Azeotropes pressure distillation

Only a fraction of the known azeotropes are sufficientiy pressure-sensitive for the conventional pressure-swing distillation process to work. However, the concept can be extended to pressure-insensitive azeotropes by adding a separating agent which forms a pressure-sensitive azeotrope and distillation boundary. Then the pressure is varied to shift the location of the distillation boundary (85). 

Methyl acetate-methanol Minimum-hoiling azeotrope Ethylene glycol monomethyl ether Element of recovery system for alternative to production of methyl acetate hy reactive distillation alternative to azeotropic, pressure, swing distillation... 

The main distillation types include atmospheric, vacuum, steam, azeotropic, extractive, and pressure distillation [45]. AU of these distillation methods can be carried out in a batch or continuous marmer with the exception of extractive distillation, which is solely continuous by nature. Gomplex solvent systems often require the use of multiple distillation columns in series to purify certain solvents that are not easily separated. The energy consumption in distillation columns can therefore be quite large because of the continuous operation of condensers and reboilers over extended periods of time. In order to cut down on these costs, both vacuum and steam distillation can be employed ]45]. 

Commercial grades of DMF may be purified initially by azeotropic distillation with benzene (CAUTION). Distil a mixture of 1 litre of DMF and 100 ml of benzene at atmospheric pressure and collect the water benzene azeotrope which distils between 70 and 75 °C. Shake the residual solvent with powdered barium oxide or with activated alumina (Grade I), filter and distil under nitrogen at reduced pressure collect the fraction having b.p. 76°C/39mmHg or 40°C/10mmHg. The distillate is best stored over a Type 4A molecular sieve. 

Tellurium Tetrabutoxide1 10 g (135 mmol) of butanol are added to 2.71 g (7.4 mmol) of tellurium tetraisopropoxide and the mixture is diluted with benzene. After the exothermic reaction has subsided, the mixture is heated under reflux for 2 h and then the isopropanol/benzene azeotrope is distilled from the mixture at 12°. Excess butanol and benzene arc distilled off, the residue is dried under reduced pressure at 20°, and the resultant colorless liquid is distilled yield 2.9 g (92%) b.p. 150°/ 0.8 torr. 

The older method to raise the azeotrope containing 95 per cent weight to 99.9 per cent ethanol consists in absorbing the water on lime (CaO). This costly process has been abandoned. Azeotropic distillation in the presence of benzene is employed today. In principle, simple low pressure distillation should allow the separation of alcohol and water, because the azeotrope disappears below 2kPa. For ethanol concentrations between 95 and 100 per cent, however, the liquid am vapor phase compositions are substantially the same, implying extremely high reflux rates and a large number of trays. 

A key feature of the Halcon process is the use of low pressure distillation (less than 80 kPa = 12 psi) to break the phenol-aniline azeotrope and allow economical separation of aniline from phenol (67). 

Since the position of an azeotropic point is not stable, besides the azeotropic and extractive distillations a change of the external thermal conditions (temperature or pressure) in the form of a vacuum or pressure distillation may be effected to make the special point disappear. Schuberth [44] has reported relations by means of which the... 

Azeotropic dehydration and condensation polymerization (route 2 in Figure 8.2) yields directly high molar mass polymers. The procedure, patented by Mitsui Toatsu Chemicals [13, 14], consists of the removal of condensation water via a reduced pressure distillation of lactic acid for 2-3h at 130°C. The catalyst (in high amounts) and diphenyl ester are added and the mixture is heated up to reflux for 30-40 h at 103°C. Polycondensated PLA is purified to reduce residual catalyst content to the ppm range [5,10,15]. 

For the separation of azeotropes or mixtures with relative volatilities that lie below about 1.4, which are difficult to separate, special distillation processes are available, such as two-pressure distillation and extractive and azeotropic rectification. 

Two-pressure distillation is especially promising when the azeotrope-forming components have very different heats of evaporation. The thus resulting differing slopes of the vapor-pressure curves leads to a pressure dependence of different azeotropic compositions. 

Low pressure distillation. At a pressure of about 75 mmHg the ethanol/water azeotrope disappears so that if it were possible to condense at that pressure dry ethanol could be produced. A more practical use of this property of ethanol would be that at 100 mmHg the azeotrope would only contain 0.16% w/w water which boils at 35 °C. Conventional extractive distillation. If the feed to an extractive distillation contains very much more of one component than the other it is almost always better to remove the smaller one. In this case this indicates that water should be removed with the entrainer from the first column and removed from the top product from the second. Since water is very strongly the more polar this calls for a polar entrainer such as ethylene glycol, glycerine, ethyl Cellosolve or DME An entrainer that can be stripped from water at atmospheric pressure, such as DMF, would be suitable. 

High pressure distillation. The THF/water azeotrope (Fig. 16.24) at lOOpsig contains 12% water compared to only 4.6% at atmospheric pressure so that by alternate distillations at high pressure and low it is possible to have a residue of... 

Figure 7.36 Pressure-swing distillation for the separation of a minimum-boiling azeotrope (a) T-y-x curves at pressures Pi and P2 for minimum-boiling azeotrope (b) distillation sequence for minimum-boiling azeotrope.

Sinple distillation columns are not able to conpletely separate mixtures when azeotropes occur, and the columns are very e5q>ensive when the relative volatility is close to 1. Distillation columns can be coupled with other separation methods to break the azeotrope. This is discussed in the first section. Extractive distillation, azeotropic distillation, and two-pressure distillation are methods for modifying the equilibrium to separate these conplex mixtures. These three methods are described in Sections 8.2 to E2 of this chapter, hi Section 8.8 we discuss the use of a distillation column as a chemical reactor, to simultaneously react and separate a mixture. 

Before the relatively recent development of detailed and accurate VLE correlations, most VLE data were only available at one atmosphere thus, many azeotropic systems have probably not been explored as candidates for two-pressure distillation. Fortunately, it is fairly easy to simulate two-pressure distillation with a process simulator (see Lab 7, part A in the appendix to Chapter 81. Methods for estimating VLE and rapidly screening possible systems are available fFrank. 19971. Because two-pressure distillation does not require a mass separating agent, it is a prefered method when it works. If two-pressure distillation were routinely considered as an option for breaking azeotropes, we would undoubtedly discover additional systems where this method is economical. 

G4. The system water-acetonitrile (C2H3N) forms an azeotrope. This system can be separated using a two-pressure distillation system. Both columns have total condensers and ketde-type reboilers. Valid phases are vapor-liquid. Use NRTL for VLE data. The feed to the system is at 1.0 atm and 60°C. The feed is 15 mol% water and 85 mol% acetonitrile, and the flow rate of the feed is 200 kmol/h. This feed and the recycle stream labeled D2 in Figure 8-6 are input into column 1 (see... 

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