Chapter 6, Problem 6.99P

Acetone is lo be extracted with n-hexane from a 40.0 wt% acetone—0.0 wt% water mixture at 25°C. The acetone distribution coefficient (mass fraction acetone in the hexane-rich phase/mass fraction acetone in the water-rich phase) is 0.34. Water and hexane may be considered immiscible. Three different processing alternatives are to be considered: a two-stage process and two single-stage processes.

(a) In the first stage of the proposed two-stage process, equal masses of the feed mixture and pure hexane are blended vigorously and then allowed to settle. The organic phase is withdrawn and the aqueous phase is mixed with 75% of the amount of hexane added in the first stage. The mixture is allowed to settle and the two phases are separated. What percentage of the acetone in the original feed solution remains in the water at the end of the process?

(b) Suppose all of the hexane added in the two-stage process of Part (a) is instead added In the feed mixture and the process is carried out in a single equilibrium stage. What percentage of the acetone in the feed solution remains in the water al the end of the process?

(c) Finally, suppose a single-stage process is used but it is desired to reduce the acetone content of the rater to the final value of Pan (a). How much hexane must be added to the feed solution?

(d) Under what circumstances would each of the three processes be the most cost-effective? What additional information would you need to make the choice?