Azeotropic distillation tetrahydrofuran

Figure 9.19 Breaking the ethanol-water azeotrope with tetrahydrofurane by a two columns sequence. The process takes place in two distillation regions.

There are many important industrial applications of azeotropic separations, which employ a variety of methods. In this book we discuss several of these chemical systems and demonstrate the application of alternative methods of separation. The methods presented include pressure-swing distillation, azeotropic distillation with a light entrainer, extractive distillation with a heavy entrainer (solvent), and pervaporation. The chemical systems used in the numerical case studies included ethanol-water tetrahydrofuran (THF)-water, isopropanol-water, acetone-methanol, isopentane-methanol, n-butanol-water, acetone-chloroform, and acetic acid-water. Economic and dynamic comparisons between alternative methods are presented for some of the chemical systems, for example azeotropic distillation versus extractive distillation for the isopropanol-water system. 

Pressure Swing Distillation The third method to break an azeotrope is pressure swing distillation, which relies on the fact that the location of the azeotropic point depends in some cases relatively strongly on the total pressure. In contrast to extractive or azeotropic distillation, the azeotrope is not broken by a solvent, but is jumped over by altering the pressure, and thus the distillation can continue. For example, the system tetrahydrofuran (TH F) and water is at first distilled at 1 bar up to approximately 80% THF (Figure 3.3.22a), that is, just before the azeotropic point is reached. The distillate rich in THF with a composition close to the azeotropic... 

To remove water, the benzene was azeotroped and distilled over CaH2. The n-pentane was stored over LiAlH and distilled over CaH2. Toluene was distilled over CaH2. Toluene from Burdick Jackson, Muskegon, MI could also be used for dilute solution characterization without any adverse effects on polymer solubility. Tetrahydrofuran (THF) was dried over molecular sieves and doubly distilled over CaH2. The solvents were blanketed with nitrogen to maintain dryness. 

Px relation of Raoult s law, and the system therefore exhibits negative deviations. When the deviations become sufficiently large relative to the difference between the two pure-species vapor pressures, the Px curve exhibits a minimum, as illustrated in Fig. 12.96 for the chloroform/tetrahydrofuran system at 30°C. This figure shows that the Py curve also has a minimum at the same point. Thus at this point where x - y the dew-point and bubble-point curves are tangent to the same horizontal line. A boiling liquid of this composition produces a vapor of exactly the same composition, and the liquid therefore does not change in composition as it evaporates. No separation of such a constant-boiling solution is possible by distillation. The term azeotrope is used to describe this state. 

General information on pressure-swing distillation can be found in Van Winkle (Distillation, McGraw-Hill, New York, 1967), Wankat (Ejmilibrium-Stased Separations, Elsevier, New York, 1988), and Knapp and Doherty [Ina. Eng. Chem. Res., 31, 346 (1992)]. Only a relatively small fraction of azeotropes are sufficiently pressure-sensitive for a pressure-swing process to be economical. Some applications include the minimum-boiling azeotrope tetrahydrofuran-water [Tanabe et al. ... 

Water can be removed from methanol by a membrane of polyvinyl alcohol cross-linked with polyacrylic acid, with a separation factor of 465.204 A polymeric hydrazone of 2,6-pyridinedialdehyde has been used to dehydrate azeotropes of water with n- and /-propyl alcohol, s- and tort butyl alcohol, and tetrahydrofuran.205 The Clostridium acetobutylicum which is used to produce 1-butanol, is inhibited by it. Pervaporation through a poly(dimethyl-siloxane) membrane filled with cyclodextrins, zeolites, or oleyl alcohol kept the concentration in the broth lower than 1% and removed the inhibition.206 Acetic acid can be dehydrated with separation factors of 807 for poly(4-methyl-l-pentene) grafted with 4-vinylpyridine,207 150 for polyvinyl alcohol cross-linked with glutaraldehyde,208 more than 1300 for a doped polyaniline film (4.1 g/m2h),209 125 for a nylon-polyacrylic acid membrane (5400 g/m2h), and 72 for a polysulfone.210 Pyridine can be dehydrated with a membrane of a copolymer of acrylonitrile and 4-styrenesulfonic acid to give more than 99% pyridine.211 A hydrophobic silicone rubber membrane removes acetone selectively from water. A hydrophilic cross-linked polyvinyl alcohol membrane removes water selectively from acetone. Both are more selective than distillation.212... 

C under argon. Methanol was removed azeotropically at 56 °C at a fast rate and as the boiling point began to rise, the distillation rate was reduced to 4 drops/min. and heating continued for 15h. The polymer thus prepared precipitated out of cyclohexane. The powdery polymer was crystalline with a melting point (DSC) of 212 °C. It was insoluble in the usual organic solvents such as methylene chloride, chloroform, ether, tetrahydrofuran, ethyl acetate, acetone, dimethylformamide, and dimethylsulfoxide. 

Fig. 11.3-3 Pressure swing distillation of the azeotropic system water/tetrahydrofuran...

A process, which combines distillation with solvent extraction, is presented in Fig. 11.4-3. Such processes are used in the process industiy for the regeneration of solvents (e.g., tetrahydrofuran, THF) diluted by water (Schoemnakers 1984). The system tetrahydrofuran/water forms a minimum azeotrope at approximately 80 mol% THF. Most of the water is removed as bottoms in distillation coluiim C-1. The overhead fraction 1 is fed into the extractor for removal of the residual water by solvent extraction with concentrated Na0H/H20. The diluted sodium fraction is regenerated in the single stage distillation nnit D-1. If the specified concentration of tetrahydrofuran product is veiy high a further purification step has to be per-... 

Consider the separation of 100 kmol/hr of an equimolar stream of tetrahydrofuran (THF) and water using pressure-swing distillation, as shown in Figure 7.37. The tower T1 operates at 1 bar, with the pressure of the tower T2 increased to 10 bar. As shown in the T-x-y diagrams in Figure 7.38, the binary azeotrope shifts from 19 mol% water at 1 bar to 33 mol% water at 10 bar. Assume that the bottoms product from T1 contains pure water and that from D2 contains pure THF. Also, assume that the distillates from T1 and T2 are at their azeotropic compositions. Determine the unknown flow rates of the product and internal streams. Note that data for the calculation of vapor-liquid equilibria are provided in Table 7.5. 

However, the scheme presented in Fig. 6 comprises a possible industrial application of the separation of ILs from other compounds. It can be used in breaking of azeotropes such as tetrahydrofuran (A) - water (B) mixture in the presence of an IL. Here the IL acts as an extractant (or entrainer) to wash down compound B and removed at the bottom of the column while compound A is released and distilled as a pure compound at the top of the column. In this case, the IL can be recycled into the process after removing residual B in a falling film evaporator [BASF, 2011 Maase, 2008]. 

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