Article pubs.acs.org/jced
The Combined Effect of Thermodynamic Promoters Tetrahydrofuran and Cyclopentane on the Kinetics of Flue Gas Hydrate Formation Nagu Daraboina and Nicolas von Solms* Department of Chemical and Biochemical Engineering, Center for Energy Resources Engineering (CERE), Technical University of Denmark, DK2800, Kgs. Lyngby, Denmark ABSTRACT: Carbon dioxide (CO2) capture through hydrate crystallization is a promising method among the new approaches for mitigating carbon emissions into the atmosphere. In this work, we investigate a combination of tetrahydrofuran (THF) and cyclopentane (CP) on the kinetics of flue gas (CO2:20 mol %/N2) hydrate formation using a rocking cell apparatus. Hydrate formation and decomposition kinetics were investigated by constant cooling (hydrate nucleation temperature) and isothermal (hydrate nucleation time) methods. Improved (synergistic) hydrate formation kinetics (hydrate nucleation and growth) were observed when THF and CP were present together compared to the individual THF and CP systems. Moreover, the complete hydrate decomposition temperature of CO2/N2/CP/THF hydrate was found to be slightly higher compared to the individual promoter (CO2/N2/CP and CO2/N2/THF) systems. The combined use of these two promoters is favorable both thermodynamically and kinetically for hydrate formation from flue gas.
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INTRODUCTION According to the intergovernmental panel on climate change (IPCC),1 the major source of greenhouse gas emissions is due to energy related activities like the combustion of fossil fuels for power generation. It is urgently necessary to reduce carbon emissions produced by human activities, the root cause of the climate change problem. CO2 capture through hydrate crystallization is a promising method among the new approaches for reducing CO2 emissions due to the higher gas holding capacity of hydrates (a volume of hydrate can hold ∼160 times of gas at STP (standard temperature and pressure)), the cheap working fluid (water), and the modest energy requirements for hydrate formation/decomposition.2−5 Moreover, this technology would be more environmentally friendly and energy-efficient compared to the amine-based absorption process, a currently proven technique which is often cited as a benchmark process. However, the requirement of high-pressure and slow crystallization kinetics are two important barriers to the commercialization of this technology.6 Several gas and liquid thermodynamic promoters have been reported in the literature to reduce the pressure requirement.7−9 CP and THF are two of the most efficient pressurereducing additives in classical hydrate-forming promoters.2,4,10−14 Recently, Herslund et al.10,15 suggested that the simultaneous presence of tetrahydrofuran and cyclopentane in CO2 hydrate forming systems provides an enhanced thermodynamic promotion of the gas hydrate phase. A synergistic effect was reported, at which the combination of the two thermodynamic gas hydrate promoters provided lower hydrate dissociation pressures than either of the two individual promoters. However, the kinetics of flue gas hydrate formation have not been studied, which are important to understand the speed of the process in the absence and presence of promoters. © 2014 American Chemical Society
The combined information on thermodynamics and kinetics are necessary for design and operation of capture or separation processes. Here we have investigated hydrate formation kinetics in the presence of this mixed promoter system in both constant cooling and isothermal experiments.
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EXPERIMENTAL SECTION Materials. A conventional power plant emits typical flue gas of composition (15 to 20) % CO2, (0.5 to 1) % sulfur dioxide (SO2), (5 to 9) % oxygen (O2), and rest nitrogen (N2). Treated flue gas will be free of sulfur components. The gases N2 and O2 form hydrate at approximately the same conditions.4,7,16−18 The gas composition was chosen so that they represent close to industrial compositions. The gas mixture CO2 (20 mol %)/ N2 (80 mol %) used in the present study was UHP grade and supplied by Air Liquide. THF (> 99.9 % purity) and CP (> 97.9 % purity) were supplied from Sigma-Aldrich. Milli-Q Plus 185 water was used to prepare all of the solutions to minimize the effect of impurities in the solution phase. Procedure. A rocking cell with five test cells (RC-5; PSL Systemtechnik, Germany) was used to test the effect of mixed promoters on flue gas hydrate formation. Figure 1 illustrates a schematic of the rocking cells used in the current work. Each test cell has a volume of 40.13 mL and is capable of operating up to 20 MPa pressure. In order to induce thorough mixing, a stainless steel ball (diameter: 17 mm) is placed inside and rolls Special Issue: In Honor of E. Dendy Sloan on the Occasion of His 70th Birthday Received: June 10, 2014 Accepted: September 30, 2014 Published: October 8, 2014 247
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commences at 20 rocks/min and is maintained throughout the experiment. The onset of hydrate nucleation is observed as a sudden pressure drop due to hydrate crystal formation. After 15 h the temperature was slowly raised to 25 °C at the rate of 0.1 °C·min−1 at the same rocking rate to observe the hydrate decomposition.
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RESULTS AND DISCUSSION Figure 2 illustrates a constant cooling experiment at 5.5 MPa initial pressure inside the cell. In the presence of pure water, no
Figure 1. Schematic of experimental apparatus. PT: pressure transducer; TT: thermocouple; PI: pressure indicator; TC: temperature control (chiller).
back and forth along the length of the rocking cell to agitate the solution inside the cell. The mixing in the cells was controlled rocking the cells back and forth between angles of −45° and +45°. Once the cells are loaded with the desired solution, they were placed in a cooling bath controlled by an external refrigerator, which can be operated between −20 °C to +60 °C. The pressure and temperature of cells were continuously monitored by data acquisition (DAQ) system throughout the experiment. Milli-Q Plus 185 (organic content < 5 parts per billion) water was used to prepare all of the solutions to minimize the effect of impurities in the solution phase. To remove the air in the cells, pressurizing with (0.2 to 0.3) MPa of gas in rocking cells, and they were evacuated using a vacuum pump. The details of the experimental setup and procedures are described elsewhere.19 Two different temperature programs were used to study the hydrate formation: Temperature ramping experiments were performed with a constant heating or cooling rate; i.e., once the cell was loaded with solutions and pressurized, temperature decreased from (25 to 1) °C or increased from (1 to 25) °C at the rate of 0.05 °C·min−1. The rocking rate was held constant at 20 rocks/min throughout the rocking experiment. During a temperature ramping (cooling cycle) experiment, initially the pressure decreased linearly in the cell due to thermal contraction. At hydrate nucleation, the gas is consumed due to hydrate formation, and the pressure decreases more rapidly. The onset of hydrate nucleation is observed as a sudden deviation from the linear trend. The hydrates formed during cooling were then decomposed by heating. The pressure increases linearly due to thermal expansion until the hydrate starts to decompose. The pressure rises rapidly as the hydrate decomposes. After complete decomposition of the hydrate, the pressure follows the linear trend again. The effect of promoters on hydrate nucleation and growth can also be expressed in terms of the nucleation time before hydrate nucleation at given constant driving force (isothermal run). Two different temperatures [(16 and 12) °C] were chosen for the isothermal experiments based on the results from the ramping experiments to elucidate the effect of CP and THF separately. The experimental temperature is reached by adjusting the external chiller without rocking the cells. Once the desired temperature is reached, rocking
Figure 2. Typical pressure (p) and temperature (T) change with time (t) inside the rocking cell during temperature ramping test. A: cooling cycle (hydrate formation), B: heating cycle (hydrate decomposition).
hydrate formation (no sudden pressure drop) was observed during cooling. This is expected since the hydrate equilibrium temperature at 5.5 MPa is approximately 273.7 K. In the presence of 5 mol % CP (blue curve, Figure 2A) hydrate nucleation was observed at 286.9 K. In the presence of 5 mol % THF (red curve, Figure 2A) hydrate nucleation was observed at 290.7 K. Interestingly, the mixed promoter system THF + CP (brown curve, Figure 2A) showed hydrate nucleation at 291.3 K, slightly higher than the pure THF system. It may seem 248
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obvious that the temperature is higher since 5 + 5 mol % promoter is used instead of 5 %. However, it has been shown that the largest pressure reduction is obtained for systems containing around 5 mol % (near optimum) THF in the aqueous phase.13,20 Hydrate forming conditions are often independent of or depend only slightly on the amount of cyclopentane present in the system. So the presence of CP improves the performance of THF. Similar behavior was observed at (7.5 and 10) MPa, and the results are summarized in Table 1. As seen from Table 1, the hydrate nucleation temperature increases with an increase in pressure as expected. Table 1. Hydrate Nucleation Temperatures T/K in Temperature Ramping Experiments at Different Experimental Pressures p/MPa p/MPa system
5.5
7.5
water water/CP water/THF water/THF/CP
not formed 286.9 ± 0.6 290.7 ± 0.2 291.3 ± 0.3
not formed 289.9 ± 0.5 292.2 ± 0.3 293.4 ± 0.4
Figure 4. Typical temperature (T) and pressure (p) drop profiles in isothermal experiments at 285 K.
10 275.1 293.2 293.9 294.8
± ± ± ±
0.2 0.1 0.3 0.2
The promotion effect was also studied in terms of hydrate nucleation time at constant driving force (isothermal experiments). Two different experimental temperatures [(285 and 289) K] were selected for isothermal runs at 5.5 MPa based on the results obtained from ramping measurements. The hydrate nucleation did not occur in a pure water system in either isothermal run at (285 and 289) K. The hydrate nucleation in the presence of THF occurred instantaneously at 285 K and 20 min at 289 K. The hydrate nucleation was delayed 100 min at 285 K and hydrate did not form at all at 289 K in the presence of CP. However, in the mixed promoter system the hydrate nucleation was instantaneous in both runs at (289 and 285) K (Figures 3 and 4). These isothermal results were in good agreement with the results obtained in ramping experiments. The gas hydrate growth was estimated by pressure change inside the rocking cells after the hydrate nucleation during temperature ramping and isothermal experiments. The pressure change was continuously monitored in all experiments, and typical temperature ramping curves after nucleation are shown in Figure 5. As seen from the figure, the large pressure drop
Figure 5. Typical pressure (P) drop profiles 6 h after the nucleation in temperature ramping experiments at 5.5 MPa.
occurs in the first 100 min after the nucleation due to rapid hydrate formation. The pressure drop reached a plateau in single promoter systems compared to mixed promoter, where constant slower pressure drop was observed (Figure 5). The pressure drop in the presence of THF and CP is (1.8 and 2.2) MPa, respectively, 300 min after nucleation. The pressure drop in the mixed promoter system is 2.6 MPa 300 min after nucleation and slightly higher compared to the individual promoter system at any given time. However, during temperature ramping experiments pressure drop decreases due to both temperature cooling and hydrate formation. For better comparison isothermal experiments were conducted for 600 min at 285 K and 5.5 MPa, and the growth curves are shown in Figure 6. As seen here, the hydrate growth in THF is slightly faster initially and reached a plateau after 50 min the pressure drop in CP is slow compared to other systems. The pressure drop in the presence of THF and CP is (1.4 and 1.2) MPa, respectively, 600 min after nucleation. The pressure drop in the mixed promoter system decreases rapidly until 100 min, and a slow growth phase is observed before reaching the plateau. Overall, a 2.2 MPa pressure drop was observed 600 min after nucleation, which is higher compared to the individual promoter systems.
Figure 3. Typical temperature (T) and pressure (p) drop profiles in isothermal experiments at 289 K. 249
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tetra-n-butyl ammonium bromide (TBAB) and CP. They reported shorter induction times and improved gas uptake in the mixed promoter system compared to the single promoter systems. Herslund et al.10,15 reported that adding THF to the water/CP/CO2 reduces the hydrate equilibrium pressure at a given temperature significantly. They concluded that different formation mechanisms for these polar (THF) and nonpolar (CP) hydrate formers can cause the synergistic effect. So the mixed hydrate promoter system can increase the driving force compared to the individual promoter systems; hence the hydrate nucleation was faster in the mixed promoter system. However, the exact mechanism for this synergistic behavior is still a mystery. An optimization of the mixed promoter system in terms of gas hydrate kinetics could be more important in a carbon dioxide capture process based on gas hydrate formation.
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SUMMARY The effect of a mixture of the two thermodynamic promoters THF and CP on the kinetics of CO2/N2 gas hydrate formation and decomposition was investigated using constant cooling and isothermal temperature methods in a standard rocking cell apparatus. The results obtained from the two different methods are in good agreement. The hydrate formation kinetics (both nucleation and growth) were enhanced in the mixed promoter system compared to the individual promoter systems. Moreover, the hydrates formed in the mixed promoter system were decomposed completely at slightly higher temperatures compared to individual systems. The combined use of these promoters thus not only affects the thermodynamics but also improves the kinetics significantly, so optimization of the mixed promoter system in terms of gas hydrate kinetics could be useful in carbon dioxide capture using hydrate technology.
Figure 6. Typical pressure (p) drop profiles 10 h after the nucleation isothermal experiments at 5.5 MPa.
The hydrates formed in the absence or presences of additives were decomposed by controlled heating in ramping and isothermal experiments. The complete decomposition of hydrate formed in the presence of CP took longer than hydrate formed in the presence THF (Figure 2B; Table 2). Table 2. Complete Hydrate Decomposition Temperatures T/K in Temperature Ramping Experiments at Different Experimental Pressure p/MPa p/MPa system
5.5
7.5
10
water/CP water/THF water/THF/CP
293.1 ± 0.1 291.2 ± 0.3 293.5 ± 0.2
295.3 ± 0.2 293.5 ± 0.3 295.7 ± 0.1
296.8 ± 0.1 295.6 ± 0.3 297.5 ± 0.2
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AUTHOR INFORMATION
Corresponding Author
*Phone: +45 4525 2867. Fax: +45 4588 2258. E-mail: NVS@ kt.dtu.dk.
These observations are expected; as the thermodynamic stability of CO2/N2/CP hydrate is higher (2 K) compared to CO2/N2/THF hydrate. Interestingly, the complete hydrate decomposition temperature of CO2/N2/CP/THF hydrate was slightly higher compared to the individual promoter (CO2/N2/ CP) system. This may be due to the fact that more hydrate formed in the presence of the mixed promoter system, which then takes longer to decompose. On the other hand, complete hydrate decomposition slightly at longer time and increased temperatures may depend on hydrate compositional and structural changes. Further molecular level studies are necessary to confirm this behavior. THF is cyclic ether and is completely miscible, forming homogeneous liquid mixtures with water. CP is a cycloalkane and is not miscible with water due to its hydrophobic properties. Both THF and CP are known to form sII hydrate. However, in the presence of CO2 gas molecules, these additives stabilize the large cavities and allow the gas to occupy the small cavities. The presence of THF and CP increases the hydrate nucleation temperature significantly for a given pressure. These additives are well-known thermodynamic promoters and known to increase the driving force for hydrate nucleation by shifting the hydrate phase boundary to higher temperature and lower pressures. Interestingly, the combination of THF/CP further increases the hydrate nucleation temperature compared to the individual. Li et al.3,21 also observed synergistic effect in the kinetics of hydrate formation with fuel gas mixture using
Notes
The authors declare no competing financial interest.
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