Effect of Different Amine Solutions on Performance of Post-Combustion CO₂ Capture

Abstract

Carbon dioxide (CO₂) is the primary component contributing to anthropogenic greenhouse gas emissions, necessitating the adoption of effective mitigation strategies to promote environmental sustainability. Among the various carbon capture methodologies, chemical absorption is acknowledged as the most scalable solution for post-combustion applications. This investigation presents a thorough, comparative, and scenario-based evaluation of both singular and blended amine solvents for CO₂ capture within packed absorption–desorption columns. A validated rate-based model employing monoethanolamine (MEA) functions as the benchmark for executing process simulations. Three sequential scenarios are meticulously examined to switch the solvents and see the results. In the preliminary scenario, baseline performance is assessed by applying MEA to achieve the designated 73% removal target. Then the implementation of alternative solvents is examined—piperazine (PZ), a combination of methyldiethanolamine (MDEA) and PZ, and a blend of MEA and PZ—under uniform design parameters to ascertain their relative effectiveness and performance. In the second scenario, the design of the system is changed to reach a CO₂ removal efficiency for MEA of 90%, and then MEA is switched to other solvents. In the final scenario, critical design parameters, including column height and diameter, are adjusted for each solvent system that did not meet the 90% capture efficiency in Scenario 2 to achieve 90% CO₂ capture. A comprehensive sensitivity analysis is subsequently conducted on the adjusted systems to evaluate the influence of critical operational variables such as temperature, flue gas and solvent flow rates, and concentrations. Importantly, the MEA + PZ blend also demonstrated the lowest specific reboiler duty, as low as 4.28 MJ/kg CO₂, highlighting its superior energy efficiency compared to other solvents in the condition that the system in this study is pilot-scale, not commercial-scale, and due to this reason, the energy consumption of the system is slightly higher than the reported value for the commercial-scale systems. The results yield invaluable insights into the performance trade-offs between singular and blended amines, thereby facilitating the development of more efficient CO₂ capture systems that function within practical constraints.

1. Introduction

Due to the extensive use of fossil fuels since the mid-20th century, the rate of energy consumption has increased, leading to a rise in greenhouse gas emissions into the atmosphere. CO₂ is the primary greenhouse gas responsible for significant climate change, which causes global warming [1,2]. Carbon capture and storage (CCS) and carbon capture, utilisation, and storage (CCUS) technologies are the best options for maintaining a clean environment in the short to medium term [3]. According to data from the Global CCS Institute, as of February 2025, 65 active facilities worldwide currently capture approximately 57 million tonnes of CO₂ annually (Mtpa). An additional 42 projects are under development; once completed, these will increase global capture capacity by 44 Mtpa, projecting cumulative capacity to exceed 100 Mtpa [4].

Various methods of CO₂ separation are currently in use, including absorption, adsorption, membrane separation, cryogenic separation, chemical looping combustion, and hydrate-based gas separation. Absorption is widely recognised as the most superior method for CO₂ capture due to its remarkable versatility, adaptability, and technological advancements across a wide array of industrial applications. This method proficiently addresses various capture scenarios, including pre-combustion, post-combustion, and direct air capture, by facilitating the interaction of flue gases with chemical solvents (such as alkanolamines or ionic liquids) within absorption columns, wherein CO₂ is absorbed and subsequently liberated as a purified stream through regulated alterations in temperature and pressure within desorption columns [3,5].

This methodology capitalises on decades of industrial enhancement, presenting established scalability and dependability in extensive operations such as natural gas processing and power plant emission management, where it surpasses alternative methods in terms of ease of integration and operational consistency. Notwithstanding the energy-intensive nature of regeneration processes, absorption continues to dominate as the prevailing commercial solution due to its elevated capture efficiency, compatibility with existing infrastructure, and ongoing advancements in solvent technology that optimise both performance and cost [6]. In addition to these established technologies, recent research has highlighted the potential of high-temperature molten carbonate systems for post-combustion CO₂ capture, particularly for industrial or energy-intensive sources. Molten carbonate solvents operate at temperatures between 450 and 700 °C and offer robust thermal stability, high CO₂ uptake, and suitability for integration with waste heat or electrochemical processes [7,8]. While still at an earlier stage of industrial deployment compared to amine-based absorption, molten carbonate systems are being explored for settings where solvent degradation or energy efficiency may limit traditional technologies.

1.1. Solvent Selection in Absorption Technology

In absorption technology, the solvent has a direct impact on the efficiency and total cost of the operation due to its effect on parameters such as CO₂ absorption capacity, unit operation size, and required energy for regeneration. Therefore, the design and selection of solvents are vitally important in the CO₂ capture system [9]. The solvents are divided into three groups based on the reaction between the solvent and CO₂: chemical, physical, and combination solvents [9]. Different characteristics can also be used to categorise solvents and help in their selection [10]; for example, solubility of CO₂ in the solvent, high reactivity, low energy requirements for regeneration, low cost, low toxicity, and negligible thermal degradation are characteristics that should be present in a suitable solvent [11]. Between different solvents, chemical solvents are more promising as this type of solvent exhibits a reasonably high degree of selectivity. Amines are the most important family of chemical solvents frequently used for CO₂ absorption processes, derived from ammonia, with the replacement of a hydrogen atom by an alkyl group, such as alkyl2 or alkyl3 [9].

According to the literature, MEA is the most utilised amine for CO₂ absorption [12,13,14,15]. MEA maintains superiority over alternative amine compounds in industrial applications primarily due to its advantageous combination of rapid CO₂ reaction kinetics, acceptable regeneration energy consumption, and favourable economic attributes, collectively establishing it as the industry standard for acid gas treatment processes despite the availability of numerous alternative amine formulations [16]. Although MEA-based scrubbing technology is suitable for post-combustion CO₂ capture (PCC) removal from the flue gas of power stations, it is hindered by several drawbacks, including an elevated enthalpy of reaction, corrosion problems, low absorption capability, and thermal and oxidative degradation [17]. However, one significant drawback is the considerable amount of energy required for regeneration in commercial-scale (industrial) absorption–stripping systems (for example, large power plant units) for 90% carbon capture, approximately 4 GJ/ton CO₂. Degradation is also a serious problem which considerably increases the cost of the process and causes corrosion problems in the system. Researchers tried to reduce the regeneration energy and degradation rate and enhance solvent efficiency in capturing CO₂ by using different process modifications and degradation inhibitors, and designing new promoters and solvents.

Consequently, there is a need for the assessment and application of different solvents in the CO₂ absorption process to identify promising alternatives for MEA [18]. As mentioned before, selection of promising solvents for CO₂ capture depends on various physical and chemical properties. These properties include absorption capacity, reaction kinetics between the amine and CO₂, reaction enthalpy, stability and toxicity [19]. These properties all have a direct influence on both the efficiency and the cost of an industrial-scale process [17]. Mixed amine solvents are being evaluated as prospective substitutes for MEA in CO₂ capture due to their rapid absorption rates of primary/secondary amines and the higher equilibrium capacity of tertiary amines and lower energy required for regeneration of tertiary amines due to weaker bonding with CO₂ [2,3]. Primary, secondary, tertiary and hindered amines are different types of amines, and their use has increased in recent years [9]. Combinations of primary and tertiary amines, such as MEA and MDEA, or secondary and tertiary amines, for example, diethanolamine (DEA) and MDEA, have been proposed for industrial gas treatment operations [16].

Table 1. Summary of selected experimental studies performed on the CO₂ absorption process using a mixture of amines.

Solvent	Equipment Specifications	Operating Conditions	Research Remark	Ref.
1MEA + PZ DEA + PZ AMP + PZ 1MDEA + PZ		Total gas pressure of 100 kPa (with CO2 partial pressure of 12 kPa) and temperature of 40 °C	10 wt.%-AMP/20 wt.%-PZ blend recorded the highest absorption capacity of 0.99 mol CO2/mol amine	[18]
2-dimethylamino-2-methyl-1-propanol 2DMA2M1P + MEA + AMP	Mass flow controller, gas mixer, saturation unit, reactor, condenser	Gas mixtures (CO2 + N2) with PCO2 between 5 and 60 kPa. A constant flow rate of 500 mL/min. The regeneration rate at T = 373 K. MEA 5 M as a benchmark aqueous solution for comparison purposes.	MEA-2DMA2M1P-AMP (1:2:2) has the fastest desorption kinetics, the highest cyclic capacity and the lowest energy demand	[19]
MEA + DIPA MEA + TEA MEA + AMP MEA + PZ	The bubble column as the scrubber, gas-flow feed system, liquid-flow system, pH meter, gas heating system, liquid cooling system	Tg = 50 °C Solvent concentration: 1–2.5 M Ratio of mixed amines: 5–20 wt.% Gas flow rate: 3–12 L/min Liquid flow rate: 150–30 mL/min	The optimum conditions to reach 100% absorption efficiency: Solvent = MEA + AMP Ratio of mixed amines: 20 wt.% Solvent concentration: 2.5 M Gas flow rate: 3.0 L/min Liquid flow rate: 300 mL/min	[20]
AMP + PZ	Inner diameter, height, and packing height of absorption and stripping columns are 0.04 m, 1.3 m and 084 m, respectively. Packing material is nonreactive metal HELI-PAK.	Pabs: atmospheric Tabs: 298–308 K Tdes: 368–383 K Pdes: 50–55 kPa PCO2: 10, 12 and 15 kPa	CO2 loading capacity increases with the increase in the ratio of PZ in the mixture A higher concentration of solvent has a significantly lower regeneration efficiency.	[21]
bis (3-aminopropyl) amine (APA) + MDEA	Kinetic experiments of CO2 absorption A wetted-wall model contactor Deuterium oxide (D2O) was used for NMR measurement	T (303 to 323 K). a concentration range of 0.1–0.5 kmol m−3 and different CO2 partial pressures.	The kinetic rate constants of APA indicate that it has great potential as an activator with AMP and MDEA A small amount of fast-reacting APA added to a much larger amount of MDEA significantly enhances the absorption rate	[22]
TEPA-MEA TMBPA-PZ	Absorber unit. Stripping unit.	Absorption experiments at 40 °C and 9.5 kPa CO2 partial pressure. Desorption experiments at 80 °C down to 1.0 kPa CO2 partial pressure.	TEPA-MEA showed the best performance in terms of absorption rate. TMBPA-PZ showed the best potential for efficient CO2 removal at reduced cost among all systems tested. Cyclic capacity in mol CO2/mol amine 70%.	[23]
1,5diazabicyclo [4.3.0] non-5-ene (DBN), 1,2,4-triazole (Tz), DBN/Tz	Tz (99%), DBN (98%). A vacuum pump to a pressure lower than 3 kPa. An absorption equilibrium.	Tdesorber: 80 °C, vibrational frequency analysis at 25 °C and 100 kPa.	Capacity 0.19 g CO2/g DES at 25 °C.	[24]
TEMDA (N,N,N′,N′-Tetra methylethylenediamine) and space-hindered amine DACH (trans-1,4-Diaminocyclohexane)	L166 concentration of Chlorella sp. 0.45 μm membranes. SPSS software.	Drying at 105 °C for 48 h.	CO2 conversion efficiency increased to 43.29%. Carbon sequestration capacity increased by 49.07%. Carbon sequestration capacity: 123.27 mg/L.day	[25]

and

Table 2. Summary of selected modelling studies conducted on the CO₂ absorption process using an amine mixture.

Mixture of Amines	Modelling Performed, Process Equipment	Software	Modelling Studies and Conditions	Research Remark	Ref.
PZ-AMP PZ-NH3	Reaction film simulator.	MATLAB	Modelling of mass transfer during CO2 absorption.	The study found that the model successfully predicted the absorption flux of CO2 in mixed solvents, especially when reactants were significantly reduced within the film.	[26]
DEA and MDEA in water.	14–16 stages in the absorber and stripper units. Flash tank.	Aspen HYSYS	P 110–120 kPa. CO2 removal efficiency % 85%. Minimum temperature 5 K. Kent–Eisenberg for mixed amines. Peng–Robinson for the vapour phase.	CO2 removal efficiency of 84–86%. 1.15 M tons of CO2/year is removed.	[27]
Mixed amines-water	Aspen Plus. Absorption–desorption modelling. Absorber unit Stripper unit.	Aspen Plus	Gas flow rate 160 kg/h. 15–20 vol% CO2. CO2 capture %90.	The absorption of CO2 into a solvent is highly dependent on the partial pressures of all the species involved, as well as on maintaining a constant volume or pressure for the combination.	[28]
(2EAE- TMPAD, 2EAE-DEAB, 2EAE-1DMA2P, 2MAE-2DMAE, 2EAE-2DMAE, and DMCA-MCA)	A molecular dynamics simulation model.	Material Studio	T (313, 3232, and 333) K.	2EAE-TMPAD has the highest diffusion rate. 10%DMCA–20%MCA has a higher interaction strength.	[29]

illustrate some experimental and modelling studies performed on CO₂ absorption–desorption or CO₂ absorption only (without stripping) using mixtures of amines, respectively.

Based on stoichiometry, where carbamate is the last product of the process, the maximum CO₂ loading in MEA cannot exceed 0.5 mol of CO₂ per mole of amine [30]. It is worth noting that tertiary amines do not produce carbamate ions in the process due to their molecular structures. For instance, MDEA, a tertiary amine, exhibits reduced reaction rates when exposed to CO₂, which can be beneficial in specific operational scenarios. MDEA has several promising characteristics over MEA, such as a reduced heat of reaction with CO₂ and a great equilibrium loading capacity of about 1.0 mol of CO₂ per mole of amine [31]. Therefore, when combined with MEA, the mixture can attain both a high CO₂ loading and moderate energy requirements. Bairq et al. [32] demonstrated that the utilisation of MEA-MDEA mixtures can reduce the amount of energy required for regeneration by up to 20% when compared to MEA alone. The mixture leverages the reduced heat capacity of MDEA to reduce total energy consumption [32]. A slow reaction rate with CO₂ due to the lack of carbamate formation is a weakness of tertiary amines, which are well-suited for the selective absorption of CO₂ in the presence of H₂S, given their high absorption capability and economic viability for the regeneration process. N,N-diethylethanolamine (DEEA) is another tertiary amine, that has a low heat of absorption due to its slow rate of reaction with CO₂. As a result, various agents, including rapidly reacting primary and secondary amines as well as polyamines, are utilised to increase the CO₂ absorption rate [33].

In addition to the polyamine-based solvent, PZ has the potential for a high absorption rate, superior absorption capability, and minimal volatility due to its multiple effective sites for reacting with CO₂. As a result, numerous scientific investigations recommend using PZ in the CO₂ absorption process because it is a useful activator for tertiary amines [34] and hindered amines [35]. The impact of adding PZ as an agent to further amines was illustrated by Rayer et al. [36] through a comparison of experimental data from scientific papers at a steady temperature. According to the authors’ findings, the combination of PZ and hindered amines results in an elevated absorption capability and absorption amount for CO₂. Furthermore, sterically hindered amines, such as 2-amino-2-methyl-1-propanol (AMP), are a type of chemical absorbent that has been proposed as an effective solvent for removing acid gases from sour gas streams [31]. Reboiler heat needed for solvent regeneration and CO₂ recovery for a blend of AMP with PZ and MEA was estimated by Artanto et al. [37]. The authors based their estimates on lean CO₂ loading at a constant liquid-to-gas percentage. Their findings demonstrate that a mixture of AMP and PZ required smaller reboiler heat duty when lean CO₂ loading was decreased, indicating the potential of AMP/PZ as an alternative to MEA. However, the reboiler heat duty as a function of L/G ratio did not change for either absorption liquid at elevated lean CO₂ loading. Most studies on AMP/PZ experiments have found that at smaller lean CO₂ loadings, the required heat for the condensation stage and reasonable heat are the primary sources of heat for the reboiler heat duty.

It is reported that PZ is a highly effective solvent for CO₂ collection when combined with primary amines [38]. The cyclic structure of PZ offers a distinct kinetics profile that has the potential to improve the overall performance of the absorption [39]. Adeosun et al. [18] demonstrated that MEA + PZ blends have higher absorption rates than MEA, whereas with 30 wt.% of MEA, the absorption rate was 1.09 mmol/(mol.s), and with MEA + PZ 10:20, 15:15 and 20:10, the absorption rates were 1.7, 1.65 and 1.6 mmol/(mol.s), respectively.

Several commercial amine mixtures are widely used for CO₂ capture in various industries, particularly in natural gas processing, ammonia production, and post-combustion carbon capture. Some examples are UCARSOL™ AP, UCARSOL™ HS, ADIP-X, ADIP Ultra, CESAR1 and CESAR2.

Table 3. Commercially applied solvents and their corresponding gas-treating processes: composition and industrial applications.

Industrial Solvents	Solvent	Applications	Suppliers	Commercial Process Examples
UCARSOL™	MDEA	Natural gas processing, refinery operations	Dow Chemical Company	Used to treat raw sour gas to meet pipeline specifications (<4 ppm H2S and <2% CO2)
UCARSOL™ AP (802/804/806/ 810/814)	Proprietary tertiary amine blend with activators	Acid gas removal in natural gas processing	Dow Chemical Company	Used in natural gas sweetening in high-pressure systems
UCARSOL™ HS (101/102/103/115)	Proprietary hindered amine formulation	Ammonia synthesis (removal of CO2 and H2S from feed gas)	Dow Chemical Company	Ammonia plants for syngas purification
Selexol™	Dimethyl ethers of polyethylene glycol	Acid gas removal in syngas and natural gas processing	Honeywell UOP	Used in integrated gasification combined cycle (IGCC) power plants
CESAR1	30 wt.% MEA + 10 wt.% PZ	Advanced post-combustion CO2 capture	European CESAR Project (Collaborative)	Pilot-scale CO2 capture at coal-fired power stations
CESAR2	30 wt.% AMP + 10 wt.% PZ	Post-combustion carbon capture	European CESAR Project (Collaborative)	Demonstrated in post-combustion CO2 capture pilot plants
Rectisol®	Methanol (CH3OH), cryogenic solvent	For the physical absorption of CO2, H2S, and other impurities. It is particularly effective for applications requiring high-purity gas streams	Linde	Methanol-based purification in Fischer–Tropsch processes
Amisol®	Proprietary amine solution (likely MDEA-based)	Acid gas removal in chemical and petrochemical industries	BASF	BASF’s Amisol process for H2S and CO2 removal
KS Series	Proprietary hindered amine blend	Post-combustion carbon capture	Mitsubishi Heavy Industries	Mitsubishi’s CO2 capture process (such as Petra Nova)
OASE® sulfexx	Amine-based	Natural gas processing, refinery, off-gas treatment, synthesis gas treatment	ExxonMobil and BASF	Ammonia production, methanol plants, and hydrogen production facilities
FLEXSORB™ (ExxonMobil)	Proprietary sterically hindered amines	Widely used in natural gas and sulphur recovery operations	ExxonMobil	ExxonMobil’s FLEXSORB™ process in gas-treating plants
ADIP-X	MDEA + PZ	CO2 and H2S removal in natural gas processing	Shell Global Solutions	Shell’s ADIP process for gas treating
ADIP Ultra	MDEA + PZ	CO2 and H2S removal in natural gas, sulphur recovery	Shell Global Solutions	Shell’s ADIP Ultra process in sulphur recovery units

presents the industrial uses, applications, and developers of mixed amine solvents.

1.2. Motivation of the Current Study

This study presents an Aspen Plus rate-based model of the post-combustion CO₂ capture process using aqueous amines, such as MEA, PZ, MEA + PZ, and MDEA + PZ, from a flue gas consisting of N₂, O₂, CO₂, and H₂O. The model contains an absorber and a stripper. Pilot-scale data from the literature is used in this study to validate the benchmark model using the MEA solution [40]. The study examines three scenarios for CO₂ absorption-desorption processes using different solvents: Scenario 1 (S1), which uses MEA as a benchmark, is validated against experimental data [40]. The same model is then used for other solvents—PZ, MEA + PZ, and MDEA + PZ—while maintaining constant absorber and stripper sizes. The required chemical reactions and physical properties are incorporated into the model for each solvent. Therefore, only the solvent is switched in this scenario, and the sizes of the absorber and stripper columns remain unchanged. Scenario 2 (S2) involves modifying and adjusting the absorber and stripper sizes to achieve a 90% carbon capture level (CCL) using the MEA solution. These sizes are then applied to the other solvents to assess their performance under the same conditions. Scenario 3 (S3) applies to solvents that fail to reach the 90% carbon capture level in Scenario 2 (S2). In this case, either the absorber or stripper sizes are further adjusted, or the inlet liquid flow rate is altered to meet the target for the solvents that did not meet 90% CCL in S2.

A sensitivity analysis is conducted using S3, as this scenario ensures that all solvents reach the 90% carbon capture level. Sensitivity analysis for different operating parameters is also performed. In summary, the main objective and novelty of this study are to evaluate the performance of various amines and their mixtures using identical unit operation sizes. This approach provides engineers and researchers in the field of carbon capture with valuable insights into the behaviour of amine solutions across the complete absorption–desorption cycle.

2. Methodology

2.1. Process Description

The configuration of the CO₂ absorption–desorption system in this study, which was developed in Aspen Plus V14, is depicted in Figure 1. Similarly to a standard configuration, the flue gas (inlet gas) is introduced into the bottom of the packed bed absorber. The lean amine solution is created by combining amine (fresh amine) with the lean out (LEANOUT), and it is introduced into the top section of the absorber. RICHOUT points to the CO₂-rich amine, which is transferred by a pump to the lean/rich amine heat exchanger (HX). To facilitate CO₂ desorption, the solvent rich with absorbed species is heated before it enters the stripper column in exchange for energy from the desorption column’s bottom stream. The heat exchange unit could minimise the need for external energy sources. In HX, the rich outflow (RICHOUT) is heated by a heat exchanger with the recycled lean amine (LEANOUT), and the resulting heated stream is referred to as the rich in (RICHIN) stream. The heat exchanger transfers the rich amine solution from the bottom of the absorber to the stripper.

2.2. Process Modelling

Simulations in Aspen Plus V14.0 evaluated four amine solvents, MEA, PZ, MEA + PZ, and MDEA + PZ, for CO₂ capture from power plant flue gas (N₂, O₂, CO₂, H₂O). The electrolyte NRTL model governed liquid-phase thermodynamics, while the Redlich–Kwong equation of state characterised vapour-phase behaviour. RadFrac columns employed rate-based modelling for absorption and stripping, incorporating true ionic species, activity-based reaction kinetics, electrolyte transport properties, and structured packing hydraulics. The design parameters in this study are illustrated in

Table 4. Stream composition and operating properties of the base case (benchmark) study [40].

Property	Flue Gas	Inlet Lean MEA	Makeup Water
Temperature (°C)	48.01	40.01	43.87
Pressure (mbar)	1004.49	2000	~1000
Total flow (kg/h)	72.0	201.3	30.75
MEA (wt.%)	0	27.5	00.31
H2O (wt.%)	7.1	67.3	99.54
CO2 (wt.%)	8.50	5.2	0
N2 (wt.%)	74.3	0	0
O2 (wt.%)	10.1	0	0
Molar CO2 loading	--	0.265	--

and

Table 5. Design parameters of the equipment of the base case study [40].

Items	Unit	Value
Absorber and stripper diameters	m	0.125
Absorbing section height	m	4.2
Washing section height (absorber)	m	0.42
Stripping section height	m	2.5
Washing section height (stripper)	m	0.42
Water outlet flow rate	kg/h	28.53
Number of stages (absorber and stripper)	stages	20
Packing type	-	FLEXIPAC® 250Y
Distillate rate	kg/h	6.93
Boil-up ratio	-	0.064

. In addition, operational parameters were derived from experimental research [41], defining flue gas and solvent feed conditions. The adjustment process was carried out using the Design Spec utility in Aspen Plus, a powerful sensitivity analysis tool that enables precise tuning of process variables to achieve targeted performance criteria within the simulation. Key CO₂–solvent reactions are classified in

Table 6. Chemical reactions for MEA, MDEA, and PZ solvents.

Chemical Solvent	Chemical Reactions	Type of Reaction
MEA	${\mathrm{M}\mathrm{E}\mathrm{A}{\mathrm{H}}^{+}+\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{H}}_3{\mathrm{O}}^{+}+\mathrm{M}\mathrm{E}\mathrm{A}$	Equilibrium
	$\mathrm{M}\mathrm{E}\mathrm{A}{\mathrm{C}\mathrm{O}\mathrm{O}}^{-}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{M}\mathrm{E}\mathrm{A}+\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$	Equilibrium
	$\mathrm{M}\mathrm{E}\mathrm{A}+\mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\to \mathrm{M}\mathrm{E}\mathrm{A}{\mathrm{C}\mathrm{O}\mathrm{O}}^{-}+{\mathrm{H}}_3{\mathrm{O}}^{+}$	Kinetic
	$\mathrm{M}\mathrm{E}\mathrm{A}{\mathrm{C}\mathrm{O}\mathrm{O}}^{-}+{\mathrm{H}}_3{\mathrm{O}}^{+}\to \mathrm{M}\mathrm{E}\mathrm{A}+\mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$	Kinetic
MDEA	${\mathrm{M}\mathrm{D}\mathrm{E}\mathrm{A}\mathrm{H}}^{+}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow \mathrm{M}\mathrm{D}\mathrm{E}\mathrm{A}+{\mathrm{H}}_3{\mathrm{O}}^{+}$	Equilibrium
	$\mathrm{M}\mathrm{D}\mathrm{E}\mathrm{A}+{\mathrm{H}}_2\mathrm{O}+\mathrm{C}{\mathrm{O}}_2\to \mathrm{M}\mathrm{D}\mathrm{E}\mathrm{A}{\mathrm{H}}^{+}+\mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-}$	Kinetic
	$\mathrm{M}\mathrm{D}\mathrm{E}\mathrm{A}{\mathrm{H}}^{+}+\mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-}\to \mathrm{M}\mathrm{D}\mathrm{E}\mathrm{A}+{\mathrm{H}}_2\mathrm{O}+\mathrm{C}{\mathrm{O}}_2$	Kinetic
PZ	$\mathrm{P}\mathrm{Z}{\mathrm{H}}^{+}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow \mathrm{P}\mathrm{Z}+{\mathrm{H}}_3{\mathrm{O}}^{+}$	Equilibrium
	$\mathrm{H}\mathrm{P}\mathrm{Z}\mathrm{C}\mathrm{O}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow \mathrm{P}\mathrm{Z}\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}+{\mathrm{H}}_3{\mathrm{O}}^{+}$	Equilibrium
	$\mathrm{P}\mathrm{Z}+\mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\to \mathrm{P}\mathrm{Z}\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}+{\mathrm{H}}_3{\mathrm{O}}^{+}$	Kinetic
	$\mathrm{P}\mathrm{Z}\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}+{\mathrm{H}}_3{\mathrm{O}}^{+}\to \mathrm{P}\mathrm{Z}+\mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$	Kinetic
	$\mathrm{P}\mathrm{Z}\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}+\mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\to \mathrm{P}\mathrm{Z}{\left(\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}\right)}_2+{\mathrm{H}}_3{0}^{+}$	Kinetic
	$\mathrm{P}\mathrm{Z}{\left(\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}\right)}_2+{\mathrm{H}}_3{\mathrm{O}}^{+}\to \mathrm{P}\mathrm{Z}\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}+\mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_{2}0$	Kinetic
Other reactions that must be considered for all the solvents	${2\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{O}\mathrm{H}}^{-}$	Equilibrium
	$\mathrm{C}{\mathrm{O}}_2+{2\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{H}}_3{\mathrm{O}}^{+}+\mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-}$	Equilibrium
	${\mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-}+\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{H}}_3{\mathrm{O}}^{+}+\mathrm{C}{\mathrm{O}}_3^{2-}$	Equilibrium
	${\mathrm{C}\mathrm{O}}_2+\mathrm{O}{\mathrm{H}}^{-}\to \mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-}$	Kinetic
	$\mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-}\to {\mathrm{C}\mathrm{O}}_2+\mathrm{O}{\mathrm{H}}^{-}$	Kinetic

.

3. Results and Discussion

In this section, the results observed for CO₂ absorption using MEA, PZ, MEA + PZ, and MDEA + PZ are provided. Throughout the modelling procedures, except during the validation phase, the lean amine stream was maintained at a concentration of 27.5 wt.%.

3.1. Validation

As the first step of modelling, validation of the base case results is performed by comparing the simulation findings with experimental data.

Table 7. Comparison of experimental and modelled CO₂ partial pressure.

Zpacking/m	${\mathit{P}}_{{\mathit{C}\mathit{O}}_2,\mathit{e}\mathit{x}\mathit{p}}$	${\mathit{P}}_{{\mathit{C}\mathit{O}}_2,\mathit{m}\mathit{o}\mathit{d}\mathit{e}\mathit{l}}$	REP (%)
0.411	16.502	15.510	6.008
1.272	18.977	19.506	2.791
2.111	23.928	25.746	7.599
2.945	35.066	33.962	3.148
3.806	44.142	44.285	0.323
		AREP	3.974

presents the results of experimental and modelling CO₂ partial pressure at the packing height of the packed bed and estimates the percentage of error using Equation (1). The base case was validated using both practical and Aspen Plus results at an inlet temperature of 48.01 °C and a pressure of 1004.49 mbar, with a flow rate of 72 kg/h of flue gas and MEA lean stream concentration of 27.5 wt.%. Figure 2 displays a comparison of CO₂ partial pressures between the absorber and the experimental study [40]. As can be seen from the figure, a strong consistency is evident between the findings of this investigation and those of Notz et al. [40], with an absolute relative error percentage (Equation (1)) of approximately 3.97%.

$$REP\left(\%\right)={\displaystyle \frac{P_{{CO}_2,model}-P_{{CO}_2,exp.}}{P_{{CO}_2,exp.}}}\times 100$$

(1)

3.2. Scenario 1: Benchmark Using MEA and Switching the Solvent

Figure 3 illustrates the CCL (Equation (2)) across four distinct solvent systems: MEA, MEA combined with PZ, MDEA combined with PZ, and PZ alone. Among these formulations, the MEA + PZ combination achieves the highest CCL, surpassing the performance of both individual amines (MEA and PZ) and the MDEA + PZ configuration. This observation corroborates findings documented in the existing literature, which indicate that the incorporation of PZ into MEA significantly enhances the performance of CO₂ absorption due to PZ’s pronounced promoting effect on the reaction kinetics and capacity. For instance, empirical investigations have revealed that MEA + PZ mixtures not only enhance capture efficiency but also diminish the energy required for regeneration relative to MEA alone, rendering them advantageous for utilisation in industrial post-combustion capture scenarios [41,42]. The effectiveness of the MDEA + PZ blend, although superior to that of PZ alone, does not surpass that of MEA + PZ, which is consistent with prior modelling and experimental studies that suggest that MEA-based blends with PZ provide an optimal equilibrium between absorption rate and energy demand. These findings substantiate the assertion that solvent blending, particularly involving MEA and PZ, can yield substantial enhancements in CO₂ capture efficiency, thereby facilitating the ongoing transition from conventional MEA systems to sophisticated blended amine formulations in both academic research and industrial applications [43].

In particular, the MEA + PZ blend, which achieves a peak CCL of approximately 84%, is in accordance with comprehensive studies that illustrate piperazine’s role as an effective kinetic promoter when combined with MEA. This enhancement is attributed to PZ’s provision of additional reactive sites and the acceleration of reaction kinetics. At the same time, MEA contributes significantly to CO₂ solubility, thereby engendering a synergistic interaction that optimises both the absorption rate and capacity [44]. The baseline MEA performance, recorded at approximately 73% CCL, serves as the standard reference point, consistent with its prevalent industrial application despite its limitations concerning regeneration energy requirements [43]. The moderate performance exhibited by MEA reflects a balance of adequate absorption capacity (0.50 molCO₂/mol amine) and acceptable reaction kinetics, albeit it lacks the enhanced characteristics that optimised blends possess [18].

The MDEA + PZ blend, exhibiting performance akin to that of MEA (~73%), can be ascribed to the inherently slower reaction kinetics associated with MDEA, which constrains the overall system performance notwithstanding PZ’s promotional influences. While MDEA + PZ blends reveal comparable performance to benchmark solvents in exhaustive studies [45], the kinetic limitations inherent to MDEA preclude this combination from surpassing the performance of MEA + PZ under identical operational conditions. Conversely, the comparatively lower performance of pure PZ (~69%) may appear counterintuitive in light of literature findings indicating that PZ achieves a capacity of 1.06 molCO₂/mol amine, which is 66% greater than that of MEA [18]. Nonetheless, this outcome likely reflects the operational challenges associated with concentrated PZ solutions, which include solubility constraints, increased viscosity, and potential precipitation issues that can inhibit effective CO₂ capture under practical column operating conditions. While PZ demonstrates superior performance in laboratory-scale absorption capacity assessments, its application as a standalone solvent in packed columns encounters operational limitations that constrain its CCL performance in comparison to optimally balanced blended systems.

$$\mathrm{C}\mathrm{C}\mathrm{L}\mathrm{\%}=\left({\displaystyle \frac{y_{{CO}_2}^{in}-y_{{CO}_2}^{out}}{y_{{CO}_2}^{in}}}\right)\times 100$$

(2)

The reboiler duty, which constitutes a pivotal parameter in the assessment of the energy efficiency of amine-based CO₂ capture systems, exerts a direct influence on the operational cost and the overall feasibility of post-combustion capture methodologies. Consequently, the reboiler duty of the scenarios under investigation is also subjected to analysis.

In Scenario 1, as shown in

Table 8. The results of CO₂ absorption for S1.

Parameters	Amine Solvents
	MEA	PZ	MEA + PZ (5:1)	MDEA + PZ (5:1)
Outlet CO2 concentration (%) in treated gas stream	1.49	1.73	0.9	1.50
Lean amine loading (mol/mol)	0.24	0.56	0.33	0.056
Rich amine loading (mol/mol)	0.36	0.72	0.55	0.24
Lean amine circulation (m3/h)	0.20	0.20	0.22	0.25
Outlet CO2 flow rate (kg/h)	4.51	4.23	5.13	4.54
Stripper reboiler duty (kJ/h)	26,773	25,244	27,472	28,344
Reboiler duty (MJ/kg CO2)	5.93	5.97	5.35	6.24
CO2 removal (%)	72.57	68.08	83.45	72.46

, the MEA + PZ blend demonstrates the lowest specific reboiler duty (5.35 MJ/kg CO₂) among all examined solvents, notwithstanding a marginally elevated total reboiler duty due to enhanced CO₂ removal efficiency (83.45%). This observation is corroborated by studies indicating that the amalgamation of MEA with PZ can augment CO₂ absorption kinetics and enhance cyclic capacity, thereby diminishing the energy requisite for solvent regeneration per unit of CO₂ captured [46,47]. The superior performance is ascribed to the synergistic interaction between MEA’s rapid absorption rate and PZ’s elevated capacity and thermal stability, facilitating more efficient stripping processes and reduced regeneration energy requirements.

PZ also exhibits a commendable reboiler duty (5.97 MJ/kg CO₂), which is comparable to that of MEA (5.93 MJ/kg CO₂). Nonetheless, PZ’s elevated lean and rich loadings suggest a greater cyclic capacity, which is advantageous for process efficiency; however, its slightly diminished CO₂ removal percentage (68.08%) implies that operational conditions or solvent management practices might be further refined. This observation aligns with findings that PZ, while exhibiting high efficacy, may necessitate meticulous process control to fully harness its potential benefits [46]. Conversely, the MDEA + PZ blend, despite benefiting from the incorporation of PZ as an activator, registers the highest specific reboiler duty (6.24 MJ/kg CO₂). This phenomenon is likely attributable to MDEA’s lower reaction rate with CO₂, resulting in increased energy demands for regeneration, particularly at elevated cyclic capacities. The literature substantiates that although MDEA blends can mitigate solvent degradation and corrosion, their diminished absorption rates may lead to heightened regeneration energy requirements unless process conditions are judiciously optimised [48].

Overall, these findings underscore the critical significance of solvent selection and process optimisation in minimising energy consumption. The MEA + PZ blend is particularly notable for its capacity to achieve both substantial CO₂ removal and a low specific reboiler duty, rendering it a promising candidate for energy-efficient post-combustion CO₂ capture methodologies.

3.3. Scenario 2: CCL 90% for MEA and Switched Solvents

To enhance the efficacy of post-combustion CO₂ capture methodologies, it is imperative to systematically assess the performance of alternative amine solvents and their respective blends under uniform operational conditions. Scenario 2 is dedicated to a comparative analysis of both single and blended amine systems, specifically including PZ, MEA + PZ, and MDEA + PZ, utilising the same packed column dimensions that have been optimised for MEA. This methodological framework yields significant insights regarding the comparative efficiency and operational constraints of each solvent system when applied in practical absorber configurations. Figure 4 illustrates the flow chart of the second scenario.

Increasing the diameter of the packed bed column from 0.1322 m to 0.1917 m in this investigation resulted in a notable enhancement in CO₂ removal efficiency utilising MEA, elevating the CCL from 75.49% to 90.33% (Figure 5). This phenomenon can be ascribed to the improved mass transfer and hydrodynamic performance observed in the enlarged column. A larger column diameter facilitates augmented gas and liquid throughput, thereby diminishing the likelihood of flooding and channelling, while simultaneously enhancing the distribution of both phases throughout the packing. Consequently, this leads to an expanded effective interfacial area for gas–liquid interaction, which is paramount for the absorption of CO₂ into the MEA solution [49,50]. Existing literature substantiates that the mass transfer coefficient (KG_a) and overall absorption efficacy are intricately correlated with column geometry, wherein larger diameters promote superior phase contact and elevated removal rates under otherwise uniform operating conditions [51]. Furthermore, an increased diameter can alleviate wall effects and maldistribution, thereby further fostering uniform flow and optimising the utilisation of the packing’s surface area for CO₂ capture. These observations align with established research indicating that the optimisation of column dimensions, particularly diameter, can be as pivotal as the selection of solvent in attaining elevated post-combustion CO₂ removal efficiencies utilising MEA in packed columns [49,51].

The documented findings regarding CCL employing PZ (71.03%), MEA + PZ (90.75%), and MDEA + PZ (70.74%) with the same optimised column dimensions as MEA (90.33%) are robustly corroborated by contemporary literature. Figure 6 illustrates the results for the other solvents based on the MEA 90% CCL parameters. The elevated CCL realised with the MEA + PZ combination is consistent with a multitude of studies illustrating that piperazine serves as a potent promoter for MEA, significantly augmenting absorption kinetics and cyclic capacity. Orangi et al. [43] demonstrated that the incorporation of PZ into MEA not only enhances CO₂ removal efficiency but also diminishes regeneration energy demands, with the MEA + PZ blend achieving superior cyclic capacity and reduced energy consumption relative to pure MEA [43]. Additionally, Jiang et al. [52] further validate that PZ amplifies the absorption rate and capacity of MEA-based solvents, reinforcing the exceptional performance of the MEA + PZ blend [52]. The lower CCL for pure PZ, despite its elevated intrinsic reactivity, can be attributed to operational impediments such as increased viscosity, precipitation, and potential mass transfer constraints in real-world absorber configurations. The literature indicates that while PZ exhibits the highest reactivity towards CO₂ among prevalent amines, its deployment as a solitary solvent is frequently limited by these practical challenges, which can adversely affect its effective capture efficiency in packed columns [52]. For the MDEA + PZ blend, the observed CCL is likewise consistent with documented findings. Although MDEA + PZ systems benefit from the kinetic enhancement provided by PZ, the inherently slower base reaction kinetics of MDEA restrict the overall absorption rate. Research has indicated that although MDEA + PZ blends can achieve commendable CO₂ loading and enhanced regeneration efficiency, their removal efficacy under identical absorber conditions may not rival that of MEA + PZ, particularly when the system is not specifically optimised for the characteristics of MDEA [53].

In summary, these findings are corroborated by existing literature, which underscores that the efficacy of amine-based CO₂ capture systems is profoundly influenced by both the chemical properties of the solvent and the intricacies of process design. Blended solvents such as MEA + PZ demonstrate superior performance by synergistically harnessing the benefits of both constituent components. In contrast, pure PZ and MDEA + PZ necessitate further process enhancements to fully realise their potential in large-scale packed column implementations.

In Scenario 2, the thermal energy requirements for each solvent system were markedly affected by both the specific amine composition and the resultant efficiency of CO₂ extraction (

Table 9. The results of CO₂ absorption for the S2.

Adjusted Results	MEA	PZ	MEA + PZ (5:1)	MDEA + PZ (5:1)
Outlet CO2 concentration (%) in treated gas stream	0.53	1.57	0.49	1.59
Outlet CO2 flow rate (kg/h)	5.54	4.40	5.58	4.41
Stripper reboiler duty (kJ/h)	25,565	25,260	23,783	22,037
Reboiler duty (MJ/kg CO2)	4.61	5.74	4.28	5
CO2 removal (%)	90.33	71.03	90.75	70.74

). The MEA + PZ formulation exhibited the lowest specific thermal energy requirement at 4.28 MJ/kg CO₂, followed closely by MDEA + PZ at 5.00 MJ/kg CO₂, and PZ at 5.74 MJ/kg CO₂. Importantly, the MEA + PZ formulation also accomplished the highest CO₂ extraction efficiency at 90.75%, in contrast to 71.03% for PZ and 70.74% for MDEA + PZ.

These empirical results align with existing literature that suggests that the incorporation of PZ with MEA can substantially enhance absorption kinetics and augment the cyclic loading capacity, thereby facilitating a greater proportion of CO₂ extraction with diminished energy requirements for solvent regeneration [46,47]. The enhanced efficacy of the MEA + PZ formulation, in terms of CO₂ extraction and thermal energy requirements, can be ascribed to the synergistic interplay between MEA’s rapid absorption kinetics and PZ’s elevated CO₂ capacity and thermal resilience. This synergy promotes more effective solvent regeneration and mitigates the energy expenditure per unit of CO₂ sequestered. Moreover, although MDEA + PZ exhibited a moderate thermal energy requirement (5.00 MJ/kg CO₂), it did not reach the same degree of CO₂ extraction efficiency as MEA + PZ. This observation corroborates prior research indicating that while MDEA demonstrates lower reactivity with CO₂, the inclusion of PZ as an activator enhances its performance, albeit not to the same magnitude as the MEA + PZ blend [48]. The comparatively reduced thermal energy requirement for MDEA + PZ relative to PZ alone may be attributable to a diminished overall CO₂ extraction, which subsequently lessens the total energy demands for regeneration.

PZ alone, notwithstanding its favourable cyclic loading characteristics and commendable thermal stability, registered the highest specific thermal energy requirement in this scenario. This phenomenon may stem from suboptimal process conditions or the necessity for increased energy input to achieve the requisite regeneration at the evaluated concentration and flow rates.

Collectively, these findings underscore that the MEA + PZ formulation provides a distinct advantage in both energy efficiency and CO₂ capture efficacy under the parameters tested. The results emphasise the critical nature of solvent selection and process optimisation in minimising operational expenditures and maximising capture efficiency in industrial contexts.

It is crucial to acknowledge that the reboiler duty values delineated in Scenario 2 (for instance, 4.61 MJ/kg CO₂ for MEA) exceed the conventional range referenced for industrial-scale MEA-based systems (approximately 3.7–4.2 GJ/t CO₂). This discrepancy arises from two principal factors. Firstly, the goal of this scenario is to attain a 90% CO₂ capture rate by modifying column geometry (such as diameter) without altering other operational parameters including solvent flow rate, temperature, or concentration. Although these geometric modifications enhance mass transfer and facilitate superior capture efficiency, they may also lead to an increase in solvent circulation and the concomitant thermal energy requirements.

Secondly, the current investigation simulates a pilot-scale system, which intrinsically lacks the sophisticated heat integration and energy recovery mechanisms that are typically found in commercial-scale facilities. Consequently, the energy consumption metrics are anticipated to be marginally elevated. This should not be construed as a direct detriment of the evaluated solvent systems, but rather as a reflection of the controlled design parameters and the simulation scale implemented in this study. Subsequent research may delve into the optimisation of operating conditions and the incorporation of heat integration strategies to mitigate energy demand while preserving high CO₂ capture efficiency.

3.4. Scenario 3: CCL 90% for Solvents That Failed to Reach 90% in Scenario 2

Building upon the insights derived from the preceding scenario, Scenario 3 (Figure 7) investigates the prospects for enhanced optimisation of CO₂ removal by methodically varying both the height and diameter of the packed bed column across distinct solvent systems. Through the incremental alteration of these critical design parameters, this scenario aims to determine the ideal absorber configuration necessary to achieve a 90% CCL for each solvent, thereby elucidating the intricate relationship between solvent chemistry and column architecture in enhancing capture efficacy.

3.4.1. MDEA + PZ as Solvent

The findings about the MDEA + PZ solvent system reveal a distinct pattern: systematic augmentations in both packed bed height and diameter notably enhance the CCL, ultimately reaching the 90% objective at a height of 15.94 m and a diameter of 0.35 m. This advancement aligns with recognised mass transfer theory and substantiated empirical evidence within the existing literature. Figure 8 illustrates this trend due to the increase in height and diameter of the absorber.

Initially, increasing the packed bed height from 4.15 m to 8.00 m (while maintaining a constant diameter of 0.125 m) increases the CCL from 73.43% to 78.03%. This enhancement is ascribed to the prolonged contact duration between the flue gas and the solvent, thereby augmenting the degree of CO₂ absorption. As corroborated by Rochelle [47], an increase in column height directly correlates with an escalation in the number of transfer units (NTU), facilitating enhanced CO₂ removal as the gas navigates a longer absorption pathway. Nevertheless, the rate of enhancement wanes as the system approaches equilibrium, signifying that height alone cannot perpetually offset the mass transfer constraints imposed by a constricted column diameter. Upon attaining a height of 8.00 m, additional increments in diameter from 0.125 m to 0.21 m lead to a pronounced increase in CCL from 78.03% to 83.33%. An enlarged diameter mitigates wall effects and channelling, optimises liquid and gas distribution, and amplifies the cross-sectional area available for mass transfer. This results in a more uniform exploitation of the packing material and an elevated effective interfacial area for CO₂ absorption. The literature consistently indicates that optimal absorber efficacy is realised when both height and diameter are meticulously calibrated to the kinetics of the solvent and the rates of process flow.

Further augmentations in both height and diameter yield diminishing yet still notable enhancements. At a constant diameter of 0.21 m, increasing the height to 13.20 m elevates the CCL to 87.96%. Subsequent increases in diameter to 0.29 m and 0.35 m yield marginal enhancements, culminating in a CCL of 88.72% at a height of 13.20 m and a diameter of 0.35 m. Ultimately, extending the height to 15.94 m with a diameter of 0.35 m achieves the target CCL of 90.00%. This iterative methodology is substantiated by the interaction between the mass transfer driving force (augmented by height) and hydrodynamic performance (enhanced by diameter). As articulated by Abu-Zahra et al. [54], such staged optimisation is imperative for solvents with inherently slower kinetics, such as MDEA, even when facilitated by PZ, to surmount intrinsic absorption rate constraints. These observations are congruent with the broader literature, which underscores that for solvents or blends exhibiting slower reaction rates, such as MDEA + PZ, the geometry of the absorber must be meticulously designed to optimise performance [47]. Although PZ serves as an efficacious promoter, the overall absorption rate remains constrained by the reaction kinetics of MDEA, necessitating larger column dimensions to attain elevated removal efficiencies. The evident diminishing returns observed at greater dimensions further substantiate the principles of mass transfer theory, which postulates that incremental gains diminish as the system nears equilibrium.

In conclusion, the findings regarding MDEA + PZ substantiate that achieving elevated CO₂ capture efficiency when employing slower-reacting solvents requires a judicious and iterative optimisation of both packed bed height and diameter. This methodology is robustly corroborated by mass transfer theories and empirical investigations, thereby underscoring the significance of absorber design in the context of post-combustion CO₂ capture methodologies.

3.4.2. PZ as Solvent

The empirical findings regarding PZ indicate that a systematic augmentation of both the packed bed height and diameter results in a consistent enhancement of CO₂ removal efficiency, ultimately attaining a CCL of 90.19% at a height of 10.585 m and a diameter of 0.909 m (Figure 9). Initially, an increase in diameter at a constant height (from 0.125 m to 0.230 m at a height of 5.925 m) engenders a substantial elevation in CCL from 78.44% to 84.97%, which aligns with mass transfer theories and existing literature that posits that a larger diameter improves gas–liquid distribution, mitigates channelling effects, and amplifies the interfacial area conducive for absorption [55,56]. Subsequent increments in height, while maintaining a constant diameter (0.230 m), yield marginal enhancements in the CCL, escalating from 85.81% at 6.470 m to 88.35% at 10.585 m, reflecting the increased contact duration and the number of transfer units available for mass transfer, corroborated by investigations into packed bed and rotating packed bed absorbers [57,58]. The final notable augmentation of diameter to 0.909 m at the highest tested height propels the CCL slightly above 90%, illustrating that for solvents with high reactivity, such as PZ, both the column height and the cross-sectional area necessitate optimisation to mitigate operational constraints such as thermal bulges and potential mass transfer impediments at elevated loadings [58]. The literature consistently indicates that while PZ demonstrates enhanced absorption kinetics and capacity in comparison to MEA, achieving markedly high removal efficiency within practical columns necessitates meticulous calibration of both geometric and operational parameters to fully leverage these inherent advantages [55,56,58].

As tabulated in

Table 10. The results of CO₂ absorption for the S3.

Adjusted Results	PZ	MDEA + PZ
Outlet CO2 concentration (%) in treated gas stream	0.549	0.553
Outlet CO2 flow rate (kg/h)	5.520	5.519
Stripper reboiler duty (kJ/h)	29,800	25,380
Reboiler duty (MJ/kg CO2)	5.40	4.60
CO2 removal (%)	90.21	90.19

, for Scenario 3, both PZ and MDEA + PZ exhibited commendable CO₂ removal efficiencies (90.21% and 90.19%, respectively), accompanied by exceptionally low outlet CO₂ concentrations in the treated gas (0.549% for PZ and 0.553% for MDEA + PZ). Nonetheless, a significant disparity in reboiler duty is observed: PZ necessitated 5.40 MJ/kg CO₂, whereas MDEA + PZ required merely 4.60 MJ/kg CO₂.

This decrement in specific reboiler duty for the MDEA + PZ combination, in contrast to PZ alone, is thoroughly substantiated by the existing literature. The incorporation of piperazine as an activator to MDEA markedly augments the absorption kinetics and enhances the cyclic capacity, thereby facilitating more efficient CO₂ desorption with diminished energy input [59,60]. Empirical studies have demonstrated that MDEA + PZ blends can surpass both pure PZ and MEA regarding energy consumption, particularly when optimised for concentration and operational conditions. The enhanced performance is ascribed to the propensity of PZ to accelerate the reaction with CO₂, while MDEA offers a high equilibrium loading coupled with a lower heat of regeneration, culminating in a synergistic effect that mitigates the overall reboiler duty [61].

Moreover, the literature suggests that increasing the proportion of MDEA in the blend can further diminish energy consumption and enhance both the cyclic capacity and desorption rate [61]. The stability and resistance to degradation of the MDEA + PZ blend additionally reinforce its applicability for industrial implementations, as corroborated by recent comparative investigations [62,63]. In conclusion, the outcomes for Scenario 3 validate that the MDEA + PZ blend provides a substantial advantage in terms of energy efficiency compared to PZ alone, while concurrently upholding comparable CO₂ removal performance. This finding is consistent with both experimental and simulation-based studies, thereby underscoring the significance of blended amine systems for economically viable and sustainable post-combustion CO₂ capture.

3.5. Sensitivity Analysis

In this section, an extensive sensitivity analysis is undertaken to assess the impact of critical operational and design parameters on the efficacy of post-combustion CO₂ capture systems. Sensitivity analysis serves as an essential instrument in process optimisation, facilitating the identification of variables that exert the most significant influence on CO₂ removal efficiency and the overall stability of the system. By methodically varying parameters such as amine concentration within the lean stream, the CO₂ concentration of the flue gas, the temperature of the lean liquid stream, and the aggregate flow rates of both the flue gas and lean liquid streams, this analysis aspires to clarify the relative significance of each factor. The insights derived from this exploration furnish a basis for informed process modifications and underscore the potential for enhancing the robustness and efficiency of amine-based CO₂ capture operations across diverse industrial contexts.

3.5.1. Lean Amine Concentration

Figure 10 shows the effect of varying lean amine concentration on CCL. The findings from the sensitivity analysis indicate that an increase in lean amine concentration is correlated with a substantial enhancement in the CCL across all solvent systems, with the most pronounced effects evident at lower concentrations, which subsequently plateau as concentrations approach 30–35%. In the case of MEA, the CCL escalates from 87.42% at a concentration of 15% to 90.87% at 35%. Analogous trends are discernible for PZ, MEA + PZ, and MDEA + PZ, all of which converge toward or marginally exceed 90% at the highest concentrations examined. The observed correlation of increasing CCL with elevated lean amine concentration is robustly substantiated by existing literature. A plethora of studies have illustrated that an increase in amine concentration amplifies the number of reactive sites available for CO₂, thereby enhancing both the absorption capacity and the driving force for mass transfer [47,64]. For instance, Dugas [64] demonstrated that elevating MEA concentration from 15 wt.% to 30 wt.% resulted in a significant enhancement of CO₂ removal efficiency in pilot-scale absorber evaluations. Comparable outcomes have been documented for PZ and blended amines, wherein higher concentrations yield increased CO₂ loading and accelerated absorption rates [65]. Nonetheless, the phenomenon of diminishing returns and the plateauing of CCL at concentrations exceeding 30 wt.% is also corroborated by literature findings, which attribute this phenomenon to elevated solution viscosity, diminished diffusivity, and operational complications such as corrosion and solvent degradation [66]. Collectively, these factors delineate an optimal concentration range for industrial operations, striking a balance between the advantages of enhanced CO₂ capture and the necessity for process stability and economic viability.

3.5.2. CO₂ Concentration

The outcomes of the sensitivity analysis indicate that as the CO₂ concentration in the flue gas increases from 3% to 12%, the CCL for all solvent systems, MEA, PZ, MEA + PZ, and MDEA + PZ, experiences a significant decline (Figure 11). For instance, the CCL of MEA decreases from 99.19% at 3% CO₂ to 64.92% at 12% CO₂, with analogous trends observed in the other solvents. This inverse correlation is well substantiated by existing literature: elevated CO₂ concentrations in the feed gas increase both the partial pressure and the total CO₂ loading required of the solvent, which can rapidly saturate the absorption capacity and diminish removal efficiency within a singular absorber stage [67,68]. Research on amine-based post-combustion capture consistently indicates that as the CO₂ content in flue gas escalates, the driving force for mass transfer initially increases; however, the solvent approaches equilibrium loading more swiftly, resulting in a reduction in the achievable removal percentage within a fixed absorber configuration. Furthermore, heightened inlet CO₂ concentrations may exacerbate operational challenges, such as temperature fluctuations, increased solvent degradation, and increased energy requirements for regeneration, all of which further constrain the system’s capacity to sustain elevated CCLs at higher CO₂ levels [68]. These findings are congruent with both experimental and simulation studies concerning MEA, PZ, and blended amines, validating that the design of the absorber and solvent must be meticulously aligned with the anticipated flue gas composition to maintain high capture efficiencies in practical applications [67,68].

3.5.3. Lean Amine Temperature

The conducted sensitivity analysis indicates that an elevation in the temperature of the lean amine stream yields a marginal yet consistent reduction in the CCL across all evaluated solvent systems (Figure 12). For instance, the CCL for MEA diminishes from 90.95% at 30 °C to 90.49% at 100 °C, with analogous trends observed for PZ, MEA + PZ, and MDEA + PZ. This inversely proportional relationship is substantiated by existing literature, which elucidates that increased lean amine temperatures diminish the solubility of CO₂ within the solvent and elevate the overall temperature profile within the absorber, consequently reducing absorption efficiency. Elevated temperatures induce a shift in the CO₂–amine equilibrium towards desorption, thereby diminishing the driving force for CO₂ absorption and resulting in a reduced removal efficiency [69,70]. Furthermore, elevated temperatures may exacerbate solvent degradation and increase amine emissions, which in turn affect the overall performance of the process. These findings corroborate the premise that optimal cooling of the lean amine stream is imperative for maximising CO₂ capture within amine-based systems, as consistently evidenced in both experimental and simulation studies [70].

3.5.4. Flue Gas Temperature

Figure 13 illustrates the effect of flue gas temperature on CO₂ removal efficiency. The findings indicate that an increase in the temperature of the flue gas stream is associated with a progressive decrease in the CCL across all evaluated solvent systems. For example, the CCL of MEA reduces from 90.19% at 30 °C to 89.24% at 100 °C, with analogous declining patterns discerned for PZ, the MEA + PZ blend, and the MDEA + PZ combination. This inverse correlation is congruent with existing literature, which elucidates that the absorption of CO₂ by amine-based solvents is characterised as an exothermic reaction; elevated flue gas temperatures diminish the thermodynamic driving force for CO₂ solubility and shift the equilibrium towards desorption, thus reducing the overall absorption capacity [70,71]. Empirical investigations have demonstrated that augmenting the temperature from 25 °C to 45 °C can substantially impair the absorption capacity of MEA solutions, with this phenomenon becoming more acute as temperatures approach or surpass typical flue gas conditions [72]. Furthermore, increased flue gas temperatures may intensify solvent degradation and elevate the vapour pressure of CO₂, further compromising the efficacy of the absorption process [73]. Consequently, pre-cooling the flue gas before it enters the absorber is widely advocated as a strategic approach to enhance CO₂ removal efficiency in practical post-combustion capture frameworks [70].

3.5.5. Gas Flow Rate

The findings indicate that an increase in the total flow rate of the flue gas stream is associated with a marked reduction in the CCL across all solvent systems (Figure 14). For instance, the CCL of MEA decreases from 97.32% at a flow rate of 50 kg/h to 68.72% at 100 kg/h, with similar trends observed for PZ, MEA + PZ, and MDEA + PZ. This inverse correlation is robustly corroborated by existing literature, which illustrates that elevated gas flow rates diminish the residence time of the gas within the absorber, thereby restricting the available contact time for CO₂ absorption by the solvent [71,74]. As the flow rate escalates, the mass transfer driving force may exhibit an initial increase due to a heightened CO₂ partial pressure [75]; however, the concomitant reduction in gas–liquid contact time and the potential for maldistribution or channelling within the packed bed ultimately compromise the overall absorption efficiency [76,77]. Empirical studies affirm that at elevated flow rates, the system can become mass-transfer-limited, and the absorber may fail to attain the same removal efficiency as observed at lower flow rates, particularly if the solvent flow is not proportionally increased [71]. These observations underscore the necessity of optimising the gas–liquid ratio and the design of the absorber to sustain elevated capture efficiencies under diverse flue gas throughput conditions.

3.5.6. Lean Amine Flow Rate

As illustrated in Figure 15, an increase in the lean amine flow rate results in a significant enhancement in the CCL for all solvent systems, especially at lower flow rates. For example, MEA’s CCL ascends from 38.11% at 100 kg/h to 90.64% at 200 kg/h, while similar patterns are noted for PZ, MEA + PZ, and MDEA + PZ. This positive correlation is well substantiated by the literature, which indicates that a heightened lean amine flow rate amplifies the availability of fresh solvent, thereby augmenting the driving force for CO₂ absorption and diminishing the CO₂ loading in the lean amine that enters the absorber [78]. As the flow rate increases, the solvent becomes less saturated with CO₂, facilitating more effective absorption per passage through the column. Nonetheless, the results also reveal diminishing returns at elevated flow rates (exceeding 200 kg/h), where further increments yield only marginal improvements in CCL. This phenomenon aligns with findings suggesting that, beyond a specific threshold, the process becomes constrained by mass transfer or absorber design rather than solvent availability [79]. Furthermore, excessively high solvent flow rates may escalate operational costs and energy requirements for solvent regeneration, thus emphasising the critical need to optimise the liquid-to-gas (L/G) ratio for both efficiency and economic viability. In conclusion, these results validate that the lean amine flow rate constitutes a pivotal parameter for maximising CO₂ capture. However, it necessitates careful consideration alongside process design and energy factors for optimal operational performance.

4. Conclusions

This investigation meticulously examined the performance of different solvents for post-combustion CO₂ capture utilising MEA, PZ, MEA + PZ, and MDEA + PZ solvent systems. The findings unequivocally demonstrate that both column geometry, particularly the height and diameter of the packed bed, and process variables, such as amine concentration, flue gas composition, temperature, and flow rates, exert a significant influence on CO₂ removal efficiency. Among the solvents assessed, the MEA + PZ blend achieved the highest capture levels, reaching up to 90.75% CCL under optimised conditions, whereas MDEA + PZ and PZ required larger column dimensions to approach the 90% target. Sensitivity analyses indicated that increasing the lean amine concentration from 15% to 35% resulted in a notable enhancement in CCL for MEA (from 87.42% to 90.87%) and MDEA + PZ (from 85.18% to 90.75%). In contrast, an increase in the flue gas flow rate from 50 kg/h to 100 kg/h prompted a significant reduction in CCL for MEA (from 97.32% to 68.72%). Conversely, elevated CO₂ concentrations in the flue gas (from 3% to 12%) led to a decline in MEA’s CCL from 99.19% to 64.92%, thereby emphasising the process’s sensitivity to feed gas composition. A key outcome of this research is the identification of the MEA + PZ blend as the most energy-efficient solvent system, with specific reboiler duties as low as 4.28 MJ/kg CO₂ at high capture rates by considering that the system studied in this paper is a pilot-scale unit and this energy consumption will be smaller if commercial-scale units are considered. This represents a significant reduction in energy consumption compared to conventional individual MEA and PZ systems, which exhibit reboiler duties typically above 5 MJ/kg CO₂ under similar conditions. These findings highlight the importance of solvent selection and process optimisation in minimising energy consumption and operational costs. These results underscore that while enhanced amine concentration and lean amine flow rate substantially improve capture efficiency, practical constraints such as solvent viscosity, mass transfer resistance, and energy consumption necessitate meticulous optimisation. Collectively, this research provides pragmatic insights for the design and operational management of industrial-scale CO₂ capture systems, emphasising the need for a balanced methodology that optimises efficiency while ensuring process stability and economic viability. The results provide a solid framework for future innovations in solvent selection and absorber design, facilitating the ongoing evolution of more efficient and sustainable carbon capture technologies.