**2. State of the art**

Cryogenic air separation is a fully developed process for the separation of air into its components for large production rates and high purity. **Table 1** gives an overview of the different types of air separation processes and their typical production rates and purities.

Cryogenic air separation systems consist of at least four blocks (**Figure 1**): air compression and purification, main heat exchanger, cryogenic distillation column, and product compression (internal or external).

Dustless air (the typical composition of dry air is given in **Table 2**) enters the air compression block and is compressed in a multistage compressor with interstage cooling. In the subsequent purification block, all chemical components within the air, which will freeze during the liquefaction of air, have to be removed. Particular attention should be given to the contents of water and carbon dioxide: must be <0.1 ppm for H2O and <1 ppm for CO2 [10]. Thermal swing adsorption (TSA) or pressure swing adsorption (PSA) is used for the purification, which consists of two vessels filled with granular adsorbents. The compressed air enters one bed, while the second bed is regenerated. For the regeneration, a so-called waste nitrogen stream is used. This stream is a side product stream of the column block. Depending on the used adsorption process, the waste nitrogen stream is either heated or pressurized in order to desorb the adsorbed impurities from the bed.


#### **Table 1.**

*Production range of cryogenic and non-cryogenic air separation processes (data adopted from [8]).*

**161**

analyses are reported in [11–13].

*Composition of dry air (adopted from [9]).*

*\*vppm: volume parts per million.*

**Table 2.**

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

**Component Volume fraction** Nitrogen 78.08 vol.% Oxygen 20.95 vol.% Argon 0.93 vol.% Carbon dioxide 400 vppm\* Neon 180 vppm Helium 5 vppm Methane 1.8 vppm Krypton 1.1 vppm Hydrogen 0.5 vppm Nitrous oxide 0.3 vppm Carbon monoxide 0.2 vppm Xenon 0.09 vppm

For the liquefaction of air, a temperature of −172°C is required. In the main heat exchanger (multi-stream heat exchanger; typically, plate and fin heat exchanger design), the air is cooled and partially liquefied. The heat transfer processes within the main heat exchanger are quite complex, due to the large number of streams in different passages and the high number of channels and interactions. Detailed

The partially liquefied air leaves the main heat exchanger and enters the distillation column block, which is a double-column system [14]. It consists of a highpressure column (operation pressure is around 5–6 bar) and a low-pressure column (operation pressure is around 1.3 bar). The condenser of the high-pressure and the reboiler of the low-pressure column are thermally coupled. The different boiling points of nitrogen and oxygen lead to the production of gaseous nitrogen at the top of the high-pressure column and an oxygen-enriched mixture at the bottom of the high-pressure column. The gaseous nitrogen is partially or totally liquefied in the condenser. A part is fed back to the high-pressure column as reflux; a second part leaves the system as liquid nitrogen and a third part enters the low-pressure column

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

*General structure of an air separation unit.*

**Figure 1.**

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

**Figure 1.**

*Low-temperature Technologies*

cutting gas in the laser technology, etc.

and product compression (internal or external).

exergetic and economic analyses.

**2. State of the art**

**Component Capacity Nm<sup>3</sup>**

and purities.

chemical industry for production of ethylene glycol. Oxygen is required for water and waste water treatment for welding and cutting, as well as an oxidizer [4].

Argon is used as an extinguishing working fluid, packaging gas in the food industry, filling gas for light bulbs, carrier gas for gas chromatography, inert and

Finally, nitrogen is used in the medicine, cryotherapy, and food industry.

In the chemical and metallurgical industries, nitrogen is mainly used as inert or flushing gas. It is also used for temperature control purposes in chemical reactions.

In this chapter, two different schematics for an air separation unit are analyzed with external and internal compression [5–7]. The systems are evaluated using

Cryogenic air separation is a fully developed process for the separation of air into its components for large production rates and high purity. **Table 1** gives an overview of the different types of air separation processes and their typical production rates

Cryogenic air separation systems consist of at least four blocks (**Figure 1**): air compression and purification, main heat exchanger, cryogenic distillation column,

Dustless air (the typical composition of dry air is given in **Table 2**) enters the air compression block and is compressed in a multistage compressor with interstage cooling. In the subsequent purification block, all chemical components within the air, which will freeze during the liquefaction of air, have to be removed. Particular attention should be given to the contents of water and carbon dioxide: must be <0.1 ppm for H2O and <1 ppm for CO2 [10]. Thermal swing adsorption (TSA) or pressure swing adsorption (PSA) is used for the purification, which consists of two vessels filled with granular adsorbents. The compressed air enters one bed, while the second bed is regenerated. For the regeneration, a so-called waste nitrogen stream is used. This stream is a side product stream of the column block. Depending on the used adsorption process, the waste nitrogen stream is either heated or pressurized in order to desorb the adsorbed impurities from the bed.

Nitrogen 5–5000 <99.99 Pressure swing

Argon Cryogenic air

*Production range of cryogenic and non-cryogenic air separation processes (data adopted from [8]).*

concentrations down to ppb1 range

concentrations down to ppb range, oxygen content mostly >95

100–5000 <95 Vacuum pressure

200–400,000 Any with residual

Oxygen 1000–150,000 Any with residual

**/h Purity mol% Separation method Load range %**

adsorption

Cryogenic air separation

swing adsorption

Cryogenic air separation

separation

30–100

60–100

30–100

60–100

1–1000 <99.5 Membrane 30–100

**160**

**Table 1.**

*1*

*ppb: parts per billion.*

*General structure of an air separation unit.*


#### **Table 2.**

*Composition of dry air (adopted from [9]).*

For the liquefaction of air, a temperature of −172°C is required. In the main heat exchanger (multi-stream heat exchanger; typically, plate and fin heat exchanger design), the air is cooled and partially liquefied. The heat transfer processes within the main heat exchanger are quite complex, due to the large number of streams in different passages and the high number of channels and interactions. Detailed analyses are reported in [11–13].

The partially liquefied air leaves the main heat exchanger and enters the distillation column block, which is a double-column system [14]. It consists of a highpressure column (operation pressure is around 5–6 bar) and a low-pressure column (operation pressure is around 1.3 bar). The condenser of the high-pressure and the reboiler of the low-pressure column are thermally coupled. The different boiling points of nitrogen and oxygen lead to the production of gaseous nitrogen at the top of the high-pressure column and an oxygen-enriched mixture at the bottom of the high-pressure column. The gaseous nitrogen is partially or totally liquefied in the condenser. A part is fed back to the high-pressure column as reflux; a second part leaves the system as liquid nitrogen and a third part enters the low-pressure column

#### *Low-temperature Technologies*

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.

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 compression [16]:


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 instead of gaseous 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 potential of hazards.

In literature, different systems have been studied from the energetic and exergetic points of view.

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 and 0.639 kW/Nm3 . A specific power consumption of 0.38 kWh/Nm3 for a largescale air separation unit located in China is reported in [18].

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

**163**

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

distillation blocks have the highest exergy destruction. An exergetic analysis is applied to an air separation unit that produces gaseous oxygen and nitrogen in [23]. The results demonstrate that the air compression system causes 38.4% of the total exergy destruction, while the distillation system is responsible for 28.2% of the total exergy destruction. A double-column and a single-column air separation unit are analyzed from the exergetic point of view in [24]. The paper discusses the effect of the air pressure on the exergy destruction within the main heat exchanger. Information about noncryogenic processes can be found in [1, 2, 25–28].

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

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>°

For each stream, the mechanical, thermal, and chemical exergies are calculated

*<sup>P</sup>*,*<sup>k</sup>* + *E*̇

*<sup>D</sup>*,*tot* + *E*̇

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

*<sup>F</sup>*,*<sup>k</sup>* = *E*̇

*<sup>P</sup>*,*tot* + *E*̇

*<sup>F</sup>*,*tot* is the exergy of fuel of the overall system, and *E*̇

*<sup>L</sup>*,*tot* is the exergy loss from the overall system.

*E*̇ *F*

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

*<sup>F</sup>*,*tot* = *E*̇

*<sup>F</sup>*,*k* is the exergy of fuel of the k-th component; *E*̇

C and

*<sup>D</sup>*,*<sup>k</sup>* (1)

*<sup>D</sup>*,*k* is the exergy destruction within the k-th

*<sup>D</sup>*,*tot* represents the exergy destruction within the

*<sup>L</sup>*,*tot* (2)

*<sup>P</sup>*,*k* represents the exergy

*<sup>P</sup>*,*tot* is the exergy

(3)

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

**3. Evaluation methods**

method includes the following analyses:

• conventional exergetic analysis

• advanced exergetic analysis

• exergoenvironmental analysis

• exergy-risk-hazard analysis

component and the overall system:

*E*̇

of product of the k-th component; *E*̇

of product of the overall system; *E*̇

The exergetic efficiency is

<sup>ε</sup> <sup>=</sup> *<sup>E</sup>*̇ \_\_\_*<sup>P</sup>*

*E*̇

**3.1 Exergy analysis**

*p*<sup>0</sup> = 1.0134 bar.

Here, *E*̇

component; *E*̇

overall system; *E*̇

• exergoeconomic analysis

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

distillation blocks have the highest exergy destruction. An exergetic analysis is applied to an air separation unit that produces gaseous oxygen and nitrogen in [23]. The results demonstrate that the air compression system causes 38.4% of the total exergy destruction, while the distillation system is responsible for 28.2% of the total exergy destruction. A double-column and a single-column air separation unit are analyzed from the exergetic point of view in [24]. The paper discusses the effect of the air pressure on the exergy destruction within the main heat exchanger.

Information about noncryogenic processes can be found in [1, 2, 25–28].
