**6. Results and discussions**

## **6.1 Energy analysis**

The total power consumption is calculated as 17.5 MW for Case A and 15.9 MW for Case B, while the specific power consumption is 2.31 kWh/NmGOX 3 and 2.11 kWh/NmGOX 3 for Cases A and B, respectively. Due to the fact that the amount of produced gaseous oxygen is the same in both systems, the specific power consumption per gaseous oxygen decreases from Case A to Case B. The production rates of the product streams, as well as their purities, are given in **Figures 4** and **5**. The mass flow rates of the gaseous and liquid oxygen are kept constant for both systems.

**Figure 6** gives an overview of the specific power consumption per produced oxygen obtained from the literature. The large deviations in the results obtained for Cases A and B are related to several reasons:

**Figure 4.** *Mass flow rates of the product streams.*

• In Cases A and B, the gaseous oxygen and nitrogen streams leave the systems at 20 bar. For the data obtained from the literature, it is not clearly indicated whether the product streams leave the system at atmospheric pressure or at a higher pressure level. Solely in [38], it is mentioned that the oxygen leaves the system at atmospheric pressure.

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*Comparative Evaluation of Cryogenic Air Separation Units from the Exergetic and Economic…*

• In some publications, the specific power consumption is calculated per produced oxygen, which also includes the liquid oxygen stream but is not identified as this.

• The production of oxygen and nitrogen with high purity and the additional production of liquids have a very large influence on the power consumption. In Case A, the additional cooling cycle significantly affects the overall power

• Another reason is the size of the air separation units. Both analyzed systems are small-scale plants, which tend to have higher specific power consumption in

The power consumption/generation of the turbomachines in Cases A and B is given in **Figure 7**. In Case A, NC1 and NC2 have the highest power consumption. In Case B, the compressor with the highest power consumption is NC5. This compressor requires more power in comparison to NC5 in Case A because the mass flow rate of the gaseous nitrogen is twice as high as in Case B. The differences in the power consumption of the components AC1 and AC2 are also related to the higher air mass

The results of the overall system are given in **Table 4**. The exergetic efficiency of Cases A and B amounts to 28.4 and 31.1%, respectively. In [39], an exergetic efficiency of 26.6% is reported for a single air separation unit, which is in the same

The difference in the exergy of fuel between Cases A and B (**Table 4**) is related to the slightly different total power consumption. The exergy of product for both overall systems is the same. Due to the fact that the amount of gaseous and liquid oxygen is identical in both systems, the significantly higher amount of gaseous nitrogen in Case B compensates the product stream of the liquid nitrogen, which is not available in Case B. The exergy loss is significantly higher in Case B. This is due to the fact that the mass flow rate of the waste nitrogen stream is significantly higher in Case B than in

Case A in order to produce the same amount of liquid and gaseous oxygen.

*DOI: http://dx.doi.org/10.5772/intechopen.85765*

consumption.

**Figure 6.**

**6.2 Exergy analysis**

comparison with large-scale units.

*Specific power consumption obtained from literature [35–37].*

flow rate in Case B in comparison to Case A.

range as the results obtained for Cases A and B.

*Comparative Evaluation of Cryogenic Air Separation Units from the Exergetic and Economic… DOI: http://dx.doi.org/10.5772/intechopen.85765*

#### **Figure 6.**

*Low-temperature Technologies*

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**Figure 5.**

**Figure 4.**

*Mass flow rates of the product streams.*

atmospheric pressure.

*Purity of the product streams.*

• In Cases A and B, the gaseous oxygen and nitrogen streams leave the systems at 20 bar. For the data obtained from the literature, it is not clearly indicated whether the product streams leave the system at atmospheric pressure or at a higher pressure level. Solely in [38], it is mentioned that the oxygen leaves the system at

*Specific power consumption obtained from literature [35–37].*


The power consumption/generation of the turbomachines in Cases A and B is given in **Figure 7**. In Case A, NC1 and NC2 have the highest power consumption. In Case B, the compressor with the highest power consumption is NC5. This compressor requires more power in comparison to NC5 in Case A because the mass flow rate of the gaseous nitrogen is twice as high as in Case B. The differences in the power consumption of the components AC1 and AC2 are also related to the higher air mass flow rate in Case B in comparison to Case A.

#### **6.2 Exergy analysis**

The results of the overall system are given in **Table 4**. The exergetic efficiency of Cases A and B amounts to 28.4 and 31.1%, respectively. In [39], an exergetic efficiency of 26.6% is reported for a single air separation unit, which is in the same range as the results obtained for Cases A and B.

The difference in the exergy of fuel between Cases A and B (**Table 4**) is related to the slightly different total power consumption. The exergy of product for both overall systems is the same. Due to the fact that the amount of gaseous and liquid oxygen is identical in both systems, the significantly higher amount of gaseous nitrogen in Case B compensates the product stream of the liquid nitrogen, which is not available in Case B. The exergy loss is significantly higher in Case B. This is due to the fact that the mass flow rate of the waste nitrogen stream is significantly higher in Case B than in Case A in order to produce the same amount of liquid and gaseous oxygen.

**Figure 7.** *Power consumption/generation.*


#### **Table 4.**

*Results obtained from the exergetic analysis of the overall system, Cases A and B.*

A graphical representation of the exergy streams is given in **Figures 8** and **9**. For each component, the inlet and outlet exergy streams associated with a material stream, the power for the turbomachines, and the exergy destruction are shown. The high-pressure and low-pressure columns, the condenser/reboiler, sub cooler (in Case B), and some throttling vales are summarized as column block (CB). **Figure 8** shows that the components within the nitrogen liquefaction block have the highest exergy destruction in Case A. The exergy destruction ratio of this block accounts for 60.8% of the total exergy destruction. In Case B (**Figure 9**), the ICN has the highest exergy destruction among all components.
