Sectional and overall reachability for systems with S‐shaped distillation lines

Abstract

Total and minimum reflux bounds ser®e as limiting case scenarios for determining feasibility in azeotropic distillation columns. Specifically, all reachable feed compositions in a staged rectifying or stripping section ha®e been reported as the union of points enclosed by the distillation and pinch-point cur®es through a desired product. By eigen®alue considerations of pinch bifurcations and geometric analysis of fundamental region structures, the region of all reachable compositions has seemingly been identified. Additional insights are needed to fully characterize the feasible compositions in a subset of azeotropic mixtures. This article describes how sectional profiles can actually cross pinch-point cur®es and generate a region of additional feasibility outside the one pre®i-ously reported. It also demonstrates how a substantially larger region between the distillation line and pinch-point cur®e after the intersection is only accessible by composition profiles going through the extended region. This extended reachable region is explained by e®aluating the conditions for a fixed point in composition space. The effect on sectional profiles is illustrated for both an artificially parametric VLE manifold and a nonideal azeotropic system with more than one simple distillation region. Sectional profiles extended beyond the traditional bounds affect coupled sections in a distillation cascade. IntroductionThe preliminary step in process design is identifying types of equipment that might achieve a set of desired objectives. It is important that a complete list of alternative structures is identified in the synthesis step to ensure that all possible process configurations are considered for the final design. With this exhaustive set of alternatives in hand, each structure is then evaluated in terms of feasibility. This step asks whether a particular process configuration can meet the design goals Ž . that is, product specifications . The intent of a feasibility check is to quickly establish, without carrying out a detailed design, whether or not a process alternative has the capability to perform the desired task. Once the set of structures is narrowed down to a few feasible alternatives, a more in-depth design is necessary to determine which structure is most suitable in terms of economics, process control, and operability.Significant research efforts have focused on developing process synthesis methods and feasibility criteria for distilla-Ž . tion systems. Westerberg and Wahnschaft 1996 review colCorrespondence concerning this article should be addressed to S. Hauan. umn sequencing algorithms that have been established for separating multicomponent ideal mixtures. Synthesis tools for systems exhibiting azeotropic behavior are largely based on Ž the thermodynamic topological analysis Serafimov, 1987; . Hilmen, 2000 described in the Russian literature. Although this geometric approach has been generalized for n-component systems, most research efforts toward a systematic synthesis method for azeotropic mixtures have...

An Efficient Method of Constructing Pinch Point Curves and Locating Azeotropes in Nonideal Distillation Systems

“…Highly nonideal systems can exhibit more complex behavior. 25 1.2.2. Distillation Synthesis and Design.…”

“…Hoffmaster and Hauan 25 have reported the use of the software package AUTO97 54 (software for continuation and bifurcation analysis of ODEs; AUTO is currently in its 2007 incarnation, known as AUTO-07p) to solve the collinearity criterion for pinch points. The collinearity criterion is exactly equivalent to solving for dx i /dn = 0, in eq 3, since it simply means that the mixing vector and separation vector sum to zero.…”

“…The impact of design variables on the possible operation of a distillation column is undoubtedly an important consideration. Other workers − have done extensive work on operating leaves (sometimes also called distillation regions in the literature), on ways of extending or manipulating them, and on applying them to systematic column sequencing. The principle of operating leaves can be explained simply, as follows: for a fixed product composition in a ternary system, there are two boundaries that demarcate which compositions can be attained within a column section, one is at infinite reflux and the other is where the column pinches; both are at opposite ideal extremes of operation.…”

“…For relatively ideal systems, these two curves also intersect at another composition and create a leaf-shaped region in the composition space, hence the name “operating leaves”. Highly nonideal systems can exhibit more complex behavior …”