Week 5 Flashcards
Used when the materials to be separated have a high boiling point and decomposition may occur if direct distillation was to be employed.
Steam distillation
The type of distillation steam distillation is
Differential distillation
In steam distillation, steam is introduced directly into the liquid, providing that the solubility of steam in the liquid is ____.
low
In steam distillation, steam is introduced directly into the liquid, providing that the ____ of steam in the liquid is low.
solubility
In steam distillation, the effect of adding steam is to ____ the boiling point by reducing the effective total pressure of the still.
Lower
In steam distillation, the effect of adding steam is to lower the boiling point by ____ the effective total pressure of the still.
Reducing
In steam distillation, the effect of adding steam is to lower the boiling point by reducing the effective total ____ of the still.
pressure
Expensive alternative to steam distillation
Vacuum distillation
Two types of steam which can be injected into the column for steam distillation
- Saturated steam
- Superheated steam
Steam distillation
Some of the steam condenses to provide the heat to vapourise the material
Saturated steam
The steam can provide enough heat of vapourisation for the material concerned without itself condensing.
Superheated steam
Where steam is injected into
The base of the column
Basic principle of packed column
- surface of the packing, over a certain height, is equivalent to a theoretical plate
- transfer occurs at the interface between the liquid and vapour
Types of column packing
- Random packing
- Structured packing : arranged, ordered & stacked
Examples of random packing
- Raschig rings and saddles
- “through flow”
Which column packing type has the lowest relative cost?
Raschig rings and saddles
Which column packing type has the highest relative cost?
structured
Which column packing type has the lowest efficiency?
Raschig rings and saddles (moderate)
Which column packing type has the highest efficiency?
Structured (very high)
Which column packing type has the lowest pressure drop?
Structured (very low)
Which column packing type has the highest pressure drop?
Raschig rings and saddles (moderate)
Which column packing type has the lowest vapour capacity?
Raschig Rings and saddles (fairly high)
Which column packing type has the highest vapour capacity?
“through flow” & structured (high)
Typical turndown ratio for Raschig rings and saddles, “through flow” and structured
2
When packed columns are used when…
- the column diameter is less than 2 feet
- the pressure drop is low
- corrosion considerations favour ceramic based materials
- low liquid hold up is desirable (safety concerns may require low inventory)
Disadvantages to packed columns
- the points at which inputs and outputs are sited are more difficult to establish. Therefore sidestreams become a problem
- the turndown ratio is not as good as plate columns
- the flow paths through packing involve wall effects (more flow at walls) and therefore care has to be taken with liquid redistribution.
- packed bed are not predictable
- poor cooling
- cannot handle foams
- scale - up can be more problematic than with plate columnms
- cleaning and maintenance can be problematic
Height of an equivalent theoretical plate (HETP)
packed height/ number of equivalent equilibrium stages
Rate of change of composition for packing columns is dependent upon
- equipment used
- the conditions of the vapour and liquid flows
- the rate of diffusion across the gas-liquid interface
Height of the column
height of transfer unit * number of transfer unit
Z = (HOG)(NOG)
Z = (HOL)(NOL)
Height of a gas transfer unit
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
Height of a liquid transfer unit
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
Number of gas transfer units
NOG = ∫dy/ ye - y
y = mole fraction of the most volatile component in the vapour
Number of liquid transfer units
NOL = ∫dx/ x - xe
