Close boiling mixtures

The convergence rate depends somewhat on the problem and on the initial estimates used. For mixtures that are not extremely wide-boiling, convergence is usually accomplished in three or four iterations,t even in the presence of relatively strong liquid-phase nonidealities. For example, cases 1 through 4 in Table 1 are typical of relatively close-boiling mixtures the latter three exhibit significant liquid-phase nonidealities. 

Torres-Marchal [110] and [111] present a detailed graphical solution for multicomponent ternary systems that can be useful to establish the important parameters prior to undertaking a more rigorous solution with a computer program. This technique can be used for azeotropic mixtures, close-boiling mixtures and similar situations. 

Peivaporation is a relatively new process that has elements in common with reverse osmosis and gas separation. In peivaporation, a liquid mixture contacts one side of a membrane, and the driving force for the process is low vapour pressure on the permeate side of the membrane generated by cooling and condensing the permeate vapour. The attraction of peivaporation is that the separation obtained is proportional to the rate of permeation of the components of the liquid mixture through the selective membrane. Therefore, peivaporation offers the possibility of separating closely boiling mixtures or azeotropes that are difficult to separate by distillation... 

The equations for estimating nucleate boiling coefficients given in Section 12.11.1 can be used for close boiling mixtures, say less than 5°C, but will overestimate the coefficient if used for mixtures with a wide boiling range. Palen and Small (1964) give an empirical correction factor for mixtures which can be used to estimate the heat-transfer coefficient in the absence of experimental data ... 

This equation applies to vaporization of single components, but can be used for close boiling mixtures without too much error. Coefficients for wide boiling mixtures will be overestimated. 

For heat pumping to be economic on a stand-alone basis, it must operate across a small temperature difference, which for distillation means close boiling mixtures. In addition, the use of the scheme is only going to make sense if the column is constrained to operate either on a stand-alone basis or at a pressure that would mean it would be across the pinch. Otherwise, heat integration with the process might be a much better option. Vapor recompression schemes for distillation therefore only make sense for the distillation of close boiling mixtures in constrained situations3. 

The first application of pervaporation was the removal of water from an azeotropic mixture of water and ethanol. By definition, the evaporative separation term /3evap for an azeotropic mixture is 1 because, at the azeotropic concentration, the vapor and the liquid phases have the same composition. Thus, the 200- to 500-fold separation achieved by pervaporation membranes in ethanol dehydration is due entirely to the selectivity of the membrane, which is much more permeable to water than to ethanol. This ability to achieve a large separation where distillation fails is why pervaporation is also being considered for the separation of aromatic/aliphatic mixtures in oil refinery applications. The evaporation separation term in these closely boiling mixtures is again close to 1, but a substantial separation is achieved due to the greater permeability of the membrane to the aromatic components. 

Figure 10.1 shows typical distillate composition profiles for close boiling mixtures (binary) and implications of using CBD column for such mixtures. 

Figure 10.2 shows typical distillate composition profiles for close boiling mixtures using a solvent in a CBD column. The CBD process becomes a conventional BED process with the addition of the solvent. Due to the addition of solvent, the components can be separated at high purity using a small column with low reflux ratio. See Safrit and Westerberg (1997) and Low and Sorensen (2002) for unconventional BED processes. 

Figure 10.1. CBD Column Processing Close Boiling Mixtures...

Example 1 Minimum Time Problem - Close Boiling Mixture... 

In liquid-liquid extraction (Fig. 3.6) two miscible solutes are separated by a solvent, which preferentially dissolves one of them. Close-boiling mixtures that cannot withstand the temperature of vaporization, even under vacuum, may often be separated by this technique. Like other contact processes the solvent and the mixture of solutes must be brought into good contact to permit transfer of material and then separated. The extraction method utilizes differences in the solubility of the components in the solvent. 

The low exergetic efficiency is typical for distillation systems with close boiling mixtures and with high energy requirements in the reboiler. An alternative is to use reboiler-liquid flashing. A compressor is used to return the reboiled vapor to the bottom of the column. The required reboiler duty is somewhat larger than the required condenser duty, and so an auxiliary steam-heated reboiler is needed. Thus, a trade-off is made between the power used in the compressor and the large reduction in reboiler steam. 

Example 4.25 Column Exergy efficiency Propylene-propane mixture is a close boiling mixture. A reflux ratio of 15.9 (close to minimum) and 200 equilibrium stages are necessary. Table 4.13 shows the enthalpy and entropies of the saturated feed and saturated products from the simulation results with the Redlich-Soave equation of state. The reboiler and condenser duties are 8274.72 and 8280.82 kW, respectively. The reference temperature is 294 K. The lost work ZTFis obtained from Eq. (4.198) as... 

The formation of two liquid phases within some temperature range for close-boiling mixtures is generally an indication that the system... 

The extrapolation is to what is called pervaporation, where the feed mixture is a liquid, but the permeate vaporizes during permeation, induced by the relatively low pressure maintained on the permeate side of the membrane. Accordingly, the reject or retentate remains a liquid, but the permeate is a vapor. Thus, there are features of gas permeation as well as hquid permeation. The process is eminently apphcable to the separation of organics and to the separation of organics and water (e.g., ethanol and water). In the latter case, either water vapor may be the permeate, as in dehydration, or the organic vapor may be the permeate. The obvious, potential application is to the separation of azeotropic mixtures and close-boiling mixtures—as an alternative or adjunct to distillation or liquid-liquid extraction methods. 

T xtractive and azeotropic distillation in different types of chemical industry has become more important as more separations of close-boiling mixtures and azeotropic ones are encountered. Extractive distillation is used more because it is generally less expensive, simpler, and can use more solvents than azeotropic distillation. Solvent selection for azeotropic distillation has recently been discussed by Berg (I). Therefore, solvent screening for extractive distillation is discussed here. 

Azeotropes, discussed in Chapter 1, form constant boiling mixtures and are, therefore, impossible to separate unless the azeotrope is broken by the addition of an external agent, using either extractive or azeotropic distillation as in the case of close boiling mixtures. 

Membranes can be used to separate molecules that differ in size, polarity, ionic character, hydrophilicity, and hy-drophobicity.100 Their use is less energy-intensive than distillation. They can often separate azeotropes and close-boiling mixtures. They can sometimes replace traditional methods, such as solvent extraction, precipitation, and chromatography, that can be inefficient, expensive, or may result in the loss of substantial amounts of product. Thermally and chemically sensitive molecules can be handled. Membranes can be porous or nonporous, solid or liquid, organic or inorganic. 

The presence of an azeotrope is one indication that a mixture is not ideal, diet it has deviations from RaouiCs Jaw (ene Chapter 1 and Section 5.2 of the present chapter). Close-boiling mixtures are more likely to exhibit azeohopism than wide-boiling mixtures whan there is more than 30°C boiling point difference, it is quite unlikely that an azeotrope will be present. Thus, the combination of close-boiling and nonideality is oris dial can land to lha presence of an azeotrope. 

This term usually is applied to cases where an extraneous material, called an entrainer, is added to a mixture to make a distillation separation feasible. In this way, a problem involving a close-boiling mixture is made tractable by the addition of an enltainer that will azeotrope with one of the components to give, in effect, a respectable relative volatility between the nonazeotropiag component and the azeotrope (treated as a pseudocomponent). Typically, the azeotrope has the higher volatility and becomes the distillate product. 

Applications of azeotropiaand extractive distillation have continued to expand because many very close Boiling mixtures may be separated economically by use of these techniques. The slparation of such mixtures by conventional distillation methods is usually uneconomical because of the large number of stages which would be required to effect such separations. 

Maximum-boiling azeotropes are less common. They occur for relatively close-boiling mixtures when negative deviations from Raoult s law arise such that 7, < 1.0. Criteria for their formation are derived in a manner similar to that of minimum-boiling azeotropes. At X = 1, where species 2 is more volatile,... 

It is possible to generate x-y equilibrium curves such as Fig. 3.3 using (3-9) by assuming that the relative volatility is a constant independent of temperature. This is convenient for close-boiling mixtures forming ideal solutions, but can lead to erroneous results for mixtures of components with widely different boiling points because it assumes that both P] and P] are identical functions of T. For example, inspection of the vapor pressure data for the hexane-octane system, Table 3.1, reveals that a varies from 101/16 = 6.3 at G. TC to 456/101 = 4.5 at 125.7°C. Calculation of relative volatilities by more accurate methods will be considered in Chapter 4. 

The algorithm of Fig. 7.8 is very successful when (7-11) is not sensitive to Tv. This is the case for wide-boiling mixtures such as those in Example 7.2. For close-boiling mixtures (e.g., isomers), the algorithm may fail because (7-11) may become extremely sensitive to the value of Ty. In this case, it is preferable to select (fi as the tear variable and solve (7-11) iteratively for Ty. 

---