Energy & Fuels: Distillation

Question 1

Why are fuels distilled?

Answer 1

Crude oil by itself is made of many products, we want certain compounds within this mixture in order to produce useful products

Question 2

What is the C range of Gas? What are it’s boiling points? What is it used for?

Answer 2

1-4, less than 40, fuel + plastics

Question 3

What is the C range of Gasoline? What are it’s boiling points? What is it used for?

Answer 3

5-12, 40-200, fuel, solvents

Question 4

What is the C range of Kerosene? What are it’s boiling points? What is it used for?

Answer 4

12-16, 200-275, Diesel + Jet fuel, home heating and cracking to gasoline

Question 5

What is the C range of Lubricating oil? What are it’s boiling points? What is it used for?

Answer 5

17-20, 300-350, Lubricants

Question 6

What is the C range of Residue? What are it’s boiling points? What is it used for?

Answer 6

20+, 350+, Paraffin, asphalt

Question 7

What is the basic process of distillation?

Answer 7

The separation of components of a mixture on the basis of boiling point/volatility of the components of crude oil

Question 8

How do longer molecules influence boiling point?

Answer 8

Longer chain = greater area for interactions therefore more electrostatic attractions requiring more energy to seperate

Question 9

How do longer molecules influence the density?

Answer 9

Longer chain = higher density as there is more area for interactions therefore stronger electrostatic attractions attracting molecules closer together

Question 10

How does the shape of a molecule affect it’s boiling point + density (branched vs straight molecule)

Answer 10

With more bend/branches there are lower area for interactions therefore less electrostatic attractions so lower boiling point and it disrupts the stacking together of adjacent molecule therefore decreases density

Question 11

Define vaporisation

Answer 11

The process of change of state from a liquid/solid to a vapour

Question 12

Define boiling point

Answer 12

The temperature at which pure substance changes completely from liquid to vapour

Question 13

Define volatility

Answer 13

The tendency of a liquid to vaporise (e.g. liquids that have a higher volatility have a lower boiling point)

Question 14

Define Saturation vapour pressure (Psat)

Answer 14

The pressure exerted by a gas in equilibrium with it liquid/solid

Question 15

Define partial pressure

Answer 15

the pressure exerted by individual components of a gas mixture the SUM is total pressure

Question 16

What is the SI unit of pressure?

Answer 16

Pascal (Pa)

Question 17

What is a phase diagram?

Answer 17

A diagram that shows the temperature and pressure regions where each of the phases of a substance will exist

Question 18

What is the triple point?

Answer 18

It is where the lines of solid, liquid and gas equilibrium all intersect so ALL THREE will be present

Question 19

What is the critical point?

Answer 19

Point where gas begins to have half the density of the liquid and begins to exhibit gas and liquid like properties (e.g. pass through solids like gas but dissolve solvents like liquid)

Question 20

What is the difference between a vapour and a gas?

Answer 20

A vapour is compressible into a liquid (e.g. before the critical point on the x axis) while a gas will only be compressed into a critical fluid (as it is past the critical point on the x axis)

Question 21

When a liquid is in a closed environment, are it’s states static? What is it doing?

Answer 21

No, it is in constant equilibrium between liquid and vapour

Question 22

What is the name of the formula used to predict the vapour pressure?

Answer 22

Antoine equation

Question 23

What is the formula to predict the vapour pressure? Explain the components of the equation

Answer 23

logPsat = A - B/(T+C)
Pst = vapour pressure
A, B and C are constants
T = temperature of substance in kelvin

Question 24

What is the name of the formula used to predict the molar ratio of vapour composition?

Answer 24

Raoult’s Law

Question 25

What is Raoult’s Law? explain the components

Answer 25

Pt * y = Psat * x
Pt = total vapour pressure of entire system
y = mole fraction of ONE component in vapour
Pst = saturation pressure of component y at given temp
x = mole fraction of component y as liquid

Question 26

What does a Vapour Liquid Equilibrium diagram (VLE diagram) show?

Answer 26

It shows the composition of the liquid and vapour based on the temperature for a given pressure

Question 27

On a VLE diagram, what does the below the graphs line show?

Answer 27

The temperature for when the substance is a liquid

Question 28

On a VLE diagram, what does above the graph line show?

Answer 28

The temperature for when the substance is a vapour

Question 29

On a VLE diagram, what does the bottom graph line show?

Answer 29

When the substance is boiling

Question 30

On a VLE diagram, what does the top graph line show?

Answer 30

When the substance is condensing

Question 31

On a VLE diagram, what does between the graph lines show? What is this region called?

Answer 31

• When the substance is in equilibrium between being a liquid and a vapour
• 2-phase region

Question 32

What does the overall mole fraction of both the vapours and the liquids have to equal?

Answer 32

1 mole fraction (eg 100% of the vapour and liquid)

Question 33

When a substance is brought to a certain temperature within the 2-phase region, if it is closer to the bottom line or top line, what will the composition of the substance tend towards?

Answer 33

• If it is closer to the top line there will be more vapour

- If it is closer to the bottom line there will be more liquid

Question 34

What is the formula for calculating the mole fraction of vapour when it is within the 2-phase region? Explain each component

Answer 34

Fr(vap) = (Q - M) / (P - M)
Fr(vap) = Fraction of vapour
Q = temperature and pressure point of substance
P = mole fraction of vapour (horizontal line to condensation point)
M = mole fraction of liquid (horizontal line to boiling point)

Question 35

What is the formula for calculating the mole fraction of liquid when it is within the 2-phase region? Explain each component

Answer 35

Fr(liq) = (P - Q) / (P - M)
Fr(liq) = Fraction of liquid
Q = temperature and pressure point of substance
P = mole fraction of vapour (horizontal line to condensation point)
M = mole fraction of liquid (horizontal line to boiling point)

Question 36

How does single stage distillation work in order to enrich, for example, liquid?

Answer 36

1 - Heat the liquid mixture (contains substance A and B where we want to enrich A) to a temperature where there are 2 phases (heat into 2-phases region)
2 - Seperate the vapour from the liquid and keep the liquids ( A(L) and B(L) )
3 - Cool the vapour to the liquid only zone, A(l) will now be at a higher concentration than before (as there will be less B(l) )

Question 37

How can a substance be further enriched past the single stage distillation?

Answer 37

Repeat the processes multiple times, each time cooling it into the liquid zone then reheating into the 2-phase region

Question 38

What is the temperature gradient of a distillation column? Where do the different substance from the distillation come out of the column?

Answer 38

• Lowest temperatures at the top and highest at the bottom
• Most volatile products (e.g. gases) come out the top and progressively going down the column heavier substances are refined

Question 39

When the boiling and condensation equilibrium points intersect what is this called and what does it mean for the distillation?

Answer 39

• Called and azeotrope

- The substances cannot be purified past this point