Overview of Cryogenic Air Separation Authors: Jared Watkins,
Overview of Cryogenic Air Separation
Authors: Jared Watkins, Juan Tellez
1. Equilibrium Condition
2. Mass Balance
Key steps of the cryogenic air distillation process including air
compression, air cooling and purification, heat exchange, refrigeration,
internal product compression and rectification were investigated. The
study of this process has become increasingly popular in accordance
with the rise in demand for these components within industry such as
mining and semiconductors. Within the separation unit the air is
separated via distillation at very low temperatures to take advantage of
the boiling point differences of the air components in a process known
as rectification. The distillation column can be designed depending on
the specific products needed, with the most common design being the
double column system with an adjacent argon unit. The energy needed
for the very low temperatures constitutes most of cost of production and
so it is of interest to optimize the process and obtain maximum
efficiency. While involving a high initial capital cost air separation units
have relatively high yields and can obtain large volumes of high purity
gases or liquids.
3. Energy Balance
Rectifying Section Operating Line
Stripping Section Operating Line
Peng Robinson EOS
Where =vapor phase
composition of component i, and the liquid phase, n=molar flow rate,
h=enthalpy, and R=reflux ratio
Figure 7: Column Tray schematic.
Retrieved from Roffel, 2000.
Liquid oxygen flows down and
becomes more concentrated as
nitrogen vapor rises. Modern plants
use structured packing's to offer
maximum surface area and a low
Figure 2: Process flow diagram of cryogenic air distillation. Shows the step by step procedure of this process. Retrieved from Linde Engineering.
Table 1: Air composition and thermodynamic properties of its components
at 101.1 Pa. Retrieved from Agrawal, 2000.
Figure 1: Vapor Pressure
curve for atmospheric
gasses, showing the dew
point and bubble point
of air and its major
from Linde Engineering.
Ambient air is filtered to
remove dust particles and then
fed to a series of turbo
compressors with an intercooler.
The air is compressed to an
operational pressure of about 6
Air Cooling and
Process air is cooled with
water in a direct contact
cooler also removing soluble
air impurities. Cooling water
is prepared in an evaporation
cooler against dry nitrogen
waste gas from the
rectification process. CO2,
water vapor and
hydrocarbons are removed
periodically via molecular
sieve absorbers. These
impurities otherwise would
freeze and disrupt equipment.
Further cooling of process air
in heat exchangers by means
of countercurrent exchange
with nitrogen waste gas from
the rectification process. The
air is cooled to nearly
A refrigeration process
provides further cooling for
the cold temperatures
required for air separation.
Side streams are drawn off
and compressed further in an
air booster compressor and
then expanded in turboexpanders taking advantage
of the Joule-Thompson effect
to supply the refrigeration.
Rectification of Air
Rectification of Argon
A double-column design with
a combined condenser and reboiler is used to obtain
oxygen and nitrogen. The
partially liquefied air enters
the bottom of the high
pressure column, oxygen
enriched liquid forms in the
column sump and pure
nitrogen gas at the column
top. Further separation takes
place in the low pressure
column with pure oxygen gas
at the bottom
A side stream enriched in
Argon and Oxygen is taken
from the bottom of the low
pressure column and fed to
the crude argon column. High
purity Argon is produced at
the top and liquid oxygen at
the sump. The oxygen is
refluxed back to the lowpressure column.
Table 3: Explanation of each step on process flow diagram. Retrieved from Linde Engineering.
Cryogenic air separation takes advantage of the differing boiling points of
its components as shown in Table 1. In order to start the separation process,
a large quantity of the incoming air needs to be liquefied. This is
accomplished by cooling the air by decreasing its temperature and
manipulating the pressure until condensation begins. Figure 1 displays a
curve delineating the range at which air condenses when approaching from
the right (decreasing temperature). The air can then be separated into its
components(Table 1), mainly Nitrogen, Oxygen and Argon. The first
process developed by Linde in 1902 separated oxygen from air , and
developed into the double column mechanism in 1910 and its Argon
constituent in 1913 as to be discussed. Figure 2 displays the general
overview of the process.
Low amount of electricity per
Produces very high purity
Can generate liquid nitrogen
for storage on site
Table 2: Optimal air separation
results from developed
optimization model. Retrieved
from Zhu, 2006.
Optimization plays a role in
economically running a column.
Energy is the biggest cost input
and therefore a maximum return
is desired. A high recovery at a
high purity is ideal. This can be
accomplished by controlling the
processes different variables,
including the column pressure,
top composition and bottoms
Large site space and utility
High capital cost
Limited scaleability in
Long start-up and shutdown
Low to moderate capital cost
High maintenance equipment
production of relativity high
Quick installation and start-up
Uneconomical for high purity
Low capital cost
Production output is very
Uneconomical for large
Quick installation and start-up
Requires relatively large
Easy to vary purity and flow
amount of electricity per unit
Table 4: Advantages and disadvantages of different methods of air
separation. Retrieved from Jaya, 2013.
AIR SEPARATION OPTIMAL RESULTS
Figure 3: X-Y Diagram N2-O2 at P=1.4atm
Figure 4: X-Y Diagram N2-O2 at P=5atm
Using vapor-liquid equilibrium data for
Oxygen and Nitrogen K values are derived as
seen in Equation 1. The Ratio of these K
values defines the relative volatility, , from
which the line is plotted.
This Diagram is produced the same way
presented in Figure 3, however the pressure is
now 5 atm. This represents the conditions in
the high pressure column. Nitrogen is more
volatile than oxygen, resulting in a good
vapor-liquid separation as seen in the above
This diagram represents, with respect to
nitrogen, the amount of nitrogen vapor in
equilibrium with the liquid nitrogen at 1.4
atm. This represents the conditions of the low
Figure 5: X-Y Diagram Ar-O2 at P=1.4atm
Argon is not as volatile as Nitrogen but still
more volatile than oxygen. As a result the
vapor-liquid separation isnt as well defined
as in the previous Figures. Separation is still
possible but more trays are needed.
Figure 6: McCabe-Thiele Analysis
The VLE diagrams can be used in column
analysis. The operating lines expressed in
Equations 4 and 5 were plotted along with a
horizontal line, q=1, at the feed composition,
x=0.79 for Nitrogen. The desired distillate
purity acts as the starting point The operating
lines and equilibrium line act as the lower and
upper boundary's from which subsequent
material and equilibrium balance lines, or
steps, are used to get to the desired bottoms
composition. The number of steps is the
number of theoretical stages.
Agrawal, R., & Herron, D. (2000). Air Liquefaction: Distillation. Allentown,
PA, USA. Retrieved from
Amarkhail, Sher shah. Air Separation Diploma project. Retrieved from
Jaya, A. (2013, January). Air Separation Units (Engineering Design Guideline.
Linde Engineering. (n.d.). History and technological progress. Cryogenic air
separation. Retrieved from
Roffel, B., Betlem, B., & Ruijter, J. (2000, March). First Principles dynamic
modeling and multivariable control of a cryogenic distillation process.
Zhu, Y., Liu, X., & Zhou, Z. (2006, June). Optimization of Cryogenic Air
Separation Distillation Columns. Dalian, China. Retrieved from
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