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).     

Kinetics of CO2 Adsorption/Desorption of Polyethyleneimine‐Mesoporous Silica

The kinetics of adsorption of CO₂ on solid sorbents based on polyethyleneimine/mesoporous silica (PEI/MPS) was studied by following the mass gain during CO₂ flow. Linear (PEI‐423) and branched (PEI‐10k) polymers were studied. The solid sorbents were synthesized by impregnating the PEI into MPS foam. The kinetics of adsorption was fitted with a double‐exponential model. In contrast, the desorption process obeyed first‐order kinetics. The activation energy of desorption of PEI‐423 was lower than that of PEI‐10k, presumably because the branched polymer required more energy to expose its nitrogen to CO₂. To increase the CO₂ sorption capacity, the MPS was treated with nonionic surfactant materials prior to impregnation with PEI. This also lowered the maximum sorption temperature and desorption activation energies.     

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.     

Innovative Carbon Dioxide‐Capturing Organic Solvent: Reaction Mechanism and Kinetics

The reaction rates of CO₂ with an innovative CO₂‐capturing organic solvent (CO₂COS), consisting of blends of 2‐tert‐butyl‐1,1,3,3‐tetramethylguanidine (BTMG) and 1‐propanol, were obtained as function of BTMG concentration and temperature. A stopped‐flow apparatus with conductivity detection was used. The reaction was modeled by means of a modified termolecular reaction mechanism which resulted in a second‐order rate constant, and activation energies were calculated for a defined temperature range. Quantum chemical calculations at the B3LYP/6‐31G(d) level also produced the activation energy of this reaction system which strongly supports the experimental findings.     

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.     

Synthesis of V‐MCM‐41 Catalysts and Their Application in CO2‐Assisted Isobutane Dehydrogenation

A series of V‐MCM‐41 were in situ synthesized. The textual properties and vanadium species were systematically characterized. The results showed that the catalysts could still maintain a mesoporous structure and high specific surface area. The vanadium species were highly dispersed on the surface, with the coexistence of polymeric vanadium species. Isobutane dehydrogenation was performed by employing CO₂ as a soft oxidant. The results revealed an almost linear relationship between the activity and surface vanadium sites. Isolated vanadium species were more active for isobutane dehydrogenation. The isobutane dehydrogenation reaction in the presence of CO₂ could proceed simultaneously through oxidative dehydrogenation and non‐oxidative dehydrogenation followed by the reverse water gas shift reaction.     

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.     

Dynamic Methanation of CO2 – Effect of Concentration Forcing

Investigating the effect of concentration forcing of the CO₂ methanation is not only relevant for power‐to‐gas plants but also for the study of dynamic phenomena of this reaction. In this study a Ni/Al₂O₃ catalyst is investigated under concentration forcing at industrially relevant conditions. The dynamic experiments allow an evaluation in terms of the reaction rate and enable the study of the reaction mechanism. The experiments show that no methane is formed in the CO₂‐rich part of the cycle, whereas a fast hydrogenation of carbonaceous species to methane takes place upon switching to H₂.     

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.     

Synthesis of High‐Performance Pebax®‐1074/DD3R Mixed‐Matrix Membranes for CO2/CH4 Separation

This work deals with the incorporation of deca‐dodecasil 3 rhombohedral (DD3R) zeolite as an inorganic filler into the Pebax®‐1074‐based polymer matrix to enhance the performance of the pure polymeric membrane in CO₂/CH₄ separation. The membranes were fabricated with different concentrations of DD3R. Separation performances of the membranes were investigated at various feed pressures and temperatures. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC) analysis of the prepared membranes were performed. In the best case, selectivity for CO₂/CH₄ separation was improved, while the permeability decreased. Membranes with 1 and 5 wt % DD3R were located in the acceptable region beyond the Robeson plot (1991) for CO₂/CH₄ gas pairs.     

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     

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.     

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.     

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.     

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.     

CO/CO2加氢制芳烃的研究进展

将CO/CO₂直接转化为芳烃是一种极具挑战性的非石油路线合成途径。本文主要对CO/CO₂通过不同反应途径制取芳烃过程中复合催化剂的开发和反应机理的研究进展进行了综述。阐述了利用反应耦合思想,构筑的复合催化剂在CO/CO₂的高效转化和产物调控等方面取得了突破性的进展。重点介绍了复合催化剂用于CO加氢制芳烃主要的两种反应途径,活性金属的类别、分子筛的结构与酸性和活性组分的组装方式与接触度对CO₂加氢制芳烃催化性能的影响。指出协同加氢与芳构化反应活性的匹配是影响催化剂性能的关键。提出开发高效稳定的催化剂用于提高CO/CO₂的转化率和芳烃产物的产率以及反应机理的探索仍然是未来研究的重点。