Closing the loop on stoichiometric balance: A VLE-based feedforward strategy for extractive–reactive distillation

Reactive–extractive distillation (RED) refers to a class of intensified reaction and separation systems while one of its potential industrial applications is constrained by the difficulty of maintaining stoichiometric ratio balance under feed disturbances. This work proposes a modified extractive–reactive distillation (ED–RD) system for the separation of a tetrahydrofuran (THF)/ethanol (EtOH)/water (H 2 O), together with an advanced thermodynamics-based feedforward control framework. By maximizing THF recovery to 99.99 mol.% in the extractive distillation column (EDC), the reactive distillation column (RDC) is reduced to a ternary system, enabling a control formulation derived directly from vapor–liquid equilibrium (VLE) fundamentals rather than empirical correlations. A VLE-based calculator is developed to infer real-time H 2 O molar composition from temperature shifts induced by infinitesimal pressure and solvent variations. This enables thermodynamics-based estimation of key stoichiometric variables without online composition analyzers. Dynamic simulations show that the proposed control structure (CS3) maintains EtOH purity at approximately 99.6 mol.% under ± 10% feed throughput and composition disturbances while eliminating expensive composition analyzers. To address transient mismatch arising from stage-wise transport dynamics, a synchronization mechanism is introduced in the solvent recycle loop, improving consistency during disturbance propagation and strengthening stoichiometric regulation. Although the modified ED–RD comes at the expense of a slightly higher total annual cost (TAC), it delivers significantly improved dynamic robustness where conventional temperature controller (TC)-only control structures fail. A holistic quantitative comparison of the proposed control structures is presented, followed by discussion of the framework’s industrial applicability, limitations, and potential extension to other distillation systems.