Tetra-n-butylphosphonium Bromide Research Articles

Molecular Dynamics Simulation to Evaluate the Stability of Tetra-n-butyl Ammonium/Phosphonium Bromide Semiclathrate Hydrates in the Presence and Absence of Methane, Carbon Dioxide, Methanol, and Ethanol Molecules

Stability of tetra-n-butyl ammonium bromide (TBAB) and tetra-n-butyl phosphonium bromide (TBPB) semiclathrate hydrates with a hydration number of 38 is studied by the molecular dynamics (MD) simulation in the absence and presence of methane, carbon dioxide, methanol, and ethanol molecules. All of the simulation runs are performed under NVT (constant number of atoms, volume, and temperature) and NPT (constant number of atoms, pressure, and temperature) conditions using optimized potentials for liquid simulations-all-atom (OPLS-AA) force field with nonpolarizable water models (including SPC, TIP3P, TIP4P, and TIP4P/Ice), where the operating conditions include a temperature range of 250–350 K and pressures of 0.1 and 50 MPa. The potential energy and structural analysis, mean square displacement (MSD), diffusion coefficient, radial distribution function (RDF), lattice parameter, and the average number of hydrogen bonds between water–water molecules are evaluated by employing MD strategy. According to the potential energy results, the order for the water models in both TBAB and TBPB hydrate stabilities is as follows: TIP4P/Ice > TIP4P ≈ SPC > TIP3P. This order agrees well with the previous studies of gas hydrates and ices. In the absence and presence of methane and carbon dioxide guest molecules, the height of the peaks in the RDF of oxygen–oxygen atoms decreases with lowering pressure and increasing temperature; greater MSD, diffusion coefficient, and lattice parameter values are also achieved. Thus, in the absence and/or presence of guest gas molecules, the stability of TBAB and TBPB hydrate cages decreases with increasing temperature and reducing pressure. A good match is noticed between the results of MD simulation and previous experimental and theoretical studies, confirming the accuracy and validation of the simulation method. Reduction of the hydrogen bond balance between water molecules and formation of new hydrogen bonds between the water and the hydroxyl groups of methanol and/or ethanol molecules in the cages indicate that methanol and ethanol molecules have an inhibitory effect on TBAB and TBPB hydrates. Due to different hydrophilic versus hydrophobic behaviors of alcohols, the extent of disruption of hydrogen bonds between water–water molecules of TBAB and TBPB hydrates in the presence of methanol is larger than that in the presence of ethanol, which is consistent with the MD simulation results of clathrate hydrate. This study can help to further understand the semiclathrate hydrate stability or dissociation conditions at a molecular scale for better design and operation of corresponding processes.

Effect of polyvinyl alcohol on aggregation and flow characteristics of tetra-n-butylammonium bromide and tetra-n-butylphosphonium bromide semi-clathrate hydrate slurries formed under CO2 bubbling

The rheological properties of tetra-n-butylammonium bromide (TBAB) and tetra-n-butylphosphonium bromide (TBPB) semi-clathrate hydrate (SCH) slurries with polyvinyl alcohol (PVA) were measured using a viscometer to evaluate the anti-agglomeration effect of PVA. The TBAB and TBPB hydrate slurries exhibited a stronger dilatant trend with yield stress and more aggregates with increasing solid fraction. Only TBPB had plate-like crystals, and the frictional force between particles and aggregability in TBPB hydrate slurries were higher than those in TBAB hydrate slurries. The appropriate degree of polymerization (DP) of PVA was 1000, and the appropriate PVA (DP of 1000) concentrations were 0.53 wt% for TBAB hydrate and 1.8 wt% for TBPB hydrate among the SCH solid fractions examined. CO2 bubbling weakens the anti-agglomeration effect of PVA by promoting particle aggregation and enhancing the interactions between plate-like particles. However, adding PVA changed the rheological properties of slurries to a Bingham fluid even under CO2 bubbling conditions.

An equation of state and a phase diagram for gas-hydrate-forming slurries in gas-shortage

Slurries of gas hydrates, especially mixed hydrates of CO2 and TBPB (Tetra-n-Butyl Phosphonium Bromide), are currently under consideration for secondary refrigeration. Secondary refrigeration circuits contain wide sections where the two-phase slurry (solid + liquid), separated from the gas phase, undergoes transformations that would if gas were present, lead to both hydrate formation and gas dissolution. The absence of gas hinders these processes: the slurry is in gas shortage. The analysis demonstrates that, although thermodynamically distant from three-phase equilibria, slurries in gas-shortage are in equilibrium. For describing their thermodynamic state, the notion of CO2 partial pressure in a gas mixture is introduced, which leads to an equation of state and a new phase diagram with temperature or enthalpy versus CO2 mass-fraction. This diagram complements those commonly used for three-phase systems. The thermal behavior of slurries in gas shortage is then investigated, leading to the possibility of future experimental validation of the present analysis.

Gas Hydrate Phase Equilibrium Data for the CO2 + TBPB + THF + Water System

In this study, phase equilibria of gas hydrates of CO2 in the presence of tetra n-butyl phosphonium bromide (TBPB) and tetrahydrofuran (THF) have been investigated. The experiments were performed using the isochoric pressure search method at varying compositions of promoters. The phase equilibrium data have been reported at 0.01, 0.05, and 0.1 mass fractions of TBPB in aqueous solutions at the temperature and pressure ranges of 278–295 K and 1.1–4.5 MPa, respectively. The results showed that adding the aqueous mixtures of TBPB and THF to the system changes the hydrate phase equilibrium conditions as expected. In comparison with the system including only TBPB, the promotion effect of the applied mixture is low at all studied concentrations of TBPB in the presence of 0.01 mass fraction of THF. However, at 0.05 mass fraction of THF, the hydrate equilibrium pressures at constant temperatures decrease dramatically.

A Calorimetric Study on the Phase Behavior of Tetra-n-butyl Phosphonium Bromide + CO2 Semiclathrate Hydrate and Evaluation of CO2 Consumption─Impact of a Surfactant

This work presents data on the phase behavior of tetra-n-butyl phosphonium bromide (TBPB) semiclathrate hydrate formed in the presence of CO2 at a stoichiometric concentration (2.57 mol % TBPB). A high pressure microdifferential scanning calorimeter (HP μ-DSC) was employed to measure the phase equilibrium data and identify the dissociation behaviors of TBPB + CO2 semiclathrate hydrate. It was found that the phase equilibrium conditions for TBPB + CO2 semiclathrate hydrate formed at 2.57 mol % TBPB were lower than CO2 hydrate formed in pure water and TBPB + CO2 semiclathrate hydrate formed at 0.5 mol % TBPB. The coexistence of CO2 hydrate (sI) and TBPB + CO2 semiclathrate were identified in the 2.57 mol % TBPB solution. When adding 500 ppm sodium dodecyl sulfate (SDS) into the 2.57 mol % TBPB solution, CO2 hydrate disappeared and more TBPB + CO2 semiclathrate hydrate was formed. However, under the same temperature and pressure conditions, CO2 consumption during the formation of TBPB + CO2 semiclathrate hydrate was reduced when 500 ppm of SDS was added to the 2.57 mol % TBPB solution. Therefore, future work should aim to increase the CO2 storage capacity of TBPB semiclathrate hydrate.

Kinetic study of semiclathrate hydrates formed with CO2 in the presence of tetra-n-butyl ammonium bromide and tetra-n-butyl phosphonium bromide

The kinetics of semiclathrate hydrates formation with CO2 in the presence of tetra-n-butyl ammonium bromide (TBAB), tetra-n-butyl phosphonium bromide (TBPB), and TBAB + TBPB were investigated. The experiments were conducted at the stoichiometric concentration of TBAB hydrate (2.57 mol%) and three subcoolings (ΔT = 6 K, 9 K, and 12 K) with the initial pressure fixed at 2.8 MPa. It was found that adding TBPB into the TBAB solution can promote hydrate nucleation, and this promotion effect at a lower subcooling (ΔT = 6 K) was stronger compared to that at higher subcoolings (ΔT = 9 K and 12 K). The gas consumption obtained in TBAB, TBPB, and TBAB + TBPB solutions decreased with the increase of subcooling, despite the increase of the rate of hydrate growth. At a given subcooling, gas consumption (CO2 uptake) in TBAB + TBPB solutions was greater than that in TBAB and TBPB solutions, indicating that hydrate formation kinetics and the CO2 storage capacity were improved in the TBAB + TBPB solution. The largest gas consumption was obtained at ΔT = 6 K in the TBAB + TBPB solution, which was 28.7% and 17.2% larger than those obtained in TBAB and TBPB solutions, respectively. Therefore, a mixture of TBAB + TBPB is a promising option to improve the kinetics of semiclathrate hydrate formation with CO2.

Hydrate Stability Conditions of CO2 + TBPB + Cyclopentane + Water System: Experimental Measurements and Thermodynamic Modeling

The current study provides experimental data and thermodynamic modeling of hydrate stability conditions of CO2 + tetra n-butylphosphonium bromide (TBPB) + cyclopentane (CP) + water system. The expe...

Phase equilibrium data for semiclathrate hydrates formed with tetra-n-butylammonium (bromide or chloride) and tetra-n-butylphosphonium (bromide or chloride) under hydrogen + carbon dioxide pressure

Semiclathrate hydrates are host–guest materials that can be used for CO2 capture due to their unique gas selectivity. CO2 capture from H2 + CO2 mixed gas is possible by semiclathrate hydrates formed by tetra-n-butylammonium (TBA) and tetra-n-butylphosphonium (TBP) salts. TBA bromide (TBAB), TBA chloride (TBAC), TBP bromide (TBPB) and TBP chloride (TBPC) are widely available ionic substances for semiclathrate hydrate formation. In order to understand their gas capture properties, phase equilibrium data of these materials are necessary. Here we report experimental three phase (gas–hydrate–aqueous phase) equilibrium data measured in (TBAB, TBAC, TBPB or TBPC) + H2 + CO2 + H2O systems. The comparison with the literature data suggested that H2 can be incorporated in the TBAB and TBAC hydrates together with CO2, but TBPB and TBPC hydrates reject H2. The hydrate phases were identified by single crystal X-ray diffraction analyses. The TBAB, TBPB and TBPC form the same or similar hydrate phases, i.e., orthorhombic and hexagonal phases. The TBAC formed the tetragonal hydrate phase. The present results suggest that different CO2 capture properties are obtained by these semiclathrate hydrates.

Morphology and kinetic investigation of TBAB/TBPB semiclathrate hydrates formed with a CO2 + CH4 gas mixture

This work presents a morphological and kinetic investigation of tetra-n-butyl ammonium bromide (TBAB) or tetra-n-butyl phosphonium bromide (TBPB) semiclathrate hydrates formed with a CO2 + CH4 gas mixture at the stoichiometric concentration (2.57 mol%). A 3-stage procedure was proposed and employed in the experiments. All experiments were performed at 2.8 MPa and a subcooling of 12 K was used as driving force for hydrate nucleation and growth. It was found that the shapes of hydrate crystals formed in TBPB and TBAB solutions were quite different. Besides, the TBPB semiclathrate hydrate grew laterally at gas/liquid interface and quickly covered the interface while the TBAB semiclathrate hydrate initially formed around the gas/liquid interface and then sank to the reactor bottom. As a consequence, gas uptake and the rate of hydrate growth obtained in TBAB solutions were higher than that in TBPB solutions. It was also found that CO2 content in hydrate phase, CO2 recovery, and separation factor obtained in TBAB solutions were higher than those obtained in TBPB solutions. Therefore, TBAB semiclathrate hydrate formed at the stoichiometric concentration and quiescent conditions will be a promising approach to improve the hydrate based CO2 separation from the CO2 + CH4 gas mixture.

Impacts of the surfactant sulfonated lignin on hydrate based CO2 capture from a CO2/CH4 gas mixture

In the present work, we report an investigation of using an anionic surfactant sulfonated lignin (SL) to promote gas hydrate formation for CO2 capture from a CO2/CH4 gas mixture. The experiments were performed at 277.15 K in a stirred tank reactor. The initial overpressure (driving force) for hydrate formation was fixed at ΔP = 2.5 MPa. The impacts of the SL presence on hydrate growth behavior and CO2 separation efficiency were elucidated. The results indicated that hydrate nucleation was prolonged in the presence of SL but this stage was accelerated when adding 1.0 mol% tetrahydrofuran (THF) into the SL solution. Compared to THF/SL and sodium dodecyl sulfate (SDS) solutions, CO2 recovery obtained in SL solutions was increased to 63.5 ± 2.9% and the separation factor was 4.0 ± 0.8. Gas consumption obtained in SL solutions was higher than that obtained in other systems like liquid water, SDS solution, THF solution, and tetra-n-butyl phosphonium bromide (TBPB) solution. Therefore, sulfonated lignin can be used as a promising surfactant to improve hydrate formation kinetics as well as the efficiency of CO2 capture from CO2/CH4 gas mixture, but there still exists a space to increase the CO2 selectivity in the presence of SL.

Management of vapor release in secondary refrigeration processes based on hydrates involving CO2 as guest molecule

When used as secondary refrigerants for cold storage and refrigeration applications, hydrate slurries offer high-energy densities due to their significant latent heat of fusion. In this context, gas hydrates exhibit a feature not yet explored in the literature: vapor is released when hydrate crystals melt in the heat exchanger supplying cold to end users. This novel feature is investigated in this paper. First, the separation of the released vapor from the solid–liquid slurry is described. This is followed by a study of how the design of the storage system can be adapted to the use of gas hydrate slurries as secondary fluid and specifically the consequences on its volume. A model is developed, based on energy and mass balances, to determine the required storage volume. Two types of hydrates involving a gas are considered: the single hydrate of carbon dioxide and the mixed hydrate of tetra-n-butylphosphonium bromide (TBPB) plus CO2. The results show that adding vapor compression to the process can save up to 75% of the total storage volume.

Energy analysis of two-phase secondary refrigeration in steady-state operation, part 2: Exergy analysis and effects of phase change kinetics

A great deal of attention is paid to secondary refrigeration as a means of reducing excessively high emissions of refrigerants (most of which have a potent greenhouse effect) due to leaks in large cooling units. Among the environmentally friendly fluids that can be used in secondary circuits for transporting and storing cold, hydrate slurries offer the advantage of significant latent heats of fusion associated with good fluidity. Research programs have focused attention on hydrate systems, including CO2, TBPB (tetra-n-butyl-phosphonium-bromide), and mixed CO2-TBPB hydrates. In addition to feasibility concerns, energy efficiency is also a crucial concern requiring an objective analysis of the improvements likely to result from these new materials. A numerical model of secondary refrigeration system was built for slurries ranging from ice-slurry to TBPB-CO2 mixed hydrate slurries. The circuit was designed for maximizing performance under various constraints relating to power rate, heat transfer areas, and flow ability. The effects of phase change kinetics on thermal exchanges were introduced in the model and in the exergy balance. The results, analyzed in terms of exergy losses, demonstrate the couplings through which kinetics influences global performance.

Energy analysis of two-phase secondary refrigeration in steady-state operation, part 1: Global optimization and leading parameter

A great deal of attention is paid to secondary refrigeration as a means of reducing excessively high emissions of refrigerants (most of which have a potent greenhouse effect) due to leaks in large cooling units. Among the environmentally friendly fluids that can be used in secondary circuits for transporting and storing cold, hydrate slurries offer the advantage of significant latent heats of fusion associated with good fluidity. Research programs have focused attention on hydrate systems, including CO2, TBPB (tetra-n-butyl-phosphonium-bromide), and mixed CO2-TBPB hydrates. In addition to feasibility concerns, energy efficiency is also a crucial concern requiring an objective analysis of the improvements likely to result from these new materials. An impartial framework was thus constructed based on the principles of optimization methods. This approach was applied to these three hydrate slurries as well as to the well-known ice slurry for comparison purposes. A numerical model of secondary refrigeration system in steady state was built, on the basis of which optimized systems subjected to common external constraints can be designed for each slurry according to its thermophysical properties. Global performance can then be compared on a sound basis, which also makes it possible to identify the hydrate property that is most influential on energetic performance. Part 2 of this study is dedicated to exergy analysis and phase change kinetics.

Structure-driven CO2 selectivity and gas capacity of ionic clathrate hydrates

Ionic clathrate hydrates can selectively capture small gas molecules such as CO2, N2, CH4 and H2. We investigated CO2 + N2 mixed gas separation properties of ionic clathrate hydrates formed with tetra-n-butylammonium bromide (TBAB), tetra-n-butylammonium chloride (TBAC), tetra-n-butylphosphonium bromide (TBPB) and tetra-n-butylphosphonium chloride (TBPC). The results showed that CO2 selectivity of TBAC hydrates was remarkably higher than those of the other hydrates despite less gas capacity of TBAC hydrates. The TBAB hydrates also showed irregularly high CO2 selectivity at a low pressure. X-ray diffraction and Raman spectroscopic analyses clarified that TBAC stably formed the tetragonal hydrate structure, and TBPB and TBPC formed the orthorhombic hydrate structure. The TBAB hydrates showed polymorphic phases which may consist of the both orthorhombic and tetragonal hydrate structures. These results showed that the tetragonal hydrate captured CO2 more efficiently than the orthorhombic hydrate, while the orthorhombic hydrate has the largest gas capacity among the basic four structures of ionic clathrate hydrates. The present study suggests new potential for improving gas capacity and selectivity of ionic clathrate hydrates by choosing suitable ionic guest substances for guest gas components.

Tetrahydrofuran Hydrate Crystal Growth Inhibition with Synergistic Mixtures: Insight into Gas Hydrate Inhibition Mechanisms

Various mixtures of two chemicals have been tested for their ability to prevent tetrahydrofuran (THF) hydrate crystal growth and compared to their ability as gas hydrate kinetic hydrate inhibitors (KHIs) using a structure-II-forming natural gas mixture, the same structure obtained with THF hydrates. The THF hydrate results for mixtures of poly(N-vinylcaprolactam) (PVCap) with tetra-n-butylammonium bromide (TBAB), tetra-n-butylphosphonium bromide (TBPB), or hexa-n-butylguanidinium chloride (Bu6GuanCl) showed clear synergy effects in two different types of tests: an isothermal test at −0.5 °C and a variable temperature test. In these tests, the key parameters are the lowest concentration of a mixture and the highest subcooling for which complete THF hydrate inhibition is observed, respectively. The synergistic similarities between the THF and previously obtained structure-II-forming gas hydrate results suggest that the dominant inhibition mechanism operating in these mixtures in the gas hydrate system is cr...

Preferential enclathration of CO2 into tetra-n-butyl phosphonium bromide semiclathrate hydrate in moderate operating conditions: Application for CO2 capture from shale gas

This work presents an investigation of using tetra-n-butyl phosphonium bromide (TBPB) semiclathrate hydrate for CO2 capture from simulated shale gas that is composed of 40% CO2 and 60% CH4 in mole fraction. The experiments were performed in a stirred tank reactor with the mass fraction of TBPB (wTBPB) varying from 5.0% to 33.2%. The phase equilibrium data of TBPB semiclathrate hydrate formed in the presence of the 40% CO2/CH4 gas mixture were measured and reported. The effects of TBPB concentration on gas solubility, hydrate formation kinetics, and CO2 separation efficiency were investigated in the temperature range of 278.1–284.2K with the initial pressure fixed at 2.8MPa. The results indicated that at a fixed driving force (overpressure), the gas uptake obtained at the induction time was nearly constant, despite the variation in TBPB concentration, while the rate of hydrate formation increased with the increase in TBPB concentration. Compared to the results obtained at a mole fraction of 1% THF, it was found that enhanced hydrate formation kinetics, a higher CO2 separation factor (31.7±3.3), and a higher operating temperature (284.2K) were obtained at the stoichiometric TBPB concentration (wTBPB=33.2%). Therefore, TBPB may be used as a promising promoter for hydrate-based CO2 capture from the simulated shale gas (CO2/CH4 gas mixture).

Efficient CO2 Capture from a Simulated Shale Gas using Tetra-n-butylphosphonium Bromide Semiclathrate Hydrate

In this work, tetra-n-butylphosphonium bromide (TBPB) semiclathrate hydrate was employed to capture CO2 from a simulated shale gas (40 mol% CO2/CH4) for the first time. New liquid-hydrate-vapor phase equilibrium data at 5, 10, and 20 wt% TBPB were experimentally determined and reported. Hydrate formation kinetics and CO2 separation efficiency at different TBPB concentrations were studied at a fixed initial pressure of 2.8MPa with the temperature varying from 278.1K to 283.4K. The results indicated that 20 wt% TBPB solution was more favorable for CO2 separation compared to 5 and 10 wt% TBPB solutions as the largest separation factor (29.3) was obtained. The preferential enclathration of CO2 into the TBPB semiclathrate hydrate indicates that the hydrate-based CO2 capture from the CO2/CH4 gas mixture is more efficient in the presence of TBPB as compared to that in the presence of tetrahydrofuran (THF).

Phase Equilibrium and Formation Behavior of the CO2–TBPB Semiclathrate Hydrate for Cold Storage Applications

Cold thermal storage is primarily used for demand management on electricity grids due to air conditioner use. The CO2 hydrate has been widely studied as a cold storage medium for its proper phase equilibrium temperature range and large latent heat. This study presents experimental work on the phase equilibrium behavior of the CO2 hydrate under applicable pressures (<10 bar) for the potential to use in air conditioning systems. The formation of hydrate was aided by tetra-n-butylphosphonium bromide (TBPB) at mass fractions of 10, 20, 30, and 37 wt % to lower the required pressure. Instead of using complex calorimeters, the enthalpy of the CO2–TBPB hydrate was measured using a modified T-history method in a self-fabricated pressure tube. Through this test, the enthalpy of the CO2–TBPB hydrate was found to be much higher than for other normally used phase change materials. However, the hydrate formation was also manifested in a large degree of supercooling and a long induction delay. To reduce them, secondary...

Modelling of CO2 capture and separation from different gas mixtures using semiclathrate hydrates

ABSTRACTIn this work we present a model for predicting hydrate formation condition to separate carbon dioxide (CO2) from different gas mixtures such as fuel gas (H2+CO2), flue gas (N2+CO2), and biogas gas (CH4+CO2) in the presence of different promoters such as tetra-n-butylammonium bromide (TBAB), tetra-n-butylammonium chloride (TBAC), tetra-n-butylammonium fluoride (TBAF), tetra-n-butyl ammonium nitrate (TBANO3), and tetra-n-butylphosphonium bromide (TBPB). The proposed method was optimized by genetic algorithm. In the proposed model, hydrate formation pressure is a function of temperature and a new variable in term of Z, which used to cover different concentrations of studied systems. The study shows experimental data and predicted values are in acceptable agreement.

Characterisation of thermal properties and charging performance of semi-clathrate hydrates for cold storage applications

Thermal storage for air conditioning applications has potential to flatten peak load on electricity grids and improve energy savings of cooling systems. Phase change materials (PCMs) for thermal storage have much greater energy storage density than sensible thermal storage materials. It is proven that semi-clathrate hydrates of tetra-n-butylammonium fluoride (TBAF) can offer appropriate phase change temperatures and rapid formation rates, thus they are considered favourable for cold storage. To demonstrate the feasibility of TBAF hydrate as a cold storage medium, its thermal properties are determined. TBAF aqueous solutions are tested at a mass fraction range of 15–40wt% in the presence or absence of another two quaternary ammonium/phosphonium salts – tetra-n-butylammonium bromide (TBAB) and tetra-n-butylphosphonium bromide (TBPB). The T-history method is employed to determine the enthalpy and heat capacity of the material while the heat diffusion equation is used to obtain the material’s thermal conductivity. The material with the highest formation enthalpy was selected for further analysis. For this material, a model of hydrate formation in a cold storage tube is established using the enthalpy method. By this means, the accumulated cooling capacity and charging rate of the tube is predicted, and the influencing factors on the charging performance are studied. The results indicate that careful selection of the heat transfer fluid (HTF) temperature, tube diameter, thermal conductivity and thickness of the tube wall can all potentially shorten the required charging time of the cold storage tube.