Page 100 - Separation process engineering

separation problem.
The existing flash drum has its dimensions h total and D specified. Solving Eqs. (2-66) and (2-67) for a

vertical drum for V, we have

(2-70)
This vapor velocity is the maximum for this existing drum, since it will give a linear vapor velocity equal
to u perm .

The maximum vapor capacity of the drum limits the product of (V/F) multiplied by F, since we must have

(2-71)

If Eq. (2-71) is satisfied, then use of the drum is straightforward. If Eq. (2-71) is violated, something has
to give. Some of the possible adjustments are:
a. Add chevrons or a demister to increase V Max or to reduce entrainment (Woinsky, 1994).

b. Reduce feed rate to the drum.

c. Reduce V/F. Less vapor product with more of the more volatile components will be produced.
d. Use existing drums in parallel. This reduces feed rate to each drum.

e. Use existing drums in series (see Problems 2.D2 and 2.D5).
f. Try increasing the pressure (note that this changes everything—see Problem 2.C1).

g. Buy a different flash drum or build a new one.
h. Use some combination of these alternatives.

i. The engineer can use ingenuity to solve the problem in the cheapest and quickest way.

2.11 Summary—Objectives

This chapter discussed VLE and the calculation procedures for binary and multicomponent flash
distillation. At this point you should be able to satisfy the following objectives:

1. Explain and sketch the basic flash distillation process
2. Find desired VLE data in the literature or on the Web

3. Plot and use y-x, temperature-composition, enthalpy-composition diagrams; explain the relationship
between these three types of diagrams
4. Derive and plot the operating equation for a binary flash distillation on a y-x diagram; solve both

sequential and simultaneous binary flash distillation problems
5. Define and use K values, Raoult’s law, and relative volatility

6. Derive the Rachford-Rice equation for multicomponent flash distillation, and use it with Newtonian
convergence to determine V/F
7. Solve sequential multicomponent flash distillation problems

8. Determine the length and diameter of a flash drum
9. Use existing flash drums for a new separation problem