Article pubs.acs.org/IECR
Experimental Investigation of the Formation of CyclopentaneMethane Hydrate in a Novel and Large-Size Bubble Column Reactor Qiu-Nan Lv, Xiao-Sen Li,* Chun-Gang Xu, and Zhao-Yang Chen Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou Institute of Energy Conversion, and Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China ABSTRACT: The effects of the flow rates of cyclopentane (CP) and methane gas, operating pressure, and experimental temperature on the formation of cyclopentane-methane hydrate were investigated using a novel bubble column reactor. The results indicated that, with either an increase of the operating pressure or a decrease of the experimental temperature, the induction time decreased whereas the cumulative gas consumption and gas conversion ratio increased. In addition, with an increase of the flow rate of methane gas, the induction time decreased, whereas the cumulative gas consumption increased. The gas conversion ratio increased first from 61.5% to 93.1% when the flow rate of methane gas increased from 159 to 234 mL/min and then decreased from 93.1% to 83.2% when the flow rate of methane gas continuously increased from 234 mL/min to 308.9 mL/min. This decrease was attributed to an increase of the discharge rate of methane from the reactor. It was noted the average methane consumption rate could be effectively enhanced with an increase of the flow rate of gas at relatively low pressure and high temperature. The flow rate of CP had a minimal effect on the cumulative gas consumption and gas conversion ratio. However, an increase of the flow rate of CP could reduce the induction time.
1. INTRODUCTION Gas hydrates are nonstoichiometric compounds consisting of water molecules and small gas molecules. Water molecules form cavities through hydrogen bonding, and small gas molecules become entrapped as guests in the cavities through van der Waals forces. The small gases include hydrogen (H2), methane (CH4), carbon dioxide (CO2), nitrogen (N2), and hydrogen sulfide (H2S). Presently, four different hydrate structures are known: structure I (sI), structure II (sII), structure H (sH), and semiclathrate hydrate (sc). For example, CH4 forms sI hydrate under certain conditions. According to published data, a natural gas hydrate with a volume of 1 m3 can contain more than 180 m3 (at standard temperature and pressure) of natural gas.1 Presently, technologies based on gas hydrates are being studied widely in the fields of gas storage and transportation,2,3 wastewater treatment,4 desalination,5 gas separation,6 solution concentration,7 and thermal storage air conditioning.8 However, gas hydrate formation is a complex process involving heat and mass transfer in the gas, liquid, and solid phases. The application of gas hydrate technology faces some challenges. One of the main challenges focuses on reducing the induction time of hydrate formation and increasing the hydrate formation rate. Many methods (including mechanical and chemical methods) have been used to eliminate obstacles. Among mechanical methods, both stirring gas/water mixtures by mechanical agitation and spraying water droplets into the gas phase can increase the contact between the gas and liquid.9 For example, mechanical agitation was used by Linga et al.10 to capture CO2 from fuel gases through gas hydrate technology. Nevertheless, although stirring can enhance the rate of gas hydrate formation, problems inherent in mechanical agitation make it impractical. For example, sufficient torque must be transmitted from the shaft of stirrer to mix the dense hydrate © 2012 American Chemical Society
slurry, and gas leakage must be prevented. Another problem is that the power required for stirring increases with the fifth power of the impeller size, which hinders the scaleup of this type of reactor.11 Instead of stirring, spraying water into the gas phase and bubbling gas into the liquid phase have been considered as possible approaches to increase the gas/liquid interface and improve hydrate formation. Murakami et al.12 injected precooled water and methylcyclohexane independently into high-pressure methane by of spraying with two liquidcirculation loops. Even though spraying enhanced the rate of gas hydrate formation, the orifices for jetting might be blocked by hydrates around their mouths. To overcome this disadvantage, Fujita et al.13 sprayed water on the surface of a cooled metal block that was exposed to the gas, so that the heat released from hydrate formation could be directly removed from the plate surface. The methods of stirring and spraying have their disadvantages of high energy consumption and low energy efficiency. Therefore, it is necessary to further seek an effective method and provide insight into its effects on the application of hydrate technology. Takahashi et al.14 studied the effect of shrinking microbubbles on xenon gas hydrate formation. Their results showed that microbubbles promoted gas dissolution and accelerated hydrate nucleation. Moreover, the method of bubble formation does not consume any energy because gas bubbles are directly created by injecting gas into the solution under the driving force resulting from the pressure difference between the gas and the solution. In terms of chemical methods, some chemical additives can be used as promoters to enhance hydrate formation efficiently. Received: Revised: Accepted: Published: 5967
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Duc et al.15 used tetra-n-butylammonium bromide (TBAB) as an additive to capture CO2 from a gas mixture by hydrate crystallization. Li et al.16 presented a new method for capturing CO2 from simulated flue gas with cyclopentane/water mixtures. In this article, a novel technique based on hydrate technology is developed to prepare hot brine in situ seafloor for marine natural gas hydrate (NGH) production.17 The basic idea is as follows: A vertical steel tube is installed in the ocean extending from the ocean surface to the seafloor. The steel tube is divided into six parts from the bottom to the top: the hot brine zone, hydrate formation zone, hydrate uplift or brine water descending zone, hydrate decomposition zone, freshwater zone, and hydrate former zone. Seawater and hydrate former form hydrates spontaneously under the high-pressure ocean conditions in the hydrate formation zone. The exothermicity and desalination of the process of hydrate formation raise the temperature and salinity of the residual seawater. The salinity of the residual seawater is higher than that of the seawater. Therefore, the residual seawater sinks to the hot brine zone, and the hydrates float to the hydrate uplift zone because of buoyancy. After the hydrates float to the hydrate dissociation zone, they are thoroughly dissociated. The water and hydrate former are regenerated by decreasing the pressure and increasing the temperature. The increase in temperature results from the high-temperature seawater at the ocean surface. Consequently, the hot brine in the hot brine zone is pumped from the bottom of the steel tube and then injected into the NGH deposits under the seafloor for producing the NGH. Compared to the conventional NGH exploitation technology, this technique prevents a significant heat loss along the pipeline during transmission of the heat from the ocean surface to the NGH regions, and the energy efficiency of the NGH recovery is improved drastically. However, the technique must meet the requirements of a high rate and a high enthalpy of hydrate formation. In the novel technique, bubbling is carried out to improve hydrate formation. On one hand, bubbling the gas into the water increases the interface between gas and water efficiently, and it further improves the heat and mass transfer observably. On other hand, bubbling leads to a kind of turbulence in the water, and the turbulence accelerates the distribution of the gas into the water. Thus, in this work, we developed a large-size bubble column reactor. The features of the reactor include the following: (1) The water in the bubble column reactor need not be circulated, saving the pumping energy for water circulation. (2) The gas is automatically introduced into the water from the gas feed cylinder owing to the pressure difference. (3) Gas bubbles have a relatively long stagnation period in the liquid phase, and the hydrates are almost completely formed during the rising of the gas bubbles. Therefore, the hydrates can be formed rapidly with low energy consumption and high energy efficiency in the equipment. In addition, according to previous work,18 cyclopentane (CP) is an effective hydrate formation additive that can accelerate gas hydrate formation, and the hydrate formation enthalpy of CP + methane is as high as 130.25 kJ·mol−1 at 285 K. The formation enthalpy of methane-CP hydrate is higher than those of methane + methylcyclohexane (74.73 kJ·mol−1, 285 K),19 methane + cyclohexane (92.52 kJ·mol−1, 285 K),19 and synthetic natural gas + methylcyclohexane (74.40 kJ·mol−1, 285 K).19 Thus, CP + methane hydrate was studied using the large-size bubble column reactor in this work. The effects of the operating pressure, experimental temperature, and the flow
rates of CP and methane gas on the cumulative gas consumption, gas conversion ratio, and induction time were investigated based on kinetic experiments. The results offer a constructive reference for optimizing the technological conditions of preparing hot brine in situ seafloor for marine NGH production.
2. EXPERIMENTAL SECTION 2.1. Experimental Materials. Methane with 99.99% purity was supplied by Foshan Huate Gas Co., Ltd. CP with a purity of higher than 99.0% was supplied by Chengdu Best Reagent Co., Ltd. Deionized water with a resistivity of 18.25 mΩ·cm−1 was produced with an ultrapure water system supplied by Nanjing Ultrapure Water Technology Co., Ltd. 2.2. Apparatus. The experimental apparatus is shown in Figure 1. The cuboid bubble reactor with a height of 4.0 m and
Figure 1. Schematic of the experimental apparatus. (1) Gas cylinder, (2) mass flow meter, (3) fluid container, (4) controlled volume pump, (5) refrigeration system, (6) water bath, (7) static mixer, (8) perforated plate, (9) coolant recycle pump, (10) reactor, (11) vacuum pump, (12) PID pressure controller, (13) gas collection cylinder, (14) data acquisition system.
an inner volume of 40.0 L was made of 316 stainless steel. It was divided into four segments, labeled from the bottom to the top as segment I, segment II, segment III, and segment IV. Four glass windows were installed on the front and back sides of each segment to allow for visual observation. The maximum working pressure of the reactor was 4.0 MPa. The temperature of the coolant tank could be regulated from −5 to 40 °C with an accuracy of ±0.2 °C. The coolant tank supplied the circulating cooling medium to the vessel jacket. The temperature in the reactor was controlled by circulating a cooling medium through the vessel jacket. A Pt1000 thermoprobe with 5968
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Table 1. Experimental Conditions and Results run
T (K)
P (MPa)
Qg (mL·min−1)
QCP (mL·min−1)
t (min)
ti (min)
ΔnCH4 (mol)
rav (mol·min−1·m−3)
η (%)
1 2 3 4 5 6 7 8 9 10 11 12
277.05 277.05 277.05 277.05 277.03 277.15 276.45 277.25 277.05 277.25 278.45 280.15
1.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 1.5 2.5 2.0 2.0
83.2 83.8 159 159.2 159.2 234 234.6 308.9 233.2 312.4 234.5 235.1
90 90 90 36 23 36 36 36 36 36 36 36
120 100 85 85 75 155 160 130 165 140 150 160
13.8 10.8 3.3 4.1 5.3 2.2 0.8 1.2 2.5 0.67 3 4.2
0.41422 0.41354 0.60054 0.59862 0.52733 1.71731 1.85158 1.84241 1.50455 2.19453 1.4297 1.34399
0.0959 0.1149 0.1963 0.1956 0.1953 0.3078 0.3214 0.3937 0.2533 0.4354 0.2648 0.2333
41.8 47.2 59.5 61.5 58.1 93.1 96.0 83.2 68.5 94.6 75.6 61.1
Figure 2. Formation process of methane/cyclopentane (CP) hydrate monitored by digital camera with QCP = 36 mL/min and Qg = 234 mL/min at 277.15 K and 2.0 MPa. 5969
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Figure 3. Schematic drawing of hydrate formation using bubbling.
±0.05 °C accuracy was placed on the top of each segment to measure the temperature. The pressure in the experimental apparatus was measured using a pressure transducer, with a range of 0−10 MPa and an accuracy of ±0.02 MPa. The pressure was controlled by a proportional−integral−derivative (PID) controller through a pressure-regulated valve (Tescom ER3000). CP was pumped into the reactor by a metering pump with a range of 0−60 L/h and an accuracy of ±1% (full scale). The flow rate of methane was controlled with a gas mass flowmeter (SYM-3030A/V) with an accuracy ±1% (full scale). The CP and gas methane were mixed in a static mixer (SK DN15/L) and then introduced into the reactor through a perforated plate with a porous distribution of 20 μm. Discharged gas was separated by a gas−liquid separator and collected in a gas collecting cylinder. 2.3. Procedure. The experimental apparatus was cleaned using deionized water before each experiment. Then, deionized water was introduced into the bubble column reactor using a vacuum pump until the liquid level reached the desired height (h). After the temperature of the liquid phase had stabilized at the desired value for 2 h, methane gas was introduced into the system rapidly from a gas feed cylinder until the system pressure reached the desired experimental pressure. Subsequently, CP and methane gas were introduced into the bubble reactor simultaneously. The experimental time at this time is defined as t0 = 0. Methane gas in the gas feed cylinder was supplied to the bubble reactor at the desired flux. The pressure in the reactor was kept constant by discharging the excess gas from the top of the column reactor through a PIDcontrolled pressure-regulated valve. The temperature, pressure, and gas and liquid flow rates were recorded during each experiment. The behaviors of the formation and accumulation of the hydrate in the bubble reactor were observed directly. In each experiment, pictures were taken with a digital camera. The average methane gas consumption rate during the reaction was calculated as rav =
ΔnCH4 tV
=
η=
nCH4,in
× 100% (2)
3. RESULTS AND DISCUSSION In this work, 12 experimental runs were carried out under different conditions, including the flow rate of CP (QCP) (runs 3−5), the experimental pressure (runs 1, 2, 8, and 10), the experimental temperature (runs 6, 7, 11, and 12), and the flow rate of methane gas (Qg) (runs 2, 4, 6, and 8). The experimental conditions and results are summarized in Table 1. 3.1. Hydrate Formation Process. Figure 2 shows images of characteristic changes in hydrate appearance during the process of hydrate formation at 277.15 K and 2.0 MPa. The flow rates of CP and methane were 36 mL/min and 234 mL/ min, respectively. The CP and methane gas mixtures left the perforated plate and entered the bubble reactor simultaneously in the form of macroscopic bubbles and droplets, respectively. After 5 min, the image of the hydrate in segment I was recorded, as shown in Figure 2a. It can be seen from Figure 2a that the methane bubbles were encapsulated by the CP membranes. CP in the form of small spheres distributed in the water phase strongly promotes two-phase (water/CP, CP/ methane) interfaces and improves the contact of the three phases (water/CP/methane). CP droplets and methane bubbles existed in the bubble column reactor. The images in Figure 2b−d were recorded in segments I, II, and III, respectively, after 10 min. As can be seen from Figure 2b, the image in segment I taken after 10 min is similar to that taken in segment I after 5 min. As can be seen from Figure 2c, a thin hydrate film first formed on the outside of the CP spheres and made the CP spheres translucent and irregular. This phenomenon further demonstrates that a hydrate shell formed at the CP/water phase boundary. As can be seen from Figure 2d, the numbers of crystals increased obviously. Panels e−g of Figure 2 show pictures taken in segments I−III, respectively, after 25 min. The densities of hydrate particles in the segments corresponding to Figure 2e−g are higher than those in the corresponding segments in Figure 2b−d on account of the continuous considerable formation of hydrate. Compared to Figure 2c, the methane gas bubbles gradually shrank and even disappeared in Figure 2f. This phenomenon means that the methane gas encapsulated by the CP membranes gradually occupied the cavities of the hydrate. The consumption of methane gas described in the following discussion can further support this result. Panels h−j of Figure 2 show images recorded in segments I−III, respectively, after 50 min. As can be seen from Figure 2e,f, the translucent surfaces of the rising droplets and bubbles changed to become turbid and rough
nCH4,in − nCH4,out tV
nCH4,in − nCH4,out
(1)
where rav (mol·min−1·m−3) is the average methane gas consumption rate; nCH4,in, nCH4,out, and ΔnCH4 (mol) are the numbers of moles of methane gas charged into the reactor, discharged from the reactor, and consumed by hydrate formation, respectively; V (m3) is volume of the water in the reactor; and t (min) is the reaction time. In addition, redundant methane gas was collected by the gas collecting cylinder. The final methane gas conversion ratio (η) was calculated as 5970
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from CP competing with methane to occupy the hydrate cavities during this period. 3.3. Effect of CP Flow Rate. Figure 5 shows a comparison of the cumulative gas consumption at different flow rates of CP
surfaces. In this period, hydrate particles formed continuously, gradually grew, then aggregated with each other along the column reactor because of their adherence force,20 and finally formed a hydrate slurry. As can be seen from Figure 2j, the whole liquid phase was full of flocculent hydrate. After 150 min, hydrate particles floated and accumulated on top of the liquid surface and formed a hydrate layer in segment IV, as shown in Figure 2k. Various configurations in the hydrate formation process are illustrated in Figure 3. CP-CH4 hydrates start to form at the CP droplet surface, and then methane enter the structure. 3.2. System Temperature Change and Induction Time. Figure 4 shows the changes in the temperatures and
Figure 5. Cumulative gas consumption for hydrate formation with Qg = 159.1 ± 0.1 mL/min and different values of QCP at 277.05 K and 2.0 MPa.
at 277.05 K and 2.0 MPa with Qg = 159.1 ± 0.1 mL/min for runs 3−5. As can be seen from Figure 5, the cumulative number of moles of gas consumed was almost constant at a fixed time as QCP increased from 23 to 90 mL/min. CP and methane form sII hydrates as the large-molecule guest substance and guest gas, respectively.23 According to the structural formula of sII hydrate, the calculated volume ratio of CP to methane gas should be approximately 1:476.8 at normal temperature and pressure conditions. However, the volume ratios of the CP to methane gas achieved in this work were 1:13.352, 1:8.53, and 1:3.412. This indicates that the volume of CP per minute was excessive for methane/CP hydrate formation at 277.05 K and 2.0 MPa with Qg = 159.1 ± 0.1 mL/min. Thus, the cumulative gas consumption was the same at different CP flow rates at a given time for runs 3−5. However, the induction time decreased with increasing flow rate of CP, as shown in Table 1. This illustrates that the addition of CP promotes the nucleation of gas hydrate and the nucleation rate increases with the amount of CP.21 3.4. Effect of Operating Pressure. Figure 6 shows the effect of operating pressure on the cumulative gas consumption at 277.15 ± 0.1 K, for runs 1, 2, 8, and 10. As can be seen from Figure 6, the cumulative number of moles of gas consumed increased with the operating pressure. This behavior can be attributed to the fact that an increase in the operating pressure means an increase in the driving force for gas hydrate formation. The driving force is the difference between the system pressure and the equilibrium hydrate formation pressure. According to Henry’s law, the amount of dissolved gas around the shrinking bubble increases with the gas pressure. Therefore, the cumulative gas consumption corresponding to gas hydrate formation increases with the operating pressure. Figure 7 shows the effect of the operating pressure on the gas conversion ratio. As can be seen from Figure 7, the gas conversion ratio increased with the operating pressure at fixed flow rate of methane gas. For example, as the operating pressure increased from 1.0 to 2.0 MPa, the gas conversion
Figure 4. Changes in temperatures and cumulative gas consumption for hydrate formation with QCP = 90 mL/min and Qg = 159 mL/min at 277.05 K and 2.0 MPa.
the cumulative gas consumption during hydrate formation with QCP = 90 mL/min and Qg = 159 mL/min at 277.05 K and 2.0 MPa. T1, T2, and T3 denote the temperatures of segments I−III, respectively. Temperatures T1, T2, and T3 decreased slightly in the starting stage of the experiment (0−3.3 min), possibly because of the disturbance between the gas phase and the liquid phase caused by the introduction of methane gas into the liquid. Because the temperature of the water close to the inner wall of the vessel jacket was lower, the disturbance accelerated the heat change between the inner wall and the center, which resulted in the decrease of temperatures T1, T2, and T3. The period from 0 to 3.3 min describes the process of gas dissolution and hydrate nucleation. Similar characteristics were found previously.22 During this period, the slight increase of methane gas consumption comes from the gas dissolution. From 3.3 to 38.8 min, the temperature first increased and then decreased. This period involves a process of phase transformation (among the gas, CP, and hydrates) that produces the heat of hydrate formation. From 3.3 to 22.2 min, the system temperature gradually rose because the heat (from the system absorbed by the ambient) was less than that released from hydrate formation. Then, the temperature decreased gradually from 22.2 to 38.8 min because the heat (from the system absorbed by the ambient) was greater than that released from hydrate formation in the system.22 However, after 38.8 min, the temperature increased sharply. This means that, after 38.8 min, large quantities of hydrates formed, associated with the release of considerable heat. Nevertheless, the consumption rate of methane gas increased only slightly. This might have resulted 5971
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Figure 8. Cumulative gas consumption for hydrate formation at different experimental temperatures at 2.0 MPa with QCP = 36 mL/ min and Qg = 234.5 ± 1 mL/min.
Figure 6. Cumulative gas consumption for hydrate formation at different operating pressures and 277.15 ± 0.1 K for fixed Qg.
Figure 7. Gas conversion ratio for hydrate formation at different operating pressures and 277.15 ± 0.1 K for fixed Qg. Figure 9. Methane gas conversion ratio for hydrate formation at different experimental temperatures and 2.0 MPa with QCP = 36 mL/ min and Qg = 234.5 ± 1 mL/min.
ratio increased from 41.8% to 47.2% at Qg = 109 mL/min. As the operating pressure increased from 2.0 to 2.5 MPa, the gas conversion ratio increased from 83.2% to 94.6% at Qg = 310 ± 2.5 mL/min. In addition, as discussed above, higher operating pressure resulted in a higher driving force. Thus, a shorter induction time can be obtained with a greater driving force.24 As shown in Table 1, the induction times for runs 8 and 10 were 1.2 and 0.67 min when the operating pressures were 2.0 and 2.5 MPa, respectively. The induction time of 0.67 min is quite short, and the results offer reliable data for the application of gas production in industry. 3.5. Effect of Experimental Temperature. Figure 8 shows the cumulative numbers of moles of gas consumed at different experimental temperatures with QCP = 36 mL/min and Qg = 234.5 ± 1 mL/min at an operating pressure of 2.0 MPa for runs 6, 7, 11, and 12. Generally, the cumulative gas consumption increased linearly with the hydrate formation time at a given temperature. However, it also increased as the temperature decreased from 280.15 to 276.45 K. The reason is that a higher driving force was obtained at the lower temperature when the operating pressure was fixed and a high driving force can promote hydrate formation. A similar phenomenon was also reported elsewhere.25 Figure 9 shows a comparison of the gas conversion ratios obtained at different temperatures. The experimental results indicate that lower
temperature resulted in a higher gas conversion ratio at a pressure of 2.0 MPa and Qg = 234.5 ± 1 mL/min. In addition, from a comparison of runs 6, 7, 11, and 12, one can see that the induction time decreased with decreasing experimental temperature, as shown in Table 1. This phenomenon indicates that lower temperature is favorable for shortening the induction time and enhances the cumulative gas consumption and gas conversion ratio. 3.6. Effect of Flow Rate of Methane Gas. As can be seen from Table 1, the induction times of runs 2, 4, 6, and 8 decreased from 10.8 to 1.2 min, respectively, as Qg increased from 83.8 to 308.9 mL/min. Figure 10 shows the numbers of moles of gas consumed at different Qg values at an operating pressure of 2.0 MPa and an experimental temperature of 277.15 ± 0.1 K for runs 2, 4, 6, and 8. As shown in Figure 10, the cumulative gas consumption increased as Qg increased from 83.8 to 308.9 mL/min. Because the cumulative methane gas consumption increased linearly with the reaction time, the average methane consumption rate corresponding to the hydrate formation rate increased with increasing methane gas flow rate. For example, at the same experimental temperature and operating pressure, the r av value for run 4 was 5972
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suitable flow rate of methane gas that not only promotes hydrate formation but also improves the gas conversion ratio. In addition, a comparison among the effects of gas flow rate, operating pressure, and experimental temperature on the average methane gas consumption rate was made. As shown in Figure 12, the rav value for run 2 was approximately 0.12
Figure 10. Cumulative gas consumption for hydrate formation at 277.15 ± 0.1 K and 2.0 MPa for different Qg values.
approximately 0.20 mol·min−1·m−3 with Qg = 159.2 mL/min, whereas the rav value for run 8 was approximately 0.39 mol·min−1·m−3 with Qg = 308.9 mL/min. These results can be attributed to the fact that a high flow rate of methane gas markedly enhances the flow turbulence and interphase mixing, thereby enhancing the gas−liquid interface26 and accelerating both mass and heat exchange among the gas, liquid, and solid phases.27 Increasing the flow rate of methane gas can yield a higher concentration of gas in the liquid at a given time and result in a larger contact area between gas and liquid, further supporting hydrate nucleation. Figure 11 shows the effect of the
Figure 12. Comparison of rav for run 2 at 277.05 K and 2.0 MPa with Qg = 83.8 mL/min and rav for run 9 at 277.05 K and 1.5 MPa with Qg = 233.2 mL/min.
mol·min−1·m−3 at an experimental temperature of 277.05 K, an operating pressure of 2.0 MPa, and a Qg value of 83.8 mL/min. The rav value for run 9 was approximately 0.25 mol·min−1·m−3 at a temperature of 277.05 K, a pressure of 1.5 MPa, and a Qg value of 233.2 mL/min. Thus, the operating pressure for run 2 was 0.5 MPa higher than that for run 9, but the rav value for run 2 was lower than that for run 9 because the Qg for run 2 (83.8 mL/min) was higher than that for run 9 (233.2 mL/min). In addition, a comparison of the rav values for runs 2 and 12 is shown in Figure 13. The rav value for run 12 was approximately 0.23 mol·min−1·m−3 at an experimental temperature of 280.15
Figure 11. Gas conversion ratio for hydrate formation at 277.15 ± 0.1 K and 2.0 MPa for different Qg values.
flow rate of methane gas on the methane conversion ratio. For Qg values of 83.8, 159, and 234.3 mL/min, the gas conversion ratios were 47.2%, 59.5%, and 94.7%, respectively. However, the gas conversion ratio decreased to 83.2% when Qg was increased further to 308.9 mL/min. The high flow rate of methane gas is helpful for mixing the gas and liquid thoroughly and promotes heat and mass exchange, further accelerating hydrate formation. Thus, the gas conversion ratio increased with increasing flow rate of methane gas. However, when the flow rate of methane gas was too high, more methane gas was discharged directly into gas collection cylinder before entering the hydrate structure. Consequently, it is important to select a
Figure 13. Comparison of rav for run 2 at 277.05 K and 2.0 MPa with Qg = 83.8 mL/min and rav for run 12 at 280.15 K and 2.0 MPa with Qg = 235.1 mL/min. 5973
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K, an operating pressure of 2.0 MPa, and a Qg value of 235.1 mL/min. Thus, the rav value for run 2 was lower than that for run 12 even though the temperature for run 2 was 3.1 K lower than that for run 9. It is noted that the average methane consumption rate corresponding to the hydrate formation rate can be effectively enhanced by increasing the flow rate of gas even at relatively low pressure and high temperature. Moreover, from a comparison of the methods used to promote the gas hydrate formation according to the above results, the energy consumed for increasing the gas flow rate reasonably is lower than the amounts of energy consumed for increasing the system pressure and decreasing the system temperature. Furthermore, the results offer a constructive suggestion for hydrate-based application technology in the future.
4. CONCLUSIONS In this work, the investigation into gas hydrate formation of methane in a large-size bubble column reactor was carried out in the presence of cyclopentane (CP). The cumulative methane gas consumption increased linearly with the hydrate formation time for each experimental run. It was found that a thin hydrate shell formed at the CP/water phase boundary. The flow rate of CP had a minimal effect on the gas consumption under the experimental conditions, whereas the induction time decreased with the flow rate of CP. The methane gas consumption, conversion ratio, and induction time were affected by the operating pressure, experimental temperature, and flow rate of methane gas. The gas conversion ratio moderately increased as Qg changed from 83.8 to 234.3 mL/min. However, it no longer increased with a further increase of Qg from 234.3 to 308.9 mL/ min. The cumulative methane gas consumption and gas conversion ratio increased with either a decrease of the experimental temperature or an increase of the operating pressure. The average methane consumption rate corresponding to the hydrate formation rate can be effectively enhanced by increasing the flow rate of gas even at relatively low pressure and high temperature. Thus, concerning low energy consumption and high energy efficiency, a reasonable flow rate of feed gas is helpful for the rapid formation of gas hydrate.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86-20-87057037. Fax: +86-20-87034664. E-mail: lixs@ ms.giec.ac.cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (51076155) and Science & Technology Program of Guangdong Province (2009B050600006).
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REFERENCES
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dx.doi.org/10.1021/ie202422c | Ind. Eng. Chem. Res. 2012, 51, 5967−5975
Industrial & Engineering Chemistry Research
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dx.doi.org/10.1021/ie202422c | Ind. Eng. Chem. Res. 2012, 51, 5967−5975
