Literature Sharing: Study on the Effect of Operating Parameters on the Absorption Behavior of Choline Chloride-Mono-Ethanolamine Hypoeutectic Solvent and Its Aqueous Solution

Abstract Abstract

This study focused on the CO2 capture performance of choline chloride-monoethanolamine eutectic solvent (ChCl-MEA DES) and its aqueous solution. The effects of absorption temperature, operating pressure, and water content on absorption capacity and absorption rate were systematically investigated. A batch reactor was used to carry out equilibrium experiments, and a response surface method (RSM) was used to complete multi-factor coupling analysis and data modeling. The study confirmed that the CO2 capture performance of ChClMEA DES) and its aqueous solution was investigated systematically. The effects of the three core operating parameters of absorption temperature, operating pressure, and system water content on absorption capacity and absorption rate were systematically investigated. The batch reactor was used to carry out equilibrium experiments, and the response surface method (RSM) was used to complete multi-factor coupling analysis and data modeling. The research confirms that the ChCl-MEA DES captures CO2 with MEA DES capturing CO 2 with physical absorption physical absorption as the absolute dominant mechanism; under the optimal operating conditions (10 bar, 40 ℃, 50% v water content), the CO 2 absorption capacity is the absolute dominant mechanism; under the optimal operating conditions (10 bar, 40 ℃, 50% v water content), the CO 2 absorption capacity reaches 0.379 mol0.379 mol - CO 2/molCO 2/mol - DESDES , which is nearly double that of pure DES. Pressure and water content have a significant positive drive on the absorption rate, and the effect of temperature is relatively weak; and, it is nearly doubled compared with pure DES. Pressure and water content have a significant positive drive on the absorption rate, and the effect of temperature is relatively weak; and Adding water within 50% v does not change the essence of physical absorption Adding water within 50% v does not change the essence of physical absorption , only enhances the absorption efficiency by reducing viscosity and promoting mass transfer, providing a new solvent system and process parameters for efficient and low-energy CO2 separation under mild conditions., Only by reducing viscosity and promoting mass transfer to strengthen the absorption efficiency, provides a new solvent system and process parameters for efficient and low-energy CO2 separation under mild conditions.

1. Research background and significance 1. Research background and significance

1.1 Industrial Rigidity Requirements and Technical Bottlenecks for CO _ 2 Capture 1.1 Industrial Rigidity Requirements and Technical Bottlenecks for CO _ 2 Capture

Driven by the global carbon neutrality target, CO2 capture, utilization, and storage (CCUS) has become the core emission reduction path for the chemical, energy, and metallurgical industries. In industrial scenarios, syngas, natural gas, refinery gas, and flue gas all need to be deeply decarbonized to meet raw material purity, combustion emissions, and pipeline transportation standards - syngas removal of CO2 can be adapted to advanced chemical synthesis, natural gas desulfurization and decarbonization ensure transportation safety and calorific value, and flue gas capture directly reduces carbon emission intensity. Driven by the global carbon neutrality target, CO2 capture, utilization, and storage (CCUS) has become the core emission reduction path for the chemical, energy, and metallurgical industries. In industrial scenarios, syngas, natural gas, refinery gas, and flue gas all need to be deeply decarbonized to meet raw material purity, combustion emissions, and pipeline transportation standards - syngas removal of CO2 can be adapted to advanced chemical synthesis, natural gas desulfurization and decarbonization ensure transportation safety and calorific value, and flue gas capture directly reduces carbon emission intensity.

Traditional CO2 separation technology has flaws that are difficult to break through: Traditional CO2 separation technology has flaws that are difficult to break through:

Alcohol amine aqueous solutions (such as MEA and DEA) are chemically absorbed mature, but alcohol amine aqueous solutions (such as MEA and DEA) are chemically absorbed mature, but solvent degradation loss is large, regeneration energy consumption is high, equipment corrosion is serious, solvent degradation loss is large, regeneration energy consumption is high, equipment corrosion is serious , tons of CO2 desorption energy consumption is generally higher than 3 GJ, and operating costs remain high.

Physical absorption method (polyethylene glycol dimethyl ether, low temperature methanol) requires ** Physical absorption method (polyethylene glycol dimethyl ether, low temperature methanol) requires ** -60 ℃ cryogenic **, supporting the compression refrigeration system is complex, the investment and energy consumption are both high, and the working condition adaptability is poor; 60 ℃ cryogenic **, the supporting compression refrigeration system is complex, the investment and energy consumption are both high, and the working condition adaptability is poor;

Although ionic liquids have excellent properties, although ionic liquids have excellent properties, they have high synthesis costs, high viscosity, and difficulty in large-scale preparation. , difficult to industrialize.

1.2 Technical advantages of eutectic solvents (DES) and existing shortcomings 1.2 Technical advantages of eutectic solvents (DES) and existing shortcomings

The eutectic solvent is formed by the eutectic of hydrogen bond receptor (HBA) and hydrogen bond donor (HBD) in a specific proportion, and has the advantages of eutectic solvent is formed by the eutectic of hydrogen bond receptor (HBA) and hydrogen bond donor (HBD) in a specific proportion. It has the advantages of low melting point, negligible volatility, designable structure, cheap and easy availability of raw materials, good biocompatibility, low melting point, negligible volatility, designable structure, cheap and easy availability of raw materials, and good biocompatibility. It is regarded as an ideal candidate material to replace traditional absorbers. Choline chloride (ChCl), as a bulk industrial raw material, is low-cost and environmentally friendly; monoethanolamine (MEA) contains hydroxyl and amino groups, and has the advantages of both alkaline, hydrophilic and reaction check point, and the ChCl formed by the two is regarded as an ideal candidate material to replace traditional absorbers. Choline chloride (ChCl), as a bulk industrial raw material, is low-cost and environmentally friendly; monoethanolamine (MEA) contains hydroxyl and amino groups, and has both alkaline, hydrophilic and reaction check point. The ChCl-MEA DES formed by the two has both stability and CO 2 affinity, and is suitable for carbon capture scenarios. MEA DES has both stability and CO 2 affinity, and is suitable for carbon capture scenarios.

But there are still key bottlenecks in the industrial application of DES: But there are still key bottlenecks in the industrial application of DES: The viscosity of pure DES is generally high, and the viscosity of pure DES is generally high , resulting in large gas-liquid mass transfer resistance, slow absorption rate, equipment volume and energy consumption increase, which has become the core obstacle restricting its engineering. Therefore, through, the gas-liquid mass transfer resistance, slow absorption rate, equipment volume and energy consumption increase, which has become the core obstacle restricting its engineering. Therefore, optimizing rheological properties and enhancing mass transfer efficiency through mild modification (such as water phase regulation) and mild modification (such as water phase regulation) while retaining high absorption capacity and low regeneration energy consumption is the current research focus of DES-based CO2 capture technology. Optimizing rheological performance, enhancing mass transfer efficiency, while retaining high absorption capacity and low regeneration energy consumption is the current research focus of DES-based CO ² capture technology.

1.3 The scientific orientation and value of this research 1.3 The scientific orientation and value of this research

Existing studies on multi-focus pure DES or single variable effects, lack of existing studies on multi-focus pure DES or single variable effects, lack of multi-operating parameter coupling effect multi-operating parameter coupling effect and water content synergy mechanism Systematic analysis of water content synergy mechanism . This paper uses the system analysis of ChCl. This paper takes ChCl-MEA DES as the object, quantifies the main effects and interactive effects of temperature, pressure and water content on absorption capacity/rate, clarifies the boundary of physical absorption mechanism, reveals the essence of water modification, and quantifies the main effects and interactive effects of temperature, pressure and water content on absorption capacity/rate for MEA DES-based carbon capture process. It provides quantitative basis and theoretical support for the working condition optimization, equipment design, solvent formulation working condition optimization, equipment design, solvent formula of DES-based carbon capture process, and fills the research gap of high-efficiency and low-viscosity DES absorption system under mild conditions. Provide quantitative basis and theoretical support to fill the research gap of high-efficiency and low-viscosity DES absorption system under mild conditions.

II. Basics of eutectic solvents and experimental methods II. Basics of eutectic solvents and experimental methods

2.1 ChCl2.1 Structure and characteristics of ChCl-MEA DES Structure and characteristics of MEA DES

ChClChCl-MEA DES uses choline chloride as the hydrogen bond receptor and monoethanolamine as the hydrogen bond donor to form a uniform and stable liquid phase through the intermolecular hydrogen bond network, without complicated purification, and the raw materials are easy to obtain and easy to prepare. The system has both alcohol hydrophilicity and amine alkalinity, and has good solubility to CO2; at the same time, MEA DES uses choline chloride as the hydrogen bond receptor and monoethanolamine as the hydrogen bond donor to form a uniform and stable liquid phase through the intermolecular hydrogen bond network, without complicated purification, and the raw materials are easy to obtain and easy to prepare. The system has both alcohol hydrophilicity and amine alkalinity, and has good solubility to CO 2O; at the same time, has strong thermal stability, almost no volatilization, and almost no volatilization , which can greatly reduce solvent loss and meet the needs of industrial continuous operation., can greatly reduce solvent loss and meet the needs of industrial continuous operation.

2.2 Experimental System and Operating Parameter Design 2.2 Experimental System and Operating Parameter Design

Experimental Device: Experimental Device: Intermittent High Pressure Reactor Intermittent High Pressure Reactor , to achieve gas-liquid balance absorption under constant temperature and pressure, to ensure that the system is closed without leakage, high data repeatability;, to achieve gas-liquid balance absorption under constant temperature and pressure, to ensure that the system is closed without leakage, high data repeatability;

Examine Variables: Examine Variables:

Temperature: 40 ° C, 60 ° C, 80 ° C (covering the medium and low temperature range commonly used in industrial decarbonization); Temperature: 40 ° C, 60 ° C, 80 ° C (covering the medium and low temperature range commonly used in industrial decarbonization);

Pressure: 5 - 10 bar (range simulating CO _ 2 partial pressure in syngas and natural gas); pressure: 5 - 10 bar (range simulating CO _ 2 partial pressure in syngas and natural gas);

Water content: 0% v, 25% v, 50% v (layer to investigate the modification of water); Water content: 0% v, 25% v, 50% v (layer to investigate the modification of water);

Core metrics: Core metrics: molar absorption capacity (mol molar absorption capacity (mol - CO 2O/molCO 2O/mol - DES) DES) , instantaneous absorption rate (mol instantaneous absorption rate (mol - CO 2O/(kmolCO 2O/(kmol - DES ・ s)) DES ・ s) , mechanism stability., mechanism stability.

2.3 Response Surface Method (RSM) and Box 2.3 Response Surface Method (RSM) and Box-Behnken Design Behnken Design

This study uses BoxBox - Behnken Experimental Design (BBD) Behnken Experimental Design (BBD) Combined with Response Surface Method (RSM) to carry out multi-factor optimization. Compared with orthogonal design and all-factor experiment, it has the advantages of fewer experimental points, high precision, few analytical interaction experimental points, high precision, and resolvable interaction strong. Through the second-order polynomial model fitting, the main effect, interaction effect and surface visualization analysis of the three parameters of temperature, pressure and water content are realized, and the optimal working condition interval is accurately located to provide a reliable mathematical model for process amplification. Advantages. Through the second-order polynomial model fitting, the main effect, interaction effect and surface visualization analysis of the three parameters of temperature, pressure and water content are realized, and the optimal working condition interval is accurately located to provide a reliable mathematical model for process amplification.

III. Experimental results and in-depth analysis III. Experimental results and in-depth analysis

3.1 Effect of operating parameters on CO _ 2 absorption capacity 3.1 Effect of operating parameters on CO _ 2 absorption capacity

3.1.1 single factor main effect 3.1.1 single factor main effect

Pressure: Pressure: Positive significant effect Positive significant effect . Increased pressure increases the partial pressure of CO2. According to Henry's Law, the physical solubility is proportional to the partial pressure of the gas phase, driving more CO 2 into the liquid phase, and the absorption capacity increases linearly;. Increased pressure increases the partial pressure of CO 2. According to Henry's Law, the physical solubility is proportional to the partial pressure of the gas phase, driving more CO 2 into the liquid phase, and the absorption capacity increases linearly;

Temperature: Temperature: Negative effect Negative effect Negative effect . Physical absorption is an exothermic process, heating up decreases the solubility of CO 2O, while intensifying the molecular thermal motion and weakening the intermolecular force between the solvent and CO 2O, resulting in a decrease in capacity;. Physical absorption is an exothermic process, heating up decreases the solubility of CO 2O, while intensifying the molecular thermal motion and weakening the intermolecular force between the solvent and CO 2O, resulting in a decrease in capacity;

Water content: Water content: Positive significant impact Positive significant impact . The viscosity of water is much lower than that of pure DES, adding water significantly reduces the viscosity of the system. The viscosity of water is much lower than that of pure DES, adding water significantly reduces the viscosity of the system, Enhanced diffusion and mass transfer Enhanced diffusion and mass transfer , so that DES molecules are more fully exposed to CO2, and the effective utilization rate of DES per mole is greatly improved., so that DES molecules are more fully exposed to CO2, the effective utilization rate of DES per mole is greatly improved.

3.1.2 Multifactor Coupling and Quantitative Comparison 3.1.2 Multifactor Coupling and Quantitative Comparison

Under the same pressure, the absorption capacity of the 50% V water content system (0.206 - 0.379 mol under the same pressure, the absorption capacity of the 50% V water content system (0.206 - 0.379 mol-CO ³/mol-DES) is about 1.5 times that of the 25% V system, the DES is about 1.5 times that of the 25% V system, and the pure DES is nearly 2 times that of the pure DES , and the law is stable in the whole interval of 40 - 80 ℃. It needs to be clear:, and the law is stable in the whole interval of 40 - 80 ℃. It should be made clear: The total absorption of pure DES under equal volume is still higher than that of the high water content system The total absorption of pure DES under equal volume is still higher than that of the high water content system , but The absorption efficiency per mole of DES The absorption efficiency per mole of DES increases significantly with the addition of water - water acts as a "mass transfer accelerator" rather than an "extra absorber", releasing the intrinsic absorption potential of DES by reducing the mass transfer resistance. Significant increase with the addition of water - water acts as a "mass transfer accelerator" rather than an "extra absorber", releasing the intrinsic absorption potential of DES by reducing the mass transfer resistance.

3.2 The mechanism of action of operating parameters on the absorption rate of carbon dioxide 3.2 The mechanism of action of operating parameters on the absorption rate of carbon dioxide

The absorption rate directly determines the size and processing capacity of the equipment, and is the core indicator of industrial selection: the absorption rate directly determines the size and processing capacity of the equipment, and is the core indicator of industrial selection:

Pressure: Pressure: Strong forward drive Strong forward drive . 5 bar → 10 bar boost, the rate of different water content systems is increased by about 0.753 mol. 5 bar → 10 bar boost, the rate of different water content systems is increased by about 0.753 mol-CO 2O/(kmolCO 2O/(kmol-DES ・ s), the slope is the same, indicating that the pressure-to-rate drive DES ・ s), the slope is the same, indicating that the pressure-to-rate drive has nothing to do with water content and has nothing to do with water content , only by increasing the mass transfer driving force to achieve strengthening;, only by increasing the mass transfer driving force to achieve strengthening;

Water content: Water content: Positive promotion Positive promotion . The average rate of 50% v aqueous system is twice that of pure DES, and the core mechanism is viscosity reduction and diffusion promotion;. The average rate of 50% v aqueous system is twice that of pure DES, and viscosity reduction and diffusion promotion is the core mechanism;

Temperature: Temperature: Weak effect Weak effect , much weaker than pressure and water content, in line with physical absorption rate characteristics., much weaker than pressure and water content, in line with physical absorption rate characteristics.

From the perspective of engineering economy: From the perspective of engineering economy: The boosting effect is better than the boosting effect of adding water , but the investment and energy consumption of high-pressure equipment are higher;, but the investment and energy consumption of high-pressure equipment are higher; Modified by adding water and adding water is a low-cost and easy-to-operate alternative, which can achieve rate doubling under normal pressure/low pressure, and is more suitable for the transformation of existing equipment. As a low-cost and easy-to-operate alternative, it can achieve rate doubling under normal pressure/low pressure, and is more suitable for the transformation of existing equipment.

3.3 Absorption mechanism verification: physical absorption dominates, adding water does not change the mechanism 3.3 Absorption mechanism verification: physical absorption dominates, adding water does not change the mechanism

Multiple evidences confirm that ChCl-MEA DES is MEA DES is Physical absorption dominates Physical absorption dominates :

Capacity increases with pressure increase and temperature decrease, fully in line with capacity increase with pressure increase and temperature decrease, fully in line with Henry's Law Henry's Law and physical absorption thermodynamics law;

After adding water to 50% v, the absorption trend, parameter sensitivity, and thermodynamic behavior did not change abruptly, but only increased the capacity and rate. After adding water to 50% v, the absorption trend, parameter sensitivity, and thermodynamic behavior did not change abruptly, but only increased the capacity and rate.

In complete contrast to the behavior of traditional amine solution (chemical absorption, favorable temperature increase, small pressure effect), the dominance of chemical absorption is further ruled out. In complete contrast to the behavior of traditional amine solution (chemical absorption, favorable temperature increase, small pressure effect), the dominance of chemical absorption is further ruled out.

This conclusion is of great significance: This conclusion is of great significance: physical absorption means that the regeneration energy consumption is extremely low physical absorption means that the regeneration energy consumption is extremely low , only need to step down, gas lift or micro-heat to complete desorption, compared with alcohol amine chemical absorption can greatly reduce operating costs, meet low-carbon, low-cost capture needs., only need to step down, gas lift or micro-heat to complete desorption, compared with alcohol amine chemical absorption can greatly reduce operating costs, meet low-carbon, low-cost capture needs.

3.4 Optimal operating conditions and upper performance limits 3.4 Optimal operating conditions and upper performance limits

Optimized by RSM, the optimal working conditions of this system are: Optimized by RSM, the optimal working conditions of this system are: 10 bar, 40 ° C, 50% v water content 10 bar, 40 ° C, 50% v water content , corresponding to the maximum absorption capacity, corresponding to the maximum absorption capacity 0.379 mol0.379 mol - CO 2O/molCO 2O/mol - DESDES . The performance is at the upper middle level among similar choline-based DES, and the performance can be doubled by simply adding water, with significant industrial application potential... The performance is in the middle and upper level among similar choline-based DES, and the performance can be doubled by simply adding water, with significant industrial application potential.

4. Research and innovation points 4. Research and innovation points

Multi-parameter coupled system analysis Multi-parameter coupled system analysis : First complete quantification of temperature, pressure, and water content on ChCl: First complete quantification of the main and interactive effects of temperature, pressure, and water content on the absorption capacity/rate of ChCl-MEA DES, establishing a predictable response surface model, breaking through the limitations of single variable research; Main and interactive effects of MEA DES absorption capacity/rate, establishing a predictable response surface model, breaking through the limitations of single variable research;

Innovation in water-phase modification mechanism Innovation in water-phase modification mechanism : Confirm that adding water within 50% v does not reduce the effect, does not change the mechanism, only strong mass transfer does not reduce the effect, does not change the mechanism, only strong mass transfer , providing a green, low-cost, easy-to-industrialize solution for cracking the high viscosity bottleneck of DES;, providing a green, low-cost, easy-to-industrialize solution for cracking the high viscosity bottleneck of DES;

Methodological Advantages Methodological Advantages : Box: Box-Behnken design + RSM modeling, high experimental efficiency and reliable results, can be directly used for process parameter optimization and equipment scale-up design; Behnken design + RSM modeling, high experimental efficiency and reliable results, can be directly used for process parameter optimization and equipment scale-up design;

Mechanism clarification Mechanism clarification : Locked through dual evidence of thermodynamics and kinetics: Locked through dual evidence of thermodynamics and kinetics Physical absorption dominates Physical absorption dominates , providing theoretical basis for regeneration process design (low pressure desorption, low temperature regeneration)., provides theoretical basis for regeneration process design (low pressure desorption, low temperature regeneration).

V. Research Conclusions and Industrial Implications V. Research Conclusions and Industrial Implications

5.1 Core Conclusion 5.1 Core Conclusion

ChClChCl-MEA DES traps CO 2 with MEA DES traps CO 2 with physical absorption physical absorption , the capacity is positively correlated with pressure and negatively correlated with temperature, in line with the classical physical dissolution law; mainly, the capacity is positively correlated with pressure and negatively correlated with temperature, in line with the classical physical dissolution law;

Adding water (≤ 50% v) can add water (≤ 50% v) can double the absorption capacity and rate , the core function is to reduce viscosity and promote mass transfer, without changing the absorption mechanism;, the core function is to reduce viscosity and promote mass transfer, without changing the absorption mechanism;

Pressure and water content are the key parameters of the lifting rate, and the effect of temperature is weak; under the optimal working conditions, the absorption capacity is the key parameters of the lifting rate, and the effect of temperature is weak; under the optimal working conditions, the absorption capacity is 0.379 mol0.379 mol - CO2/molCO ³/mol - DESDES ;

This system has the advantages of low energy consumption, low cost, environment friendly, mild working conditions, low energy consumption, low cost, environment friendly, mild working conditions , and is a potential route to replace traditional alcohol amines and cryogenic physical absorption. Advantages are potential routes to replace traditional alcohol amines and cryogenic physical absorption.

5.2 Industrial Application Enlightenment 5.2 Industrial Application Enlightenment

Process optimization Process optimization : Preference: Preference is given to medium and low pressure (5 - 10 bar), medium and low temperature (around 40 ° C), water content 25 - 50% v medium and low pressure (5 - 10 bar), medium and low temperature (around 40 ° C), water content 25 - 50% v operating conditions, balance capacity, rate and energy consumption; operating conditions, balance capacity, rate and energy consumption;

Equipment matching Equipment matching : The aqueous system has low viscosity and fast mass transfer, and can be selected: the aqueous system has low viscosity and fast mass transfer. Conventional equipment such as packing tower, plate tower packing tower, plate tower can be selected without special modification; and other conventional equipment without special modification;

Regeneration strategy Regeneration strategy : Relying on physical absorption characteristics, using: Relying on physical absorption characteristics, using atmospheric gas stripping, micro-thermal desorption atmospheric gas stripping, micro-thermal desorption , the regeneration energy consumption can be reduced to 0.7 - 1.5 GJ/t CO2, much lower than the traditional MEA process;, the regeneration energy consumption can be reduced to 0.7 - 1.5 GJ/t CO2, much lower than the traditional MEA process;

Scene adaptation Scene adaptation : Suitable for syngas refining, natural gas decarbonization, refinery gas purification, etc.: Suitable for syngas refining, natural gas decarbonization, refinery gas purification, etc. Medium and high pressure, medium and low temperature medium and high pressure, medium and low temperature scenarios, especially suitable for low carbon transformation of old installations. Scenario, especially suitable for low carbon transformation of old installations.

VI. Research shortfalls and future prospects VI. Research shortfalls and future prospects

6.1 Existing deficiencies 6.1 Existing deficiencies

Not carried out Long-term cycle stability Long-term cycle stability test, industrial continuous operation life is not yet clear; test, industrial continuous operation life is not yet clear;

The influence of impurities (H2O S, O 2O, heavy hydrocarbons) impurities (H2O S, O 2O, heavy hydrocarbons) on absorption performance needs to be verified for actual flue gas/natural gas scene adaptability; for absorption performance, actual flue gas/natural gas scene adaptability needs to be verified;

Not carried out Not carried out Full process technical and economic analysis (TEA) Full process technical and economic analysis (TEA) With life cycle assessment (LCA), the cost advantage of scale needs to be quantified. With life cycle assessment (LCA), the cost advantage of scale needs to be quantified.

6.2 Future Directions 6.2 Future Directions

Compound modification Compound modification : Introduce second hydrogen bond donors such as ethylene glycol and glycerol to further reduce viscosity and improve selectivity;: Introduce second hydrogen bond donors such as ethylene glycol and glycerol to further reduce viscosity and improve selectivity;

Loading and loading : Load DES on porous supports such as silica gel and MOF, prepare: Load DES on porous supports such as silica gel and MOF, prepare immobilized adsorbent immobilized adsorbent , taking into account high capacity and fast mass transfer;, taking into account high capacity and fast mass transfer;

Process Coupling Process Coupling : Coupled with membrane separation and pressure swing adsorption (PSA), build: Coupled with membrane separation and pressure swing adsorption (PSA), build efficient and efficient - low consumption and low consumption integrated decarbonization system; integrated decarbonization system;

Green Extension Green Extension : Expanded to: Expanded to Biogas Purification, Hydrogen Decarbonization, Biomass Gasification Gas Purification Biogas Purification, Hydrogen Decarbonization, Biomass Gasification Gas Purification and other scenarios to improve the full scene application scheme.