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OPERATING MANUAL - VOLUME A - SECTION A- 03 THEORITICAL ELEMENTS – THE
DISTILLATION COLUMNS
1.
2.
3.
THE PRINCIPLE OF DISTILLATION
2
1.1 Theoretical Trays
1.2 Structured Packing
2
3
THE DISTILLATION COLUMN
4
THE DOUBLE DISTILLATION COLUMN
6
3.1
3.2
3.3
3.4
6
7
8
9
The main vaporizer
The LP and the MP column
The “Minaret”
Assessment matter
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INTRODUCTION
The principle of distillation is to separate a mixture in its various components more or less volatile, i.e. which do not
vaporize at the same temperature (with pressure given).
1.
1.1
THE PRINCIPLE OF DISTILLATION
Theoretical Trays
The separation of gases by distillation is done by means of a set of distillation trays whose purpose is to achieve a
transfer of heat and material between an ascending vapor and a descending liquid.
The liquid coming from a tray has the same composition as the liquid on the tray.
The ascending vapor Vo enters the tray through perforations and comes into contact with the liquid descending from
the upper tray L2.
During this exchange, part of the vapor Vo condenses, part of the liquid on the tray vaporizes, with a resulting
ascending vapor V1 which is in equilibrium with the liquid on the tray thus with L1, this vapor being richer in light
elements than L1.
When the tray is in continuous operation the material balance must be even, this being achieved for a known flow
and composition of liquid L2.
Vapor V2
liquid L2
Vapor V1
Ponctual contact
Liquid/Vapor
liquid L1
vapor V0
liquid L2
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OPERATING MANUAL - VOLUME A - SECTION A- 03 THEORITICAL ELEMENTS – THE
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1.2
Structured Packing
Structured Packing is an internal device of distillation columns, facilitating a close contact of liquid with vapor. This
type of packing is made of corrugated metal sheet, criss-crossing one another
The use of this technology for cryogenic separation has been successfully developed and industrially validated by Air
Liquide at the beginning of the 1980s.
The technological advantages, as compared to trays, are:

their low pressure drop and consequently significant energy savings

their greater « flexibility » (good performance over wider range of liquid and vapor flow)

their lower « inertia » (less liquid inventory), permitting quick change of operating modes.
Recently a new generation of Air Liquide improved structured packing has been issued and validated allowing a
significant reduction of column diameter.
The limit of structured packing operation is flooding. It appears when the gas or liquid flow rates become too high.
Physically, it corresponds to a carry-over of the downward liquid film by the gas stream flowing in the opposite
direction. In the vicinity of flooding, pressure drop increases dramatically with the slightest increase in vapor load.
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OPERATING MANUAL - VOLUME A - SECTION A- 03 THEORITICAL ELEMENTS – THE
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2.
THE DISTILLATION COLUMN
A column consists of a stack of structured packing elements through which passes a flow of ascending gas, and a
descending liquid. In other words, the heavy products are condensed and the light ones are vaporized.
For a mixture O2 + N 2
Condenser
Most volatile product
(O2+N2)
Structured
Packing
Less volatile product
Heater
Depending on what we have to treat (liquid or gas) there are two possibilities:
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OPERATING MANUAL - VOLUME A - SECTION A- 03 THEORITICAL ELEMENTS – THE
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
The only use of the heater
Residual ( O2 + N2)
Vapor
(O2+N2)
Liquid
We obtain at the bottom of the column , liquid
oxygen (its purity depends on the number of
structured packing elements and on the reflux
ratio) and at the head, the residual: vapor of
oxygen and nitrogen
liqui
de
Less volatile product (O2)
Liquid
Heater

The only use of the condenser
Condenser
Most volatile product (N2)
Vapor
We obtain at the top of the column a vapor
made up of nitrogen (of which the purity
depends among other things of the number of
structured packing elements and on the reflux
ratio) and at the bottom, a liquid residual
containing N2 andO2 (it is the Rich Liquid)
(O2+N2)
Vapor
Residual ( O2+N2)
Liquid
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OPERATING MANUAL - VOLUME A - SECTION A- 03 THEORITICAL ELEMENTS – THE
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2.
THE DOUBLE DISTILLATION COLUMN
The idea is to combine the two described models above, in a double distillation column : by vaporizing oxygen with
the heat of the gas nitrogen, which would be condensed.
3.1
The main vaporizer
The idea enabling the combined operation of the two columns described above, is the condensation of nitrogen gas
with the vaporization of liquid oxygen in the main vaporizer
LP column
It is located between the 2 columns. It is a
bath exchanger of counter-current type.
Liquid oxygen is in the bath, passages are
opened. The liquid enters at the bottom of
the exchanger and is driven by the
thermosiphon effect. It is the formation of
the gas bubbles by vaporization which
generate this effect.
Venting of
GOX: Gas Oxygen
incondensable
gas
LOX exit
LOX: Liquid Oxygen
GAN: Gas Nitrogen
LIN: Liquid Nitrogen
MP
Column
Colonne
MP
Lean Liquid (low oxygen
content)
Rich Liquid (rich in oxygen)
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3.2
1st Assembly of the double column
Waste (O2 +N2):
With O2 ~15%
The oxygen content in the rich liquid is about 40 %
(for an average pressure of 6 bars).
The oxygen content of the LP column waste is about 15
%
This represents a significant oxygen loss.
The 2nd assembly will decrease these losses and
increase the oxygen recovery (ratio of the number of
molecules produced on the number of entering
molecules)
LP Column
GOX
LOX
Incondensable
gas
GAN
LIN
MP Column
Gas air
Rich Liquid
With O2 ~ 40%
O2 production flow X (purity [ % ]/100)
recovery O2 = ------------------------------------------------------------- X 100 [ % ]
air flow X 0.2096
3.3
2nd Assembly of the double column
To improve the oxygen recovery, we distil gas going up in the column LP, to reduce the quantity of oxygen contained
in the waste.
Thus, It is necessary to add a section above the previous LP column, and to feed the head of this additional section
by a liquid to enable the distillation.
This liquid is taken in the MP column, with a low O2 content. The optimum purity of this liquid corresponds roughly at
twice the waste O2 content.
We notice the increase of oxygen recovery due to the low O2 content of the LP waste.
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Waste:
Nitrogen impure
With O2 < 0.5%
LP Column
GOX
Lean Liquid
With O2 ~ 1%
LOX
GAN
MP Column
LIN
Lean liquid
Gas air
Rich liquid
Notes:

more the nitrogen production in MP column is significant ( >10% of the air flow) more the equivalent content
oxygen of waste will be high.

Typically : Lean liquid ~1% O2, Waste N2~0.5% O2.
3.3
3rd Assembly of the double column: MINARET
It is used to produce pure nitrogen in great quantity at low pressure, by continuing the distillation of LP waste N2.
Some structured packing are added at the head of the LP column.
Only a part of the waste N2 flow is treated so, the section of this section is slightly lower than that of the remainder of
the column.
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OPERATING MANUAL - VOLUME A - SECTION A- 03 THEORITICAL ELEMENTS – THE
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GAN at Low pressure
Minaret
Waste N2
LP Column
GOX
LOX
Incondensable
gas
GAN
Liquid
Nitrogen
MP column
Lean Liquid
liquid
LIN
Gas air
Rich liquid
3.4
O 2
Assessment matter
Q
 Goxpurity  QLOX  Loxpurity
 100 GOX
0.2096QAIR
QAIR
The principal parameter which fixes the
performance of the cold box is the oxygen
recovery.
= Air flow in the cold box
The majority of Air Liquid plant realizes an oxygen extraction output near to 100 % in production total gas + liquid.
The oxygen purity is generally close to 99.5 % (pure O2) or 95% (impure O2).
All the oxygen molecules are not in the oxygen production. However, oxygen recovery is close to 100 % because few
molecules are lost in the waste N2.
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OPERATING MANUAL - VOLUME A - SECTION A- 03 THEORITICAL ELEMENTS – THE
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3.5
Importance of the backward flow
It is the ratio:
L
V
Liquid flow
Vapor flow
Vapor Liquid
When L/V increases in a section of a column, it involves:
- at the top of the section a reduction in concentration of the least volatile elements
- at the bottom of this section an increase of content in least volatile element.
The result is reversed if L/V decreases.
In MP Column:
Gas air enter at the bottom, thus the vapor composition arriving in bottom of the column is fixed.


A L/V growing will decrease O2 content at head, thus improve the purity of the product.
Therefore a reduction of L/V will reduce the purity of the product.
In LP Column:
The compositions are those of the liquid backward flows coming from the MP column.
The L/V increases in the bottom part of the LP column: it will reduce the purity of produced oxygen.
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OPERATING MANUAL - VOLUME A - SECTION A- 03 THEORITICAL ELEMENTS – THE
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3.6
Profile of vapor composition in a double column
The oxygen content in vapor decreases as this vapor crosses the packing structured upwards
Impur nitrogen content
residual
LP
Decrease of LOX production
RL
GOX
GOX content
LOX
GAN
GAN & LIN content
PL content
Gas air
AIR
20
LR content
40
60
80
% oxygen
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