Authors: Jared Watkins, Juan Tellez

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.

INTRODUCTION

1. Agrawal, R., & Herron, D. (2000). Air Liquefaction: Distillation. Allentown, PA, USA. Retrieved from https://www.thevespiary.org/library/Files_Uploaded_by_Users/Sedit/Chemical%20Analysis/Encyclopedia%20of%20Separation%20Science/Level%20III%20-%20Practical%20Applications/AIR%20LIQUEFACTION%20-%20DISTILLATION.pdf

2. Amarkhail, Sher shah. Air Separation Diploma project. Retrieved from http://kchbi.chtf.stuba.sk/ODAOld/doc/Diploma%20work%20of%20Shershah%20Amarkhail.pdf

3. Jaya, A. (2013, January). Air Separation Units (Engineering Design Guideline. Retrieved from http://kolmetz.com/pdf/EDG/ENGINEERING_DESIGN_GUIDELINE_Air_Separation_Units_Rev01web.pdf

4. Linde Engineering. (n.d.). History and technological progress. Cryogenic air separation. Retrieved from http://www.linde-engineering.com/internet.global.lindeengineering.global/en/images/AS.B1EN%201113%20-%20%26AA_History_.layout19_4353.pdf

5. Roffel, B., Betlem, B., & Ruijter, J. (2000, March). First Principles dynamic modeling and multivariable control of a cryogenic distillation process. Retrieved from http://tbk.eldoc.ub.rug.nl/FILES/root/2000/CompChemEngRoffel/2000CompChemEngRoffel.pdf

6. Zhu, Y., Liu, X., & Zhou, Z. (2006, June). Optimization of Cryogenic Air Separation Distillation Columns. Dalian, China. Retrieved from http://people.tamu.edu/~wangzhu/YuZhu_WCICA2006.pdf

REFERENCES

Overview of Cryogenic Air Separation

Air CompressionAir Cooling and

Purification Heat ExchangeInternal Compression

and Refrigeration Rectification of Air Rectification of Argon

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 bar.

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 liquefaction temperature.

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 turbo-expanders taking advantage of the Joule-Thompson effect to supply the refrigeration.

A double-column design with a combined condenser and re-boiler 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 low-pressure column.

EQUATIONS

Figure 3: X-Y Diagram N2-O2 at P=1.4atm (Amarkhail, 2009)

Figure 4: X-Y Diagram N2-O2 at P=5atm (Amarkhail, 2009)

Figure 5: X-Y Diagram Ar-O2 at P=1.4atm (Amarkhail, 2009)

Figure 6: McCabe-Thiele Analysis (Amarkhail, 2009)

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, α, from which the line is plotted.

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 pressure column.

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 figure.

Argon is not as volatile as Nitrogen but still more volatile than oxygen. As a result the vapor-liquid separation isn’t as well defined as in the previous Figures. Separation is still possible but more trays are needed.

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.

Figure 2: Process flow diagram of cryogenic air distillation. Shows the step by step procedure of this process. Retrieved from Linde Engineering.

Figure 1: Vapor Pressure curve for atmospheric gasses, showing the dew point and bubble point of air and its major components. Retrieved from Linde Engineering.

AIR SEPARATION OPTIMAL RESULTS

Optimal Results

Air total input, m3/h 3150

Gas O2 product output, m3/h 516.7

O2 product Purity 99.6%

Gas N2 product output, m3/h 1449

N2 product Purity .999

Liquid O2 product output, m3/h 30.05

Liquid O2 product Purity 99.75%

Argon product output 9.5

Argon product purity 95.09%

1. Equilibrium Condition 2. Mass Balance

3. Energy Balance 4. Rectifying Section Operating Line 5. Stripping Section Operating Line 6. Peng –Robinson EOS 7. Joule-Thompson Effect Where =vapor phase

composition of component i, and the liquid phase, n=molar flow rate, h=enthalpy, and R=reflux ratio

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 composition (Amarkhail).

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 pressure drop.

Table 4: Advantages and disadvantages of different methods of air separation. Retrieved from Jaya, 2013.

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.

Background

Process Advantages Disadvantages

Cryogenic

• Low amount of electricity per unit nitrogen

• Produces very high purity nitrogen

• Can generate liquid nitrogen for storage on site

• Large site space and utility requirements

• High capital cost• Limited scaleability in

production• Long start-up and shutdown

PSA

• Low to moderate capital cost• Cost-effective nitrogen

production of relativity high purities

• Quick installation and start-up

• High maintenance equipment• Noisy operation• Limited scalability

Membrane

• Low capital cost• Production output is very

flexible• Quick installation and start-up• Easy to vary purity and flow

rate

• Uneconomical for high purity requirements

• Uneconomical for large outputs

• Requires relatively large amount of electricity per unit nitrogen

Table 3: Explanation of each step on process flow diagram. Retrieved from Linde Engineering.

Table 1: Air composition and thermodynamic properties of its components at 101.1 Pa. Retrieved from Agrawal, 2000.

Temperature (K)

Pres

sure

(Pa)