**3. Evaluation methods**

*Low-temperature Technologies*

compression [16]:

instead of gaseous state.

potential of hazards.

getic points of view.

and 0.639 kW/Nm3

as reflux. At the top of the low-pressure column also gaseous nitrogen is gained, which is fed to the main heat exchanger. The liquid and gaseous oxygen leave the column system at the bottom of the low-pressure column either before or after the reboiler. The two gaseous product streams are fed to the main heat exchanger in order to cool and partially liquefy the air. In addition, a waste nitrogen stream leaves

the low-pressure column and is also heated within the main heat exchanger.

oxygen in the main heat exchanger against high-pressure air.

compressed to the required pressure using a compressor.

The air separation process can be extended in order to obtain specified product requirements. The integration of an additional cooling cycle is also possible [15]. Air separation units can be distinguished regarding the kind of product

• *Internal compression* or "pumped LOX cycle," where the product oxygen is produced at an elevated pressure by using a pump and heating high-pressure liquid

• *External compression* or "low-pressure GOX cycle," where the product oxygen is taken as a gas from the bottom of the low-pressure column and is subsequently

Nowadays, most of the air separation plants use the internal compression of oxygen [17, 18]. The internal compression has several advantages from the thermodynamic and safety points of view. From the thermodynamic point of view, the increase of the oxygen pressure requires less power if it is pressurized in liquid state

In addition, there are safety-related problems in conjunction with the oxygen compressors, which lead to higher costs, lower efficiency, and reliability in comparison to air and/or nitrogen compressors [17, 19]. The internal compression of oxygen has a second advantage from the safety viewpoint. Due to the fact that hydrocarbons accumulate in the bottom of the column, an explosion could occur. Therefore, in air separation plants where only gaseous products are produced, a small amount of liquid oxygen has to be withdrawn from the bottom to decrease the potential of hazards [16]. In contrast, in air separation units with internal compression, the liquid oxygen is continuously withdrawn from the sump and thus decreases the

In literature, different systems have been studied from the energetic and exer-

In [20], schematics of air separations units are evaluated, which differ regarding (a) the kind of product compression (internal or external) and (b) the amount of produced gaseous oxygen. The specific power consumption varies between 0.464

. A specific power consumption of 0.38 kWh/Nm3

An air separation unit with a nitrogen liquefaction block is analyzed from exergetic point of view in [15]. The nitrogen liquefaction block is the subsystem with the highest exergy destruction. The total exergy destruction ratio is 51%. In [21], two cryogenic air separation units are analyzed from the exergetic point of view. The paper evaluates a two- and a three-column system (as part of an integrated gasification combined cycle), which produces one gaseous oxygen and three gaseous nitrogen streams at different pressure levels (88, 25, and 1.3 bar). The highest exergy destruction is reported for the preprocessing feed subsystem (air compressors, interstage cooler, and purification system), which amounts to 47 and 54% for the two- and three-column system, respectively. An air separation unit with an internal compression unit is analyzed from the energetic, exergetic, and economic points of view in [22]. The exergetic analysis shows that the air compression and

scale air separation unit located in China is reported in [18].

for a large-

**162**

The exergy-based methods are powerful tools for identifying thermodynamic and cost inefficiencies, as well as environmental impacts and risks associated with the inefficiencies within energy conversion systems [29, 30]. This evaluation method includes the following analyses:


#### **3.1 Exergy analysis**

In order to apply an exergetic analysis, the reference environment needs to be defined. In this chapter, the average European conditions are chosen: *T*<sup>0</sup> <sup>=</sup> <sup>15</sup>° C and *p*<sup>0</sup> = 1.0134 bar.

For each stream, the mechanical, thermal, and chemical exergies are calculated according to [29, 31]. For all components, the exergies of fuel and product are defined, and the exergy destruction is calculated using exergy balances for the *k*-th component and the overall system:

$$
\dot{E}\_{F,k} = \dot{E}\_{P,k} + \dot{E}\_{D,k} \tag{1}
$$

$$
\dot{E}\_{F, \text{tot}} = \dot{E}\_{P, \text{tot}} + \dot{E}\_{D, \text{tot}} + \dot{E}\_{L, \text{tot}} \tag{2}
$$

Here, *E*̇ *<sup>F</sup>*,*k* is the exergy of fuel of the k-th component; *E*̇ *<sup>P</sup>*,*k* represents the exergy of product of the k-th component; *E*̇ *<sup>D</sup>*,*k* is the exergy destruction within the k-th component; *E*̇ *<sup>F</sup>*,*tot* is the exergy of fuel of the overall system, and *E*̇ *<sup>P</sup>*,*tot* is the exergy of product of the overall system; *E*̇ *<sup>D</sup>*,*tot* represents the exergy destruction within the overall system; *E*̇ *<sup>L</sup>*,*tot* is the exergy loss from the overall system.

The exergetic efficiency is

$$\mathbf{e} = \frac{\dot{E}\_P}{\dot{E}\_F} \tag{3}$$

The following exergy destruction ratios are used in the analysis according to [29].

$$\mathcal{Y}\_k = \frac{\dot{E}\_{D,k}}{\dot{E}\_{F,\text{tot}}} \tag{4}$$

$$\mathcal{Y}\_k^\* = \frac{\dot{E}\_{D,k}}{\dot{E}\_{D,\text{act}}} \tag{5}$$

#### **3.2 Economic analysis**

The economic analysis is performed according to the total revenue requirement (TRR) method [29]. First, the purchased equipment costs (PEC) and the bare module costs (C BM) have to be estimated for all components. Afterward, the fixed capital investment (FCI) and total capital investment (TCI) are determined. The FCI consists of the direct and indirect costs, whereas the direct costs are further divided into onsite and offsite costs. The total revenue requirement (TRR) consists of the sum of the levelized carrying charges (CC L), the levelized operating and maintenance costs (OMC L), and the levelized fuel costs(FC L) .

#### **4. Process description**

System Case A (CA) is a conventional air separation unit with two distillation columns, a nitrogen liquefaction block, and an external compression unit of the product. Case B (CB) is an air separation unit with two distillation columns and an internal compression unit [5–7]. The flowsheets of Cases A and B are given in **Figures 2** and **3**. The key values for the simulations are based on [15, 32].

#### **4.1 Air compression and purification block (ACPB)**

In both systems, the dustless air is compressed (in a two-stage compression process with interstage cooling) to approximately 6 bar and purified in the adsorption block (AD). In Case A, the "pure" air enters the main heat exchanger (MHE). In Case B, the "pure" air is divided into two parts: Stream 11 enters the main heat exchanger, while stream 17 is further compressed in the booster air compressor (BAC or AC3). The three air streams (streams 12, 14, and 18) are fed into the main heat exchanger.

In Case A, the heat exchanger 3 (HE3) is also assigned to the air compression and purification block because this component is required in order to heat the waste nitrogen to 170°C, which is required for the desorption of the water vapor and carbon dioxide from the adsorption beds. A temperature between 150 and 200°C is required for the desorption [32, 33].

In Case A, the air compression and purification block consists of AC1, AC2, IC1, IC2, AD, and HE3. In Case B, the following components belong to the air compression and purification block: AC1, AC2, IC1, IC2, AD, and AC3.

#### **4.2 Main heat exchanger (MHE)**

The main heat exchanger is the core component of an air separation unit where the cleaned air is cooled to −173.4°C and partially liquefied using the streams leaving the column block. In Case A, these are two gaseous nitrogen streams, one gaseous oxygen stream, and a waste nitrogen stream. In Case B, the three air streams are cooled using one gaseous nitrogen, one gaseous oxygen, and one waste nitrogen stream. Stream 15 (28.1% of the air mass flow rate, i.e., stream 11) leaves the MHE at a temperature of −120°C and is expanded in the expander (EXP1).

**165**

**Figure 3.** *Flowsheet of case B.*

**Figure 2.** *Flowsheet of case A.*

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

The share of this mass flow rate is slightly higher in comparison to the data available in literature. As reported in [32], a mass portion of 10–20% at a temperature of −100 to −130°C is common for this stream. In [24], it is mentioned that the air is divided at a temperature level of −140°C and fed to the expander. After the expander, stream 16 enters the low-pressure column (LPC) at an intermediate sieve tray.

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

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

**Figure 2.** *Flowsheet of case A.*

*Low-temperature Technologies*

**3.2 Economic analysis**

**4. Process description**

heat exchanger.

required for the desorption [32, 33].

**4.2 Main heat exchanger (MHE)**

*yk <sup>=</sup> <sup>E</sup>*̇ \_\_\_\_ *D,k*

*yk*

*E*̇ *F,tot*

<sup>∗</sup> *<sup>=</sup> <sup>E</sup>*̇ \_\_\_\_\_ *D,k E*̇ *D,tot*

The economic analysis is performed according to the total revenue requirement

(TRR) method [29]. First, the purchased equipment costs (PEC) and the bare module costs (C BM) have to be estimated for all components. Afterward, the fixed capital investment (FCI) and total capital investment (TCI) are determined. The FCI consists of the direct and indirect costs, whereas the direct costs are further

(TRR) consists of the sum of the levelized carrying charges (CC L), the levelized operating and maintenance costs (OMC L), and the levelized fuel costs(FC L) .

System Case A (CA) is a conventional air separation unit with two distillation columns, a nitrogen liquefaction block, and an external compression unit of the product. Case B (CB) is an air separation unit with two distillation columns and an internal compression unit [5–7]. The flowsheets of Cases A and B are given in

In both systems, the dustless air is compressed (in a two-stage compression process with interstage cooling) to approximately 6 bar and purified in the adsorption block (AD). In Case A, the "pure" air enters the main heat exchanger (MHE). In Case B, the "pure" air is divided into two parts: Stream 11 enters the main heat exchanger, while stream 17 is further compressed in the booster air compressor (BAC or AC3). The three air streams (streams 12, 14, and 18) are fed into the main

In Case A, the heat exchanger 3 (HE3) is also assigned to the air compression and purification block because this component is required in order to heat the waste nitrogen to 170°C, which is required for the desorption of the water vapor and carbon dioxide from the adsorption beds. A temperature between 150 and 200°C is

In Case A, the air compression and purification block consists of AC1, AC2, IC1, IC2, AD, and HE3. In Case B, the following components belong to the air compres-

The main heat exchanger is the core component of an air separation unit where

the cleaned air is cooled to −173.4°C and partially liquefied using the streams leaving the column block. In Case A, these are two gaseous nitrogen streams, one gaseous oxygen stream, and a waste nitrogen stream. In Case B, the three air streams are cooled using one gaseous nitrogen, one gaseous oxygen, and one waste nitrogen stream. Stream 15 (28.1% of the air mass flow rate, i.e., stream 11) leaves the MHE at a temperature of −120°C and is expanded in the expander (EXP1).

**Figures 2** and **3**. The key values for the simulations are based on [15, 32].

**4.1 Air compression and purification block (ACPB)**

sion and purification block: AC1, AC2, IC1, IC2, AD, and AC3.

divided into onsite and offsite costs. The total revenue requirement

(4)

(5)

**164**

#### **Figure 3.** *Flowsheet of case B.*

The share of this mass flow rate is slightly higher in comparison to the data available in literature. As reported in [32], a mass portion of 10–20% at a temperature of −100 to −130°C is common for this stream. In [24], it is mentioned that the air is divided at a temperature level of −140°C and fed to the expander. After the expander, stream 16 enters the low-pressure column (LPC) at an intermediate sieve tray.

The air stream at high pressure leaves the MHE (stream 19) and is expanded within a throttling valve (stream 20). In Case A, the oxygen and nitrogen streams are heated to 15°C, and the waste nitrogen leaves the main heat exchanger at the temperature of 33°C, which results in a minimal temperature difference of 2 K. In Case B, the liquid oxygen stream is vaporized and heated within the main heat exchanger and leaves it also at 15°C. The waste nitrogen is heated to 170°C, which results in a minimal temperature difference of 2.7 K for the MHE.
