Week 5

Question 1

Used when the materials to be separated have a high boiling point and decomposition may occur if direct distillation was to be employed.

Answer 1

Steam distillation

Question 2

The type of distillation steam distillation is

Answer 2

Differential distillation

Question 3

In steam distillation, steam is introduced directly into the liquid, providing that the solubility of steam in the liquid is ____.

Answer 3

low

Question 4

In steam distillation, steam is introduced directly into the liquid, providing that the ____ of steam in the liquid is low.

Answer 4

solubility

Question 5

In steam distillation, the effect of adding steam is to ____ the boiling point by reducing the effective total pressure of the still.

Answer 5

Lower

Question 6

In steam distillation, the effect of adding steam is to lower the boiling point by ____ the effective total pressure of the still.

Answer 6

Reducing

Question 7

In steam distillation, the effect of adding steam is to lower the boiling point by reducing the effective total ____ of the still.

Answer 7

pressure

Question 8

Expensive alternative to steam distillation

Answer 8

Vacuum distillation

Question 9

Two types of steam which can be injected into the column for steam distillation

Answer 9

1. Saturated steam
2. Superheated steam

Question 10

Steam distillation

Some of the steam condenses to provide the heat to vapourise the material

Answer 10

Saturated steam

Question 11

The steam can provide enough heat of vapourisation for the material concerned without itself condensing.

Answer 11

Superheated steam

Question 12

Where steam is injected into

Answer 12

The base of the column

Question 13

Basic principle of packed column

Answer 13

• surface of the packing, over a certain height, is equivalent to a theoretical plate
• transfer occurs at the interface between the liquid and vapour

Question 14

Types of column packing

Answer 14

• Random packing
• Structured packing : arranged, ordered & stacked

Question 15

Examples of random packing

Answer 15

• Raschig rings and saddles
• “through flow”

Question 16

Which column packing type has the lowest relative cost?

Answer 16

Raschig rings and saddles

Question 17

Which column packing type has the highest relative cost?

Answer 17

structured

Question 18

Which column packing type has the lowest efficiency?

Answer 18

Raschig rings and saddles (moderate)

Question 19

Which column packing type has the highest efficiency?

Answer 19

Structured (very high)

Question 20

Which column packing type has the lowest pressure drop?

Answer 20

Structured (very low)

Question 21

Which column packing type has the highest pressure drop?

Answer 21

Raschig rings and saddles (moderate)

Question 22

Which column packing type has the lowest vapour capacity?

Answer 22

Raschig Rings and saddles (fairly high)

Question 23

Which column packing type has the highest vapour capacity?

Answer 23

“through flow” & structured (high)

Question 24

Typical turndown ratio for Raschig rings and saddles, “through flow” and structured

Answer 24

2

Question 25

When packed columns are used when…

Answer 25

1. the column diameter is less than 2 feet
2. the pressure drop is low
3. corrosion considerations favour ceramic based materials
4. low liquid hold up is desirable (safety concerns may require low inventory)

Question 26

Disadvantages to packed columns

Answer 26

1. the points at which inputs and outputs are sited are more difficult to establish. Therefore sidestreams become a problem
2. the turndown ratio is not as good as plate columns
3. the flow paths through packing involve wall effects (more flow at walls) and therefore care has to be taken with liquid redistribution.
4. packed bed are not predictable
5. poor cooling
6. cannot handle foams
7. scale - up can be more problematic than with plate columnms
8. cleaning and maintenance can be problematic

Question 27

Height of an equivalent theoretical plate (HETP)

Answer 27

packed height/ number of equivalent equilibrium stages

Question 28

Rate of change of composition for packing columns is dependent upon

Answer 28

• equipment used
• the conditions of the vapour and liquid flows
• the rate of diffusion across the gas-liquid interface

Question 29

Height of the column

Answer 29

height of transfer unit * number of transfer unit

Z = (HOG)(NOG)
Z = (HOL)(NOL)

Question 30

Height of a gas transfer unit

Answer 30

HOG = G’ / K’g a

G’ = moles of gas flowing up the column per unit area, per unit time
K’g = overall mass transfer coefficient for gas
a - interfacial surface area per unit volume of column

Question 31

Height of a liquid transfer unit

Answer 31

HOL = G’ / K’L a

G’ = moles of gas flowing up the column per unit area, per unit time
K’L = overall mass transfer coefficient for liquid
a - interfacial surface area per unit volume of column

Question 32

Number of gas transfer units

Answer 32

NOG = ∫dy/ ye - y

y = mole fraction of the most volatile component in the vapour

Question 33

Number of liquid transfer units

Answer 33

NOL = ∫dx/ x - xe