HFC-134a Hydrate Formation Kinetics during Continuous Gas Hydrate

Ind. Eng. Chem. Res. 2010, 49, 2525–2532

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HFC-134a Hydrate Formation Kinetics during Continuous Gas Hydrate Formation with a Kenics Static Mixer for Gas Separation Hideo Tajima,*,† Toru Nagata,‡ Yutaka Abe,‡ Akihiro Yamasaki,§ Fumio Kiyono,| and Kazuaki Yamagiwa† Graduate School of Science and Technology, Niigata UniVersity, 2-8050 Ikarashi, Niigata 950-2181, Japan, Graduate School of System and Information Engineering, UniVersity of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki, 305-8573, Japan, Department of Materials and Life Science, Seikei UniVersity, 3-3-1Kichijoji-kitamachi, Musashino, Tokyo, 180-8633, Japan, and Research Institute for EnVironmental Management Technology, National Institute of AdVanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki, 305-8569, Japan

Gas hydrate formation kinetics were investigated in a Kenics static mixer. When operated under thermodynamic conditions (pressure or temperature), the hydrate formation rate increased and the HFC-134a gas bubble was covered with a hydrate “shell”. This hydrate shell inhibited hydrate growth because of resistance to mass transfer. Water recycling in the hydrate reactor accelerated hydrate formation by increasing the gas-water interface during water-gas cocurrent flow and causing the continued presence of a fresh interface in the counterflow. The kinetic data indicated that the hydrate formation rate would be equal to the mass transfer rate including the rate of hydrate shedding from the gas bubble. Enriched HFC-134a gas could be continuously recovered from an HFC-134a-nitrogen mixture in a continuous hydrate formation system. The hydrate formation rate constant for the mixed gas depended on the feed gas components. 1. Introduction Gas hydrates have been widely proposed as materials to separate greenhouse gases, including freon gases1-3 and carbon dioxide.4-8 Many investigations have been carried out in batch systems. Effective gas separation, or higher selectivity, can be achieved for gas mixtures that differ largely in their hydrate formation conditions. The solid hydrate can be dissociated to recover a product gas that is highly enriched with target gas. The selectivity and production rate are key factors in determining separation performance. The selectivity is limited by the equilibrium of the hydrate phase and the feed vapor phase.1,2,4 The production rate is dependent on the hydrate formation rate and the system design. Kinetic or transport processes significantly affect and can determine the performance of hydratebased separation processes.6 The gas-liquid interfacial area, the driving force, and kinetic constant can affect hydrate formation. A way to increase these factors is necessary for improving hydrate formation. Several efficient processes to increase the interfacial area for gas hydrate formation have been demonstrated, including a spray9 or jet reactor10,11 and a bubble column.12,13 However, information on hydrate formation kinetics in these processes is limited. The abilityofadditivessuchassodiumn-alkylsulfate,14,15 tetrahydrofran,4-6 and tetrabutylammonium bromide16 to accelerate hydrate formation has also been investigated. In each case, the driving force is increased by a shift in the equilibrium conditions. However, most of these studies were carried out in batch systems. Our previous research demonstrated a new way to produce gas hydrate continuously using a Kenics type static mixer.17,18 Static mixers are motionless mixing devices with fixed mixing * To whom correspondense should be addressed. Telephone: +8125-262-7277. E-mail: [email protected]. † Niigata University. ‡ University of Tsukuba. § Seikei University. | National Institute of Advanced Industrial Science and Technology (AIST).

elements arranged in a straight pipe. The static mixer experiments demonstrated that drop/bubble and water fluids are efficiently agitated with the mixing elements and are subsequently converted to hydrate formed on the drop/bubble surface at specific temperature and pressure conditions. Compared with batch mixers, static mixers also generally provide continuous operational availability, small size and space requirements, flexibility in the process installation, and low power requirements.19 However, static mixing has not been used for gas separation, and hydrate formation kinetics in this system are not clear yet. This research aimed to study hydrate formation kinetics in a static mixer equipped with a hydrate formation and recovery system, and to apply continuous gas hydrate formation and recovery to a gas separation system. In this study, HFC-134a gas and a mixture of HFC-134a and nitrogen gases (HFC-134aN2) were used as model gases for hydrate formation. The effects of operation conditions on gas hydrate formation kinetics were investigated. In addition, HFC-134a gas separation was demonstrated by forming and recovering binary HFC-134a-N2 hydrate. 2. Experimental Section 2.1. Apparatus. Figure 1 shows the experimental setup for HFC-134a hydrate formation and recovery. The static mixer unit was installed vertically in a glass tube. HFC-134a gas was injected at the bottom of the tube and stirred while it rose. The HFC-134a gas was converted to gas hydrate in the glass tube, as shown in our previous research this glass tube and static mixer makes a good hydrate reactor.18 Transport and recovery of HFC134a hydrate was carried out with the hydrate in slurry form. Because the HFC-134a hydrate was denser than water, HFC134a hydrate particles that formed in the slurry settled out and could be recovered in the hydrate recovery vessel. The recovery vessel was set up in a manner to prevent the gas hydrate blocking the gas supply nozzle or the reactor. The recovery

10.1021/ie901613h  2010 American Chemical Society Published on Web 01/25/2010

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Ind. Eng. Chem. Res., Vol. 49, No. 5, 2010

Figure 1. Schematic drawing of experimental apparatus: T-1 and T-2, temperature sensors; PIA-1, pressure sensor; v-1-v-8, valves; v-9, back pressure valve; v-10, check valve; MFC, gas mass flow controller; and MFM, gas mass flow meter.

vessel was removable for gas recovery by hydrate decomposition. Deionized water was supplied and recycled by a water supply pump, which also controlled water flow rates. The supply pump could also alter the direction of water flow to be counter to or cocurrent with the gas flow. The reactor, the recovery vessel, and the water supply pump were all placed in a low temperature thermostatic chamber to control the system temperature. In this study, the gas recovery (hydrate decomposition) process separate from the hydrate formation process for the gas recovery test. If liquid water produced from hydrates for the gas recovery process is recycled to form gas hydrate, it will become a better continuous system of hydrate formation and decomposition for gas separation because an additional water supply is not required. 2.2. Materials and Method. Gas hydrate was formed in the static mixer using deionized water and HFC-134a gas (>99.999%). Kenics-type mixing elements of a SUS316 static mixer (Noritake Co. Ltd., Japan) were used in this study. There were 24 mixing elements and these were inserted into a Pyrex glass tube (455 mm, i.d. 11.0 mm). The total system was designed for operation under pressure at about 1.0 MPa. The HFC-134a gas flow rate was controlled at 150-300 cm3/min by a mass flow controller. The water flow rate was operated in the range of 0-148 cm3/ min, either counter or cocurrent to the gas flow direction. At a water flow rate of zero, this system is regarded as a semibatch system. Pressure and temperature conditions for HFC-134a hydrate formation were selected according to previous data.1,18 Outlet gas flow rates were measured by a mass flow meter after the gas had passed through the static mixer unit. Gas hydrate formation was confirmed by both visual observations and variations in outlet gas flow rates. In all experimental runs, the temperature was controlled within (0.1 K by the thermostatic chamber. Experimental pressure was controlled within (0.01 MPa by a pressure-regulating valve installed on the downstream side of the static mixer unit. Pressure, temperature, and outlet gas flow rate were recorded on a data logger system. The fluid patterns in the static mixer unit were recorded on a digital video camera recorder.

Figure 2. Typical gas consumption line for HFC-134a gas hydrate formation with water: (conditions) operation pressure 0.20 MPa, operation temperature 276.2 K, gas flow rate 200 cm3/min, and water flow rate 148 cm3/min (counter flow).

2.3. Calculation of Gas Hydrate Formation Rate. The gas uptake rate was determined using the difference between inlet and outlet gas flow rates, assuming that all the gas molecules are used to form hydrate and gas uptake rate is equal to overall gas hydrate formation rate (rhy). The definition of gas uptake rate is the same as the hydrate growth rate in available literature.4-6 The difference in gas flow rate theoretically includes HFC-134a molecules dissolved into water. However, the water solubility of HFC-134a is very low, and the water in this experiment was saturated with HFC-134a before use, therefore the water dissolution effect is negligible. The point at which hydrate formation started and the outlet gas flow rate reduced was used as an arbitrary zero point for the elapsed time. A typical gas consumption trend line for HFC-134a gas hydrate formation with water is shown in Figure 2. During hydrate slurry formation, the amount of HFC-134a gas consumed increased linearly and continuously with the formation of gas hydrate. The rhy was calculated from the slope of the gas consumption line. When a hydrate plug formed and blocked the reactor, the amount of gas consumption initially increased linearly and then the slope decreased gradually with elapsed time. The overall hydrate formation rate when a hydrate plug formed was determined from the early largest slope of the gas consumption line. The rhy is expressed by the chemical potential difference between formation and equilibrium as the driving force.15,16,20 rhy ) -

dn ) aK*(µg - µeq) dt

(1)

where n is the number of moles of HFC-134a gas consumed in the gas phase, t is elapsed time, aK* is the hydrate formation rate constant, a is the interfacial area, K* is the overall kinetics constant, and µg and µeq are chemical potentials of guest molecules in the gas phase and hydrate phase, respectively. The overall kinetics constant K* is expressed using the mass transfer coefficient kL and the hydrate crystal growth constant kf. 1 1 1 ) + K* kL kf

(2)


Because the chemical potential terms can be reduced to the fugacity of the gas, eq 1 can be easily transformed to the following form. rhy ) -

()

fg dn ) aK*RT ln dt feq

(3)

where R is the gas constant, T is the temperature, and fg and feq are the fugacities of the guest molecules in the gas phase and in hydrate phase, respectively. The fugacity feq is equal to that under equilibrium. Because fugacity is expressed by the pressure and fugacity coefficient φ, f ) φP rhy

( )

Pg dn ≈ aK*RT ln )dt Peq

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2.4. HFC-134a Separation and Recovery. HFC-134a gas was separated from a HFC-134a-N2 mixture gas by formation of a gas hydrate, which was recovered in the recovery vessel. The decomposition of hydrate formed was carried out at room temperature (about 293 K) by reducing pressure inside the recovery vessel up to atmospheric pressure (about 0.10 MPa). After decomposition of the recovered hydrate by reducing pressure, the generated gases were sampled and analyzed by a gas chromatograph (Shimadzu GC-8A) equipped with a Porapak Q column (Waters) for gas components. The gas recovery factor Rf in the hydrate phase is expressed by4 Rf )

nhy nfeed

(5)

(4)

where Pg and Peq are the pressure in the gas phase and in equilibrium, respectively. Equation 4 was used to calculate aK* using the experimental rhy, Pg, and literature data21 for Peq at the experimental temperature. For instance, aK* for the experimental data shown in Figure 2 was 2.08 × 10-8 mol2/(s · J), which was same order of magnitude as xenon hydrate15 and one order of magnitude lower than that of CO2 hydrate with an aqueous solution of additive.16

Figure 3. Formation of hydrate plug and slurry in the static mixer system.

where nhy and nfeed are the total number of moles of HFC-134a gas in the hydrate phase and the feed gas. Rf is the same as the split fraction S.Fr. in the available literature.4 For the mixed gas, the separation factor SF is calculated using the following equation.4,16 SF )

H /nNH2 nHFC G nHFC /nNG2

(6)

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Figure 5. Effect of operation temperature on formation rate constant of HFC-134a hydrate with water in semibatch system: (conditions) operation pressure 0.15-0.30 MPa and gas flow rate 200 cm3/min.

Figure 4. Effect of operation pressure on formation rate of HFC-134a hydrate with water in semibatch system: (a) overall hydrate formation rate and (b) hydrate formation rate constant. (conditions) Operation temperature 276.15 ( 0.3 K and gas flow rate 200 cm3/min. H G where nHFC and nHFC are the number of moles of HFC-134a in hydrate phase and gas phase, respectively, and nNH2 and nNG2 are the number of moles of N2 in hydrate phase and gas phase, respectively.

3. Results and Discussion 3.1. Effects of Operation Conditions on Hydrate Formation in a Semibatch System. 3.1.1. Effect of Operation Pressure. As in previous research,18 both hydrate plug and hydrate slurry formation patterns were observed (Figure 3). The hydrate plug had a HFC-134a hydrate “shell” formed on the surface of the bubbles. Whereas the hydrate slurry consisted of very small HFC-134a hydrate particles in water and a hydrate shell rarely formed on the bubble surface. Hydrate slurry (formed at 0.15 MPa) turned into hydrate plug (formed at >0.25 MPa) with an increase in operation pressure. At 0.20 MPa, the hydrate formed was a mixture of slurry and plug. Figure 4 shows the effect of operation pressure on the rate of HFC-134a hydrate formation with water at a constant temperature in the semibatch system (water flow rate ) 0). The hydrate formation rate increased gradually with increasing operation pressure (Figure 4a). This was due to a high driving force with the increase in Pg in eq 4. The hydrate formation rate at 0.30 MPa was only 1.5 times as high as that at 0.15 MPa, although the operation pressure was twice as high. As a result, the hydrate formation rate constant decreased with increasing operation pressure (Figure 4b). The formation of hydrate plug meant the hydrate formation rate constant was independent of the operation pressure. These results indicate

that hydrate crystal growth and mass transfer are largely inhibited by the formation of hydrate shell at the HFC-134a bubble surface. In this case, the hydrate crystal growth rate will be higher than the mass transfer rate, which includes the shedding rate of hydrate formed at the bubble surface. The Rf was in the range of 0.14-0.16 and independent of operation pressure. 3.1.2. Effect of Operation Temperature. HFC-134a hydrate formation in water was carried out at various temperatures and constant pressure. The hydrate formation rate increased gradually with decreasing temperature, which can also be explained by the high driving force with the decrease in Peq in eq 4. At a constant pressure of more than 0.20 MPa, the hydrate plug formed at all temperatures tested. The hydrate formation rate constant at various temperatures is shown in Figure 5. At operation pressures of 0.30 and 0.25 MPa, hydrate plug formed and the hydrate formation rate constant was independent of the temperature. The gas hydrate crystal growth will be inhibited by the formation of hydrate shell at the bubble surface. In the case of hydrate slurry formation (0.20 and 0.15 MPa), the rate constant was higher than for hydrate plug formation. In addition, the slurry formation rate constant deceased with decreasing temperature and approached that of hydrate plug formation at lower temperature. Low temperature decreased the hydrate formation rate constant, although the overall hydrate formation rate increased with an increase in driving force. These results imply the inhibition of hydrate crystal growth by a low shedding rate of hydrate from the bubble surface and hydrate shell formation at the bubble surface because of the high driving force. The Rf was in the range of 0.09-0.16 and was poorly correlated to the temperature. 3.1.3. Effect of Feed Gas Flow Rate. An increase in the gas flow rate will lead to increased mixing effects in a Kenics mixer because the gas rises faster. This increase in the mixing effect could increase the interfacial area of the HFC-134a gas bubble and water and increase the overall gas hydrate formation rate. The effect of feed gas flow rate on hydrate formation was investigated (Figure 6). The hydrate formation rate constant increased with increasing feed gas flow rate (Figure 6a). In the case of hydrate slurry formation at 0.15 MPa, when the feed gas flow rate was doubled, the rate constant was also about twice as high. These results can be explained by an increase in the gas-water interfacial area and an increase in the shedding rate of hydrate from the bubble surface. At an increased operation


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Figure 6. Effect of feed gas flow rate on formation rate constant of HFC134a hydrate with water in semibatch system: (a) hydrate formation rate constant and (b) recovery factor. (conditions) Operation pressure 0.15-0.30 MPa and operation temperature 275.2-276.3 K.

Figure 7. Effect of water flow rate on the hydrate formation: (a) hydrate formation rate constant and (b) recovery factor: (conditions) operation pressure 0.15-0.2 MPa, temperature 275.8 ( 0.3 K, gas flow rate 200 cm3/ min, and hydrate slurry formation.

pressure (0.25 MPa) and with formation of hydrate plug, the feed gas flow rate had a lesser effect on the rate constant than in hydrate slurry formation (0.15 MPa). The hydrate shell at the bubble surface inhibited the hydrate crystal growth. The Rf was in the range of 0.07-0.16 and tended to decrease with increasing feed gas flow rate (Figure 6b). This could be explained by the increase in nfeed in eq 5. 3.2. Continuous Formation of HFC-134a Gas Hydrate in a Pure Gas System. 3.2.1. Effect of Water Flow Rate. As discussed above, for continuous hydrate formation and recovery, it is necessary to prevent hydrate growth inhibition by hydrate shell formation at the bubble surface. We investigated varying both the flow rate and direction of water flow compared with gas flow in the reactor. With gas-water cocurrent flow, the interfacial area and mixing effect of the static mixer are expected to increase, although the residence time of HFC-134a bubbles in the reactor decreases because they rise faster with the water flow. In contrast, a counterflow will increase the residence time. Counterflow will also result in easier peeling and shedding of the formed HFC-134a hydrate from the bubble surface because of an increase in the shear force on the gas-water interface. Both water flow directions increased the hydrate formation rate. Figure 7 shows the effects of water flow rate and direction on hydrate formation at constant pressure and temperature. A positive water flow rate indicates cocurrent flow to the HFC134a gas, and a negative flow rate indicates counterflow. In the semibatch system, the water flow rate is zero. As expected, the hydrate formation rate constant increased with increasing water flow rate (Figure 7a). The Rf was in the range of 0.12-0.37 (Figure 7b). Despite holding thermodynamic conditions (pres-

sure and temperature) constant, the HFC-134a hydrate formation was accelerated and hydrate slurry was observed for both water flow conditions. The water flow can help avoid the hydrate plug formation. In cocurrent flow, the hydrate formation rate constant and the Rf were directly proportional to the water flow rate. A main factor in acceleration of hydrate formation will be an increase in the gas-water interfacial area from bubbles breaking up. The Rf increased with the increase in the aK*. In the counterflow, the hydrate formation rate constant at 0.20 MPa initially increased with increasing water flow and then remained constant. The Rf showed a similar trend to the aK* for the water flow rate. This result can be explained by a renewal of the gas-water interfacial area. The gas-water interfacial area is renewed by hydrates peeled from the bubble surface due to turbulence of the interface with a counterflow. The fresh gas-water interfacial area remains almost constant because the inhibition of hydrate formation by hydrate shell formation is negligible. However, the effects of water flow rate on aK* and Rf were not remarkable at 0.15 MPa. The hydrate formation patterns in a semibatch system were the hydrate slurry at 0.15 MPa and the hydrate plug at 0.20 MPa. Because the hydrate shedding rate at 0.15 MPa is enough higher than the hydrate growth rate and the hydrate slurry can be formed without the help of water flow, the effects of water flow rate on the hydrate formation (renewal of surface and/or bubble breaking up) are not remarkable. The hydrate formation rate will consist mainly of the hydrate crystal growth rate that is dependent on thermodynamic conditions and the mass transfer rate that is dependent on mechanical conditions. With an increase in

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Figure 8. Demonstration of HFC-134a hydrate recovery in the recovery vessel.

pressure or decease in temperature, the hydrate growth rate increases and leads to hydrate plug formation because the bubble surface is rapidly covered with a strong hydrate shell. The overall hydrate formation rate will be equal to the mass transfer rate including hydrate shedding from the bubble surface. Water flow conditions can accelerate the mass transfer rate. 3.2.2. Gas Hydrate Recovery System with Water Flow. The ideal unit cell formula of HFC-134a hydrate is known to be 8HFC-134a · 136H2O, the hydrate crystal structure II. Since gas hydrates are nonstoichiometric compounds, the density of HFC-134a hydrate is greatly affected by the large cage occupancy of the HFC-134a gas molecule. Assuming that HFC134a gas molecules fill the hydrate large cages and that the lattice parameter of HFC-134a hydrate is 17.3 Å according the literature,22 the density of HFC-134a hydrate is expected to be 1047 kg/m3. Likewise, HFC-134a hydrate with a large cage occupancy of 87.5% will allow seven molecules of HFC-134a to be included in eight cages, and the density is 1014 kg/m3. This makes hydrate denser than water and means that HFC134a hydrate particles in a slurry can be recovered by settling out in the hydrate recovery vessel. However, the hydrate shell formation can cause not only the decrease in the aK* for hydrate kinetics but also the decrease in the density of hydrate particles because of the captured gas inside hydrate shell. The formation of hydrate plug should be avoid by water flow. We observed HFC-134a hydrate sedimentation in the hydrate recovery vessel (Figure 8). In this case, the HFC-134a hydrate slurry was formed at 0.25 MPa and 278.0 K with a cocurrent water flow rate (90 cm3/min). When this hydrate slurry reached the hydrate recovery vessel, hydrate particles in the slurry settled out. Over time, the hydrate particles built up at the bottom until at 840 s the hydrate recovery vessel was filled with HFC-134a hydrate particles. This sedimentation of hydrate particles in the recovery vessel was consistently observed under various operation conditions. This indicates that the hydrate particles prepared were denser than water and that hydrate was easily recovered by a solid-liquid separation using sedimentation of the particles. HFC-134a hydrate was easily transported with the formation of hydrate slurry and, water was recycled to form hydrate slurry in the reactor. 3.3. Recovery of HFC-134a gas from a Gaseous Nitrogen Mixture. 3.3.1. Hydrate Formation from Gaseous Mixtures in a Semibatch System. To estimate how the HFC134a concentration in a feed mixture gas affected hydrate formation kinetics, HFC-134a hydrate was formed using a gaseous mixture in the semibatch system. The operation pressure was set at HFC-134a partial pressure of 0.15 MPa. The HFC-

Figure 9. Effect of HFC-134a concentration in feed gas on hydrate formation: (a) hydrate formation rate constant and (b) HFC-134a concentration in hydrate. (conditions) Operation pressure 0.15 MPa, operation temperature 275.6 ( 0.3 K, and gas flow rate 200 cm3/min.

134a hydrate formation rate was corrected using the amount of gas recovered because any reduction in the mixed gas outlet flow rate was due to hydrate formation. Figure 9 shows the effect of HFC-134a concentration (nitrogen balance) in feed gas on hydrate formation. The hydrate formation rate constant decreased gradually with decreasing HFC-134a concentration in the mixed feed gas (Figure 9a). This occurred because the amount of HFC-134a at the bubble surface decreased when the HFC-134a concentration in the feed gas decreased, although the total gas-water interfacial area was almost constant under all experimental conditions. To recover


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Table 1. Hydrate Formation Rate Constant, Mole Fraction, and Recovery Ratio of HFC-134a Recovered from a Gaseous Mixture in the Continuous System operation temp [K]

operation pressure [MPa]

partial pressure [MPa]

HFC-134a in feed gas [-]

QL [cm3/min]

275.6 274.2 275.5 274.9 274.6 274.8 275.8 276.2 276.3

0.32 0.32 0.34 0.34 0.25 0.25 0.15 0.20 0.20

0.156 0.171 0.187 0.188 0.165 0.165

0.487 0.537 0.550 0.552 0.659 0.659 1.000 1.000 1.000

65.0 -148.0 148.0 -65.0 -90.0 148.0 0 -148.0 148.0

a large amount of hydrate when the HFC-134a concentration in feed gas is decreased, a high pressure to increase the driving force or a long residence time for bubbles in the reactor is required. The concentration of HFC-134a in the gas recovered from the gaseous mixture, and therefore in the hydrate, was higher than that in feed gas (Figure 9b). The separation factor SF was 6.7-8.2. This HFC-134a enriched gas was easily recovered by formation and decomposition of the hydrate. A small amount of N2 was present in the recovery gas in all cases except for when the input gas was pure. Both pure N2 and pure HFC-134a are known to form structure II hydrate, and HFC-134a molecules occupy only large cages due to their size. HFC-134a hydrate is stabilized at lower equilibrium pressures or higher temperatures than N2 hydrate. Therefore, binary N2-HFC-134a hydrate is stabilized by HFC-134a molecule occupation in the large cages and slight N2 molecule uptake in small cages,1 resulting in HFC-134a enrichment in the hydrate phase. 3.3.2. Continuous Recovery of HFC-134a Gas from Gaseous Mixture. As a better way to form and recover the hydrate, HFC-134a gas was recovered from an approximately 1:1 gaseous mixture (HFC-134a + N2) by formation and decomposition of gas hydrate. Hydrate formation was accelerated by water flow conditions. The hydrate formation rate constant, and the mole fraction and recovery ratio of HFC-134a recovered from a gaseous mixture in the continuous system were summarized in Table 1. The data on hydrate formation rate constant for pure HFC-134a gas were also shown for comparison with mixed gas system. The hydrate formation rate constant of gaseous mixture was about half that of hydrate formation in pure HFC-134a gas. However, the HFC-134a Rf was in the range of 0.22-0.36, which was similar to those for pure HFC-134a gas. The separation factor SF was 5.3-9.6, which was similar to those in the semibatch system. HFC-134a was concentrated in the mole fraction range of 87-94%, which in recovery gas is close to the equilibrium data for the HFC-134a-N2-hydrate system.1,2 These results indicate that high concentration of HFC-134a gas is recovered with continuously gas hydrate formation in this system. Further optimization of several conditions could improve the recovery factor for HFC-134a gas. 4. Conclusions HFC-134a hydrate formation kinetics were investigated in a Kenics static mixer. The rate constant of HFC-134a hydrate formation was the same order of magnitude, 10-8 mol2/(s · J), as literature data for xenon hydrate and one order of magnitude lower than that of CO2 hydrate with an aqueous solution of additive. When operated under thermodynamic conditions (pressure or temperature) and with an increase in

HFC-134a in hydrate [-] 0.885 0.887 0.867 0.922 0.934 0.940

Rf [-]

SF [-]

aK* [10-8 mol2/(s J)]

0.248 0.306 0.292 0.258 0.224 0.358 0.140 0.263 0.372

8.1 6.8 5.3 9.6 7.3 8.1

1.13 1.04 1.34 0.95 1.06 2.20 2.07 2.48

the hydrate formation rate, the HFC-134a gas bubble was covered with hydrate “shell”. This shell inhibited hydrate growth and the hydrate formation rate constant decreased. Water recycling in the hydrate reactor increased the hydrate formation rate constant and the hydrate recovery factor Rf. The acceleration was due to an increase in the gas-water interface in gas-water cocurrent flow, and the continued presence of a fresh interface in counterflow. The kinetic data indicated that the hydrate formation rate would be equal to the mass transfer rate, which included the rate of hydrate shedding from the gas bubble. Enriched HFC-134a gas could be separated and continuously recovered from a HFC-134aN2 mixture in the continuous hydrate formation system. Although the hydrate formation rate constant for the HFC134a-N2 mixture gas depended on the feed gas components, the HFC-134a recovery factor and separation factor in the continuous system were nearly equal to those in the semibatch system. High concentration of HFC-134a gas was recovered with continuously forming gas hydrate in this system. Acknowledgment The authors are grateful to the Japan Society for the Promotion of Science (JSPS). This work was partly supported through the Grant-in-Aid for Young Scientists B (No.21710074). Literature Cited (1) Seo, Y.; Tajima, H.; Yamasaki, A.; Takeda, S.; Ebinuma, T.; Kiyono, F. A New Method for Separating HFC-134a from Gas Mixtures Using Clathrate Hydrate Formation. EnViron. Sci. Technol. 2004, 38, 4635. (2) Nagata, T.; Tajima, H.; Yamasaki, A.; Kiyono, F.; Abe, Y. An analysis of gas separation processes of HFC-134a from gaseous mixtures with nitrogen-Comparison of two types of gas separation methods, liquefaction and hydrate-based methods, in terms of the equilibrium recovery ratio. Sep. Purf. Technol. 2009, 64, 351. (3) Seo, Y.; Lee, H. A New Hydrate-Based Recovery Process for Removing Chlorinated Hydrocarbons from Aqueous Solutions. EnViron. Sci. Technol. 2001, 35, 3386. (4) Linga, P.; Kumar, R.; Englezos, P. The Clathrate Hydrate Process for Postcombustion Capture of Carbon Dioxide. J. Hazardous Mater. 2007, 149, 625. (5) Linga, P.; Adeyemo, A.; Englezos, P. Medium Pressure Clathrate Hydrate/ Membrane Hybrid Process for Postcombustion Capture of Carbon Dioxide. EnViron. Sci. Technol. 2007, 42, 315. (6) Linga, P.; Kumar, R.; Englezos, P. Gas Hydrate Formation from Hydrogen/ Carbon Dioxide and Nitrogen/ Carbon Dioxide Gas Mixtures. Chem. Eng. Sci. 2007, 62, 4268. (7) Kang, S.-P.; Lee, H. Recovery of CO2 from flue gas hydrate: thermodynamic verification through phase equilibrium measurements. EnViron. Sci. Technol. 2003, 34, 4397. (8) Duc, N. H.; Chauvy, F.; Herri, J.-M. CO2 Capture by Hydrate Crystallization - A Potential Solution for Gas Emission of Steelmaking Industry. Energy ConVers. Manage. 2007, 48, 1313. (9) Fukumoto, K.; Tobe, J.; Ohmura, R.; Mori, Y. H. Hydrate formation using water spraying in a hydrophobic gas: A preliminary study. AIChE J. 2001, 47, 1899.

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ReceiVed for reView October 16, 2009 ReVised manuscript receiVed January 7, 2010 Accepted January 14, 2010 IE901613H

HFC-134a Hydrate Formation Kinetics during Continuous Gas Hydrate

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