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operating manual - volume A - section A- 03 theoritical elements – THE distillation COLUMNS
1.The principle of distillation
1.1Theoretical Trays
1.2Structured Packing
2.The distillation column
3.The double distillation column
3.1The main vaporizer
3.2The LP and the MP column
3.3The “Minaret”
3.4Assessment matter
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.The principle of distillation
1.1Theoretical 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.
1.2Structured 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 sizesdiameter.
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.
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
Depending on what we have to treat (liquid or gas) there are two possibilities:
- The only use of the heater
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
- The only use of the condenser
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)
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.
3.1The 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
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.
GOX: Gas OxygenLOX: Liquid Oxygen
GAN: Gas Nitrogen
LIN: Liquid Nitrogen
Lean Liquid (low oxygen content)
Rich Liquid (rich in oxygen)
3.21st Assembly of the double column
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)
O2 production flow X (purity [ % ]/100)recoveryO2 = ------X 100 [ % ]
air flow X 0.2096
3.32nd 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.
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.
3.33rd 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.
3.4Assessment matter
The principal parameter which fixes the performance of the cold box is the oxygen recovery.
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.
3.5Importance of the backward flow
It is the ratio:
LV
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.
3.6Profile of vapor composition in a double column
The oxygen content in vapor decreases as this vapor crosses the packing structured upwards
E-TPL-10-10-10 – rev A