(423c) Optimal Design of Energy-Efficient Integrated Distillation Processes for Multicomponent Ideal and Non-Ideal Mixtures by Use of the Vmin-Diagram

Abstract for AIChE
Annual Meeting 2011, separations division

Optimal
design of energy-efficient integrated distillation processes for multicomponent
ideal and non-ideal mixtures by use of the Vmin-diagram.

I.J. Halvorsen¹,
S. Skogestad², I. Dejanović³, ?. Olujić⁴

¹SINTEF ICT, Applied Cybernetics, Trondheim, Norway, ²Norwegian
University of Science and Technology, Department of Chemical Engineering,
Trondheim, Norway

³University of Zagreb, Department of Chemical
Engineering and Technology, Zagreb, Croatia

⁴Delft University of Technology, Process & Energy
Laboratory, Delft, The Netherlands

The dividing wall
distillation columns (DWC) find increased use in industrial practice. The
benefits are the combined savings in both energy and capital cost. Most of the
industrial applications are three-product columns, but also 4-product DWCs have
been constructed. A simulation study has shown that a four-product DWC with
multiple partition walls for an aromatics feed can save up to 50% energy
compared to conventional sequences. The internal arrangement is more complex
and it is important to predict the necessary internal flow rates both for
making proper sizing of the internals and also for control structure design.
Thus, there is a need to focus on development of simple-to-use methods for
making assessment on how complex columns like the DWC will perform for any
given application. This paper will demonstrate the usability of the Vmin-diagram
method for quick assessment of column configurations for multicomponent separation.

The Vmin-diagram is
obtained from feed data only, and gives a direct picture of the minimum vapor
requirement for any specification of product splits. It is particularly well
suited for fully thermally coupled distillation configurations like the generalized
Petlyuk arrangements and dividing wall columns. The Vmin-diagram was first
presented by Halvorsen and Skogestad (2003). The deduction was based on the
simplifying assumptions of constant molar flows, constant pressure and constant
relative volatility and then the Underwood equations were used to formulate
analytical expressions for calculation of any property of the system. The
concept, however, is by no mean limited to ideal assumptions and the Underwood
equations. We show how the Vmin-diagram can be made by a few simple simulations
on any commercial process simulator.

The construction and interpretation
of the diagram and usage for design of DWC columns will be presented in some
detail. Some typical design questions that can be answered are:

What is the minimum
vapor flow in a DWC for a set of feed conditions?

How shall the internal
flows be distributed?

How can the operating
conditions be chosen for the various sections of a DWC column?

What kind of
flexibility can be expected, for example what kind of feed variations can be
handled and what is the tolerance for operational parameters like the vapor
split and the liquid split?

The intention is to
provide a versatile and reliable tool to simplify the process of designing and
operating DWC columns and thereby lower the threshold for industrial use of
this energy- and capital-saving technology.