Hydrate Formation by Water Spraying in a Methane + Ethane +

Energy & Fuels 2005, 19, 869-876

869

Hydrate Formation by Water Spraying in a Methane + Ethane + Propane Gas Mixture: An Attempt at Promoting Hydrate Formation Utilizing Large-Molecule Guest Substances for Structure-H Hydrates Hideyuki Tsuji,†,§ Takehito Kobayashi,† 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, 2-17-2-1 Tsukisamu-Higashi, Toyohira-ku, Sapporo 062-8517, Japan Received August 28, 2004. Revised Manuscript Received December 27, 2004

An experimental study of hydrate formation has been performed, using a simulated natural gassa mixture of methane, ethane, and propane in a 90:7:3 molar ratiosand a liquid largemolecule guest substance (LMGS), which provides guest molecules to fit into the 51268 cages of a structure-H hydrate. Except for the use of the gas mixture, the hydrate-forming procedure used in this study was the same as that tested in our previous studies [Ohmura et al., Energy Fuels 2002, 16, 1141-1147; Tsuji et al., Energy Fuels 2004, 18, 418-424], i.e., spraying liquid water downward through a gas phase onto a liquid-LMGS layer lying on a pool of water under a prescribed temperature-pressure condition (275 K, 2.9 MPa) in a chamber into which the gas mixture was being supplied to compensate for its loss due to hydrate formation. We selected three LMGSs for comparison: tert-butyl methyl ether (TBME), which was found to give the highest rate of hydrate formation when pure methane is used as the guest gas [Tsuji et al., Energy Fuels 2004, 18, 418-424], 2,2-dimethylbutane (neohexane), and methylcyclohexane (MCH). The rate of hydrate formation from the gas mixture was determined to be increased by the presence of any of these LMGSs, compared to the rate observed in the absence of any LMGS. An unexpected fact was found in the rate of hydrate formation from the gas mixture plus an LMGS, compared with the rate observed with pure methane plus the same LMGS under the same temperaturepressure condition. That is, the former rate may be much higher or, on the contrary, appreciably lower than the latter rate, depending on the species of the LMGS used. Accordingly, it turns out that the rate of hydrate formation from the gas mixture plus neohexane or MCH is higher than that from the same mixture plus TBME, in strong contrast to the nature of hydrate formation from pure methane plus an LMGS that we previously revealed [Tsuji et al., Energy Fuels 2004, 18, 418-424].

Introduction This paper describes an experimental study that we have performed to explore the possible advantage of storing and transporting natural gas in the form of structure-H clathrate hydrates (abbreviated as sH). If the natural gas available is so methane-rich as to form, together only with water, a structure-I (sI) hydrate at elevated pressures (J3.14 MPa at a temperature of 275 K), we find a definite advantage of utilizing an sH hydrate instead of the sI hydrate: an ∼20%-70% reduction in the pressure required for hydrate formation. Forming such an sH hydrate requires the use of an additional guest substance that has a 0.75-0.98 nm molecular size, which should fit into the 51268 cages of * Author to whom correspondence should be addressed. Telephone: +81-45-566-1522. Fax: +81-45-566-1495. E-mail: [email protected]. † Keio University. § Current address: Technology Development Department, Tokyo Gas Co., Ltd., 3-13-1 Minamisenju, Arakawa-ku, Tokyo 116-0003, Japan. ‡ National Institute of Advanced Industrial Science and Technology.

sH lattices and, with few exceptions, takes the form of an oily liquid under hydrate-forming thermodynamic conditions. Following our preceding paper1 in this series, we call any substance that fits into the 51268 cages a large-molecule guest substance (LMGS). Our preceding studies described by Ohmura et al.2 and Tsuji et al.1 were devoted to examining the characteristics of sHhydrate formation from pure methane plus an LMGS in a semibatch operation, in which methane was continuously supplied to a reactor initially charged with specified amounts of the LMGS and water. Water was continuously pumped out of the reactor, cooled to a prescribed temperature, and sprayed back into the reactor to contact methane and the LMGS. The rate of methane fixation into the hydrate formed inside the reactor could be determined from the rate of methane supply into the reactor, while maintaining the pressure inside the reactor constant. The preceding studies (1) Tsuji, H.; Ohmura, R.; Mori, Y. H. Energy Fuels 2004, 18, 418424. (2) Ohmura, R.; Kashiwazaki, S.; Shiota, S.; Tsuji, H.; Mori, Y. H. Energy Fuels 2002, 16, 1141-1147.

10.1021/ef049785a CCC: $30.25 © 2005 American Chemical Society Published on Web 02/17/2005

870

Energy & Fuels, Vol. 19, No. 3, 2005

revealed that such a water-spraying system is suitable for continuous sH-hydrate formation and that, depending on the selection of the species of the LMGS, the rate of methane fixation by sH-hydrate formation at a given temperature (T ) 275 K) and pressure (p ) 2.8-2.9 MPa) may far exceed that for sI-hydrate formation at the same temperature but at a higher pressure (p ) 3.7 MPa). Although the results of our previously mentioned studies suggest the potential advantage of using sH hydrates for storing natural gas, we still need to investigate the effect of an indefinite chemical composition of the natural gas. Generally, commercially available natural gas contains, in addition to methane, which typically constitutes some 90 mol % of the gas, heavier hydrocarbons such as ethane, propane, and iso-butane, and, hence, it possibly forms a structure-II (sII) hydrate, instead of an sI hydrate, in the absence of any LMGS. Recent phase-equilibrium studies3,4 strongly suggest that the crystallographic structure of the hydrate formed from natural gas that is comprised of ∼86 mol % methane is sII, even if an LMGS coexists with the gas. However, we should further note that the gas composition inside a hydrate-forming reactor must be different from that of the feed gas, because of the preferential fixation of heavier components into the hydrate that is formed. According to a simple mass-balance-based estimation,5 for example, methane that constitutes 88 mol % of a feed gas may be concentrated to ∼99 mol % in the gas phase inside a reactor in a continuous and steady operation at T ) 275 K, in the absence of any LMGS. If the reactor is used in a batch operation or, as in our previous and present studies, in a semibatch operation, the gas composition inside the reactor should change with time. Thus, it is difficult to predict whether an sH hydrate will actually form in the course of each hydrate-forming operation, using a reactor charged with an LMGS in addition to natural gas (or a natural-gassimulating hydrocarbon mixture) and water, and to estimate what effect the LMGS will have, if any, on the rate of gas fixation. The present study is intended to investigate this issue by means of experiments that generally follow our previous experiments with pure methane plus each species of LMGS,1,2 except for the replacement of pure methane by a methane/ethane/ propane mixture that simulates natural gas. The experimental results obtained indicate the general effectiveness of using some LMGS in enhancing the gas fixation, presumably as the result of a change in hydrate structure from sII to sH in the course of each semibatch experimental operation. The results include the unexpected finding of a substantial difference in LMGS effectiveness between the hydrate formation from pure methane and that from the methane/ethane/propane mixture. This means that an LMGS that has been determined to be quite effective in increasing the gasfixation rate in the system using pure methane is not necessarily as effective in the system using the gas mixture, and, in contrast, another LMGS that is less (3) Østergaard, K. K.; Tohidi, B.; Burgass, R. W.; Danesh, A. C.; Todd, A. C. J. Chem. Eng. Data 2001, 46, 703-708. (4) Tohidi, B.; Østergaard, K. K.; Danesh, A. C.; Burgass, R. W. Can. J. Chem. Eng. 2001, 79, 384-391. (5) Mori, Y. H. J. Chem. Ind. Eng. (China) 2003, 54 (Suppl.), 1-17.

Tsuji et al.

Figure 1. Cross-sectional view (schematic) of the spray chamber.

effective than other LMGSs in the former system may be the most effective in the latter system. Experimental Section Outline of Experiments. The apparatus and procedure used in the present study are essentially the same as those used in our previous studies1,2 and are detailed elsewhere.2 Thus, we describe below the apparatus and the operation in each experiment only briefly. On the other hand, we describe some details of the procedure of running successive experiments, which are not described in our previous papers1,2 but may have affected the results of the present experiments in which a gas mixture was used. Figure 1 illustrates the spray chamber in which a guest gas (a simulated natural gas nominally composed of 90 mol % methane, 7 mol % ethane, and 3 mol % propane), liquid water, and an LMGS were brought into contact to form a hydrate. The chamber was 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 was 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-cone-type spray nozzle (H. Ikeuchi and 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) were immersed in a temperature-controlled water bath, so that the circulating water could be cooled to the water-bath temperature before being sprayed into the chamber, although the temperature in the water pool inside the chamber was, more or less, higher than that in the water bath, because of the exothermic hydrate formation inside the chamber. For each LMGS, several experimental runs were performed successively typically at one-day intervals without exchanging the test liquids (water and the LMGS) with which the chamber and

Hydrate Formation by Water Spraying

Energy & Fuels, Vol. 19, No. 3, 2005 871

Table 1. Chemical Species Selected as Large-Molecule Guest Substances (LMGSs) and Their Samples Used in the Experiments chemical species

abbreviation

manufacturer

certified purity (wt %)

tert-butyl methyl ether 2,2-dimethylbutane methylcyclohexane

TBME Neohexane MCH

Aldrich Chemical Co., Milwaukee, WI Aldrich Chemical Co., Milwaukee, WI Junsei Chemical Co., Tokyo

99.7 99.0 99.0

the water circulation loop were once charged. In advance of the first run, the chamber and the water circulation loop were once-evacuated, and the chamber was then charged with 500 cm3 of water and 100 cm3 of an LMGS. 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), a guest gas was supplied from a high-pressure cylinder via a pressure-regulating valve to the chamber to maintain the pressure at 2.9 ( 0.05 MPa. The instantaneous flow rate of the guest gas into the chamber was continuously measured by a mass-flow meter (Brooks, model 5860E) within an accuracy of (10.0 cm3/min NTP (converted to a volumetric flow rate at a temperature of 273.15 K and pressure of 101.3 kPa). Measurement of the gas flow rate, as well as video recording of the inside of the chamber through its windows, was continued throughout each experiment until a hydrate slurry or the LMGS layer approached the bottom of the chamber, so that we had to stop the plunger pump to prevent the slurry or the LMGS from being sucked into the water circulation loop, or until 75 min had passed after the inception of hydrate formation. When the first run was thus over, the gas supply to the 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 an external Bunsen burner to be burned away. After the gas burning ceased, the chamber and the gas-supply/ discharge tubings were evacuated, leaving the liquids 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 was then shut off by closing all valves nearest to the chamber on the gas-supply/discharge tubings and was left at a temperature of 275 ( 0.5 K overnight until preparation for the second experimental run began. This preparation started by evacuating the chamber and the tubings again, to expel the gas that had been released from the liquids and, if any, the residual hydrates during the overnight resting period. The chamber was then pressurized to 2.9 ( 0.05 MPa by supplying the guest gas from the highpressure cylinder. The succeeding experimental operation in the second run simply followed that in the first run. The succession from the second run to the third run, and even later run-to-run successions, were performed in just the same way as that in the succession from the first run to the second run. Note that the experimental system conditions could never be completely initialized during each one-day interval between successive runs. Instead, the mutual dissolution of the test fluids possibly progressed from run to run. The so-called memory effect could also occur in the second and later runs to promote hydrate nucleation. Thus, we intended to observe, in each series of successive experiments, how the hydrateformation behavior changes (or does not change) with the repeated hydrate-forming operations. The three speciessmethane, ethane, and propanesthat comprise the gas mixture supplied to the test chamber never dissolve into the liquids (the LMGS and, to a lesser extent, water) in the same proportion as that in the original mixture (90:7:3). Instead, ethane and propane should preferentially dissolve into the LMGS, thereby making the gas phase inside the spray chamber increasingly methane-rich, compared to the original mixture fed to the chamber.6 This methane-concentration effect in the gas phase should occur most intensively in

the first run in which the chamber was initially charged with a fresh LMGS. Unless the gas-mixture components dissolved in the LMGS phase in the preceding run are completely discharged in the following evacuation procedure, their amounts that remain in the LMGS phase at the inception of each run should increase asymptotically from earlier to later runs, thereby reducing the methane-concentration effect on the gas phase in the later runs. Based on this presumption, we later note how the hydrate-formation behavior varied from run to run in each set of successive experimental runs assigned to each LMGS species. For comparison with the hydrate formation in the presence of an LMGS, we also performed a set of experiments in the absence of any LMGS. The procedure in these experiments was essentially the same as that outlined previously, except that the amount of liquid water initially poured into the spray chamber was 600 cm3, instead of 500 cm3. This was done to assign the same space inside the chamber (354 cm3) to the gas phase, irrespective of whether an LMGS was used. Materials Used. The guest gas used in the present study was a mixture of 90.04 mol % methane, 6.91 mol % ethane, and 3.05 mol % propane (synthesized and analyzed by Takachiho Chemical Industrial Co., Ltd., Tokyo). Among the five LMGSs tested in our previous studies,1,2 three species were selected for use in the present study: tert-butyl methyl ether (TBME), 2,2-dimethylbutane (neohexane), and methylcyclohexane (MCH). TBME is the LMGS that showed the highest rate of sH-hydrate formation in our previous studies in which pure methane was used as the guest gas.1 The rates of hydrate formation obtained with neohexane and MCH were much lower than that obtained with TBME. The reason for our selection of neohexane and MCH in the present study lies in the fact that these substances can be addressed by CSMHYD, which is a program package for estimating phase equilibria in hydrate-forming systems,8 which allows us to attempt to interpret the hydrate formation experimentally observed with each LMGS on the basis of the relevant phase equilibria (although this attempt is not described in this paper). The LMGS samples used in the present study are specified in Table 1. The water used in hydrate formation was a deionized and distilled water that was prepared in the laboratory. No special precautions were taken to degas the liquid samples before pouring them into the spray chamber. Phase Equilibria. It is reasonable to assume that the rate of hydrate formation is dependent, more or less, on the system subcooling (∆Tsub), which is the deficit in the experimental temperature (Tex) relative to the equilibrium temperature (Teq) that corresponds to the system pressure, pex (which was 2.9 ( 0.05 MPa throughout the present experiments). Here, the experimental temperature Tex denotes the temperature at which the contents of the spray chamber were initially adjusted and, at the same time, water at the spray nozzle was held constant throughout each experiment, both within (0.5 (6) The presumption that ethane and propane preferentially dissolve into an LMGS, thereby enriching the gas phase with methane, is supported by an equation-of-state-based flash calculation, with which we can determine the compositions of the gas and LMGS phases in mutual equilibrium.7 CSMHYD8 is one of the tools for such a calculation. (7) Sloan, E. D., Jr. Clathrate Hydrates of Natural Gases, 2nd Edition; Marcel Dekker: New York, 1998; Section 5.2.3. (8) Sloan, E. D., Jr. CSMHYD, a program package accompanying the book cited as ref 7.

872


Tsuji et al.

Table 2. Equilibrium Temperature (Teq) in a System Charged with a Guest Gas (90% C1 + 7% C2 + 3% C3 Mixture or Pure Methane) and an LMGS at a Pressure of 2.9 MPa and the Relevant System Subcooling ∆Tsub (∆Tsub ≡ Teq - Tex, where Tex ) 275 K)a LMGS

gas:LMGS molar ratio

No LMGS TBME Neohexane MCH

Teq (K)

90% C1 + 7% C2 + 3% C3 as Guest Gas 283.8 1:1.47 1:1.55

No LMGS TBME Neohexane MCH

280.4 279.7 Methane as Guest Gas 276.7 278.3 280.8 280.1

∆Tsub (K)

reference

8.8

CSMHYD8

5.4 4.7

CSMHYD8 CSMHYD8

-1.7 3.3 5.8 5.1

CSMHYD8 Hu¨tz and Englezos9 CSMHYD8 CSMHYD8

a Note that, in a gas-mixture + LMGS system, the composition of the gas phase in the equilibrium state should be different from the original gas-mixture composition, because of different solubilities of the gas components in the LMGS; hence, Teq may change with the gas-to-LMGS volume (or molar) ratio. As for neohexane or MCH, Teq was estimated using a phase-equilibrium computation program called CSMHYD,8 assuming a gas:LMGS volume ratio that approximates the condition of the present experiments (4:1 at a pressure of 2.9 MPa and a temperature of ∼275 K). Because CSMHYD cannot be used with TBME, Teq for the methane + TBME system was estimated by interpolation of relevant phase-equilibrium data.9 Teq for a gas-mixture (+ LMGS) system given in the table is for a critical phase equilibrium at which the hydrate phase is vanishing. In the course of hydrate-phase growth, Teq and ∆Tsub should continuously change, depending on a change in the composition of the gas phase, which results from a difference in guest composition between the gas and hydrate phases.

K (i.e., Tex ) 275 K). Thus, a knowledge of the Teq value at which the four phases (gas, LMGS, liquid water, and hydrate phases) are mutually equilibrated is desired for interpretation of the experimental observations of hydrate formation. However, note that specifying Teq relevant to hydrate formation from a gas mixture is not as simple as in the case of hydrate formation from a pure guest gas, regardless of whether an LMGS is present. The composition in the gas phase in the spray chamber should change during the course of each hydrate-formation process, because the guest composition in the formed hydrate is different from that in the gas phase. Accordingly, the Teq value relevant to the instantaneous gasphase composition in the chamber should change during each hydrate-formation process. In case of the presence of an LMGS, dissolution of the respective gas-phase-forming species into the LMGS phase also affects the gas-phase composition and thereby Teq. Thus, in Table 2, we indicate only Teq and the corresponding ∆Tsub values for each gas-mixture + LMGS (or gas-mixture only) system, which would be available when the three (or, in the absence of any LMGS, two) fluid phases are equilibrated with an infinitesimal hydrate phase, such that the gas-phase composition is not affected by the hydrate formation. Such Teq values are determined using the CSMHYD phase-equilibrium computation program,8 except for the system including TBME, for which CSMHYD cannot be used. Table 2 also indicates, for comparison, Teq and ∆Tsub values for systems that are comprised of pure methane gas, instead of the methane/ethane/propane mixture. Here, we find no more than a minor difference in the magnitude of ∆Tsub between a gas-mixture + LMGS system and the corresponding methane + LMGS system,10 whereas the difference is significant between the two systems comprising no LMGS.

Results and Discussion We show the results of four series of experiments in the following order: (i) hydrate formation from a gas mixture (90% C1 + 7% C2 + 3% C3) without LMGS, (ii) hydrate formation from a gas mixture (90% C1 + 7% C2 + 3% C3) + TBME, (iii) hydrate formation from a gas mixture (90% C1 + 7% C2 + 3% C3) + neohexane, and (iv) hydrate formation from a gas mixture (90% C1 + 7% C2 + 3% C3) + MCH. For each series, the results are summarized in a diagram that shows gas-mixture supply data obtained (9) Hu¨tz, U.; Englezos, P. Fluid Phase Equilib. 1996, 117, 178-185.

in 5-6 successive experimental runs and a typical sequence, or typical sequences, video-recorded in one or two of these runs. The gas-mixture supply data for each run are plotted in the form of the time evolution of Vg, which is the amount of the gas mixture (measured in volume NTP) supplied into the spray chamber after the first appearance of hydrate crystals. After showing the results of the individual series of experiments in order, we compare them with each other and with the results of our previous studies1,2 in which pure methane was used as the guest gas. Hydrate Formation without LMGS. Figure 2 shows Vg(t) data recorded in five experimental runs consecutively 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. A sequence of the formation and accumulation of a hydrate, presumably in sII, video-recorded in one of the runs, is shown in Figure 3. We find no qualitative difference in formation/accumulation behavior between the sI hydrate of methane observed in our previous study2 and the mixed-gas hydrate observed this time. In Figure 2, we note the peculiar behavior of Vg(t) in the first run and, to lesser extents, in some of the later runs: dVg/dt, the rate of gas consumption due to hydrate formation, abruptly increased at a relatively early stage within a first 20-min lapse, beyond which Vg increased almost linearly with time. The discontinuous behavior of Vg(t) mentioned above was possibly caused by activation of some hydrate-nucleation sites in the spray chamber, which could be retarded the most in the first run, in which the gas dissolution into the water pool must have been slighter than in the later runs. However, we detected no direct sign of such secondary nucleation in our videographic observations, and, hence, the true mechanism of the abrupt change in dVg/dt is still left unconfirmed. Hydrate Formation from Gas Mixture + TBME. The Vg(t) data compiled in Figure 4 demonstrate that the rate of gas consumption due to hydrate formation was increased appreciably in the presence of TBME, compared to that in the LMGS-free system under the same pressure-temperature condition. Although such


Figure 2. Compiled gas-mixture supply data obtained in five experimental runs in the absence of any large-molecule guest substance (LMGS). Each curve represents the time evolution of 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, and Tex ) 275 ( 0.5 K. The five runs were successively performed in numerical order (from M-1 to M-5).


Figure 4. Compiled gas-mixture supply data obtained in five experimental runs using tert-butyl methyl ether (TBME) as an LMGS; pex ) 2.9 ( 0.05 MPa, and Tex ) 275 ( 0.5 K. The five runs were successively performed in numerical order (from MT-1 to MT-5).

Figure 5. Sequential videographs of hydrate formation in the presence of a TBME layer lying on a water pool (run MT-1). Figure 3. Sequential videographs of hydrate formation in the absence of any hydrate (run M-4). The time t indicated below each picture is the lapsed time after the first appearance of hydrate crystals inside the spray chamber. The downward spreading of the darker area in the liquid-water phase indicates an accumulation of hydrate particles in the form of a water-suspended slurry. Hydrate formation is also observed on the glass windows above the level of the gas/water interface.

an abrupt change in dVg/dt as that observed in the first run for the LMGS-free system (Figure 2) is no longer observed here, we still note, in some of the runs, a substantial increase in dVg/dt in the period of t ) 1020 min, which is followed by an almost-linear increase in Vg. As recognized in Figure 5, in comparison with Figure 3, the vertical spreading of the water-suspended hydrate slurry down from the TBME/water interface was rather slow, which enabled us to continue each experimental run until Vg increased beyond the final Vg values recorded with the LMGS-free system. Hydrate Formation from Gas Mixture + Neohexane. The Vg(t) data recorded in six successive runs are compiled in Figure 6. These data obtained with neohexane are qualitatively different from those obtained with no LMGS and with TBME, in that they are widely scattered, demonstrating a substantial change in hydrate formation rate from run to run over the first

three runs (MN-1 to MN-3). The first run (MN-1) exhibited an abrupt change in dVg/dt, similar to that observed in the first run (M-1) in the experiment series with no LMGS (Figure 2). However, such a change was no longer observed in the succeeding runs. The interrun variation in Vg(t) observed in the later four runs (MN-3 through MN-6) is erratic, and any tendency to increase or decrease in hydrate formation rate with repetitions of the experimental runs is no longer recognized there. This fact indicates that the initial state of mutual gas/neohexane/water dissolution changed little from run to run after the second run was over. The downward spreading of hydrate slurry in each run was fast, as demonstrated in Figure 7, so that we had to cease the experimental operations at levels of Vg(t) lower than those in the other series of experiments. Hydrate Formation from Gas Mixture + MCH. The Vg(t) data compiled in Figure 8 are apparently scattered most widely throughout all series of experiments performed in this study. However, they show a rather regular manner of change from run to run: i.e., the slope averaged over each Vg(t) curve substantially increased through the first three runs (MM-1 through MM-3) and changed no more than slightly thereafter. It may be said that the hydrate formation from the gas mixture + MCH is the most sensitive to the initial state

874


Figure 6. Compiled gas-mixture supply data obtained in six experimental runs using neohexane as an LMGS; pex ) 2.9 ( 0.05 MPa, and Tex ) 275 ( 0.5 K. The six runs were successively performed in numerical order (from MN-1 to MN6).

Tsuji et al.

Figure 8. Compiled gas-mixture supply data obtained in five experimental runs using methylcyclohexane (MCH) as an LMGS; pex ) 2.9 ( 0.05 MPa, and Tex ) 275 ( 0.5 K. The five runs were successively performed in numerical order (from MM-1 to MM-5).

Figure 7. Sequential videographs of hydrate formation in the presence of a neohexane layer lying on a water pool (run MN3).

of mutual gas/LMGS/water dissolution, which should have progressed through a few early runs. In Figure 9, we compare hydrate-formation sequences that have been video-recorded in two extreme runs, Runs MM-1 and MM-5, in which the time-averaged rate of gas consumption was the lowest and the highest, respectively. Consistent with the gas-consumption rates in these runs deviating greatly from each other, we find a large difference between these runs in each of the following characteristics: opaqueness in the gas-filling space, turbidity in the MCH layer, and the rate of downward spreading of hydrate slurry in the water phase. Interpretation of Run-by-Run Change in Hydrate Formation Rate. As demonstrated previously, the rate of gas consumption, or the rate of hydrate formation, showed a marked tendency to increase from the first run to the second run, and from the second run to the third run, when neohexane or MCH was used together with the methane/ethane/propane mixture. This tendency may be ascribed, as indicated in the Experimental Section, to a reduction in the selective gas-absorption capacity of the LMGS phase with repetition of the experimental runs. That is, we can assume that the hydrate-formation rate has a tendency to increase as the preferential absorption of ethane and

Figure 9. Sequential videographs of hydrate formation in the presence of an MCH layer lying on a water pool. The two sequences (a) and (b), shown here, were recorded in two extreme runs, MM-1 and MM-5, in which the time-averaged rate of gas consumption was the lowest and the highest, respectively, in this series of experiments.

propane by the neohexane or MCH phase decays run by run, resulting in an asymptotic decrease in the methane concentration in the gas phase and its leveling off at later runs. This interpretation is qualitatively consistent with the fact that even the lowest hydrateformation rate recorded in the first run (run MN-1 or


run MM-1) is still higher than the corresponding rate observed with pure methane gas instead of the methane/ ethane/propane mixture. Thus, it turns out that the hydrate-formation rate monotonically increases as the methane concentration in the gas phase decreases from 100%, as long as neohexane or MCH is present in the hydrate-forming system. However, note that the above-stated interpretation is still open to question from at least two aspects. First, the interpretation is not necessarily consistent with the results of the experiments with TBME (see Figure 4). These results show no tendency of the hydrate-formation rate to increase with repetition of the experimental runs. Furthermore, the hydrate-formation rates obtained with the methane/ethane/propane mixture + TBME are appreciably lower than those obtained with pure methane + TBME under the same pressuretemperature condition, which is demonstrated in the next section. At present, we cannot provide a coherent explanation of the experimental results obtained with all of the three LMGSs we tested. A second question may be raised about the mechanism through which the methane concentration in the gas phase affects the rate of hydrate formation. As indicated in Table 2, a decrease in methane concentration from 100% to 90% may not change the temperature driving force for hydrate formation (∆Tsub) very extensively, compared with the observed change in the hydrate-formation rate in the system using neohexane or MCH.10 Thus, the mechanism of present interest is possibly related to some gas-phase-composition-dependent matter other than the heat removal from the hydrate-formation sites. The mechanism may be related to the structural multiplicity of hydrates formed from the gas mixture in the presence of an LMGS. Not only sH hydrate but also sII and sI hydrates may form simultaneously or alternately during each experimental run, depending on the instantaneous gas-phase composition.11 The transition from one structure to another, or from one multiple-structure state (e.g., sH + sII) to another state, during each run or in successive runs may have some role in significantly changing the rate of gas consumption. This issue is still left for future investigation, which may require analyses of the gas mixture and hydrate crystals coexisting in hydrate-forming processes. Comparison of Gas Consumption Rates Obtained with Different Gas + LMGS Pairs. For the convenience of discussing the gas consumption rates due (10) According to CSMHYD, the four-phase equilibrium temperature (Teq) for the gas-mixture + LMGS pair is predicted to be slightly lower than that for the methane + LMGS pair, when the LMGS is neohexane or MCH (see Table 2). From this observation, it turns out that the system subcooling (∆Tsub), which is the thermal driving force for hydrate formation, is slightly decreased, instead of increased, by changing the guest gas from pure methane to the methane/ethane/ propane mixture (90% C1 + 7% C2 + 3% C3). We should be careful in viewing this consequence, because, for mixed hydrates, the average absolute deviation of equilibrium pressure, at a given temperature, predicted by CSMHYD is estimated to be (9% (see Section 5.1.8 of the book cited as ref 7). The pressure deviation of (9% at a temperature of ∼280 K almost corresponds to a deviation of (0.8 K in Teq, and, hence, it is uncertain if ∆Tsub is actually decreased by replacing pure methane with the mixture. Nevertheless, it seems to be reasonably conservative to assume that ∆Tsub will not be increased by more than a few degrees by the replacement of pure methane with the mixture. (11) The structural multiplicity of hydrates formed from a gas mixture in the presence or absence of an LMGS is discussed in detail in a separate paper that is currently under preparation.


Figure 10. Comparison of gas-supply rates during hydrate formation from different guest-gas + LMGS pairs and from guest gases in the absence of any LMGS. The quantity V˙ g as the ordinate denotes the gas-supply rate time-averaged over each hydrate-forming process. For every series of experiments using the gas mixture, the arithmetic mean of the V˙ g data obtained in the third and later runs is represented by a vertical bar. Tex ) 275 ( 0.5 K, and pex ) 2.9 ( 0.05 MPa, except for hydrate formation from pure methane gas with no LMGS, which was tested at a higher pressure (pex ) 3.7 ( 0.05 MPa). Except for methane + MCH, the data relevant to pure methane, instead of the mixture, were obtained in our previous studies already reported.1,2 The data for methane + MCH demonstrated here have not been reported so far; they were newly obtained in separate experiments for comparison, because our previous data for methane + MCH were obtained at a slightly lower pressure (2.8 ( 0.05 MPa).2

to hydrate formation from various guest-gas + LMGS pairs, we evaluated the average rate of gas supply to the spray chamber in each experimental run (V˙ g), by dividing the final value of Vg in the run by its duration after the first hydrate formation. Figure 10 compares the levels of V˙ g obtained with six guest-gas + LMGS pairs and two guest gases (90% C1 + 7% C2 + 3% C3 mixture and pure methane) with no LMGS. For every series of experiments using the gas mixture, the arithmetic mean of the V˙ g data obtained in the third and later runs is represented by a vertical bar to focus our attention on the V˙ g level to be maintained in periodic hydrate-forming operations. Here, we recognize some important and/or unexpected facts, which may be summarized as follows: (1) V˙ g for the hydrate formation with the gas mixture is increased by the presence of any of the three LMGSs tested, compared to its value available in the absence of any LMGS under the same pressure-temperature condition. (2) Among the three LMGSs, MCH increases V˙ g the most, whereas TBME increases it the least. This is opposite to the case of hydrate formation with pure methane, in which TBME and MCH increase V˙ g the most and the least, respectively.1 (3) In the presence of neohexane or MCH, the V˙ g value for hydrate formation with the gas mixture is much higher than that for hydrate formation with pure methane. However, the opposite is true in the case of hydrate formation in the presence of TBME.

876


Although none of these observations is physically (or physicochemically) understood yet, they must have practical significance in developing hydrate-based technology for storing and transporting natural gas. Concluding Remarks This study has experimentally investigated the hydrate formation from a natural-gas-simulating methane/ ethane/propane mixture and a large-molecule guest substance (LMGS)sa heavy structure-H (sH) hydrate formersin an isobaric system that used continuous water spraying into the gas mixture. The results of this study provided us with the prospect that the rate of natural-gas fixation by hydrate formation can be significantly increased (up to a few times) by forcing an LMGS such as tert-butyl methyl ether (TBME), 2,2dimethylbutane (neohexane), or methylcyclohexane (MCH) to interact with the hydrate formation. As far as water-spraying, semibatch operations (such as those used in this study) are concerned, MCH is presumably more effective than TBME and neohexane in promoting the hydrate formation under a continuous supply of natural gas containing ∼90 mol % methane. However, note that the advantage of using MCH instead of TBME or neohexane will be lost if the feed gas is much more methane-rich. An unexpected finding in this study was a significant deviation of the rate of mixed-gas fixation from the corresponding rate of methane fixation under the same pressure-temperature condition. Furthermore, the de-

Tsuji et al.

viation is variant with the species of the LMGS selected. Presently, we have no means that allows us to reasonably estimate the natural-gas fixation rate in the presence of an LMGS from the methane-fixation rate observed with pure methane gas plus the same LMGS. The role of model experiments using pure methane as a substitute for natural gas should be restricted to obtaining a qualitative understanding of hydrate formation behavior. This study suffered from a lack of information on the gas-phase composition, as well as the structure of hydrate crystals formed, in the course of each hydrate formation process, which prevented us from going into the mechanisms underlying the phenomenological findings reported in this paper. This weakness came from technical restrictions that were due to the design of the experimental facilities that we used in this study. Some attempts to compensate for the weakness are underway, with the expectation that we will find a way to gain a better scientific understanding of the formation of hydrates from natural gas in the presence of an LMGS. Acknowledgment. This study was performed as a part of a research project “Development of an Advanced Hydrate-Production Technology for Natural-Gas Storage” proposed to and adopted by the Japan Oil, Gas and Metals National Corporation (JOGMEC, formerly JNOC). We appreciate the financial support of the project by the JOGMEC. EF049785A

Hydrate Formation by Water Spraying in a Methane + Ethane +

Recommend Documents