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Clathrate Hydrate Formation by Water Spraying in a Methane + Ethane + Propane Gas Mixture: Search for the Rate-Controlling Mechanism of Hydrate Formation in the Presence of Methylcyclohexane Takehito Kobayashi,†,§ Naotaka Imura,† Ryo Ohmura,†,‡ and Yasuhiko H. Mori*,† Department of Mechanical Engineering, Keio UniVersity, Yokohama 223-8522, Japan, and National Institute of AdVanced Industrial Science and Technology (AIST), Sapporo 062-8517, Japan ReceiVed September 11, 2006. ReVised Manuscript ReceiVed January 14, 2007
In a previous paper (Tsuji et al. Energy Fuels 2005, 19, 869-876), we reported that the rate of clathrate hydrate formation from a simulated natural gas;a mixture of methane, ethane, and propane in a 90:7:3 molar ratio;in the presence of methylcyclohexane (MCH) significantly changed with the repetition of hydrateforming operations and that its asymptotic level substantially exceeds the rate available in the absence of MCH. Here, we report new results in which continual chromatographic analyses of the gas-phase composition inside the hydrate-forming spray chamber gave insight into the mechanism by which the presence of MCH influences the rate of hydrate formation. The observed evolution of the gas-phase composition during each hydrate-forming operation indicates that the hydrate formed over the major portion of the operation is in structure II, although MCH may provide guest molecules to fit into the 51268 cages of a structure-H hydrate. The difference in the rate of hydrate formation from operation to operation was found to be related to the difference in the initial gas-phase composition due to the preferential dissolution of ethane and propane from the gas phase into the liquid MCH phase before the inception of hydrate formation; the higher the initial concentrations of ethane and propane in the gas phase are, the higher the rate of successive hydrate formation is.
Introduction The formation of clathrate hydrates from natural gas and water is a key process in the yet-to-be established technology for storing and transporting natural gas in the form of hydrates.1 Because natural gas is generally a mixture of some hydrateforming substances including methane, ethane, propane, and so forth, the process of hydrate formation is inevitably complicated by a fractionation effect. Except for the case in which a hydrateforming reactor in a continuous operation has achieved a steady state, both the gas-phase composition and the guest-molecule composition in the hydrate product should vary with time during each hydrate-forming process. Sufficient understanding of such time evolution of the states of gas and hydrate phases during each hydrate-forming process is required for designing and constructing any hydrate-forming system for industrial use. In a previous paper published in this journal,2 we reported an experimental study of hydrate formation from a simulated natural gas;a mixture of methane, ethane, and propane in a 90:7:3 molar ratio;in the presence of a large-molecule guest substance (LMGS), an oily liquid with molecules that stabilize the 51268 cages of structure-H hydrates. Water was continuously * Corresponding author tel.: +81-45-566-1522; fax: +81-45-566-1495; e-mail:
[email protected]. † Keio University. ‡ Formerly National Institute of Advanced Industrial Science and Technology (AIST). § Current address: Toyota Motor Corp., 1 Toyota-cho, Toyota-shi, Aichi Prefecture 471-8571, Japan. (1) Mori, Y. H. J. Chem. Ind. Eng. (China) 2003, 54 (Suppl.), 1-17. (2) Tsuji, H.; Kobayashi, T.; Ohmura, R.; Mori, Y. H. Energy Fuels 2005, 19, 869-876.
sprayed into the gas phase in a high-pressure chamber into which the gas mixture of the above-stated composition was continuously supplied to compensate for the loss of the gas due to hydrate formation and thereby to maintain constant pressure inside the chamber. The rate of hydrate formation, as evaluated from the rate of gas supply into the chamber, was generally increased by the use of any of the three LMGSs tested;tertbutyl methyl ether, 2,2-dimethylbutane (neohexane), and methylcyclohexane (MCH);beyond the rate of hydrate formation in the absence of any LMGS. In addition to the experimental study, we performed thermodynamic simulations of isobaric, continuous or semi-batch hydrate-forming operations relevant to our experiments.3 The simulations indicated the evolutions of the gas-phase composition in the hydrate-forming reactor, the structure of hydrate crystals instantaneously formed, and the corresponding phase-equilibrium temperature in the course of each hydrate-forming operation. These simulations could substantially help our understanding of how a transient or steady hydrate-forming process is proceeding inside the reactor. However, the validity of the simulations for LMGS-involved hydrate-forming operations has not been confirmed yet.4 There is still room for further modifying the scheme of such simula(3) Tsuji, H.; Kobayashi, T.; Okano, Y.; Ohmura, R.; Yasuoka, K.; Mori, Y. H. Energy Fuels 2005, 19, 1587-1597. (4) The general validity of such thermodynamic simulations of hydrate formation from a gas mixture in the absence of any LMGS was indicated in a recent paper by Kobayashi and Mori,5 in which a thermodynamic simulation of a batch-type operation of hydrate formation from a methane + propane mixture is compared to the relevant experimental result reported by Uchida et al.6
10.1021/ef060461r CCC: $37.00 © 2007 American Chemical Society Published on Web 03/07/2007
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Figure 1. Schematic of experimental apparatus.
tions, depending on the progress in our experiment-based knowledge and understanding of hydrate formation from a gas mixture plus an LMGS. As a matter of fact, the current simulation scheme does not take into account the issue discussed below. In the previous experimental study outlined above,2 we noted that the rate of hydrate formation in the presence of neohexane or MCH exhibited a consistent run-by-run change; that is, as the experimental system first charged with fresh samples of the test liquids (water and neohexane or MCH) was subjected to successive hydrate-forming runs at 1-day intervals, the rate of hydrate formation showed a marked tendency to increase from run to run. We tentatively ascribed this tendency to a reduction in the selective gas-absorption capacity of the LMGS phase with repetition of the experimental runs. However, the details of its mechanism remained to be clarified. In order to clarify this issue, we need to obtain experimental information on the evolution of the gas-phase composition and/or the structure of hydrate crystals instantaneously formed during each hydrate-forming operation. This paper reports our latest experimental study aimed at the above task. As for the experimental framework, this study generally followed our previous studies in this series2,7,8 in which semi-batch water-spray operations were employed. As the LMGS, we exclusively used MCH with which the run-by-run change in hydrate formation rate had been observed to be the most extensive in the preceding study.2 The principal feature of this study was the employment of continual chromatographic analyses of the gas-phase composition inside the hydrateforming spray chamber during each hydrate-forming operation. On the basis of the evolution of the gas-phase composition thus measured, we can estimate the structure of the hydrate crystals instantaneously formed inside the spray chamber. Applying this composition-analysis procedure to a set of hydrate-forming experiments systematically arranged, we have arrived at an unexpected consequence; that is, the hydrate dominantly formed over the major portion of each hydrate-forming operation is in structure II, instead of structure H, contrary to the prediction of (5) Kobayashi, T.; Mori, Y. H. Energy ConVers. Manage. 2007, 48, 242250. (6) Uchida, T.; Moriwaki, M.; Takeya, S.; Ikeda, I. Y.; Ohmura, R.; Nagao, J.; Minagawa, H.; Ebinuma, T.; Narita, H.; Gohara, K.; Mae, S. AIChE J. 2004, 50, 518-523. (7) Ohmura, R.; Kashiwazaki, S.; Shiota, S.; Tsuji, H.; Mori, Y. H. Energy Fuels 2002, 16, 1141-1147. (8) Tsuji, H.; Ohmura, R.; Mori, Y. H. Energy Fuels 2004, 18, 418424.
our simulation study.3 We also found that the difference in the rate of hydrate formation from run to run was related to the difference in the initial gas-phase composition due to the preferential dissolution of ethane and propane from the gas phase into the liquid MCH phase before the inception of hydrate formation. These issues are described and discussed in this paper. Experimental Section Apparatus. Figure 1 illustrates the experimental apparatus used in the present study. Except for the gas-chromatography assembly connected to the spray chamber, this apparatus is the same waterspray-type hydrate-forming apparatus that we exclusively used in our previous studies of this series.2,7,8 Its details are described in the earliest report,7 and hence we give below only its outline in addition to some details of the newly added gas-chromatography assembly. The major portion of the apparatus is a spray chamber in which a gas mixture (a simulated natural-gas nominally composed of 90 mol % methane, 7 mol % ethane, and 3 mol % propane), liquid water, and an LMGS (MCH in the present study) are brought into contact to form a hydrate. The chamber is a vertically oriented stainless-steel cylinder, 80 mm in diameter and 190 mm in height (inner dimensions), equipped with two rectangular glass windows, each 25 mm wide and 100 mm high, located opposite each other. The chamber is connected to a closed loop to allow water to circulate at a constant flow rate of 90 cm3/min through the chamber, a nonpulsating double-plunger pump (Nihon Seimitsu Kagaku Co., model NP-LX-300), a helical-tube heat exchanger, and a full-conetype spray nozzle (H. Ikeuchi & Co., model 1/4MJ006BW) installed at the top of the chamber. The chamber and the major portion of the water-circulation loop (including the helical-tube heat exchanger) are immersed in a temperature-controlled water bath so that the circulating water is cooled to the water-bath temperature before being sprayed into the chamber, although the temperature in the water pool inside the chamber may be slightly higher than that in the water bath due to the exothermic hydrate formation inside the chamber. A mass flow meter (Brooks 5860E) is installed on the gas supply line to the spray chamber such that the instantaneous flow rate of the feed gas into the chamber was continuously measured within an accuracy of (10.0 cm3/min NTP (converted to a volumetric flow rate at 273.15 K and 101.3 kPa). A highspeed gas chromatograph with a built-in thermal conductivity detector (Agilent 3000 Micro Gas Chromatograph) is connected to one of the ports installed on the top cover of the spray chamber such that the gas inside the chamber can be periodically sampled for analysis during each hydrate-forming operation. A needle valve and a pressure-reducing valve are installed on the stainless-steel tubing from the chamber to the gas chromatograph. The internal
Hydrate Formation by Water Spraying volume of this tubing is as small as 9.5 cm3 so that we can limit the amount of gas to be sampled each time in order to minimize the effect of sampling on the hydrate formation process inside the chamber. Procedure. The experiments in the present study were planned to follow those in the preceding study2 concerning the setting of nominal operational conditions such as the amounts of water (600 or 500 cm3) and MCH (0 or 100 cm3) with which the spray chamber was initially charged, the flow rate of water pumped to, and sprayed from, the spray nozzle (90 cm3/min), and the temperature and pressure inside the chamber (275 K and 2.9 MPa). Under these conditions, we performed three distinct series of experiments, which are termed M1, MM1, and MM2 in this paper. Each series consisted of five to eight experimental runs performed at 1-day intervals. MCH was not used in series M1. After each experimental run in this series, the spray chamber and the water-circulation loop (including the pump, the helical-tube heat exchanger, and the spray nozzle) were purged and then charged with freshly distilled water to be used in the next experimental run. In the other two series, MCH was used. In the case of series MM1, the spray chamber and the water-circulation loop were purged after each run and then charged with fresh MCH and water for use in the next run. In the case of series MM2, all successive runs were performed without exchanging the test liquids; that is, MM2 generally followed the way of successive run-to-run operations employed in our previous study.2 More details of the experimental operations are described below. In advance of the first run in any series, the chamber and the water-circulation loop were once evacuated. The chamber was then charged with 600 cm3 of water (in the case of series M1) or 500 cm3 of water plus 100 cm3 of MCH (in the cases of series MM1 or MM2). After the chamber was cooled to within (0.5 K of the prescribed temperature, 275 K (as detected by two thermocouples inserted into the chamber), the feed gas was supplied from a highpressure cylinder via a pressure-regulating valve to the chamber to maintain the pressure there at 2.9 ( 0.05 MPa. The plunger pump was turned on to make water spray from the nozzle into the chamber at a constant flow rate, 90 cm3/min. The instantaneous flow rate of the feed gas into the chamber was continuously measured by the mass flow meter. Measurement of the gas flow rate, as well as video recording of the inside of the chamber through its windows, was continued throughout each run until a hydrate slurry or the MCH layer approached the bottom of the chamber so that we had to stop the plunger pump to prevent the slurry or the MCH being sucked into the water-circulation loop, or until 180 min had passed after the inception of hydrate formation. At every 30 min during each run, 110-120 cm3 (NTP) of the gas inside the chamber was sucked into the tubing connected to the gas chromatograph. A part of this gas was used to purge the tubing, and the residue was supplied to the gas chromatograph to be analyzed. Some operational details of the gas-chromatographic measurements are described later. When the first run was over, the gas supply to the spray chamber was stopped, and the chamber was depressurized to atmospheric pressure by releasing the gas inside the chamber and the gas-supply tubing (i.e., the gas already stored therein and that being generated by hydrate dissociation in the chamber) to the outside to be burned away. Subsequently, the chamber and the gas-supply/discharge tubings were evacuated, leaving the liquids (water or water plus MCH) inside the chamber and the water-circulation loop as they were, to expel the residual gas inside these parts. The chamber plus the water-circulation loop were then shut off by closing all valves nearest to the chamber on the gas-supply/discharge tubings and were left at a temperature of 275 ( 0.5 K overnight until preparation for the second experimental run began. The preparation for the second experimental run was different from series to series as described below. (i) In the case of series M1, the water left in the chamber and the water-circulation loop was drained away. The chamber, the water-circulation loop, and the gas-supply/discharge tubings were then evacuated, and the chamber was charged with 600 cm3 of water.
Energy & Fuels, Vol. 21, No. 2, 2007 547 (ii) In the case of series MM1, the liquids left in the chamber and the water-circulation loop were drained away. The chamber was then charged with 600 cm3 of freshly distilled water. After the chamber was pressurized to 0.3 MPa with nitrogen gas, the plunger pump was turned on for 10 min to wash the water-circulation loop. Such a 10-min washing operation was repeated five times, each time being preceded by the replacement of the used water with a fresh 600 cm3 of water. Subsequently, the chamber, the watercirculation loop, and the gas-supply/discharge tubings were evacuated. The chamber was then charged with 500 cm3 of water plus 100 cm3 of MCH. (iii) In the case of series MM2, the liquids left in the chamber and the water-circulation loop were reserved for use in the subsequent experimental runs. However, the chamber, the watercirculation loop, and the gas-supply/discharge tubings were once evacuated to expel the gas that had been released from the liquids and, if any, the residual hydrates during the overnight resting period. After any of the three preparatory operations listed above, the chamber was then pressurized to 2.9 ( 0.05 MPa by supplying the feed gas from the high-pressure cylinder. The succeeding experimental procedure in the second run simply followed that in the first run. Subsequent run-to-run successions were performed in the same manner as that in the succession from the first to the second run. Materials Used. The feed gas was a mixture of 89.96 mol % methane, 7.04 mol % ethane, and 3.00 mol % propane (synthesized and analyzed by Japan Fine Products Corp., Oyama, Tochigi-ken). A reagent-grade MCH of 99.0% (mass basis) certified purity was used as received from the supplier (Junsei Chemical Co., Tokyo). The water used in hydrate formation was a deionized-and-distilled water prepared in the laboratory. No special precaution was taken to degas the liquid samples before pouring them into the spray chamber. Gas Chromatography Details. The Agilent 3000 gas chromatograph was loaded with an Agilent PLOT Q column, an analytical column suitable for detecting light hydrocarbons. The carrier gas was helium. The injection temperature and detection temperature were 80 and 100 °C, respectively. The calibration of the gas chromatograph was done, using the same methane + ethane + propane mixture as that used in the hydrate-forming experiments. The analysis of each sample was completed within 90 s, and this was repeated 10 times so that we could evaluate the mole fractions of the three species;this paper reports xC1 for methane, xC2 for ethane, and xC3 for propane;by averaging the 10 sets of data. The repeatability of the analysis (as measured in terms of the standard deviation in the 10 data sets for each sample) was within ( 0.07%. MCH was not detected by the above analysis procedure. Considering the vapor pressures of MCH and water being substantially low at the system temperature (275 K), we neglected their inclusion in the gas phase and assumed that xC1 + xC2 + xC3 ) 1. Effect of Gas Sampling on Hydrate-Formation Processes. It should be noted that the periodic gas sampling for the chromatographic analyses should have more or less affected the transient behavior of hydrate formation in each experimental run. At each sampling, a certain quantity of the gas mixture was taken away from the spray chamber, and this loss of the gas mixture with the current gas-phase composition inside the chamber was immediately compensated by the feed gas with the fixed composition (90% C1 + 7% C2 + 3% C3). It therefore turned out that, qualitatively speaking, the sampling should have retarded the chronological change in the gas-phase composition inside the chamber that would have occurred otherwise. However, the quantity of the gas mixture sampled each time was only 1.0-1.1% (mass or mole basis) of the total quantity of the gas mixture occupying the 354 cm3 gas phase inside the chamber, and hence the effect of the sampling on the chronological change in the gas-phase composition could not be substantial. Thus, we can reasonably assume that the hydrate formation processes observed in this study were not substantially distorted from those that would occur in the normal semibatch hydrate-forming operations accompanied by no in-process gas-phase sampling.
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Estimating the Phase-Equilibrium Temperatures. For analyzing and interpreting the observations of hydrate formation processes, we need to know Teq, the temperature at which the gas phase having the same composition as that determined by the gas-chromatographic analysis of each sampled gas mixture would be in equilibrium with a hydrate, the water-rich liquid and, in the case of series MM1 or MM2, the MCH-rich liquid under the experimental system pressure, pex ) 2.9 MPa. The estimation of Teq was done, alternatively using two phase-equilibrium calculation programs, CSMHYD9 and HWHydrate,10 for comparison. In using either program, we alternatively assumed the hydrate to be in the three crystallographic structures;structure I (abbreviated as sI), structure II (sII), and structure H (sH);such that we could compare the Teq values corresponding to the three structures and could determine with which structure the highest Teq value was predicted by the program. We define the deficit in the experimental system temperature, Tex ) 275 K, from Teq as the system subcooling ∆Tsub, a thermodynamic driving force for the hydrate formation, and generally evaluate it on the basis of the highest Teq value predicted.
Results and Discussion We show the results of three series of experiments in the following order: (i) series M1, hydrate formation from the gas mixture (90% C1 + 7% C2 + 3% C3) accompanied by water renewal in advance of every run, (ii) series MM1, hydrate formation from the gas mixture (90% C1 + 7% C2 + 3% C3) + MCH accompanied by water and MCH renewal in advance of every run, and (iii) series MM2, hydrate formation from the gas mixture (90% C1 + 7% C2 + 3% C3) + MCH with no liquid renewal during intervals between successive runs. For each series, the results are summarized in (a) a diagram showing the time evolution of Vg, the amount of the gas mixture (measured in volume NTP) supplied into the spray chamber after the first appearance of hydrate crystals in each of 5-8 successive experimental runs, (b) a diagram showing the change in the composition of sampled gas (as indicated by xC1, xC2, and xC3) with Vg in each of three arbitrarily selected runs, and (c) a diagram showing the change in the phase-equilibrium temperature, Teq, with Vg in one typical run. In b and c, we could use t, the elapsed time after the first appearance of hydrate crystals in the spray chamber, instead of Vg. However, we have considered Vg or its ratio to Vgc, the quantity of gas-phaseforming mixture in the spray chamber (∼10.8 × 103 cm3 NTP), to be a more pertinent index to the progress of the transient change in the gas-phase composition in the chamber than t. Because it is difficult to accurately estimate Vgc for a gas phase with variable composition, we simply use Vg, instead of Vg/ Vgc, in preparing diagrams b and c. On the basis of the results arranged in this way, we discuss the thermodynamic and kinetic aspects of the hydrate-formation processes in each of the three series (particularly, series MM1 and MM2). The mechanism underlying the run-by-run change in hydrate-formation rate observed in series MM2 is considered in the course of the above discussion. Hydrate Formation from Gas Mixture (Series M1). Figure 2 shows Vg(t) data recorded in five experimental runs performed without using any LMGS, where t denotes the elapsed time after the first appearance of hydrate crystals in the spray chamber in each run. These data indicate relatively good repeatability in hydrate formation behavior in the absence of any LMGS. Figure (9) CSMHYD, a phase-equilibrium calculation program package accompanying the following book: Sloan, E. D., Jr. Clathrate Hydrates of Natural Gases, 2nd ed.; Marcel Decker Inc.: New York, 1998. (10) HWHydrateGUI (ver. 1.1), a phase-equilibrium calculation program developed at the Centre for Gas Hydrate Research, Heriot-Watt University, U.K., 2005.
Figure 2. Compiled gas-mixture supply data obtained in five experimental runs in series M1, in which no LMGS was used. For graphical clarity, the data are plotted at 3-min intervals, although the data acquisition was done at 1-min intervals throughout each experimental run. Each curve represents the time evolution of Vg, the amount of the gas mixture (measured in volume NTP) supplied into the spray chamber after the first appearance of hydrate crystals in an experimental run. pex ) 2.9 ( 0.05 MPa, Tex ) 275 ( 0.5 K.
3 plots the simultaneous changes in xC1, xC2, and xC3 in the sampled gas mixture. Slight deviations in xC1, xC2, and xC3 at t ) 0 (Vg ) 0) from their values in the feed gas (0.9, 0.07, and 0.03, respectively) are ascribable to the individual dissolution of methane, ethane, and propane from the gas phase into water in advance of the inception of hydrate formation. Here, we can recognize an increase in xC1 and decreases in xC2 and xC3 with the progress of hydrate formation. This fact shows that ethane and propane were instantaneously consumed by hydrate formation at molar proportions higher than 7% and 3%, respectively, while methane was consumed at a proportion less than 90%. Because propane cannot be guest molecules of sI hydrates, we can conclude from the above fact that the hydrate dominantly formed in the experiments in series M1 was in sII, although we cannot exclude the possibility of simultaneous formation of an sI hydrate to a lesser extent on the basis of the Gibbs phase rule (see the discussion in section 4.2 in our previous paper cited as ref 3). Figure 4 shows the change in the phaseequilibrium temperature, Teq, during run M1-3. In using CSMHYD and HWHydrate for determining Teq, we alternatively specified the hydrate structure to be sI and sII. As exemplified in Figure 4, higher Teq values were predicted for the equilibrium with an sII hydrate than that with an sI hydrate throughout the hydrate-formation process observed in every run in series M1, indicating that larger degrees of system subcooling ∆Tsub were available for the formation of an sII hydrate than for the formation of an sI hydrate. This result is consistent with the above-stated conclusion that sII hydrate formation was predominant in the experiments in series M1. Hydrate Formation from Gas Mixture + MCH (Series MM1). The Vg(t) data recorded in eight runs each performed with freshly prepared test liquids are compiled in Figure 5. Comparing Figure 5 to Figure 2, we recognize that the rate of gas-mixture supply was generally lowered by the addition of pure MCH in which none of the feed-gas components (methane, ethane, and propane) had been dissolved in advance. Figure 6 plots the simultaneous changes in xC1, xC2, and xC3 in the gas mixture continually sampled during three of the eight runs. The
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Figure 4. Change in the phase-equilibrium temperature Teq with an increase in Vg during run M1-3. The Teq values plotted here were calculated by CSMHYD9 or HWHydrate10 on the basis of the gasmixture composition data shown in Figure 3.
Figure 3. Simultaneous changes in mole fractions of methane, ethane, and propane in the gas mixture sampled from the gas phase inside the spray chamber. For graphic clarity, only the data obtained in three runs in series M1 are plotted here. The sampling was performed every 30 min during each run.
deviations in xC1, xC2 and xC3 at t ) 0 (Vg ) 0) from their values in the feed gas (0.9, 0.07, and 0.03, respectively) are much more significant than those in the absence of MCH (cf. Figure 3); xC2 and xC3 were reduced to almost half of 0.07 and less than one-third of 0.03, respectively, while xC1 showed a severalpercent increase beyond 0.9. These deviations are reasonably ascribed to the preferential dissolution of propane and ethane, compared to methane, into MCH before the inception of hydrate formation in each run. The changes in xC1, xC2, and xC3 during hydrate-forming processes (t g 0, Vg g 0) are qualitatively the same as those in the absence of MCH (cf. Figure 4); that is, xC1 increases, while xC2 and xC3 decrease, with the progress of hydrate formation. However, this fact conflicts with our thermodynamic simulations of hydrate-formation processes relevant to the experiments in series MM1 (see Table 4, and
Figure 5. Compiled gas-mixture supply data obtained in eight experimental runs in series MM1, in which water and MCH were renewed in advance of every run. pex ) 2.9 ( 0.05 MPa, Tex ) 275 ( 0.5 K. For graphical clarity, the data are plotted at 3-min intervals, although the data acquisition was done at 1-min intervals throughout each experimental run.
Figures 4 and 6 in ref 3). The implication of the changes in xC1, xC2, and xC3 shown in Figure 6 is discussed below. Possible crystallographic structures of the hydrates formed in the present experimental system are sI, sII, and sH. If an sH hydrate was exclusively formed at early stages in each run as predicted by our simulations,3 xC1 would decrease and both xC2 and xC3 would increase with an increasing Vg; this is because, among the three feed-gas components, only methane may be consumed in sH-hydrate formation. Only the formation of an sII hydrate could preferentially consume both ethane and propane molecules, resulting in simultaneous decreases in xC2
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Figure 7. Change in the phase-equilibrium temperature Teq with an increase in Vg during a typical run in series MM1. The Teq values plotted here were calculated by CSMHYD9 or HWHydrate10 on the basis of the gas-mixture composition data shown in Figure 6.
Figure 6. Simultaneous changes in mole fractions of methane, ethane, and propane in the gas mixture sampled from the gas phase inside the spray chamber. For graphic clarity, only the data obtained in three runs in series MM1 are plotted here. The sampling was performed every 30 min during each run.
and xC3 and thereby an increase in xC1. Therefore, it is reasonable to presume that the hydrate mainly formed throughout each experimental run was sII rather than sH. At the same time, however, we should note the possibility that an sH hydrate was also formed, presumably to a lesser extent, along with the sII hydrate. This possibility is supported by the fact that, compared to the corresponding results for series M1 (Figure 3), the rates of changes in xC1, xC2, and xC3 with an increase in Vg were substantially reduced. It is reasonable to assume that the reduction of changes in xC1, xC2, and xC3 resulted from the sHhydrate formation with which only methane molecules in the gas phase must have been consumed. This issue may further be discussed consulting Figure 7, where the changes in Teq values relevant to the three hydrate structures are plotted for mutual comparison.
Because of the increase in the methane concentration in the gas phase during the induction time, Teq for the equilibrium with an sII hydrate at t ) 0 (Vg ) 0) is lower than that in the absence of MCH (series M1) by ∼4 K. The decrease in Teq during the hydrate-formation process (t g 0, Vg g 0) was subdued as a result of the above-mentioned reduction of change in xC1, xC2, and xC3. As demonstrated in Figure 7, the difference between Teq for the equilibrium with an sII hydrate and that for the equilibrium with an sH hydrate was small throughout the hydrate-formation process. Although CSMHYD predicts the former to be lower than the latter throughout the hydrateformation process,11 HWHydrate predicts the former to be higher than the latter for the major proportion of the hydrate-forming process except for its last part (the regime covering the Vg range from ∼5000 to ∼6000 cm3) in which the latter exceeds the former. Presumably, the difference between the two equilibrium temperatures thus recognized at each stage in the hydrateformation process was within the sum of the uncertainties in determining these temperatures on the basis of either phaseequilibrium calculation program. Because the two equilibrium temperatures were so close to each other and hence the driving force for hydrate formation as measured by the system subcooling ∆Tsub (t Teq - Tex) was nearly the same for sII and sH, it is likely that both sII and sH hydrates were simultaneously formed. Concerning this issue, it may be worthy noting that, as already pointed out by Tsuji et al.,3 the Gibbs phase rule allows the coexistence of hydrates in two structures while the gasphase composition in a methane + ethane + propane + LMGS (11) The prediction of higher Teq values for sH than those for sII due to CSMHYD is the reason why our simulations3 incorporating CSMHYD predicted a decrease in xC1 and increases in xC2 and xC3 with an increase in Vg during hydrate formation from a 90% C1 + 7% C2 + 3% C3 gas mixture and MCH. Because the algorithm of the simulations allows the formation of only the hydrate with the highest equilibrium temperature, even a minute difference between Teq for sH and that for sII as predicted by the incorporated phase-equilibrium calculation program crucially affects the results of the simulations.
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Figure 8. Compiled gas-mixture supply data obtained in five experimental runs in series MM2, in which neither water nor MCH was not renewed throughout. For graphical clarity, the data are plotted at 3-min intervals, although the data acquisition was done at 1-min intervals throughout each experimental run. The five runs were successively performed in numerical order (from MM2-1 to MM2-5). pex ) 2.9 ( 0.05 MPa, Tex ) 275 ( 0.5 K.
+ water system is changing under a constant system pressure. Thus, simultaneous formation of sII and sH hydrates is thermodynamically possible, even if the gas-phase composition was spatially uniform inside the chamber. In practice, some spatial variation in the gas-phase composition should occur, which in turn should enhance simultaneous hydrate formation in different structures. Hydrate Formation from Gas Mixture + MCH (Series MM2). Figure 8 compiles the Vg(t) data recorded in five runs, which were successively performed using the same test liquids repeatedly, the same as in our previous study2 except for the periodic gas sampling newly added this time. Despite some irregular scatter of these data, we still note a tendency for the rate of gas consumption averaged over the entire period of hydrate formation to increase with the repetition of experimental runs. This tendency is essentially the same as those that we previously observed in the experiments using neohexane or MCH as the LMGS.2 We seek the mechanism underlying the above tendency in Figure 9 which shows simultaneous changes in xC1, xC2, and xC3 in three of the five runs in series MM2. The changes in the first run (run MM2-1) are similar to those observed in series MM1 (Figure 6). However, the changes in the later runs considerably differ from those in series MM1;that is, the changes are more substantial, showing an intermediate nature between series M1 and series MM1. This fact may be interpreted as follows: the dissolution of the feed-gas components (methane, ethane, and propane) into the bulk of the liquid MCH phase asymptotically progressed from run to run, leading to a reduction in the capacity of the MCH phase for absorbing the feed-gas components (preferentially, propane and ethane compared to methane) during the induction time in later runs. The reduction in the gas-absorbing capacity of the MCH phase inevitably reduced the deviation in the gas-phase composition at t ) 0 (Vg ) 0) from that in the feed gas. The sharper changes in xC1, xC2, and xC3 during hydrate-forming processes (t g 0, Vg g 0) in later runs indicate that sII hydrate formation was more dominant in these runs compared to the first run (run MM2-1) and those in series MM1. This issue is further discussed below, consulting Figure 10, which exhibits the changes in Teq values relevant to the three hydrate structures throughout one
Figure 9. Simultaneous changes in mole fractions of methane, ethane, and propane in the gas mixture sampled from the gas phase inside the spray chamber. For graphic clarity, only the data obtained in three runs in series MM2 are plotted here. The sampling was performed every 30 mins during each run.
of the later runs, run MM2-3, in which the gas-composition measurements were successfully done over the most extensive Vg range (0 e Vg e 8237 cm3). Responding to the initial methane concentration in the gas phase being lower than those in series MM1 but still higher than those in series M1, Teq for sII substantially exceeds that for sH over the major proportion of the entire Vg range covered by the gas-composition measurements. At a later stage where xC1 ≈ 0.963-0.967, the decreasing Teq for sII and the increasing Teq for sH cross. It is reasonable to assume that sII hydrate formation was dominant over sH hydrate formation at stages earlier than the above Teq crossover point. Nevertheless, sH hydrate formation must have been maintained, along with sII hydrate formation, to such an extent that, if compared to the corresponding results obtained in series M1 in which no LMGS
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Figure 10. Change in the phase-equilibrium temperature Teq with an increase in Vg during a typical run in series MM2. The Teq values plotted here were calculated by CSMHYD9 or HWHydrate10 on the basis of the gas-mixture composition data shown in Figure 9.
was used, the increase in the methane concentration in the gas phase was appreciably suppressed (compare Figure 9 to Figure 3), and hence the decrease in Teq for sII was delayed (compare Figure 10 to Figure 4). At later stages beyond the Teq crossover point, sH hydrate formation possibly became dominant over sII hydrate formation. However, further evolution of the presumably multistructured hydrate formation needs to be investigated using an improved experimental setup that allows us to continue the observation of each hydrate-formation process until it falls into an asymptotic (or nearly steady) state. We then examine how the gas consumption rates observed in the five runs in series MM2 are correlated to the gas-phase composition or the system subcooling. The overall rate of gas consumption due to hydrate formation during each run is evaluated in terms of V˙ g, the average rate of gas supply to the spray chamber, which is determined by dividing Vgt, the final value of Vg in the run, by τh, the duration of the run after the first hydrate formation. The V˙ g values for the five runs are plotted versus x1, x2, and x3 in Figure 11, where we can readily recognize that V˙ g tends to increase with a decrease in methane concentration and with increases in ethane and propane concentrations, respectively, in the gas phase. Such apparent dependence of V˙ g on the gas-phase composition may coherently be accounted for in terms of the system subcooling, ∆Tsub. In Figure 12, the same V˙ g values as those displayed in Figure 11 are plotted against ∆Tsub evaluated on the basis of the highest of the three phase-equilibrium temperatures (i.e., Teq’s for sI, sII, and sH) predicted by CSMHYD9 or HWHydrate.10 Here, we find a positive correlation between V˙ g and ∆Tsub, which may explain the run-by-run change in hydrate-formation rate as observed in series MM2 as well as in the experiments in our previous study.2 That is, as the absorption of the feed-gas components into the liquid MCH phase progressed with repetition of an experimental run, the gas-absorption capacity
Figure 11. Average rate of gas-mixture supply vs mole fractions of methane, ethane, and propane in the gas phase, based on the five runs in series MM2. Each closed circle represents the V˙ g value for each run and the mole fraction of each species at the inception of hydrate formation (t ) 0, Vg ) 0) in the run. The horizontal bar attached to the circle indicates the range over which the mole fraction changed during the subsequent hydrate-formation process (0 e t e τh, 0 e Vg e Vgt).
Figure 12. Average rate of gas-mixture supply vs system subcooling ∆Tsub based on the five runs in series MM2. Each open or closed circle represents the V˙ g value for each run and the ∆Tsub value at the inception of hydrate formation (t ) 0, Vg ) 0) in the run. The horizontal bar attached to the circle indicates the range over which ∆Tsub changed during the subsequent hydrate-formation process (0 e t e τh, 0 e Vg e Vgt). ∆Tsub is evaluated on the basis of the highest of the three phaseequilibrium temperatures (i.e., Teq’s for sI, sII, and sH) predicted by CSMHYD9 or HWHydrate.10
Hydrate Formation by Water Spraying
of the liquid MCH phase decreased, thereby reducing its effect on the gas-phase composition and consequently mitigating the reduction in the phase-equilibrium temperature Teq from its level corresponding to the original feed-gas composition, and hence ∆Tsub during the hydrate-forming period (0 e t e τh, 0 e Vg e Vgt) increased. Concluding Remarks This study has experimentally investigated hydrate formation from a simulated natural gas;a mixture of methane, ethane, and propane in a 90:7:3 molar ratio;and MCH;a structureH-forming LMGS;in an isobaric system employing continuous water-spraying into the gas mixture. Each semibatch hydrateforming operation was accompanied by continual gas-chromatographic measurements of the methane/ethane/propane composition of the gas phase in contact with the liquids (MCH and water) and a hydrate product inside the spray chamber, and special attention was paid to the evolution of the composition during the operation. It was confirmed that the gas-phase composition is substantially affected by the absorption of methane, ethane, and propane into the liquid MCH phase during the induction time in each hydrate-forming operation. The change in the gas-phase composition due to the absorption tends to lower the four-phase (gas + MCH + water + hydrate) equilibrium temperature and thereby reduce the system subcooling, that is, the thermal driving force for hydrate formation. This absorption effect due to the presence of the liquid MCH phase tends to be suppressed with consecutive repetition of the hydrate-forming operation. The evolution of the gas-phase composition during each hydrate-
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forming operation indicates that the hydrate dominantly formed through the major proportion of each 60-180 min hydrateformation period was in sII, although a hydrate in sH was also formed to a lesser extent. The formation of the sH hydrate would become dominant, substituting for the sII hydrate formation, if the hydrate-formation period lasted longer. Whichever structure may be dominant, the simultaneous formation of sII and sH hydrates prevented the gas-phase composition from rapidly changing, and thereby the equilibrium temperature from rapidly decreasing, during the hydrate-formation period; this is considered to be the major advantage of using such a LMGS as MCH in forming hydrates from natural gas. We expect that the experimental results or findings as summarized above will be useful information for the engineering studies and developments in natural-gas hydrate technologies. It should be noted, however, that those results represent the transient behavior of hydrate formation during a relatively early portion of each semibatch hydrate-forming operation. This restriction on the time span of observing hydrate formation in this study is due to the function of the experimental apparatus that we used. The hydrate-formation behavior in an asymptotic (or nearly steady) regime in a sufficiently long-lasting operation is yet to be investigated. Acknowledgment. This study was supported by the Industrial Technology Research Grant Program in 2005 from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. EF060461R
