Termolecular Kinetic Model for CO2‐Alkanolamine Reactions: An Overview

The CO₂ reaction with alkanolamines has received considerable attention by both academia and industry. Commonly, the formation of the carbamate during the CO₂ reaction with primary, secondary, and sterically hindered amines is described by a two‐step zwitterion mechanism. Alternatively, a single‐step termolecular reaction mechanism can also be used to govern carbamate formation. The experimental kinetic data for several amine‐based solvents are consistent with this mechanism, and it can satisfactorily explain fractional‐order and higher‐order kinetics. However, up to now, the termolecular reaction mechanism has not been properly discussed. Here, this mechanism is described in detail, a simple procedure to estimate the kinetic parameters is outlined, and the termolecular reaction kinetics for various systems comprising individual and mixed amines is reviewed.     

Sub‐and Supercritical CO2‐Extraction of Hypericum perforatum L.

The extraction of Hypericum perforatum L. by liquid carbon dioxide (p = 80 bar, t = 15 °C) gave almost the same extract yield (1 %, w/w) as by supercritical (p = 100 bar, t = 40 °C) carbon dioxide, containing the same percentages of essential oil (about 6.4 %, w/w). The increase of the extract yield at higher pressure (250 to 350 bar) is due to the increase of extragent density, i.e., solubility. By increasing the grinding degree of the drug, a higher extract yield is obtained in the supercritical range under high pressure. GC‐MS analysis of the extract composition showed that the non‐terpene compounds have the highest contribution. The oil content in the drug, determined by steam distillation, was 0.058 %, w/w. The oil content in the extracts, calculated for the drug, was significantly higher (1.2 to 1.9 times).     

Thermal Effects During the Absorption of CO2 in Aqueous Solutions of 3‐Amino‐1‐Propanol

In this work, we study the process of CO₂ absorption, at high partial pressures, in aqueous solutions of 3‐amino‐1‐propanol (AP), with respect to the thermal effects of this operation. All of the experiments were performed in a stirred tank gas‐liquid reactor with a flat, known interface. The variables considered were the AP concentration in the range of 0.1 to 3.0 M and the temperature within the interval of 288–313 K. From the results, we deduce that the process takes place in the instantaneous nonisothermal regime, and we propose an equation which relates the experimental results of molar flux with the initial amine concentration. At the same time, we can evaluate the temperature increase at the gas‐liquid interface.     

Kinetics of CO2 absorption into a novel 1‐diethylamino‐2‐propanol solvent using stopped‐flow technique

A stopped‐flow apparatus was used to measure the kinetics of carbon dioxide (CO₂) absorption into aqueous solution of 1‐diethylamino‐2‐propanol (1DEA2P) in terms of observed pseudo‐first‐order rate constant (k_o) and second‐order reaction rate constant (k₂), in this work. The experiments were conducted over a 1DEA2P concentration range of 120–751 mol/m³, and a temperature range of 298–313 K. As 1DEA2P is a tertiary amine, the base‐catalyzed hydration mechanism was, then, applied to correlate the experimental CO₂ absorption rate constants obtained from stopped‐flow apparatus. In addition, the pK_a of 1DEA2P was experimentally measured over a temperature range of 278–333 K. The Brønsted relationship between reaction rate constant (obtained from stopped‐flow apparatus) and pK_a was, then, studied. The results showed that the correlation based on the Brønsted relationship performed very well for predicting the absorption rate constant with an absolute average deviation of 5.2%, which is in an acceptable range of less than 10%. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3502–3510, 2014     

Hydrogen Production from a Fluidized‐bed Coal Gasifier with In Situ Fixation of CO2–Part I: Numerical Model

In order to attempt to eliminate global warming effects, it is highly desirable that new technologies with lower or zero emission of CO₂ to the environment are developed. In this work, a high‐pressure fluidized‐bed coal gasifier for H₂ production with in situ fixation of CO₂ is simulated by a comprehensive two‐dimensional model. The Eddy Dissipation Concept (EDC) model is first adopted in the pulverized coal gasification model to simultaneously describe the turbulent mixing and detailed chemical kinetics. The developed model is verified with experimental results. The simulated concentrations for the gas product agree well with the experimental data. The simulated distributions for gas temperature and velocity correlate well with the reaction mechanism and experimental phenomena.     

Comparison of Various Alkaline Solutions for H2S/CO2‐Selective Absorption Applied to Biogas Purification

Biogas is a common renewable energy resource. A very important stage of biogas upgrading, studied in the present work, is its purification from H₂S traces. The selective absorption of H₂S and CO₂ into oxido‐alkaline solutions containing hydrogen peroxide and into amine solutions was compared by performing absorption test runs in a cables‐bundle scrubber at 293.15 K and atmospheric pressure. The absorption rate and selectivity for H₂S over CO₂ were determined for various solute partial pressures, different alkaline absorbents and hydrogen peroxide concentrations in the scrubbing liquid, and different pH values. Higher H₂S‐selective absorption performances with oxido‐alkaline solutions than with amine solutions could be observed provided that the solution is at a low pH value (9.5) and contains a sufficient hydrogen peroxide concentration.     

A Two‐Compartment Fluidized Bed Reactor for CO2 Capture by Chemical‐Looping Combustion

Chemical‐looping combustion (CLC) is a combustion method for a gaseous fuel with inherent separation of the greenhouse gas carbon dioxide. A CLC system consists of two reactors, an air reactor and a fuel reactor, and an oxygen carrier circulating between the two reactors. The oxygen carrier transfers the oxygen from the air to the fuel. The flue gas from the fuel reactor consists of carbon dioxide and water, while the flue gas from the air reactor is nitrogen from the air. A two‐compartment fluidized bed CLC system was designed and tested using a flow model in order to find critical design parameters. Gas velocities and slot design were varied, and the solids circulation rate and gas leakage between the reactors were measured. The solids circulation rate was found to be sufficient. The gas leakage was somewhat high but could be reduced by altering the slot design. Finally, a hot laboratory CLC system is presented with an advanced design for the slot and also with the possibility for inert gas addition into the downcomer for solids flow increase.     

CO2‐Switchable Oil/Water Emulsion for Pipeline Transport of Heavy Oil

By mixing an aqueous solution of tertiary amine, N,N‐dimethylethanolamine (DMEA), with naphthenic acid (RCOOH) derived from heavy oil, a CO₂ switchable zwitterionic surfactant (RCOO^?DMEAH⁺) aqueous system was constructed. The CO₂ switchability of this zwitterionic surfactant was confirmed by visual inspection, pH measurements, and conductivity tests, i.e., the RCOO^?DMEAH⁺ decomposed into RCOOH, DMEAH⁺ and HCO₃^? after bubbling CO₂ through but switched back to its original state by subsequent bubbling N₂ through at 80 °C to remove the CO₂. The interfacial tension tests of heavy oil in DMEA aqueous solutions indicated that the solution containing 0.5 wt% of DMEA and 0.2 wt% of NaCl resulted in the lowest interfacial tension. The O/W emulsion formed when aqueous solutions of DMEA were used to emulsify heavy oil exhibited the best performance when the oil/water volume ratio, DMEA concentration, and NaCl concentration were 65:35, 0.5 and 0.2 wt%, respectively. The feasibility of pipeline transport of the O/W heavy oil emulsion was evaluated. The results illustrated that the demulsification of the O/W emulsion after transport could be easily realized by bubbling CO₂ through. Although demulsification efficiency still needs to be improved, the recycling of the aqueous phase after demulsification by removal of CO₂ looks promising.     

Structure and Catalytic Performance of Mg‐SBA‐15‐Supported Nickel Catalysts for CO2 Reforming of Methane to Syngas

A series of Mg‐modified SBA‐15 mesoporous silicas with different MgO contents were successfully synthesized by a simple one‐pot synthesis method and further impregnated with Ni. The Mg‐modified SBA‐15 materials and supported Ni catalysts were characterized by N₂ physisorption (BET), X‐ray diffraction (XRD), temperature‐programmed desorption of CO₂ (CO₂‐TPD), temperature‐programmed H₂ reduction (H₂‐TPR), and temperature‐programmed hydrogenation (TPH) techniques and used for methane dry reforming with CO₂. CO₂‐TPD results proved that the addition of Mg increased the total amount of basic sites which was responsible for the enhanced catalytic activity over the Mg‐modified Ni catalyst. The excellent catalytic stability of Ni/8Mg‐SBA‐15 was ascribed to less coking and higher stability of the Ni particle size due to the introduction of Mg.     

CO2‐assisted polymer processing: A new alternative for intractable polymers

CO₂‐assisted polymer processing is proposed as an alternative route for intractable and high molecular weight polymers based on the plasticization effects of CO₂ and its direct effect on the melting behavior of semicrystalline polymers. A modified processing system was used to process a variety of polymers in the presence of high‐pressure CO₂. The system includes an extruder that was modified to allow for high pressures created by the injection of CO₂. The new design includes a modified feed section that allows a given mass of polymer to interact with CO₂ before and during the extrusion process. The inherent shear mixing and the presence of CO₂ allow for a specific control over the extrudate morphology. Results suggest that this alternative design provides a new and easy route to melt process high melt viscosity polymers of commercial importance, such as polytetrafluoroethylene (PTFE), fluorinated ethylene propylene copolymer (FEP), and syndiotactic polystyrene (s‐PS). The increased processability of these systems in CO₂ is related to the plasticization effect of CO₂ that was quantified through a depression in the glass‐transition temperature according to the Chow model. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1501–1511, 2004     

Reaction Mechanism and Kinetics of 1,8‐Diazabicyclo[5.4.0]undec‐7‐ene and Carbon Dioxide in Alkanol Solutions

Carbon dioxide‐binding organic liquids (CO₂BOL) are a new class of solvents with advantageous properties such as high boiling points, low specific heats, high absorption capacities, and easily reversible reactions. In order to implement these solvents in processes, the reaction characteristics must be determined a priori. This work presents an analysis of the rate constants and activation energies of the reaction between carbon dioxide and 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) in 1‐hexanol and 1‐propanol. The reactions were found to comply with a termolecular reaction mechanism and exhibited pseudo‐first‐order behavior in the presence of excess DBU and 1‐alkanol. It was concluded that DBU‐based CO₂BOL are environmentally friendly and easy‐to‐handle solvents that may provide great flexibility and improvements over conventional carbon dioxide absorption processes.     

Hydrate Formation during Pressure Release of Wet CO2,View‐Cell Observations

The objective of this work is the evaluation of the blocking risk of pipelines by hydrate crystals in processing steps. Two topics were investigated: (I) hydrate formation during pressure release and isochore cooling and (II) the influence of a contact surface on hydrate formation. The investigated materials were steel, glass, and polytetrafluoroethylene (PTFE). Experiments were carried out in a 87 mL view‐cell to observe hydrate formation in the bulk CO₂ phase, since crystals of that size can cause the problems mentioned. The starting conditions of all experiments were within the temperature range of T = 278–285 K and the pressure range of p = 6–20 MPa. The results of the experiments suggest that the major criterion for hydrate formation during pressure release is the degree of supersaturation. Visible hydrate formation can be observed at a minimum subcooling of ΔT = 7 K. With a starting temperature of T = 285 K and a starting pressure of p = 6 MPa no hydrate formation is observed. The surface properties of the material have no direct influence on the hydrate formation process. However, during pressure release hydrate crystals detach from hydrophobic materials like PTFE, whereas they stick to hydrophilic materials like glass and steel. Thus, the adhesion between hydrate crystals and hydrophilic surfaces is stronger than between hydrate crystals and hydrophobic surfaces.     

Molecular‐based virial coefficients of CO2‐H2O mixtures

We report second and third virial coefficients for the system CO₂‐H₂O, calculated via cluster integrals using quantitative molecular models taken from the literature. Considered models include (1) fits to highly accurate ab initio calculations of the potential energy surfaces, and (2) semiempirical Gaussian Charge Polarizable Models (GCPM). Three‐body effects are found to be essential for obtaining quantitative results. Good agreement with experiment is obtained for the pure‐component coefficients, and for the cross second virial coefficient. For the two cross third virial coefficients, the few experimental data available do not agree well with the calculations; it is not clear whether this is due to problems with the data or deficiencies in the three‐body potentials. The uncertain state of the experimental data, and the relative mutual consistency of values computed from ab initio and GCPM models, suggest that calculated mixture third virial coefficients could be more accurate than values from experiment. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3029–3037, 2015     

Roles of Surface and Bulk Lattice Oxygen in Forming CO2 and CO During Methane Reaction over Gadolinia-Doped Ceria

Temperature-programmed reaction of methane and temperature-programmed reduction were performed over gadolinia-doped ceria (GDC). It was found that CO₂ formation can occur at very much lower temperature than CO formation. The surface lattice oxygen acts as the active site for CH₄ adsorption. This active site has a dynamic characteristic due to the mobility of the lattice oxygen. The rates of CO and CO₂ formations can be controlled by the supply rate of the lattice oxygen from the GDC bulk; this supply rate depends on the mobility and the concentration of the bulk lattice oxygen. CO₂ formation is associated with the existing surface lattice oxygen while CO formation depends on the oxygen species coming from the bulk lattice during methane reaction.     

Solubility of CO2 in Aqueous Piperazine and its Modeling using the Kent‐Eisenberg Approach

A systematic investigation of the equilibrium solubility of CO₂ in aqueous piperazine solutions was conducted in a double‐jacketed stirred cell reactor. The solubilities of CO₂ in the solution were measured at 20, 30, 40, and 50 °C with CO₂ partial pressures ranging from 0.4–95 kPa. Generally the aqueous piperazine solution exhibits the same characteristics as conventional alkanolamines. Increasing the CO₂ partial pressure increases the gas loading, however increasing the temperature or concentration decreases the CO₂ loading. The values of the CO₂ loading obtained confirm that the piperazine forms stable carbamates. The equilibrium solubility data were analyzed using a Kent‐Eisenberg approach. Representation of the model is generally in good agreement with that of the experimental data, especially at high temperature.     

Development of a Scalable Method for Manufacturing High‐Temperature CO2 Capture Sorbents

An engineered process for scalable manufacture of a calcium aluminum carbonate CO₂ sorbent with production amounts of about 1000 g per hour has been developed. The process includes mixing and heating, solid‐liquid separation, drying and extrusion, crushing and conveying, and calcined molding steps. The sorbent preparation involves the coprecipitation of Ca²⁺, Al³⁺, and CO₃^2– under alkaline conditions. By adjusting the Ca:Al molar ratio, a series of Ca‐rich materials could be synthesized for use as CO₂ sorbents at 750 °C. A calcium acetate‐derived sorbent exhibited better cyclic stability than sorbents originating from CaCl₂ and Ca(NO₃)₂. The initial sorption capacity increased with CaO concentration. High stability of more than 90 % was maintained by the Ca:Al sorbents after 40 looping tests.     

DEGRADATION OF AQUEOUS SOLUTIONS OF ALKANOLAMINE BLENDS AT HIGH TEMPERATURE, UNDER THE PRESENCE OF CO2 AND H2S

An experimental study on the degradation of aqueous solutions of alkanolamine blends, under the presence of carbon dioxide and hydrogen sulfide, was carried out. The studied alkanolamines were: diethanolamine (DEA), methyldiethanolamine (MDEA), and 2-amino-2-methyl-1-propanol (AMP). Degradation experiments were carried out at a temperature of 200°C. The mass fraction of DEA and MDEA in the studied aqueous solutions was 10% and 35%, respectively. AMP was incorporated into the MDEA-DEA aqueous solutions, with concentrations of (0-8) mass fraction. Partially degraded alkanolamine aqueous solutions were analyzed, after about 90 hours, by gas chromatography.

It was found that in all the studied alkanolamine aqueous solutions the MDEA degrades more slowly than DEA under the same experimental conditions. Degradation of both alkanolamines was found to be almost independent of the AMP concentration. AMP exhibits an intermediate stability; it is more resistant to degradation than DEA but less than MDEA. In addition, thermal degradation of DEA and MDEA is minimal up to 200°C.     

CO2‐Abtrennung für fossil befeuerte Kraftwerke

An overview of technologies for fossil fuel power plants with drastically reduced CO₂ emissions is given. Post combustion capture, Pre combustion capture, and Oxyfuel technology are introduced and compared. Current research results indicate that Post combustion capture may lead to slightly higher losses in power plant efficiency than the two other technologies. However, retrofitting of existing plants with Oxyfuel technology is complex and costly, and retrofitting of Pre combustion capture is not possible. On the other hand, Post combustion capture is suited for retrofitting. Based on the mature technology of reactive absorption, it can be implemented on a large scale in the near future. Therefore, Post combustion capture using reactive absorption is discussed here in some detail.     

CO2驱油化学机理实验研究

由长岩心实验、微观实验以及细管实验结果，认识到了一些新的重要的CO2驱油化学机理：萃取原油中碳轻质化学成分、萃取后混相以及水气交替驱时水对混相影响机理，这些机理的认识对油田二氧化碳驱油应用具有指导意义。     

超临界CO2中CO2参与的化学反应

超临界CO2中的化学反应是目前的研究热点之一。超临界CO2既为反应介质又为反应物的化学反应的主要优势是将萃取与反应相耦合形成均相体系,排除了传质阻力,提高了反应的速率和选择性;产物与催化剂易于分离;同时CO2作为反应原料无毒、不燃、易得,不但可以代替有机溶剂和有毒原材料,而且可有效利用CO2。介绍了超临界CO2作为反应原料所参与化学反应的类型及特点,简单综述了这些化学反应的研究进展,并对未来的发展提出展望。