Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 12980−12986
pubs.acs.org/IECR
Effect of Nonionic Surfactants on F‑Gases (HFC-134a and SF6) Hydrate Formation Hyunju Lee,† Ju Dong Lee,‡ and Yangdo Kim*,§ †
Department of Mechanical Engineering, Worcester Polytechnic Institute, Worcester, Massachusetts 01609, United States Offshore Plant Resources R&D Center, Korea Institute of Industrial Technology, Busan 618-230, Korea § Department of Materials Science and Engineering, Pusan National University, Busan 609-735, Korea
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‡
ABSTRACT: Fluorinated gases (F-gases), such as 1,1,1,2-tetrafluoroethane (HFC-134a) and sulfur hexafluoride (SF6), are used widely in a variety of industrial processes but they are some of the most potent greenhouse gases. A clean process using the principle of gas hydrate formation can be a new alternative to the separation and recovery of these greenhouse gases. In this study, alcohol ethoxylate (AE), vegetable oil ethoxylate (VOE), and alkyl polymer (AP), which are known to be nonionic, less toxic, and readily biodegradable surfactants, were used as additives to improve the hydrate kinetics. All surfactants increased the kinetics of HFC-134a and SF6 hydrate formation. In particular, the rates of HFC-134a and SF6 hydrate formation was fastest when AP was added. In the case of AP addition, the inflection point at which the formation rates of SF6 hydrate increased significantly was also found. The addition of AP not only improved the rates of HFC-134a and SF6 hydrate formation but also reduced the hydrate nucleation time.
1. INTRODUCTION Fluorinated gases (F-gases) are man-made gases that are used widely in many industries. These gases can be divided into four types: hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF6), and nitrogen trifluoride (NF3). Among them, HFCs have been used mainly as replacements for ozone-depleting substances (ODSs), such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs).1 SF6 gas is also used widely as an insulating gas in electrical transformers, cleaning gas in semiconductor manufacturing processing, and covering gas in the foundry process.1,2 On the other hand, fluorinated gases, such as HFCs and SF6, are included in the “Kyoto basket” as greenhouse gases.1 The 100year time horizon-global warming potentials (GWP) for 1,1,1,2-tetrafluoroethane (HFC-134a) and SF6 are 1,300 and 23,500 (relative to CO2), respectively.3 Therefore, it is important to recover and separate the waste HFC-134a and SF6 gas. Gas hydrates are icelike crystalline compounds that are formed by physically stable interactions between the water and guest molecules. The water molecules form hydrogen-bonded cages, in which the guest molecules are incorporated. These © 2018 American Chemical Society
compounds exist in three distinct structures that contain cages of different sizes and shapes. Structure s-I and structure s-II hydrates consist of two types of cages, whereas structure s-H hydrate consists of three types of cages.4 Gas hydrates have been studied for the various field. Many researchers have reported natural gas hydrates (NGH) transportation and storage.5,6 Veluswamy et al. have proposed a hybrid combinatorial approach of methane hydrate formation utilizing the amino acid by combining stirred and unstirred reactor for solidified natural gas (SNG).7 Recently, clathrate hydrate-based desalination (HyDesal) has been proposed for seawater desalination.8−13 The process of gas hydrates formation has also been proposed for the recovery of global warming gases.14,15 Previous studies reported the effects of tetrahydrofuran (THF) and tetra-n-butyl ammonium bromide (TBAB) additives on CO2 separation and recovery from fuel gas mixtures.16,17 A SF6−N2 mixture gas hydrate equilibrium as Received: Revised: Accepted: Published: 12980
June 13, 2018 September 3, 2018 September 7, 2018 September 7, 2018 DOI: 10.1021/acs.iecr.8b02651 Ind. Eng. Chem. Res. 2018, 57, 12980−12986
Article
Industrial & Engineering Chemistry Research well as a gas separation efficiency was also investigated.18,19 Recently, a technique for geological storage of flue gas or CO2−N2 mixtures captured by hydrate has been also proposed.20 Gas hydrates including CO2 and CH4 can be formed at high pressures and low temperatures. On the other hand, HFC-134a and SF6 gases easily form hydrates under relatively low pressures and high temperatures compared to other global warming gases.21,22 Therefore, technological and economic effects can be expected for the recovery of HFC-134a and SF6 gases by gas hydrate formation. In addition, gas hydrate technology has the potential to be an eco-friendly recovery technology because it does not emit any byproducts or contaminants during processing. In this study, we investigated the potential applications of the treatment of HFC-134a and SF6 gas using gas hydrate process. Although surfactants and/or promoter have been studied to improve the hydrate formation rate, it has been limited to gases, such as CO2 and CH4.23−30 Therefore, the effects of surfactants on HFC-134a and SF6 hydrate formation were examined using alcohol ethoxylate (AE), vegetable oil ethoxylate (VOE), and alkyl polymer (AP). The results showed that the rates of HFC-134a and SF6 gas hydrate formation were increased by the addition of surfactants.
Figure 1. Experimental apparatus.
2.3. Kinetics. Gas hydrate was formed during the same temperature and pressure. First, the system was kept at the desired temperature, and then the gas was charged into the reactor. When the temperature and pressure of the reactor were kept constant, the magnetic stirring bar of the reactor was turned on and the time was measured thereafter. The number of moles of gas consumed was expressed as a function of time and pressure and was calculated as the pressure difference in the supply vessel between that at time = 0 and that at time t.31 This is given by eq 1
2. EXPERIMENTAL SECTION 2.1. Materials. HFC-134a and SF6 gases with 99.9% purity were used to form the gas hydrates. AE, VOE, and AP were used as a kinetic promoter for gas hydrate formation, and it was provided from Dow Corning. AE, VOE, and AP are a class of nonionic surfactants that are low toxicity and excellent biodegradable. Table 1 lists the properties of these surfactants.
i P yz i P yz zz − Vsv jjj zz ΔnH = Vsv jjj zRT k {0 k zRT {t
where VSV, P, and T are the volume, pressure, and temperatures of the gas in the supply vessel, and z is the compressibility factor, which was calculated using Pitzer’s correlations.32 A more detailed description of the calculation of the amount of gas consumption is reported elsewhere.33 The moles of gas consumed (ΔnH) per mole of water (nw) were used to compare the rate of hydrate formation. The eq 2 used in the calculation is shown below
Table 1. Properties of Nonionic Surfactant sample
alcohol ethoxylate (AE)
type
nonionic surfactant
pH (10% sol) surface tension (1% sol) hydrophilic− lipophilic balance (HLB)
vegetable oil ethoxylate (VOE)
alkyl polymer (AP)
nonionic nonionic surfactant, readily surfactant, readily biodegradable biodegradable
5−6 27−28 dyn/cm
7 28−30 dyn/cm
13
11
(1)
ij Δn yz Hydrate formation rate = jjj H zzz j nw z k {
7.5 28−30 dyn/cm
(2)
3. RESULTS AND DISCUSSION Figure 2 shows the cumulative moles of HFC-134a gas consumed as a function of time due to its dissolution and hydrate formation. The general shape of this curve agrees with the behavior of the CO2/N2 hydrate curve described by Linga et al.33 Generally, three methods can be used to confirm the gas hydrate nucleation time, including the change in the gas consumed, the change in temperature, and visual observation. The moles of gas consumed at point A represent the amount of gas dissolved, corresponding to the three-phase hydrate equilibrium pressure at that temperature. Subsequently, a metastable region (steady state) was observed between points A and B. Point B in Figure 2 represents the point at gas hydrate nucleation and is called the hydrate nucleation time or induction time. The aqueous solution changed to a turbid solution at point B. As a result, the rate of hydrate growth is defined as the slope of the gas-consumed curve in the B−C region after the hydrate nucleation time. Gas hydrate
2.2. Experimental Apparatus. We used a semibatch reactor for kinetic experiments. The same equipment was used in previous HFC-134a phase equilibrium experiments.31 Figure 1 presents the experimental apparatus equipped with temperature and pressure control systems. The equipment consists of a reactor (R) in which the gas hydrate reaction occurs and a supply vessel (SV) in which the gas is stored.31 During the hydrate reaction, the gas is charged from the supply vessel to the reactor. All vessels are contained in an insulated bath and temperature controlled using a chiller. The reactor and supply vessel was made from 316 stainless steel and had a volume of 350 and 518 cm3, respectively and used omega copperconstant thermocouple to measure the temperatures during all experiments.31 We also installed a baffle arrangement inside the reactor for effective stirring. 12981
DOI: 10.1021/acs.iecr.8b02651 Ind. Eng. Chem. Res. 2018, 57, 12980−12986
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Industrial & Engineering Chemistry Research
Figure 2. Gas consumption curve and temperature profile in the HFC-134a hydrate formation. Figure 4. Effect of VOE on HFC-134a hydrate formation at 277.15 K and 0.25 MPa.
formation is an exothermic process. As shown in the Figure 2 (at the point B), the temperature increases immediately after nucleation, reaching a higher level and then decreasing gradually because the temperature controller brings the temperature back to the set point value. Figure 3 shows the HFC-134a hydrate formation rates with AE addition at a temperature and pressure of 277.15 K and
Figure 5. Effect of the AP on the HFC-134a hydrate formation at 277.15 K and 0.25 MPa.
fastest when 0.05 wt % AP was added and the amount of HFC134a consumption was approximately 1.8 times higher than that of pure water. Kumar et al. reported that gas consumption was the highest at a certain concentration of nonionic surfactant (Tween-80).30 To compare the three nonionic surfactants, the rates of HFC-14a hydrate formation containing 0.1 wt % additive were confirmed. Figure 6 shows the rates of HFC-134a hydrate formation in the pure water and additive solution. As shown in Figure 6, HFC-134a gas consumption with AP addition was higher than that of AE and VOE addition. Figure 7 shows the HFC-134a hydrate nucleation time for the three surfactants. The experiments of hydrate nucleation time were performed three times, and the data were presented with standard deviations. The hydrate nucleation or induction time in gas hydrate crystallization is an important characteristic of the hydrate process. This also indirectly affects the economics of processes using hydrate technology. Several studies of the effects of surfactants on the hydrate nucleation time have been reported.34,35 On the other hand, the nucleation time was different, even under the same conditions. For example, in the case of pure water, the nucleation time ranged from 0 to 15 min. The nucleation time with surfactant addition was lower than that of pure water with the exception
Figure 3. Effect of AE on HFC-134a hydrate formation at 277.15 K and 0.25 MPa.
0.25 MPa, respectively. In all kinetic experiments, the reactor was charged with 150 cm3 of water. The data were calculated after the hydrate nucleation. The hydrate formation rates increased with the addition of AE except for 0.01 wt % AE. Figure 4 shows the moles of gas consumed of HFC-134a hydrate with VOE addition. Similar to the results of AE addition, the addition of 0.01 wt % VOE inhibited the hydrate formation rate slightly. The rate of hydrate formation of pure water was similar to that of VOE addition (0.05 and 0.1 wt %) for the first 30 min, but there was a difference subsequently. Figure 5 shows the gas uptake curves of HFC-134a hydrate with AP addition. The addition of AP at 0.01 wt % also had an inhibition effect. The rate of hydrate formation of pure water was similar to that of AP addition (0.05 and 0.1 wt %) for the first 15 min. After 15 min, however, the rate of hydrate formation with AP (0.05 and 0.1 wt %) increased significantly. In addition, the rate of HFC-134a hydrate formation was the 12982
DOI: 10.1021/acs.iecr.8b02651 Ind. Eng. Chem. Res. 2018, 57, 12980−12986
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0.01 wt % AE was added, the rate of hydrate formation increased for the first 10 min and then decreased, eventually becoming similar to that of pure water. In previous experiments, similar results show that the addition of 0.01% Tween-20 acts as a promoter for the first 20 min from nucleation and then acts as an inhibitor for the remainder.36 The rate of SF6 hydrate formation was the fastest when 0.1 wt % AE was added and the moles of SF6 consumption was approximately 1.8 times than that of pure water. Figure 9 shows the moles of gas consumed of SF6 hydrate with VOE addition. The rate of hydrate formation with VOE
Figure 6. Comparison of the surfactant effects on HFC-134a hydrate formation at 277.15 K and 0.25 MPa.
Figure 9. Effects of VOE on the SF6 hydrate formation at 276.15 K and 0.7 MPa.
of the addition of 0.05 wt % VOE. The deviation of the nucleation time was also decreased. Figure 8 shows the moles of gas consumed SF6 hydrate rates with AE addition at 276.15 K and 0.7 MPa, respectively. When
addition (0.01, 0.05, and 0.1 wt %) was similar for the first 5 min but there was a subsequent difference. The rate of SF6 hydrate formation was fastest when 0.1 wt % VOE was added and the moles of SF6 consumption was approximately 2.3 times higher than that of pure water. Figure 10 shows the gas uptake curves of SF6 hydrate with AP addition. The data were obtained after the hydrate nucleation time. The hydrate formation rates increased significantly with the addition of AP. SF6 hydrate showed two different formation rates in an aqueous solution with AP.
Figure 8. Effects of AE on SF6 hydrate formation at 276.15 K and 0.7 MPa.
Figure 10. Effects of AP on SF6 hydrate formation at 276.15 K and 0.7 MPa.
Figure 7. Comparison of surfactant effects on the HFC-134a hydrate nucleation time.
12983
DOI: 10.1021/acs.iecr.8b02651 Ind. Eng. Chem. Res. 2018, 57, 12980−12986
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Industrial & Engineering Chemistry Research As shown in Figure 10, more rapid hydrate formation rates were obtained after the point A, B, and C. The turning point occurred from approximately 40 to 70 min. In the case of SF6, this turning point was also observed in a previous study.36 Therefore, this turning point is referred to as an inflection point. In addition, the hydrate formation rates increased with increasing amounts of AP addition. When the amount of AP was increased from 0.01 to 0.1 wt %., the occurrence time of the inflection point was decreased. The gas solubility is believed to be one of the key factors in accelerating hydrate formation.36 The increased hydrate formation rates of SF6 gas are due likely to the increased SF6 gas solubility with the surfactant additives. To compare the three nonionic surfactants, the rates of SF6 hydrate formation with added 0.1 wt % additives were confirmed. Figure 11 shows the SF6 hydrate formation rates
Figure 12. Comparison of the surfactant effects on the SF6 hydrate nucleation time.
change of the hydrate growth behavior is probably due to the continuous supply of water at the interface from the capillary effect.39 Figure 13 shows the inside of the reactor after SF6 hydrate formation. As shown in Figure 13a, the hydrate was dissociated and existed as a slurry (water + hydrate) phase. On the other hand, in 0.05 wt % AP addition (Figure 13b) the hydrate was formed like a bubble, and no water was found. This difference in morphology may be due to the addition of surfactants. The addition of nonion surfactant resulted in the formation of foam; it can be decreased by the interfacial tension and increased by the contact area of the gas/liquid. As a result, it seems to have changed the kinetics and morphology of hydrate formation. Lee et al. have reported hydrate crystal growth at the bubble surface.40 In the presence of SDS, drastic changes in morphology were observed in that smokelike crystals appeared from the top of the bubble.40 Besides, SDS molecules adsorbed on the interface blocked the film formation while facilitating gas uptake that led to an increase in the hydrate formation rate.40 Although additional studies, such as structural analysis by Raman and NMR and the effects of solubility on the surfactants, will be needed, AP was found to be most effective surfactant on the hydrate kinetics, including the induction time. The study of HFC-134a and SF6 gas recovery using gas hydrate is expected to be applicable to global warming gas separation under mild conditions. In addition, the use of ecofriendly surfactants might contribute to the process configuration of other hydrate-based gas separations.
Figure 11. Comparison of the surfactant effects on SF6 hydrate formation at 276.15 K and 0.7 MPa.
in pure water and the additive solution at 276.15 K and 0.78 MPa, respectively. As shown in Figure 11, SF 6 gas consumption with AP addition was higher than that with AE and VOE addition, and the level of SF6 consumption (0.1 wt % AP) was approximately 7.3 times higher than that of pure water. Figure 12 shows the SF6 hydrate nucleation time for the three surfactants. In the case of SF6 hydrate with AP addition, the nucleation time was reduced drastically compared to pure water. In addition, the deviation of the nucleation time was less than 10 min. This means that AP not only increases the rate of hydrate formation but also decreases the nucleation time. That is, the AP surfactant had the strongest effect on the SF6 hydrate kinetics. Some studies also showed that the addition of a nonionic surfactant decreases the nucleation time, which is in good agreement with the present results.37,38 Kumar et al. have reviewed the article of surfactants in gas hydrate studies and investigated their mechanisms of hydrate formation.39 According to the literature, the addition of surfactants increases the solubility of the gas for hydrate formation, decreases the interfacial tension of the gas/liquid to promote contact between gas and water, and changes the hydrate growth behavior.39 The surface tension and capillary effect are closely related. It was also observed that the hydrates usually grow upward on the wall of the reactor by small additions of surfactant. Hence, it has been suggested that the
4. CONCLUSION The effects of nonionic surfactants, such as AE, VOE, and AP, on HFC-134a and SF 6 gas hydrate formation were investigated. The rate of HFC-134a hydrate formation was fastest when 0.05 wt % AP was added and the level of HFC134a consumption was approximately 1.8 times than that of pure water. In the case of SF6, gas consumption with AP addition was higher than that of AE and VOE addition and the level of SF6 consumption (0.1 wt % AP) was approximately 7.3 times higher than that of pure water. In addition, the inflection point at which the rates of SF6 hydrate formation increased significantly was found. The addition of AP not only improved the hydrate formation rate of SF6 and HFC-134a but also reduced the hydrate nucleation time. 12984
DOI: 10.1021/acs.iecr.8b02651 Ind. Eng. Chem. Res. 2018, 57, 12980−12986
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Figure 13. Images of SF6 hydrate in the reactor. (a) Pure water and (b) 0.05 wt % AP addition.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ju Dong Lee: 0000-0002-9567-8396 Yangdo Kim: 0000-0002-8792-2360 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (No. 2011-0014453). Authors also acknowledges the support by Korea Institute of Industrial Technology (UR180024, EO180032, UI170003). We also thank Dr. Man Sig Lee for his support in part of the surfactant study.
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