**3. Physical absorption solvents**

Physical absorption processes are highly recommended to separate CO2 in pre-combustion processes that commonly operate at elevate CO2 partial pressure. Physical solvents are able to selectively capture CO2 in contact with a gas stream without a chemical reaction occurring. As it was indicated in the introduction section, the high partial pressures of CO2 and lowtemperatures are desirable to obtain an optimized performance of the physical absorption process in terms of absorption rates and solubility equilibrium of CO2 . Then, the rich (CO2 loaded) solvent is regenerated [55].

Focusing on the pre-combustion CO2 capture process itself, seven processes using physical solvents are currently commercially available, which are discussed in the following section.

A summary of the most relevant physical properties of each solvent and the list of advantages and disadvantages can be found at the end of this chapter in **Table 8**.

column. A pre-treatment absorption column can be used to accomplish the sulfur compound

cess from which the solvent recovers its original capacity by either reducing the pressure or inert gas stripping. The recovered (lean) solvent is recycled back into the absorber, whereas

S and CO2

ammonia production, but it is not applied for the food and beverage industry. It has a high selec-

into various components depending on the final product specifications and process objectives. Rectisol™ is licensed by Lurgi AG, which is an affiliated company of Air Liquide. This technology employs chilled methanol as solvent and can be applied for low and moderate CO2 concentrated gas streams. Due to the high vapor pressure of the solvent, the absorption stage

must be carried out at very low-temperatures to reduce solvent losses [57].

and COS and can be configured to accomplish the separation of synthesis gas

exits the regeneration stages to be compressed and stored.

loaded solvent is then sent to the regeneration pro-

capture [58, 59].

• Most efficient at elevated pressures

Solvents for Carbon Dioxide Capture http://dx.doi.org/10.5772/intechopen.71443 155

• High refrigeration costs • High capital costs

• Expensive solvent

• High compression cost • Most efficient at high-pressure

• Foaming issues • Corrosive solvent • Thermal regeneration

• New process

• Amalgams formation at low T

• High solvent circulation rates

removal from syngas streams, mainly from

stream obtained can be used in urea, methanol and

separation. The CO2

**Process Advantages Disadvantages**

• Dry gas leaves from the absorber

• High chemical and thermal stability

 solubility • Non-thermal regeneration

• High chemical and thermal stability

• Non-corrosive solvent

• Non-corrosive solvent

• Simple operation • Non-corrosive solvent

• Non-corrosive solvent • Low volatility

• Low solvent circulation rate

• Low energy requirement • Non-corrosive solvent

• Low capital and operating costs

**Table 8.** Main advantages and disadvantages of physical absorption technologies available for CO2

Selexol™ • Non-thermal solvent regeneration

Rectisol™ • Non-foaming solvent

Purisol™ • Non-foaming solvent

Morphysorb™ • High solvent loading capacity

Sulfinol™ • High capacity

Fluor™ • High CO2

Ipexol-2 ™

removal prior to CO2

the high purity CO2

The Rectisol™ process is applied in H2

S, CO2

heavy oil and coal gasification. The CO<sup>2</sup>

**3.2. Rectisol™**

tivity for H2

#### **3.1. Selexol™**

The Selexol™ process has been widely used and effectively proven in the refinery industry, natural gas sweetening, syngas processing and fertilizer production since the 1960s. Recently, Selexol™ has also been used in IGCC for H2 S, COS and CO2 removal.

The Selexol™ process, licensed by Universal Oil Products (UOP), employs a mixture of different dimethyl ethers and polyethylene glycol, represented by the formulae (CH3 O(C2 H4 O)n CH3 ), with n factor ranging from 3 to 9 [2]. This physical solvent was patented by DOW chemical [56]. Selexol™ provides a selective absorption of H2 S, COS, mercaptans and CO2 from a variety of natural and synthesis gas streams. It has shown a high performance under high-pressure, lowtemperature and high acid gas process conditions.

In the Selexol™ process, the flue gas must be first dehydrated before being introduced in the absorption column. After that, the dehydrated flue gas enters the absorber at 30 atm and 0–5°C and the acid gas components are selectively absorbed into the solvent along the


**Table 8.** Main advantages and disadvantages of physical absorption technologies available for CO2 capture [58, 59].

column. A pre-treatment absorption column can be used to accomplish the sulfur compound removal prior to CO2 separation. The CO2 loaded solvent is then sent to the regeneration process from which the solvent recovers its original capacity by either reducing the pressure or inert gas stripping. The recovered (lean) solvent is recycled back into the absorber, whereas the high purity CO2 exits the regeneration stages to be compressed and stored.

### **3.2. Rectisol™**

*2.6.2.3. Lipophilic-amine-based thermomorphic biphasic solvents*

N-ethylpiperidine (EDP) (**Table 7**).

154 Carbon Dioxide Chemistry, Capture and Oil Recovery

to selectively capture CO2

**3.1. Selexol™**

**3. Physical absorption solvents**

loaded) solvent is regenerated [55].

Focusing on the pre-combustion CO2

Selexol™ has also been used in IGCC for H2

Selexol™ provides a selective absorption of H2

temperature and high acid gas process conditions.

processes that commonly operate at elevate CO2

in regeneration step allows a decrease of the energetic consumption in CO2

Physical absorption processes are highly recommended to separate CO2

process in terms of absorption rates and solubility equilibrium of CO2

and disadvantages can be found at the end of this chapter in **Table 8**.

As it was indicated in the introduction section, the high partial pressures of CO2

temperatures are desirable to obtain an optimized performance of the physical absorption

solvents are currently commercially available, which are discussed in the following section.

A summary of the most relevant physical properties of each solvent and the list of advantages

The Selexol™ process has been widely used and effectively proven in the refinery industry, natural gas sweetening, syngas processing and fertilizer production since the 1960s. Recently,

The Selexol™ process, licensed by Universal Oil Products (UOP), employs a mixture of differ-

with n factor ranging from 3 to 9 [2]. This physical solvent was patented by DOW chemical [56].

natural and synthesis gas streams. It has shown a high performance under high-pressure, low-

In the Selexol™ process, the flue gas must be first dehydrated before being introduced in the absorption column. After that, the dehydrated flue gas enters the absorber at 30 atm and 0–5°C and the acid gas components are selectively absorbed into the solvent along the

ent dimethyl ethers and polyethylene glycol, represented by the formulae (CH3

S, COS and CO2

in CO2

Lipophilic-amine-based thermomorphic biphasic solvents have shown potential advantages

Amine blends used by Zhang were mostly composed of an absorption activator: A1, dipropylamine (DPA) and a regeneration promoter: N, N-dimethylciclohexylamine (DMCA) and

 capture compared to conventional alkanolamines in terms of solvent regeneration and cyclic capacity. The improvements obtained using these types of solvent are based on its thermomorphic behavior. This phenomenon consists of the generation of two liquid phases after heating inside the reboiler. According to Zhang et al. [52], these systems can be regenerated at lower temperatures than the conventional alkanolamine blends. This temperature reduction

capture processes.

in pre-combustion

. Then, the rich (CO2

O(C2 H4 O)n CH3 ),

from a variety of

and low-

partial pressure. Physical solvents are able

in contact with a gas stream without a chemical reaction occurring.

capture process itself, seven processes using physical

removal.

S, COS, mercaptans and CO2

The Rectisol™ process is applied in H2 S and CO2 removal from syngas streams, mainly from heavy oil and coal gasification. The CO<sup>2</sup> stream obtained can be used in urea, methanol and ammonia production, but it is not applied for the food and beverage industry. It has a high selectivity for H2 S, CO2 and COS and can be configured to accomplish the separation of synthesis gas into various components depending on the final product specifications and process objectives.

Rectisol™ is licensed by Lurgi AG, which is an affiliated company of Air Liquide. This technology employs chilled methanol as solvent and can be applied for low and moderate CO2 concentrated gas streams. Due to the high vapor pressure of the solvent, the absorption stage must be carried out at very low-temperatures to reduce solvent losses [57].

In the Rectisol™ process, the raw syngas is cooled before being introduced into the absorption process. The sulfur compounds must be firstly removed using a CO<sup>2</sup> loaded solvent. After that, the sulfur exempt syngas contacts with the chilled methanol in the absorber, operating at 50 atm and temperatures in the range of [−100°C, −30°C]. The rich solvent is then sent to the regeneration stage where CO2 is released by flash desorption, reducing the pressure up to 1 bar. The lean solvent is recycled back to the absorber [2, 57].

In the Purisol™ process, H2

gas [63].

**3.6. Sulfinol™**

around 50%v/v H<sup>2</sup>

per temperatures [64].

**3.7. Morphysorb™**

Process [65, 66].

tion down to about −15°C. The CO<sup>2</sup>

The Sulfinol™ process can remove H<sup>2</sup>

This process accomplishes H2

S removal is not required prior to CO2

desorption is accomplished by stripping with an inert

, carbonyl sulfide, mercaptans and organic sulfur

. The principle of this process aims at combining

S, CO2

, COS, CS2

separation in a wide variety of compositions up to

The process can be operated at 50 bar and either at ambient temperature or with refrigera-

components from natural and synthesis gas from coal or oil gasifiers and steam reformers.

the high absorption potential of alkanolamine (chemical absorption) and the low regeneration

Sulfinol™ is licensed by Shell Oil Company and employs mixtures of diisopropylamine (DIPA) or methyldiethanolamine (MDEA) and tetrahydrothiophene dioxide (SULFOLANE) in different blends. The physical solvent used (DIPA or MDEA) has a higher absorption capacity and a low energy requirement for regeneration, thus increasing the carrying capacity due to lower solvent recycled requirements. The absorber is operated at 40°C and a pressure

at temperatures over 110°C and vacuum pressure. It should be noted that addition of antifoam is needed in the absorber and solvent degrades due to the presence of oxygen and strip-

captans and other components from coal/oil gasification syngas at IGCC facilities. This process is particularly effective for high-pressure and high acid gas applications and offers substantial savings in investment and operating cost compared to the competitive physical solvent-based processes. The operational cost is 30–40% lower than that for Selexol™

This technology is developed by Krupp Uhde GmbH in cooperation with the Institute of Gas Technology (GTI), and employs N-formyl morpholine (NFM) and N-acetyl morpholine (NAM) mixtures as solvent (manufactured by *BASF* AG). In comparison with other physical solvents, the Morphysorb solvent co-absorbs fewer heavier hydrocarbons and is also suited for simultaneous water removal from the feed gas [67]. In this process, the acid gases are removed from the absorbent by flashing and the regenerated absorbent is recycled to the absorbent. The physical absorption occurs at temperatures between −20 and +40°C and at pressure of 10–150 bar [67]. The key advantage of the Morphysorb™ technology is the high acid gas capacity together with the low solubility of C1–C3 hydrocarbons, resulting in a

S, CO2

around 60–70 bar. The rich solvent is then sent to the stripping column where CO<sup>2</sup>

S and CO2

S and above 20%v/v CO2

energy requirement of the physical solvent (physical absorption).

The Morphysorb™ process is applied for selective removal of H2

higher product yield and a lower recycle flash stream [68].

absorption occurring.

157

Solvents for Carbon Dioxide Capture http://dx.doi.org/10.5772/intechopen.71443

is released

, mer-
