input to the distillation process. A small sidestream of about 0.5 million SCFH experiences further compression to 110 psi. It subsequently expands to 7 psi to provide cryogenic refrigeration for the distillation columns.