Control Structures Evaluation for a Salt Extractive Distillation Pilot Plant: Application to Bio-Ethanol Dehydration

Abstract

This paper addresses the challenge of evaluating control structures for a salt extractive distillation column producing absolute ethanol for use as biofuel. A sensitivity analysis aided with designing a pseudo-binary distillation pilot plant and examining the conceived process and the influence of the reflux ratio on both product purity and energy consumption. We compare three control structures for inferential tracking of the distillate composition: a dual-temperature control with an RV (reflux/boilup) structure and two single-end temperature control configurations, and their performance is measured using deterministic indicators. The result is the proposal of a pilot plant design for treating 15 kg/h of a diluted mixture with mole fraction of ethanol equal to 0.2 and assuming a column efficiency of 50%. The R/F (reflux to feed ratio) configuration is the best control structure, given that its corresponding performance indicators conduct lowest steady-state errors, less oscillating responses, and reduced settling times. For this configuration, the reflux flow rate is rationed to the feed flow rate, and the temperature is controlled manipulating the distillate flow rate. Even subject to perturbations, the energy consumption of the plant remains close to the nominal value. The three evaluated control structures consistently met international quality standards for fuel ethanol and enhanced the use of salts in ethanol dehydration.

Keywords

• absolute ethanol
• bio-fuel
• salt extractive distillation
• distillation control