Enhanced Kinetics of CO2 Hydrate Formation under Static Conditions

Dec 19, 2008 - Trapping CO2 in hydrates is one of the new technologies for CO2 capture and storage. One of the primary obstacles to this option is the...
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Ind. Eng. Chem. Res. 2009, 48, 5934–5942

Enhanced Kinetics of CO2 Hydrate Formation under Static Conditions Junshe Zhang and Jae W. Lee* Department of Chemical Engineering, The City College of New York, New York, New York 10031

Trapping CO2 in hydrates is one of the new technologies for CO2 capture and storage. One of the primary obstacles to this option is the low formation rate. This work presents the rapid formation of CO2 hydrates with a small amount of cyclopentane (CP). The formation kinetics was investigated in a 474 cm3 nonstirred batch reactor with 100 and 200 cm3 of water. At volume ratios of CP to water between 0.01 and 0.1, the maximum growth rate is 0.32 mol h-1 at CO2 pressures ranging from 1.9 to 3.4 MPa and at 274 K. CO2 hydrates (sI) and CO2 + CP (sII) binary hydrates coexist at the end of hydrate growth. The mole ratio of CO2 entrapped in the sI hydrates to that in the sII binary hydrates is higher with 100 cm3 of water than with 200 cm3 of water. The same trend is also observed for the total amount of CO2 entrapped in the hydrate phase. The growth rate depends not only on the water volume but also on the pressure. The hydrate growth rate and the water conversion reach a maximum at 3.06 MPa and then decrease as the pressure increases from 1.9 to 3.4 MPa with 100 cm3 of water and 5 cm3 of CP. The water conversion to the hydrates reaches 52% within 2 h. This accelerated formation kinetics can provide a stepping-stone for developing a new hydrate-based CO2 capture and storage technique. 1. Introduction Global climate change is one of the most important environmental issues. It is believed that global climate change is due to the increased concentration of greenhouse gases (GHGs) in the atmosphere. The concentration of CO2, being responsible for about 60% of the greenhouse effect, has risen from 280 ppm at the preindustrial level to currently 360 ppm in the atmosphere.1 Anthropogenic emissions of CO2, mainly from power generation, oil and gas refining, hydrogen and ammonia processing, iron and steel manufacturing, and cement production, have contributed to the increased CO2 concentration.2,3 CO2 capture and storage (CCS) is one of the important concepts to reduce greenhouse gas emissions. The techniques currently available for CO2 capture are (1) physical adsorption, (2) chemical/physical absorption, (3) low temperature distillation, and (4) membranes.1,4-6 Making these options more efficient and developing new technologies for CCS will be necessary in order to continuously use fossil fuels, which is unavoidable at present and in the near future. One new approach for CCS is based on gas hydrate formation.6-13 Gas hydrates are nonstoichiometric crystalline compounds in which guest molecules such as carbon dioxide, nitrogen, methane, and cyclopentane (CP) stabilize the hydrogenbonded water cavities.14 If all the cavities of hydrates are occupied by gas molecules, a unit volume of hydrates can store up to ∼170 volumes of gas at 0 °C and 1 atm. Even if in a rudimentary stage, gas hydrate systems can provide an opportunity to develop innovative clean energy initiatives, including CO2 capture,10 ocean CO2 sequestration,9 and CO2/N2 separation.6 One challenge to implementing the gas hydrate technology for CCS is to realize the rapid formation of CO2 hydrates in a reasonable time frame. The hydrate formation process includes two steps: hydrate nucleation and crystal growth. Hydrate nucleation is stochastic in nature since the induction time varies from a few second to several days, whereas crystal growth is usually controlled by the transport of gas molecules from the gas phase to the hydrate/ * To whom correspondence should be addressed. Tel.: (212) 6506688. Fax: (212) 650-6660. E-mail: [email protected].

liquid interface.14 Thus, the long formation time makes the hydrate-based technology for CCS less economically feasible compared with other options such as adsorption and absorption. Increasing the driving force for hydrate crystallization can reduce the time required for gas hydrate formation. One way to increase the driving force is to enforce higher operating pressures or lower operating temperatures. Another path is to reduce the equilibrium pressures by the addition of a small amount of thermodynamic promoters such as tetrahydrofuran (THF)8,10,15 and tetrabutylammonium bromide (TBAB).7,16 The disadvantage of using these thermodynamic promoters is that the amount of CO2 captured in the hydrate form decreases since the thermodynamic promoters occupy some water cavities. This can be overcome when using a kinetic promoter. Some surfactants such as sodium dodecyl sulfate (SDS) were found to be excellent kinetic promoters for hydrocarbon hydrates.17-19 However, until now, no studies have indicated that SDS could promote CO2 hydrate formation. Seo et al.12 found that about 90% of water dispersed in silica gel with a nominal pore diameter of 30 nm is converted to CO2 hydrates in 1 h, which is due to the high degree of water dispersion. However, this process has a low efficiency of CO2 recovery because most of the reactor volume is occupied by silica gel. If the silica gel is replaced by hydrates with some cavities unoccupied and unchanged water dispersion, not only does the recovery of CO2 increase, but also the formation of pure CO2 hydrates may be accelerated. In this work, we will present the accelerated growth rate of CO2 hydrates and water conversion to the hydrates by adding a small amount of cyclopentane (CP). CP was found to be a good thermodynamic promoter.20 The merits of CP over THF and TBAB are that (1) the melting point of CP hydrates is higher than that of THF hydrates,21 (2) CP is less toxic than THF, and (3) the equilibrium pressure of CP + CO2 binary hydrates is independent of CP concentrations because CP is immiscible with water. To the best of our knowledge, this is the first work to obtain a high yield of CO2 hydrate formation without any mechanical agitation. The enhanced kinetics will be characterized by the overall hydrate growth rate, the water conversion,

10.1021/ie801170u CCC: $40.75  2009 American Chemical Society Published on Web 12/19/2008

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Figure 1. Schematic diagram of experimental setup.

and the CO2 consumption for the hydrate formation for given water volumes, pressures, and CP volumes. 2. Experimental Section 2.1. Materials. Carbon dioxide was purchased from T. W. Smith with a minimum purity of 99%. Cyclopentane (CP) used in this work has a purity of 95% and was supplied by Alfa Aesar. All chemicals were used as received without further purification. Deionized (DI) water was produced in our laboratory with a resistivity of 17 mΩ cm-1. 2.2. Apparatus. Figure 1 shows the schematic diagram of the experimental setup to investigate the formation kinetics of CO2 hydrates in the presence of CP. The high-pressure reactor with a volume of 474 cm3 was customized by Parr Instruments, and it can be operated at temperatures between 253 and 623 K and at pressures up to 22 MPa. The temperature of the reactor was controlled by circulating the coolant from an Isotemp 3006P thermostat (Fisher Scientific) with a stability of (0.01 K inside the jacket around the reactor. The temperature of the reactor was monitored with two type-T thermocouples (Omega), where one was immersed in the liquid and the other was placed in the gas phase; the uncertainty of the temperature measurement is (0.2 K. The pressure of the reactor was measured by using a 9001PDM pressure transducer (Ashcroft, 0-34.47 MPa) with an accuracy of (0.03 MPa. CP was charged to the reactor through an ISCO 260-D syringe pump (Teledyne Isco) with an accuracy of (0.01 cm3. The amount of CO2 charged to the reactor was measured by an FMA-8508 mass flow meter (Omega, 0-5000 sccm); the uncertainty of the flowrate measurement is (1% of full scale. The temperature and pressure were sampled every 20 s by the Labview interface. 2.3. Procedures. The temperature of the thermostat was set to 273.2 K after charging 100 or 200 cm3 of DI water to the reactor, and then the reactor was purged with CO2 twice. The temperature of the thermostat was maintained at this level until formation was complete. CP was injected into the reactor at a flow rate of 1 cm3 min-1 once the temperature of reactor was stabilized, and CO2 was charged 0.5 h later to a preset pressure. No mechanical agitation was applied to all runs. More CO2 was charged to the reactor once the pressure decreased to the

equilibrium pressure at an operating temperature. The occurrence of CO2 hydrates at the end of hydrate growth was verified by dissociating hydrate mixtures at 285 K or above. 2.4. Water Conversions, CO2 Recovery, and Growth Rate. The amount of CO2 in the gas phase is calculated by using the Peng-Robinson equation of state (EOS) without considering the gas volume change in the gas phase. The solubility of CO2 in water is calculated by using the computer program CO2SOL22 at pressures below the hydrate equilibrium pressure, or CSMGem14 at pressures above the equilibrium pressure. This CO2 solubility is counted from the change of CO2 mole number in the gas phase when calculating CO2 entrapped in the hydrate phase. When the number of moles of CO2 is calculated for CO2 + CP (sII) binary hydrate formation (nCO2, sII) at the end of hydrate growth, we assume that all of CP is consumed for sII hydrate formation with CO2. The remaining mole change of CO2 from the CO2 pressure change in the gas phase is attributed to CO2 hydrate (sI) formation (mole number of CO2 in sI hydrates ) nCO2,sI). The Langmuir constant for CO2 in large cavities of sII hydrates is calculated as 51 MPa-1 at 273.6 based on Munck et al.’s equation,23 whereas the Langmuir constant of CP in large cavities is 6.5 × 105 MPa-1 at 273.6 K by integration of a spherically symmetric Kihara potential (the parameters were given by Tohidi et al.20). The latter is 4 orders of magnitude higher than the former, and the saturated vapor pressure of CP is 1.46 × 10-2 MPa at 273.6 K.24 Therefore, we can assume that CO2 only occupies the small cavities of sII hydrates at CO2 pressures below 3.5 MPa. The fractional occupancy of CO2 (θCO2) in water cavities is calculated from the following Langmuir adsorption isotherm:14 θCO2 )

CCO2fCO2 1 + CCO2fCO2

(1)

where fCO2 (MPa) is the CO2 fugacity, which is calculated by using the Peng-Robinson EOS, and CCO2 (MPa-1) is the Langmuir constant, which is obtained from CCO2 )

B A exp T T

()

(2)

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where T (K) is the temperature; A (K MPa-1) and B (K) are constants, whose values were given by Munck et al.23 Water conversion, xW, is given by xW )

100(mi - mf) mi

(3)

where mi (kg) is the initial mass of water charged to the reactor and mf (kg) is the final mass of water at the end of hydrate growth. The recovery of CO2 in hydrates, yCO2, is calculated from the following equation: yCO2 )

nCO2,H nCO2,t

(4)

where nCO2,H (mol) is the mole number of CO2 entrapped in hydrates; nCO2,t (mol) is the total amount of CO2 charged. nCO2,H is the sum of nCO2,sII + nCO2,sI. These three calculated mole numbers are confirmed by the hydrate dissociation experiments. The overall hydrate growth rate, r (mol h-1), is defined as the mole number of CO2 entrapped in hydrates divided by the growth period r)

nCO2,H tG

(5)

where tG (h) is the growth period, which is the time between the onset and the end of hydrate growth that is determined from the plots of the temperature and pressure versus reaction time. The end point is where the slope of the plots starts to decrease to zero. 3. Results and Discussion 3.1. Formation with 100 cm3 of Water. When hydrate formers contact water under conditions where hydrates are thermodynamically stable, it usually takes a long time for the appearance of a detectable volume of hydrates or a significant change in the amount of hydrate formers to occur.14 The time taken for hydrates to be detected macroscopically is defined as the induction time. Figure 2a shows a plot of the mole number of CO2 in the gas phase versus reaction time during the hydrate formation with 5 cm3 of CP. The mole number of CO2 starts to decrease rapidly at about 0.1 h after charging CO2, suggesting that the hydrate induction time is around 0.1 h. At a pressure of 2.65 MPa (point A in Figure 2a), the dissociation equilibrium temperature is about 279 K for CO2 hydrates14 and 292 K for CO2 + CP binary hydrates.25 Thus, the supercooling is around 5 K for CO2 hydrate, 6 K for CP hydrate,25 and 18 K for CO2 + CP binary hydrate nucleation at a temperature of 274 K (point A in Figure 2a). According to a hydrate nucleation theory, hydrate nucleation occurs readily in the labile region whose boundary is determined by the supercooling at a specific pressure.14 Although the labile region for CP, CO2, and CP + CO2 hydrates has not been reported and is also very hard to determine, the experimental conditions at point A in Figure 2a are most likely located in the labile region of CO2 + CP binary hydrates due to the high supercooling (18 K). Therefore, CO2 + CP binary hydrates most possibly nucleate first during the induction period. The temperature and pressure profile from the start of CO2 charge to the end of hydrate growth is given in Figure 2b. The pressure between points A and B is above the dissociation pressure of CO2 hydrates as predicted by CSMGem.14 However, the temperature at a pressure range of 1.4-1.7 MPa (a part of the curve between points B and C) is higher than the dissociation

Figure 2. Formation of CO2 and CO2 + CP mixture hydrates with 100 cm3 of water and 5 cm3 of CP at an initial pressure of 2.7 MPa. (a) Mole number of CO2 in gas phase. (b) Temperature and pressure profile. (c) Change in the liquid phase temperature and gas pressure.

temperature of CO2 hydrates. Therefore, the consumption of CO2 within this pressure range is mainly due to the growth of CO2 + CP binary hydrates (see the CO2 consumption from point B to point C in Figure 2a). Figure 2c presents the change of the temperature and pressure versus reaction time during the hydrate formation. As the hydrate growth proceeds, the pressure monotonically decreases toward a constant level in 1.8 h after charging CO2, which corresponds to the dissociation pressure of CO2 hydrates. More than one temperature peak is observed, which is because more than one type of hydrate forms, i.e., simple CO2 hydrates and binary CO2 + CP hydrates. The occurrence of more than one type of

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Figure 3. Dissociation of CO2 hydrates with 100 cm3 of water and 5 cm3 of CP at an initial pressure of 2.7 MPa. (a) Temperature and pressure profile. (b) Mole number of CO2 in the gas phase.

hydrates (sI and sII) is mainly dependent on thermodynamic conditions if we assume that the system reaches equilibrium at the end of the run. Both CO2 and CP + CO2 binary hydrates are thermodynamically stable at point C in Figure 2. The coexistence of these two hydrates is verified by dissociating the hydrate mixture as described below. The profile of pressure versus temperature after setting the temperature of the thermostat to 285 K is shown in Figure 3a. During the hydrate dissociation from points A to B, the pressure is higher than the equilibrium pressure of CO2 + CP binary hydrates but lower than that of CO2 hydrates. The number of moles of CO2 in the gas phase increases from 0.23 to 0.43 when the temperature increases from 273.7 to 285.3 K (Figure 3b). The increase in moles of CO2 in the gas phase is due to the dissociation of sI CO2 hydrates because pure CO2 hydrates are unstable and the solubility of CO2 in water at point A (0.89 mol kg-1) is less than that at point B (0.99 mol kg-1). Thus, the amount of CO2 stored in pure CO2 hydrates is about 0.2 mol. The total CO2 initially charged to the reactor is 0.59 mol. Then, the CO2 entrapped in CO2 + CP binary hydrates is around 0.08 mol if the absorption of CO2 reached equilibrium and all CP converts to hydrates at the end of dissociation. These measured mole numbers of CO2 entrapped in simple CO2 hydrates and CO2 + CP binary hydrates are close to those calculated by assuming that CO2 only occupied the small cavities of CO2 + CP binary hydrates (Table 1 with 5 cm3 of CP). The pressure is equal to the equilibrium pressure of CO2 hydrates in the temperature range of 273.7-275.3 K in Figure 3a, indicating that dissociation proceeds at equilibrium conditions. Similar behavior is also observed for the hydrate growth

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at the later stage of hydrate growth (some points close to the CO2 hydrate phase boundary from B to C in Figure 2b). This is surprising because the average heating rate in this study is about 15 K h-1, 2 orders of magnitude higher than a typical heating rate (0.1 K h-1) used for determining gas hydrate equilibrium conditions using the continuous-heating method.14 For the purpose of comparison, the formation kinetics of pure CO2 hydrates (without CP) is shown in the Supporting Information (Figure 1). The mole number of CO2 in the gas phase decreases from 0.5654 to 0.4539 in 6.3 h after charging CO2, and then it decreases to 0.4209 in another 69.3 h after charging. The pressure decreases from 2.75 to 2.15 MPa in 75.6 h. The induction time is relatively difficult to determine because the fluctuation in the temperature is less than 0.1 K after CO2 charge. If we assume that the aqueous phase is in equilibrium with the hydrate phase at the end of the run, then the mole number of CO2 in hydrates is 0.064 after 75.6 h by excluding CO2 dissolved in the aqueous phase. This is also observed for the formation of CO2 hydrates with SDS solutions (Figure 2 in the Supporting Information). The water conversion, the CO2 recovery, and the hydrate growth rate with the three volumes of CP used are listed in Table 1. Both the water conversion and the CO2 recovery increase with increasing CP amount, while the ratio of CO2 entrapped in sI hydrates to that in sII hydrates (nsI/nsII) decreases as more CP is added and more CO2 + CP binary hydrates form. The total mole number of CO2 entrapped in the hydrate phase (nCO2,H) with 5 and 10 cm3 of CP is about twice that with 2 cm3 of CP for the first charge of CO2. The pressure approaches the equilibrium pressure of CO2 hydrates with 5 and 10 cm3 of CP, but it is higher than the equilibrium pressure with 2 cm3 of CP at the end of run. The different formation kinetics with different amounts of CP can be explained by the role of CP during the hydrate formation. First, the CO2 + CP binary hydrates may nucleate readily due to the high supercooling as mentioned before, and then CO2 hydrates nucleate spontaneously during the CO2 + CP binary hydrate growth. However, we cannot determine when the spontaneous nucleation occurs at this point. The crystal structure of CO2 hydrates (sI, body centered cubic) is different from that of CO2 + CP binary hydrates (sII, diamond lattice); therefore, the coexistence of these two types of hydrates at certain ratios makes the hydrate phase have a fine-grained or porous structure as reported in the previous studies.26-28 The CO2 and CO2 + CP binary hydrate mixtures also have a finegrained and porous structure as shown in Figure 4a, which allows CO2 to diffuse to the water film. 3.2. Formation with 200 cm3 of Water. Table 2 summarizes the results from the kinetic experiments with 200 cm3 of water and three different amounts of CP. The water conversion, CO2 recovery, and hydrate growth rate with 200 cm3 of water are lower than those with 100 cm3 of water for the first charge of CO2. The low water conversion arises from the smaller mole number of CO2 charged to the reactor than the case with 100 cm3. The pressure at the end of run with 2 cm3 of CP is higher than the equilibrium pressure of CO2 hydrates, while those with 5 and 10 cm3 of CP reach the equilibrium pressures. For the first charge of CO2, CO2 is not entrapped much in sI hydrates with 200 cm3 of water as shown in Table 2, while a lot of CO2 is entrapped in sI hydrates with 100 cm3 of water as shown in Table 1. This difference cannot be explained by the relative ratio of CP to water because (1) a larger volume of water is available for the formation of sI CO2 hydrates with 200 cm3 of water (Table 2) than with 100 cm3 of water (Table 1), (2) CP is immiscible with water and thus the phase boundary of CP +

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Table 1. Summary of Formation Kinetics with 100 cm3 of Water Vcpa (cm3)

P ib (MPa)

Tfc (K)

Pfc (MPa)

nCO2,td (mol)

nCO2,sIe (mol)

nCO2,sIIe (mol)

nsI/nsII

nCO2,Hf (mol)

100xw

100yCO2

2 5

2.78i 2.73i 3.41i 2.68i 3.41j

273.46 273.67 273.65 274.31 273.81

1.79 1.31 2.73 1.34 2.95

0.5737 0.5909 1.1053 0.5928 1.1103

0.1393 0.2428 0.4176 0.1779 0.2925

0.0294 0.0673 0.0796 0.1341 0.1601

4.74 3.61 5.25 1.33 1.83

0.1687 0.3101 0.4972 0.3120 0.4526

21.17 41.90 59.92 50.27 61.99

29.40 52.48 44.99 52.63 40.84

10

tig (h)

tgg (h)

r (mol h-1)

∆Tmaxh (K)

0.05 0.10

0.84 1.74

0.2008 0.1782

4.5 4.0

0.04

1.40

0.2229

10.7

a Volume of CP. b Pressure after CO2 charged. c Temperature and pressure at the end of runs. d Total mole number of CO2 charged. e Mole number of CO2 in sI or sII hydrates. f Mole number of CO2 in the total hydrate phase. g Induction time and growth period. h Maximum temperature increase. i First charge. j Second charge.

Figure 4. CO2 and CO2 + CP mixture hydrates with (a) 100 cm3 of water and 10 cm3 of CP after two CO2 charges, (b) 200 cm3 of water and 2 cm3 of CP after a single CO2 charge, and (c) 200 cm3 of water and 10 cm3 of CP after three CO2 charges. Table 2. Summary of Formation Kinetics with 200 cm3 of Water Vcpa (cm3)

P ib (MPa)

Tfc (K)

Pfc (MPa)

nCO2,td (mol)

nCO2,sIe (mol)

nCO2,sIIe (mol)

nsI/nsII

nCO2,Hf (mol)

100xw

100yCO2

2 5

2.69i 2.71i 3.47i 2.67i 3.41j 3.42k

273.41 273.60 273.43 273.61 273.56 273.56

2.26 1.38 2.81 1.37 1.31 3.17

0.4364 0.4532 0.8734 0.4579 0.8844 1.2590

0.0000 0.0519 0.2271 0.0031 0.4841 0.4901

0.0309 0.0685 0.0802 0.1365 0.1348 0.1631

0.00 0.76 2.83 0.02 3.59 3.00

0.0309 0.1204 0.3073 0.1396 0.6189 0.6523

3.09 10.54 19.81 15.60 41.81 41.46

7.08 26.57 35.18 30.47 69.99 51.88

10

tig (h)

tgg (h)

r (mol h-1)

∆Tmaxh (K)

0.14 0.02

0.42 1.34

0.0736 0.0899

0.4 1.9

0.17

1.67

0.0836

1.4

a Volume of CP. b Pressure after CO2 charged. c Temperature and pressure at the end of runs. d Total mole number of CO2 charged. e Mole number of CO2 in sI or sII hydrates. f Mole number of CO2 in the total hydrate phase. g Induction time and growth period. h Maximum temperature increase. i First charge. j Second charge. k Third charge.

CO2 and CO2 hydrates is independent of the amount of CP, and (3) the same operating temperature and pressure give almost the same driving force for the hydrate formation regardless of the ratio. A plausible explanation for this difference is that the amount of CO2 entrapped in the hydrates depends on the growth habit according to the temperature difference between the reactor wall and bulk liquid as explained below.

Hydrates usually form at the liquid-liquid interface when both organic liquid and gas are present in the systems.14 The CO2 + CP binary hydrates possibly nucleate first due to the high supercooling as mentioned before and then CO2 + CP binary hydrate growth and CO2 hydrate growth occur simultaneously. The temperature of the liquid phase increases dramatically once the growth of hydrates starts, as given in Table 1

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Figure 6. Change in temperature of the liquid phase and pressure during hydrate formation with 200 cm3 of water and 10 cm3 of CP at an initial pressure of 2.7 MPa.

Figure 5. Change in temperature of liquid phase and pressure during hydrate formation with 2 cm3 of CP at an initial pressure of 2.7 MPa. (a) Change in pressure with reaction time. (b) Change in temperature of the liquid phase.

(∆T max ) 4.5, 4.0, and 10.7 K with respect to CP volumes, where ∆Tmax is the maximum temperature difference between the top of temperature peaks and the end of hydrate growth as shown in Figure 2c), because the heat released from the hydrate formation cannot be removed immediately. The temperature near the reactor wall is the lowest, and it is the closest to the coolant temperature under static conditions because of the coolant circulation in the jacket. Therefore, the driving force of hydrate formation is highest near the reactor wall and more hydrates are accumulated at this position than at the other locations. Hydrates accumulated along the reactor wall have a porous structure that draws water from the bulk phase to the crystallization front by a capillary force, and thus hydrate growth continues along the reactor as shown in Figure 4a. The same growth habit was also reported for propane17 and methane hydrates.18 On the other hand, if the temperature increase (∆Tmax) is relatively small as shown in Table 2 (∆Tmax ) 0.4, 1.9, and 1.4 K with respect to CP volumes), the growth rate is almost homogeneous across the section of reactor. Therefore, the whole interface may be covered with a thin layer of hydrates that reduces the diffusion of CO2 into the bulk water as shown in Figure 4b. The changes of temperature and pressure with reaction time with 100 and 200 cm3 of water are given in Figure 5. The maximum temperature fluctuation is 0.4 K with 200 cm3 of water and 4.5 K with 100 cm3 as shown in Figure 5b. This trend was the same for the different volumes of CP (Tables 1 and 2) for the first CO2 charge. More CO2 was charged to the reactor when the pressure decreased to the equilibrium pressure of CO2 hydrates with 5 and 10 cm3 of CP. At the end of the last charge, the pressure is higher than the equilibrium one and water conversion is less

than 100% (Tables 1 and 2). Snowlike hydrates were observed after the second and third charges (Figure 4a,c) when we opened the reactor immediately after reaching steady states. The growth habit is strongly dependent on the amount of water in the system. The entire reactor wall is covered with a layer of hydrates and the center of the reactor is empty with 100 cm3 of water (Figure 4a); however, about two-thirds of the reactor is full of hydrates with 200 cm3 of water (Figure 4c). This different growth habit is due to the difference in magnitude of the temperature spike with respect to different amounts of water during hydrate growth (Tables 1 and 2). The larger is the magnitude of the temperature spike, the larger relative amount of hydrates is accumulated along the reactor water as discussed before. After the third charge of CO2 with 10 cm3 of CP and 200 cm3 of water, the pressure decreases slowly but continuously, and no steady state is reached at the end of the run (Figure 6). This may be due to water occluded within the hydrate phase, which makes it difficult for CO2 to transport through the hydrate phase, thereby increasing the time needed to reach steady state. 3.3. Formation at Different Pressures. The effect of CO2 pressure on the formation kinetics is summarized in Table 3. The induction time is less dependent on the pressure. However, the pressure has a significant effect on the water conversion, the CO2 recovery, and the growth rate. Both the CO2 recovery and the mole number of CO2 entrapped in hydrates reach maximum values at a pressure of 3.06 MPa as the pressure increases from 1.9 to 3.41 MPa. The ratio of the number of moles of CO2 entrapped in sI to that sII and the maximum temperature increase (∆Tmax) follow this trend for the given pressure range. According to the argument presented in the previous section, the growth habit is affected by the temperature difference between the reactor wall and bulk liquid. If the temperature difference is small, the relative amount of hydrates accumulated near the reactor wall is also small. Therefore, the whole interface is possibly covered with a thin layer of hydrates that reduces the growth rate significantly. The ratio of CO2 entrapped in sI to sII is 0.24 at an initial pressure of 3.41 MPa, indicating that most of the hydrates are CO2 + CP binary hydrates with ∆Tmax less than 0.2 K (Table 3). The occurrence of this layer is verified by heating the reactor to disrupt the layer and then cooling the reactor again (refer to Figure 3 in the Supporting Information). The mole number of CO2 entrapped in sII hydrates increases from 0.02 to 0.23 after setting the temperature of the thermostat to 278.2 K and then

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Table 3. Summary of Formation Kinetics with 100 cm3 of Water and 5 cm3 of CP Pia (MPa) Tfb (K) Pfb (MPa) nCO2,tc (mol) nCO2,sId (mol) nCO2,sIId (mol) nsI/nsII nCO2,He (mol) 100xw 100yCO2 tif (h) tgf (h) r (mol h-1) ∆Tmaxg (K) 1.90 2.35 2.73 3.06 3.41

273.53 273.71 273.67 273.53 273.76

1.24 1.28 1.31 1.48 3.07

0.3603 0.4685 0.5909 0.6693 0.8405

0.0095 0.1109 0.2428 0.2923 0.0195

0.0664 0.0669 0.0673 0.0698 0.0810

0.14 1.66 3.61 4.19 0.24

0.0759 0.1778 0.3101 0.3621 0.1005

16.48 27.53 41.90 47.12 17.51

20.61 37.95 52.48 54.09 11.95