Distillation regions at infinite reflux

The original oversimplified view on feasible azeotropic mixtures splits consists of the following the feed point and product points have to belong to one distillation region (xd e Reg° and Xb e Reg°° if Xp e Reg° ). This view is quite accurate if the separatrix of distillation regions is linear. In a general case, at curvilinear separatrixes, the feed point can lie in one distillation region at infinite reflux and... 

The region of two-directed distillation (pitchfork region Regpuch) is located between the hne of the best product compositions (pitchfork distillation boundary) and boundary of distillation regions at infinite reflux Reg°° (Castillo et al., 1998 Davydyan, Malone, Doherty, 1997 Wahnschafft et al., 1992). 

Rule of connectedness condition satisfied to product points at R = oo and N = oo. The stable node of the top product boundary element of distillation region at infinite reflux Reg and the unstable node Njj of the bottom product boundary element of distillation region at infinite reflux Reg should coincide (N = iVjj), or should be connected with each other by the bond (N — Nj ) or chain of bonds in direction to the bottom product. 

Feasible separation alternatives identified at total reflux remain valid for finite reflux. However, if the distillation boundary at infinite reflux has a strong curvature, it can be crossed at finite reflux (Wahnschafft and Westerberg, 1992). This means that the top and bottom products may belong to different residue curves in different distillation regions. Thus, it seems that we might speak about an effective distillation border that is a function of reflux. There is no simple way to predict it, but its extent can be framed by simulation. This topic will be illustrated in the next example. 

As before, tracing the DCM of a ternary mixture involving azeotropes may lead to one of more distillation regions. Numerical investigation demonstrates that distillation and residue curves are, in general, close to each other. In fact, both are related with the variation of concentration in a distillation column operated at infinite reflux, RCM for a packed column, and DCM for a frayed column. 

In literature, several different terms for distillation regions at the infinite reflux are used simple distillation regions, basic regions of distillation, and regions of closed distillation. We use a longer but more exact term - distillation region at the infinite refiux (for the sake of briefness, we sometimes use just distillation region -... 

Let s examine three-component azeotropic mixtures with one binary azeotrope and with two regions of distillation at infinite reflux Reg°° (Fig. 3.6a). There is some region (triangle to the right of separatrix) where two points of the bottom product corresponding to one top product point exist. This fact is explained by the 5-shape of c-hnes in this region (Fig. 3.6b, points xb(2) and xb(3>). 

Trajectories of adiabatic distillation at finite reflux for given product points should be located in concentration space in the region limited by trajectories at infinite reflux and by trajectories of reversible distillation (Petlyuk, 1979 Petlyuk Serafimov, 1983). 

For azeotropic mixtures, the main difficulty of the solution of the task of synthesis consists not in the multiplicity of feasible sequences of columns and complexes but in the necessity for the determination of feasible splits in each potential column or in the complex. The questions of synthesis of separation flowsheets for azeotropic mixtures were investigated in a great number of works. But these works mainly concern three-component mixtures and splits at infinite reflux. In a small number of works, mixtures with a larger number of components are considered however, in these works, the discussion is limited to the identification of splits at infinite reflux and linear boundaries between distillation regions Reg° . Yet, it is important to identify all feasible splits, not only the spUts feasible in simple columns at infinite reflux and at linear boundaries between distillation regions. It is important, in particular, to identify the spUts feasible in simple columns at finite reflux and curvilinear boundaries between distillation regions and also the splits feasible only in three-section columns of extractive distillation. 

Reg°°) region of concentration simplex filled with one bundle of distillation lines (residue curves) at infinite reflux (N => N+). 

But by analogy with extractive distillation, it can be expected that a second feed point would drastically widen the product region at a finite reflux ratio and thus also increase the conversion. Between the two feed points, the column profile is perpendicular to the distillation lines (Fig. 2.5). Since this effect is based on the finite nature of the reflux ratio employed, we can expect product purity and conversion to first increase with an increase in the reflux ratio and then slowly decrease again. The limiting value that is established for an infinite reflux ratio is determined by the azeotrope concentration in the methyl acetate/methanol system. 

At some value of parameter (L/ trajectory bundles of sections Reg and Regi adjoin each other by their boundary elements - separatrix min-reflux regions Reg , s (5 => A+, shortly 5 - A+) and Re = (S N, shortly - N+), mostly remote from product points xd and xb, if one uses for determination of (L/ y) " the model in Fig. 5.29b, or if validity of condition (Eq. [5.18]) is achieved between some points of these boundary elements, if one uses the model in Fig. 5.29a. At this value of the parameter (L/F)J ", the distillation process becomes feasible in infinite column at set product compositions. Such distillation mode is called the mode of minimum reflux. It follows from the analysis of bundle dimensionality 5 - 1V+ and that, at separation without distributed... 

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