Page 235 - Separation process engineering

(5-35a)

which gives K = 2.0065, K nC5 = 1.5325, K nC6 = 0.5676. Then
iC6

This is too high. To find the next temperature, use Eq. (5-34).

Solving Eq. (5-35a) for T,

(5-35b)

and we obtain T = 577.73°R. Using this for the new guess, we can continue. The final result is T
1 1
= 576.9°R = 47.4°C. This result is within the error of Eq. (2-30) when compared to the 47.0°C

found from the DePriester charts. Equation (5-35b) is valid for all the hydrocarbons covered by the
DePriester charts except n-octane and n-decane.
F. Generalize. If K values depend on composition, then an extra loop in the trial-and-error procedure
will be required. When K values are in equation form such as Eq. (2-30), bubble-point calculations
are easy to solve with a spreadsheet. With a process simulator, one of the vapor-liquid equilibrium
(VLE) correlations (see Table 2-4) will be used to find the bubble-point temperature and the y i

values.

If only LNKs are present, we should step off stages going down the column. Then the liquid mole
fractions are determined from dew-point calculations and the vapor mole fractions are found from the
operating equations. Otherwise, the procedure is very similar to going down the column with constant
relative volatilities.

For dew-point calculations, all y are known. We need to find the temperature T at which
i,j
j

(5-36)

We determine the temperature for the next guess by calculating the reference component K value.

(5-37)

Remember that convergence of the stage-by-stage distillation calculation is easy only when the NKs are
all heavy or are all light. Both the dew-point and bubble-point methods can be solved with a spreadsheet
(see the Appendix for Chapter 5).

5.5 Summary—Objectives

At the end of this chapter you should be able to satisfy the following objectives: