Acetone-Methanol-Water Phase Equilibrium

Design and Control of Distillation Systems for Separating Azeotropes. By William L. Luyben and I-Lung Chien Copyright 2010 John Wiley Sons, Inc. 

Knapp and Doherty present the economic optimum design of an acetone-methanol separation using water as the extractive solvent. The design used in this chapter is based on their work. Kossack et al. presented a systematic synthesis framework for extractive distillation systems and the acetone-methanol system was considered. 

The other difference between this design and the Knapp and Doherty design is the reflux ratios in the two columns. They report 2.76 and 1.06, while the present design values are 3.44 and 1.61. This difference is probably due to our higher solvent flowrate. 

These results are generated with a fixed number of trays in each column. The reboiler heat duties are Qr and Qjo, in the low and high-pressure columns, respectively. This feature is a clear advantage of the pressure-swing process. 

The economics of the extractive distillation process are presented in the second column of Table 11.1. Notice that the column diameters are smaller, heat exchanger areas are smaller, and energy consumption is smaller in the extractive system compared with the pressureswing system. The TAC is about 17% lower for the extractive system. 

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The first crystalline dioxirane 4 has been reported to melt at 62-64 °C <1997JA7265>. The authors used HPLC analysis (reversed phase/methanol-water) for isolation of 4 from the reaction mixture. Bis-dioxirane 28 was found to be more stable than the corresponding mono-dioxirane 27 <2006JOC5796>. It has been observed that 27 decomposes rapidly at 0 °C, while at the same temperature 28 has been found to be stable for about 30 min. Zeller et al. used NMR experiments with different isotope-labeled acetones to demonstrate that DMDO lb and acetone are not in equilibrium <2005EJ05151>. 

Since pentane and water exhibit immiscibility, we might consider decantation as the first step. If it worked, it would be an inexpensive one to carry out. But a rigorous three-phase equilibrium calculation predicts that, in the presence of acetone and methanol, the small water fraction in the feed does not form a second liquid phase so we reject this idea. The calculation also reveals that the feed mixture is almost at the azeotropic composition for the pentane/methanol binary pair. 

Figure 6.11. About calculation minimum entrainer flow rate E/D)ram- Ki, j and E/D as functions x j (a,b,c, respectively) for extractive distillation of the acetone(l)-water(2)-methanol(3) azeotropic mixture. x[ j = x g concentration of component 1 in tear-off point of intermediate section reversible distillation trajectory on side 1-2 Ki, phase equilibrium coefficient of component i in point j, x), j and E/D, concentration of component 1 in pseudoproduct point and ratio of entrainer and overhead flow rates, respectively, if tear-off point of intermediate section trajectory xj j on side 1-2 coincide with point...

Figure 1.38 Vapor-liquid equilibrium of acetone (C3H6O) -methanol (CH4O) - water at 250 °C. The isopleths of acetone and water show a composition of equilibrium vapor phase are terminated by the critical locus where the compositions of liquid and vapor phases become equal (Griswold, J. and Wong, S.Y. (1952) Chem. Eng. Prog., Symp. Ser., n.3, 48, pp. 18-34.).

The disadvantage of such a course of action is that water builds up in the residue and will be present in the vapour leaving the still. For an immiscible solvent the distillate will separate into two phases after condensing and because of the shape of the vapour-liquid equilibrium (VLB) diagram (Fig. 5.4) no fractionating column is needed. However, a water-miscible solvent will have to be freed of water by fractionation or some other means. Further, there are only two solvents in this class that do not form azeotropes with water—methanol and acetone. The latter is difficult to separate from water by fractionation below a level of about 1.5% w/w water so that only methanol can be mixed with water without a... 

The rigid aluminum TLC frame, 10 cm plastic bag, filter paper saturator strips, aluminum developing tray, and clamp and fishhook are assembled, and the mobile phase is added. The TLC sheet is attached to the aluminum frame with the clip and lowered into the plastic bag with the fishhook. Paper clips are placed behind the sheet (between the sheet and the aluminum frame). The sheet is essentially suspended in space and is held only with the clip. The mobile phase will advance in a straight line. The sheet is allowed to stay in the bag without contacting the mobile phase for about 5 min to reach equilibrium, after which the plastic bag is pulled down to allow the mobile phase to contact the lower 1 cm of the layer. Development is carried out to within 1 cm of the top of the sheet with 15-18 ml of the mobile phase specified for the particular drug being analyzed methanol-conc. ammonium hydroxide (25 0.38), methanol-acetone-conc. ammonium hydroxide (13 17 1), or ethyl acetate-glacial acetic acid-conc. ammonium hydroxide-water (12 12 4 4). The development bag and back-to-back aluminum trays will accommodate two sheets at a time. 

Lannung (1930) gave solubility results for He, Ne, and Ar in water, methanol, ethanol, acetone, benzene, cyclohexane, and cyclohexanol for different temperatures from 5 to 45 C. He gave his results as the Bunsen absorption coefficient a. He gave the Ostwald coefficient L, where L = aT/273 and T is the r K of the measurement, but he defined L as the equilibrium ratio of the volume concentrations of the gas in the solution and in the vapor phase. ... 

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