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Hydrate phase equilibria data and hydrogen storage capacity measurement of the system H2 + tetrabutylammonium hydroxide + H2O
Fluid Phase Equilibria 361 (2014) 175–180
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Hydrate phase equilibria data and hydrogen storage capacity measurement of the system H2 + tetrabutylammonium hydroxide + H2 O Amir A. Karimi, Oleksandr Dolotko 1 , Didier Dalmazzone ∗ ENSTA ParisTech – UCP, 828, Boulevard des Maréchaux, 91762 Palaiseau Cedex, France
a r t i c l e
i n f o
Article history: Received 2 July 2013 Received in revised form 11 October 2013 Accepted 17 October 2013 Available online 27 October 2013 Keywords: Hydrogen Storage Semi-clathrate Gas hydrates
a b s t r a c t The possibility of hydrogen storage in mixed semi-clathrate hydrates formed from hydrogen + tetrabutylammonium hydroxide (TBAOH) + water was studied using high pressure differential scanning calorimetry (DSC) and isochoric reactor experiments (p–V–T). The phase diagram of the system TBAOH + water was first investigated, confirming the existence of one congruent melting hydrate and one non-congruent melting hydrate. Secondly, the effect of hydrogen pressure was studied under the range from 0 to 40 MPa, and salt mole fractions from 0.0083 to 0.0244. (p, T) equilibrium data for hydrate dissociation in the system H2 + TBAOH + water were determined. These results showed that hydrogen has a strong stabilizing effect on the non-congruent TBAOH hydrate structure, dissociation temperatures being increased by about 8 K under a pressure of 40 MPa. This effect was attributed to hydrogen enclathration into the TBAOH hydrate structure. Hydrogen pressure revealed to have much less effect on the stability of the congruent melting TBAOH hydrate, the increase in dissociation temperatures being of the order of 1 K at 40 MPa, therefore indicating an absence of H2 enclathration. A volumetric measurement of hydrogen storage capacity of hydrate was performed. The amount of gas entrapped in the hydrate phase was measured to be 0.35–0.47 wt% over the pressure range 10–20 MPa. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen as an environmentally clean and efficient energy carrier is becoming a primary topic worldwide. Being highly reactive, it has the highest energy content per unit of weight of any known element [1]. However, one of the most challenging problems which lies ahead is the storage and transport of hydrogen. Cryogenic liquid and compressed gas are the most commonly used storage systems for hydrogen [2]. The low density of hydrogen is an undesirable property which necessitates a relatively large volume of storage equipment. While compressed hydrogen gas barely contains 15 kg m−3 at 35 MPa, liquid hydrogen with higher energy content (70.8 kg m−3 at 0.101 MPa) is very energy consuming to be produced. This reveals the necessity of new storage methods for hydrogen storage.
∗ Corresponding author. Tel.: +33 01 81 87 20 00. E-mail addresses: [email protected] (A.A. Karimi), [email protected] (O. Dolotko), [email protected] (D. Dalmazzone). 1 Present address: Research Neutron Reactor ZWE FRM-II, Technische Universität München, Lichtenbergstrasse 1, D-85747 Garching n., Munich, Germany 0378-3812/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fluid.2013.10.043
Among the recent innovative storage methods such as hydrogen adsorption using nanotubes, metal hydrides or Metal Organic Frameworks (MOF), storing hydrogen in natural gas hydrate clathrates could be an attractive alternative solution. Clathrate hydrates are crystalline inclusion compounds made up by rearrangement of hydrogen bonds between water molecules. Under appropriate thermodynamic conditions, i.e. high pressure and low temperatures, water molecules form an enclosed hydrogen bonded structure containing several kinds of cavities inside which can trap external guest molecules. Depending on the size of these cavities and the thermodynamic conditions, various clathrate structures are formed, namely the cubic structures sI and sII and the hexagonal structure sH, which are capable of entrapping different guest molecules that can be low molecular gases or organic liquids. Historically it was thought that hydrogen molecule is too small to stabilize clathrate hydrates and could not form hydrates structures [3–5]. Udachin et al. [6] first experimentally reported that binary system of hydrogen with tetrahydrofuran (THF) forms sII clathrates. They reported that at 70 MPa and 210 K, THF molecules occupy large cages of clathrate structure sII, while H2 enters the small cages. In 1999, Dyadin et al. [7] demonstrated the existence of the first pure hydrogen hydrate (H2 –H2 O), confirming the formation of sII clathrate hydrates of hydrogen, with H2 :H2 O ratio of 1:6 (H2 ·6H2 O) under the pressure of about 220 MPa and
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temperature of 300 K. Storing hydrogen in the form of pure hydrogen hydrates has several advantages. Firstly the storage media is composed of water which is plenty accessible, secondly the formation and decomposition can be very fast and finally no further separation process is needed after release of hydrogen from hydrate. But the main challenge is the severe formation condition which demands high pressure and low temperature. Since the report of Dyadin et al. [7] many studies have been done on hydrogen hydrates with different promoters (guest molecules) to moderate the severe formation conditions. Although the usage of organic promoters such as THF reduces considerably the formation pressure and stabilizes them at ambient temperature, the volatility of most of these compounds causes the contamination of the released hydrogen, which demands a further separation unit. One proposed solution is to store hydrogen in semi-clathrates of quaternary ammonium salts. Semi-clathrates are a particular class of clathrates in which the guest species, generally an alkyl-substituted organic salt, also participates to the host structure through the inclusion of the anion into the water molecules lattice. Such hydrates are stable at ambient temperatures and pressures and the guest is not vaporized upon dissociation, unlike in most clathrates. In addition, some semi-clathrates have available cavities capable of entrapping gas molecules. A number of investigations have shown the evidence of hydrogen enclathration in tetrabutylammonium bromide (TBAB) semi-clathrate [8–12]. Recent studies have shown the ability of semi-clathrate hydrates of various salts, including tetrabutylammonium chloride (TBACl), tetrabutylphosphonium bromide (TBPB) [13], and tetrabutylphosphonium borohydride [14] in entrapping hydrogen molecules at moderate conditions. Most of these systems are unstable at temperatures above 293 K, and pressures lower than 20 MPa, thus requiring refrigeration to be preserved for long periods of time. In this work we have studied the storage of hydrogen in semi-clathrates of tetrabutylammonium hydroxide (TBAOH) as a function of hydrogen pressure and salt concentration. These hydrates present particularly high dissociation temperatures and could therefore provide a storage medium at ambient conditions, without cooling requirements. The phase diagram of H2 O–TBAOH binary system and H2 O–TBAOH–H2 ternary system were determined using DSC measurements, and an estimation of hydrate amount and hydrogen storage was obtained by isochoric reactor experiments. 2. Experimental 2.1. Materials TBAOH–H2 O solutions were prepared using TBAOH·30H2 O hydrates (C16 H37 NO·30H2 O, C.A.S. n◦ 147741-30-8) from Sigma–Aldrich (98 wt% purity) without further purification. Samples of desired concentrations were prepared by dissolving TBAOH·30H2 O crystals in the exact calculated ratio in freshly distilled and degassed water. Samples were kept under vacuum in air-tight laboratory bottles. In order to avoid contamination of TBAOH solution, samples were prepared just before the measurements. Hydrogen (y = 0.999999) was purchased from Linde.
Fig. 1. Schematic of hydrate cell (with courtesy of Ecole des Mines de Paris, CEP/TEP). DAU, data acquisition unit; DW; EC, equilibrium cell; G, gas cylinder; GC, gas chromatograph; HPT, high pressure transducer; LB, liquid bath; LPT, low pressure transducer; PP, platinum probe; RS, ROLSI sampler; SD, stirring device; SW, sapphire windows, TR, temperature regulator; V1, V2, V4, V5, feeding valves; V3, V6, purge valves.
Hydrogen. For this purpose all the DSC measurements were done using a high pressure differential scanning calorimeter, HP-DSC VII (SETARAM, France) which is designed for a range of temperature from 233 to 393 K and supports the pressures up to 40 MPa. DSC is an indirect method for determining the temperature of phase transitions of materials by measuring the heat flux exchanged by a sample when transformations occur upon cooling or warming programs. The advantage of using DSC is that it is much faster than the other common methods used for hydrate stability measurements, such as p–V–T isochoric reactor experiments. The technique has been used for several years for determining the dissociation temperatures of gas hydrates [13–15], as well as the kinetics of hydrates formation [16,17]. A complete description of the experimental set-up and protocol is given in [13,15]. A 0.04 g (±0.001 g) sample of aqueous solution was injected into the crucible, which was then purged several times with hydrogen gas to assure the evacuation of air; finally the desired gas pressure was set. As DSC cells are not equipped with stirrer, in spite of the small quantity of sample, hydrate formation is hindered by limiting diffusion of gas through the liquid. The formation reaction is thus very slow and enters in concurrence with the crystallization of metastable ice, which is not transfer-limited. In order to overcome this inconvenience a multi-cycle program was adjusted [13], which consisted in successively cooling the sample down to 248.15 K for providing a proper super-cooling (driving force) and then heating the sample to a temperature higher than 273.15 K and lower than the dissociation temperature of hydrates. The decrease in the magnitude of the successive melting and crystallization heat peaks in each cycle is a measure of water consumption and therefore hydrate formation. When a total conversion was reached, sample was heated up to 303.15 K to dissociate the accumulated hydrates.
2.2. Differential scanning calorimeter 2.3. Isochoric reactor A differential scanning calorimeter (DSC) was used to investigate the phase diagram of TBAOH–H2 O system for aqueous TBAOH concentrations from 0.83 mol% to 3.23 mol% under atmospheric pressure and also to measure the corresponding hydrate dissociation temperatures in the pressure range between 5 and 40 MPa of
p–V–T measurements were performed using a high pressure reactor (ARMINES, France), allowing a precise measuring of gas hydrates phase equilibrium. As it can be seen in Fig. 1 the reactor cell is equipped with a strong agitation system which facilitates hydrate
A.A. Karimi et al. / Fluid Phase Equilibria 361 (2014) 175–180
formation kinetics. Pressure is measured by two calibrated pressure transducers (GE DRUCK, type PTX611), one for low pressure ranges i.e. 5 MPa (±0.15% full scale) and the other for high pressure ranges i.e. 40 MPa (±0.15% full scale). A platinum temperature probe (PT 100, with uncertainty of 0.02 K) installed inside the cell was used to measure the temperature. Both the pressure transducer and temperature probe were connected to an acquisition unit to record the temperature, and pressure variation versus time. (50 ± 0.1) mL of aqueous solution was injected into the cell through the valve V3, using an ISCO syringe pump and then the reactor was charged with the desired pressure of hydrogen gas through valves V4 and V1. The whole set up was then submerged in the thermostatic cooling bath of silicone oil. After reaching the initial equilibrium state at a temperature higher than the hydrate formation condition, the bath was cooled down to a temperature of 275.15 K. Hydrate formation was detected by the pressure drop and temperature increase due to exothermic formation and visually observed through the two sapphire windows installed on the front and rear of reactor. For analyzing the hydrate storage capacity we have used dilute concentration of LiCl (4 mg/L) as a trace molecule in order to measure the amount of hydrate formed. As LiCl is water soluble salt that does not enter the hydrate structure, its concentration in the remaining aqueous phase increases after hydrate formation. With the help of atomic absorption analysis we have measured LiCl concentrations in the initial solutions, and in the liquid remaining at the end of hydrate formation experiments. A solution containing 0.0083 mol fraction of TBAOH was used for these measurements because, with higher concentrations, the amount of hydrates formed did not allow correctly sampling the liquid phase. The amount of liquid consumed by the hydrate formation was then calculated by the relation (1):
VL = VL,0 − VL,eq = VL,0
1−
CLi,0 CLi,eq
(1)
where VL and CLi respectively stand for the volume and Li+ ions concentration of the liquid phase in the reactor. Underscores 0 and eq indicate the initial and final conditions, respectively. Assuming that the mass of liquid consumed was converted into hydrates, the volume of hydrates formed may be approximated by: VH,eq ≈ VL
L H
(2)
In Eq. (2) the density of liquid phase L was supposed constant over the range of compositions and temperatures, and equal to 1000 kg m−3 . This assumption is justified by the low concentration of salt in the solution used for the measurements. Furthermore, we have verified that an error of 1% on the liquid density changes the final result by only 0.4%. The density of hydrates L was taken from literature data for pure TBAOH hydrate [18]. The volumes of vapor phase at initial and final conditions are given by Eqs. (3) and (4), respectively: VV,0 = VR − VL,0
(3)
VV,eq = VR − VL,eq − VH,eq
(4)
where the volume of the reactor has been determined previously to be VR = (206.0 ± 1.5) mL, including the liquid transfer lines. Finally the amount of hydrogen stored in the hydrate can be obtained from pressure and temperature measurements using Eq. (5): 2 nH H,eq =
peq VV,eq p0 VV,0 − ZRT0 ZRTeq
(5)
At the pressures investigated for these measurements, 20 MPa or less, the compressibility factor Z of hydrogen was assigned a value of 1. The number of moles entrapped was further converted
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Fig. 2. DSC heat flux curve of dissociation of 1.23 mol% solution of TBAOH.
to a percentage of the hydrate mass in order to compare the storage capacity with other media. 3. Results and discussions 3.1. H2 O–TBAOH phase diagram According to Dyadin and Udachin [18], TBAOH–H2 O system forms two hydrate structures. The more stable one (hereafter hyd1) melts congruently at 300.55 K, possesses 28.3 mol of water per mole of TBAOH (Bu4 NOH·28.3H2 O) and a density of 1063 kg m−3 . The second one (hyd2) melts incongruently at 292.15 K, possesses 32.3 mol of water per mole of TBAOH (Bu4 NOH·32.3H2 O) and a density of 1046 kg m−3 . TBAOH solutions in a mole fraction range of 0.0083–0.0323 were studied. There were two constraints for choosing this concentration range. Firstly because the purchased TBAOH salts was in form of TBAOH crystals containing 30 molecules of H2 O and therefore we could not work in more concentrated solutions. On the other hand, highly diluted solutions did not produce dissociation peaks with enough resolution. Fig. 2 shows the heat flux curve obtained upon warming up a xTBAOH = 0.0123 solution sample after 30 cycles at a rate of 1 K/min. There are three peaks detectable in this curve. The first one from the left corresponds to the melting of the eutectic mixture of (Ice + hyd2) phases. Melting of a eutectic mixture is an invariant phase transition that takes place at constant temperature and therefore presents a sharp isotherm melting peak. The melting temperature is given by the onset of the heat flux peak, around 272 K. The small peak at 288 K shows an isotherm phase transition which is attributed to the melting of the peritectic mixture of hydrates (hyd1 + hyd2). The invariance of the two first peaks implies that their corresponding phase transition temperature is independent of the salt concentration. This can be seen in Table 1, which gathers the dissociation points measured for the 9 compositions studied. The onset temperature of the first peak (H2 O (cr) + TBAOH·32.3H2 O (cr) in Table 1) for all mole fractions between 0.0083 and 0.0244 has an average value of 272.3 ± 0.5 K; the average onset temperature of the second peak (TBAOH·32.3H2 O (cr) + TBAOH·28.3H2 O (cr) in Table 1) for mole fractions between 0.0099 and 0.0323 is 288.9 ± 0.7 K. The ranges of uncertainty given above correspond to twice the standard deviation of the mean values. They may be compared to the uncertainty of temperature measurements using DSC, which is generally of 0.5 K. The last peak in this curve (right) presents the progressive dissociation of hyd1 hydrate structure. Hydrate dissociation in the presence of a liquid having a composition that differs from hyd1
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Table 1 Temperatures of (Ice + hyd2) eutectic melting, TE , (hyd1 + hyd2) peritectic melting, TP , and hyd2 dissociation, Tdiss , in the system H2 O + TBAOH at variable salt concentrations. CTBAOH (mol%)
TE (K)
TP (K)
Tdiss (K)
3.23 2.44 1.96 1.64 1.41 1.23 1.1 0.99 0.83
– 272.44 272.64 272.74 272.14 272.14 272.24 272.04 272.14
289.64 288.44 288.54 288.94 288.74 288.54 289.04 288.94 –
300.14 299.54 299.14 298.44 297.14 295.14 294.14 292.94 291.44
Fig. 4. Effect of hydrogen pressure on dissociation temperature of the second hydrate structure of TBAOH–H2 O system (hyd2).
Fig. 3. Phase diagram prediction of TBAOH–H2 O system (the dashed lines on the diagram are just a prediction of the phase diagram and are not investigated by experiment).
invariant corresponding to (hyd1 + hyd2 + liquid) equilibria, at a slightly lower temperature. As the concentration of TABOH salt reduces in the solution, the dissociation temperature of hyd1 decreases until at point P it coincides with the dissociation of hyd2. This point is the peritectic point of the phase diagram. Further dilution of the solution brings to point E which represents the eutectic melting of (Ice + hyd2) mixture. Composition of point E shown in Fig. 3 is an extrapolation of equilibrium points measured at high concentrations. Compositions of these dashed points have not been obtained as in highly diluted concentration of TBAOH solution, the corresponding peak heights were reduced dramatically which made the graphical methods very difficult and inexact to find the dissociation point. 3.2. H2 O–TBAOH–H2 phase diagram
compound results in a non-isotherm phase transition. One practical method for calculating the dissociation temperature of this kind of phase transition was proposed by the Gesellschaft für Thermische Analyze (GEFTA) [19]. It suggests passing a line parallel to the tangent of isotherm peak, from the hydrate peak and then projecting its intersection with base line to the temperature axis in order to find the corresponding temperature. Using this method we estimate that the hyd1 dissociation temperatures obtained from our experiments are given with an uncertainty of ±0.5 K. Results are presented in Table 1 (TBAOH·28.3H2 O (cr)). The resulting x − T phase diagram of the (TBAOH + H2 O) system at ambient pressure is shown in Fig. 3. Point C represents the congruent melting point of hydrate with composition of TBAOH·28.3H2 O (hyd1) reported by Dyadin and Udachin [18]. Point M represents the melting of metastable hydrate TBAOH·32.3H2 O (hyd2) reported by Dyadin, while our measurements show the
Stabilization of TBAOH–H2 O semi-clathrate hydrate in different pressures of hydrogen was investigated using DSC measurements in TBAOH solutions of mole fractions ranging from 0.0083 to 0.0244. The dissociation temperatures of the two hydrate structures at 5, 10, 20 and 40 MPa of hydrogen are presented in Table 2. Temperatures were obtained using the GEFTA method described above with an estimated uncertainty of ±1 K. These results are represented in Figs. 4 and 5 to easily compare the effect of hydrogen pressure on the two hydrate structures. As it can be seen in Fig. 4, dissociation temperature of the hyd2 hydrate structure is almost independent of TBAOH concentration, in this concentration range, while it highly depends on the hydrogen pressure. For example for an increase of pressure from 0 to 40 MPa in a solution with TBAOH mole fraction of 0.016, the dissociation temperature of hyd2 increases by about 8 K. On the other hand, Fig. 5 reveals a strong dependence of the hyd1
Table 2 Dissociation temperatures of H2 –TBAOH–H2 O semi-clathrates under different hydrogen pressures. CTBAOH (mol%)
H2 pressure (MPa) Hydrate structure hyd1
2.44 1.96 1.64 1.41 1.23 1.10 0.99 0.83
Hydrate structure hyd2
0
5
10
20
40
0
5
10
20
40
288.44 288.54 288.94 288.74 288.54 289.04 288.94 286.74
289.14 290.54 289.94 289.74 288.94 289.04 288.74 287.74
290.94 291.74 291.34 291.04 290.54 290.14 290.34 288.64
292.74 293.34 293.94 292.94 292.44 292.64 291.84 290.04
296.74 296.94 296.54 295.84 294.74 292.64 292.24 296.74
299.54 299.14 298.44 297.14 295.14 294.14 292.94 291.44
300.14 299.14 297.34 295.14 294.44 295.14 294.34 291.64
300.34 299.34 297.94 296.54 294.54 295.44 294.64 –
301.44 299.74 298.14 296.44 295.04 294.94 291.84 290.04
303.54 301.34 299.14 298.14 295.84 294.74 292.64 292.24
A.A. Karimi et al. / Fluid Phase Equilibria 361 (2014) 175–180
179
Table 3 Hydrogen storage capacity of common hydrates and their formation conditions. System
H2 content (wt%)
p (MPa)
T (K)
Reference
H2 + H2 O H2 + THF + H2 O H2 + MCHa + H2 O H2 + TBAB + H2 O H2 + TBAOH + H2 O H2 + TBACl + H2 O H2 + TBPB + H2 O
3.7 3.44 1.38 0.6 0.47 0.12 0.14
220 75 149 16 20 14.9 12.1
200–270 255 273–279 287 290 288.9 285
[21] [22] [20] [5] Present work [14] [14]
a
Methylcyclohexane.
hydrate dissociation temperature on the salt concentration, while the pressure effect is much less sensible. For example the dissociation temperature increase of the first structure for the mentioned salt concentration (xTBAOH = 0.016) is less than 1 K. These results indicate that hydrogen interactions with both the structures hyd1 and hyd2 are of different natures. Dissociation temperatures of TBAOH hydrates in the presence of hydrogen are compared with some other clathrates or semiclathrates in Fig. 6. For this comparison dissociation temperatures of hyd2 hydrate structure of TBAOH hydrate with salt mole fractions of 0.0083, 0.0123 and 0.0196 are plotted, as well as those of
hyd1 structure with the mole fraction of 0.0123. T–p curves of THF, TBAB, TBPBP and hyd2 TBAOH hydrates obey approximately a similar trend and may be fitted by exponential curves. On the other side, T–p curve of hyd1 structure is almost perfectly linear. This observation may be explained by considering the Clapeyron relation between the p and T differentiates on the equilibrium curves: diss Vdp = diss HdT
(6)
In Eq. (6), diss V (respectively diss H) represents the variation of volume (respectively enthalpy) of the system upon hydrate dissociation. diss H is independent of pressure. For a hydrate that contains no gas entrapped, diss V is the difference in volume between the hydrate and the corresponding amount of solution, which may be considered independent of pressure over the range of interest. diss V is thus constant, leading to a linear T–p curve. If the hydrate contains gas entrapped, diss V includes the volume of gas released, which varies as 1/p. In that case the classical exponential-type T-p curve of H–L–V equilibria is obtained. This is a confirmation that only hyd2 hydrate type is probably capable of accommodating gas in available cavities. At 40 MPa of H2 pressure with the sample at 1.96 mol% TBAOH, the stabilizing effect on hyd2 hydrate is such that its temperature of dissociation becomes higher than that of hyd1. As a result of this work, the pressure and temperature conditions of hydrogen storage in TBAOH hydrate are more favorable than in most of the other clathrates that were previously investigated. 3.3. Hydrogen storage capacity
Fig. 5. Effect of hydrogen pressure on dissociation temperature of the first hydrate structure of TBAOH–H2 O system (hyd1).
Fig. 6. P–T equilibrium data of various H2 + semi-clathrate hydrates.
As it was discussed earlier, the major problem in using hydrogen as a mobile energy source is its storage. While in many conventional methods stored volume of hydrogen seems satisfying, the mass density of hydrogen is that low that compressing and keeping hydrogen in the gas form does not meet the hydrogen demand practically. Considering DOE’s competitive storage goal (5.5 wt%) as one of the encouraging interests to challenging hydrogen storage capacity, several efforts are still in progress to increase the storage capacity of hydrogen in different methods. Since hydrates have been seen as a storage medium, the storage capacity of hydrogen has been an important issue. While pure hydrogen hydrate system shows maximum storage capacity of 5.3 wt% under severe thermodynamic conditions i.e. 200–300 MPa [2], stabilized hydrates with moderate formation conditions show lower storage capacity (0.14–3.44 wt%) [5,13,20,22]. To calculate the hydrogen storage capacity of TBAOH hydrates from isochoric reactor experiments we assumed that only hyd2 hydrate structure was formed, because the TBAOH concentration and temperature conditions were located in the (hyd2 + liquid) domain of the phase diagram. A value of 1046 kg m−3 was thus taken for the hydrate density in Eq. (2). Our results at 10 MPa and 20 MPa are presented in Table 3 together with those of other clathrate and semi-clathrate hydrates. As measurements were performed with a TBAOH mole fraction of 0.0083, the temperatures of dissociation are relatively low, between 288.6 and 290 K depending
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on the pressure, while only 40% of aqueous solution was converted to hydrate. If the measurements were performed with more concentrated TBAOH solutions conversion could reach higher values (up to 100% for xTBAOH = 0.0296) and dissociation temperatures would reach 290–297 K at equivalent pressures, as shown on the phase diagram on Fig. 4. With a storage capacity of less than 0.5 wt%, this material is not considerable for application as a storage medium for hydrogenfueled vehicles. Regarding the favorable formation conditions and the purity of hydrogen released, it may however be applicable for industrial storage and in-place storage of hydrogen. 4. Conclusion In this work, hydrogen storage in semi-clathrate system of hydrogen + tetrabutylammonium hydroxide + water was studied for the first time. The phase diagram of this system is presented in a mole fraction range of 0.0083–0.0323 mol% of TBAOH and temperature range of 273.15–303.15 K. The phase equilibrium data collected using differential scanning calorimetry show the capacity of hydrogen to stabilize one of the hydrate structures and therefore occupy the semi-clathrate cavities. By using a high pressure reactor cell and the help of atomic absorption analysis, hydrogen storage capacity of the system was measured to be 0.35 wt% at 10 MPa and 0.47 wt% at 20 MPa. This was compared with hydrogen storage capacity of some common hydrate structures. It is obvious that these storage capacities do not meet DOE’s 2015 goal. However, considering the favorable thermodynamic conditions of formation and dissociation of TBAOH hydrates, and the absence of any volatile promoter, it may reveal to be a good replacement for conventional storage methods for in-place storage of hydrogen. List of symbols
CLi hyd1 hyd2 n
concentration of Li+ ions in the liquid, mol L−1 Hydrate TBAOH·28.3H2 O Hydrate TBAOH·32.3H2 O number of moles
p R T V VR Z diss H diss V
pressure, MPa ideal gas constant, 8.314 J mol−1 K−1 absolute temperature, K volume, m3 , mL volume of reactor, 206 mL compressibility factor of real gas enthalpy of dissociation, kJ mol−1 volume of dissociation, m3 mol−1 density, kg m−3
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Hydrate phase equilibria data and hydrogen storage capacity measurement of the system H2 + tetrabutylammonium hydroxide + H2 O Amir A. Karimi, Oleksandr Dolotko 1 , Didier Dalmazzone ∗ ENSTA ParisTech – UCP, 828, Boulevard des Maréchaux, 91762 Palaiseau Cedex, France
a r t i c l e
i n f o
Article history: Received 2 July 2013 Received in revised form 11 October 2013 Accepted 17 October 2013 Available online 27 October 2013 Keywords: Hydrogen Storage Semi-clathrate Gas hydrates
a b s t r a c t The possibility of hydrogen storage in mixed semi-clathrate hydrates formed from hydrogen + tetrabutylammonium hydroxide (TBAOH) + water was studied using high pressure differential scanning calorimetry (DSC) and isochoric reactor experiments (p–V–T). The phase diagram of the system TBAOH + water was first investigated, confirming the existence of one congruent melting hydrate and one non-congruent melting hydrate. Secondly, the effect of hydrogen pressure was studied under the range from 0 to 40 MPa, and salt mole fractions from 0.0083 to 0.0244. (p, T) equilibrium data for hydrate dissociation in the system H2 + TBAOH + water were determined. These results showed that hydrogen has a strong stabilizing effect on the non-congruent TBAOH hydrate structure, dissociation temperatures being increased by about 8 K under a pressure of 40 MPa. This effect was attributed to hydrogen enclathration into the TBAOH hydrate structure. Hydrogen pressure revealed to have much less effect on the stability of the congruent melting TBAOH hydrate, the increase in dissociation temperatures being of the order of 1 K at 40 MPa, therefore indicating an absence of H2 enclathration. A volumetric measurement of hydrogen storage capacity of hydrate was performed. The amount of gas entrapped in the hydrate phase was measured to be 0.35–0.47 wt% over the pressure range 10–20 MPa. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen as an environmentally clean and efficient energy carrier is becoming a primary topic worldwide. Being highly reactive, it has the highest energy content per unit of weight of any known element [1]. However, one of the most challenging problems which lies ahead is the storage and transport of hydrogen. Cryogenic liquid and compressed gas are the most commonly used storage systems for hydrogen [2]. The low density of hydrogen is an undesirable property which necessitates a relatively large volume of storage equipment. While compressed hydrogen gas barely contains 15 kg m−3 at 35 MPa, liquid hydrogen with higher energy content (70.8 kg m−3 at 0.101 MPa) is very energy consuming to be produced. This reveals the necessity of new storage methods for hydrogen storage.
∗ Corresponding author. Tel.: +33 01 81 87 20 00. E-mail addresses: [email protected] (A.A. Karimi), [email protected] (O. Dolotko), [email protected] (D. Dalmazzone). 1 Present address: Research Neutron Reactor ZWE FRM-II, Technische Universität München, Lichtenbergstrasse 1, D-85747 Garching n., Munich, Germany 0378-3812/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fluid.2013.10.043
Among the recent innovative storage methods such as hydrogen adsorption using nanotubes, metal hydrides or Metal Organic Frameworks (MOF), storing hydrogen in natural gas hydrate clathrates could be an attractive alternative solution. Clathrate hydrates are crystalline inclusion compounds made up by rearrangement of hydrogen bonds between water molecules. Under appropriate thermodynamic conditions, i.e. high pressure and low temperatures, water molecules form an enclosed hydrogen bonded structure containing several kinds of cavities inside which can trap external guest molecules. Depending on the size of these cavities and the thermodynamic conditions, various clathrate structures are formed, namely the cubic structures sI and sII and the hexagonal structure sH, which are capable of entrapping different guest molecules that can be low molecular gases or organic liquids. Historically it was thought that hydrogen molecule is too small to stabilize clathrate hydrates and could not form hydrates structures [3–5]. Udachin et al. [6] first experimentally reported that binary system of hydrogen with tetrahydrofuran (THF) forms sII clathrates. They reported that at 70 MPa and 210 K, THF molecules occupy large cages of clathrate structure sII, while H2 enters the small cages. In 1999, Dyadin et al. [7] demonstrated the existence of the first pure hydrogen hydrate (H2 –H2 O), confirming the formation of sII clathrate hydrates of hydrogen, with H2 :H2 O ratio of 1:6 (H2 ·6H2 O) under the pressure of about 220 MPa and
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temperature of 300 K. Storing hydrogen in the form of pure hydrogen hydrates has several advantages. Firstly the storage media is composed of water which is plenty accessible, secondly the formation and decomposition can be very fast and finally no further separation process is needed after release of hydrogen from hydrate. But the main challenge is the severe formation condition which demands high pressure and low temperature. Since the report of Dyadin et al. [7] many studies have been done on hydrogen hydrates with different promoters (guest molecules) to moderate the severe formation conditions. Although the usage of organic promoters such as THF reduces considerably the formation pressure and stabilizes them at ambient temperature, the volatility of most of these compounds causes the contamination of the released hydrogen, which demands a further separation unit. One proposed solution is to store hydrogen in semi-clathrates of quaternary ammonium salts. Semi-clathrates are a particular class of clathrates in which the guest species, generally an alkyl-substituted organic salt, also participates to the host structure through the inclusion of the anion into the water molecules lattice. Such hydrates are stable at ambient temperatures and pressures and the guest is not vaporized upon dissociation, unlike in most clathrates. In addition, some semi-clathrates have available cavities capable of entrapping gas molecules. A number of investigations have shown the evidence of hydrogen enclathration in tetrabutylammonium bromide (TBAB) semi-clathrate [8–12]. Recent studies have shown the ability of semi-clathrate hydrates of various salts, including tetrabutylammonium chloride (TBACl), tetrabutylphosphonium bromide (TBPB) [13], and tetrabutylphosphonium borohydride [14] in entrapping hydrogen molecules at moderate conditions. Most of these systems are unstable at temperatures above 293 K, and pressures lower than 20 MPa, thus requiring refrigeration to be preserved for long periods of time. In this work we have studied the storage of hydrogen in semi-clathrates of tetrabutylammonium hydroxide (TBAOH) as a function of hydrogen pressure and salt concentration. These hydrates present particularly high dissociation temperatures and could therefore provide a storage medium at ambient conditions, without cooling requirements. The phase diagram of H2 O–TBAOH binary system and H2 O–TBAOH–H2 ternary system were determined using DSC measurements, and an estimation of hydrate amount and hydrogen storage was obtained by isochoric reactor experiments. 2. Experimental 2.1. Materials TBAOH–H2 O solutions were prepared using TBAOH·30H2 O hydrates (C16 H37 NO·30H2 O, C.A.S. n◦ 147741-30-8) from Sigma–Aldrich (98 wt% purity) without further purification. Samples of desired concentrations were prepared by dissolving TBAOH·30H2 O crystals in the exact calculated ratio in freshly distilled and degassed water. Samples were kept under vacuum in air-tight laboratory bottles. In order to avoid contamination of TBAOH solution, samples were prepared just before the measurements. Hydrogen (y = 0.999999) was purchased from Linde.
Fig. 1. Schematic of hydrate cell (with courtesy of Ecole des Mines de Paris, CEP/TEP). DAU, data acquisition unit; DW; EC, equilibrium cell; G, gas cylinder; GC, gas chromatograph; HPT, high pressure transducer; LB, liquid bath; LPT, low pressure transducer; PP, platinum probe; RS, ROLSI sampler; SD, stirring device; SW, sapphire windows, TR, temperature regulator; V1, V2, V4, V5, feeding valves; V3, V6, purge valves.
Hydrogen. For this purpose all the DSC measurements were done using a high pressure differential scanning calorimeter, HP-DSC VII (SETARAM, France) which is designed for a range of temperature from 233 to 393 K and supports the pressures up to 40 MPa. DSC is an indirect method for determining the temperature of phase transitions of materials by measuring the heat flux exchanged by a sample when transformations occur upon cooling or warming programs. The advantage of using DSC is that it is much faster than the other common methods used for hydrate stability measurements, such as p–V–T isochoric reactor experiments. The technique has been used for several years for determining the dissociation temperatures of gas hydrates [13–15], as well as the kinetics of hydrates formation [16,17]. A complete description of the experimental set-up and protocol is given in [13,15]. A 0.04 g (±0.001 g) sample of aqueous solution was injected into the crucible, which was then purged several times with hydrogen gas to assure the evacuation of air; finally the desired gas pressure was set. As DSC cells are not equipped with stirrer, in spite of the small quantity of sample, hydrate formation is hindered by limiting diffusion of gas through the liquid. The formation reaction is thus very slow and enters in concurrence with the crystallization of metastable ice, which is not transfer-limited. In order to overcome this inconvenience a multi-cycle program was adjusted [13], which consisted in successively cooling the sample down to 248.15 K for providing a proper super-cooling (driving force) and then heating the sample to a temperature higher than 273.15 K and lower than the dissociation temperature of hydrates. The decrease in the magnitude of the successive melting and crystallization heat peaks in each cycle is a measure of water consumption and therefore hydrate formation. When a total conversion was reached, sample was heated up to 303.15 K to dissociate the accumulated hydrates.
2.2. Differential scanning calorimeter 2.3. Isochoric reactor A differential scanning calorimeter (DSC) was used to investigate the phase diagram of TBAOH–H2 O system for aqueous TBAOH concentrations from 0.83 mol% to 3.23 mol% under atmospheric pressure and also to measure the corresponding hydrate dissociation temperatures in the pressure range between 5 and 40 MPa of
p–V–T measurements were performed using a high pressure reactor (ARMINES, France), allowing a precise measuring of gas hydrates phase equilibrium. As it can be seen in Fig. 1 the reactor cell is equipped with a strong agitation system which facilitates hydrate
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formation kinetics. Pressure is measured by two calibrated pressure transducers (GE DRUCK, type PTX611), one for low pressure ranges i.e. 5 MPa (±0.15% full scale) and the other for high pressure ranges i.e. 40 MPa (±0.15% full scale). A platinum temperature probe (PT 100, with uncertainty of 0.02 K) installed inside the cell was used to measure the temperature. Both the pressure transducer and temperature probe were connected to an acquisition unit to record the temperature, and pressure variation versus time. (50 ± 0.1) mL of aqueous solution was injected into the cell through the valve V3, using an ISCO syringe pump and then the reactor was charged with the desired pressure of hydrogen gas through valves V4 and V1. The whole set up was then submerged in the thermostatic cooling bath of silicone oil. After reaching the initial equilibrium state at a temperature higher than the hydrate formation condition, the bath was cooled down to a temperature of 275.15 K. Hydrate formation was detected by the pressure drop and temperature increase due to exothermic formation and visually observed through the two sapphire windows installed on the front and rear of reactor. For analyzing the hydrate storage capacity we have used dilute concentration of LiCl (4 mg/L) as a trace molecule in order to measure the amount of hydrate formed. As LiCl is water soluble salt that does not enter the hydrate structure, its concentration in the remaining aqueous phase increases after hydrate formation. With the help of atomic absorption analysis we have measured LiCl concentrations in the initial solutions, and in the liquid remaining at the end of hydrate formation experiments. A solution containing 0.0083 mol fraction of TBAOH was used for these measurements because, with higher concentrations, the amount of hydrates formed did not allow correctly sampling the liquid phase. The amount of liquid consumed by the hydrate formation was then calculated by the relation (1):
VL = VL,0 − VL,eq = VL,0
1−
CLi,0 CLi,eq
(1)
where VL and CLi respectively stand for the volume and Li+ ions concentration of the liquid phase in the reactor. Underscores 0 and eq indicate the initial and final conditions, respectively. Assuming that the mass of liquid consumed was converted into hydrates, the volume of hydrates formed may be approximated by: VH,eq ≈ VL
L H
(2)
In Eq. (2) the density of liquid phase L was supposed constant over the range of compositions and temperatures, and equal to 1000 kg m−3 . This assumption is justified by the low concentration of salt in the solution used for the measurements. Furthermore, we have verified that an error of 1% on the liquid density changes the final result by only 0.4%. The density of hydrates L was taken from literature data for pure TBAOH hydrate [18]. The volumes of vapor phase at initial and final conditions are given by Eqs. (3) and (4), respectively: VV,0 = VR − VL,0
(3)
VV,eq = VR − VL,eq − VH,eq
(4)
where the volume of the reactor has been determined previously to be VR = (206.0 ± 1.5) mL, including the liquid transfer lines. Finally the amount of hydrogen stored in the hydrate can be obtained from pressure and temperature measurements using Eq. (5): 2 nH H,eq =
peq VV,eq p0 VV,0 − ZRT0 ZRTeq
(5)
At the pressures investigated for these measurements, 20 MPa or less, the compressibility factor Z of hydrogen was assigned a value of 1. The number of moles entrapped was further converted
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Fig. 2. DSC heat flux curve of dissociation of 1.23 mol% solution of TBAOH.
to a percentage of the hydrate mass in order to compare the storage capacity with other media. 3. Results and discussions 3.1. H2 O–TBAOH phase diagram According to Dyadin and Udachin [18], TBAOH–H2 O system forms two hydrate structures. The more stable one (hereafter hyd1) melts congruently at 300.55 K, possesses 28.3 mol of water per mole of TBAOH (Bu4 NOH·28.3H2 O) and a density of 1063 kg m−3 . The second one (hyd2) melts incongruently at 292.15 K, possesses 32.3 mol of water per mole of TBAOH (Bu4 NOH·32.3H2 O) and a density of 1046 kg m−3 . TBAOH solutions in a mole fraction range of 0.0083–0.0323 were studied. There were two constraints for choosing this concentration range. Firstly because the purchased TBAOH salts was in form of TBAOH crystals containing 30 molecules of H2 O and therefore we could not work in more concentrated solutions. On the other hand, highly diluted solutions did not produce dissociation peaks with enough resolution. Fig. 2 shows the heat flux curve obtained upon warming up a xTBAOH = 0.0123 solution sample after 30 cycles at a rate of 1 K/min. There are three peaks detectable in this curve. The first one from the left corresponds to the melting of the eutectic mixture of (Ice + hyd2) phases. Melting of a eutectic mixture is an invariant phase transition that takes place at constant temperature and therefore presents a sharp isotherm melting peak. The melting temperature is given by the onset of the heat flux peak, around 272 K. The small peak at 288 K shows an isotherm phase transition which is attributed to the melting of the peritectic mixture of hydrates (hyd1 + hyd2). The invariance of the two first peaks implies that their corresponding phase transition temperature is independent of the salt concentration. This can be seen in Table 1, which gathers the dissociation points measured for the 9 compositions studied. The onset temperature of the first peak (H2 O (cr) + TBAOH·32.3H2 O (cr) in Table 1) for all mole fractions between 0.0083 and 0.0244 has an average value of 272.3 ± 0.5 K; the average onset temperature of the second peak (TBAOH·32.3H2 O (cr) + TBAOH·28.3H2 O (cr) in Table 1) for mole fractions between 0.0099 and 0.0323 is 288.9 ± 0.7 K. The ranges of uncertainty given above correspond to twice the standard deviation of the mean values. They may be compared to the uncertainty of temperature measurements using DSC, which is generally of 0.5 K. The last peak in this curve (right) presents the progressive dissociation of hyd1 hydrate structure. Hydrate dissociation in the presence of a liquid having a composition that differs from hyd1
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Table 1 Temperatures of (Ice + hyd2) eutectic melting, TE , (hyd1 + hyd2) peritectic melting, TP , and hyd2 dissociation, Tdiss , in the system H2 O + TBAOH at variable salt concentrations. CTBAOH (mol%)
TE (K)
TP (K)
Tdiss (K)
3.23 2.44 1.96 1.64 1.41 1.23 1.1 0.99 0.83
– 272.44 272.64 272.74 272.14 272.14 272.24 272.04 272.14
289.64 288.44 288.54 288.94 288.74 288.54 289.04 288.94 –
300.14 299.54 299.14 298.44 297.14 295.14 294.14 292.94 291.44
Fig. 4. Effect of hydrogen pressure on dissociation temperature of the second hydrate structure of TBAOH–H2 O system (hyd2).
Fig. 3. Phase diagram prediction of TBAOH–H2 O system (the dashed lines on the diagram are just a prediction of the phase diagram and are not investigated by experiment).
invariant corresponding to (hyd1 + hyd2 + liquid) equilibria, at a slightly lower temperature. As the concentration of TABOH salt reduces in the solution, the dissociation temperature of hyd1 decreases until at point P it coincides with the dissociation of hyd2. This point is the peritectic point of the phase diagram. Further dilution of the solution brings to point E which represents the eutectic melting of (Ice + hyd2) mixture. Composition of point E shown in Fig. 3 is an extrapolation of equilibrium points measured at high concentrations. Compositions of these dashed points have not been obtained as in highly diluted concentration of TBAOH solution, the corresponding peak heights were reduced dramatically which made the graphical methods very difficult and inexact to find the dissociation point. 3.2. H2 O–TBAOH–H2 phase diagram
compound results in a non-isotherm phase transition. One practical method for calculating the dissociation temperature of this kind of phase transition was proposed by the Gesellschaft für Thermische Analyze (GEFTA) [19]. It suggests passing a line parallel to the tangent of isotherm peak, from the hydrate peak and then projecting its intersection with base line to the temperature axis in order to find the corresponding temperature. Using this method we estimate that the hyd1 dissociation temperatures obtained from our experiments are given with an uncertainty of ±0.5 K. Results are presented in Table 1 (TBAOH·28.3H2 O (cr)). The resulting x − T phase diagram of the (TBAOH + H2 O) system at ambient pressure is shown in Fig. 3. Point C represents the congruent melting point of hydrate with composition of TBAOH·28.3H2 O (hyd1) reported by Dyadin and Udachin [18]. Point M represents the melting of metastable hydrate TBAOH·32.3H2 O (hyd2) reported by Dyadin, while our measurements show the
Stabilization of TBAOH–H2 O semi-clathrate hydrate in different pressures of hydrogen was investigated using DSC measurements in TBAOH solutions of mole fractions ranging from 0.0083 to 0.0244. The dissociation temperatures of the two hydrate structures at 5, 10, 20 and 40 MPa of hydrogen are presented in Table 2. Temperatures were obtained using the GEFTA method described above with an estimated uncertainty of ±1 K. These results are represented in Figs. 4 and 5 to easily compare the effect of hydrogen pressure on the two hydrate structures. As it can be seen in Fig. 4, dissociation temperature of the hyd2 hydrate structure is almost independent of TBAOH concentration, in this concentration range, while it highly depends on the hydrogen pressure. For example for an increase of pressure from 0 to 40 MPa in a solution with TBAOH mole fraction of 0.016, the dissociation temperature of hyd2 increases by about 8 K. On the other hand, Fig. 5 reveals a strong dependence of the hyd1
Table 2 Dissociation temperatures of H2 –TBAOH–H2 O semi-clathrates under different hydrogen pressures. CTBAOH (mol%)
H2 pressure (MPa) Hydrate structure hyd1
2.44 1.96 1.64 1.41 1.23 1.10 0.99 0.83
Hydrate structure hyd2
0
5
10
20
40
0
5
10
20
40
288.44 288.54 288.94 288.74 288.54 289.04 288.94 286.74
289.14 290.54 289.94 289.74 288.94 289.04 288.74 287.74
290.94 291.74 291.34 291.04 290.54 290.14 290.34 288.64
292.74 293.34 293.94 292.94 292.44 292.64 291.84 290.04
296.74 296.94 296.54 295.84 294.74 292.64 292.24 296.74
299.54 299.14 298.44 297.14 295.14 294.14 292.94 291.44
300.14 299.14 297.34 295.14 294.44 295.14 294.34 291.64
300.34 299.34 297.94 296.54 294.54 295.44 294.64 –
301.44 299.74 298.14 296.44 295.04 294.94 291.84 290.04
303.54 301.34 299.14 298.14 295.84 294.74 292.64 292.24
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179
Table 3 Hydrogen storage capacity of common hydrates and their formation conditions. System
H2 content (wt%)
p (MPa)
T (K)
Reference
H2 + H2 O H2 + THF + H2 O H2 + MCHa + H2 O H2 + TBAB + H2 O H2 + TBAOH + H2 O H2 + TBACl + H2 O H2 + TBPB + H2 O
3.7 3.44 1.38 0.6 0.47 0.12 0.14
220 75 149 16 20 14.9 12.1
200–270 255 273–279 287 290 288.9 285
[21] [22] [20] [5] Present work [14] [14]
a
Methylcyclohexane.
hydrate dissociation temperature on the salt concentration, while the pressure effect is much less sensible. For example the dissociation temperature increase of the first structure for the mentioned salt concentration (xTBAOH = 0.016) is less than 1 K. These results indicate that hydrogen interactions with both the structures hyd1 and hyd2 are of different natures. Dissociation temperatures of TBAOH hydrates in the presence of hydrogen are compared with some other clathrates or semiclathrates in Fig. 6. For this comparison dissociation temperatures of hyd2 hydrate structure of TBAOH hydrate with salt mole fractions of 0.0083, 0.0123 and 0.0196 are plotted, as well as those of
hyd1 structure with the mole fraction of 0.0123. T–p curves of THF, TBAB, TBPBP and hyd2 TBAOH hydrates obey approximately a similar trend and may be fitted by exponential curves. On the other side, T–p curve of hyd1 structure is almost perfectly linear. This observation may be explained by considering the Clapeyron relation between the p and T differentiates on the equilibrium curves: diss Vdp = diss HdT
(6)
In Eq. (6), diss V (respectively diss H) represents the variation of volume (respectively enthalpy) of the system upon hydrate dissociation. diss H is independent of pressure. For a hydrate that contains no gas entrapped, diss V is the difference in volume between the hydrate and the corresponding amount of solution, which may be considered independent of pressure over the range of interest. diss V is thus constant, leading to a linear T–p curve. If the hydrate contains gas entrapped, diss V includes the volume of gas released, which varies as 1/p. In that case the classical exponential-type T-p curve of H–L–V equilibria is obtained. This is a confirmation that only hyd2 hydrate type is probably capable of accommodating gas in available cavities. At 40 MPa of H2 pressure with the sample at 1.96 mol% TBAOH, the stabilizing effect on hyd2 hydrate is such that its temperature of dissociation becomes higher than that of hyd1. As a result of this work, the pressure and temperature conditions of hydrogen storage in TBAOH hydrate are more favorable than in most of the other clathrates that were previously investigated. 3.3. Hydrogen storage capacity
Fig. 5. Effect of hydrogen pressure on dissociation temperature of the first hydrate structure of TBAOH–H2 O system (hyd1).
Fig. 6. P–T equilibrium data of various H2 + semi-clathrate hydrates.
As it was discussed earlier, the major problem in using hydrogen as a mobile energy source is its storage. While in many conventional methods stored volume of hydrogen seems satisfying, the mass density of hydrogen is that low that compressing and keeping hydrogen in the gas form does not meet the hydrogen demand practically. Considering DOE’s competitive storage goal (5.5 wt%) as one of the encouraging interests to challenging hydrogen storage capacity, several efforts are still in progress to increase the storage capacity of hydrogen in different methods. Since hydrates have been seen as a storage medium, the storage capacity of hydrogen has been an important issue. While pure hydrogen hydrate system shows maximum storage capacity of 5.3 wt% under severe thermodynamic conditions i.e. 200–300 MPa [2], stabilized hydrates with moderate formation conditions show lower storage capacity (0.14–3.44 wt%) [5,13,20,22]. To calculate the hydrogen storage capacity of TBAOH hydrates from isochoric reactor experiments we assumed that only hyd2 hydrate structure was formed, because the TBAOH concentration and temperature conditions were located in the (hyd2 + liquid) domain of the phase diagram. A value of 1046 kg m−3 was thus taken for the hydrate density in Eq. (2). Our results at 10 MPa and 20 MPa are presented in Table 3 together with those of other clathrate and semi-clathrate hydrates. As measurements were performed with a TBAOH mole fraction of 0.0083, the temperatures of dissociation are relatively low, between 288.6 and 290 K depending
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on the pressure, while only 40% of aqueous solution was converted to hydrate. If the measurements were performed with more concentrated TBAOH solutions conversion could reach higher values (up to 100% for xTBAOH = 0.0296) and dissociation temperatures would reach 290–297 K at equivalent pressures, as shown on the phase diagram on Fig. 4. With a storage capacity of less than 0.5 wt%, this material is not considerable for application as a storage medium for hydrogenfueled vehicles. Regarding the favorable formation conditions and the purity of hydrogen released, it may however be applicable for industrial storage and in-place storage of hydrogen. 4. Conclusion In this work, hydrogen storage in semi-clathrate system of hydrogen + tetrabutylammonium hydroxide + water was studied for the first time. The phase diagram of this system is presented in a mole fraction range of 0.0083–0.0323 mol% of TBAOH and temperature range of 273.15–303.15 K. The phase equilibrium data collected using differential scanning calorimetry show the capacity of hydrogen to stabilize one of the hydrate structures and therefore occupy the semi-clathrate cavities. By using a high pressure reactor cell and the help of atomic absorption analysis, hydrogen storage capacity of the system was measured to be 0.35 wt% at 10 MPa and 0.47 wt% at 20 MPa. This was compared with hydrogen storage capacity of some common hydrate structures. It is obvious that these storage capacities do not meet DOE’s 2015 goal. However, considering the favorable thermodynamic conditions of formation and dissociation of TBAOH hydrates, and the absence of any volatile promoter, it may reveal to be a good replacement for conventional storage methods for in-place storage of hydrogen. List of symbols
CLi hyd1 hyd2 n
concentration of Li+ ions in the liquid, mol L−1 Hydrate TBAOH·28.3H2 O Hydrate TBAOH·32.3H2 O number of moles
p R T V VR Z diss H diss V
pressure, MPa ideal gas constant, 8.314 J mol−1 K−1 absolute temperature, K volume, m3 , mL volume of reactor, 206 mL compressibility factor of real gas enthalpy of dissociation, kJ mol−1 volume of dissociation, m3 mol−1 density, kg m−3
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