A mixture of 45 mole % n-hexane and 55 mole % n-heptane is subjected to continuous fraction in a tray column at 1 atm total pressure. The distillate contains 90% n-hexane and the residue contains 10% n-hexane. The feed is saturated vapour. The relative volatility of n-hexane in mixture is 2.56. Determine the minimum reflux ratio for this separation.

100 kmoles/h of saturated liquid containing 10 mole % pentane, 55 mole % hexane, and 35 mole % heptane was separated using multistage distillation column at 380 mmHg. The top and bottom temperature of the column is 51.3°C and 76°C respectively. It is desired to have 99.5 % recovery of the hexane in the distillate. The mole fraction of the hexane in the distillate should be 0.75. The reflux ratio used was 1.2 the minimum. The θ value for Underwood’s correlation is 1.29. Using shortcut method, calculate the number of tray required and the feed tray location for this distillation. Antoine coefficient for the components was given in Table 1.

Table 1: Antoine equation coefficient

Component

A

B

C

Pentane

6.87632

1075.78

233.205

Hexane

6.87024

1168.72

224.21

Heptane

6.89385

1264.37

216.636

An equimolar mixture of hexane and heptane is continuously produced in an oil refinery at a rate of 390lbmol/h. The alkane mixture is to be separated via continuous distillation with a design specification of 2% impurity allowable in each stream. The feed is a 2-phase mixture (75 mol% liquid and 25 mol% vapor). Assume that the liquid is ideal and that the column’s operating pressure is 1 atm. Assume a full condenser and a partial reboiler are used.

Do the following. You will find L08C and L08D to be helpful.

1.Use http://app.knovel.com/web/toc.v/cid:kpYHACVPEH to find the relative volatility a when the temperature is such as to give about 1 atm vapor pressure for hexane, and another value at the temperature giving about 1 atm vapor pressure for heptane.

2.Use the average of the relative volatilities found in 1 to calculate y versus x, choosing the component giving y > x. (Hint: see slide 18 in L05B.) Plot this using whatever method you like. Include the 45^o x=y line. You’ll need 4 copies of this.

3.Graphically determine the minimum number of theoretical trays N_min in the column that would meet the specifications.

4.Graphically determine the minimum reflux ratio R_min .

5.Graphically determine the minimum boilup ratio R_B,min . (This corresponds to the minimum reflux ratio determined in 4, but using the operating line for the stripping section of the column.)

6.Let us consider the economics of distillation. There are two costs – construction and operating. The operating cost is primarily for coolant for the condenser and heating fluid for the reboiler. At total reflux, we get the lowest construction cost and a low operating cost, but no product. Thus, the cost per unit of product is infinite. At minimum reflux, the operating cost is a minimum but the column is infinitely high, and so again the cost per unit of product is infinite. The economic optimum condition is somewhere in between.

Estimate the economic optimum reflux ratio using the Gilliland Correlation (Figure 9.20 on
p 306 of the text) by selecting (R - R_min)/(R+1) = 0.4. Calculate R using your R_min, construct a McCabe-Thiele diagram using this R, and graphically determine the number of theoretical trays in the column. Compare this with that predicted by the Gilliland Correlation. What is the optimal feed tray, counting from the top?

Submit all of your calculations and plots.

We ar e separating a mi xture of 40 mol% iso-pentane, 30% n-hexane, and 30% n-h eptane

fed into the distil la tion col umn at the fe ed ra te of 1,000kmol/h. We desire a 98 m ol%

recove ry of n-he xane in the bottom prod uct and a 99% recoveryof iso-pentane in the

distillate. The feed is a two-phase mixtur e with 40% vapour,and the reflux ratio

L/D=2.5

6.1) Find D and B

6.2) Find co mpositions of distillate and bottom products

6.3) Calculate the values of L , V , L , and V , assuming thatCMO is valid