Page 221 - Separation process engineering

Find x i,dist , x i,bot , D, and B.

B. Explore. This appears to be a straightforward application of external mass balances, except there
are two variables too many. Thus, we will have to assume the recoveries or concentrations of two
of the components. A look at the DePriester charts (Figures 2-11 and 2-12) shows that the order of
volatilities is propane > n-butane > n-pentane > n-hexane. Thus, n-butane is the LK, and n-pentane
is the HK. This automatically makes propane the LNK and n-hexane the HNK. Since the recoveries

of the keys are quite high, it is reasonable to assume that all of the LNK collects in the distillate
and all of the HNK collects in the bottoms. We will estimate distillate and bottoms based on these
assumptions.
C. Plan. Our assumptions of the NK splits can be written either as

(5-8a,b)

or

(5-9a,b)

The fractional recovery of n-butane in the distillate can be used to write

(5-10)

Note that this also implies

(5-11)

For n-pentane the equations are

(5-12)

(5-13)

Equations (5-8) to (5-13) represent eight equations with ten unknowns (four compositions in both
distillate and bottoms plus D and B). Equations (5-4) give two additional equations, which we will