Distillation boundaries crossing

Selection of an entrainer such that some type of distillation boundary-crossing mechanism is employed to separate desired products that lie in different regions. 

Figure 5.21 shows that the distillation boundary has indeed been crossed (only Just) by merely operating at a finite reflux value (note X/> and lie on opposite ends of the distillation boundary). Wahnschafft and coworkers have attempted to explain this phenomenon and have concluded for distillation boundary crossing to be possible that (1) the distillation boundary has to display considerable curvature, (2) a distillate or bottoms composition has to close to or on the concave side of the boundary, and (3) the distillation column needs to operate at a certain range of reflux ratios for it to be feasible as over-refluxing the column will cause the topology to resemble that of the RCM, hence rendering the product specifications to be infeasible. 

Table 3 contains strategic separations to be considered for crossing distillation boundaries. Many of these can be eliminated after examining the pertinent physical properties and equiUbrium behavior (see Table 4) and referring to the general separation considerations. The results are summarized in Table 5. 

The feed compositions and products of each of these strategic separations remain ill-defined. The unspecified 2-propanol—water mixture, the input to each strategic separation, could be but is not necessarily the original feed composition. The MSA composition (pure hexane in this case) is such that one of the products of the strategic separation is in region II, ie, the strategic separation crosses the distillation boundary. Two opportunistic distillations from... 

The strategy for boundary crossing has been implemented however, by the addition of the hexane another critical feature has been created. Hexane must be regenerated, but it is in a different distillation region than the only remaining unprocessed stream (Ml). In this case the possible boundary crossing strategic operations are Mixer 6 and Decanter 7. Two opportunistic distillations. Fractionators 8 and 9, can also be appHed to Ml (decantation is also a possible opportunistic separation). 

In one possible sequence the MSA composition is chosen as water-saturated methylene chloride expected to be regenerated by decantation. The boundary-crossing strategic operation is to mix the feed with the MSA. The resulting two-phase mixture is opportunistically fractionated to produce the 2-propanol product as bottoms, and a mixture of water—methylene chloride as distillate. This distillate is opportunistically decanted to recover water-saturated methylene chloride MSA for recycle. The aqueous decanter phase is the water product, which optionally may be further purified by... 

Distillation boundaries for continuous distillation are approximated by simple distillation boundaries. This is a very good approximation for mixtures with nearly linear simple distillation boundaries. Although curved simple distillation boundaries can be crossed to some degree (16,25—30,32,33), the resulting distillation sequences are not normally economical. Mixtures such as nitric acid—water—sulfuric acid, that have extremely curved boundaries, are exceptions. Therefore, a good working assumption is that simple distillation boundaries should not be crossed by continuous distillation. In other words, for a separation to be feasible by distillation it is sufficient that the distillate and bottoms compositions He in the same distillation region. 

Exploitation of Boundary Curvature A second approach to boundaiy crossing exploits boundaiy curvature in order to produce compositions in different distillation regions. When distillation boundaries exhibit extreme curvature, it may be possible to design a column such that the distillate and bottoms are on the same residue curve in one distillation region, while the feed (which is not required to lie on the column-composition profile) is in another distillation region. In order for such a column to meet material-balance constraints (i.e., bottom, distillate, feed on a straight hne), the feed must be located in a region where the boundary is concave. 

One extremely powerful feature of heterogeneous distillation is the ability to cross distillation boundaries. It was noted previously that distillation boundaries divide the compositions into two regions that cannot be accessed from each other. Decanters allow distillation boundaries to be crossed, as illustrated in Figure 12.32. The feed to the decanter at F is on one side of the distillation boundary. This splits in the decanter to two-liquid phases E and R. These two-liquid phases are now on opposite sides of the distillation boundary. Phase splitting in this way is not constrained by a distillation boundary, and exploiting a two-phase separation in this way is an extremely effective way to cross distillation boundaries. 

Figure 12.32 Phase splitting can be used to cross distillation boundaries.

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. 

Some systems form two-liquid phases for certain compositions and this can be exploited in heterogeneous azeotropic distillation. The use of liquid-liquid separation in a decanter can be extremely effective and can be used to cross distillation boundaries. 

The pure products A and B are separated by a distillation boundary and belong to different distillation fields. In this case, the distillation boundary must be sufficiently curved, such that one split can cross it and gives opportunities for separating the other pure component. 

The selection of an entrainer with boundary crossing is based on the observation that in a RCM both A and B must be nodes, stable or unstable. By consequence the pure components are separated either as overhead or bottom products. Table 3.17 gives a list of recommended heuristics [13, 14]. In all cases, tbe distillation boundary must be highly curved, although how curved cannot be specified theoretically. In this case the simplest entrainer choice is a low-boiler entrainer for minimum AB azeotrope, and a high-boiler entrainer for maximum AB azeotrope, again not easy to meet in practice. 

Distillation systems for the separation of nonazeotropic mixtures are discussed in this subsection. Many of the results extend also to azeotropic mixtures when the desired splits do not attempt to break azeotropes or cross a distillation boundary. 

Since residue curves do not cross simple batch distillation boundaries, the distillate and bottoms compositions must be in the same distillation region with the mass balance line intersecting a residue curve in two places. Mass balance lines for mixing and for other separations not involving vapor-liquid equilibria, such as extraction and decantation, are of course not limited by distillation boundaries. 

Fresh aqueous ethanol feed is first preconcentrated to nearly the azeotropic composition in column C3, while producing a water bottoms product. The distillate from C3 is sent to column Cl, which is refluxed with the entire organic (entrainer-rich) layer, recycled from a decanter. Mixing of these two streams is the key to this sequence as it allows the overall feed composition to cross the distillation boundary... 

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