Control of a multiunit heterogeneous azeotropic distillation process

An investigation of the design and control of a system in which a heterogeneous azeotrope permits the use of a decanter to cross a distillation boundary is reported. The specific numerical example is the classical production of anhydrous ethanol from an ethanol/water mixture using benzene as the entrainer. The feed is an ethanol/water mixture with composition that is near the binary azeotrope, which has been produced in an upstream binary distillation column. The process consists of two columns in which the addition of benzene takes the water out the top of the first column, producing high-purity ethanol out the bottom. The overhead vapor is condensed and forms two liquid phases. The organic benzene-rich phase provides reflux to the first column. The aqueous phase is fed to a second distillation column that produces high-purity water out the bottom and recycles the distillate back to the first column. The steady-state convergence of this two-column system with two-recycles if very difficult. Convergence is achieved by using a dynamic model. A plantwide control scheme is developed and is shown to provide stable base-level regulatory control for very large disturbances. The control structure has a counter-intuitive feature. The aqueous level in the decanter is controlled by manipulating the flow rate of the aqueous stream from the decanter. The action of the level controller is the reverse of what is used conventionally, that is, if the level is increasing, the exit flow rate is decreased! (c) 2005 American Institute of Chemical Engineers AIChEJ, 52: 623-637, 2006