Residue curve maps mixture

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

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. 

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. 

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. 

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. 

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...

Fig. 21. A selection of feasible residue curve maps for ternary heterogeneous mixtures where I represents homogeneous and heterogeneous...

More Complex Mixtures. AH the sequences discussed are type I Hquid systems, ie, mixtures in which only one of the binary pairs shows Hquid—Hquid behavior. Many mixtures of commercial interest display Hquid—Hquid behavior in two of the binary pairs (type II systems), eg, secondary butyl alcohol—water—di-secondary butyl ether (SBA—water—DSBE), and water—formic acid—xylene (92). Sequences for these separations can be devised on the basis of residue curve maps. The SBA—water—DSBE separation is practiced by ARGO and is considered in detail in the Hterature (4,5,105,126). 

Exploitation of Homogeneous Azeotropes Homogeneous azeotropic distillation refers to a flowsheet structure in which azeotrope formation is exploited or avoided in order to accomplish the desired separation in one or more distillation columns. The azeotropes in the system either do not exhibit two-hquid-phase behavior or the hquid-phase behavior is not or cannot be exploited in the separation sequence. The structure of a particular sequence will depend on the geometry of the residue curve map or distillation region diagram for the feed mixture-entrainer system. Two approaches are possible ... 

As mentioned previously, ternaiy mixtures can be represented by 125 different residue curve maps or distillation region diagrams. However, feasible distillation sequences using the first approach can be developed for breaking homogeneous binaiy azeotropes by the addition of a third component only for those more restricted systems that do not have a distillation boundaiy connected to the azeotrope and for which one of the original components is a node. For example, from... 

Unsaturation, in oils, 10 826 Unsensitization phenomenon, 19 237 Unshaped refractories, 6 491 Unslaked lime, 15 29 Unstabilized liquid sulfur trioxide, 23 517 Unstable angina, 5 109 Unstable flows, 11 761-765 Unstable node, in separating nonideal liquid mixtures, 22 303 Unstable nodes, residue curve maps, 8 790 Unstable reagents, measurement strategies for, 14 621... 

Process synthesis and design of these non-conventional distillation processes proceed in two steps. The first step—process synthesis—is the selection of one or more candidate entrainers along with the computation of thermodynamic properties like residue curve maps that help assess many column features such as the adequate column configuration and the corresponding product cuts sequence. The second step—process design—involves the search for optimal values of batch distillation parameters such as the entrainer amount, reflux ratio, boiler duty and number of stages. The complexity of the second step depends on the solutions obtained at the previous level, because efficiency in azeotropic and extractive distillation is largely determined by the mixture thermodynamic properties that are closely linked to the nature of the entrainer. Hence, we have established a complete set of rules for the selection of feasible entrainers for the separation of non ideal mixtures... 

Nitromethane shows the simplest residue curve map with one unstable curved separatrix dividing the triangle in two basic distillation regions. Methanol and acetonitrile give rise two binary azeotropic mixtures and three distillation regions that are bounded by two unstable curved separatrices. Water shows the most complicated residue curve maps, due to the presence of a ternary azeotrope and a miscibility gap with both the n-hexane and the ethyl acetate component. In all four cases, the heteroazeotrope (binary or ternary) has the lowest boiling temperature of the system. As it can be seen in Table 3, all entrainers except water provide the n-hexane-rich phase Zw as distillate product with a purity better than 0.91. Water is not a desirable entrainer because of the existence of ternary azeotrope whose n-hexane-rich phase has a water purity much lower (0.70). Considering in Table 3 the split... 

As shown in Fig. 2, the still path obtained experimentally (circles) is in excellent agreement with the still trajectory calculated by simulation. The selected reflux policy permits the still path to cross the separatrix into another distillation region than the initial feed region. Hence, the still path is able to reach the ethyl acetate vertex and this component remains pure into the still at the end of the distillate removal step. Such a behavior is not possible with a homogeneous entrainer that gives rise to a similar residue curve map because the distillation process is restricted to the distillation region where the initial composition of the mixture is located. In this case, ethyl acetate could not be obtained as an isolated product. 

S. Ung and M. F. Doherty, Calculation of residue curve maps for mixtures with multiple equilibrium chemical reactions. Ind. Engng. Chem. Res.,... 

The feasibility of separations of nonideal mixtures, as well as the screening of mass-separation agents for breaking azeotropes can be rationalized by means of thermodynamic methods based on residue curve maps. The treatment was extended processes with simultaneous chemical reaction. Two comprehensive books have been published recently by Stichlmair and Frey [10], as well as by Doherty and Malone [11]. 

Plotting residue curves maps (RCM) allows the designer to anticipate problems by the separation of nonideal mixtures, namely when dealing with homogeneous and heterogeneous azeotropes. By reactor selection, it may foresee problems incurred by the recycle of some reactants. 

A residue curve map (RCM) consists of a plot of the phase equilibrium of a mixture submitted to distillation in a batch vessel at constant pressure. RCM is advantageous for analyzing ternary mixtures. More exactly, a residue curve shows explicitly the evolution of the residual liquid of a mixture submitted to batch distillation. 

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