Azeotropic liquid-separation system

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 reactor system may consist of a number of reactors which can be continuous stirred tank reactors, plug flow reactors, or any representation between the two above extremes, and they may operate isothermally, adiabatically or nonisothermally. The separation system depending on the reactor system effluent may involve only liquid separation, only vapor separation or both liquid and vapor separation schemes. The liquid separation scheme may include flash units, distillation columns or trains of distillation columns, extraction units, or crystallization units. If distillation is employed, then we may have simple sharp columns, nonsharp columns, or even single complex distillation columns and complex column sequences. Also, depending on the reactor effluent characteristics, extractive distillation, azeotropic distillation, or reactive distillation may be employed. The vapor separation scheme may involve absorption columns, adsorption units,... 

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-liquid-phase behavior or the liquid-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 ... 

In general, the steps of this separations system synthesis method for nonideal mixtures involving azeotropes include examination of the RCM representation (overlaid with vapor-liquid equlibria (VLE) pinch information, liquid-liquid equlibria (LLE) binodal curves and tie lines, and. solid-liquid equlibria (SLE) phase diagrams if appropriate) determination of the critical thermodynamic features to be avoided (e.g., pinched regions), overcome (e.g., necessary distillation... 

This tutorial paper is a review of recent advances in the synthesis of ideal and nonideal distillation-hased separation systems. We start hy showing that the space of alternative. separation processes is enormous. We discuss. simple methods to classify a mixture either as nearly ideal or as nonideal, in which case it displays azeotropic and possibly liquid/liquid behavior. 

Various processes are used for separating components that are difficult or impossible to be separated by conventional distillation. Whether the difficulty of separation arises from the components close boiling points or their tendency to form azeotropes, the separation processes must take into account the complex vapor-liquid equilibrium relationships of the system. The system to be considered involves both the components to be separated and the separating agent that, in one way or another, enhances the desired separation. The vapor-liquid equilibria of such mixtures is highly nonideal, and it is precisely this nonideality that is capitalized on to bring about the separation. 

An azeotrope is a mixture of two or more components that, when brought to boiling, issues a vapor with the same composition as the liquid. Hence, separation by simple distillation is not possible. Binary systems containing azeotropes have y-x equilibrium curves as shown in Figure 12.6. On either side of the azeotropic composition, separation by simple distillation is possible. 

In Chapter 7, the focus is on separation processes, in which the criteria for the selection of separation processes and the choices of equipment are reviewed before systematic methods of process synthesis are covered. The latter begin with sequences of ordinary distillation columns, then general vapor-liquid separation processes, and subsequently sequences that include azeotropic distillation columns. Also covered are considerations in selecting separation systems for gas mixtures and for solid-fluid systems. 

Up to this point we have looked at systems with fairly ideal vapor-liquid equilibrium behavior. The last separation system examined is a highly nonideal ternary system of methyl acetate (MeAc), methanol (MeOH), and water. Methyl acetate and methanol form a homogeneous minimum-boiling azeotrope at 1.1 atm with a composition of... 

One way to separate a mixture containing an azeotrope is to add a light entrainer into the system so that an additional azeotrope(s) can be formed that helps in the separation. In order to make the separation feasible, there are two important eharacteristics of this additional azeotrope (or one of the additional azeotropes). First, the azeotropic temperature of one additional azeotrope should be the minimum temperature of the whole ternary system. Second, this azeotrope should be heterogeneous so that natural liquid-liquid separation, without energy requiremenf can be performed in a decanter at the top of the column. 

The feasible designs of three different heterogeneous azeotropic column systems have been illustrated in this chapter with real industrial applications. With the aid of liquid-liquid separation, the products of a column sequence can be located in different distillarion regions. 

In azeotropic distillation, the third component forms an azeotrope with the system that becomes either the top or bottom product. The azeotrope is then separated into the agent and component. Sometimes such separation must be done using another process such as liquid extraction. Some typical systems (the azeotroping agent in parentheses) are acetic acid-water (butyl acetate), and ethanol-water (benzene). 

Such a process depends upon the difference in departure from ideally between the solvent and the components of the binary mixture to be separated. In the example given, both toluene and isooctane separately form nonideal liquid solutions with phenol, but the extent of the nonideality with isooctane is greater than that with toluene. When all three substances are present, therefore, the toluene and isooctane themselves behave as a nonideal mixture and then-relative volatility becomes high. Considerations of this sort form the basis for the choice of an extractive-distillation solvent. If, for example, a mixture of acetone (bp = 56.4 C) and methanol (bp = 64.7°Q, which form a binary azeotrope, were to be separated by extractive distillation, a suitable solvent could probably be chosen from the group of aliphatic alcohols. Butanol (bp = 117.8 Q, since it is a member of the same homologous series but not far removed, forms substantially ideal solutions with methanol, which are themselves readily separated. It will form solutions of positive deviation from ideality with acetone, however, and the acetone-methanol vapor-liquid equilibria will therefore be substantially altered in ternary mixtures. If butanol forms no azeotrope with acetone, and if it alters the vapor-liquid equilibrium of acetone-methanol sufficiently to destroy the azeotrope in this system, it will serve as an extractive-distillation solvent. When both substances of the binary mixture to be separated are themselves chemically very similar, a solvent of an entirely different chemical nature will be necessary. Acetone and furfural, for example, are useful as extractive-distillation solvents for separating the hydrocarbons butene-2 and a-butane. 

The relationship (6.3.182) between the separation factor Uij for the pervaporation process and the separation factors a and a . for the hypothetical thermodynamically equivalent process for Figure 6.3.30(b) can be conceptually illustrated via Figure 6.3.31(a). This figure is based on a similar one from Wijmans and Baker (1993) showing vapor-Uquid equilibrium of the azeotropic ethanol-water system at 60 °C. The plot illustrates total permeate pressure vs. alcohol concentration in the liquid phase (both feed liquid and permeate vapor). Feed liquid having molar concentrations of Cif and Cjf (mole fractions are in equi-... 

Figure A2.5.5. Phase diagrams for two-eomponent systems with deviations from ideal behaviour (temperature T versus mole fraetion v at eonstant pressure). Liquid-gas phase diagrams with maximum (a) and minimum (b) boiling mixtures (azeotropes), (e) Liquid-liquid phase separation, with a eoexistenee eurve and a eritieal point.

The choice of separation method to be appHed to a particular system depends largely on the phase relations that can be developed by using various separative agents. Adsorption is usually considered to be a more complex operation than is the use of selective solvents in Hquid—Hquid extraction (see Extraction, liquid-liquid), extractive distillation, or azeotropic distillation (see Distillation, azeotropic and extractive). Consequentiy, adsorption is employed when it achieves higher selectivities than those obtained with solvents. 

If the molecular species in the liquid tend to form complexes, the system will have negative deviations and activity coefficients less than unity, eg, the system chloroform—ethyl acetate. In a2eotropic and extractive distillation (see Distillation, azeotropic and extractive) and in Hquid-Hquid extraction, nonideal Hquid behavior is used to enhance component separation (see Extraction, liquid—liquid). An extensive discussion on the selection of nonideal addition agents is available (17). 

An example of heterogeneous-azeotrope formation is shown in Fig. 13-13 for the water-normal butanol system at 101.3 kPa. At liquid compositions between 0 and 3 mole percent butanol and between 40 and 100 mole percent butanol, the liquid phase is homogeneous. Phase sphtting into two separate liquid phases (one with 3 mole percent butanol and the other with 40 mole percent butanol) occurs for any overall hquid composition between 3 and 40 mole percent butanol. A miuimum-boihug heterogeneous azeotrope occurs at 92°C (198°F) when the vapor composition and the over l composition of the two liquid phases are 75 mole percent butanol. 

---