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Energy & Fuels 2006, 20, 2198-2201
Effect of Cyclodextrins on Hydrate Formation Rates Yusuke Kuji,† Akihiro Yamasaki,*,‡ and Yukio Yanagisawa† School of Frontier Science, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 Japan, and Institute for EnVironmental Management Technology, National Institute of AdVanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, 305-8569 Japan ReceiVed February 1, 2006. ReVised Manuscript ReceiVed July 3, 2006
Effects of cyclodextrins (CD) on hydrate formation kinetics were investigated. Several kinds of cyclodextrins, such as R-CD, β-CD, β-CD polymers, and β-CD modified with methyl and triacetyl groups were tested as additives for hydrate formation of xenon, with a concentration range of 0-5 wt %. Induction time for hydrate formation was reduced by the addition of any of the CDs; however, no clear relationship between the structure of CDs and the reduction effect was observed. The three-phase equilibrium pressure (H-V-Lw) was unaffected by the addition of the CDs, except for β-CDs modified with methyl or triacetyl groups. The hydrate formation rate, which was expressed as a rate constant based on the chemical potential, increased with the addition of R-CD, β-CD, and β-CD polymers. β-CD polymer showed the highest acceleration effect on the xenon hydrate at about 1 wt %; the rate constant was about three times larger than that for pure water. Contrastingly, modified β-CD and dextrin with a straight chain structure reduced the hydrate formation rate. β-CD polymer was subsequently applied as an acceleration additive for the methane hydrate formation process, and it was found that the addition of β-CD polymer increased the methane hydrate formation rate up to five times as compared to that for pure water.
Introduction Various applications of gas hydrates have been proposed for use in the field of energy and the environment. Gudmunddson and Borrehaug1 proposed a new concept of transportation of natural gas in the form of gas hydrates (NGH transportation) as a substitute method for conventional LNG transportation. However, it is necessary to reduce the power consumption for hydrate formation to make hydrate-related technology more feasible. One effective measure to reduce the power consumption for the hydrate formation process would be to accelerate the hydrate formation rates using additives. Surfactants such as SDS (sodium dodecyl sulfate) have been tested for methane hydrate formation, and acceleration effects have been reported.2 Although detailed mechanisms of the acceleration effect of such surfactants have not been elucidated, it can be suggested that the amphiphilic structure of the surfactants would enhance the uptake rate of the guest molecules from the vapor phase. In this study, effects of cyclodextrins on the hydrate formation kinetics were investigated experimentally. Cyclodextrins are cyclic oligosaccharides with a bucket-like structure and have a cavity with the diameter of about 0.5-0.7 nm.3 The inside of the cavity is hydrophobic, and the outside is hydrophilic; CDs can be recognized as a kind of amphiphilic compound. It can be anticipated that the hydrophobic cavity in CDs could induce uptake rates of the guest gas molecules into the water phase * Corresponding author. Phone: +81-29-861-9409. Fax: +81-29-8618727. E-mail:
[email protected]. † The University of Tokyo. ‡ National Institute of Advanced Industrial Science and Technology. (1) Gudmunddson, J.; Borrehaug, A. Frozen hydrate for transport of natural gas. Proceedings of the 2nd International Conference on Natural Gas Hydrate; 1996; pp 415-429. (2) Zhong, Y.; Rogers, R. E. Surfactant effects on gas hydrate formation. Chem. Eng. Sci. 2000, 55, 4175-4187. (3) Bender, M. L.; Komiyama, M. Cyclodextrin Chemistry; SpringerVerlag: Tokyo, 1978.
when the CDs are dissolved in the aqueous phase. Therefore, the hydrate formation rate could be increased by CD addition. In this study, CDs with different structures were tested as additives for the hydrate formation of xenon and methane, and the effect of the structure of the CDs on the hydrate formation process was examined. Experimental Section Materials. The following cyclodextrins and their derivatives purchased from Wako Chemical Co., Japan, and Ensuiko Sugar Refining Co., Japan. They were used without further purification: (a) CD monomers (i) R-CD: (C6H10O5)6 (ii) β-CD: (C6H10O5)7 (b) Modified β-CDs (i) hydroxyethyl β-CD: C2H4OH, H (ii) methyl β-CD: CH3, H (iii) triacetyl β-CD: (CH3COO)3, H (c) β-CD polymers: ((C6H10O5)7)n (i) ligo β-CD: n ) 2-3 (ii) poly β-CD: n ) 5-10 Experimental Apparatus and Procedures. A schematic drawing of the experimental apparatus is shown in Figure 1. Measurements of the hydrate formation rates were by a constant-volume system. The hydrate reactor is a high-pressure vessel with an inner volume of 100 mL. A magnetic stirring unit inside the reactor had an adjustable stirring speed of up to 1000 rpm. A known amount of water was fed in the reactor and was purged several times with a feed gas to replace the gas phase. The purge-valve was then closed, and the feed gas was introduced to the reactor until the system pressure reached a prefixed pressure. The initial pressure and the amount of water were determined so that the three-phase equilibrium (H-LW-V) could be attained. The experimental temperature was controlled by immersing the reactor in a constanttemperature bath, of which the temperature was controlled with an accuracy of ( 0.1 K. The system pressure changes as well as
10.1021/ef060046z CCC: $33.50 © 2006 American Chemical Society Published on Web 09/02/2006
Effect of Cyclodextrins on Hydrate Formation
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Table 1. Effect of CD Addition on the Induction Time for the Xenon Hydrate Formation concentration (wt %) CD
0
0.1
0.5
1.0
R-CD β-CD oligo β-CD poly β-CD hydroxyethyl-β-CD methyl-β-CD triacetyl-β-CD dextrin
1200-32000 1200-32000 1200-32000 1200-32000 1200-32000 12000-32000 12000-32000 12000-32000
10-540 100-150 950-2500 10-150 0 0-50 140-460 (C ) 0.01%) 21000(C ) 0.2%)
10-30 150-390 2340-2730 10-250
140-330 0-170 810-7080 0-90 10-20 0-6000
temperature changes were monitored and recorded by a pressure indicator and thermometer. Hydrate formation was initiated by stirring, and the induction time was determined as the period the stirring started to the pressure decrease due to hydrate formation. The hydrate formation rate was determined from the pressure decrease based on the kinetic equation described in the next section. Xenon and methane were used as guest components for hydrate formation. Xenon hydrate formation was investigated for all the CDs as well as dextrin for screening effective CD. Methane hydrate formation experiments were subsequently carried out with the most effective CD for the hydrate formation acceleration. Of note is that both hydrates of xenon and methane have the same structure sI. Deionized water and pure grade gas (Xe, CH4) were used for all runs.
Hydrate Formation Rate Constant The hydrate formation rate can be expressed in terms of fugacity difference during formation and at equilibrium.4 In this study, a more general term of chemical potential difference between the gas phase and the hydrate phase at equilibrium condition (µgas - µeq) was employed as the driving force of hydrate formation.5 The formation rate, rf, could therefore be expressed as
( dndt ) ) ak (µ
rf ) -
f
gas
- µeq)
(1)
where (-dn/dt) is the gas consumption rate, a is the interfacial area for the hydrate formation, and kf is the kinetic rate constant. The chemical potential difference could be related to the fugacity ratio, fgas/feq:
( dndt ) ) ak (µ
rf ) -
f
gas
fgas feq
- µeq) ) akfRT ln
(2)
where R is the gas constant, T is temperature, fgas is the fugacity of the gas phase, and feq is the fugacity of the hydrate phase at equilibrium condition. The effect of the additives on hydrate formation rates could be compared in terms of the kinetic rate constant, kf. However, it might be difficult to separate the term a and kf from the experimental results for the fugacity change. The interfacial area could be altered with the progress of hydrate formation. The time variation of the interfacial area was eliminated by extrapolating the hydrate formation rate to t f 0. The interfacial area, a, could also depend on the stirring speed. It was confirmed that the apparent rate constant, akf, was found to be constant when the stirring speed was over 500 rpm. Therefore, the kinetic constant with the interfacial area, akf, measured under a stirring speed of 500 rpm was used for the comparison of the effects of (4) Englezos, P.; Kalogerakis, N.; Dholabhai, P. D.; Bishnoi, P. R. Kinetics of formation of methane and ethane gas hydrate. Chem. Eng. Sci. 1987, 42, 2647-2658. (5) Sloan, E. D. Clathrate Hydrates of Natural Gases, 2nd ed.; Dekker: New York, 1997.
25000
surfactant, and the effects of CDs can be compared and discussed in terms of the parameter, akf. Results and Discussion Induction Time for Xenon Hydrate Formation. Effect of CD addition on the induction time for xenon hydrate formation is summarized in Table 1. Although the experimentally observed induction time was distributed over a wide range, the induction time for hydrate formation was significantly reduced by CD addition as compared with that of pure water (which was over 1000 s). Especially the CD polymer showed a relatively consistent induction time, which was under 200 s for any used concentration. However, no clear correlation between the CD structure or concentrations and induction time was observed. The reduction of the induction time could be due to the induced gas uptake by the CD dissolved in the aqueous phase. Three-Phase Equilibrium Pressure of Xenon Hydrate. Figure 2 shows the results of the three-phase (H-LW-V) equilibrium pressure at 274 K. The dotted line in Figure 2 indicates the equilibrium pressure for pure water. In most cases, the three-phase equilibrium pressure was equal to that for the case with pure water. Addition of methyl β-CD or triacetyl β-CD increased the equilibrium pressure. Effect of the Addition of CDs on Formation Rates of Xenon Hydrate. Figure 3 shows the effect of the addition of CD monomers and dextrin on the rate constant of xenon hydrate formation (akf) at 274 K. The dotted line indicates the rate constant for pure water. The hydrate formation rate of xenon was accelerated by the addition of R-CD (except at 0.1 wt %), and the rate constant showed a maximum value at a concentration of 0.5 wt %. The rate constant was almost unaffected by the addition of β-CD. On the other hand, the rate constant decreased with an increase in the dextrin concentration.
Figure 1. Schematic drawing of experimental apparatus. This is a constant-volume system; the main part being a high-pressure vessel (inner volume of 100 mL) with a magnetic stirring unit: 1, gas cylinder; 2, gas reservoir; 3, pressure-regulating valve; 4, hydrate formation reactor; 5, constant-temperature bath; 6, filter; 7, stirring unit; 8, pressure sensor; 9, thermometer; 10, dry silica gel.
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Figure 2. Relation between equilibrium pressure of xenon hydrate and concentration of the added CD type. Temperature ) 274 K.
Figure 3. Relation between the rate constant of xenon hydrate formation and weight concentration (wt %) of the added CD monomers. Temperature ) 274 K.
Figure 4. Relation between the rate constant of xenon hydrate formation and weight concentration (wt %) of the added modified β-CD. Temperature ) 274 K.
Figure 4 shows the effects of the addition of modified β-CDs on the rate constant for the xenon hydrate formation. The rate constant was significantly reduced by the addition of methyl β-CD or triacetyl β-CD, while the rate constant was slightly affected by the addition of hydroxyethyl β-CD as well as unmodified β-CD. Figure 5 shows the effect of the addition of β-CD polymers on the rate constant of xenon hydrate formation. The hydrate formation rate of xenon was greatly accelerated by the addition of β-CD polymers. The acceleration effect of β-CD oligomer was almost equivalent with that of β-CD polymer at lower concentrations up to 0.5 wt %, and after that the rate constant decreased with increasing concentrations of β-CD oligomer.
Kuji et al.
Figure 5. Relation between the rate constant of xenon hydrate formation and weight concentration (wt %) of the added polymerized β-CD. Temperature ) 274 K.
Figure 6. Relation between the rate constant of xenon hydrate formation and weight concentration (wt %) of the added poly β-CD. Temperature ) 274 K.
From the above results for screening CDs, the highest acceleration effect was shown by the poly β-CD addition on the hydrate formation rate of xenon. The effects of the concentration of poly β-CD addition on the xenon hydrate formation rates were investigated with wider range of concentrations, and the results are shown in Figure 6. The rate constant showed a maximum value at 1 wt %, where the rate constant was 2.5 times larger than that for pure water and decreased after that. Interestingly, the addition of β-CD polymer decreased the rate constant when the concentration was higher than 4 wt. %. The above experimental results demonstrated that most CDs reduced the induction time for hydrate formation. This is because the addition of CDs would increase the heterogeneity of the water phase, which may induce the nucleation of hydrate in water. The three-phase equilibrium pressure was unchanged by the addition of most of CDs. These CDs have an acceleration effect on the formation rate of xenon hydrate. The acceleration effect could be best explained in terms of enhancement of gas uptake rate by the addition of CDs into the aqueous phase; the hydrophobic cavities in CDs would capture guest gas molecules more quickly compared to pure water. The captured gas molecule would not be stabilized in the cavity, but instead they would be transferred to the aqueous phase to form a cluster, a precursor for hydrate formation (Figure 7). As a result, the hydrate formation rate would be accelerated by the addition of these CDs. However, when the concentration of CDs in the aqueous phase was high, the hydrophilic part of the CD would retard the formation of a water cluster, just like the inhibition effect by ionic compounds.5 This could be the reason for a maximum acceleration effect against the CD concentration. The
Effect of Cyclodextrins on Hydrate Formation
Figure 7. Model for the effect of CD on hydrate formation.
addition of β-CD polymers showed a higher acceleration effect than that of β-CD monomers. The reason for the higher acceleration effect of β-CD polymers would be explained as follows. The guest gas molecules would be captured into the cavities of the CD. The captured gas molecules in the cavities of CD polymer would be located more closely to each other than in the case of the CD monomer. As a result, a water cluster can grow more rapidly by including guest molecules to form a hydrate structure. The modified CDs with hydrophobic groups, such as methyl or hydroxyethyl groups, were found to significantly decrease the hydrate formation rate as well as to elevate the equilibrium pressure for hydrate formation. These modified CDs are both kinetic and equilibrium inhibitors for hydrate formation. The inhibition of hydrate formation could be explained by the exclusion effect by hydrophobic groups on the CD, which would retard the gas uptake and the formation of a water cluster near the CD. Effect of the Addition of CDs on Methane Hydrate Formation Rates. The hydrate formation rate was most effectively accelerated by addition of β-CD polymers. The effect of addition of β-CD polymer was investigated on methane hydrate formation rates. Figure 8 shows the results for the rate constant of methane hydrate formation against the weight concentration (wt %) of poly β-CD polymers. The rate of methane hydrate formation was accelerated by addition of β-CD
Energy & Fuels, Vol. 20, No. 5, 2006 2201
Figure 8. Relation between the rate constant of methane hydrate formation and weight concentration (wt %) of the added poly β-CD. Temperature ) 274 K.
polymers, and the enhancement effect showed a maximum with poly β-CD at a concentration of 1.0 wt %, where the rate constant was about 5 times as large as that for pure water. The enhancement effect of poly β-CD is more prominent for methane hydrate formation than for xenon hydrate formation. This result suggests that the enhancement of the gas uptake by the CD polymer would be more effective for methane because the solubility of methane in water is much lower than that of xenon. From these results, the addition of CD can be viewed as a way to reduce the power consumption for hydrate formation. However, the understanding of mechanism remains to be solved. Conclusions The induction time of xenon hydrate formation was reduced by the addition of CDs used in this study, although no clear correlation between the induction time and the concentration or structure of CDs was observed. The equilibrium pressure of hydrate was unaffected by the addition of CDs, except for methyl β-CD or triacetyl β-CD. The hydrate formation rate of xenon was accelerated by addition of the R-CD or the β-CD polymer and the β-CD oligomer, while the rate was reduced by the addition of methyl and triacetyl β-CD. The addition of poly β-CD increased the methane hydrate formation rate about 5 times as compared to that for pure water. EF060046Z
