Multicomponent Distillation

Question 1

Hengstebeck’s Assumptions

Answer 1

Considers that each of the non key components approach constant mole fractions thus are limiting components.

Treats the system as a binary separation between the keys.

Question 2

What is the order at which each correlation is used in the short cut method? what does it calculate?

Answer 2

1. Fenske’s Equation - calculates min number of stages
2. Underwoods Equation - calculates the minimum reflux ratio
3. Erbar-Maddox Correlation- finds the real number of stages and reflux ratio.
4. Kirkbrides Equation - calculates the feed location

Question 3

What does MESH stand for?

Answer 3

• Material: or flow rate balance equations (both component and total)
• Equilibrium equations including the bubble and dew point equations
• Summation or stoichiometric equations or composition constraints
• Heat or Enthalpy or Energy balance equations

Question 4

A complex column is when it has at least one feature?

3 total it could be

Answer 4

• Inter-Condensers
• Inter-reboilers
• Side Products

Question 5

Feed stage model is…

Answer 5

The distribution of vapour and liquid within a column is found by an adiabatic flash at the feedstage pressure.

If feed is all
Liquid - it will be added to liquid flow entering the stage
Vapor - it will be added to vaour flow entering the stage

Question 6

Feed stage model assumes

Answer 6

1. ) the liquid mixes with the liquid entering the feed stage
2. ) While the feed vapour mixes with the vapour leaving the stage

Question 7

Feed stage model, when it is subcooled or superheated what happens?

Answer 7

Subcooled liquid condenses some of the vapour ascending from the stage below the feed stage

Superheated feed, vapourises some of the liquid descending from the stage above the feed stage.

Question 8

If there is tray P, where is the vapour or liquid stream product to be drawn from?

Answer 8

If Vapour Product the product will be drawn from the stage above tray P.

If Liquid Product the product will be drawn from the stage below tray P.

Question 9

What are the three different Bubble Point Methods?

Answer 9

• Theta Method
• Kb Method
• The constant composition method

Question 10

What is the first 5 exponential numbers for number of sequences?

Answer 10

1 , 2 , 5 , 14 , 42

Question 11

Direct Sequence vs Indirect sequence? Which is less energy intensive?

(4 reasons how to choose)

Answer 11

The direct sequence often requires less energy…

1. do the most difficult separation last
2. favour the direct sequence
3. The larger fraction of the feed should be removed first
4. Wanting the molar flow of both top and bottom products to be as equal as possible.

Question 12

Process constraints may reduce the amount of options:

Answer 12

1. Hazardous needs to be removed first
2. reactive and heat sensitive need to be removed first
3. corrosive components removed first
4. Finished product taken from top of column (avoid thermal decomposition)
5. Remove light components before undergoing refrigeration or high pressures (very costly otherwise)

Question 13

When can a side stream occur?

Answer 13

When Middle product is in excess (i.e. >50%) and components C&A are present in small quantities

if A&raquo_space; B (volatilities) then take the vapour product side stream

if B&raquo_space; C (volatilities then take the liquid product side stream

Question 14

What is Thermo-coupled distillation sequences?

Answer 14

Saving energy through the interconnection of liquid and vapour streams between the two columns.

Question 15

What is double effect distillation column?

Answer 15

Direct heat integration between the reboiler of the first column and the condenser of the second column. (Combined into one HX)

Second Column is operated at higher Pressure

Question 16

What are 4 tricks to improve energy/capacity performance?

Answer 16

1. Feed Location
   Wrong place of Feed position negatively affects energy and capacity. A small change in feed location can save on large reboiler duty.

2.Feed condition
Adding or removing heat from the feed can change the reboiler and condenser requirements. May allow for easier heat recovery

3. Use of Side draws:
   Taking the middle product at high concentration from a side draw may drastically reduce the heat duty of the column.
4. Treating Pressure as a variable
   Relative volatilities vary with pressure, sometimes it is useful to lower column average operating pressure to reduce heat duty. (cost vs benefit tradeoff)