Amine Technology

2.1.1 Process Description of Amine Based CO

Capture from Flue Gas

The process of amine-based PCC is consisting of several mass and heat transfer operations. Primarily there is a process of chemical absorption of CO² into the aqueous amine solution. Figure 2.1 illustrates the process flow of a general CO² capture process, which is used for the CO² removal from flue gas.

The flue gas coming from the power plant with the pressure close to atmospheric pressure [29] is sent to the absorber column bottom. The CO²-lean aqueous amine solution comes to the top of the absorber and they meet countercurrently. There is a mass transfer across the gas/liquid interface in that CO² in the flue gas migrate into the CO² -lean amine solution. The concentration difference of CO² between the flue gas stream and the solution is the main driving force for the mass transfer. CO² reacts with amine and forms several species of carbamate, carbonate and bicarbonate. The carbamate formation varies depending on the type of amine used as the solvent. Then the CO²-rich amine solution goes through the lean/rich heat exchanger to increase the stream temperature before it goes to the desorber. It is an advantage to save some energy through this heat exchanger to reduce the overall energy demand of the process. The captured CO² is released in the desorber/stripper column. Heat is given through the reboiler to reverse the carbamate formation reaction to release CO². The typical operating conditions for aqueous MEA solvent is 115-120 ℃ at the stripper bottom. Desorption is an energy-intensive process, which represent up to 70-80% of the plant operational cost [26, 30].

The liquid stream coming out from the desorber contains CO²-lean amine solution at a high temperature. The stream is sent through the lean/rich heat exchanger to recover some of the heat before it is recycled back into the absorber. The CO² taken out from the desorber needs to be compressed and transported to the storage facility. In commercial scale, tanks, pipelines and ships are used for gaseous and liquid CO². For pipelines, the operating pressures are between 10 to 80 MPa [5]. CO² has to be compressed up to 150-250 bars prior to export [31]. CO² has been used in various sectors including chemical and oil, food, mineralization, power and pharmaceutical [32]. In the oil industry, EOR application has a high demand for CO². The purity of CO² is crucial in the industries of food and pharmaceutical.

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Figure 2.1: Post-combustion CO² capture process with absorption and desorption:

Svendsen and Eimer [33]

2.1.2 Amine as an Absorbent

The use of amine in the removal of acid gas of CO², H²S and other sulphur species from natural gas is a well-established chemical absorption technology [34, 35]. The general formula of the amine is NR¹R²R³ where R¹, R² and R³ can be either hydrogen or hydrocarbon groups. Primary amines with general formula NH²R are considered as the most reactive amines, followed by secondary (NHR¹R²) and tertiary amines (NR¹R²R³) [35]. Alkanolamines are commonly used in acid gas treating due to the enhanced water solubility and reduced volatility from the hydroxyl group [36]. MEA (monoethanol amine, H²NC²H⁴OH) is a primary amine with a high absorption rate, it is relatively cheap and relatively low hazardous to the environment compared to other amines [29]. The chemical absorption with MEA has been highly studied through laboratory experiments and process simulations for CO² removal from flue gas. MEA is regarded as the benchmark solvent of PCC to evaluate other potential absorbents by considering the different characteristics of the absorption rate of CO², absorption capacity, degradation and corrosion. The main drawbacks of MEA are its high regeneration energy due to the stable carbamate formation during the reaction with CO², oxidative and thermal degradation and a high corrosion tendency. Typical process parameters of MEA in acid gas treating are shown in Table 2.1.

DEA (diethanol amine) is a secondary amine with a high CO² absorption rate [37]. The regeneration energy of DEA is less compared to MEA but DEA has a lower absorption rate than that of MEA [35]. Aqueous DEA solvent is not considered as the best choice for CO² capture due to the irreversible side reactions and formation of corrosive products.

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11 Table 2.1: Operating parameters for MEA system [35].

Operating parameter

wt% (weight percentage) 15 to 25

Rich amine acid gas loading (mol acid gas ⸳ mol MEA^-1) 0.45 to 0.52 Acid gas pickup (mol acid gas ⸳ mol MEA^-1) 0.33 to 0.4 Lean solution residual acid gas (mol acid gas ⸳ mol MEA^-1) 0.12 ±

Tertiary amines have gained higher attention as they show several characteristics that are in favour of optimizing the CO² capture process. Tertiary amines have a low energy demand for the regeneration and higher absorption capacity compared to other amines [38]. The reaction between CO² and aqueous tertiary amines reveal a high CO² loading value of up to 1 mol CO² / mol amine as it does not form carbamate as primary and secondary amines [39]. Since there is no hydrogen atom attached to the nitrogen atom, the carbamation reaction cannot take place. This leads to the formation of bicarbonate that releases lower heat than that of carbamate formation [40]. The rate of CO² absorption is low in tertiary amines compared to primary and sterically hindered amines [41].

MDEA (N-methyldiethanolamine) is the most used tertiary amine in acid gas removal [29].

The use of one amine with H²O as a solvent cannot fulfil all requirements that are favourable in PCC. The aqueous blends of primary, secondary with tertiary amine are studied to achieve an acceptable level of CO² absorption rate, absorption capacity and low energy demand to make this technology more economically feasible for PCC [38, 42-44].

2.1.3 CO

Absorber

The chemical absorption of CO² into amine takes place in the absorber. The absorber can either consist of plates or packing materials. In acid gas treating, a column with structured packing is preferred owing to high efficiency, high capacity and low pressure drop [29]. The exothermic reaction between the amine and CO² results in high temperature at the bottom compared to the top. These temperature variations affect the physicochemical properties of amine + H²O + CO² system. Svendsen and Eimer [33]

demonstrate the variations of gas and liquid temperature, CO² loading, mole fraction and partial pressure in an absorber through simulations as shown in Figure 2.2. The effect of CO² loading on physical properties of density, viscosity and surface tension has been widely studied for various amine, water mixtures for years. This results in variations of overall mass transfer between gas and liquid interface along the absorber column. It is common practice to use an average overall mass transfer coefficient in the absorber design, but this can lead to high uncertainty in the characteristics of the column such as packing height, column diameter and pressure drop. This issue has been addressed by Nookuea, et al. [45] who suggested an optimized design procedure for absorbers in PCC to reduce the uncertainty. A detailed analysis of uncertainties related

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to absorber design, cost estimation and physical properties has been discussed by Øi [29]. Karunarathne, et al. [46] studied the propagation of uncertainty of physical properties through the mass transfer and interfacial area correlations. Still, there is a research gap to fill by analysing the propagation of uncertainty of physical properties through the mass transfer and interfacial area correlations in the design calculations of packed bed height and column diameter calculations.

Figure 2.2: Variation of the gas and liquid temperature, CO² loading, mole fraction and partial pressure in an absorber: Svendsen and Eimer [33]

2.1.4 CO

Desorber

The reboiler applies heat to reverse the carbamate formation reaction to release CO² into the gas phase. The operating temperature and pressure of the desorber depend on the type of amine that is used in the absorption desorption cycle. For an aqueous MEA system, it is rather high as 120 ^oC [47] due to the high heat of reaction between MEA and CO². The theoretical background of chemical desorption and the required conditions of chemical absorption theory to apply on chemical desorption were discussed by Astarita and Savage [48]. The experimental study performed by Jamal, et al. [49], [50] revealed the possibility to use desorption experimental data to determine forward and backward rate constants. An Aspen Plus simulation model for 30 mass% MEA was built and validated using experimental data by Garcia, et al. [51] to predict stripped CO² and loading of the CO²-lean solution.

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