9.  Design problem

 

Design a continuous distillation column (plate) to recover acetone from a 50-50 mole % acetone-water mixture available at 30°C. The feed stream flow rate is 25,000 kg/h. The top product should contain at least 95 mole % acetone and the bottom product should contain <1 % acetone by mole. Consider reboiler as equivalent to one stage. This column is operated at atmospheric pressure (top tray). Column efficiency of 60% and pressure drop per plate of 1.25 kPa may be assumed. You can take the minimum liquid flow as 70% of the maximum rate both above and below the feed plate. The vapor liquid equilibrium (VLE) data for the acetone-water system at atmospheric pressure is provided in Table 7.2.

Data given:

Latent heat of water= 41,360 J/mol; latent heat of acetone= 28,410 J/mol

Specific heat of water=75.3 J/mol°C (mean); Specific heat of acetone 128 J/mol°C (mean)

 

Table 7.2: VLE data for the acetone-water system at 1 atm.

Step #1: Mass balance and determination of n umber of theoretical stage

Bubble point of feed (from the data shown in table) = 59.95°C

Latent heat of the feed = 28,410 X 0.5 + 41,360 X (1 - 0.5) = 34,885 J/mol

Specific heat of the feed = (128 X 0.5) + 75.3 X (1 - 0.5)= 101.75 J/mol °C

Heat required to vaporize 1 mole of the given feed = (59.95 - 30) X 101.75 + 34,885=37933 J

 

Here, the top operating line just touches the equilibrium curve at the point of tangency of the rectifying section operating line at which the minimum reflux takes place.