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Promoting effect of super absorbent polymer on hydrate formation
Journal of Natural Gas Chemistry 19(2010)251–254
Promoting effect of super absorbent polymer on hydrate formation Fei Long, Shuanshi Fan, Yanhong Wang,
Xuemei Lang∗
The Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, Guangdong, China [ Manuscript received September 8, 2009; revised March 25, 2010 ]
Abstract The effect of super absorbent polymer (SAP) on the formation of tetrahydrofuran (THF) hydrate was studied by the successional cooling method. It was found that THF solution samples with 0.004 wt% and 0.03 wt% of SAP formed THF hydrate completely during the same cooling process. The corresponding induction time was 16–29 min, 14–31 min, respectively, which was obviously shorter than that of THF solution samples without SAP (25–62 min). It indicated that SAP accelerated the formation of THF hydrate. At the same time, the pictures of hydrate formation with and without SAP had been compared. It was found that SAP did not change the morphology of the hydrate. Finally, the mechanism of SAP promoting effect on the formation of THF hydrate was suggested. Key words super absorbent polymer; tetrahydrofuran; hydrate; promoting effect
1. Introduction Hydrates are nonstoichiometric crystalline compounds, in which small guest molecules are trapped by host water molecules. Hydrate technologies are mainly used in gas storage and transportation, the separation of mixed gas, cold storage in air conditioning, desalination of seawater etc. The application and popularization of hydrate technologies are hindered by some drawbacks, such as harsh formation conditions, long induction time, slow hydration rate, low gas storage density and separation efficiency. Lots of researchers are trying to use additives to promote hydrate generation. Surfactant is in common use, for examples, SDS [1−4], TBAB and TBAF [5,6], LABS, CTAB, ENP [7], Tween [8], Span20 [9] etc. They can speed up the hydrate formation in some extent. Structural revulsant is another kind of promoter, such as silver iodide [10] and potassium oxalate monohydrate [11]. The former may provide the nucleus and the latter is used as the template agent. Porous medium can also accelerate the hydrate formation, such as glass powder [12], silica gel [13], molecular sieve [14], polyHIPE [15], dry water and dry gel and so on [16,17]. They promote hydrate formation through the enhancement of mass transfer. The hydro gel formed by poly (acrylic acid) sodium salt (PSA) and Tetrahydrofuran-water (THF-H2 O) solution greatly increases the hydrogen storage rate and density, and these materials can be used repeatedly [18]. However, the ∗
effect of PSA on hydrate induction time has not been studied and there is few mechanical analysis in the references. So the effect of super absorbent polymer (SAP, crosslinking copolymerized by sodium acrylate and acrylic acid) on THF solution was studied in this paper. The suggested mechanism for its promotion effect was also discussed. 2. Experimental 2.1. Experimental materials Tetrahydrofuran (THF, AR) was purchased from Sinopharm Chemical Reagent Co., Ltd. Super absorbent polymer (SAP) was kindly provided by Anhui Huajing new materials Co., Ltd. Sodium dodecyl sulfate (SDS, AR) was from Tianjin Chemical Reagent Co., Ltd. Distilled water was produced in our laboratory. 2.2. Experimental apparatus Figure 1 is the schematic diagram of experimental system. The low temperature test chamber (Xutemp Temptech Co., Ltd.) provides cold air bath, the temperature fluctuation of which is smaller than 0.5 ◦ C and the temperature control range is –30 ◦ C–40 ◦ C. There is a window on one side of chamber, from which we can observe the reactions.
Corresponding author. Tel: +86-20-22236581; Fax: +86-20-22236581; E-mail: [email protected]
Copyright©2010, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(09)60074-8
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Fei Long et al./ Journal of Natural Gas Chemistry Vol. 19 No. 3 2010
Figure 1. Schematic diagram of experimental system. 1—Low temperature test chamber, 2—Thermocouple, 3—Glass bottle, 4—Data acquisition unit, 5—Computer
Data acquisition system contains data acquisition unit (Agilent 34970A), type J thermocouple (made by ourselves, deviation is smaller than 0.1 ◦ C) and computer. Except for apparatuses introduced above, electronic balance (BT224S, Sartorious AG, precision is ±0.1mg, and measuring range is 0– 220 g), digital camera (Sony, DSC-H9), volumetric flask and glass bottle (Figure 2) were also used.
Figure 2. Picture of glass bottle inserted by a thermocouple (taken outside of the chamber)
2.3. Experimental process
platform due to the exothermic nature of the THF hydrate or ice formation. The initial temperature platform of distilled water is −0.5 ◦ C and the THF is 3.4 ◦ C, which are all below their corresponding phase equilibrium point, because of the heat exchange between the solutions and the cold air in chamber. This result is similar to that reported by Vincent Ayel et al. [19]. Teq in Figure 3 is the phase equilibrium temperature, 4.4 ◦ C, of the 19%THF solution; teq is the corresponding time; tc is the time when the nucleation starts and leads to sudden increase in temperature. The induction time, t0 , is defined as the difference between teq and tc , t0 = tc – teq .
Figure 3. Temperature evolution of different solutions during successional cooling
3.2. Ef fect of SDS and SAP on THF hydrate induction time 4.4 ◦ C
at 19%THF solution (phase equilibrium point is 1 atm) and THF solutions with additives (SDS or SAP) were prepared first to make sure the mass ratio of THF to water 19 : 81 in all mixtures. Then 15 ml solutions were taken in the glass bottles respectively. Glass bottles were inserted by thermocouples and put in the chamber. The temperature of chamber was raised to 35 ◦ C and kept for 30 min later. Finally chamber was cooled continuously. The temperatures of the solutions were collected by the data acquisition unit during cooling. The final temperature of chamber was about −30 ◦ C. At the same time, the hydration processes were taken photos by the digital camera from the window of chamber. Every solution was tested 15 times, and 40 bottles can be measured in one time by our apparatus.
Figure 4 shows the result of effect of SDS and SAP on the induction time of THF hydrate formation.
3. Results and discussion 3.1. Def inition of induction time Temperature evolutions of distilled water, 19%THF solution and THF solutions-SAP mixtures during successional cooling are displayed in Figure 3. There is a temperature
Figure 4. Effect of SDS and SAP on the induction time of THF hydrate formation. Nt represents the number of samples that have not formed hydrates at induction time t, N0 = 15
Journal of Natural Gas Chemistry Vol. 19 No. 3 2010
Nt in Figure 4 represents the number of samples that have not formed hydrates at induction time t, and N0 is the total number of samples (here N0 = 15). This definition is the same with Refs. [20,21]. Huang Zeng et al. [20,21] considered that Nt /N0 (i.e., the uncrystallized fraction) vs. time represents the inhibition activity of the additive, and the slower the curve declines, the stronger the inhibition. So here the faster the curve declines, the stronger the promotion. The effect of SDS on the induction time of THF hydrate formation was studied first. The curve of THF with 0.8% SDS solution in Figure 4 declines faster than pure THF solution, which proves that the basic theory is correct. The effect of SAP addition ranging from 0.004% to 0.32% was further studied. It is found that all of the THF solutions with SAP decline faster than the THF solution without SAP (Figure 4), indicating that SAP can promote the hydrate formation and shorten the induction time. The THF solution samples with 0.004 wt% and 0.03 wt% SAP decline faster than others, and the corresponding induction time is 16–29 min and 14–31 min, respectively, which is obviously shorter than that of the pure THF solution samples with the corresponding induction time of 25–62 min. There is some intersection between different curves and we consider that it is caused by the randomicity of the static system. The induction of hydrate formation is stochastic [22,23], and this kind of random is more conspicuous under low driving force [24,25].
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The balance of static system must be broken before the hydrate forms, which is affected by lots of random environmental disturbances. At the same time, because of the insolubility of SAP, the inhomogeneity of SAP would be inevitable, although all of the solutions were agitated several times before test. 3.3. Ef fect of SAP on morphology of the THF hydrate The hydration process pictures of 19% THF solution and the THF solution with 0.004% SAP are all presented in Figure 5. Figure 5(a1 ) is the starting cooling point of THF solution, Figure 5(b1 ) is the starting cooling point of THF solution with 0.004% SAP, and the thermocouple can be seen clearly. Hydrate was generated instantaneously as we can see in Figure 5(a2 ) and Figure 5(b2 ), and acicular hydrate growing around the thermocouple and the thermocouple can not be seen clearly. Figure 5(a3 ) and Figure 5(b3 ) were the hydration terminations, and Figure 5(a4 ) and Figure 5(b4) were the pictures of successional cooling after the hydration processes were completed. There is no obviously different between these two hydration processes, suggesting that SAP does not change the morphology of the hydrate.
Figure 5. a1 –a4 : the hydration process of 19% THF solution; b1 –b4 : the hydration process of THF solution with 0.004% SAP. (a1 ) Time: 12:14:01, T : 35.0 ◦ C; (a2 ) Time: 13:22:34, T : 3.4 ◦ C; (a3 ) Time: 13:50:36, T : 2.1 ◦ C; (a4 ) Time: 14:21:14, T : –22.6 ◦ C; (b1 ) Time: 12:14:10, T : 34.9 ◦ C; (b2 ) Time: 13:18:26, T : 3.3 ◦ C; (b3 ) Time: 13:47:50, T : 2.2 ◦ C; (b4 ) Time: 14:21:20, T : –22.8 ◦ C
3.4. Mechanism of SAP promoting ef fect on hydrate formation Hydrate is produced when the guest molecules enter cage formed by hydrogen-bonded water molecules under certain conditions. THF hydrates are structure II which is consisted of large and small cages, and THF molecules can only be in-
cluded as guests in the large cages (consisting of 12 pentagons and 4 hexagons, 512 64 , see Figure 6(a)). There is a long induction time when hydrate forms in pure water. When SAP is added to THF solution, SAP and water can form hydro gel with steric network structure containing lots of carboxyl groups. Because the hydrogen bonds form between carboxyl
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Fei Long et al./ Journal of Natural Gas Chemistry Vol. 19 No. 3 2010
groups and water molecules, water molecules are attracted to the carboxyl groups, then form water clusters; it is very easy to form pentagonal ring and hexatomic ring in hydrate cages (Figure 6(b)). These rings can be the template of hydrate and induce the hydrate formation, which is similar to the case of potassium oxalate monohydrate [11]. Meanwhile, the THF molecules can be bound by the hydrogen bond, which surrounded by water molecules links. Therefore, the appearance of hydrate is relatively easy. Furthermore, we consider that the template agent plays the leading role when SAP is in low concentration, and when SAP is in high concentration, water molecules bond together around THF plays the dominant function. The free water molecules will decrease if the SAP concentration increases, so the promoting effect is not stronger at higher concentration. Consequently, there are two optimal concentrations of 0.004% and 0.03%.
Figure 6. Suggested mechanism model for SAP promoting effect on the hydrate formation. (a) Large cage of THF hydrate(512 64 ), (b) Ring structure formed by SAP and water
4. Conclusions The effect of SAP on the formation of THF hydrate has been studied by the successional cooling method. It is found that THF solution samples with 0.004% and 0.03% SAP respectively first form hydrate completely, and the induction time of which is 16–29 min, 14–31 min, respectively. Meanwhile, the induction time of 19% THF solution samples without SAP is 25–62 min, which is obviously longer than that of THF solution samples with SAP. The results clearly indicate that SAP can promote the THF hydrate formation. At the same time, the pictures of hydrate formation with and without SAP additive are compared. It is found that SAP does
not change the morphology of the hydrate. The mechanism of SAP prompting effect on the THF hydrate formation is discussed. We suggest that there are two factors that may be responsible for its promoting effect. One is SAP and water can form hydro gel with steric network structure containing lots of carboxyl groups. Because the hydrogen bonds form between carboxyl groups and water molecules, water molecules are attracted to the carboxyl groups, and then form water clusters; it is very easy to form pentagonal ring and hexatomic ring, which can be the template of hydrate and induce the hydrate formation. The other is that the THF may be bound by the hydrogen bond, which surrounded by water molecules links. The appearance of hydrate is relatively easy. Further study on the suggested mechanism is now underway in our laboratory. References [1] Gayet P, Dicharry C, Marion G, Graciaa A, Lachaise J, Nesterov A. Chem Eng Sci, 2005, 60: 5751 [2] Zhang J S, Lee S Y, Lee J W. Ind Eng Chem Res, 2007, 46: 6353 [3] Zhang J S, Lo C, Somasundaran P, Lu S, Couzis A, Lee J W. J Phys Chem C, 2008, 112: 12381 [4] Ajay M. and Sukumar L. Energy Fuels, 2008, 22: 2527 [5] Li S, Fan S, Wang J, Lang X, Liang D. J Natur Gas Chem, 2009, 18: 15 [6] Fan S S, Li S F, Wang J Q, Lang X M, Wang Y H. Energy Fuels, 2009, 23: 4202 [7] Ganji H, Manteghian M, zadeh K S, Omidkhah M R, Mofrad H R. Fuel, 2007, 86: 434 [8] Zhang B, Wu Q, Sun D. Journal of China University of Mining and Technology, 2008, 18: 18 [9] Du J, Tang C, Fan S, Liang D. Journal of Xi’an Jiaotong University (Xi’an Jiaotong Daxue Xuebao), 2008, 42: 6610 [10] Wilson P W, Lester D, Haymet A D J. Chem Eng Sci, 2005, 60: 2937 [11] Zhang C S, Fan S S, Liang D Q, Guo K H. Fuel, 2004, 83: 2115 [12] Watanabe K, Yokokawa K, Muto Y. 13th International Conference on Cold Regions Engineering, 2006 [13] Kang S P, Seo Y. Energy Fuels, 2009, 23: 3711 [14] Zhang X Y, Fan S S, Liang D Q, Li D L, Chen G J. Science in China Series B: Chemistry, 2008, 51: 893 [15] Su F B, Bray C L, Tan B, Cooper A I. Adv Mater, 2008, 20: 2663 [16] Wang W X, Bray C L, Adams D J, Cooper A I. J Am Chem Soc, 2008, 130: 11608 [17] Carter B O, Wang W, Adams D J, Cooper A I. Langmuir, 2010, 26(5): 3186 [18] Su F, Bray C L, Carter B O. Adv Mater, 2009, 21: 2 [19] Ayel V, Lottin O, Popa E, Peerhossaini H. Int J Therm Sci, 2005, 44: 11 [20] Zeng H, Wilson L D, Walker V K, Ripmeester J A. J Am Chem Soc, 2006, 128: 2844 [21] Zeng H, Lu H L, Huva E, Walker V K, Ripmeester J A. Chem Eng Sci, 2008, 63: 4026 [22] Fan S. The Storage and Transportation Technology of Natural Gas Hydrate. Beijing: Chemical Industry Press, 2005. 75 [23] Chen G, Sun C, Ma Q. The Science and Technology of Gas Hydrate. Beijing: Chemical Industry Press, 2008. 124 [24] Zhang W, Creek J L, Koh C A. Meas Sci Technol, 2001, 12: 1620 [25] Barlow T W, Haymet A D J. Rev Sci Instrum, 1995, 66: 2996
Promoting effect of super absorbent polymer on hydrate formation Fei Long, Shuanshi Fan, Yanhong Wang,
Xuemei Lang∗
The Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, Guangdong, China [ Manuscript received September 8, 2009; revised March 25, 2010 ]
Abstract The effect of super absorbent polymer (SAP) on the formation of tetrahydrofuran (THF) hydrate was studied by the successional cooling method. It was found that THF solution samples with 0.004 wt% and 0.03 wt% of SAP formed THF hydrate completely during the same cooling process. The corresponding induction time was 16–29 min, 14–31 min, respectively, which was obviously shorter than that of THF solution samples without SAP (25–62 min). It indicated that SAP accelerated the formation of THF hydrate. At the same time, the pictures of hydrate formation with and without SAP had been compared. It was found that SAP did not change the morphology of the hydrate. Finally, the mechanism of SAP promoting effect on the formation of THF hydrate was suggested. Key words super absorbent polymer; tetrahydrofuran; hydrate; promoting effect
1. Introduction Hydrates are nonstoichiometric crystalline compounds, in which small guest molecules are trapped by host water molecules. Hydrate technologies are mainly used in gas storage and transportation, the separation of mixed gas, cold storage in air conditioning, desalination of seawater etc. The application and popularization of hydrate technologies are hindered by some drawbacks, such as harsh formation conditions, long induction time, slow hydration rate, low gas storage density and separation efficiency. Lots of researchers are trying to use additives to promote hydrate generation. Surfactant is in common use, for examples, SDS [1−4], TBAB and TBAF [5,6], LABS, CTAB, ENP [7], Tween [8], Span20 [9] etc. They can speed up the hydrate formation in some extent. Structural revulsant is another kind of promoter, such as silver iodide [10] and potassium oxalate monohydrate [11]. The former may provide the nucleus and the latter is used as the template agent. Porous medium can also accelerate the hydrate formation, such as glass powder [12], silica gel [13], molecular sieve [14], polyHIPE [15], dry water and dry gel and so on [16,17]. They promote hydrate formation through the enhancement of mass transfer. The hydro gel formed by poly (acrylic acid) sodium salt (PSA) and Tetrahydrofuran-water (THF-H2 O) solution greatly increases the hydrogen storage rate and density, and these materials can be used repeatedly [18]. However, the ∗
effect of PSA on hydrate induction time has not been studied and there is few mechanical analysis in the references. So the effect of super absorbent polymer (SAP, crosslinking copolymerized by sodium acrylate and acrylic acid) on THF solution was studied in this paper. The suggested mechanism for its promotion effect was also discussed. 2. Experimental 2.1. Experimental materials Tetrahydrofuran (THF, AR) was purchased from Sinopharm Chemical Reagent Co., Ltd. Super absorbent polymer (SAP) was kindly provided by Anhui Huajing new materials Co., Ltd. Sodium dodecyl sulfate (SDS, AR) was from Tianjin Chemical Reagent Co., Ltd. Distilled water was produced in our laboratory. 2.2. Experimental apparatus Figure 1 is the schematic diagram of experimental system. The low temperature test chamber (Xutemp Temptech Co., Ltd.) provides cold air bath, the temperature fluctuation of which is smaller than 0.5 ◦ C and the temperature control range is –30 ◦ C–40 ◦ C. There is a window on one side of chamber, from which we can observe the reactions.
Corresponding author. Tel: +86-20-22236581; Fax: +86-20-22236581; E-mail: [email protected]
Copyright©2010, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(09)60074-8
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Fei Long et al./ Journal of Natural Gas Chemistry Vol. 19 No. 3 2010
Figure 1. Schematic diagram of experimental system. 1—Low temperature test chamber, 2—Thermocouple, 3—Glass bottle, 4—Data acquisition unit, 5—Computer
Data acquisition system contains data acquisition unit (Agilent 34970A), type J thermocouple (made by ourselves, deviation is smaller than 0.1 ◦ C) and computer. Except for apparatuses introduced above, electronic balance (BT224S, Sartorious AG, precision is ±0.1mg, and measuring range is 0– 220 g), digital camera (Sony, DSC-H9), volumetric flask and glass bottle (Figure 2) were also used.
Figure 2. Picture of glass bottle inserted by a thermocouple (taken outside of the chamber)
2.3. Experimental process
platform due to the exothermic nature of the THF hydrate or ice formation. The initial temperature platform of distilled water is −0.5 ◦ C and the THF is 3.4 ◦ C, which are all below their corresponding phase equilibrium point, because of the heat exchange between the solutions and the cold air in chamber. This result is similar to that reported by Vincent Ayel et al. [19]. Teq in Figure 3 is the phase equilibrium temperature, 4.4 ◦ C, of the 19%THF solution; teq is the corresponding time; tc is the time when the nucleation starts and leads to sudden increase in temperature. The induction time, t0 , is defined as the difference between teq and tc , t0 = tc – teq .
Figure 3. Temperature evolution of different solutions during successional cooling
3.2. Ef fect of SDS and SAP on THF hydrate induction time 4.4 ◦ C
at 19%THF solution (phase equilibrium point is 1 atm) and THF solutions with additives (SDS or SAP) were prepared first to make sure the mass ratio of THF to water 19 : 81 in all mixtures. Then 15 ml solutions were taken in the glass bottles respectively. Glass bottles were inserted by thermocouples and put in the chamber. The temperature of chamber was raised to 35 ◦ C and kept for 30 min later. Finally chamber was cooled continuously. The temperatures of the solutions were collected by the data acquisition unit during cooling. The final temperature of chamber was about −30 ◦ C. At the same time, the hydration processes were taken photos by the digital camera from the window of chamber. Every solution was tested 15 times, and 40 bottles can be measured in one time by our apparatus.
Figure 4 shows the result of effect of SDS and SAP on the induction time of THF hydrate formation.
3. Results and discussion 3.1. Def inition of induction time Temperature evolutions of distilled water, 19%THF solution and THF solutions-SAP mixtures during successional cooling are displayed in Figure 3. There is a temperature
Figure 4. Effect of SDS and SAP on the induction time of THF hydrate formation. Nt represents the number of samples that have not formed hydrates at induction time t, N0 = 15
Journal of Natural Gas Chemistry Vol. 19 No. 3 2010
Nt in Figure 4 represents the number of samples that have not formed hydrates at induction time t, and N0 is the total number of samples (here N0 = 15). This definition is the same with Refs. [20,21]. Huang Zeng et al. [20,21] considered that Nt /N0 (i.e., the uncrystallized fraction) vs. time represents the inhibition activity of the additive, and the slower the curve declines, the stronger the inhibition. So here the faster the curve declines, the stronger the promotion. The effect of SDS on the induction time of THF hydrate formation was studied first. The curve of THF with 0.8% SDS solution in Figure 4 declines faster than pure THF solution, which proves that the basic theory is correct. The effect of SAP addition ranging from 0.004% to 0.32% was further studied. It is found that all of the THF solutions with SAP decline faster than the THF solution without SAP (Figure 4), indicating that SAP can promote the hydrate formation and shorten the induction time. The THF solution samples with 0.004 wt% and 0.03 wt% SAP decline faster than others, and the corresponding induction time is 16–29 min and 14–31 min, respectively, which is obviously shorter than that of the pure THF solution samples with the corresponding induction time of 25–62 min. There is some intersection between different curves and we consider that it is caused by the randomicity of the static system. The induction of hydrate formation is stochastic [22,23], and this kind of random is more conspicuous under low driving force [24,25].
253
The balance of static system must be broken before the hydrate forms, which is affected by lots of random environmental disturbances. At the same time, because of the insolubility of SAP, the inhomogeneity of SAP would be inevitable, although all of the solutions were agitated several times before test. 3.3. Ef fect of SAP on morphology of the THF hydrate The hydration process pictures of 19% THF solution and the THF solution with 0.004% SAP are all presented in Figure 5. Figure 5(a1 ) is the starting cooling point of THF solution, Figure 5(b1 ) is the starting cooling point of THF solution with 0.004% SAP, and the thermocouple can be seen clearly. Hydrate was generated instantaneously as we can see in Figure 5(a2 ) and Figure 5(b2 ), and acicular hydrate growing around the thermocouple and the thermocouple can not be seen clearly. Figure 5(a3 ) and Figure 5(b3 ) were the hydration terminations, and Figure 5(a4 ) and Figure 5(b4) were the pictures of successional cooling after the hydration processes were completed. There is no obviously different between these two hydration processes, suggesting that SAP does not change the morphology of the hydrate.
Figure 5. a1 –a4 : the hydration process of 19% THF solution; b1 –b4 : the hydration process of THF solution with 0.004% SAP. (a1 ) Time: 12:14:01, T : 35.0 ◦ C; (a2 ) Time: 13:22:34, T : 3.4 ◦ C; (a3 ) Time: 13:50:36, T : 2.1 ◦ C; (a4 ) Time: 14:21:14, T : –22.6 ◦ C; (b1 ) Time: 12:14:10, T : 34.9 ◦ C; (b2 ) Time: 13:18:26, T : 3.3 ◦ C; (b3 ) Time: 13:47:50, T : 2.2 ◦ C; (b4 ) Time: 14:21:20, T : –22.8 ◦ C
3.4. Mechanism of SAP promoting ef fect on hydrate formation Hydrate is produced when the guest molecules enter cage formed by hydrogen-bonded water molecules under certain conditions. THF hydrates are structure II which is consisted of large and small cages, and THF molecules can only be in-
cluded as guests in the large cages (consisting of 12 pentagons and 4 hexagons, 512 64 , see Figure 6(a)). There is a long induction time when hydrate forms in pure water. When SAP is added to THF solution, SAP and water can form hydro gel with steric network structure containing lots of carboxyl groups. Because the hydrogen bonds form between carboxyl
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Fei Long et al./ Journal of Natural Gas Chemistry Vol. 19 No. 3 2010
groups and water molecules, water molecules are attracted to the carboxyl groups, then form water clusters; it is very easy to form pentagonal ring and hexatomic ring in hydrate cages (Figure 6(b)). These rings can be the template of hydrate and induce the hydrate formation, which is similar to the case of potassium oxalate monohydrate [11]. Meanwhile, the THF molecules can be bound by the hydrogen bond, which surrounded by water molecules links. Therefore, the appearance of hydrate is relatively easy. Furthermore, we consider that the template agent plays the leading role when SAP is in low concentration, and when SAP is in high concentration, water molecules bond together around THF plays the dominant function. The free water molecules will decrease if the SAP concentration increases, so the promoting effect is not stronger at higher concentration. Consequently, there are two optimal concentrations of 0.004% and 0.03%.
Figure 6. Suggested mechanism model for SAP promoting effect on the hydrate formation. (a) Large cage of THF hydrate(512 64 ), (b) Ring structure formed by SAP and water
4. Conclusions The effect of SAP on the formation of THF hydrate has been studied by the successional cooling method. It is found that THF solution samples with 0.004% and 0.03% SAP respectively first form hydrate completely, and the induction time of which is 16–29 min, 14–31 min, respectively. Meanwhile, the induction time of 19% THF solution samples without SAP is 25–62 min, which is obviously longer than that of THF solution samples with SAP. The results clearly indicate that SAP can promote the THF hydrate formation. At the same time, the pictures of hydrate formation with and without SAP additive are compared. It is found that SAP does
not change the morphology of the hydrate. The mechanism of SAP prompting effect on the THF hydrate formation is discussed. We suggest that there are two factors that may be responsible for its promoting effect. One is SAP and water can form hydro gel with steric network structure containing lots of carboxyl groups. Because the hydrogen bonds form between carboxyl groups and water molecules, water molecules are attracted to the carboxyl groups, and then form water clusters; it is very easy to form pentagonal ring and hexatomic ring, which can be the template of hydrate and induce the hydrate formation. The other is that the THF may be bound by the hydrogen bond, which surrounded by water molecules links. The appearance of hydrate is relatively easy. Further study on the suggested mechanism is now underway in our laboratory. References [1] Gayet P, Dicharry C, Marion G, Graciaa A, Lachaise J, Nesterov A. Chem Eng Sci, 2005, 60: 5751 [2] Zhang J S, Lee S Y, Lee J W. Ind Eng Chem Res, 2007, 46: 6353 [3] Zhang J S, Lo C, Somasundaran P, Lu S, Couzis A, Lee J W. J Phys Chem C, 2008, 112: 12381 [4] Ajay M. and Sukumar L. Energy Fuels, 2008, 22: 2527 [5] Li S, Fan S, Wang J, Lang X, Liang D. J Natur Gas Chem, 2009, 18: 15 [6] Fan S S, Li S F, Wang J Q, Lang X M, Wang Y H. Energy Fuels, 2009, 23: 4202 [7] Ganji H, Manteghian M, zadeh K S, Omidkhah M R, Mofrad H R. Fuel, 2007, 86: 434 [8] Zhang B, Wu Q, Sun D. Journal of China University of Mining and Technology, 2008, 18: 18 [9] Du J, Tang C, Fan S, Liang D. Journal of Xi’an Jiaotong University (Xi’an Jiaotong Daxue Xuebao), 2008, 42: 6610 [10] Wilson P W, Lester D, Haymet A D J. Chem Eng Sci, 2005, 60: 2937 [11] Zhang C S, Fan S S, Liang D Q, Guo K H. Fuel, 2004, 83: 2115 [12] Watanabe K, Yokokawa K, Muto Y. 13th International Conference on Cold Regions Engineering, 2006 [13] Kang S P, Seo Y. Energy Fuels, 2009, 23: 3711 [14] Zhang X Y, Fan S S, Liang D Q, Li D L, Chen G J. Science in China Series B: Chemistry, 2008, 51: 893 [15] Su F B, Bray C L, Tan B, Cooper A I. Adv Mater, 2008, 20: 2663 [16] Wang W X, Bray C L, Adams D J, Cooper A I. J Am Chem Soc, 2008, 130: 11608 [17] Carter B O, Wang W, Adams D J, Cooper A I. Langmuir, 2010, 26(5): 3186 [18] Su F, Bray C L, Carter B O. Adv Mater, 2009, 21: 2 [19] Ayel V, Lottin O, Popa E, Peerhossaini H. Int J Therm Sci, 2005, 44: 11 [20] Zeng H, Wilson L D, Walker V K, Ripmeester J A. J Am Chem Soc, 2006, 128: 2844 [21] Zeng H, Lu H L, Huva E, Walker V K, Ripmeester J A. Chem Eng Sci, 2008, 63: 4026 [22] Fan S. The Storage and Transportation Technology of Natural Gas Hydrate. Beijing: Chemical Industry Press, 2005. 75 [23] Chen G, Sun C, Ma Q. The Science and Technology of Gas Hydrate. Beijing: Chemical Industry Press, 2008. 124 [24] Zhang W, Creek J L, Koh C A. Meas Sci Technol, 2001, 12: 1620 [25] Barlow T W, Haymet A D J. Rev Sci Instrum, 1995, 66: 2996