**4.5 Nitrogen liquefaction block (NLB)**

Stream 31 leaves the MHE and is fed to the nitrogen liquefaction block where it is mixed with streams 44 and 47, which are recycled streams within the nitrogen liquefaction block to stream 32. This stream is heated in the heat exchanger 1 (HE1) and compressed in a three-stage compression process with interstage cooling to 38 bar. Afterward, stream 38 is divided into two streams (stream 39 and 45). Stream 45 is cooled within HE1, while stream 39 is first compressed to 45 bar in NC4 and is then cooled within HE1. At the outlet of the HE1, stream 41 is split into streams 42 and 21. Streams 46 and 42 are expanded in expanders 1 and 2 (EXP1 and EXP2), respectively, which are connected with the nitrogen compressors 3 and 4

**167**

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

(NC3 and NC4). Stream 21 is cooled in the HE2 by stream 43, which is afterward mixed with streams 47 and 31. The cooled stream leaves the HE2 as stream 48 and is throttled and split into streams 58 and 59. Stream 58 is directly fed to the HPC,

In Case A, the nitrogen liquefaction block is formed by the following compo-

Some components shown in **Figures 2** and **3** cannot be assigned to any of the afore-mentioned blocks. These components from the remaining block are in Case A

The two systems were simulated using Aspen Plus [34]. For the equation of state, the Peng-Robinson-equation was selected. The general assumptions made for

**Variable Unit Case A Case B**

Mass flow rate kg/s 16.39 33.50

Inlet temperature °C 15 Inlet pressure bar 1.013

Composition mol/mol *x*N2 = 0.7720

Isentropic efficiency (compressor) % 84 Isentropic efficiency (expander) % 90 Isentropic efficiency (pump) % 70 Mechanical efficiency % 99

The total power consumption is calculated as 17.5 MW for Case A and 15.9 MW

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

**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

3 and 2.11 kWh/NmGOX

*x*O2 = 0.2080 *x*Ar = 0.0905 *x*H2O = 0.0102 *x*CO2 = 0.0003

3

for Case B, while the specific power consumption is 2.31 kWh/NmGOX

of the gaseous and liquid oxygen are kept constant for both systems.

Cases A and B are related to several reasons:

nents: HE1, HE2, NC1, NC2, NC3, NC4, IC3, IC4, EXP1, EXP2, and MIX1.

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

TV3 and TV4, and in Case B EXP1 and TV1.

the simulation are given in **Table 3**.

**6. Results and discussions**

*General assumptions for the simulation of the systems.*

**6.1 Energy analysis**

Turbomachines

**Table 3.**

**5. Simulation**

Air

while stream 59 is throttled again and then enters the LPC.

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

(NC3 and NC4). Stream 21 is cooled in the HE2 by stream 43, which is afterward mixed with streams 47 and 31. The cooled stream leaves the HE2 as stream 48 and is throttled and split into streams 58 and 59. Stream 58 is directly fed to the HPC, while stream 59 is throttled again and then enters the LPC.

In Case A, the nitrogen liquefaction block is formed by the following components: HE1, HE2, NC1, NC2, NC3, NC4, IC3, IC4, EXP1, EXP2, and MIX1.

Some components shown in **Figures 2** and **3** cannot be assigned to any of the afore-mentioned blocks. These components from the remaining block are in Case A TV3 and TV4, and in Case B EXP1 and TV1.
