(516c) Innovative Cryogenic Air Separation Processes

Cryogenic Air Separation Units (ASUs) are widely used for the high-volume supply of O₂, N₂ and rare gases in industrial processes such as steel plants, gasification plants, fertilizer plants and petrochemical plants. More recently, due to the increased concern about CO₂ emissions, a new large scale ASU application is the O₂ supply in CO₂-capturing power plants by oxy-combustion or pre-combustion. The cryogenic ASUs are causing considerable energy penalties related to CO₂ capture. New research activities on improving the performance of ASUs targeted for CO₂-capturing plants are essential.

Traditional cryogenic ASUs are based on the double-column distillation scheme as developed by Carl von Linde in 1910. In this scheme, air is crudely separated in a higher pressure (HP) column and then further separated in a lower pressure (LP) column. The high pressure N₂ from the top of the HP column can be condensed against the boiling O₂ at the bottom of the LP column. The condensed N₂ liquid is used as reflux for the two distillation columns. The double-column distillation scheme has solved the most considerable challenge in sub-ambient distillation processes, i.e. the production of reflux. However, the air feed has to be generally compressed to 400-600 kPa in order to be separated in the HP column. This causes unnecessary compression of O₂ in the air feed, thus wasting expensive compression work, in applications such as coal based power plants with oxy-combustion, where O₂ slightly above atmospheric pressure is sufficient.

An exergy analysis of a traditional double-column air distillation cycle indicates that the actual energy consumption (0.199 kWh/kgO₂) is around 4 times the theoretical minimum (around 0.05 kWh/kgO₂) when all unit operations are assumed reversible [1]. The compression of the air feed and the distillation system are responsible for the two largest exergy losses: 38.4% and 28.2% respectively. The irreversibility in distillation columns can be reduced by using “distributed reboiling”. The energy consumption can be reduced by 10% when dual reboilers are used. For the air compression process, the energy consumption can be reduced by (i) improving the compressor efficiency, (ii) reducing the compression ratio, (iii) lowering the operating temperature, and (iv) reducing the mass flow through the compressor. This paper focuses on the last of these measures, i.e. ways to reduce the mass flow through the air compressor. A recuperative vapor recompression (RVRC) heat pump distillation scheme is proposed for the separation of air. As a result, only one column is used for air separation. In addition, the principle of “distributed reboiling” is applied to reduce irreversibilities in the distillation column.

In a simple RVRC heat pump distillation scheme, the air feed is slightly compressed to compensate for pressure losses along its flow path. The air is separated into N₂ and O₂ in a single distillation column. The reflux for the column is produced in the following way: the N₂ from the top of the column is heated to ambient temperature; a portion of the heated N₂ is compressed and cooled to its dew point after the compression heat is removed (by cooling water); the compressed and cooled N₂ is then condensed against the boiling O₂ in the bottom of the distillation column; the condensed N₂ liquid is then sub-cooled if necessary, expanded and finally sent back to the distillation column as reflux. In this case, the O₂ in the air feed is not significantly compressed. When this scheme is used for the production of O₂ (125 kPa) with a purity of 95 mole%, the energy consumption is calculated to be 0.184 kWh/kgO₂, a 7.5% reduction compared to a traditional double-column air distillation process [2, 3].  The energy consumption is reduced to 0.165 kWh/kgO₂ (17.1% reduction) when an intermediate RVRC heat pump is applied for distributing the reboiling. In addition, the energy consumption can be further reduced by efficient organization of the refrigeration cycle, e.g. using a portion of the air feed instead of the N₂ as the refrigerant.

The RVRC heat pump distillation scheme can also be applied for the production of high purity O₂. An argon side column can be added in this case. The energy consumption is calculated to be 0.232 kWh/kgO₂ in the case that the air feed is separated into the following products: O₂-99.4 mole%, N₂-99.5 mole% and argon-98.2 mole%. A 5.3% reduction in energy consumption has been achieved compared to a traditional three-column air separation cycle [4]. Since much more reflux is required in the high purity O₂ production process than in the low purity (95 mole%) case, the energy saving potential is expected to be less for high purity than for low purity O₂ production.

For the production of high pressure O₂, a simple way is to compress the gaseous O₂ (GOX) to the desired pressure in a low pressure O₂ production cycle. The pumped liquid O₂ (LOX) cycle is more commonly applied in recent years for the reason of safety and cost. In a pumped LOX cycle, the O₂ is produced in liquid phase and then pumped to the desired pressure. The pumped LOX is then vaporized against a portion of the air feed and heated to ambient temperature. When the RVRC heat pump distillation scheme is applied for producing high pressure O₂, a pumped LOX process can be similarly applied. The energy consumption is calculated to be 0.301 kWh/kgO₂ when O₂ (95 mole%) is produced at 4000 kPa, while the value is reported to be 0.313 kWh/kgO₂ when the double-column distillation scheme is used as the basis [5].

In conclusion, a recuperative vapor recompression heat pump distillation scheme is proposed to substitute the traditional double-column distillation scheme in cryogenic air separation processes. When the scheme is applied for the production of O₂ with low purity and low pressure, the energy consumption can be substantially reduced by avoiding the compression of O₂.  The scheme can also be applied for producing O₂ with high purity and/or high pressure, however, the energy saving potential is smaller since the mass to be compressed is larger.

References

[1] Fu, C.; Gundersen, T. Using exergy analysis to reduce power consumption in air separation units for oxy-combustion processes. Energy 2012, in press.

[2] Fu, C.; Gundersen, T.; Eimer, D. Air separation. GB Patent, application number: GB1112988.9, 2011.

[3] Fu, C.; Gundersen, T. Using PSE to develop innovative cryogenic air separation processes. Proceedings of the 11th International Symposium on Process Systems Engineering, 15-19 July 2012, Singapore.

[4] Beysel, G. Enhanced cryogenic air separation: a proven process applied to oxyfuel. 1st International Oxyfuel Combustion Conference, 8-11 September 2009, Cottbus, Germany.

[5] Dillon, D. J.; White, V.; Allam, R. J.; Wall, R. A.; Gibbins, J. Oxy combustion processes for CO₂ capture from power plant; IEA GHG report 2005/9, 2005.