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Chem Eng - Y1.S2 > Principles - Volatility / Boiling point Separation > Flashcards

Principles - Volatility / Boiling point Separation Flashcards

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1
Q

What’s distillation?

A

The separation of components of a mixture on the basis of boiling (/bubble) point or volatility.

The distillation column has stages for different equilibriums.

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2
Q

What’s vapourisation?

A

Vapourisation - The process of change of state from a liquid (or solid) to a gas (vapour).

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3
Q

What’s boiling point?

A

Boiling point - The temperature at which a pure substance changes completely from liquid to vapour.
• The boiling point is the temperature where thermal excitement overcomes the attractive forces between the molecules of the liquid.
• The temperature of boiling at 1 atm is the ‘normal boiling point’.

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4
Q

What’s volatility?

A

The tendency of a liquid to vaporise.

Liquids that have a high volatility have low boiling points.

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5
Q

What’s saturation vapour pressure?

A

The pressure exerted by a gas in equilibrium with its liquid (or solid).

• It is a measure of the volatility of a liquid.

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6
Q

What’s partial pressure?

A

The pressure exerted by individual components of a gas mixture.

Their sum is the total pressure.
p1 + p2 + p3 +…. = P
Dalton’s Law of Partial Pressures

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7
Q

What is vapour pressure, P*?

A

The pressure exerted by a vapour in equilibrium with its solid or liquid phase. (Depend on temperature)

A liquid standing in a sealed beaker is actually a dynamic system:
• Some molecules of the liquid are evaporating to form vapour and
some molecules of vapour are condensing to form liquid.
• At equilibrium, the rates of the two processes are equal and the
system appears to be stationary.

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8
Q

What’s a saturated gas?

A

When the gas contains the maximum amount of liquid vapour it can hold.

– For air-water systems, this is called “humidity”
– The saturated vapour pressure changes with temperature

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9
Q

How is vapour pressure calculated?

A

By using the Antoine equation.

LogP* = A - B/(T + C)

Where:
P* = vapour pressure exerted by pure liquids (Pa, atm, mmHg – depends on the correlation used)

T = temperature (°C, K, °F, R – depends on the correlation used)

A, B, C = values listed in tables. Watch out for units!

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10
Q

How do evaporation and boiling differ?

A

Evaporation - bubbles can’t form as vapour pressure is below atmospheric pressure.

Boiling - bubbles can form as vapour pressure can overcome atmospheric pressure / it is equal to 1 atm.

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11
Q

How many mmHg are in 1 atm?

A

1 atm = 760 mmHg

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12
Q

What is normal boiling point?

A

Boiling point at 1 atm

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13
Q

What happens to fluid vapours when 2 liquids are mixed?

A

When two liquids are mixed, the vapour above them will also be a mixture of the two components, but in different proportions than in the liquid.

There is an uneven distribution of components in the vapour and liquid phases. This fact is the key to separations by distillation.

• Vapour is enriched in the lower boiling point compound (y is greater)
This is the more volatile compound/component (MVC, higher vapour pressure, P*)

• Liquid is enriched in the higher boiling point compound (x is greater)
This is the less volatile compound/component (LVC, lower vapour pressure, P*)

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14
Q

How are mole fractions found?

A

Mole of component in phase / total moles in phase

(Can be x, y, z)
Where:
y = mol fraction in vapour phase 
x = mol fraction in liquid phase 
z = overall mol fraction
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15
Q

How can the distribution of components in vapour and liquid phases be predicted?

A

There is an uneven distribution of components in the vapour and liquid phases.
This fact is the key to separations by distillation.

• Vapour is enriched in the lower boiling point compound (y is greater)
- This is the more volatile compound/component (MVC, higher vapour pressure, P*)

• Liquid is enriched in the higher boiling point compound (x is greater)
- This is the less volatile compound/component (LVC, lower vapour pressure, P*)

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16
Q

What’s an ideal solution?

A

A gas-liquid system in which the VLE (vapour-liquid equilibrium) for each species obeys either Henry’s law or Raoult’s law.

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17
Q

What’s Raoult’s law?

A

A law stating that the vapour pressure of an ideal solution is proportional to the mole fraction of solvent.

Partial pressure of component A = mole fraction of component A in vapour * total pressure

= mole fraction of component A in liquid * saturation vapour pressure of pure component A (at a given temperature)

Pₐ =yₐ.P = xₐ.Pₐ*(T)

Where:
Pₐ* - vapour pressure of pure liquid A at temperature T 
yₐ - mole fraction of A in the gas phase
xₐ - mole fraction of A in liquid phase
P - total system pressure
Pₐ - partial pressure of A

This means that the composition of the vapour phase (yₐ) is determined by the composition of the liquid phase (xₐ) and the Saturation Vapour Pressure of component A (Pₐ*).

[It’s an approximation that is valid when xA is close to 1 (when liquid is nearly pure A).

It can also be valid for ranges of similar substances such as hydrocarbons – these are ‘ideal’ solutions, where the components in the mixture are chemically similar, like hexane and octane (straight chain hydrocarbons).]

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18
Q

What’s Henry’s law?

A

A gas law that states that the amount of dissolved gas is proportional to its partial pressure in the gas phase.

Pₐ = yₐ.P = xₐ.Hₐ,ₚₓ (T )

Where:
Pa - partial pressure of A
yₐ - mole fraction of A in gas phase
P - total system pressure
xₐ - mole fraction of A in liquid phase
Hₐ,ₚₓ - Henry's law constant for A in a specific solvent at temp' = T (and typically = partial pressure [gas] / mole fraction x [liquid])
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19
Q

What is the bubble point?

A

[The conditions] When the first bubbles of gas form in a liquid

Vapour pressure = atmospheric pressure

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20
Q

What’s dew point?

A

[The conditions]

When the first droplets of liquid form from a vapour

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21
Q

When is Raoult’s law valid?

A

It’s an approximation that is valid when xA is close to 1 (when liquid is nearly pure A).

It can also be valid for ranges of similar substances such as hydrocarbons – these are ‘ideal’ solutions, where the components in the mixture are chemically similar, like hexane and octane (straight chain hydrocarbons).

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22
Q

What do yₐ and xₐ represent (in separations and distillation)?

A

yₐ - mole fraction of a in gas/vapour phase

xₐ - mole fraction of a in liquid phase

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23
Q

What does Raoult’s law state?

A

Raoult’s law states that the vapor pressure of a solvent above a solution is equal to the vapor pressure of the pure solvent at the same temperature scaled by the mole fraction of the solvent present.

Pₐ = yₐ.P = xₐ.Pₐ*(T)

Where:
Pₐ* - vapour pressure of pure liquid A at temperature T 
yₐ - mole fraction of A in the gas phase
xₐ - mole fraction of A in liquid phase
P - total system pressure
Pₐ - partial pressure of A
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24
Q

When should Henry’s law be used (instead of Raoult’s law)?

A

Henry’s law works best at low solute concentration (close to 5% or less)

Valid for any species present at low concentration. Limited to low to moderate pressure.

For species whose critical T is less than the application temperature, Raoult’s law is not appropriate as Raoult’s law requires a P* - saturated pressure value at application temperature.

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25
Q

When should Raoult’s law be used (instead of Henry’s law)?

A

Raoult’s law works best at non-low concentration of the solute (10−50%)

It also only works for ideal solutions (vapour phase is an ideal gas and liquid phase is ideal solution).

Valid for systems at low to moderate pressure and systems of chemically similar species.

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26
Q

How do Henry’s law and Raoult’s law differ?

A

Raoult’s law:
- relates vapour pressure, P, above the solution to the mole fraction (of given substance) and the vapour pressure of the pure solution!, P*

  • valid for systems at low/moderate pressure and systems with chemically similar species

Henry’s law:
- relates vapour pressure, P, above the solution to the mole fraction (of given substance) and the Henry’s law constant!, Hₐ,ₚₓ

  • valid for any species present at low/moderate pressure
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27
Q

What does Henry’s law state?

A

The partial pressure of the species in the vapor phase is directly proportional to its liquid phase mole fraction.

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28
Q

By what method are bubble point temperature & pressure and dew point temperature & pressure calculated?

A

By iteration (trial and error)

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29
Q

How is bubble point temperature calculated?

A

By trial and error for the value of Tbp that satisfies Raoult’s law equation (Pₐ = xₐ.Pₐ*(Tbp))

All that is needed is a set of relationships for P*(T) such as Antoine Equations or Vapour Pressure Charts.

[Form a chart with columns for temperature, P* of each component and the calculated function value, f(Tbp).

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30
Q

How is bubble point pressure calculated?

A

Once Tbp is known, the composition of the vapour phase can be determined by calculating the partial pressures of each component from the equation:

yᵢ = Pᵢ/P

yᵢ = Pᵢ / Pᵦₚ

= xᵢP*ᵢ (T) / Pᵦₚ

31
Q

How is dew point temperature calculated?

A

It can be found by an analogous method. If a gas contains condensable components A, B, C, … at fixed pressure, P, let yi be the mole fraction of component i in the gas. If the gas mixture is cooled slowly to its dew point, Tdp, it will be in equilibrium with the first liquid that forms.

(From Raoult’s law)

xᵢ = yᵢP / P*ᵢ (Tdp)

and the sum of mole fractions = 1

The value of Tdp can be found by trial and error once expressions for P*(T) have been substituted.

32
Q

How is dew point pressure calculated?

A

P dp, which relates to the condensation brought about by increasing system pressure at constant temperature, can be found from Raoults law,

xᵢ = yᵢP / P*ᵢ (Tdp)

P dp = 1 / ((yₐ/Pₐ (T)) + (yᵦ/Pᵦ (T)+…)

33
Q

What’s the difference between evaporation and boiling?

A

Evaporation - occurs at the fluid surface. Bubbles can’t form as the vapour pressure is less than atmospheric pressure.

Boiling - bubbles can form and rise in the fluid since vapour pressure is greater than atmospheric pressure. A liquid boils at a temperature at which the vapour pressure of the liquid equals the total pressure of the atmosphere above it.

34
Q

What’s a VLE diagram?

A

A vapour-liquid equilibrium diagram.

The composition of the vapour in equilibrium with a liquid of a given mixture of two components is shown in a binary phase diagram, either as a Txy (temp) or Pxy (pressure) diagram.

35
Q

What’s a Txy and Pxy diagram?

A

A Txy diagram is a plot of the equilibrium temperature versus the mole fraction of the more volatile component with curves for both the liquid phase (T versus xA) and the vapour phase (T versus yA).

Alternatively, equilibrium pressure can be plotted against mol fraction for a fixed temperature to generate a Pxy diagram.

36
Q

What do VLE diagrams consist of?

A

Mole fraction plotted against temperature or pressure.

2 lines are plotted, one for saturated vapour, y, and one for saturated liquid, x.
The area between the lines shows binary phase (both liquid and gas present). The regions above and below show liquid or vapour only.

37
Q

How is relative volatility defined?

A

= vapour pressure
= partial pressure/ mole fraction

E.g.
volatility of A = Pₐ(T) = Pₐ/xₐ
and
volatility of B = Pᵦ(T) = Pᵦ/xᵦ

Therefore (using Raoult’s law) relative velocity, a:

aₐᵦ = Pₐ(T) / Pᵦ

= Pₐxᵦ / Pᵦxₐ

∴ yₐ/ᵦ = a (xₐ/xᵦ)

[Overall relationship between yₐ and xₐ]
yₐ = xₐaₐᵦ / (1 + (aₐᵦ - 1)xₐ)

38
Q

What’s the relationship between yₐ and xₐ

A

yₐ = xₐaₐᵦ / (1 + (aₐᵦ - 1)xₐ)

This can be used to draw a VLE plot.

  • The relative volatility is a measure of the ease of separation.
  • Relative volatility must be greater than 1 for separation
39
Q

How can the fraction of vapour, or fraction of liquid (at a point Q in the 2-phase region of a VLE diagram be found using the Lever rule?

A

Fraction of vapour:
Fr vap = (Q - M) / (P - M)

Fraction of liquid:
Fr liq = (P - Q) / (P - M)

[On a Txy diagram]

40
Q

What’s an azeotrope?

A

A mixture of two liquids which has a constant boiling point and composition throughout distillation.

Max purity occurs at this point (where dew point = bubble point)

41
Q

How are bubble and dew points determined by a Txy diagram for a binary mixture?

A

The bubble point temperature T of a solution of two components, A+B, is determined at a fixed pressure P, and several values of xA, and the equilibrium vapour composition yA is determined for each liquid composition.

e.g. To determine a bubble point temperature for a certain liquid composition, go to the liquid curve on the Txy diagram at that composition, then read the desired temperature from the y-axis. This is the bubble point temperature! For the vapour composition, move horizontally to the right to the vapour curve. Then, drop vertically down to determine the composition of the vapour in equilibrium with liquid.

42
Q

What happens in batch distillation?

A

A partial enrichment of the more volatile component takes place.
The liquid is heated to a temperature where 2 phases form.
The vapour is then separated from the liquid, collected somewhere and condensed. - The resulting liquid is enriched in that specific compound, compared to the original mixture.

43
Q

What is flash distillation?

What occurs?

A

It is usually a continuous process, where: feed is usually pumped through a fired heater and enters the vessel (the still) through a pressure reduction valve.
The still acts as a separator where the liquid and vapour produced by the reduction in pressure have time to reach equilibrium.

Vapour is then removed from the top and liquid is removed from the bottom.

44
Q

What are the main mass balances (for a component A)

A

yₐ = (F xfₐ - S xₐ) / V

&

xₐ = (P - Pᵦ) / (Pₐ - P*ᵦ)

Therefore:
V / F = (x fₐ - xₐ) / (yₐ - xₐ)
Rearranging, the gradient of a straight line to determine vapour composition can be found.

45
Q

What is f, in binary flash distillation calculations?

A

Fractional vaporisation of feed

f = V / F
where V and F are the vapour product product and initial feed flowrate respectively

46
Q

What are F, V and S in binary flash distillation calculations?

A

Feed flowrates (initial, vapour product and liquid product feed flowrates)

47
Q

What happens on each tray of a distillation column?

A

Equilibrium will tend to be reached because some of the less volatile component (LVC) condenses from the rising vapour into the liquid and the heat given up causes some of the more volatile component (MVC) in the liquid to vapourise.

We have equimolar counter-diffusion: Since the heat given out by one molecule of condensing vapour is approximately equal to the heat required to vaporise one molecule of liquid exists, we can sometimes (not always) assume that the molar fluxes of the MVC and LVC are equal, but moving in the opposite direction to each other.

48
Q

What are the top and bottom sections of the distillation column known as?

A

Top - rectifying

Bottom - stripping

49
Q

What happens at the top of a distillation column?

Rectifying section

A

Vapour from the top tray is condensed, and part of the condensed stream is returned as liquid to the top plate (reflux).

Part is taken off as top product.

50
Q

What happens at the bottom of a distillation column?

Stripping section

A

Liquid is withdrawn. Internal or external heating can occur.

External heating involves passing the bottoms through a reboiler prior to it leaving as bottoms product.

This causes the more volatile material to return as vapour in the still. Thus there’s a continual movement and contacting of vapour and liquid in the column.

51
Q

What’s the McCabe Thiele method used for?

A

To determine the theoretical number of plates/stages needed in a column for separation.

It assumes constant molar overflow, which implies:
• A constant molar heat of vaporisation = molar heat of vaporisation of the liquid phase is equal to that of the vapour phase.
• for every mole of vapour condensed, a mole of liquid is vaporised (constant molar reflux flow in any section of the column, so Vn = Vn+1, Ln = Ln+1 and so on).
• heat effects are negligible (i.e. no heat of mixing and no heat loss).

52
Q 
What do the nomenclatures used below represent?
L
V
W
D
F
m
n
d
f
w
q

(for separation by boiling point)

A

L - liquid flowrate

V - vapour flowrate

W - bottom product flowrate

D - distillate (top) product flow rate

F - feed flow rate

m - plates below the feed line

n - plates above the feed line

d - distillate (top product)

f - feed

w - bottom product

q - intersection of operating lines

53
Q

What is the name of the top and bottom section of the distillation column?

A

Top: rectifying section

Bottom: stripping section

54
Q

How is the TOL (top operating line) found for a distillation column?

A

By drawing a line on the graph from where xd intersects the y = x line, to the y intercept.

Complete balance on more volatile component (MVC):
yₙVₙ = xₙ₊₁Lₙ₊₁ + xdD

Assuming constant molar overflow (so Lₙ = Lₙ₊₁ etc).

yₙVₙ = xₙ₊₁Lₙ + xdD
so yₙ = (xₙ₊₁Lₙ + xdD)/Vₙ
which represents a straight line.
When xₙ₊₁ = xd and xₙ₊₁ = 0,

The TOL is drawn through: (xd, xd) and (0, xdD/Vₙ)

55
Q

How is the BOL (bottom operating line) found for a distillation column?

A

1) Carry out a total balance:
Lm₊₁ = W + Vm

and a component balance on the MVL (more volatile compound).

2) xm₊₁Lm₊₁ = xwW + ymVm

assume constant molar overflow - Lₘ = Lm₊₁ etc)

Therefore,
ym = xm₊₁(Lm/Vm) - xw(W/Vm)

This can therefore be drawn as a straight line of gradient L/V with y intercept -xw(W/Vm)

It will pass through (xw, xw) and (0, -xw(W/Vm)

56
Q

How can mass balances be formed to link the top and bottom of a distillation column?

A

Considering around the feed point for equimolar counter-diffusion:

Generally,
Lm = Ln+ qF
and
Vn = Vm + (1-q)F

So at bubble point / saturated liquid:

q = 1, (1-q) = 0, 
Lm = Ln + F
Vm = Vn

At dew point,
q = 0, (1-q)=1
Lm = Ln
Vn = Vm + F

57
Q

What does constant molar overflow suggest?

A

For every mole of vapour condensed, a mole of liquid will be vaporised.

58
Q

What is the y intercept of a McCabe-Thiele plot (on a graph of liquid and vapour mole fractions) equal to?

A

xd / (R + 1)

Where:
xd is the mole fraction of the distillate (top product)

R is the reflux ratio (equal to L/D)

59
Q

How do you construct a McCabe-Thiele plot?

A

A VLE diagram is needed. This can be given or plotted.

  1. Add a y = x line
  2. Mark xd, xf and xw
  3. Add the q-line
  4. Add the TOL
  5. Add the BOL
  6. Step off from the top to find the number of stages
60
Q

What is the feed line?

A

Aka the q line.
It’s the intersect of the top and bottom operating lines.
The q-line gives indication of the feed condition (how much of feed is
liquid and how much of it is vapour).

61
Q

How can the q/feed line mathematically defined?

A

q = (Lm - Ln)/F

It is also defined,
q = heat to vapourise 1 mole of feed / molar latent heat of feed.

q = (H bp - H feed + H vap)/H vap

For saturated vapour, numerator = 0, q = 0

For saturated liquid, denominator = 0, q = 1

62
Q

How is the optimal feed plate located?

A

It should be Lm the stage closest to the intersection of top and bottom operating lines.

This ensures the maximum enrichment per plate. If the feed is specified elsewhere, it will result in more plates being needed to effect a given separation.

63
Q

What’s the reflux ratio?

A

The ratio of the flow returned as reflux (Ln) to the quantity of top product taken off (D)

R = L/D

64
Q

What happens if reflux ratio is increased?

A

The MVC in the vapour is increased since vapour ascending is contacted with a descending boiling liquid, which is rich in MVC.

It increases the purity of the top product but has higher energy costs.

If reflux ratio is high, a larger column diameter is needed since more liquid is returned to the column.

Number of stages needed depends on R

65
Q

What’s the y intercept of a McCabe Thiele graph equal to?

A

Xd/(R + 1)

= Xd*D/Vn

66
Q

What does total reflux show (with regards to number of stages in a distillation column)?

A

The minimum number of theoretical stages.

The number of plates at total reflux can be calculated graphically or by using Fenske’s method.

67
Q

What is total reflux?

A

When all condensate is returned to the column as reflux, and there is no product.

This shows the minimum number of theoretical stages needed for separation.
(Not practical)

68
Q

What is the miminum reflux ratio?

A

The minimum reflux for which enrichment can occur.
Below Rm, the desired enrichment cannot be obtained.

As the reflux ratio is reduced, a ‘pinch point’ occurs where the separation can only be achieved by an infinite number of stages.

It can be found graphically or by the Underwood-Fenske method.

69
Q

How is Rmin determined graphically?

A

Dr aw in a top operating line which passes through (xd, xd) and the intersection of the ‘q’ line and the equilibrium line (Point A).

Point A is also known as the ‘pinch point’.

This operating line will intersect the y axis at the point B; from the value of y intercept, Rm can be calculated.

70
Q

What’s the optimum reflux ratio?

A

A practical R between R min and total reflux.

As the reflux ratio decreases towards minimum reflux, Rm, the number of plates tends to infinity (∞).

Increasing the reflux ratio towards total reflux decreases the number of plates required in the distillation column.

A trade off must be found.

A typical value for R is 1.5*Rmin

71
Q

How is total cost of a distillation column system determined?

A

= capital + operating costs.

There’s a trade off between the 2.

Capital cost is determined by number of plates and the size of
column.
Operating cost takes into account the steam and cooling water required by the reboiler and condenser.

Also, as the reflux ratio is increased, the number of plates required is reduced, reducing the capital cost of the column.
However, at high values of reflux ratio, the capital cost rises since there is only a small reduction in the number of plates, but the diameter, and hence the area continually increase because vapour load becomes greater.

− Therefore, the condenser and reboiler will also be larger and more expensive.
− The steam required per mole of product is proportional to (R+1). Therefore, the steam cost increases linearly with reflux ratio.

72
Q

How is overall column efficiency calculated?

A

Eo = number of idea, stages / number of real stages

73
Q

What are the Murphree plate efficient equations?

A

E.my = (yn - y n-1)/(ye - y n-1) (vapour)

E.mx = (x n+1 - x n)/(x n+1 - xe) (liquid)

n is the plate and composition on that plate
e is equilibrium composition

Chem Eng - Y1.S2 (10 decks)

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