Multicomponent Distillation

Question 1

RoT for designing distillation column.

Answer 1

1. Safety first
2. If α is not 1.05 - 1.1, it’s the worst.
3. Do extreme P or T
4. Split large α because it’s easy.
5. Excess
6. Remove volatile (low b.p)
7. Remove binary.
8. Favour 50:50 split.
9. Product is distillate.

Question 2

Two major assumptions made in distillation calculations?

Answer 2

Mass is conserved between the inlet and outlet flow streams.

The components of the outlet vapour and liquid streams are in equilibrium.

Question 3

Why is a mesh pad included at the top of the drum?

Answer 3

De-entrainment

of liquid droplets carried along with vapour flow.

Question 4

Compare and contrast the rate-limiting steps for the three types of
separation processes: 1) multi-stage counter-current (column) distillation,
2) absorption, and 3) adsorption.

Answer 4

The rate limiting step for all three separation process is a mass transfer process from one phase to another – typically
liquid to/from vapour for distillation and absorption (in solvents), and fluid (eg gas or liquid) to/from solid in
adsorption on pellets [1 mark]
In all three cases the rate of mass transfer is typically controlled by a film at the boundary between the phases [1
mark], though for adsorption on porous solids the mass transfer might alternatively be limited by diffusion in the
pores [1 mark].
The temperature will affect rate of mass transport in all cases [1 mark], but many of the other process parameters
affecting mass transport are different. For example, pore size is important in adsorption on solid adsorbents [1
mark].

Question 5

Define relative volatility.

Answer 5

K1 / K2

Question 6

How does a non ideal liquid mixture show on a McCabe Thiele plot for the distillation of a two component mixture?

Answer 6

Draw diagram

K value is a function of composition

Equilibrium line is not symmetric and wiggly.

Question 7

Why is constant temperature in a distillation column unrealistic?

Answer 7

To achieve separation, liquid and vapour compositions must vary from tray to tray.

Since the bubble point temperature of a liquid mixture depends on its composition, and the liquid on each tray is close to its bubble point, the tray temperatures must also vary.

Therefore, the assumption of constant temperature across the column is inconsistent with the physical reality of changing compositions along the column height.

Question 8

What are the three assumptions of relative volatility of components?

Answer 8

Constant T

Constant P

Ideal mixtures

Question 9

What determines if a mixture is non-ideal?

Answer 9

Does not obey Raoult’s law.

The strength of intermolecular forces determines if it’s ideal.