Residue curve

The dominance of distiHation-based methods for the separation of Hquid mixtures makes a number of points about RCM and DRD significant. Residue curves trace the Hquid-phase composition of a simple single-stage batch stiHpot as a function of time. Residue curves also approximate the Hquid composition profiles in continuous staged or packed distillation columns operating at infinite reflux and reboil ratios, and are also indicative of many aspects of the behavior of continuous columns operating at practical reflux ratios (12). 

Fig. 3. Combined residue curve and phase equilibria diagrams for the systems where Al—4 represent a2eotropes, (—) is the distillation boundary, the...

D/B = distillate-to-bottoms ratio, RCM = residue curve map, DRD = distillation region diagram, and MSA = mass separating agent. 

Residue curves can only originate from, terminate at, or be deflected by the pure components and azeotropes in a mixture. Pure components and azeotropes that residue curves move away from are called unstable nodes (UN), those where residue curves terminate are called stable nodes (SN), and those that deflect residue curves are called saddles (S). 

Fig. 3. Residue curve map for a ternary mixture with a distillation boundary mnning from pure component D to the binary azeotrope C.

Residue curve maps would be limited usebilness if they could only be generated experimentally. Fortunately that is not the case. The simple distillation process can be described (14) by the set of equations ... 

Even though the simple distillation process has no practical use as a method for separating mixtures, simple distillation residue curve maps have extremely usehil appHcations. These maps can be used to test the consistency of experimental azeotropic data (16,17,19) to predict the order and content of the cuts in batch distillation (20—22) and, in continuous distillation, to determine whether a given mixture is separable by distillation, identify feasible entrainers/solvents, predict the attainable product compositions, quaHtatively predict the composition profile shape, and synthesize the corresponding distillation sequences (16,23—30). By identifying the limited separations achievable by distillation, residue curve maps are also usehil in synthesizing separation sequences combining distillation with other methods. 

Residue curve maps exist for mixtures having more than three components but cannot be visualized when there are more than four components. However, many mixtures of industrial importance contain only three or four key components and can thus be treated as pseudo-temary or quaternary mixtures. Quaternary residue curve maps are more compHcated than thek ternary counterparts but it is stiU possible to understand these maps using the boiling point temperatures of the pure components and azeotropes (31). 

The second step is to sketch the residue curve map for the mixture to be separated. The residue curve map allows one to determine whether the goal can be reached and if so how to reach it, or whether the goal needs to be redefined. The addition of a separating agent to meet a separation objective carries with it the additional responsibiHty of finding an effective method for its recovery for reuse. 

As an example, consider the residue curve map for the nonazeotropic mixture shown in Eigure 2. It has no distillation boundary so the mixture can be separated into pure components by either the dkect or indkect sequence (Eig. 4). In the dkect sequence the unstable node (light component, L) is taken overhead in the first column and the bottom stream is essentially a binary mixture of the intermediate, I, and heavy, H, components. In the binary I—H mixture, I has the lowest boiling temperature (an unstable node) so it is recovered as the distillate in the second column and the stable node, H, is the corresponding bottoms stream. The indkect sequence removes the stable node (heavy component) from the bottom of the first column and the overhead stream is an essentially binary L—I mixture. Then in the second column the unstable node, L, is taken overhead and I is recovered in the bottoms. 

Fig. 5. The acetone—2-propanol—water system where I represents the 2-propanol—water azeotrope, (a) Residue curve map (34) (b) material balance lines...

The overwhelming majority of all ternary mixtures that can potentially exist are represented by only 113 different residue curve maps (35). Reference 24 contains sketches of 87 of these maps. For each type of separation objective, these 113 maps can be subdivided into those that can potentially meet the objective, ie, residue curve maps where the desired pure component and/or azeotropic products He in the same distillation region, and those that carmot. Thus knowing the residue curve for the mixture to be separated is sufficient to determine if a given separation objective is feasible, but not whether the objective can be achieved economically. 

All extractive distillations correspond to one of three possible residue curve maps one for mixtures containing minimum boiling azeotropes, one for mixtures containing maximum boiling azeotropes, and one for nonazeotropic mixtures. Thus extractive distillations can be divided into these three categories. 

Minimum Boiling Azeotropes. AH extractive distillations of binary minimum boiling azeotropic mixtures are represented by the residue curve map and column sequence shown in Figure 6b. Typical tray-by-tray composition profiles are shown in Figure 7. 

Fig. 10. Residue curve map for separating a maximum boiling azeotrope using a high boiling solvent where (-----------------) represents the distillation boundary and...

Fig. 11. Separation of nitric acid, HNO, and water, H2O, using sulfuric acid, H2SO4, as the solvent, (a) Residue curve map and material balance lines where...

As a starting point for identifying candidate solvents, all compounds having boiling points below that of any component in the mixture to be separated should be eliminated. This is necessary to yield the correct residue curve map for extractive distillation, but this process implicitly rules out other forms of homogeneous azeotropic distillation. In fact, compounds which boil as much as 50°C or more above the mixture have been recommended (68) in order to minimize the likelihood of azeotrope formation. On the other hand, the solvent should not bod so high that excessive temperatures are required in the solvent recovery column. 

Historically azeotropic distillation processes were developed on an individual basis using experimentation to guide the design. The use of residue curve maps as a vehicle to explain the behavior of entire sequences of heterogeneous azeotropic distillation columns as weU as the individual columns that make up the sequence provides a unifying framework for design. This process can be appHed rapidly, and produces an exceUent starting point for detailed simulations and experiments. 

Residue Curve Maps. Residue curve maps are useful for representing the infinite reflux behavior of continuous distillation columns and for getting quick estimates of the feasibiHty of carrying out a desired separation. In a heterogeneous simple distillation process, a multicomponent partially miscible Hquid mixture is vaporized ia a stiH and the vapor that is boiled off is treated as being ia phase equiHbrium with all the coexistiag Hquid phases. 

The vapor is thea withdrawa from the stiH as distillate. The changing Hquid composition is most coavenieafly described by foUowiag the trajectory (or residue curve) of the overall composition of all the coexistiag Hquid phases. An exteasive amouat of valuable experimental data for the water—acetoae—chloroform mixture, including biaary and ternary LLE, VLE, and VLLE data, and both simple distillation and batch distillation residue curves are available (93,101). Experimentally determined simple distillation residue curves have also been reported for the heterogeneous system water—formic acid—1,2-dichloroethane (102). 

Fig. 16. Residue curve map calculated for the ethanol—water—benzene mixture where A is the end poiat of the vapor line I represents a homogeneous...

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