Application of an interval optimization method 
for studying feasibility of batch extractive distillation

Erika Frits*, Endre Rév*, Zoltán Lelkes*, Mihály Markot#, Tibor Csendes#

*Budapest Univ. Techn. Econ., Dept. Chem. Eng., 1521 Budapest
#Szeged University, Inst. Appl. Inf., 6701, Szeged

ABSTRACT

A short-cut methodology for studying feasibility of batch extractive distillation has recently been published and successfully applied by Dept. Chem. Eng., BUTE. The methodology involves numerical solution of first order differential equations. Series of phase curve maps are computed and displayed in function of operation parameters namely reflux ratio R and feed ratio F. Feasibility of the chemical process is limited to a narrow region of these two variables. This region is constrained by bifurcation of the map in function of the parameters. Minimum feed ratio Fmin at infinite reflux ratio R is an important value to determine; and minimum reflux ratio Rmin at a given F is an other one. These two data are enough to estimate the feasibility region and to start detailed study and design. However, determination of these important parameters is tedious with the given methodology. The singular points, especially saddle points, cannot always be determined with satisfactory precision; moreover, some details of the map are missed because unstable nodes are not searched for (saving time), and singular points out of a physically interpretable region are not studied. The bifurcation cannot always be recognized because the computed phase curve maps are not detailed enough. 

Looking for a more reliable methodology, a global and reliable interval optimization tool with Branch and Bound technique, developed at Inst. of Appl. Informatics, Szeged Univ., and based on C++ toolbox of Hammer R., 1994, and the Profil/Bias routine library, is tried and successfully applied. The new methodology verified some already known singular point paths and bifurcations, but also discovered new paths and bifurcations hitherto hidden. Phase curve maps and singular point paths, together with their interpretation will be presented.

The results obtained till now with the new tool give hope for extending its application to direct search for the bifurcation points, and constructing a full bifurcation plot that would provide much more information on the feasibility region than earlier we had. These expected results will contain information useful for optimal preliminary design of the process.