Azeotropic feasibility

In the first class, azeotropic distillation, the extraneous mass-separating agent is relatively volatile and is known as an entrainer. This entrainer forms either a low-boiling binary azeotrope with one of the keys or, more often, a ternary azeotrope containing both keys. The latter kind of operation is feasible only if condensation of the overhead vapor results in two liquid phases, one of which contains the bulk of one of the key components and the other contains the bulk of the entrainer. A t3q)ical scheme is shown in Fig. 3.10. The mixture (A -I- B) is fed to the column, and relatively pure A is taken from the column bottoms. A ternary azeotrope distilled overhead is condensed and separated into two liquid layers in the decanter. One layer contains a mixture of A -I- entrainer which is returned as reflux. The other layer contains relatively pure B. If the B layer contains a significant amount of entrainer, then this layer may need to be fed to an additional column to separate and recycle the entrainer and produce pure B. 

The majority of successful processes are those in which the entrainer and one of the components separate into two Hquid phases on cooling if direct recovery by distillation is not feasible. A further restriction in the selection of an azeotropic entrainer is that the boiling point of the entrainer is 10—40°C below that of the components. 

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. 

The first step in the synthesis of a homogeneous azeotropic distillation sequence is to determine the separation objective. Eor example, sometimes it is deskable to recover all of the constituents in the mixture as pure components, other times it is sufficient to recover only some of the pure components as products. In other cases an azeotrope may be the desked product. Not every objective is attainable and those that are feasible may requke different distillation sequences. 

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. 

Sy.stem visualization. Location of distillation boundaries, azeotropes, distillation regions, feasible products, and liquid-hquid regions. 

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... 

FIG. 13-64 Feasible distillation region diagrams for breaking homogeneous binary azeotrope A-B, a) Low-boiling entranoes. 

The transformed variables describe the system composition with or without reaction and sum to unity as do Xi and yi. The condition for azeotropy becomes X, = Y,. Barbosa and Doherty have shown that phase and distillation diagrams constructed using the transformed composition coordinates have the same properties as phase and distillation region diagrams for nonreactive systems and similarly can be used to assist in design feasibility and operability studies [Chem Eng Sci, 43, 529, 1523, and 2377 (1988a,b,c)]. A residue curve map in transformed coordinates for the reactive system methanol-acetic acid-methyl acetate-water is shown in Fig. 13-76. Note that the nonreactive azeotrope between water and methyl acetate has disappeared, while the methyl acetate-methanol azeotrope remains intact. Only... 

This is an equilibrium process and two techniques are used to drive the reaction to completion. One is to use a large excess of the alcohol, which is feasible for simple and inexpensive alcohols. The second method is to drive the reaction forward by irreversible removal of water, and azeotropic distillation is one way to accomplish this. Entries 1 to 4 in Scheme 3.5 are examples of acid-catalyzed esterifications. Entry 5 is the preparation of a diester starting with an anhydride. The initial opening of the anhydride ring is followed by an acid-catalyzed esterification. 

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. 

Pressure shift should always be explored as the first option when separating an azeotropic system. Adding extraneous components to a separation should always be avoided if possible. Unfortunately, most azeotropes are insensitive to change in pressure, and at least a 5% change in composition with pressure is required for a feasible separation using pressure shift1. 

Thong DYC and Jobson M (2001) Multi-component Azeotropic Distillation 1. Assessing Product Feasibility, Chem Eng Sci, 56 4369. 

Figure 4.9) to synthesize isopropyl myristate. This is enabled by a continuous feed of 2-propanol to the reactor, which forms an azeotropic mixture with water. This mixture is distilled, thereby removing the water produced. Afterwards, the immobilized enzyme used can be easily removed by filtration. The feasibility of this technique is also illustrated within the synthesis of glucose stearate. A mixture of ethyl methylketone and hexane as solvent is used, forming an azeotropic mixture with the water produced [42],... 

It can be noted from Fig. 2a that ProCAMD needed only 1.97 seconds to generate 5614 molecular structures and after evaluating them ended-up with 111 feasible candidates. The time also includes the process calculations related to miscibility, azeotrope verification as well as solvent loss and selectivity. 

Two feasible methods for removal of as much water as desired from the azeotrope are depicted on Figure 13.27. The dual pressure process takes advantage of the fact that the azeotropic composition is shifted by change of pressure operations at 100 and 760Torr result in the desired concentration of the mixture. In the other method, trichlorethylene serves as an entrainer for the water. A ternary azeotrope is formed that separates into two phases upon condensation. The aqueous layer is rejected, and the solvent layer is recycled to the tower. For economic reasons, some processing beyond that shown will be necessary since the aqueous layer contains some acetonitrile that is worth recovering or may be regarded as a pollutant. 

The overhead stream of the distillation column may be a low-boiling binary azeotrope of one of the keys with the entrainer or more often a ternary azeotrope containing both keys. The latter kind of operation is feasible only if condensation results in two liquid phases, one of which contains the bulk of one of the key components and the other contains virtually all of the entrainer which can be returned to the column. Figure 13.29(a) is of such a flow scheme. When the separation resulting from the phase split is... 

It appears that the pressures needed to make higher than azeotropic composition are beyond the strength of available membranes. A pressure of 1000 psi (68 atm) is feasible. With this pressure the concentrations of solute on the two sides of the membrane are related by... 

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... 

Alternative methods of integrated concentration have been developed that do not require separate dehydrating agents. These processes differ from the direct process in that weak acid reacts with concentrated nitrogen dioxide to produce an acid that is sufficiently superazeotropic that distillation into concentrated acid is economically feasible. The weaker azeotropic acid may be recycled for concentration or used as it is91. 

Section 4.2 is focused on phase equilibrium-controlled vapor-liquid systems with kinetically or equihbrium-controlled chemical reactions. The feasible products are kinetic azeotropes or reactive azeotropes, respectively. 

For example 2, Figs. 4.4(a) and (b) show the bifurcations of all singular points with respect to the Damkohler numbers of the reactive condenser and the reactive reboiler, respectively. As can be seen from the feasibility diagram in Fig. 4.4(c), at Damkohler numbers Dac > 0.830, two possible condenser products - that is, the top products of a fully reactive distillation column, are predicted. The kinetic azeotrope in the reactive reboiler is always the possible bottom product of a column. 

Fig. 4.8(b)). At Damkohler numbers Dac> 0.085 and Dar> 0.166, pure isobutene and pure MeOH are feasible top and bottom products, respectively. At Dar< 0.166, both pure MeOH and a kinetic azeotrope (i.e., the mixture on the branch from MTBE to the pinch point) are possible bottom products, while another kinetic azeotrope (i.e., the mixture on the branch between isobutene and the nonreactive azeotrope isobutene-MeOH) is the possible top product. 

Figure 4.9(a) and (b) illustrate the system behavior at a total pressure of 15 atm and 8 atm, respectively. As can be seen from the location of the PSPS, this system has similar features as the ideal system example 1 which has an elhpse-shaped PSPS (see Fig. 4.2(a)), as discussed above. Due to the boiling sequence of the reaction components, the PSPS is fully located outside the physically relevant composition space and, as a consequence, no reactive azeotrope can appear. It is worth noting that inside the phase-splitting region, the PSPS of the real heterogeneous system and the PSPS of the pseudohomogeneous system are different However, this does not affect the feasible top and bottom products of a fully reactive distillation column. 

New phenomena compared to nonreactive Langmuir systems are the same as in the binary case - that is, the existence of combined waves due to the occurrence of inflection points of the equilibrium functions y(x) or Y(X) and limitations on feasible product composition due to adsorptivity reversal similar to azeotropic distillation. Nonreactive examples for the latter were treated in Refs. [6 - 8], reactive examples will be discussed in the next section. 

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]. 

In contrast, when dealing with azeotropic mixtures the feasibility of separation is not guaranteed. Entrainer selection and feasibility is the central problem. The... 

The separation problem is defined as finding at least one feasible separation sequence for breaking a binary azeotrope AB by means of an entrainer C using only homogeneous distillation. The solution of this problem depends greatly on the existence of distillation boundaries. There are two possibilities ... 

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