Kinetics measurements and in situ Raman spectroscopy of formation of hydrogen–tetrabutylammonium bromide semi-hydrates

The kinetics of formation of H₂–TBAB semi-clathrate hydrates was studied in this work in order to elucidate their potential for H₂ storage. The influence of pressure (5–16 MPa), TBAB concentration (2.6 mol% and 3.7 mol%) and formation method (T-cycle method and T-constant method) on the hydrate nucleation, hydrate growth and H₂ storage capacity was determined. The results showed that kinetics is favored at higher pressures and solute concentrations. Additionally, the hydrate phase formation and dissociation was study for a solution of 2.6 mol% of TBAB in situ by using the Raman spectroscopy technique. The inclusion of H₂ in the semi-hydrate phase was confirmed. The results showed the importance of H₂ mass transfer on the storage capacity of the H₂–TBAB semi-hydrates.     

High-efficiency separation of a CO2/H2 mixture via hydrate formation in W/O emulsions in the presence of cyclopentane and TBAB

CO₂/H₂ mixtures, such as integrated gasification combined cycle (IGCC) syngas, were separated via hydrate formation in water-in-oil (W/O) emulsions. The oil phase was composed of diesel and cyclopentane (CP). Span 20 was used to disperse the aqueous phase or hydrate in the oil phase, and tetra-n-butyl ammonium bromide (TBAB) was added to produce a synergistic effect with CP. The experimental results show that the presence of TBAB can remarkably increase the separation ability and improve the flow behavior of the hydrate slurry. The most suitable contents of TBAB in the aqueous phase and water in the emulsion were determined to be 0.29 mol% and 35 vol%, respectively. The maximum separation factor of CO₂ over H₂ was 103, which is much higher than the literature values for separating CO₂/H₂ gas mixture via hydrate formation. After a two-stage separation, hydrogen was enriched from 53.2 to 97.8 mol%. The influence of temperature, pressure, and the initial gas–liquid volume ratio on the separation ability and hydrate formation rate were investigated in detail. In addition, a criterion for choosing the suitable operation conditions was suggested based on both phase equilibrium and kinetic factors. Based on this criterion, the suitable operation temperature, pressure, and gas–liquid volume ratio for the separation of CO₂/H₂ are approximately 270.15 K, 3–5 MPa, and 80–100, respectively.     

Gas hydrate formation process for pre-combustion capture of carbon dioxide

In this study, gas hydrate from CO₂/H₂ gas mixtures with the addition of tetrahydrofuran (THF) was formed in a semi-batch stirred vessel at various pressures and temperatures to investigate the CO₂ separation/recovery properties. This mixture is of interest to CO₂ separation and recovery from Integrated Gasification Combine Cycle (IGCC) power plants. During hydrate formation the gas uptake was determined and composition changes in the gas phase were obtained by gas chromatography. The impact of THF on hydrate formation from the CO₂/H₂ was observed. The addition of THF significantly reduced the equilibrium formation conditions. 1.0 mol% THF was found to be the optimum concentration for CO₂ capture based on kinetic experiments. The present study illustrates the concept and provides thermodynamic and kinetic data for the separation/recovery of CO₂ (pre-combustion capture) from a fuel gas (CO₂/H₂) mixture.     

Tetra-n-butyl ammonium bromide semi-clathrate hydrate process for post-combustion capture of carbon dioxide in the presence of dodecyl trimethyl ammonium chloride

To determine the appropriate operating conditions for separating carbon dioxide from flue gas via the hydrate formation, the effects of the concentrations of dodecyl trimethyl ammonium chloride (DTAC) in 0.29 mol% Tetra-n-butyl ammonium bromide (TBAB) aqueous solution and the initial pressures on the induction time of the hydrate formation and CO₂ separation efficiency are investigated. The experiments are conducted at the DTAC concentration range of 0–0.056 mol%, initial pressures range of 0.66 MPa–2.66 MPa and temperature range of 274.95 K–277.15 K. The results indicate that the initial pressure of 1.66 MPa in conjunction with the concentration of 0.028 mol% DTAC is most favorable for CO₂ separation. At the condition, the induction time of forming the hydrate can be shortened considerably and CO₂ can be purified from 17.0 mol% to 99.4% with the two-stage hydrate separation process. CO₂ split fractions for Stage 1 and Stage 2 are 0.54 and 0.39, respectively, and the separation factors are 9.60 and 62.25, respectively.     

Hydrate-based pre-combustion carbon dioxide capture process in the system with tetra-n-butyl ammonium bromide solution in the presence of cyclopentane

Effects of 0.29 mol% tetra-n-butyl ammonium bromide (TBAB) solution in conjunction with cyclopentane (CP) on the hydrate-based pre-combustion CO₂ capture are investigated by the measurements of the gas uptakes, CO₂ separation efficiencies and induction time of the hydrate formation at the different temperature-pressure conditions. The results show that the volume of the TBAB has an effect on the CO₂ separation and the induction time, and the addition of the CP into the TBAB solution remarkably enhances the CO₂ separation and shortens the induction time. The system with the CP/TBAB solution volume ratio of 5 vol% and TBAB solution/reactor effective volume ratio of 0.54 is optimum to obtain the largest gas uptake and the highest CO₂ separation efficiency at 274.65 K and 4.0 MPa. Compared to the results with tetrahydrofuran (THF) as an additive [1], the gas uptake is enhanced by at least 2 times and the induction time is shortened at least 10 times at the similar temperature-pressure condition. In addition, the CO₂ concentration in the decomposed gas from the hydrate slurry phase reaches approximately 93 mol% after the first-stage separation at 274.65 K and 2.5 MPa. The gas uptakes of more than 80 mol% are obtained after 400 s at the temperature range of 274.65-277.65 K and the pressure range of 2.5-4.5 MPa.     

Macroscopic kinetics of hydrate formation of mixed hydrates of hydrogen/tetrahydrofuran for hydrogen storage

Hydrogen/Tetrahydrofuran mixed hydrate formation studies were conducted in a stirred tank reactor. Hydrate formation kinetics at driving forces of 2 MPa, 5 MPa and 7 MPa were studied. Tetrahydrofuran (THF) concentration was varied between 1 mol% and 5 mol% and its effect on hydrate formation kinetics was investigated. With an increase in the driving force there was an increase in gas uptake till super saturation. However, the increased driving force had little effect on the reduction of induction time even at high promoter (THF) concentration. 5 mol% THF solutions behave distinctly exhibiting an increased hydrate growth compared to low promoter concentrations due to the occurrence of multiple nucleation events (observed based on temperature spikes and gas uptake). Rate of hydrate formation increased with an increase in driving force for a given concentration of THF promoter. Water conversion to hydrates in the range of 2.8–10.8% was achieved for all the experiments. Addition of Sodium Dodecyl Sulphate (SDS) surfactant had no effect in improving the kinetics of mixed hydrogen/THF hydrates. This study highlights the kinetic challenges that need to be overcome in storing hydrogen as clathrate hydrates.     

Synergic effect of cyclopentane and tetra-n-butyl ammonium bromide on hydrate-based carbon dioxide separation from fuel gas mixture by measurements of gas uptake and X-ray diffraction patterns

The synergic effect of Cyclopentane (CP) and Tetra-n-butyl Ammonium Bromide (TBAB) on the hydrate-based carbon dioxide (CO₂) separation from IGCC (Integrated Gasification Combined Cycle) syngas is investigated by measuring the gas uptake and the power X-ray diffraction (PXRD) patterns in this work. The CP with CP/TBAB solution ratio of 5 vol% added into the 0.29 mol% TBAB solution can remarkably increase the gas uptake at 4.0 MPa and 274.65 K. The PXRD patterns of the semi-clathrate (sc) hydrate and structure II (sII) hydrate are obtained for the CP/TBAB/gas/H₂O system. The synergic effect of the CP and the TBAB includes two aspects: On one hand, the CP molecules housed in the hollow centers of the large cavities together with TBAB cations (TBA⁺) make the sc hydrate more stable. On the other hand, the TBA⁺ displaced out of the large cavities by the CP molecules make the ionization reaction of TBAB in the solution going toward the reverse direction. Thus, the more TBAB molecules exist in the solution and form the more sc hydrate, resulting in the considerable increase of the gas uptake.     

Hydrate-based separation of the CO2 + H2 mixtures. Phase equilibria with isopropanol aqueous solutions and hydrogen solubility in CO2 hydrate

The gas hydrates' ability to preferentially bind one of the components of a gas mixture into a hydrate state makes it possible to consider hydrate-based technology as promising for the separation of gas mixtures. When a hydrate is obtained from a gas mixture, mixed hydrates with a complex composition inevitably occur. Issue of their composition determination stays apart. This a rather difficult task, which is complicated by the dissolution of small molecules such as hydrogen in the hydrate phase. This, in turn, impedes the analysis of the data obtained. In this work, the solubility of hydrogen in carbon dioxide hydrate in the range of 269.7–275.7 K and at partial H₂ pressure up to 4.5 MPa was experimentally determined. Hydrate composition was found to be CO₂·(0.01X)H₂·6.5H₂O at H₂ pressure of X MPa. The equilibrium conditions of hydrates formation in the systems of H₂O – CO₂ – H₂ and H₂O – 2-propanol – CO₂ – H₂ were also determined in a wide range of hydrogen concentrations. Hydrogen seems to be an indifferent diluent gas regarding CO₂ hydrate equilibrium pressure. The compositions of the equilibrium phases have been determined as well. It was shown that isopropanol does not form a double hydrate with СО₂, only sI СО₂ hydrate occurred in the studied systems. The obtained dependencies will be useful in analyzing the process of СО₂ + Н₂ gas mixtures separation by the hydrate-based method.     

The energy efficiency of carbon dioxide fixation by a hydrogen-oxidizing bacterium

Fixing carbon dioxide (CO₂) with solar hydrogen (H₂) is a novel alternative to conventional photosynthesis of plants and microalgae. The energy efficiency of CO₂ fixation by a hydrogen-oxidizing bacterium was investigated in a closed reactor system. The molar ratio of consumed H₂ and CO₂ was measured under mass transfer limitation in atmospheres of sufficient H₂, low CO₂, and a broad range of O₂. The energy efficiency, ranging from 10% to 60%, was primarily affected by the oxygen concentration (6–30 mol%). The research revealed a clear trend that a low oxygen concentration gave high energy efficiency, but slow gas consumption. A high energy efficiency of 50% was measured under a moderate oxygen concentration (10 mol%). Based on 10% solar hydrogen efficiency, a 5% overall efficiency from solar energy to biomass can therefore be achieved.     

Syngas production from catalytic gasification of waste polyethylene: Influence of temperature on gas yield and composition

The catalytic steam gasification of waste polyethylene (PE) from municipal solid waste (MSW) to produce syngas (H₂ + CO) with NiO/γ-Al₂O₃ as catalyst in a bench-scale downstream fixed bed reactor was investigated. The influence of the reactor temperature on the gas yield, gas composition, steam decomposition, low heating value (LHV), cold gas efficiency and carbon conversion efficiency was investigated at the temperature range of 700–900 °C, with a steam to waste polyethylene ratio of 1.33. Over the ranges of experimental conditions examined, NiO/γ-Al₂O₃ catalyst revealed better catalytic performance as a view of increasing product gas yield and of decreasing char and liquid yields in the presence of steam. Higher temperature resulted in more H₂ and CO production, higher carbon conversion efficiency and product gas yield. The highest syngas (H₂ + CO) content of 64.35 mol%, the highest H₂ content of 36.98 mol%, and the highest CO content of 27.37 mol%, were achieved at the highest temperature level of 900 °C. Syngas produced with a H₂/CO molar ratio in the range of 0.83–1.35, was highly desirable as feedstock for Fischer–Tropsch synthesis for the production of transportation fuels.     

Hydrate-based removal of carbon dioxide and hydrogen sulphide from biogas mixtures: Experimental investigation and energy evaluations

This paper presents an experimental study on the application of gas hydrate technology to biogas upgrading. Since CH₄, CO₂ and H₂S form hydrates at quite different thermodynamic conditions, the capture of CO₂ and H₂S by means of gas hydrate crystallization appears to be a viable technological alternative for their removal from biogas streams. Nevertheless, hydrate-based biogas upgrading has been poorly investigated. Works found in literature are mainly at a laboratory scale and concern with thermodynamic and kinetic fundamental studies. The experimental campaign was carried out with an up-scaled apparatus, in which hydrates are produced in a rapid manner, with hydrate formation times of few minutes. Two types of mixtures were used: a CH₄/CO₂ mixture and a CH₄/CO₂/H₂S mixture. The objective of the investigation is to evaluate the selectivity and the separation efficiency of the process and the role of hydrogen sulphide in the hydrate equilibrium. Results show that H₂S can be captured along with CO₂ in the same process. The maximum value of the separation factor, defined as the ratio between the number of moles of CO₂ and the number of moles of CH₄ removed from the gas phase, is 11. In the gas phase, a reduction of CO₂ of 24.5% in volume is achievable in 30 min.Energy costs of a real 30-min separation process, carried out in the experimental campaign, are evaluated and compared with those obtained from theoretical calculations. Some aspects for technology improvement are discussed.     

Thermodynamic analysis of steam assisted conversions of bio-oil components to synthesis gas

The aim of this study is to investigate the thermodynamics of steam assisted, high-pressure conversions of model components of bio-oil – isopropyl alcohol, lactic acid and phenol – to synthesis gas (H₂ + CO) and to understand the effects of process variables such as temperature and inlet steam-to-fuel ratio on the product distribution. For this purpose, thermodynamic analyses are performed at a pressure of 30 bar and at ranges of temperature and steam-to-fuel ratio of 600–1200 K and 4–9, respectively. The number of moles of each component in the product stream and the product composition at equilibrium are calculated via Gibbs free energy minimization technique. The resulting optimization problems are solved by using the Sequential quadratic programming method. The results showed that all of the model fuels reached near-complete conversions to H₂, CO, CO₂ and CH₄ within the range of operating conditions. Temperature and steam-to-fuel ratio had positive effects in increasing hydrogen content of the product mixture at different magnitudes. Production of CO increased with temperature, but decreased at high steam-to-fuel ratios. Conversion of model fuels in excess of 1000 K favored molar H₂/CO ratios around 2, the synthesis gas composition required for Fischer–Tropsch and methanol syntheses. It was also possible to adjust the H₂/CO ratios and the amounts of CH₄ and CO₂ in synthesis gas by steam-to-fuel ratio, the value depending on temperature and the fuel type. Product distribution trends indicated the presence of water–gas shift and methanation equilibria as major side reactions running in parallel with the steam reforming of the model hydrocarbons.     

Electrochemical separation of hydrogen from reformate using PEM fuel cell technology

This article is an examination of the feasibility of electrochemically separating hydrogen obtained by steam reforming a hydrocarbon or alcohol source. A potential advantage of this process is that the carbon dioxide rich exhaust stream should be able to be captured and stored thereby reducing greenhouse gas emissions. Results are presented for the performance of the anode of proton exchange membrane (PEM) electrochemical cell for the separation of hydrogen from a H₂–CO₂ gas mixture and from a H₂–CO₂–CO gas mixture. Experiments were carried out using a single cell state-of-the-art PEM fuel cell. The anode was fed with either a H₂–CO₂ gas mixture or a H₂–CO₂–CO gas mixture and hydrogen was evolved at the cathode. All experiments were performed at room temperature and atmospheric pressure. With the H₂–CO₂ gas mixture the hydrogen extraction efficiency is quite high. When the gas mixture included CO, however, the hydrogen extraction efficiency is relatively poor. To improve the efficiency for the separation of the gas mixture containing CO, the effect of periodic pulsing on the anode potential was examined. Results show that pulsing can substantially reduce the anode potential thereby improving the overall efficiency of the separation process although the anode potential of the CO poisoned and pulsed cell still lies above that of an unpoisoned cell.     

Design of a novel biohythane process with high H2 and CH4 production rates

A biohythane process based on wheat straw including: i) pretreatment, ii) H₂ production using Caldicellulosiruptor saccharolyticus, iii) CH₄ production using an undefined consortium, and iv) gas upgrading using an amine solution, was assessed through process modelling including cost and energy analysis. According to simulations, a biohythane gas with the composition 46–57% H₂, 43–54% CH₄ and 0.4% CO₂, could be produced at high production rates (2.8–6.1 L/L/d), with 93% chemical oxygen demand (COD) reduction, and a net energy yield of 7.4–7.7 kJ/g dry straw. The model was calibrated and verified using experimental data from dark fermentation (DF) of wheat straw hydrolysate, and anaerobic digestion of DF effluent. In addition, the effect of gas recirculation was investigated by both wet experiments and simulation. Sparging improved H₂ productivities and yields, but negatively affected the net energy gain and cost of the overall process.     

CO2 sequestration in depleted methane hydrate deposits with excess water

The recent increase in atmospheric CO₂ concentration makes it necessary to investigate new ways to reduce CO₂ emissions. Simultaneously, natural gas hydrate mining technology is developing rapidly. The use of depleted methane hydrate (MH) deposits as potential sites for CO₂ storage is relatively safe and economical. This method can alleviate the shortage of hydrate displacement gas with CO₂. The purpose of this study was to investigate CO₂ hydrate formation characteristics during the seepage process—in reservoirs with excess water—and their effect on CO₂ storage. The experimental process can be divided into 5 parts: MH formation, water injection, MH dissociation, CO₂ hydrate formation, and CO₂ hydrate dissociation. Magnetic resonance imaging was employed to monitor the distribution of liquid water, and the effects of different parameters on the formation and dissociation of CO₂ hydrates were analyzed. It was found that a state of initial water saturation can effectively control hydrate saturation in artificial MH reservoirs for hydrate reservoirs with excess gas. In the process of CO₂ flow, initial water saturation was not the main controlling factor for CO₂ hydrate formation. Increasing the flow pressure and reducing the flow rate were beneficial for CO₂ hydrate formation. This study is of great significance for advancing the science of CO₂ geological storage in the form of deep‐sea hydrates.     

CO2 separation from the mixture of CO2/H2 using gas hydrates: Experimental and modeling

Hydrate formation is a new technique to separate hydrogen from carbon dioxide. In this way, modeling and prediction of gas hydrate kinetics is very important. Several experiments have been conducted to study the hydrate formation from pure carbon dioxide and mixture of hydrogen and carbon dioxide in a stirred reactor in different temperatures, pressures and compositions. The mass transfer approach model was used to predict the mass transfer coefficient for each experiment, and the dependency of temperature and pressure has been studied. It was observed that the mass transfer coefficient of CO₂ in the mixture is close to the pure system. The result of this work shows that the pure data on the kinetics for CO₂ hydrate formation is applicable for the case of CO₂ separation from the mixture of carbon dioxide and hydrogen.     

Enhanced reversible hydrogen storage performance of NbCl5 doped 2LiH–MgB2 composite

2LiBH₄ + MgH₂ system is considered as an attractive candidate for reversible hydrogen storage with high capacity and favorable thermodynamics. However, its reaction kinetics has to be further improved for the practical application. In this work, we investigated the effect of NbCl₅ additive on the de/hydrogenation kinetics and microstructure refinement in 2LiH–MgB₂ composite systematically. The hydrogenation and dehydrogenation kinetics of 2LiH–MgB₂ composite can be significantly enhanced with the increase of NbCl₅ content. The 3 mol% NbCl₅ doped 2LiH–MgB₂ composite exhibits the superior reversible hydrogen storage performance, which requires 50 min to uptake 9.0 wt% H₂ at 350 °C and release 8.5 wt% H₂ at 400 °C, respectively. In contrast, the undoped 2LiH–MgB₂ sample uptakes 6.2 wt% H₂ and releases 3.1 wt% H₂ under identical measurement conditions. Moreover, the 3 mol% NbCl₅ doped 2LiH–MgB₂ composite can release more than 9.0 wt% H₂ within 300 min at 400 °C without obvious degradation of capacity over the first 10 cycles. Microstructure analyses clearly indicate that NbCl₅ additive first reacts with LiH to form Nb and LiCl during ball-milling process, and then NbH is formed after the first hydrogenation and stabilized upon further de/hydrogenation cycling. The well-distributed NbH active species play an important role in the improvement of de/hydrogenation kinetics for Li–Mg–B–H system through facilitating hydrogen diffusion rapidly as well as prevent the particles from further growth in the subsequent hydrogenation and dehydrogenation processes.     

Hydrogen production from biogas reforming and the effect of H2S on CH4 conversion

Biogas produced during anaerobic decomposition of plant and animal wastes consists of high concentrations of methane (CH₄), carbon dioxide (CO₂) and traces of hydrogen sulfide (H₂S). The primary focus of this research was on investigating the effect of a major impurity (i.e., H₂S) on a commercial methane reforming catalyst during hydrogen production. The effect of temperature on CH₄ and CO₂ conversions was studied at three temperatures (650, 750 and 850 °C) during catalytic biogas reforming. The experimental CH₄ and CO₂ conversions thus obtained were found to follow a trend similar to the simulated conversions predicted using ASPEN plus. The gas compositions at thermodynamic equilibrium were estimated as a function of temperature to understand the intermediate reactions taking place during biogas dry reforming. The exit gas concentrations as a function of temperature during catalytic reforming also followed a trend similar to that predicted by the model. Finally, catalytic reforming experiments were carried out using three different H₂S concentrations (0.5, 1.0 and 1.5 mol%). The study found that even with the introduction of small amount of H₂S (0.5 mol%), the CH₄ and CO₂ conversions dropped to about 20% each as compared to 65% and 85%, respectively in the absence of H₂S.     

Sequential dark–photo fermentation and autotrophic microalgal growth for high-yield and CO2-free biohydrogen production

Dark fermentation, photo fermentation, and autotrophic microalgae cultivation were integrated to establish a high-yield and CO₂-free biohydrogen production system by using different feedstock. Among the four carbon sources examined, sucrose was the most effective for the sequential dark (with Clostridium butyricum CGS5) and photo (with Rhodopseudomonas palutris WP3-5) fermentation process. The sequential dark–photo fermentation was stably operated for nearly 80 days, giving a maximum H₂ yield of 11.61 mol H₂/mol sucrose and a H₂ production rate of 673.93 ml/h/l. The biogas produced from the sequential dark–photo fermentation (containing ca. 40.0% CO₂) was directly fed into a microalga culture (Chlorella vulgaris C–C) cultivated at 30 °C under 60 μmol/m²/s illumination. The CO₂ produced from the fermentation processes was completely consumed during the autotrophic growth of C. vulgaris C–C, resulting in a microalgal biomass concentration of 1999 mg/l composed mainly of 48.0% protein, 23.0% carbohydrate and 12.3% lipid.     

Mesoporous CexMn1−xO2 composites as novel alternative carriers of supported Co3O4 catalysts for CO preferential oxidation in H2 stream

The mesoporous Co₃O₄ supported catalysts on Ce–M–O (M = Mn, Zr, Sn, Fe and Ti) composites were prepared by surfactant-assisted co-precipitation with subsequent incipient wetness impregnation (SACP–IWI) method. The catalysts were employed to eliminate trace CO from H₂-rich gases through CO preferential oxidation (CO PROX) reaction. Effects of M type in Ce–M–O support, atomic ratio of Ce/(Ce + Mn), Co₃O₄ loading and the presence of H₂O and CO₂ in feed were investigated. Among the studied Ce–M–O composites, the Ce–Mn–O is a superior carrier to the others for supported Co₃O₄ catalysts in CO PROX reaction. Co₃O₄/Ce_0.9Mn_0.1O₂ with 25 wt.% loading exhibits excellent catalytic properties and the 100% CO conversion can be achieved at 125–200 °C. Even with 10 vol.% H₂O and 10 vol.% CO₂ in feed, the complete CO transformation can still be maintained at a wide temperature range of 190–225 °C. Characterization techniques containing N₂ adsorption/desorption, X-ray diffraction (XRD), H₂ temperature-programmed reduction (H₂-TPR) and scanning electron microscopy (SEM) were employed to reveal the relationship between the nature and catalytic performance of the developed catalysts. Results show that the specific surface area doesn’t obviously affect the catalytic performance of the supported cobalt catalysts, but the right M type in carrier with appropriate amount effectively improves the Co₃O₄ dispersibility and the redox behavior of the catalysts. The large reducible Co³⁺ amount and the high tolerance to reduction atmosphere resulted from the interfacial interaction between Co₃O₄ and Ce–Mn support may significantly contribute to the high catalytic performance for CO PROX reaction, even in the simulated syngas.