Extractive Distillation Methanol Separation Section

The distillate D2 composition (22.8 mol% methanol) is near the azeotropic composition at the 2 bar pressure. Reboiler heat input and condenser heat removal are 19.5 and 24.5 MW, respectively. The column diameter is 4.2 m. 

Distillate D2 is fed to the C5 recovery column C3. This column operates at a pressure of 10 bar, which shifts the azeotropic composition so that the distillate stream from column D3 has a composition of 34.2 mol% methanol. Higher and lower pressures were explored to see their effect on the economics. The 10 bar pressure seems to be about the optimum because going above this pressure does not shift the azeotrope significantly and raises the base temperature, which would require higher temperature energy input. 

The separation is a fairly easy one, so using only 10 stages and a reflux ratio of 1 yield a bottoms impurity of 0.01 mol% methanol. This bottoms stream B3 is the C5 product stream containing the inert isopentane. The reflux drum temperature is 373 K at this high pressure, which means that some heat integration between C2 and C3 may be economical (the base of C2 is at 356 K and the reflux drum of C3 is at 373 K). This possibility is not considered. 

The reactor and column Cl are identical to those used in the pressure-swing process. The methanol-containing distillate Di from the top of the reactive column is fed to stage 6 of a 12-stage extraction column. Water is fed on the top tray at a rate of 1050 kmol/h and a temperature of 322 K, which is achieved by using a cooler (heat removal = 1.24 MW). The column is a simple stripper with no reflux. The column operates at 2.5 atm so that cooling water can be used in the condenser (reflux-dmm temperature = 326 K). Reboiler heat input is 5.96 MW. The overhead vapor is condensed and it is the C5 product stream. 

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Let us consider the minimum-boiling acetone/methanol separation discussed in Section 5.1, where extractive distillation was used. The first thing to find out is the pressure dependence of the azeotrope. Figure 5.26 gives Txy diagrams at two pressures 1 and 10 atm. The azeotropic compositions are 77.6 and 37.5 mol% acetone at these two pressures. This significant shift indicates that pressure swing should be feasible. 

Figure 2.14a shows a flowsheet of the column of extractive distillation and, in Fig. 2.14b, an example of acetone(l)-water(entrainer)(2)-methanol(3) mixture with section trajectories is shown. This mixture, which is impossible to separate sharply into acetone (xd) and methanol-water mixture (xb) in the single-feed column, may be separated into these products in the column with an extractive section located between two feed inlets. 

The apphcation of extractive distillation is of great practical importance because it ensures the possibility of sharp separation of some types of azeotropic mixtures into zeotropic products, which is impossible in a colunm with one feeding. The mixture acetone(l)-water(2)-methanol(3) is an example of this type of mixture. Trajectories of reversible distillation of three sections of extractive distillation column, the feeding of which is binary azeotrope acetone-methanol, the extractive... 

The RCMs and the equivolatility curves of this chemical system ean be seen in Figure 13.1, where the numbers in the equivolatility emwes denote the relative volatility of acetone versus methanol in the presence of water. The RCM indicates that any mixture of acetone and methanol, even premixed with water, will produce the acetone-methanol azeotrope at the top of the column. However, by continuously adding water (a heavy entrai-ner) into the column, it can be seen from the equivolatility curves that the acetone is becoming more and more volatile than the methanol in the extractive section. Acetone and methanol can then be separated in the extractive section if the number of trays in this section is sufficient. Acetone will go toward the top of the column while methanol will be carried with the water toward the column bottom. In the rectifying section, owing to the lack of methanol in this section, only the separation of acetone and water is performed. Pure acetone will preferably go to the top of the batch extractive distillation column. After the draw-off of the acetone product and a slop-cut period, where the acetone in the column is completely depleted, the methanol product can be collected at the top of the column. The heavy entrainer (water) can be collected at the column bottom. 

The column liquid composition profile at the end of each operating step can be seen in Figure 13.7. Notiee that there is no methanol in the upper rectifying section at the end of Step 1 (time = 3.17 h). This is due to the continuous feeding of the entrainer into the column to push the methanol to appear only in the lower extractive section of the column. At the end of Step 2 (time = 5.34 h), the column composition profile has moved from the acetone comer toward the methanol comer. The top Uquid composition is close to the methanol comer at the end of Step 3 (time = 8.48 h). At this time, there is almost no acetone inside the column, so the separation is just like regular batch distillation. At the end of Step 4 (time = 12.26 h), the methanol pmity in the P2 product tank can no longer be maintained at its specification. At the same time, the bottom product has already satisfied the water purity specification. The column is shut down and the bottom product is collected. 

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