Azeotropic distillation total reflux

In a 2-1. flask fitted with a total-reflux, variable-take-off distillation head is placed a solution of 53 g. (0.472 mole) of dihydroresorcinol (Note 1), 2.3 g. of -toluenesulfonic acid monohydrate and 250 ml. of absolute ethanol in 900 ml. of benzene. The mixture is heated to boiling and the azeotrope composed of benzene, alcohol, and water is removed at the rate of 100 ml. per hour. When the temperature of the distilling vapor reaches 78° (Note 2), the distillation is stopped and the residual solution is washed with four 100-ml. portions of 10% aqueous sodium hydroxide which have been saturated with sodium chloride. The resulting organic solution is washed with successive 50-ml. portions of water until the aqueous washings are neutral and then concentrated under reduced pressure. The residual liquid is distilled under reduced pressure. The yield of 3-ethoxy-2-cyclohexenone (Note 3), b.p. 66-68.5°/0.4 mm. or 115-121°/11 mm., Mq 1.5015, is 46.6-49.9 g. (70-75%). 

Castillo FJL, Thong DYC and Towler GP (1998) Homogeneous Azeotropic Distillation 1. Design Procedure for Single-Feed Columns at Non-total Reflux, Ind Eng Chem Res, 37 987. 

To trimethyl(vinyl)silane (26.0 g, 259 mmol) and tert-butyl peroxybenzoate (1.02 g, 5 mmol) in MeOH (100 mL) was slowly added NaHSOs (52.6 g, 505 mmol) in HjO (100mL) at rt. The mixture was refluxed for 48 h, carefully concentrated (CAUTION peroxides ) and H2O was removed by azeotropic distillation with MeOH (3 x) by rotary evaporation. The residue was extracted with MeOH (3 x with a total of 250 mL), and the resulting soln was taken to dryness and dried at 150 °C under reduced pressure giving sodium 2-(trimethylsilyl)ethanesulfonate yield 37.0 g (70%). 

Acetone and methanol are impossible to separate by simple distillation due to the presence of an azeotrope. However, the addition of water near the top of a column allows these two components to be separated. Five sets of steady-state operating data for the extractive distillation of an acetone-methanol azeotrope in a laboratory scale column have been provided by Kumar et al. (1984). A schematic diagram of the column is provided in Figure 14.19. The column had a diameter of 15 cm and was fitted with 13 bubble cap trays, a total condenser and a thermosiphon (equilibrium) reboiler. Unlike many experimental distillation studies, these experiments were not carried out at total reflux the acetone-methanol feed entered the column on the eleventh stage from the top (the condenser counts as the first stage) and the water was introduced on stage six. The column was operated at atmospheric pressure for all five runs. Additional details of the column, operational specifications, and computed product compositions for one of these experiments can be found in Table 14.9. 

The occurence of azeotropes, such as the one in the ethyl acetate-benzene system above, results in difficulties in separations by distillation. For example, suppose it is desired to produce a distillate containing 98 mol % ethyl, acetate and a bottoms product containing 98 mol % benzene from a feed that contains 40 mol % ethyl acetate and 60 mol % benzene. Rather than consider detailed distillation calculations at various reflux ratios, we will consider only the case of total reflux, q — oo, which you should remember results in the minimum number of stages to accomplish a given separation. Also, if the desired separation cannot be made at total reflux, it will not be possible to accomplish the separation at any lower reflux ratio. 

Figure 10.2-2 Graphical tray-to-tray calculation for the ethyl acetate-benzene mixture at 1.013 bar and total reflux, showing that as the feed is of lower composition than the azeotropic point, the highest purity possible of the distillate is the azeotropic composition.

Figure 8-7 shows the characteristic pattern of distillation curves for ideal or close to ideal VLE with no azeotropes. All of the systems considered in Chapters 5. 6, and 7 follow this pattern. The y-axis (Xg = 0) represents the binary A-C separation. This starts at the reboiler (x = 0.01 is an arbitrary value) and requires only the reboiler plus 4 stages to reach a distillate value of x = 0.994. The x axis (x = 0) represents the binary B-C separation, which was started at the arbitrary value Xg = 0.01 in the reboiler. The maximum in B concentration should be familiar from the profiles shown in Chapter 5. Distillation curves at finite reflux ratios are similar but not identical to those at total reflux. Note that the entire space of the diagram can be reached by starting with concentrations near 100% C (the heavy boiler). 

After a one-hour period of reflux, distillation is started at the rate of about 20 ml. of distillate per hour, until a total of 170 ml. of liquid has been collected. To the residual solution is added 200 ml. of toluene, and the alcohol is distilled off completely as an azeotrope with toluene. Eventually a yellow suspension is obtained. The yellow crystalline solid is collected on a fritted glass of medium porosity and washed with ether. Care should be taken not to expose the compound to atmospheric moisture. The compound is dried under vacuum at 50°, giving a yield of 11.1 g. (95.4%). Anal. Calcd. for Na2-[(MobC18)(OC2H )6] C, 12.25 H, 2.57. Found C, 12.81 H, 2.81. 

A feed stream at the rate of 100 kmol/h contains 50% mole acetone and 50% mole chloroform. The two components form a maximum boiling azeotrope which prevents their separation by conventional distillation. It is proposed to separate them by extractive distillation using benzene as a solvent, at a rate of 800 kmol/h. Both the main feed and the solvent are at 75 C and 110 kPa, and the column pressure is assumed uniform, also at 110 kPa. A total condenser is used, with a reflux ratio of 4. The distillate composition is specified at 95% mole acetone and the bottoms at 5% mole acetone on a solvent-free basis. Using the pseudo-binary... 

---