Liquid Absorption Systems Research Articles

Highly efficient CO2 capture by a non-aqueous amine-based absorbent of 3 aminopropanol/polyethylene glycol 200: Experimental and computational simulation

In the present work, a novel non-aqueous 3-aminopropanol/polyethylene glycol 200 (3AP/PEG200) absorbent for efficient carbon dioxide (CO2) capture was designed using environment-benign and non-toxic polyethylene glycol 200 (PEG200) as an alternative to water. The mechanism, physical, thermodynamic, and kinetic properties of absorption process was investigated through a combination of experimental and multi-scale computational simulation approaches including molecular dynamics (MD) simulation and quantum chemical (QC) calculations. It was found that the addition of PEG200 not only enhanced the thermal stability of the absorbent, but also did not significantly increase the viscosity. In addition, kinetic studies revealed that the absorption process conforms to a pseudo-first-order equation, with the activation energy (Ea) value of 20.40 kJ/mol, significantly lower than that of the benchmark 30 % monoethanolamine (MEA) aqueous solution used in industrial CO2 capture. Furthermore, the enthalpy change (ΔH) and entropy change (ΔS) values were found to be more negative than those of other liquid absorption systems, indicating that the 3AP/PEG200 absorbent is favorable for CO2 absorption even at low partial pressures. Most importantly, the absorption system exhibited good regeneration capability, as demonstrated by five absorption–desorption cycle experiments. In conclusion, the non-aqueous 3AP/PEG200 absorbent exhibits low viscosity, high thermal stability, enhanced absorption capacity, and excellent cyclic performance, positioning it as a promising candidate for CO2 absorption in industrial applications.

Dynamic simulation and exergy-economic assessment of solar thermal refrigeration systems in different climates

In this paper, the performance of solar-thermal cooling systems for air conditioning application is investigated in different climates of Iran. Diverse climate types are considered which makes the results applicable to other countries. Available commercial refrigeration cycles that are capable of integration with solar-thermal collector systems including the closed liquid absorption cycle, closed solid adsorption cycle, and desiccant cycle are studied. First, a solar and climatic map is proposed and the representative cities have been selected for each climate. Then, the climate parameters related to each region are extracted to create an hourly database. Also, the cooling load of a reference building is modeled based on the climate database, each refrigeration cycle is simulated using thermodynamic equations, and the dynamic model is generalized daily for the hot season in each city. In the last step, each cooling system is examined using an exergy-economic analysis, and technical and economic indicators are compared. The results show that the most suitable solar-thermal refrigeration cycle that can be installed in the central plateau and semi-arid areas of Iran is the closed-cycle liquid absorption system. The annual average exergy-economic factor and the annual solar fraction in this cycle are 0.7578 and 0.57, respectively. In the southern coastal areas, due to the high humidity and air temperature in summer, the solid desiccant refrigeration system with an exergy-economic factor of 0.6978 and a solar fraction of 0.2416 is preferred. The suitable solar-thermal refrigeration cycle in the cold and mountainous regions of Iran is a solid adsorption system with a silica gel absorber. The annual solar fraction and average annual exergy-economic factor in this cycle are 0.5158 and 0.7394. Also, in northern moderate regions, the solid adsorption refrigeration system with an average annual exergy-economic factor of 0.409 and average exergy efficiency of 0.12 is comparatively beneficial.

Hydrodynamics and mass transfer performance of gas–liquid microflow in viscous liquids

Mass transfer performance is markedly improved in microchannels compared with traditional gas–liquid chemical devices. The residence time is a crucial parameter for determining the mass transfer performance. However, investigation of the residence time in gas–liquid absorption systems remains insufficient, especially in viscous gas–liquid systems with obvious flow non-uniformity between the two phases. Herein, the microscale gas–liquid flow and mass transfer performance in viscous liquids (9.0–55.4 mPa·s) based on the true residence time are investigated. Owing to the broad operating conditions of the gas–liquid flow ratio (0.2–6.7), both Taylor flow and bubbly flow patterns were observed. The results show that the bubble velocity gradually decreases to the minimum value with increasing flow distance in the Taylor flow pattern owing to gas absorption. Subsequently, bubble velocity gradually increases under the bubbly flow pattern, and the dominant factor of bubble velocity is pointed out. The true bubble residence time initially increases and then gradually decreases compared to the superficial time as the gas holdup decreases, based on the superficial velocity using the initial flow rates. The transition point occurs at a gas holdup of 0.2. In addition, the turning point of the different rules of the pressure drop occurs at a gas holdup of 0.5. Finally, a new dimensionless equation was proposed to predict the mass transfer coefficient and reveal the equation differences between low- and high-viscosity liquids. This work enriches the understanding of microflow characteristics and mass transfer performance in viscous solutions and provides a reliable guide for microreactor design.

Development of novel nanoabsorbents by amine functionalization of Fe3O4 with intermediate ascorbic acid coating for CO2 capture enhancement

Novel nanoabsorbents are developed CO2 capture enhancement by functionalized magnetite nanoparticles in a water-based nanofluid system. The functionalization of magnetic nanoparticles was performed using different organic and inorganic reagents: (3-Aminopropyl) triethoxysilane (APTES), tetraethyl orthosilicate (TEOS), and L(+)-ascorbic acid (AA). The first coating used AA followed by a secondary shell of SiO2, and then amine functionalization via APTES, creating the amine-functionalized double-coated magnetite (Fe3O4 @AA@SiO2-NH2). The CO2 adsorption capacity of the synthesized material was enhanced by 11% at ambient pressure, compared with that of amine functionalization without AA (Fe3O4 @SiO2-NH2). To evaluate the absorption enhancement of Fe3O4 @AA@SiO2-NH2 at 25 °C, concentrations ranging from 0.1 to 0.4 wt% were compared with those of the base fluid (water). The highest enhancement of 59.2% was obtained at a concentration of 0.3 wt%. A cyclic performance evaluation showed that the initial absorption capacity was reduced by only 7%. Owing to its nanoabsorbent stability, significant CO2 absorption enhancement, and high recyclability, Fe3O4 @AA@SiO2-NH2 will be a milestone in physical liquid absorption systems for a carbon free environment.

Experimental analysis of a high-performance open sorption thermal storage system with absorption-crystallization-adsorption processes

Sorption thermal energy storage is one of the promising solar thermal energy storage considering its long-term storage ability. Since the common sorption thermal storage suffers from low sorption capacity, a high energy storage density enhancement mechanism is always essential for a cost-effective and compact storage system. With three-phase sorption thermal storage, high energy storage density from the crystallization sorption process can be achieved. However, such a sorption process brings significant challenges, including crystal blockage issues in absorption system or solution leakage handling problems in adsorption system, which need to be resolved. Here, we showed a high-performance sorption thermal storage that can attain high energy storage density via a full liquid-to-solid state sorption process. Such a process is achieved by adopting moist air as heat and mass transfer media, which enables liquid absorption, crystallization, and solid adsorption in a simple configuration simultaneously. The lab-scale model achieved high energy storage density in the range of 333–405 kWh/m3 resulting from the heat of absorption and crystallization of LiCl, which is the record value for liquid absorption systems. The energy storage density obtained from the dual liquid and solid sorption processes is enhanced by 278 % compared with that of the conventional cycle at 70 % RH (relative humidity) and temperature lift of 10 °C. With simple construction and high energy storage density, the proposed system has great potential in real applications.

Model analysis for development of piperazine activated sodium sarcosinate solutions for environmental sustainability

Model analysis for piperazine (PZ) activated sodium sarcosinate (Na-Sar) for CO2 capture and sequestration for environmental sustainability is challenging and required in-depth analysis. Here, we performed experiments and developed a model for correlating physical properties such as density, refractive index, surface tension, and viscosity of PZ activated Na-Sar for CO2 capture. Physical properties were investigated at different temperatures from 298.15 to 333.15 K and different mass fractions (w1 + w2) of aqueous blends of PZ + Na-Sar were used (0.02 + 0.10, 0.05 + 0.10, 0.10 + 0.10, 0.02 + 0.20, 0.05 + 0.20, 0.10 + 0.20, 0.02 + 0.30, 0.05 + 0.30, and 0.10 + 0.30). All physical properties decreased as temperature increased; however, as PZ in Na-Sar solutions increased, physical properties tended to increase. Refractive index, density, and surface tension showed a linear relationship, while viscosity showed an exponential behavior with the temperature. Empirical correlations were established for all measured properties as a function of temperature and solvent concentration. The correlations showed that the experimentally measured and correlated values were in good agreement. These properties are not available in the literature and are important for the designing of gas–liquid absorption systems. This work can be a useful contribution towards the global efforts to mitigate CO2 from various industrial streams.

Assessment of turbulent contact absorber hydrodynamics with application in carbon capture

Chemical gas–liquid absorption systems are one of the most employed technologies for CO2 removal from post-combustion processes, due to the high capture efficiency. This work presents a complex hydrodynamics model used to describe a novel CO2 capture technique that relies on the use of a three phase, gas–solid-liquid absorption column, in which the gas phase acts as the continuous phase, the liquid as the dispersed phase and the low-density solid particles are inert.To determine and analyze the hydrodynamic parameters of the process: minimum fluidization velocity, fluidized bed height, pressure drop, fluid retention, an air–water system was used. The developed hydrodynamics model was validated against experimental data from a pilot column (correlation coefficients above 0.97).For the study of the chemisorption process, the liquid phase consists of NaOH solution and the gas phase is a mixture of CO2 and air. The implementation of the hydrodynamic model along with the mass transfer model contributes to increasing the correlation coefficient for the outlet CO2 concentration to a value above 0.99. In this case, the increased value of the mass transfer area leads to a value up to 10 times higher of the transferred flow of carbon dioxide between the two phases, compared to regular packed bed columns.The model proves useful in the analysis of the influence of liquid phase and solid phase characteristics on the system’s performance. The results show that the capture rate is highly dependent on the fluidized bed height. Increasing the size or density of solid particles leads to a decrease in the height of the fluidized bed resulting in a 20 – 40% decrease in the carbon capture rate.

Sustained-release of interlayer chloride in iron oxychloride for mercury oxidation from industrial flue gas

Conversion of gaseous elemental mercury (Hg0) to its higher valence state is essential for mercury control using the current air pollution devices from industrial flue gas. Traditional technologies often use selective catalytic reduction (SCR) units to enhance the Hg0 oxidation performance but highly depend on the high-temperature catalysts, the presence of HCl, and the downstream liquid absorption system. This study proposes a novel method coupled with a low-temperature catalyst and a dust removal device to realize mercury immobilization. The iron oxychloride (FeOCl) 2D-nanosheets are put into practice as the low-temperature catalyst. The results indicate superior Hg0 oxidation performance at low oxidizing conditions and a wide temperature window (100–300 °C). The FeOCl catalyst can further combine with MnO2 to achieve mercury fixation. The outstanding property is attributed to the chloride with high activity between the Van Der Waals layer of FeOCl. The oxidative products HgCl2(g) in high yield are attributed to the low Fe-Cl bonding energy in FeOCl relative to metal chloride. This study highlights the conversion process of gaseous Hg0 to HgCl2 in oxidized form and provides a new idea that can be used in conjunction with dust removal equipment to expand the removal method of flue gas mercury.

Investigation on the thermal performances of [Na(TX-7)]SCN/NH3 absorption systems based on physical properties measurement of the working fluid

The application of ionic liquid in the ammonia absorption system is current research hotspot. The low solubilities of ammonia in common ionic liquids lead to poor thermal performances of absorption systems, which is the main obstacle faced by ionic liquid absorption systems. Hence, we proposed a new idea of using ionic liquids containing metal cations to enhance the thermal performances of the absorption systems. The specific ionic liquid being studied is [Na(TX-7)]SCN. The vapor–liquid equilibrium and heat capacities of [Na(TX-7)]SCN/NH3 were studied by experimental methods. Based on above physical properties, the thermal performances of absorption refrigeration and absorption heat transformer using [Na(TX-7)]SCN/NH3 were simulated and compared with those of other systems. The thermal performances of [Na(TX-7)]SCN/NH3 absorption refrigeration are better than NH3/H2O system and other ionic liquid systems, but is a bit lower than NaSCN/NH3 system. The thermal performances of [Na(TX-7)]SCN/NH3 absorption heat transformer are better than those of the [mmim]DMP/H2O and [mmim]DMP/CH3OH systems, similar to those of the NaSCN/NH3 system, and slightly lower than those of the LiBr/H2O system. Overall, the proposed working fluid obviously improves the thermal performances of ionic liquid system and solves the crystallization problem of NaSCN/NH3 and LiBr/H2O systems.

Cooperative CO2 Absorption Isotherms from a Bifunctional Guanidine and Bifunctional Alcohol.

Designing new liquids for CO2 absorption is a challenge in CO2 removal. Here, achieving low regeneration energies while keeping high selectivity and large capacity are current challenges. Recent cooperative metal–organic frameworks have shown the potential to address many of these challenges. However, many absorbent systems and designs rely on liquid capture agents. We present herein a liquid absorption system which exhibits cooperative CO2 absorption isotherms. Upon introduction, CO2 uptake is initially suppressed, followed by an abrupt increase in absorption. The liquid consists of a bifunctional guanidine and bifunctional alcohol, which, when dissolved in bis(2-methoxyethyl) ether, forms a secondary viscous phase within seconds in response to increases in CO2. The precipitation of this second viscous phase drives CO2 absorption from the gas phase. The isotherm of the bifunctional system differs starkly from the analogous monofunctional system, which exhibits limited CO2 uptake across the same pressure range. In our system, CO2 absorption is strongly solvent dependent. In DMSO, both systems exhibit hyperbolic isotherms and no precipitation occurs. Subsequent 1H NMR experiments confirmed the formation of distinct alkylcarbonate species having either one or two molecules of CO2 bound. The solvent and structure relationships derived from these results can be used to tailor new liquid absorption systems to the conditions of a given CO2 separation process.

Investigation of the mutual diffusion coefficients of [mmim]DMP/H2O and [mmim]DMP/CH3OH at atmospheric pressure

The mutual diffusion coefficient (D12) of [mmim]DMP (1-methyl-3-methylimidazolium dimethylphosphate)/H2O and [mmim]DMP/CH3OH at ionic liquid (IL) mass fraction (ω1) of 0.005–0.995, atmospheric pressure of 101.5kPa, and temperature (T) of 298.15–328.15K were measured using the holographic interferometry. It is the first time to employ holographic interferometry to measure the D12 of binary solutions containing ionic liquid. The influence of the meniscus on the determination of the initial time for diffusion is discussed for the first time. And this influence is eliminated by using a fitting method. D12 of [mmim]DMP/H2O and [mmim]DMP/CH3OH increase with the increase of temperature, and decrease with the increase of the IL mass fraction. The experimental data of [mmim]DMP/H2O and [mmim]DMP/CH3OH were correlated using the modified group contribution model, with the total deviations below 4.7% and 4.2%, respectively. The relationship of the mutual diffusion coefficients is [mmim]DMP/CH3OH>[mmim]DMP/H2O>LiBr/H2O on the same conditions. The studies on D12 establish the foundation for the design and optimization of the absorber of ionic liquid absorption systems.

Process intensification characteristics of a microreactor absorber for enhanced CO2 capture

Gas separation processes, including post-combustion carbon capture (PCCC) by chemical absorption using liquid solvents can be substantially enhanced using high performance micro-structured surfaces to enhance the surface area available for reaction. The present paper studies the hydrodynamics and mass transfer performance of gas–liquid absorption of CO2 into aqueous diethanolamine in a micro-structured reactor. The system was designed to comprise 15 straight parallel channels in a cross flow inlet configuration. The hydraulic diameter of each channel was 456μm. The performance of the reactor was studied with respect to the absorption efficiency, mass transfer coefficient, acid gas loading ratio, and pressure drop. A flow pattern map was developed using available regime transition criteria. Parametric studies varying the gas and liquid flow rates, as well as their respective concentrations at the reactor inlet, were conducted. The two-phase pressure drop was compared against the predictions of a piecewise model and a reasonably good agreement was obtained. Absorption efficiencies close to 100% were observed under certain operating conditions. The presently achieved values of liquid-side volumetric mass transfer coefficients were between 1–3 orders of magnitude higher than those reported for most conventional gas–liquid absorption systems, which can be attributed to the inherent high specific interfacial area provided through micro-structured surfaces. The results reported here indicate the substantial levels of process intensification that can be achieved using microreactors.

Assessment of chemical absorption/adsorption for post-combustion CO2 capture from Natural Gas Combined Cycle (NGCC) power plants

The fossil fuel power generation sector is facing critical decisions to significantly reduce CO2 emissions by implementing carbon capture technologies at industrial scale for transition to low carbon economy. This paper assesses the usage of reactive absorption/adsorption systems for post-combustion CO2 capture from Natural Gas Combined Cycle (NGCC) power plants. As reactive gas–liquid absorption system assessed for post-combustion CO2 capture, the alkanolamine-based gas–liquid absorption was evaluated (MDEA was considered as illustrative example). As reactive gas–solid adsorption system, the innovative Calcium Looping (CaL) method was considered. The work evaluates how chemical absorption/adsorption influence the techno-economic performances of NGCC power plants. As benchmark option used to quantify the carbon capture energy and cost penalties, NGCC plant without CO2 capture was considered. The post-combustion carbon capture options have at least 90% carbon capture rate. As the results show, CaL concept exhibits improved environmental performances (e.g. >98 vs. 90% carbon capture rate) and economic indicators (e.g. 969 vs. 1238 €/kW net power as specific capital investment, 42.82 vs. 46.24 €/MWh as O&M costs, 56.91 vs. 66.12 €/MWh as electricity cost etc.) compared to MDEA case.

Hydrodynamics and mass transfer performance of a microreactor for enhanced gas separation processes

Chemical absorption processes for gas separation applications can be substantially enhanced using microreactors that utilize micro-structured surfaces/channels and fluid feed systems. The present paper focuses on the absorption of CO2 using aqueous diethanolamine solvent in microreactors having hydraulic diameters ranging from 254 to 762μm. The performance of the reactor was studied with respect to the absorption efficiency, pressure drop, mass transfer coefficient, interfacial area, enhancement factor, and Sherwood number. Parametric studies were conducted varying the channel hydraulic diameter, liquid solvent concentration, and CO2 concentration in the gas phase. The two-phase pressure drop was compared against an available empirical model and a reasonably good agreement was obtained for the present range of channel diameters. An empirical model for the Sherwood number was developed and compared against experimental data. Absorption efficiencies close to 100% were observed under certain operating conditions. Liquid-side volumetric mass transfer coefficients close to 400s−1 were achieved, which is between 2 and 4 orders of magnitude higher than those reported for most conventional gas–liquid absorption systems. Such high levels of process intensification were attributed to the increased area per unit volume offered by microchannels, thus increased gas–liquid specific interfacial area at reduced channel diameters. Interfacial areas close to 15,000m2/m3 were achieved which is between 1 and 2 orders of magnitude higher than those of conventional absorption systems.

Removal of nitric oxide in rotating packed bed by ferrous chelate solution

As a major pollutant flue gas, nitric oxide (NO) emission has received great attention of the world. Various technologies have been developed to reduce NO emission. Among them, wet scrubbing approach seems to be promising concept. Ferrous chelate solution has been demonstrated as a suitable and efficient option in flue gas denitrification process due to its high reaction rate with NO and good regeneration ability. However, NO absorption rate is strongly affected by mass transfer limitation of NO and FeII(EDTA) solution in traditional reactors. In this study, RPB is firstly introduced as a gas–liquid reactor to enhance the NO removal efficiency by FeII(EDTA) solution. Preliminary experimental results are presented on the absorption of NO by investigating the effects of operating parameters on NO removal efficiency in RPB. The removal efficiency peaks when the pH value of the absorbent reaches 7. The optimum temperature for this gas–liquid absorption system is in the range of 293–303K. When the gravity level of the RPB is higher than 150g, the removal efficiency could reach 87%. In addition, the NO removal efficiency decreases with the increase of the gas–liquid ratio. The simultaneous increase of the gas and the liquid flow rates has no obvious effect on the removal efficiency. The obtained results imply a great potential of RPB in the removal of NO from flue gases.