Distillation lines

A more complex distillation line map is shown in Figure 12.7b. This involves two binary azeotropes. The closeness of the dots on a distillation line is indicative of the difficulty of separation. As an azeotrope is approached, the... 

The distillation lines in the distillation line map were in this case developed by carrying out a balance around the bottom of the column, as indicated in Figure 9.13. Equally well, the distillation line could have been developed by drawing an envelope around the top of the column at total reflux, and the calculation developed down the column in the direction of increasing temperature. 

Figure 12.11 superimposes distillation lines and residue curves for the same ternary systems. Figure 12.11a shows the system n-pentane, n-hexane and n -heptane, which is a relatively wide boiling mixture. It can be observed in Figure 12.11a that there are significant differences between the paths of the distillation lines and the residue curves. By... 

Figure 12.11 Comparison of distillation lines and residue curves.

This time, the distillation lines and residue curves follow each other fairly closely because the difficult separation means that the changes from stage to stage in a staged column become smaller and approach the continuous changes in a packed column. It is important to note that distillation lines and residue curves have the same properties at fixed points (when the distillation lines and residue curves converge to a pure product or an azeotrope). 

The rectifying or stripping section of a column must operate somewhere between total reflux and minimum reflux conditions. The range of feasible operation of a column section can thus be defined for a given product composition. It can be seen in Figure 12.19 that these section profiles are bounded for a stage column by the distillation line and the pinch point curve. As noted previously, the pinch point curve provides a minimum reflux boundary for both staged and packed columns,... 

Also shown in Figure 12.19 is the residue curve projected from the same product composition. The area enclosed within the residue curve and the pinch point curve thus provides the feasible compositions that can be obtained by a packed column section from a given product composition. For any given product composition, the operation leaf of feasible operation for a column section can be defined by plotting the distillation line (or residue curve) and the pinch point curve1314. The column section must operate somewhere between the total and minimum reflux conditions. 

Figure 12.19 The operation leaf is bounded by the distillation line or residue curve and the pinch point curve. (From Castillo F, Thong DY-C, Towler GP, 1998, Ind Eng Chem Res, 37 987 reproduced by permission of the American Chemical Society).

All of the discussions so far regarding distillation lines, residue curves and distillation boundaries have assumed equilibrium behavior. Real columns do not work at equilibrium, and stage efficiency must be accounted for. Each component will have its own stage efficiency, which means that each composition will deviate from equilibrium behavior differently. This means that if nonequilibrium behavior is taken into account, the shape of the distillation lines, residue curves and distillation... 

Thus, while it is possible in theory to cross a curved distillation boundary as shown in Figure 12.35, it is generally more straightforward to follow designs that will be feasible over a wide range of reflux ratios and in the presence of uncertainties. Such designs can be readily developed using distillation line and residue curve maps. 

Thus, distillation line and residue curve maps are excellent tools to evaluate feasibility of azeotropic separations, with just one exception, namely, the use of high-boiling entrainers for separation. In such cases, the equi-volatility curves discussed in this chapter are a better way of determining separation feasibility. 

Sketch the distillation line map (residue curve map) for the system ethanol-ethyl acetate-methanol at 1 atm and 5 atm from the data in Table 12.1. Does the system have a distillation boundary Is the position of the boundary sensitive to pressure ... 

Me(Ph)SiCl2 reacts with dimethyl phosphonate in a 1 1-mixture at 100°C to the O.O-disilylated phosphonate. In contrast to this observation the O-alkyl O-silyl phosphonate is formed solely by reaction of both compounds in acetonitrile at 80°C (line 7,8). An excess of the corresponding phosphonate as polar solvent leads to the mixed ester 2 only. The excess of the dialkyl phosphonate is removed from the mixture by vacuum distillation (line 9). 

Distillation into curved boundary, general separation heuristics for, 22 318 Distillation lines (residue curve maps), 8 790-793... 

Pelkonen S, Kaesemann R, Gorak A. Distillation lines for multicomponent separation in packed columns—theory and comparison with experiment. Ind Eng Chem Res 1997 36 5392-5398. 

Similarly, a distillation line map (DLM) shows the distribution of liquid composition on the stages of a continuous distillation column at infinite reflux and for infinite number of stages. DLM is obtained even simpler by computing successive dew and bubble points as described by the relation ... 

Figure A.2 Construction of the distillation lines for nonreactive (left) and reactive mixtures (right).

A similar representation is based on distillation tines [1], which describe the composition on successive trays of a distillation column with an infinite number of stages at infinite reflux (°°/°° analysis). In contrast with relation (A.8) the distillation lines may be obtained much easier by algebraic computations involving a series of bubble and dew points, as follows ... 

Figure A.2 (left) shows the construction of a distillation for an ideal ternary system in which A and C are the light (stable node) and the heavy (unstable node) boilers, while B is an intermediate boiler (saddle). The initial point xiA produces the vapor y, that by condensation gives a liquid with the same composition such that the next point is xi 2 = y,, etc. Accordingly, the distillation line describes the evolution of composition on the stages of a distillation column at equilibrium and total reflux from the bottom to the top. The slope of a distillation line is a measure of the relative volatility of components. The analysis in RCM or DCM leads to the same results.

Figure 2. (a) Internal profile in the eomposition simplex eorresponding to the base design with phase separator and water-rieh phase reeyele (Flysys) (b) Distillation lines eorresponding to the distillate eomposition of the methanol eolumn. System methanol/ethanol/water+l-pentanol at 101.3 kPa (DISTIL). 

Figure 9.1 Residue curves and distillation lines analogy...

Schemes 16.4a, b, and e do not suffer from the above instability. This is particularly true for schemes 16.4a and e, where reflux flow is maintained steady by a flow controller. In the Fig. 19.3 arrangement, these schemes can be configured with either the distillate or the reflux on overflow. If reflux is an overflow, the reflux flow controller manipulates a valve in the distillate line (e.g., Fig. 19.4a).

At the same time, the transformation eliminates the reaction term in the balance equation. The operating line for the rectifying section of a reaction column is formally identical to the operating line of a non-reactive column. An infinite reflux ratio gives an expression that is formally identical to the one for calculating conventional distillation lines [5, 6]. Accordingly, we will refer to lines that have been calculated by this procedure as RD lines. These analogies are found for all the relationships that are important in distillation [7, 8]. 

These analogies become particularly clear if we look at the synthesis of methyl acetate (MeAc) from methanol (MeOH) and acetic acid (HAC) as an example. Essentially, we see diagrams that are similar to the distillation line diagrams of non-reactive systems. As a result of the transformation, the four pure substances lie at the comers of a square and the non-reactive binary systems lie along the edges. 

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. 

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