Extending McCabe-Thiele diagrams to multicomponent distillation systems

Abstract

A novel graphical technique for the preliminary design of multicomponent distillation systems has been developed in this work. The graphical technique is akin to McCabeThiele diagrams for binary distillation systems. The method is presented in a twodimensional (2D) scheme and is simple, intuitive and non-iterative in nature, hence retaining all the inherent characteristics of the McCabe-Thiele method.

Several graphical techniques have been developed for the preliminary design of multicomponent distillation systems in 2D. The methods have started with the fundamentals of the binary McCabe-Thiele method but have lost vital information inherent in their simplifying assumptions. The novel graphical technique presented in this work is not limited by simplifying assumptions, and retains the simplicity and inherent elements of the binary McCabe-Thiele method as well as visually depicting the behaviour of all the components in the system.

This work postulates a novel approach to calculating and depicting the equilibrium relationship between vapour and liquid compositions as a series of contours of vapourliquid equilibrium (VLE) allowing designers and students alike to analyse systems behaviour as well as determine preliminary design variables in order to initialize rigorous process simulations.

The method of plotting the contours of VLE is presented such that the behaviour of each component in a system can be analysed in order to identify if a separation would be simple or difficult, and contain azeotropes and tangent pinches. The graphical technique is applied to ternary systems to develop a method of calculating the minimum number of stages at total reflux for both ideal and non-ideal systems.

A simple graphical method is proposed to allocate the minimum reflux for ternary ideal and non-ideal systems for transition, sharp direct and indirect splits using the transition point theory presented by Stichlmair (2005). Once the minimum reflux is calculated, the method of determining the number of theoretical stages and feed stage location is unpacked. The method is simple and can be applied to both ideal and nonideal systems. The preliminary design parameters obtained from the graphical technique were initialised and converged as a feasible column design using Aspen PlusTM. The graphical method has also been extended to the design of quaternary systems. A general method has been presented for the design of higher order systems for feed stage location although this work depicts applications to systems with less than five components.

Finally, the finite reflux graphical method was applied to determine design parameters for ternary reactive-hybrid distillation processes. The notion of reactive difference point cascades presented by Lee et al., (2000) was extended to ternary reactive-hybrid columns for ideal and non-ideal systems such that the powerful visual insights of nonreactive distillation processes are retained. Using the reactive difference point cascades, a simple graphical technique is derived to determine design parameters for different reactive-hybrid distillation column configurations. Particular attention to columns with reaction on the feed stage and in the rectifying section only is analysed. In addition, the method of determining design parameters for reaction in the rectifying section and feed stage as well as stripping section only is presented. Besides reaction on the feed stage, the other column configurations have been applied only to ideal systems with no net change in the number of moles. The design parameters obtained from the graphical technique for reaction on the feed stage were initialised and produced a feasible design that correlated with simulations on Aspen PlusTM for a nonideal system with a net change in the number of moles.

The novel graphical method in this thesis are practical for early stage design such that rigorous simulators can be initialised with the design parameters obtained. The graphical method is applicable to reactive and non-reactive systems.