NGL Demethanizer Control

The recovery of valuable heavier hydrocarbons from natural gas is accomplished in a series of distillation columns. The first column is a cryogenic high-pressure column in which the methane is taken overhead and the ethane and heavier hydrocarbons are produced as a bottoms product called “natural gas liquid” (NGL). This demethanizer column uses compression and expansion to achieve the required low temperatures (190 K) for achieving liquids at the operating pressure of 25 atm. The column has a complex configuration of side-reboilers that use the ambient-temperature natural-gas feed stream to partially reboil the column and precool the feed. Additional feed precooling occurs using the cold methane gas product stream from the top of the column and using an external refrigerant. Finally a portion of the cold feed is expanded in a turbine and fed near the top of the column while the remainder is flashed and fed to the top tray as reflux. The system has many components and is extensively energy integrated. This paper studies the dynamic control of this complex interacting system. An effective control structure is developed and tested using conventional PI controllers.