Synthesis and Evaluation of Two New Kinetic Hydrate Inhibitors

Oct 15, 2015 - Cha, Shin and Seo , Shin and Kang. 2013 117 (51), pp 13988–13995. Abstract: The effect of the concentration of kinetic hydrate inhibi...
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Synthesis and Evaluation of Two New Kinetic Hydrate Inhibitors Hui-Bo Qin,†,‡ Zhen-Feng Sun,†,‡ Xiao-Qin Wang,§,∥ Jing-Li Yang,‡ Chang-Yu Sun,*,‡ Bei Liu,‡ Lan-Ying Yang,‡ and Guang-Jin Chen*,‡ ‡

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, People’s Republic of China School of Energy Resources, China University of Geosciences, Beijing 100083, People’s Republic of China ∥ College of Science, China University of Petroleum, Beijing 102249, People’s Republic of China §

ABSTRACT: Two new kinetic hydrate inhibitors (KHIs) named PVP-B and PVP-BP were synthesized successfully and compared with commercial KHIs such as PVP and Inhibex 55W. PVP-B and PVP-BP are both the ramifications of PVP. They were obtained by introducing a t-butyl group and both t-butyl and phenyl groups into PVP molecules, respectively. The hydrate inhibition performances of KHIs were assessed in a sapphire cell through two kinds of onset times of hydrate formation: TVO, the time when hydrate crystals are initially observed by naked eyes, and TPD, the time when rapid and continuous dropping of system pressure begins. TVO was used to evaluate an KHI’s inhibition performance to hydrate nucleation. The length of time period from TVO to TPD was used to evaluate a KHI’s inhibition performance to hydrate growth. The results demonstrate that both PVP-B and PVP-BP are superior to PVP and Inhibex 55W; PVP-BP has better inhibition performance than PVP-B because of the stronger steric hindrance effects of the phenyl group in PVP-BP molecules than those of the t-butyl group in PVP-B molecules. Additionally, we found TVO does not always increase with the increasing dosage of KHI while TPD does. The most suitable dosage of PVP-BP was determined to 0.5 wt % or so, at which the nucleation of hydrate is inhibited most perfectly. Finally, we demonstrated glycol could be used as synergist to improve the performance of PVP-BP remarkably. The most suitable dosage of glycol was determined to be 0.9 wt % or so. Our work presents not only two new KHIs but also important insights into the inhibition mechanism of KHIs.

1. INTRODUCTION

water phase, which almost does not affect the thermodynamic conditions of hydrate formation.13,14 Generally KHIs are water-soluble polymers.15 The clathrate hydrate inhibition performance of poly(N-vinylpyrrolidone) (PVP) was first tested and reported.12 Up to now, a series of KHIs have been developed, such as Gaffix VC-713, polyvinylcaprolactan (PVCap), and so on. The most common classes of polymers, which are used in commercial KHI syntheses, are homopolymers and copolymers of the Nvinyllactams, N-vinylpyrrolidone (VP), and N-vinyl caprolactam (VCap). For example, one commercial KHI, Luvicap 55W, is the copolymers of 1:1 vinyl caprolactam:vinylpyrrolidone.12,16 Inhibex 55W, to date, proved to be one of the successful commercial KHIs that was developed more than a decade ago.17 Up to now, the inhibition mechanisms of KHIs are still not very clear. Different researchers have their own points of view. A proposed mechanism is that hydrophobic hydrocarbyl groups on side-chains of KHI polymers fit as pseudoguest molecules in incomplete clathrate hydrate cavities through van der Waals interactions, with extra binding to the surface caused by hydrogen bonds from nearby amide groups.18 Some studies reported that polymers with adjacent alkyl groups of 3−5 carbon atoms are probably optimal for KHI performance. These alkyl groups are of optimum size to interact with structure II (sII) hydrate 51264 cages.19 In this work, we will investigate the kinetic inhibition performance of

Clathrate hydrates (or gas hydrates) are a group of nonstoichiometric, ice-like crystalline compounds formed through a combination of water and suitably sized “‘guest’” molecule(s), such as methane, under suitable temperatures and pressures.1,2 Most oil and gas wells always produce undesired water along with hydrocarbons. It may form solid hydrates with light hydrocarbons in wells or transportation pipelines under elevated pressures and low temperatures. The formation of hydrates could cause severe problems, such as plugging of pipelines.3,4 Efficient measures to deal with hydrate formation and plugging in oil and gas pipelines have been a wide concern in the oil and gas industries since hydrate plugging was discovered by Hammerschmidt in 1934.5,6 Now, hydrate formation is one of the major issues in flow assurance of oil and gas production and transportation.7 The oil and gas industry spends over 200 million US dollars annually to prevent hydrate formation and aggregation to maintain flow assurance.2,8 Traditionally, hydrate formation has been inhibited by injecting large quantities of methanol or glycol into the pipelines so that the hydrate equilibrium phase boundary is shifted to lower temperature and higher pressure.9,10 This method is not only expensive but also unfriendly to the environment. These disadvantages of thermodynamic inhibitors promote the development of more effective low dosage hydrate inhibitors (LDHIs) including kinetic hydrate inhibitors (KHIs) and antiagglomerants (AAs).3,11,12 LDHIs are usually used at a dosage of ca. 0.1−1.0 wt % (active component) based on the © XXXX American Chemical Society

Received: August 21, 2015 Revised: October 11, 2015

A

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monomers and solvent by vaporing. This vaporing process was repeated three times, and then we obtained the refined KHIs. During the process of synthesizing KHIs, we introduced t-butyl group into PVP and named the product as PVP-B. And then, in the same way, we introduced phenyl group into PVP-B as well and named the product as PVP-BP. For PVP-B, we use FT-IR (Fourier Transform Infrared Spectroscopy) to further analysis which is shown in Figure 1. As the sample reason, we dropped some ethanol into the KHIs which could form hydrogen bond by free hydroxide radical. That made us easily to find two broad peaks at ∼3300−3400 cm−1 and ∼700 cm−1. While the wave numbers ∼1060−1100 cm−1 is the C−O characteristic peak. There are many t-butyl groups in the polymer molecule contributing ∼2850−2960 cm−1 and ∼1450−1465 cm−1, (−CH3,-CH2−). Our KHIs are the ramification of PVP. Hence we could find ∼1680 cm−1, -CO and ∼1300 cm−1, −C-N. As there is not any peak at ∼3010 cm−1, -CC−H, this proved thorough removal of monomer NVP. 2.3. Performance Test of KHIs. Figure 2 illustrated a schematic of the experimental apparatus used in the evaluation of kinetic inhibition behaviors of the synthesized KHIs which has been described particularly in previous papers.23−25 The main part of the apparatus is a cylindrical high pressure (up to 40 MPa) transparent sapphire cell which is 2.54 cm in diameter with an effective volume 59 cm3. It is mounted in an air bath with a view window and equipped with a magnetic stirrer for accelerating the hydrate formation. The uncertainties of the temperature and pressure measurements are ±0.1 K and ±0.01 MPa, respectively. The air bath temperature is stabilized with a fluctuation within ±0.1 K. Before tests, the transparent sapphire cell was rinsed twice with ethanol and then with petroleum ether to remove possible residual impurities. After that, it was dried by nitrogen purging to ensure that the cell was thoroughly cleaned. The desired quantities of prepared aqueous solution with given dosage of KHI is charged into the sapphire cell. Subsequently, the vapor space of the cell is vacuumed to ensure the absence of air and the stirrer is turned on to ensure the KHI is completely dissolved in aqueous solution. At the same time, the temperature of air bath is adjusted to the desired value. When the temperature in the cell maintained constant, the stirrer is turned off and the gas is injected into the sapphire cell until the desired pressure is achieved. Afterward, the stirrer is turned on again at a constant speed of 40 rpm. The recording of the pressures and observing of the cell by naked eyes commences with the start-up of stirring.

two derivatives of PVP with different hydrocarbyl groups which are synthesized by ourselves in the lab, and compare them with some commercial KHIs such as PVP and Inhibex 55W. According to the experimental results, we will give our opinions on the inhibiting mechanism of KHIs in different systems.

2. EXPERIMENTAL SECTION 2.1. Materials. The Inhibex 55W was obtained from ISP (International Specialty Products), Inc. USA. PVP was obtained from sigma-aldrich. Deionized water was produced in our laboratory. N-vinyl-2-pyrrolidone (NVP), azobis(isobutyronitrile), ethanol and glycol were all analytical reagent (AR) purchased from Aladdin Industrial (United States). All the reagents were utilized without further purification. Two natural gases and pure methane were used as hydrate forming gases in our work. The compositions of natural gases are determined by a Gas Chromatography HP 6890,20 which are shown in Table 1. Hydrate equilibrium conditions of three hydrate

Table 1. Composition of Different Hydrate Forming Gases NG1

NG2

Methane

Component

(mol %)

(mol %)

(mol %)

CH4 C2H6 C3H8 i-C4 n-C4 i-C5 n-C5 CO2 N2

91.0432 3.8366 0.7324 0.1910 0.1324 0.0276 0.0196 2.0051 2.0121

86.7798 3.7838 6.5281 0.2147 0.3276 0.0426 0.0374 1.7376 0.5484

99.99

0.01

forming gases in pure water were predicted by the Chen−Guo hydrate model,21,22 which could be used to determine the subcooling of hydrate formation systems. 2.2. Synthesis of Kinetic Inhibitors. The new KHIs were synthesized in appropriate solvent. The reactant, NVP and appropriate solvent were mixed evenly in a three-neck round-bottom flask under nitrogen atmosphere at 343 K. After about 6 h, the reaction was ended and the system was cooled down to room temperature. Afterward, certain quantity of ethanol was added to the flask and then the diluted product was transferred to a rotary evaporator to remove unreacted

Figure 1. FT-IR of PVP-B with a little ethanol. B

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Figure 3. Pressure−time profile of hydrate forming process.

Figure 2. Schematic of experimental apparatus.

3. RESULTS AND DISCUSSION 3.1. Onset Time of Hydrate Formation. The kinetic inhibition performance of a KHI is usually evaluated with onset time for hydrate formation. Different methods have been reported for determining the onset time of hydrate formation.26−30 In this work, the onset times of hydrate formation are determined by both visual observation and pressure drop profile methods, which were abbreviated as “TVO” (onset time by visual observation) and “TPD” (onset time by pressure drop profile), respectively. Usually, system pressure variation undergoes three stages for a hydrate formation system in an autoclave: the first rapid dropping stage indicating the dissolving of hydrate forming gas in aqueous solution, a stable stage indicating the process of hydrate nucleation, and the second rapid dropping stage referring to the fast and continuous growth of hydrates. The time from the beginning of gas dissolving in the aqueous phase to the start of the second rapid drop stage is usually taken as the induction time,31−33 which is named as TPD in this work. A typical example is illustrated in Figure 3. We also observed what happened in the sapphire cell continually by naked eyes in each experimental run. In this study, the solution used after the addition of KHIs is usually turbid. As the hydrate formation happens at the surface of gas and liquid, with the stirring of the solution, the hydrate crystal will, in general, first appear on the inside wall of the sapphire cell which was always wetted by the solution. The typical formation process of hydrate crystal on the inside wall of the sapphire cell is shown in Figure 4. When we found a trace of hydrate appeared on the inside wall of the sapphire cell, the

Figure 4. Typical formation and growth of hydrate crystal on the inside wall of the sapphire cell: (1) No hydrate formation; (2) Hydrate crystal just appearing on the wall; (3) Hydrate growth under the inhibition effect of KHI; (4) Generation of a large amount of hydrates.

TVO was obtained. As shown in Figure 3, the TVO is always earlier than the TPD. Before the TVO, the KHI plays a role of retarding the nucleation of hydrate. After that, KHIs play a dominant role of inhibiting hydrate growth. The very slowly decreasing of pressures between TVO and TPD indicates this role of the KHI. 3.2. Comparison of New KHIs with Reported Ones. At first, a series of hydrate formation experiments were performed for comparing the kinetic inhibition performance of our new inhibitors PVP-B and PVP-BP with PVP and Inhibex 55W, which are well-known KHIs. We measured TVO and TPD for C

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Energy & Fuels each kinetic inhibitor at the same range of subcooling, about 8.7−9.0 K, using natural gas NG1, which forms sII hydrate. The subcooling is defined as the difference between the equilibrium hydrate formation temperature under the present system pressure and the experimental temperature. The equilibrium hydrate formation temperature was calculated using the ChenGuo model.21,22 The experimental results are listed in Table 2. Table 2. Onset Time of Hydrate Formation for Natural Gas NG1 from Aqueous Solution with 3.92 wt % Ethanol in the Presence of Different Kinds of KHIs Onset time Inhibitor PVP PVP-B

PVP-BP Inhibex 55W

KHI dosage (wt %)

Texp (K)

Pexp (MPa)

Subcooling (K)

TVO (min)

TPD (min)

0.5 0.9 0.5 0.5 0.5 0.9 0.5 0.9 0.5 0.9

275.9 275.9 275.9 275.9 275.9 275.9 275.9 275.9 276.0 275.9

4.53 4.53 4.59 4.60 4.58 4.61 4.58 4.60 4.51 4.58

8.81 8.82 8.92 8.94 8.86 9.04 8.90 9.00 8.70 8.89

16 13 200 178 238 175 308 319 115 136

258 281 321 306 480 1017 122 162

Figure 5. Pressure−time profiles of the hydrate forming process for natural gas NG1 from 0.5 wt % PVP-B + 3.92 wt % ethanol at subcooling of approximately 8.9 K.

phenyl group. As there are N and O atoms on the PVP monomers, they could easily form hydrogen bonds with water molecules, which facilitates the adsorption of PVP-B or PVP-BP molecules on the hydrate crystal surface. Due to the steric hindrance effect of the hydrophobic groups in KHI molecules, the nucleation and growth of hydrate are inhibited. Because a larger phenyl group has stronger steric hindrance effects than a t-butyl group, PVP-BP has better inhibition performance than PVP-B, as shown in Table 2. To demonstrate that our kinetic inhibitor PVP-BP has the same inhibition performance for different hydrate forming gases, we performed another group of experiments on PVP-BP using natural gases NG1 and NG2 (their compositions are shown in Table 1), respectively. The dosage of PVP-BP and the content of ethanol in aqueous solution were fixed at 0.5 and 2.35 wt %, respectively. The experimental results are shown in Table 3. One can see that, although the subcooling for NG2 is a

As our KHIs (PVP-B and PVP-BP) are difficult to dissolve in water, a small quantity of ethanol was added to water to realize their thorough dissolving. In order to keep PVP and Inhibex 55W to be on par with them, to be on par with them, in this work, the solvent in Inhibex 55W was removed and the same quantity of ethanol was added to water. Deionized water of 17 g was added to the sapphire cell for each experimental run. Two dosages of inhibitors, 0.5 and 0.9 wt % main active component excluding ethanol in the initial aqueous phase, were chosen to check the influence of dosage of KHI upon the inhibition behavior for each KHI. The content of ethanol in aqueous solution was fixed to 3.92 wt % for this group of experimental runs. From Table 2, one can see that, for a given dosage of KHI, PVP-B and PVP-BP are obviously superior to PVP and Inhibex 55W, indicated by their much longer TVO. As the subcooling was too big for PVP, the induction time of hydrate formation was very short and the hydrates grew fast as soon as the hydrate crystals appeared, which made TPD difficult to determine. This phenomenon demonstrates that PVP has little inhibiting effect on the growth of hydrate crystals at high subcooling. On the contrary, our new KHIs, PVP-B and PVP-BP, have remarkable inhibition effects, as their TPDs are obviously longer than TVOs, especially for PVP-BP. In addition, to check the repeatability of the experimental data, two more sets of tests were performed for the onset time of hydrate formation for natural gas NG1 from 0.5 wt % PVP-B + 3.92 wt % ethanol at subcooling of approximately 8.9 K. The pressure−time profiles of the hydrate forming process for these three runs under similar experimental conditions are shown in Figure 5, and the onset time results are listed in Table 2. The comparisons for these three runs imply the repeatability of the hydrate nucleation at a higher subcooling investigated in this study, although it is deemed as a stochastic phenomenon. PVP-B and PVP-BP made in our lab are both the ramifications of PVP. Hence, they not only have the property of PVP but also have the functions of the t-butyl group or

Table 3. Influence of Compositions of Natural Gases upon Inhibition Performance of PVP-BP with Fixed Ethanol Content of 2.35 wt % in Aqueous Solution Onset time Hydrate forming Gas

KHI dosage (wt %)

Texp (K)

Pexp (MPa)

Subcooling (K)

TVO (min)

TPD (min)

NG 1 NG 2

0.5 0.5

275.7 275.8

5.03 2.89

9.83 10.02

66 113

120 163

little larger than that for NG1, both TVO and TPD are much longer than those for NG1. The reason why PVP-BP manifests even better inhibition behavior for NG2 might be attributed to the much lower initial pressure when using NG2 under fixed subcooling. Lower initial pressure causes lower solubility of gas molecules in aqueous solution, which might result in longer induction time and slow growth rate. For comparison of our new KHIs with commercial inhibitor Inhibex 55W, we performed the third group of experiments. This time we used pure methane as the hydrate forming gas, which forms a structures I (sI) hydrate. The dosage of KHI and the content of ethanol for each experimental run are fixed at 0.5 and 2.35 wt %, respectively. The experimental results are listed in Table 4. One can see that, among PVP-B, PVP-BP, and D

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TPD always increases with increasing dosage of PVP-BP in our experimental range. After the formation of hydrate nuclei, the KHI molecules are adsorbed on their surfaces through hydrogen bonding. These adsorbed KHI molecules will inhibit the growth of hydrate nuclei. It is reasonable that the number of adsorbed KHI molecules increases with the concentration of KHI in aqueous solution. Thus, the inhibition effect on the macroscopic growth of hydrate also strengthens with the increase in KHI dosage. 3.4. Synergic Effect of Alcohols. It has been reported that some alcohols could work as synergist to improve the kinetic inhibition behavior of KHI.35−37 Considering glycol is a widely used thermodynamic inhibitor, here we chose it as the potential synergist for PVP-BP. To test whether glycol has synergic effect and determine its suitable content, we performed the last group of experiments, in which natural gas NG1 was used as hydrate forming gas. 2.35 wt % ethanol was added to dissolve PVP-BP in water. The content of glycol in aqueous solution changed from 0.0 to 4.43 wt %. By the way, we compared them with Inhibex 55W containing 2.35 wt % ethanol. The experimental results are tabulated in Table 6. From them, one can see that

Table 4. Evaluation of Different KHIs Using Methane as Hydrate Forming Gas under Subcooling of 6.1 K or so Onset time

Inhibitor PVP-B PVP-BP Inhibex 55W

KHI dos. (wt %)/ ethanol cont. (wt %)

Texp (K)

Pexp (MPa)

Subcooling (K)

TVO (min)

TPD (min)

0.5/2.35 0.5/2.35 0.5/2.35

275.3 275.4 275.4

5.96 5.98 5.88

6.19 6.14 5.89

45 98 54

282 456 189

Inhibex 55W, PVP-BP displays the best kinetic inhibition performance. In this group of experiments, the subcooling was set to 6.1 K or so, which is much lower than those set in the foregoing two groups of experiments shown in Tables 2 and 3. However, TVOs are still shorter than those in the former cases for each KHI. From the above experiments we could conclude that the nucleation period depends on not only the subcooling but also the structure of the hydrate formed. On the other hand, as shown in Table 4, TPD is much longer than TVO, respectively, for each KHI, which implies that the inhibition effect on the growth of hydrate crystal is more conspicuous. This phenomenon indicates that, unlike the case of nucleation, the inhibition effect on the growth of hydrate mainly depends on subcooling. 3.3. Effect of Dosage of KHI. From the above experiments, we found that PVP-BP works best. We thus used PVPBP to do more experiments to investigate the effect of its dosage on the inhibition behavior. In this group of tests, we restricted the subcooling to about 8.7−9.0 K. The dosage of PVP-BP changed from 0.301 wt % to 0.907 wt %. The content of ethanol in aqueous solution was fixed to 3.92 wt %. Natural gas NG1 was used as hydrate forming gas. The experimental results are listed in Table 5. One can see that the TVO

Table 6. Onset Time of Hydrate Formation for Natural Gas NG1 from Aqueous Solution with 0.53 wt % PVP-BP, 2.35 wt % Ethanol, and Different Contents of Glycol under Subcooling 9.8 K or so Onset time

Table 5. Onset Time of Hydrate Formation for Natural Gas NG1 from Aqueous Solution with Different Dosages of PVPBP and 3.92 wt % Ethanol under Subcooling 8.9 K or so Onset time KHI dosage (wt %)

Texp (K)

Pexp (MPa)

Subcooling (K)

TVO (min)

TPD (min)

0.301 0.485 0.535 0.647 0.907

275.9 276.0 276.0 276.0 275.9

4.60 4.54 4.52 4.52 4.60

9.00 8.74 8.71 8.71 9.00

98 262 352 357 319

123 314 540 600 1017

Glycol content (wt %)

Texp (K)

Pexp (MPa)

Subcooling (K)

TVO (min)

TPD (min)

0 0.41 0.91 1.25 1.81 2.41 3.53 4.43 Inhibex 55W

275.7 275.8 275.8 275.7 275.6 275.6 275.6 275.7 275.6

5.03 4.98 5.02 4.98 5.00 4.96 4.97 5.00 4.90

9.83 9.70 9.75 9.76 9.88 9.82 9.84 9.78 9.74

66 223 251 101 192 282 302 317 34

120 328 443 163 210 330 420 610 39

glycol has a very remarkable synergic effect when its content was lower than 1.0 wt %, which can prolong the onset time of hydrate formation obviously. However, this synergic effect declined when further increasing the content of glycol to higher than 1.25 wt %. This effect rebounded at further high glycol content (>1.8 wt %). The reason might be the thermodynamic inhibition performance of glycol became remarkable in this case. It should be noted that the subcoolings given in Table 6 were determined without considering the influence of the additives in water. The actual subcoolings should decrease with the increasing content of glycol. It has been reported that, for the thermodynamic inhibitors, when their contents are low, their thermodynamic inhibition effects are not obvious. However, they can enhance the rate of hydrate formation in a certain content range.38 Hence, glycol may play three roles: KHIs synergist, kinetic promoter, and thermodynamic inhibitor. When the dosage of glycol is very low (2.0 wt %), its thermodynamic inhibition performance became dominant and TVO rebounded rapidly. When we used glycol as synergist of PVP-BP, the best content should be 0.9 wt % or so.

4. CONCLUSIONS Two new kinetic hydrate inhibitors named PVP-B and PVP-BP were synthesized successfully, which are both the ramifications of PVP. The evaluation experimental results demonstrate that both PVP-B and PVP-BP are much superior to PVP. PVP-BP has better inhibition performance than PVP-B. The reason is attributed to the bigger surface area of the phenyl group in PVP-BP molecules, which may have a stronger steric hindrance effect on water molecules to connect to hydrate nuclei. This result also implies that the steric hindrance effect plays an important role in the kinetic inhibition mechanism of KHI. It has also been demonstrated that PVP-BP has better inhibition performance than commercial KHI Inhibex 55W. TVO does not always increase with the increasing dosage of KHI, while TPD does. We think the coexistence of hydrophilic groups and hydrophobic groups in PVP-BP molecules makes it work as a surfactant. When its concentration is higher than the critical micelle concentration, it forms micelles, which can solubilize gas molecules and then promote the nucleation of hydrate. The comparison among experimental results with respect to different hydrate forming gases also shows that higher pressure leads to higher gas solubility in water and then shorter nucleation period (i.e., TVO) for a given KHI under the same subcooling. On the other hand, the KHI molecules are adsorbed on nuclei surfaces through hydrogen bonding. These adsorbed KHI molecules will inhibit the growth of hydrate nuclei. The number of adsorbed KHI molecules increases with the concentration of KHI in aqueous solution. Thus, the inhibition effect on the macroscopic growth of hydrate also strengthens with the increase of KHI dosage. Finally, we demonstrated that glycol could be used as synergist to improve the performance of PVP-BP remarkably. Considering its thermodynamic inhibition and kinetic promotion effects, the most suitable dosages of glycol were determined to be 0.5 wt % or so.

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AUTHOR INFORMATION

Corresponding Authors

*Fax: +86 10 89733156. E-mail: [email protected] (C. Y. Sun). *Fax: +86 10 89733156. E-mail: [email protected] (G. J. Chen). Author Contributions †

H.-B.Q. and Z.-F.S. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support received from the National Natural Science Foundation of China (Nos. 51376195, U1162205, 21276272) and National 973 Project of China (No. 2012CB215005) is gratefully acknowledged.

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REFERENCES

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DOI: 10.1021/acs.energyfuels.5b01916 Energy Fuels XXXX, XXX, XXX−XXX