Principles - Volatility / Boiling point Separation Flashcards
What’s distillation?
The separation of components of a mixture on the basis of boiling (/bubble) point or volatility.
The distillation column has stages for different equilibriums.
What’s vapourisation?
Vapourisation - The process of change of state from a liquid (or solid) to a gas (vapour).
What’s boiling point?
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’.
What’s volatility?
The tendency of a liquid to vaporise.
Liquids that have a high volatility have low boiling points.
What’s saturation vapour pressure?
The pressure exerted by a gas in equilibrium with its liquid (or solid).
• It is a measure of the volatility of a liquid.
What’s partial pressure?
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
What is vapour pressure, P*?
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.
What’s a saturated gas?
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
How is vapour pressure calculated?
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!
How do evaporation and boiling differ?
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.
How many mmHg are in 1 atm?
1 atm = 760 mmHg
What is normal boiling point?
Boiling point at 1 atm
What happens to fluid vapours when 2 liquids are mixed?
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*)
How are mole fractions found?
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
How can the distribution of components in vapour and liquid phases be predicted?
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*)
What’s an ideal solution?
A gas-liquid system in which the VLE (vapour-liquid equilibrium) for each species obeys either Henry’s law or Raoult’s law.
What’s Raoult’s law?
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).]
What’s Henry’s law?
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])
What is the bubble point?
[The conditions] When the first bubbles of gas form in a liquid
Vapour pressure = atmospheric pressure
What’s dew point?
[The conditions]
When the first droplets of liquid form from a vapour
When is Raoult’s law valid?
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).
What do yₐ and xₐ represent (in separations and distillation)?
yₐ - mole fraction of a in gas/vapour phase
xₐ - mole fraction of a in liquid phase
What does Raoult’s law state?
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
When should Henry’s law be used (instead of Raoult’s law)?
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.
When should Raoult’s law be used (instead of Henry’s law)?
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.
How do Henry’s law and Raoult’s law differ?
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
What does Henry’s law state?
The partial pressure of the species in the vapor phase is directly proportional to its liquid phase mole fraction.
By what method are bubble point temperature & pressure and dew point temperature & pressure calculated?
By iteration (trial and error)
How is bubble point temperature calculated?
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).