Research on CO₂ Desorption Using Microwave-Assisted Novel Blended Alkanolamine Formulations

Abstract

Traditional alkanolamine absorption methods for CO₂ capture suffer from significant absorbent loss and high regeneration energy consumption. To address this issue, novel blended alkanolamine formulations based on monoethanolamine (MEA), methyldiethanolamine (MDEA) and 2–amino–2–methyl–1–propanol (AMP) were investigated. Based on the optimization of CO₂ absorption conditions, a low–temperature and high–efficiency microwave heating desorption method for CO₂ was proposed, and the microwave heating desorption process of a CO₂ alkanolamine absorption solution was optimized. The results show that when the mass ratio of monoethanolamine (MEA), methyldiethanolamine (MDEA) and 2–amino–2-methyl–1–propanol (AMP) was 4:5:1, the composite alkanolamine solution with a concentration of 20% had the best absorption effect at an absorption temperature of 30 °C. The desorption efficiency of this group of formulations at 95 °C reached 89% in 4 min. Compared with the traditional heating desorption method, the CO₂ desorption rate of the microwave heating method at 95 °C increased by 62%, the desorption time was significantly shortened, and the energy consumption was significantly reduced. This study provides a new research direction for the efficient and low-energy desorption of CO₂ by blended alkanolamine.

1. Introduction

With the intensification of global climate change, the reduction in carbon dioxide (CO₂), one of the main greenhouse gasses, has become an urgent global task [1]. Among the currently available technologies, CO₂ capture stands out as the most economically viable and effective method for reducing point-source carbon emissions from industrial flue gasses [2]. Chemical absorption has the characteristics of high absorption efficiency and large processing capacity, making it the most suitable technology for large-scale carbon capture across various industries, with the most widely used and effective being the amine method [3]. However, the traditional desorption process using amine absorbents to capture CO₂ has the disadvantages of significant absorbent loss and high energy consumption for regeneration. Therefore, improving the amine absorbent to have high absorption efficiency and designing a feasible desorption process with low energy consumption for regeneration are of great significance for industrial carbon capture and environmental protection.

The organic amine absorption method first uses amines (primary/secondary/tertiary amines, polyamino, sterically hindered amines) to absorb CO₂, and then desorbs CO₂ through traditional heating methods. The selection of absorbents is a crucial step in the CO₂ capture process. In searching for suitable alcohol amine absorbents, researchers have conducted studies on the properties of alcohol amines and the reaction mechanism of CO₂ absorption [4] and found that there are carbamate, NH4⁺, and HCO₃⁻/CO₃²⁻ formations in the reaction of CO₂ and alcohol amine [5] (the reaction mechanism Equations (1) and (2)):

$$C{O}_2+2R{R}^{’}NH\leftrightarrow R{R}^{’}CO{O}^{-}+R{R}^{’}{NH}_2^{+}$$

(1)

$$C{O}_2+R{R}^{’}{R}^{\prime \prime }N+H_{2}O\leftrightarrow R{R}^{’}{R}^{\prime \prime }N{H}^{+}+HC{O_3}^{-}$$

(2)

Monoethanolamine (MEA) absorbs CO₂ under simple conditions but with rapid reactions, primarily forming carbamate ions [6,7,8]. During desorption, the energy consumption for carbamate desorption is relatively high, and in conventional thermal desorption, up to 75% of carbamate remains un-desorbed [9,10]. Methyldiethanolamine (MDEA) absorbs CO₂ by forming carbonate ions, which are less stable than carbamate ions, making the complete release of CO₂ during desorption easier; however, the CO₂ absorption rate is slower [11,12]. 2–Amino–2–methyl–1–propanol, abbreviated as AMP, is a sterically hindered amine that also belongs to the class of primary alkanolamines. AMP not only retains the fast CO₂ absorption characteristic of primary alkanolamines but also, due to its steric hindrance, exhibits reduced stability, which can lower the energy required for alkanolamine regeneration in decarbonization processes [13,14,15]. Based on the different absorption characteristics of CO₂ by different alkanolamines, the selection of alkanolamines with high CO₂ absorption rates, large absorption capacities, and low desorption energy consumption for blending can result in a composite alkanolamine solvent with excellent comprehensive performance.

In the optimization of a CO₂ desorption process, it was found that microwave heating occurs via direct molecular interactions with electromagnetic radiation, whereby the main advantages are an instantaneous and volumetric heating without the heat transfer restrictions and heat losses associated with the conventional conductive or convective heating modes [16]. Compared to conventional conductive heating, microwave-assisted CO₂ regeneration offers distinct advantages such as uniform heating, rapid speed, reduced thermal losses, and swift operational response [17]. The fundamental principle of microwave heating involves the reorientation of polar dielectric molecules’ dipoles in the presence of a rapidly oscillating electric field [18,19]. Phase lag between molecular thermal motion and the electric field causes friction and collision among adjacent molecules, ultimately leading to the dissipation of electromagnetic energy into heat. Since its first application in the field of CO₂ regeneration in 2014, it has been used in both solid adsorbents and chemical absorbent systems, demonstrating faster desorption rates and lower energy consumption compared to traditional heating methods [20]. Scholars researching the CO₂ desorption rate of microporous activated carbon found that microwave heating could quadruple the desorption rate, making the overall desorption process four times faster than traditional thermal desorption [21]. Similar conclusions were confirmed in studies of the CO₂ desorption process in porous molecular sieve 13X, which also found that the apparent activation energy for microwave regeneration (15.8–18.1 kJ/mol) is significantly lower than that for traditional thermal regeneration (41.5 kJ/mol), and it has been theoretically proven that microwave heating is more efficient than conventional heating [22]. Therefore, microwaves can regenerate the solution faster and desorb CO₂ with lower energy consumption than traditional heating, potentially reducing overall process costs (Figure 1).

In order to improve the CO₂ capture process and reduce the energy consumption of alcohol amine solution regeneration, herein, we investigated novel blended alkanolamine formulations based on monoethanolamine (MEA), methyldiethanolamine (MDEA) and 2–amino–2–methyl–1–propanol (AMP), and combined them with microwave–assisted desorption technology to improve CO₂ desorption efficiency and reduce energy consumption. Firstly, based on single-factor experiments, the response surface method was used, with the masses of monoethanolamine (MEA), methyldiethanolamine (MDEA) and 2–amino–2–methyl–1–propanol (AMP) used as experimental factors and the CO₂ absorption rate and desorption rate as indicators, to optimize the blending ratio of amine solvents. Furthermore, the absorption and desorption conditions (amine solution mass fraction and absorption desorption temperature) were optimized, and the CO₂ desorption performance and energy consumption under microwave heating and traditional heating methods were compared, developing an efficient and low–energy CO₂ capture process.

2. Materials and Methods

2.1. Materials and Instruments

The materials used in the experiments are as shown in

Table 1. Experimental materials.

Name	Molecular Formula	Manufacturer	Type
Monoethanolamine (MEA)	NH2CH2CH2OH	Sinopharm Group	Chemically Pure
N-Methyldiethanolamine (MDEA)	(CH3)2NCH2CH2OH	Sinopharm Group	Analytical Grade
2–Amino–2–methyl–1–propanol (AMP)	CH3)2C(NH2)CH2OH	Sinopharm Group	Analytical Grade
Antifoaming agent		Sinopharm Group	Analytical Grade
Deionized water	H2O	Self-Made	18 MΩ
Carbon dioxide	CO2	Zhongke Gas Co.	99.999%

.

Instruments: Microwave oven (Model XH–100B, Shanghai, China); electronic balance (Mettler Balance MS304TS/02, Zurich, Switzerland); intelligent round-bottom flask heater (ZNHW–500 mL, Shanghai Yuezhong, Shanghai, China).

2.2. Experimental Setup and Procedure

The following diagram shows (Figure 2) the experimental setup and experimental procedure for the absorption desorption experiments conducted in this paper.

2.3. Experimental Study on Alkanolamine CO₂ Absorption

A specific ratio of the absorbent solution was added to a three-necked flask, and an antifoaming agent was included to stabilize the reaction by controlling foaming. The flask was placed in an electric heating mantle, adjusted to the experimental temperature, and CO₂ was introduced until saturation. The CO₂ capture performance was studied using the gravimetric method [23], where the mass difference in the solution before and after CO₂ introduction was measured with an electronic analytical balance, with the mass of the saturated CO₂ denoted as m₁.

To determine the optimal blend ratio of blended alkanolamines, the response surface methodology (RSM) was employed. Based on single experiments, the factors considered were the mass of alkanolamine A (A), alkanolamine B (B), and AMP (C), using a Box–Behnken design for the three variables. A total of 17 experiments were conducted to obtain the optimal process parameters, which were then experimentally validated.

Using the optimal formula calculated by RSM, we further investigated the effects of absorption temperature (30 °C, 35 °C, 40 °C, 45 °C, 50 °C) and the concentration of the blended alkanolamine aqueous solution (20 wt%, 25 wt%, and 40 wt%) on the absorption rate and CO₂ loading capacity.

2.4. Experimental Study on Microwave Desorption of Alkanolamine-Rich Liquor

After the optimal blend ratio of the blended alkanolamine solution at different concentrations (20 wt%, 25 wt% and 40 wt%) absorbed CO₂ until saturation, the rich liquor was placed in a microwave or heating furnace. The solution was heated to the required temperature (85 °C, 90 °C, 95 °C and 100 °C), with a reaction time set to 10 min. The volume of CO₂ expelled was measured using the displacement method and recorded. The mass of the actually desorbed CO₂, m₂, was calculated, and the CO₂ desorption rate c was determined using Equation (3):

$$c={\displaystyle \frac{m_2}{m_1}}\times 100\%$$

(3)

2.5. Regeneration Performance Study

Following the optimal experimental formulation, the prepared absorbent solution was placed in an electric heating mantle for CO₂ absorption. After the absorption experiment was completed, the rich liquor was desorbed in the microwave apparatus. The microwave parameters were adjusted to instantaneously heat the solution to 95 °C, and the reaction time was set to 10 min. The volume of CO₂ expelled was measured using the displacement method and recorded. The absorbent solution was reused for six cycles of the above operations to verify the cyclic decarbonization performance and efficiency of the solution.

3. Results and Discussion

3.1. Comparison of CO₂ Absorption and Desorption Performance Between Blended and Single Alkanolamines

In terms of absorption performance, as shown in Figure 3 (left), at 30 °C, single MEA has a lower CO₂ absorption loading, and single MDEA has a slightly faster absorption rate and, comparatively, a better absorption loading effect due to its molar mass. However, in actual experiments, the absorption amount is relatively low, with about 3.45 g of CO₂ absorbed after 15 min. The blended MEA and MDEA absorbent has the fastest absorption rate in the early stages, reaching near saturation within 9 min, but has a lower ability to absorb CO₂, resulting in low absorption loading. When AMP is further added, the ternary blended alkanolamine absorbent, although not as fast as the binary MEA and MDEA blend, significantly increases the absorption loading compared to the binary mixture. Experimental results show that the ternary mixture of MDEA, MEA, and AMP saturates within 15 min and has a higher total absorption loading than the single MEA component and the binary mixture, with MDEA having a strong CO₂ absorption capacity. However, based on the current optimization direction to develop novel adsorbents with high CO₂ absorption rates and low regeneration energy consumption [24], the single-component MDEA has high desorption energy consumption; therefore, the ternary composite absorbent with better overall capability is chosen.

Regarding the desorption regeneration experiment, as shown in Figure 3 (right), under microwave desorption conditions at 95 °C, comparative experiments with and without AMP doping reveal that the MDEA:MEA:AMP blend shows a significant improvement in desorption rate. After 4 min of desorption, the rate reaches up to 79%, and it can be observed that with microwave desorption, all experimental groups reach desorption equilibrium within 4 min.

3.2. Determination of the Optimal Blend Ratio of Blended Alkanolamines–Response Surface Analysis

The optimal blend ratio of blended amines is determined using the response surface methodology (RSM), which allows for the simultaneous adjustment of multiple factors to obtain optimal absorption performance based on combinations of independent variables [25]. The three factors in RSM are the mass ratios of MDEA, MEA, and AMP, with the target being the amount of CO₂ absorbed; due to laboratory operational constraints, the maximum value for MDEA is set at 6 and the minimum at 2; for MEA, the maximum is 5 and the minimum is 1; and for AMP, the maximum is 3 and the minimum is 1. Following software (Design-Expert Version 13) computation of different ratios of alkanolamines, experimental outcomes for absorption amounts are obtained. The software is then used to perform response surface and curve-fitting corrections, and the optimal mass ratio is determined by comprehensively comparing absorption and desorption amounts. Within the experimental range, one variable is fixed while the other two are varied to obtain a visualized response surface map as shown in Figure 4, ultimately determining the optimal mass ratio of MDEA, MEA, and AMP to be 4:5:1, with the coordinates representing the mass (g) of each substance.

As can be seen from the figure, the corresponding response surface is a convex surface opening downward, indicating that within the considered range, there are maximal values for CO₂ absorption and desorption rates. Verification experiments using optimized process parameters confirm the optimal blend ratio of MEA, MDEA, and AMP.

3.3. The Influence of Temperature on Alkanolamine Solutions Absorbing CO₂

As shown in Figure 5, the absorbent has a relatively rapid rate of CO₂ absorption. At the beginning of the reaction, the rate of absorption is quite distinct between different temperatures. However, as the absorption time increases, the amount of CO₂ absorbed by the solvent at 30 °C steadily grows, with this temperature demonstrating a clearly better sustained absorption effect, reaching the maximum total amount within 20 min, and achieving the highest absorption loading.

3.4. The Influence of Concentration on Alkanolamine Solutions Absorbing CO₂

The concentration of the amine solution affects the viscosity of the absorbent; the higher the viscosity, the poorer the fluidity of the solution, and the fewer the reactive molecules involved in absorbing CO₂, leading to a diminished CO₂ absorption capacity. As shown in Figure 6(left), with increasing concentration, there is a marked increase in absorbent viscosity. When the concentration of the absorbent is at 20 wt%, the viscosity is the lowest, and the mass transfer capability is stronger, resulting in greater reactivity in the CO₂ absorption reaction. As shown in Figure 6(right), after 15 min of reaction, the 20% concentration of the blended alkanolamine solution has the highest CO₂ absorption loading. The absorption capacity decreases progressively with increasing concentration.

3.5. Impact of Microwave Heating and Conventional Heating on the Desorption of CO₂

To compare the desorption performance of absorbents under traditional heating and microwave–assisted desorption, the same reaction conditions were established at 95 °C. Conventional electrical heating mantles were used to thermally assist the desorption of alkanolamine-rich solutions in flasks [26]. This was compared to desorption rates of CO₂ in a microwave reactor. The results, as illustrated in Figure 7, show that the desorption rate of the absorbent is significantly affected by the heating method. Under equivalent temperature conditions, the volume and rate of desorption achieved through microwave heating are substantially greater than those achieved by conventional heating, indicating better desorption effects. Hence, microwave heating is more suitable for the CO₂ desorption process with this formulation.

3.6. Effect of Temperature on the Desorption of CO₂ by Alkanolamine Solutions

Research on the effect of temperature on the desorption of CO₂ from alkanolamine-rich solutions involved gradient experiments set between 85 °C and 100 °C, with the results shown in Figure 8.

The study revealed that, at the onset of the reaction (within 2 min), the reaction proceeds swiftly. As the temperature increases, so does the desorption rate. By 4 min, the reaction is nearly complete, with desorption time being relatively short. The desorption rates for each group reach their maximum between 4 and 6 min, as depicted in the figure. At 85 °C, the desorption rate reaches its maximum of 67% in 6 min. At 90 °C, the desorption rate is slower, reaching 72% at four minutes and a maximum of 79% at 6 min. Higher temperatures correspond to faster desorption rates, with equilibrium reached within 4 min at both 95 °C and 100 °C. At 95 °C, the desorption rate reaches 89%, and at 100 °C, it reaches 99%, nearing complete desorption. However, higher temperatures result in evaporative losses of the absorbent solvent and higher energy consumption for desorption, failing to meet the objective of reducing desorption energy consumption. Therefore, 95 °C is considered the optimal operation temperature for microwave–assisted desorption.

3.7. Effect of Concentration on the Desorption of CO₂ by Alkanolamine Solutions

In this experiment, each group’s desorption reaction time was 10 min; however, as shown in Figure 9, rapid desorption occurs within the first 2 min. Thus, excluding time as a factor and after the reaction’s completion, the relationships between desorption concentration, temperature, and rate are assessed. The experimental results conclude that a 20 wt% alkanolamine solution exhibits the best absorption and desorption effects, followed by the 25 wt% solution, while the 40 wt% solution performs comparably to the first two concentrations in high-temperature desorption but with inferior absorption capacity. At 95 °C, the 20 wt% concentration alkanolamine solution shows the best CO₂ absorption and desorption effects, with a desorption rate as high as 89%.

The observed phenomenon can be attributed to the fact that with increasing concentration, the viscosity of the absorbent also increases, which diminishes the fluidity between molecules and slows the rate at which alkanolamine molecules react with CO₂. Therefore, within the same time frame, a low–concentration alkanolamine absorbent has a superior CO₂–carrying capacity compared to a high–concentration alkanolamine absorbent. Moreover, during desorption, the lower the concentration, the faster the molecular motion rate, the greater the chance of collisions, and the more energy produced. This not only accelerates the desorption rate and improves the desorption rate of CO₂ but also reduces energy consumption, allowing for better desorption effects at lower temperatures.

3.8. Regeneration Performance

Figure 10 presents the cyclic repeatability experiment for the optimal formulation, indicating good repeatability for the formulation. During six cycles of experimentation, the CO₂ absorption load of the absorbent group, after an initial decline following the first absorption, remains stable, with only minor variations in the quality of desorption over multiple cycles. At the initial stage of the cycling experiment, there was no carbon dioxide loading in the absorption lean solution, so the initial absorption was large. In the subsequent absorption process, the carbon dioxide previously remaining in the absorption solution that was not desorbed decreased the absorption of carbon dioxide by the alkanolamine solution; this phenomenon continued to appear during the cycling experiment and had a similar effect on the subsequent multiple cycles of the absorption experiments. Therefore, in the regeneration experiments, the absorption loading decreased and stabilized after the first cycle.

The absorption and desorption performance of the absorbent is minimally affected by the cyclic experiment, demonstrating the formulation’s good repeatability and stability.

3.9. Comparison of Desorption Energy Consumption Calculations

The energy consumption for desorption at 95 °C was compared between microwave-assisted desorption and traditional electric heating desorption. Desorption energy consumption was calculated using Formula (2), where the power of the microwave oven and the electric heating mantle, both applied to the absorbent fluid, can be simply estimated as the total energy P is transferred to the liquid [27], t is the operating time, and n is the amount of CO₂ desorbed:

$$E={\displaystyle \frac{P\cdot t}{n}}$$

(4)

Comparison of the average desorption energy consumption using heating energy as the standard for calculating desorption energy consumption is shown in the table below. Although the microwave heating requires higher power, the desorption equilibrium can be reached within 4 min. In contrast, under the conventional heating condition, the energy consumption of desorption is greatly increased due to the lower heating power, longer preheating time and lower desorption rate, and the energy consumption of conventional heating is 9.45 times of that of microwave desorption when complete desorption is achieved. As time passes and the CO₂ desorption rate increases, the energy consumption decreases, but not indefinitely. When near-equilibrium desorption is reached in 30 min with conventional heating, the energy consumption is calculated to be as high as 12.31 kw-h/mol. Therefore, microwave heating is less energy intensive and more efficient than conventional heating (

Table 2. Comparison of average energy consumption of microwave heating and conventional heating at 95 °C.

Microwave Heating			Conventional Heating			Energy Consumption Ratio Conventional/Microwave
Power (W) 1	RE (%) 2	Desorption time/min	Power (W)	RE (%)	Desorption time/min	9.45
850	89%	4	300	27%	10

¹ Power: energy consumption. ² RE: CO₂ recovery.

).

3.10. Result Aggregation and Operational Optimization

After the summary of the experimental results, the optimal reaction conditions for the alcohol amine solvent absorption and microwave-assisted desorption of carbon dioxide can be obtained, i.e., the composite alcohol amine absorber formulated with a mass ratio of 4:5:1 for MDEA, MEA, and AMP has the optimal absorption composite at a concentration of 20 wt% and an absorption temperature of 30 °C, while it has the optimal rate of carbon dioxide desorption under microwave-assisted heating at 95 °C. The results are summarized in

Table 3. Summary of optimal reaction conditions.

Reaction Condition	MDEA, MEA, and AMP Quality Ratio	Solution Concentration	Absorption Temperature	Desorption Temperature
optimal value	4:5:1	20 wt%	20 °C	95 °C

.

The results show that the composite formulation of alkanolamine absorber has good absorption performance and can be reused; the low-energy-consumption advantage of the microwave desorption technology results in a greater potential for application in industrial production. The progress of this study provides a new perspective for CO₂ capture technology and a scientific basis for realizing a more economical and environmentally friendly carbon capture and storage (CCS) technology for automobiles.

In experiments exploring the relationship between desorption rate and temperature, we found that in the early stages of desorption, the energy released by electromagnetic waves is rapidly absorbed by the absorbent, facilitating the desorption process. Thus, in the initial stage of the reaction (within the first 2 min of this experimental operation), a rapid reaction can occur at relatively low temperatures. However, in the middle and later stages of the reaction, as the majority of the rich solution has been consumed and the CO₂ and alkanolamine have been regenerated, more energy is required to promote the reverse chemical reaction, necessitating a moderate increase in temperature. The microwave heating is set to start at 80 °C to avoid damaging the molecular structure of the absorbent, with a termination temperature set at 95 °C. Intermittent temperature variation not only ensures that the reaction proceeds stably and orderly, but also further saves on economic costs and reduces energy consumption.

In this experiment, the rapid reaction of alkanolamine-rich liquid in the first 2 min of desorption suggests that a low–temperature operation is more conducive to the stability of the reaction. However, once the reaction approaches stability, the low temperature is insufficient to provide the necessary desorption kinetics, and it is appropriate to increase the reaction temperature later in the process to favor regeneration. Thus, an optimized variable temperature operation was designed to improve upon the deficiencies of constant temperature operation. The optimized desorption operation entails raising the temperature in 5 °C increments every 2 min (as can be seen from Figure 11), and the experimental results demonstrate that intermittent temperature increases have a positive overall effect on desorption. Among them, the 20 wt% absorbent still exhibits the best desorption performance, with a desorption rate reaching 91%, higher than any concentration absorbent under constant 95 °C operation. Therefore, the feasibility of optimized intermittent variable temperature desorption operation is confirmed. This discovery will provide new ideas for the subsequent research and exploration of microwave–assisted desorption processes.

4. Conclusions

This study focuses on the development of a novel blended alkanolamine formulation and explores its performance in CO₂ desorption under microwave assistance. By optimizing the alkanolamine formulation, we aim to increase the CO₂ absorption capacity and desorption efficiency, while reducing the energy consumption in the desorption process. This study determined the optimal mass ratio of the blended alkanolamine absorbent by comparing the CO₂ absorption and desorption performance of blended alkanolamine with that of single alkanolamine, combined with the response surface method. On this basis, temperature and concentration comparison experiments were used to investigate the effects of temperature and concentration on absorption, and desorption. Under the optimal experimental conditions, the advantages of microwave heating desorption were proven in comparison with conventional heating; the reusability of the blended alkanolamine absorbent was proven by cycling experiments. The conclusions of the experimental demonstration are as follows:

(1) When the mass ratio of the blended alkanolamine absorbent MDEA:MEA:AMP was 4:5:1, the absorbent was superior to the single-component absorbent in terms of absorption rate and absorption loading, and the absorption loading was increased by 25% compared with that of the industry-standard one–component MEA aqueous solution under the conditions of absorption at 30 °C and 20 wt%.

(2) With the assistance of microwave desorption technology, the absorbent has better desorption performance, the desorption rate reaches 89% when heated to 95 °C at a microwave power of 850 w. Optimization of intermittent variable-temperature desorption can further enhance the desorption effect to 91%, which increases the CO₂ desorption rate by 3.37 times and reduces the regeneration energy consumption by 89.5% compared with the traditional heating desorption method.