Simultaneous Optimal Design and Control of an Extractive Distillation System for the Production of Fuel Grade Ethanol Using a Mathematical Program with Complementarity Constraints

Abstract

The simultaneous optimal design and control problem of an extractive distillation system is studied using a rigorous model and solved when the feed is subjected to composition disturbances. First, the problem is formulated as a mixedinteger dynamic optimization (MIDO) problem, and then transformed into a mathematical program with complementarity constraints (MPCC) problem using orthogonal collocation on finite elements. The model allows simultaneous determination of the number of trays, feed tray locations, structural parameters of the column, heat exchanger areas, set points of both the controlled and manipulated variables, and the optimal control policy in order to obtain an optimal design which maximizes profit and also guarantees feasible dynamic operation. The study also demonstrates how the distillation column design problem can be addressed as a MPCC problem instead of as a mixed-integer nonlinear programming (MINLP) problem, containing thousands of continuous variables and constraints, and how can it be solved in reasonable times using state-of-the-art algorithms. The problem is modeled in GAMS and solved using IPOPT. Finally, results obtained by the simultaneous strategy are compared to those obtained by addressing the design and control problem sequentially.

Review of extractive distillation. Process design, operation, optimization and control

“…The non-linear dynamic optimisation problem that needs to be solved is very challenging (computationally speaking) and thus simplified models as well as efficient solving strategies are required. Ramos et al (2013Ramos et al ( , 2014 studied the optimal control profiles of the extractive dehydration of ethanol using glycerol as entrainer when the azeotropic feed was subject to step-wise and sinusoidal disturbances. The goal was to help tailoring nonlinear model-predictive control strategy implemented in a real production facility.…”

Optimization for the flexibility analysis of processes: Application to the acetone-ethanol-butanol producing process

“…For distillation column, we adapted MPCC dynamic model to steady-state operation presented by Ramos et al (2014). The MPCC model has at its core an equilibriumbased model, so-called the MESH model (Mass, equilibrium, fraction summations and enthalpy equations).…”

“…Thus, in a MINLP-modeling context this issue that could limit the procurement of optimal solutions. Recently, Ramos et al (2014) studied the optimal design and control of nonideal extractive distillation columns using a rigorous equilibrium model and developed an efficient Mathematical Program with Complementary Constraints (MPCC)-based methodology in order to deal with the discrete decisions related to column design. This MPCC model is used to study the flexibility of a three distillation columns sequence for the production of bioethanol by maximizing/minimizing the biobutanol throughput, subject to column structural and hydraulics constraints as well as purity constraints.…”