Effects of Synthesis Temperature and Support Material on CO2 and CH4 Permeation through SAPO-34 Membranes

In this research, continuous SAPO-34 membranes were synthesized via secondary growth method onto both α-Al₂O₃ and mullite supports at three levels of synthesis temperature: 185, 195, and 220°C for 24 h. The synthesized membranes were characterized using XRD and SEM analysis and single gas permeation experiments. It was found out that support material and synthesis temperature both have significant effects on the membrane performance. At higher synthesis temperature, SAPO-34 crystals grown over the mullite support become more uniform and smaller in size but those grown on the α-Al₂O₃ support become larger. Effect of synthesis temperature on single gas permeation properties of the synthesized SAPO-34 membranes was also studied. For the mullite supported membranes, the CH₄ and CO₂ permeances decrease as synthesis temperature increases; but in the case of the alumina supported membranes, by increasing synthesis temperature, CH₄ and CO₂ permeances first decrease up to 195°C and then increase up to 220°C. Even in equal membrane thicknesses, the mullite supported membrane shows lower gas permenaces. Increasing synthesis temperature decreases CO₂/CH₄ ideal selectivity for the α-Al₂O₃ supported membranes, while increases for the mullite supported membranes. Under optimum synthesis conditions, at room temperature and 2 bar feed pressure, the CO₂ permeance through the α-Al₂O₃ and the mullite supported SAPO-34 membranes are 8.2 × 10^?7 and 8.5 × 10^?8 (mol/m² · s · Pa), respectively, and CO₂/CH₄ ideal selectivities are 51 and 61, respectively.     

Synthesis,kinetic study,and reaction mechanism of Li4SiO4 with CO2 in a slurry bubble column reactor

Abstract

This study was performed to investigate the synthesis, kinetic and reaction mechanism of Li₄SiO₄ with CO₂ in a slurry bubble column reactor. The Li₄SiO₄ powder sample was prepared via a solid-state reaction. The sample was characterized via X-ray diffraction (XRD) analysis and verified as a single phase. The median diameter of the sample was measured using the laser diffraction and scattering method as about 20?μm. The synthesized sample was suspended in binary molten carbonate of Li₂CO₃–K₂CO₃ having a molar ratio of 38:62. The experimental results show that Li₄SiO₄ in the slurry bubble column absorbed approximately a stoichiometric amount of CO₂. The kinetic study shows that the CO₂ reaction behavior on the Li₄SiO₄ surface was fitted to a double exponential model and the limiting step of the reaction was lithium diffusion. The mass transfer coefficient of CO₂ and rate constant of reaction with Li₄SiO₄ were studied to understand the overall absorption mechanism in the reactor. The resistance for the direct reaction of CO₂ on the Li₄SiO₄ was much smaller than the resistance for the mass transfer of CO₂ to the Li₄SiO₄. We can conclude that the direct contact of CO₂ with Li₄SiO₄ was the main path for the reaction.     

Effect of Regeneration Temperature on the Composition and Carbon Dioxide Sorption Ability of a K2CO3/Al2O3 Solid Sorbent in a Bubbling Fluidized Bed Reactor

The effect of the regeneration temperature (150°, 250°, and 350°C) during multiple CO₂ cyclic sorption-regeneration cycles of a K₂CO₃/Al₂O₃ solid sorbent in a bubbling fluidized bed reactor was evaluated in terms of the CO₂ capture capacity and chemical composition of the solid sorbent. The CO₂ capture capacity after regeneration at 150° and 250°C decreased with increasing cycle numbers, reaching approximately 57 and 78%, respectively, and 19.0 and 39.3%, respectively, of the original capacity after one and five regeneration cycles. This decline in the CO₂ capture capacity was due to the accumulation of KHCO₃ (at 150°C) and KAl(CO₃)₂(OH)₂ (150° and 250°C) from their incomplete degradation back to the K₂CO₃/Al₂O₃ solid sorbent. When regenerated at 350°C, the CO₂ capture capacity remained essentially constant in each cycle number because of complete desorption (no residual KHCO₃ and KAl(CO₃)₂(OH)₂). The formation mechanism of complex structure occurred similar to the one in a fixed bed reactor/thermogravimetric analyzer with lower regeneration temperature. The general operation conditions for K₂CO₃/Al₂O₃ solid sorbents are summarized.     

Synthesis of Silica Nanofluid and Application to CO2 Absorption

Abstract

This study focused on the synthesis of stable nanofluids and their direct application to the CO₂ absorption process. A sol-gel process was used as the synthesis method of nanoparticles in nanofluid. The particle size and stability were determined by SEM image and zeta potential of the nanofluid. Three types of nanofluids containing approximately 30 nm, 70 nm, and 120 nm particles were synthesized and all nanofluids had a stable zeta potential of approximately ? 45 mV. Addition of nanoparticles increased the average absorption rate of 76% during the first 1 minute and total absorption amount of 24% in water. The capacity coefficient of CO₂ absorption in the nanofluid is 4 times higher than water without nanoparticles, because the small bubble sizes in the nanofluid have large mass transfer areas and high solubility.     

Effects of Elevated CO2 and Temperature on Methane Production and Emission from Submerged Soil Microcosm

Incubation experiments were conducted under controlled laboratory conditions to study the interactive effects of elevated carbon dioxide (CO₂) and temperature on the production and emission of methane (CH₄) from a submerged rice soil microcosm. Soil samples (unamended soil; soil + straw; soil + straw + N fertilizer) were placed in four growth chambers specifically designed for a combination of two levels of temperature (25 °C or 35 °C) and two levels of CO₂ concentration (400 or 800 mol mol^–1) with light intensity of about 3000 Lx for 16 h d^–1. At 7, 15, 30, and 45 d after incubation, CH₄ flux, CH₄ dissolved in floodwater, subsurface soil-entrapped CH₄, and CH₄ production potential of the subsurface soil were determined. The results are summarized as follows: 1) The amendment with rice straw led to a severalfold increase in CH₄ emission rates, especially at 35 °C. However, the CH₄ flux tended to decrease considerably after 15 d of incubation under elevated CO₂. 2) The amount of entrapped CH₄ in subsurface soil and the CH₄ production potential of the subsurface soil were appreciably larger in the soil samples incubated under elevated CO₂ and temperature during the early incubation period. However, after 15 d, they were similar in the soil samples incubated under elevated or ambient CO₂ levels. These results clearly indicated that elevated CO₂ and temperature accelerated CH₄ formation by the addition of rice straw, while elevated CO₂ reduced CH₄ emission at both temperatures.     

Synthesis of soluble oligsiloxane-end-capped hyperbranched polyazomethine and their application to CO2/N2 separation membranes

Three soluble hyperbranched polyazomethines containing oligosiloxane end group HBP-PAZ-SiO_n were successfully synthesized. HBP-PAZ-SiO_ns were used as modifiers of ethyl cellulose (EC) and polysulfone (PS) membranes. Blend membranes, HBP-PAZ-SiO_n/EC and HBP-PAZ-SiO_n/PS were prepared by blending the THF solution of HBP-PAZ-SiO_n with ethanol solution of EC and dichloromethane solution of PS, respectively. Surprisingly, the permeabilities for CO₂ of the blend membranes were more than 15–16 times higher than those of pure EC and PS membranes without any drop of pemselectivity to N₂. This unusual improvement has been achieved by both enhancement of diffusivity for carbon dioxide and nitrogen by the oligosiloxane groups and enhancement of affinity of the amino groups with carbon dioxide at the end groups of HBP-PAZ-SiO_n.     

PREDICTION OF CO2 FREEZE-OUT CONDITIONS AND CO2 HYDRATE FORMATION CONDITIONS WITH A SINGLE EQUATION OF STATE

A recent study by Eggeman and Chaffin (2005 Eggeman , T. , and Chafin , S. ( 2005 ). Beware of pitfalls of CO₂ freezing predictions , Chem. Eng. Prog. , 101 ( 3 ), 39 – 44 . [Google Scholar]), which showed large discrepancies in CO₂ freeze-out conditions as predicted by several commercial simulators, prompted a reexamination of using the TBS equation of state for phase equilibrium calculations involving solids. Salim and Trebble (1994 Salim , P. , and Trebble , M. A. ( 1994 ). Modelling of solid phases in thermodynamic calculations via translation of a cubic equation of state at the triple point , Fluid Phase Equilib. , 93 , 75 – 99 .[Crossref] , [Google Scholar]) had previously presented a methodology for extending the Trebble-Bishnoi-Salim (TBS) equation of state (Salim, 1990 Salim , P. ( 1990 ). A modified Trebble-Bishnoi equation of state, M.Sc. thesis , University of Calgary . [Google Scholar]) to calculations involving a solid phase. In this study, the CO₂ freeze-out conditions in CO₂/CH₄ and CO₂/C₂H₆ mixtures are calculated from the TBS equation of state, and it is shown that they provide a better data fit than the traditional Poynting correction method. Furthermore, since the use of an equation of state in SLE/SVE calculations does not require the explicit assumption of a pure solid phase, the model was assessed for its ability to correlate CO₂ gas hydrate equilibrium conditions. Gas hydrates were simply treated as an impure solid phase, and it was seen that the predictions of gas hydrate equilibrium were in very good agreement with the experimental data. Computationally, the use of the TBS equation of state has the advantage, over the model of Yokozeki (2005 Yokozeki , A. ( 2005 ). Methane gas hydrates viewed through unified solid-liquid-vapour equation of state , Int. J. Thermophys. , 26 ( 3 ), 743 – 765 . [Google Scholar]), that it does not require a modifying factor (c_b) in the repulsive term to handle the presence of hydrates; they are instead handled using a unique binary interaction parameter for the hydrate phase.     

The kinetics of CO2 hydrogenation on a Rh foil promoted by titania overlayers

Submonolayer deposits of titania on a Rh foil have been found to increase the rate of CO₂ hydrogenation. The primary product, methane, exhibits a maximum rate at a TiO _x coverage of 0.5 ML which is a factor of 15 higher than that over the clean Rh surface. The rate of ethane formation displays a maximum which is 70 times that over the unpromoted Rh foil; however, the selectivity for methane remains in excess of 99%. The apparent activation energy for methane formation and the dependence of the rate on H₂ and CO₂ partial pressure have been determined both for the bare Rh surface and the titania-promoted surface. These rate parameters show very small variations as titania is added to the Rh catalyst. The methanation of CO₂ is proposed to start with the dissociation of CO₂ into CO_(a) and O_(a), and then proceed through steps which are identical to those for the hydrogenation of CO. The increase in the rate of CO₂ hydrogenation in the presence of titania is attributed to an interaction between the adsorbed CO, released by CO₂ dissociation, and Ti³⁺ ions located at the edge of TiO _x islands covering the surface. Differences in the effects of titania promotion on the methanation of CO₂ and CO are discussed in terms of the mechanisms that have been proposed for these two reactions.     

Electrochemically Activated CNT Sheet as a Cathode for Zn-CO2 Batteries

High demand for electrochemical storage devices is increasing the need for high-performance batteries. A Zn-CO₂ battery offers a promising solution for CO₂ reduction as well as energy storage applications. For this study, a Zn-CO₂ battery was fabricated using a Carbon Nanotube (CNT) sheet as a cathode and a Zn plate as an anode. The electrochemical activation technique was used to increase the surface area of the CNT electrode by roughly 4.5 times. Copper (Cu) as a catalyst was then deposited onto the activated CNT electrode using electrodeposition method and different Cu loadings were investigated to optimize CO₂ reduction. The final assembled Zn-CO₂ battery has a 1.6 V output voltage at a current density of 0.063 mA/cm², which is higher than most devices reported in the literature. This study demonstrates the importance of activation process which enabled more catalyst loading on the cathode resulted in additional active sites for electroreduction process. This paper presents the activated CNT sheet as a promising cathode material for Zn-CO₂ batteries.     

CO2 Hydrogenation on Rh/TiO2 Previously Reduced at Different Temperatures

Hydrogenation of CO₂ was studied on 1% Rh/TiO₂ reduced at different temperatures. The interaction of CO₂ with the catalyst and that of the CO₂+H₂ mixture was also studied. FTIR and TPD measurements revealed that CO₂ dissociation depends on the reduction temperature of the catalyst. In the surface reaction, besides Rh carbonyl hydride, formate groups and different carbonates and surface formyl species were also formed. The surface concentration of the formyl group depended on the reduction temperature. The initial rate of CO₂ hydrogenation significantly increased with increasing reduction temperature but after some time it drastically decreased. The promotion effect of the reduction temperature was explained by the formation of oxygen vacancies on the perimeter of the Rh/TiO₂ interface, which can be re-oxidized by the adsorption of CO₂ and H₂O.     

Characteristics of CO2 Absorption into Aqueous Ammonia

Abstract

Aqueous ammonia was investigated as a new absorbent of the chemical absorption process for CO₂ capture from combustion flue gas. The effects of the temperature and concentration of aqueous ammonia on CO₂ absorption in a semi‐batch reactor were studied by interpreting breakthrough curves. Raman spectroscopy analysis of CO₂ loaded aqueous ammonia provided concentration changes of bicarbonate, carbonate, and carbamate as well as CO₂ sorption capacity at given time during the absorption with 13 wt% aqueous ammonia at 25°C. It was observed that carbamate formation was dominating at the early stage of absorption. Then, the bicarbonate formation took over the domination at the later stage while the carbonate remained unchanged.     

A Novel Adsorption Cycle for CO2 Recovery: Experimental and Theoretical Investigations of a Temperature Swing Compression Process

Abstract

A novel adsorption cycle is examined experimentally and theoretically for recovering carbon dioxide from a 50 mol% mixture with carbon monoxide. Several adsorbents are considered, and zeolite NaY is chosen for the process due to its high capacity and selectivity for CO₂ in the presence of CO. The process consists of three steps. The bed is fed the gas mixture at 273 K until CO₂ breakthrough occurs. The bed then undergoes countercurrent blowdown of CO₂ while heating at 391 K and is finally cooled to the initial feed temperature once the bed has been depleted of CO₂. Results are presented from laboratory scale experiments and are described using numerical simulations. This novel cycle provides a method for capturing and producing CO₂ without the need for a purge gas and has low energy requirements if waste heat is available.     

Alcohol Synthesis from Syngas over K2CO3/CoS/MoS2 on Activated Carbon

Supported K₂CO₃/Co–MoS₂ on activated carbon was prepared by a co-impregnation technique and has been characterized by X-ray diffraction (XRD) and BET. Active ingredients ranged from 39 to 66% and included molysulfide and cobalt sulfide. XRD analysis indicates that cobalt and molybdenum sulfides are found in the Co₃S₄ and Co₉S₈ phases. These catalysts were performance tested in a fixed-bed reactor under higher alcohol synthesis conditions, 2000–2400 psig and 270–330°C. Active chemicals on the carbon extrudates decreased the surface area dramatically, as measured by BET. Surprisingly, at the high level of active chemicals, alcohol productivity and selectivity were decreased. An increase in the reaction temperature led to a decrease in the selectivity of methanol and an increase in selectivity of hydrocarbons. Total alcohol productivity was also increased as gas hourly space velocity (GHSV) was increased. Co₉S₈ may play a role in the catalyst aging process. In prolonged reaction periods (140 h), sulfur is lost from the surface, possibly as H₂S. The quantity of Co₉S₈ on the surface appears to increase as the catalyst ages.     

A Fourier-transform infrared study of CO2 and CO2/H2 interactions with caesium-doped copper catalysts

FTIR spectra are reported of CO₂ and CO₂/H₂ on a silica-supported caesium-doped copper catalyst. Adsorption of CO₂ on a “caesium”/silica surface resulted in the formation of CO₂ ⁻ and complexed CO species. Exposure of CO₂ to a caesium-doped reduced copper catalyst produced not only these species but also two forms of adsorbed carboxylate giving bands at 1550, 1510, 1365 and 1345 cm⁻¹. Reaction of carboxylate species with hydrogen at 388 K gave formate species on copper and caesium oxide in addition to methoxy groups associated with caesium oxide. Methoxy species were not detected on undoped copper catalyst suggesting that caesium may be a promoter for the methanol synthesis reaction. Methanol decomposition on a caesium-doped copper catalyst produced a small number of formate species on copper and caesium oxide. Methoxy groups on caesium oxide decomposed to CO and H₂, and subsequent reaction between CO and adsorbed oxygen resulted in carboxylate formation. Methoxy species located at interfacial sites appeared to exhibit unusual adsorption properties.     

A study of the oxidative coupling of methane over SrO-La2O3/CaO catalysts by using CO2 as a probe

20%SrO-20%La₂O₃/CaO catalyst (SLC-2), prepared by impregnation, has shown 18% CH₄ conversion and 80% C₂-selectivity for the oxidative coupling of methane (OCM) at 1073–1103 K with CH₄O₂ molar ratio=91 and total flow rate of 100 ml/min. Addition of SrO onto La₂O₃/CaO (LC) catalyst strengthens the surface basicity and leads to an increase in CH₄ conversion and C₂-selectivity. Meanwhile, the reaction temperature required to obtain the highest C₂-yield increases with increasing SrO content. The formation of carbonate on the catalyst surface is the main reason for the deactivation of LC and SLC catalysts. If the amount of CO₂ added into the feed is appropriate and the reaction temperature is high enough, there is no deactivation at all. In such case, the added CO₂ will suppress the formation of CO₂ produced via the OCM reaction, therefore, improves the C₂-selectivity. The FT-IR spectra of CO₂ adspecies recorded at different temperatures show that CO₂ interacts easily with the catalyst surface to form different carbonate adspecies. Unidentate carbonate is the main CO₂ adspecies formed on the catalyst surface. On the LC catalyst surface, the unidentate carbonate was first formed on Ca²⁺ cations at room temperature. If the temperature is higher than 473 K, it will form on La³⁺ cations. On the SLC catalyst surface, if the temperature is lower than 573 K, only the unidentate carbonate formed on Ca²⁺ cations could be observed. When the temperature is higher than 673 K, it will then form on Sr²⁺ cations. This suggests that the unidentate carbonate can migrate on the LC and SLC catalyst surface on one hand, and on the other hand, that the surface composition of SLC catalysts is dynamic in nature. On the basis of both the decomposition temperatures of the carbonate species, and the temperature dependence of the value which is the difference of symmetric and asymmetric stretching frequencies of surface carbonates, the in situ FT-IR technique offered two approaches to measure the surface basicity of the SLC catalyst. The results thus obtained are in good agreement with that of CO₂-TPD. The role of the surface basicity of the SLC catalyst is also discussed.     

Pathways for CO2 Formation and Conversion During Fischer–Tropsch Synthesis on Iron-Based Catalysts

CO₂ reaction and formation pathways during Fischer–Tropsch synthesis (FTS) on a co-precipitated Fe–Zn catalyst promoted with Cu and K were studied using a kinetic analysis of reversible reactions and with the addition of ¹³C-labeled and unlabeled CO₂ to synthesis gas. Primary pathways for the removal of adsorbed oxygen formed in CO dissociation steps include reactions with adsorbed hydrogen to form H₂O and with adsorbed CO to form CO₂. The H₂O selectivity for these pathways is much higher than that predicted from WGS reaction equilibrium; therefore readsorption of H₂O followed by its subsequent reaction with CO-derived intermediates leads to the net formation of CO₂ with increasing reactor residence time. The forward rate of CO₂ formation increases with increasing residence time as H₂O concentration increases, but the net CO₂ formation rate decreases because of the gradual approach to WGS reaction equilibrium. CO₂ addition to synthesis gas does not influence CO₂ forward rates, but increases the rate of their reverse steps in the manner predicted by kinetic analyses of reversible reactions using non-equilibrium thermodynamic treatments. Thus the addition of CO₂ could lead to the minimization of CO₂ formation during FTS and to the preferential removal of oxygen as H₂O. This, in turn, leads to lower average H_2/CO ratios throughout the catalyst bed and to higher olefin content and C₅₊ selectivity among reaction products. The addition of ¹³CO₂ to H₂/¹²CO reactants did not lead to significant isotopic enrichment in hydrocarbon products, indicating that CO₂ is much less reactive than CO in chain initiation and growth. We find no evidence of competitive reactions of CO₂ to form hydrocarbons during reactions of H₂/CO/CO₂ mixtures, except via gas phase and adsorbed CO intermediates, which become kinetically indistinguishable from CO₂ as the chemical interconversion of CO and CO₂ becomes rapid at WGS reaction equilibrium.     

Isolation of a new highly CO2 tolerant fresh water MicroalgaChlorella sp. KR-1

A fresh water microalga, which has tolerance to high concentrations of CO₂, was isolated. The KR-1 strain was identified as a genusChlorella. ThoughChlorella KR-1 showed maximum growth at 10 % (v/v) CO₂, the strain showed a good growth rate up to 50 % (v/v) CO₂. The results indicated the feasibility of the KR-1 strain for massive cultivation using condensed stack gases.     

Methanol Synthesis from CO2 Using Skeletal Copper Catalysts Containing Co-precipitated Cr2O3 and ZnO

Skeletal Cu-Cr₂O₃-ZnO catalysts have been prepared by leaching CuAl₂ alloy particles at 273 K using 6.1 M aqueous NaOH solutions containing sodium chromate (Na₂CrO₄) and sodium zincate (Na₂Zn(OH)₄). The presence of sodium chromate and sodium zincate in the caustic solution was found to affect the pore structure and surface areas of the resulting catalysts. Both BET and Cu surface areas were increased by increasing the concentration of Na₂CrO₄ and of Na₂Zn(OH)₄.Increasing the Na₂CrO₄ level from 0 to 0.06 M in a 6.1M NaOH solution containing 0.2M Na₂Zn(OH)₄ caused the content of ZnO in the catalyst to decrease from 8.8 to 3.0 wt% whilst increasing the Cr₂ O₃ content from 0 to 1.7 wt%, indicating that the presence of Na₂CrO₄ in the leach liquor not only resulted in deposition of a Cr compound but also inhibited precipitation of zinc hydroxide onto skeletal Cu catalysts. On the other hand, increasing the concentration of Na₂Zn(OH)₄ from 0 to 0.6 M in a 6.1 M NaOH solution containing 0.008 M Na₂ CrO₄ resulted in increasing the ZnO loading from 0 to 8.9wt% with an almost constant content of Cr₂ O₃ (1.3 ± 0.2%) in the catalysts, revealing that sodium zincate only led to precipitation of zinc hydroxide and did not suppress Cr₂O₃ formation.Hydrogenation of CO₂ was studied using a gas mixture of 24% CO₂ in H₂ at a total pressure of 4MPa, space velocities up to 210000L kg^-1h^-1 and temperatures in the range 493-533K. The catalysts were found to be both highly active and selective for methanol synthesis. This study confirms the role of ZnO in promoting the activity of copper for methanol synthesis from CO₂ and improving the selectivity by inhibiting the reverse water-gas shift reaction. The role of Cr₂O₃ is to improve the structural development of high surface area skeletal copper.