Self-Preservation Effect for Hydrate Dissociation in Water + Diesel Oil

Article pubs.acs.org/EF

Self-Preservation Effect for Hydrate Dissociation in Water + Diesel Oil Dispersion Systems Yi-Ning Lv,† Meng-Lei Jia,‡,§ Jun Chen,‡,∥ Chang-Yu Sun,*,‡ Jing Gong,*,† Guang-Jin Chen,*,‡ Bei Liu,‡ Ning Ren,‡ Shu-Di Guo,‡ and Qing-Ping Li⊥

Downloaded by KTH ROYAL INST OF TECHNOLOGY on August 26, 2015 | http://pubs.acs.org Publication Date (Web): August 25, 2015 | doi: 10.1021/acs.energyfuels.5b00837

†

National Engineering Laboratory for Pipeline Safety, and ‡State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, People’s Republic of China § Dalian Engineering Construction Company, Limited, Dalian, Liaoning 116000, People’s Republic of China ∥ Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, South China University of Technology, Guangzhou, Guangdong 510640, People’s Republic of China ⊥ China National Offshore Oil Corporation (CNOOC) Research Center, Beijing 100027, People’s Republic of China ABSTRACT: The self-preservation effect experiments in water + diesel oil dispersion systems for methane hydrate were carried out with the particle size ranging from tens to more than 100 μm. The influence of water cuts (low water cuts of 10, 20, and 30 vol % or high water cuts of 95, 99, and 100 vol %) and types of inhibitors (tetra-n-butylammonium bromide or Lubrizol) on the dissociation kinetics in oil and water suspensions were examined. The addition of surfactants, especially those able to lower the size of droplets or hydrate particles in low water cut suspension systems, could remarkably hinder the self-preservation effect by surface adsorption and alterations in structures and morphologies of ice film. For higher water cut systems with or without surfactants, the enhanced self-preservation effect was observed in comparison to lower water cuts. Systems with oil exhibited a declined effect in contrast to pure water systems. The ice-shielding mechanism for hydrate dissociation is supported by the effects of surfactants and water cuts on dispersion and agglomeration properties as well as the size of hydrate particles.

1. INTRODUCTION Gas hydrates are non-stoichiometric crystalline compounds that are formed by encapsulating guest molecules into host cavities combined by hydrogen bonds.1 Naturally occurring gas hydrates are considered to be one of the potential future energy resources because methane gas encapsulated in hydrates constitutes the major part of hydrocarbon resources.2−7 However, for the petroleum industry, gas hydrates are better known as unwanted reaction products between transported fluid and rest water that frequently agglomerate and plug pipelines.8,9 This issue becomes particularly troublesome when the production and transport of oil and gas moves toward deep sea areas and cold regions. The development of flow management tools or remediation methods has been initially focused around thermodynamic and more recently kinetic inhibitors.10 Alternatively, formed hydrates can be turned into flowing slurry, making pipeline transportation feasible.11,12 The gas hydrate lattice of both srtucture I (sI) and structure II (sII) can store up to one molecule per cage, which gives a maximum of 174.6 m3 for sI and 174.9 m3 for sII [standard temperature and pressure (STP)] of gas in 1 m3 of clathrate.13 From this point of view, hydrates could also be viewed as a potential gas storage and transportation method. Nevertheless, controlling hydrate slurry under atmospheric pressure and relatively high temperature is still challenging,14 and all of the above applications demand a better understanding of the stability and dissociation behaviors of hydrates. Particularly intriguing is the recently found self-preservation effect for gas hydrates dispersed in crude oil emulsions.15 The self-preservation refers to a kinetic anomaly found at a certain range of temperatures © XXXX American Chemical Society

below the melting point of ice outside the thermodynamic stability field where the further decomposition of hydrates can be greatly suppressed. The phenomena was first discovered in the 1980s16,17 and, later, was confirmed by many researchers.13,18−33 The self-preservation effect of methane hydrate was firmly established at ambient pressure and between 242 and 271 K, and the minimum dissociation rate was registered at 268.2 ± 1 K.19 The dependence of the self-preservation phenomena on guest molecules was investigated, and these phenomena were repeatedly confirmed on CH4 and CO2 hydrates of sI13,19,21,22,24−26 and synthetic natural gas hydrates of sII.23,27,28 The self-preservation effect is also related to the size of hydrate particles. The stability of hydrate particles seemed to increase with the particle size.20,24 For particle sizes below about 250 μm, all methane hydrates dissociated by about 210 K in a heating experiment from 135 K.20 The rate of methane hydrate dissociation was investigated to be inversely proportional to the granule diameter.20,25,29 The methane hydrate under anomalous preservation conditions was observed to be enveloped and stabilized by a thin layer of ice.14 The ice layer had an average thickness of about 100 μm after storage for 24 h at 253 K.14 For CO2 hydrates, an ice thickness of no larger than 5−10 μm is obtained, assuming a homogeneous coverage of grains with an average particle size of 250 μm.26 More recently, a layer of tight ice coating was reported to be as thin as Received: April 16, 2015 Revised: August 13, 2015

A

DOI: 10.1021/acs.energyfuels.5b00837 Energy Fuels XXXX, XXX, XXX−XXX

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examined. The potential application of this effect in oilcontaining dispersion systems into transportation, flow assurance technology, or effective exploitation of natural hydrate resources lies in the improved knowledge on these factors with a wider range of temperatures, pressures, and compositions.

a few micrometers that formed at the very initial state of the dissociation of methane hydrates.13 The mechanism for the self-preservation effect involved in these experiments and observations is somewhat unclear. Because this effect occurs exclusively below the melting point, the evolution of the ice film formed by water produced in initial dissociation resulted in “the ice-shielding mode”. In this mode, the preservation effect is predominantly explained to be controlled by the ability of the ice film to hinder the penetration or permeation of gas, which is tightly related to the perfection/defectiveness and crystallographic structure of ice.13,21,26 With ex situ cryo-scanning electron microscopy (cryo-SEM), Falenty et al.13,26 correlated kinetic data with the morphology of initially formed ice coatings, of which the microstructure and annealing process appeared to be a significant factor. Melnikov et al.34−38 also reported observations of the formation of metastable (supercooled) water and the growth of metastable-state hydrates during dissociation both optically and through nuclear magnetic resonance (NMR) for CH4, C3H8, CO2, and Freon-12 gases. However, in the literature, only a few works have been directed into the selfpreservation effect in oil suspensions or dissociation behaviors for systems of gas and pure water with the addition of surfactants.39−42 Lachance et al.43 investigated the emulsion stability during hydrate formation/dissociation using differential scanning calorimetry (DSC), suggesting that the formation of hydrates may destabilize emulsions. Kakati et al.44 studied the formation and dissociation kinetics in oil-in-water emulsions without looking into the self-preservation effect in these systems. Through the addition of sodium dodecyl sulfate (SDS) into non-stirred water with an aluminum or copper tube, Zhang and Rogers27 succeeded in maintaining extraordinary stability of sI methane hydrates at 1 atm and 268.2 K and sII natural gas hydrates at 1 atm and 268.2−270.2 K upon depressurization within 5 s. This ultrastability was attributed to closely packing minimized sizes of void spaces or fractures surrounded by hydrates and negligible unreacted water. Recently, in work by Stoporev et al.15 on 50% water content in crude oil emulsions with the static formation of hydrates, it showed that oil suspensions with aggregates constituted by small (a few tens of micrometers) particles of methane hydrate could also exhibit the self-preservation effect. However, the hydrodynamic and surfactant effects were not addressed in the literature. Quaternary ammonium salts can be considered as anti-agglomerants (AAs) in the reported literature.45 Tetra-nbutylammonium bromide (TBAB) is a typical quaternary ammonium salt.46 Lately, a new AA47 containing cocamidopropyl dimethylamine (effective component) and glycerin was developed that shows exceptional effectiveness in the full water content range at a low dosage (0.2 wt % in the aqueous phase) when there is no salt. This new AA was reported to be effective even when there is no oil phase in the mixture. To further clarify the effect of the presence of the liquid hydrocarbon phase and surfactant in different amounts as well as the particle size on the preservation behavior, experiments in water + oil dispersion systems with hydrate particles ranging from tens to more than 100 μm were carried out systematically in this work. The impact of different hydrate particle sizes on the preservation effect is inspected using the focused beam reflectance method (FBRM) probe. The influences of water cut (defined as the volume fraction of water in the total liquid phase), different dosage, and type of inhibitors for both low and high water cut systems on the self-preservation effect were

2. EXPERIMENTAL SECTION 2.1. Materials and Apparatus. The materials used in this work include deionized water, non-ionic surfactant of Lubrizol (provided by American Institute of Reservoir Engineering; for more detail, please refer to the study by Sun and Firoozabadi47), and cationic surfactant of TBAB (provided by Sinopharm Chemical Reagent Beijing Co., Ltd.), where the twice-distilled water was prepared in our laboratory with conductivity less than 10−4 S m−1. To eliminate the impact of resin and asphaltene compositions in crude oil, which would complicate the effects, diesel oil with well-defined composition (listed in Table 1) was

Table 1. Composition of the Diesel Oil Used in This Work C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C20 C24 C28+

component

mol %

wt %

heptanes octanes nonanes decanes undecanes dodecanes tridecanes tetradecanes pentadecanes hexadecanes heptadecanes octadecanes eicosanes tetracosanes octacosanes plus total

0.219 1.345 3.595 3.703 5.899 5.156 8.336 13.612 11.370 10.084 9.587 8.713 11.422 6.807 0.152 100.000

0.100 0.698 2.094 2.393 4.187 3.988 6.979 12.263 10.967 10.369 10.469 10.070 14.656 10.469 0.298 100.000

adopted, and thus, the effect of surfactants could be investigated exclusively. For gas phase, methane with analytical grade (99.99 vol %) was supplied by Beijing Beifen Gas Industry Company. The schematic diagram of the experimental apparatus is shown in Figure 1, which is mainly constituted by three parts: a high-pressure autoclave with glycol bath and a magnetic stirrer, particle video microscope (PVM) and FBRM probes, and data acquisition system. The effective internal volume of the autoclave is approximately 535 mL (51.84 mm in diameter and 320 mm in depth). Both PVM and FBRM D600X probes were purchased from Mettler-Toledo Lasentec. The PVM probe consists of six lasers, which illuminates a small area in front of the probe face. The probe creates digital images of the illuminated area with a field of view of 1680 × 1261 μm. The image provides clear resolution to approximately 5 μm. For the FBRM probe, there is a rotating optical lens at the probe tip, which can deflect the laser. When it starts working, the emitted laser is reflected if it scans across the surface of a particle. The chord length of droplets or particles can be determined by the product of the measured reflectance time and the laser scan speed with an uncertainty of 0.5 μm, and it is counted at a certain time interval. The chord length distribution (CLD) can then be determined. Mean chord length that stands for the size of the droplet or particle to some extent is obtained from CLD on FBRM software. For more information on the probe and the technique, please refer to the previous literature. 48,49 A secondary platinum resistance thermometer (type Pt100) and a differential pressure transducer (type Trafag 8251) were installed in the autoclave to detect the temperature and pressure with uncertainties of 0.1 K and 0.02 MPa, respectively. A glycol bath was used to control the temperature, which ranges from 253.2 to 363.2 K. The pictures captured from the PVM B


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Figure 1. Schematic diagram of the experimental device for testing the self-preservation effect in water or oil systems: (1) gas cylinder, (2) autoclave, (3) glycol bath, (4) pressure transducer, (5) FBRM probe, (6) vent, (7) electromotor, (8) computer, (9) PVM probe, and (10) thermocouple. with the liquid phase at the end of stage i. nhyd represents the total moles of gas molecules consumed by the whole hydrate formation process. nd,i (i = 2, 3, and 4) denotes the amount of dissolved gas molecules in the oil phase at the end of stage i. The method for measurement of the solubility of gas in diesel oil is similar to that by Xiang et al.51 The moles of gas in the gas phase are given by

probe, CLD from the FBRM probe, temperature, and pressure data were collected and recorded by the data acquisition system. 2.2. Experimental Procedure. The whole high-pressure reactor and each joint were first washed by distilled water and then dried with pure nitrogen. The PVM and FBRM sensors were cleaned and tested until the precision requirements in specification are fulfilled. The aqueous phase was prepared according to oil/water ratios and the mass fraction of the surfactant, with the total volume of the liquid phase being 180 mL. Thereafter, the liquid is injected into the reactor, which is then evacuated using a vacuum pump for 10 min to deprive of the residual gas in the vessel. The temperature of the cooling bath is adjusted to the required value, and the stirrer is set to 400 rpm. After the temperature of the system became constant, the stirrer was stopped and the experimental gas was injected to about 8 MPa. Afterward, the isothermal system with or without surfactant was left for hydrate formation for more than 20 h with the stirrer maintained at 400 rpm until the pressure became constant and the hydrate formation stage was ended. After the hydrate formation stage, the temperature of the cooling bath is changed to either 267.2 K to freeze the remaining fluid or 274.2 K for contrast, which was maintained constant for 4 h without stirring. Thereafter, hydrates began to dissociate under isochoric conditions. The system pressure is first lowered to slightly above the equilibrium pressure value at the corresponding temperature (the hydrate stability boundary was calculated by the Chen−Guo model50). After the pressure becomes steady, the system is promptly vented to atmospheric pressure within 5 s and then closed for over 20 h. The system remains isothermal while pressure data are recorded. Finally, the temperature will be raised to about 293.2 K for complete dissociation. 2.3. Data Analysis. The calculation of the percentage of formed or remaining hydrates is based on mass balance. The entire procedure can be divided into four stages: (1) gas injection to the desired value, (2) beginning of formation and subsequently a constant temperature maintained at 267.2 or 274.2 K, (3) gas venting to atmospheric pressure followed by dissociation isothermally for 20 h until the pressure became constant, and (4) complete dissociation after the increase of the temperature to 293.2 K. In each stage, the total mass of gas molecules in the system consisted of that in the gas phase, oil phase, and hydrate phase, as shown in the following equations: n0 = nhyd + nd,2 + ng,2

(1)

nhyd,r + nd,3 + ng,3 = nd,4 + ng,4

(2)

ng, i =

PV i i ZiRTi

(3)

where Pi, Ti, and Vi are system pressure, temperature, and gas volume in each stage, respectively. Zi indicates the compressibility factor, which is calculated by the Benedict−Webb−Rubin−Starling (BWRS) equation of state. The gas volume can be estimated by subtracting the volumes of the oil phase, hydrate phase, and remaining water from the total volume. The dissociation ratio (the ratio of the hydrates dissociated to the total hydrates formed before dissociation) d, the formation volume ratio (the volume fraction of hydrates in the entire liquid−hydrate mixed phase) f, and the mole conversion percentage of total water F can be inferred as follows: d=

f=

F=

nhyd − nhyd,r nhyd

× 100% (4)

1.25Vrw × 100% 1.25Vrw + (Vw − Vrw ) + (180 − Vw) 5.75nhyd (mj /M w )

(5)

× 100% (6)

where Vrw is the volume of reacted water, Vw is the total volume of water added, 1.25 refers to the expansion factor, 180 refers to the total volume of the liquid phase in the unit of milliliters, 5.75 refers to the hydration number, mj is the mass of water in different water cuts, and Mw is the molar weight of water. After the formation of large quantities of hydrates, the mean chord length of hydrate particles tends to be stable, which could be used to calculate the mean size of hydrate particles by relations correlated by Greaves et al.52 Because the amount of total hydrates formed nhyd and that of remaining hydrates before the temperature increase nhyd,r are both calculated, the instantaneous dissociation ratio dins during the isothermal and isobaric dissociation stage could be determined as follows to show the dissociation process in a way relative to all of the hydrates formed in each run

where nhyd,r stands for the remaining moles of gas molecules in hydrates at the end of stage 3 before eventual dissociation at stage 4. n0 is the total moles of gas initially injected in stage 1, and ng,i (i = 2, 3, and 4) is the moles of gas molecules in the gas phase in equilibrium

⎛n nhyd − nhyd,r ⎞ release,end − nrelease,ins ⎟⎟ × 100% d ins = ⎜⎜ + nhyd nhyd ⎝ ⎠ C

(7)


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Table 2. Summary of Experimental Conditions and Results for Different Water Cut Systems with or without Surfactant run

dissociation temperature (K)

1 2 3 4

267.2 267.2 267.2 267.2

5

267.2

6 7 8 9 10

274.2 267.2 267.2 267.2 267.2

11

267.2

12

267.2

13

267.2

14 15

274.2 267.2

surfactant addition 0 0 0 0.06 wt % Lubrizol 0.06 wt % TBAB 0 0 0 0 0.06 wt % TBAB 0.06 wt % Lubrizol 0.06 wt % TBAB 0.06 wt % Lubrizol 0 0.6 wt % Lubrizol

water cut (vol %)

dissociation ratio (mol %)

formation volume ratio (mol %)

conversion percentage (mol %)

mean particle size (μm)

10 20 30 20

43.64 34.18 30.55 97.18

11.58 24.80 36.49 19.40

94.80 95.94 96.22 80.77

27.44 42.89 48.31 33.70

20

94.32

16.90

85.22

38.35

20 100 99 95 99

97.05 8.30 10.90 15.01 8.90

23.70 61.57 60.60 55.90 42.63

96.03 56.18 55.90 53.40 37.26

42.32 184.35 156.36 94.07 127.06

99

25.42

56.73

51.18

100.26

100

15.73

29.92

26.38

150.47

100

20.55

50.27

44.71

142.95

100 100

81.22 23.15

60.45 49.57

55.32 43.29

180.45 115.13

where nrelease,end is the total amount of accumulated gas revolved during the isothermal and isobaric dissociation stage and nrelease,ins is the instantaneous amount of accumulated gas revolved. Both nrelease,end and nrelease,ins could be determined by taking use of recorded pressure data, dissolution measurement results, and equation of state.

3. RESULTS AND DISCUSSION A summary of experimental conditions and results are listed in Table 2 for different water cut systems with or without surfactant. The formation volume ratio, the conversion percentage, and mean size of polycrystalline hydrate particles before gas venting as well as the dissociation ratio at the end of the isothermal and isochoric dissociation after venting of the system are all listed in Table 2. 3.1. Effect of the Water Cut in Low Water Cut Systems. As shown in Table 2 for runs 1−3, three groups of experiments were conducted to examine the influence of the water cut (10, 20, and 30 vol %) on the self-preservation effect in water + diesel oil systems when no surfactant was added. It can be seen that the conversion percentage is rather high and increases with the water cut. Correspondingly, the formation volume ratio also increases with the water cut and can be over 36.49% for the 30% water cut system. However, the dissociation ratio decreases with the increase of the water cut, implying that, at a higher water cut, the self-preservation effect is more significant. The dissociation ratio also decreases with the mean size of hydrate particles, which is in accordance with Takeya’s conclusion about the influence of hydrate size on selfpreservation.20 Figure 2 shows the change of instantaneous dissociated ratio with time for different water cut systems (runs 1−3) during the isothermal and isochoric dissociation process. In these values, it should be noted that the dissociation ratio does not start from 0% at 0 min at the end of venting, because a small part of hydrates decomposed inevitably during the venting process. As shown in Figure 2, the final ratio at the end of dissociation decreases with the increase of the water cut. There remains a short time lag before hydrate dissociation starts. This is possibly because of the heat and mass transfer process involved in

Figure 2. Effect of the water cut on the dissociation ratio in low water cut systems.

hydrate dissociation. Little defectiveness in these hydrate systems may induce a lag in mass transfer through ice film and eventually release of gas. It may also need a certain amount of heat to break down the equilibrium. In addition, the free water released by initial hydrate decomposition may form an ice film or shell on the surface of hydrates, impeding the release of gas. Because the particle size increases with the water cut (as shown in Table 2), the decrease of the surface/volume ratio may result in a slower dissociation rate according to the current understanding of the self-preservation mechanism as a limited gas diffusion process by ice-coating on the surface.13,26,33 Thus, the macroscopic dissociation rate presented by the dissociation ratio versus time is shown to be lower with a higher water cut. 3.2. Effect of Surfactants and Temperature in Low Water Cut Systems. The two types of non-ionic and ionic surfactants of Lubrizol and TBAB are generally applied to prevent agglomeration of hydrates. To investigate the effect of surfactant addition for low water cut systems, experiments of 20 vol % water + 80 vol % oil with or without the addition of Lubrizol or TBAB were conducted, as runs 4−6 in Table 2. D


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Figure 3. Morphologies observed by the PVM probe for methane hydrate dissociation and formation from a 20 vol % water + 80 vol % diesel oil system with the addition of 0.06 wt % Lubrizol at 274.2 K and 8.0 MPa (run 4): (a) before hydrate formation, (b) initial hydrate formation, (c) after massive hydrate formation, and (d) hydrate dissociation after venting.

move rightward to Figure 4d. The bubbles were of different sizes, as shown in Figure 3d. The dissociation ratios for systems with 0.06 wt % Lubrizol or 0.06 wt % TBAB systems listed in Table 2 after ∼21.6 h are as high as 97.18 and 94.32%, respectively; that is, almost all hydrates dissociated for the surfactant systems, while for run 2 with the same water/oil ratio but without the addition of surfactant, the dissociation ratio is only 34.18%. When the dissociation temperature was increased to 274.2 K, the self-preservation effect obviously disappeared in comparison to the surfactant-free system in 267.2 K. Figure 5 shows the change of the dissociation ratio with time for 20 vol % water cut systems with or without surfactant during the isothermal and isochoric dissociation process. For systems with surfactant, the high initial dissociation rate could result from the association of surfactants toward the surface of hydrate particles

Figures 3 and 4 show the change in morphology and CLD in run 4 during methane hydrate dissociation and formation from

Figure 4. CLDs observed by the FBRM probe for methane hydrate for a 20 vol % water + 80 vol % diesel oil system with the addition of 0.06 wt % Lubrizol at 274.2 K and 8.0 MPa (run 4) corresponding to four stages in Figure 3: (a) before hydrate formation, (b) initial hydrate formation, (c) after massive hydrate formation, and (d) hydrate dissociation after venting.

a 20 vol % water + 80 vol % diesel oil system with the addition of 0.06 wt % Lubrizol (the dosage of surfactants is based on the mass of the solution) under 274.2 K and 8.0 MPa. As seen from Figure 3a, water droplets were able to disperse uniformly within the oil phase in emulsion, while the CLD in Figure 4a stayed stable. With the initial formation of hydrate, the CLD slightly moved to the right in Figure 4b. When massive hydrates formed, the CLD remained at Figure 4c and particles showed well dispersion, as shown in Figure 3c. Upon dissociation by venting, the emitted gas bubbles caused the CLD to further

Figure 5. Effect of the dissociation temperature and surfactant type on the dissociation ratio for systems with 20 vol % water cut. E


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particle size with a lower oil content. Figure 6 shows the variation of the dissociation ratio with time for high water cut

by the hydrophilic end. The mono- or bilayer structure formed by the hydrophobic end or its further association to other surfactant molecules constitutes a barrier preventing agglomeration of hydrates during both hydrate formation and dissociation, which, in turn, lowered the size of hydrate particles and increased their specific surface area. On the other hand, the relatively high concentration of surfactants in water may also have altered the interfacial structures upon hydrates. This could make the formation of compact hydrate/ice film disappear, causing less preservation effect and increased decomposition rate. A lowered particle size accompanied by a change in interfacial structures could contribute to observed phenomena. The mean size of hydrate particles with the addition of Lubrizol is the smallest for all 20% systems, and the dissociation ratio exhibited a sharp increase from the beginning and quickly approached total decomposition. This implies that the changes in polycrystalline hydrate particle size and interfacial structure are likely to influence the self-preservation effect. Similar observations have been reported for methane hydrate dissociation with the presence of SDS. Wang et al.42,53 discovered that the specific surface areas of formed hydrate particles could be increased by the presence of SDS and stirring, which subsequently led to the increase in the hydrate dissociation rate. According to these findings,42,53 when the temperature was kept constant, a larger dissociation area for smaller hydrate particles could be the controlling factor in the dissociation rate. Similar perspectives were obtained for linear alkyl benzenesulfonate (LABS), cationic surfactant cetyltrimethylammonium bromide (CTAB), and non-ionic surfactant ethoxylated nonylphenol (ENP).41 Therefore, it indicates that no matter whether it is a gas−water system with or without the presence of liquid hydrocarbon, the self-preservation effect could be affected by the hydrate particle size and interfacial structures that relies on stirring, the addition of surfactants, and other experimental operations in hydrate formation or freezing. In comparison to the Lubrizol-containing system, the hydrate particle size in the TBAB-containing system is slightly larger. The initial dissociation rate for the TBAB-containing system shown in Figure 5 is smaller presumably as a result of the larger particle size, but the dissociation, as dominated by the altered interfacial structure, continued over time and led to total decomposition. It has been reported many times that surfactant addition leads to an increase in the rate of hydrate formation and changes the surface structure (or reflected through morphology) of hydrate during formation.27,54 3.3. Effect of the Water Cut in High Water Cut Systems. To examine the self-preservation effect at high water cuts, the hydrate formation and dissociation in oil/water or pure water systems with high water cuts (100, 99, and 95%) were conducted without the addition of inhibitors at 267.2 K, corresponding to runs 7−9 in Table 2. It can be seen that, in comparison to the low water cut systems, the dissociation ratio is much lower and the mean hydrate particle size is much higher for high water cut systems. This is probably due to the difference in hydrate formation and freezing processes in high water cut systems. In the presence of excessive water, continuous growth of hydrate particles could be sustained and ice films with more compact structures are likely to occur in the freezing process. The formation volume ratio is high, although a lower conversion percentage emerges for higher water cut systems. In accordance with low water cut systems, the dissociation ratio decreases with the increase of the water cut, which is most likely related to the increase in the hydrate

Figure 6. Effect of the water cut on the dissociation ratio in high water cut systems.

systems without the addition of surfactant. In line with low water cut systems, the final dissociation ratio during the period investigated (∼21.6 h) decreases with the increase in the water cut. Therefore, it can be inferred that the self-preservation effect is generally enhanced with increasing water cuts in both waterin-oil (w/o) and oil-in-water (o/w) systems without surfactants. This is perhaps because, following the formation of hydrates, part of the free water phase turned into ice and attached to or covered the hydrates, impeding the mass transfer of gas molecules from the hydrate phase into the gas phase. The initial water film on the hydrate surface and its subsequent conversion into ice and thickening increased the transfer resistance gradually, leading to the stop or decrease in dissociation. The presence of the oil phase, on the other hand, improved the flow ability of the system and caused the hydrate particle size to decrease sharply, which favoring the initial rapid dissociation. Additionally, it is noticed that the dissociation ratio of the pure water system (run 7 shown in Figure 6) seems to continue increasing in the end. It could possibly occur that the dissociated ratio in run 7 would become higher than those in runs 8 and 9 for a longer time than what is shown in the figure. This seems to imply that, to be accurate, the above conclusions about the influences of water cuts on the self-preservation effect are exclusively based on the period investigated (∼21.6 h). Although the change in the oil content in these systems is small, the dissociation curve exhibited a significant difference, mainly owing to the different dispersion properties of the systems. The broad composition of oil might also contribute to hindering the self-preservation effect in line with Stoporev’s claim about the influence of the decane content in oil.15 3.4. Effect of the Surfactant Type in High Water Cut Systems. The formation of hydrate aggregates could, to some extent, be hindered by the addition of surfactants, thus weakening this effect. Runs 8, 10, and 11 were conducted for 99% water cut systems to examine the influence of surfactant type on the self-preservation effect. The kinetic curves for these experiments are given in Figure 7. In runs 7, 12, and 13, experiments of pure water systems with or without surfactants at 267.2 K were conducted. For comparison, run 14 was performed at 274.2 K and the trends of the dissociation ratio F


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142 μm at 100% water cut) compared to non-addition cases. This facilitates higher flow ability in dissociation and better contact of particles with free water and gas phases. Consequently, systems (99 or 100% water cut) with Lubrizol (runs 11 and 13) exhibited the highest initial dissociation rate and final ratio of decomposition. With oil addition in run 11, this dispersion effect is reinforced, as illustrated in Figure 9,

Figure 7. Effect of the surfactant type on the dissociation ratio in high water cut systems.

are shown in Figure 8. The final dissociation ratio at 274.2 K without additive is as high as 81.22%, implying no self-

Figure 9. Effect of the surfactant type on the dissociation ratio in 99% water cut and pure water systems.

comparing the influences of surfactant type and water cut. The phenomena that a higher surface/volume ratio in smaller sized particle systems resulted in an increased dissociation pressure is similar to the work by Takeya et al.20 on self-preservation about hydrate size. The phenomena in TBAB systems are more complicated, and several possible mechanisms, including kinetic inhibition,55−57 AA, and semi-clathrate formation, may be involved. The dissociation ratio for the system with TBAB at 99% water cut (in Figure 7) is anomalously smaller than the surfactant-free system. This may be explained by enhanced formation of ice shielding around hydrates as a result of metastable water assembly seeking the alternative nearest free energy minimum. The interaction between quaternary ammonium bromide and tetrahydrofuran (THF) molecules was reported55 to be in favor of ice formation. Oil components and TBAB molecules may interact similarly; otherwise, the AA feature would have prevailed. This would cause heat- and mass-transfer resistance to increase, thereby enhancing the self-preservation effect. When oil is added, amphiphilic TBAB molecules may also absorb the hydrocarbon molecules surrounding hydrate surfaces. This external oil film and internal water film formed by dissociation could together increase mass-transfer resistance in dissociation. The mole conversion percentage indicates that the initial formation of hydrates was partly inhibited by TBAB, making the subsequent formation of ice and dissociation processes even more complicated. On the other hand, the AA function of TBAB as a quaternary ammonium salt55 might be limited in high water cuts with the presence of oil. Mostly, the AA feature of TBAB was reported in either low water cut systems58−62 or pure water systems.59,63 In a TBAB-containing system with 100% water (run 12 in Figure 8), the AA effect appeared to prevail (mean particle size of 150 μm comparable to 143 μm for run 13) and the formation volume ratio and the mole conversion percentage are lower, while the dissociation ratio of 15.73% is slightly smaller than 20.55% for the Lubrizol-

Figure 8. Effect of the dissociation temperature and surfactant type on the dissociation ratio in pure water systems.

preservation effect (the final pressure at the end of the isothermal dissociation process at 274.2 K in run 14 is around 2.5 MPa; there exists a small portion of hydrates undecomposed as a result of the significantly decreased driving force for hydrate dissociation), compared to 8.30% at 267.2 K. It can be seen that, in most cases where surfactant was added, the self-preservation effect did not disappear but was undermined, except in run 10 when this effect was enhanced. The reason for this considerable difference with low water cut systems may primarily be the concentration of surfactants. For the 20% water cut systems with 0.06 wt % TBAB or Lubrizol (runs 4 and 5), the self-preservation effect disappeared, explained by the lowered interfacial tension and suppressed massive agglomeration of hydrates. This can be further caused by a higher concentration and absorption of surfactants at hydrate surfaces in the 20% water cut systems because the amount of these water-soluble surfactants is proportional to the mass of the liquid phase. In high water cut systems, however, the tendency for hydrate particles to agglomerate can be higher and the AA feature of surfactants is relatively limited at a lower concentration. Relatively speaking, Lubrizol addition enabled formed hydrate particles to be considerably smaller (156 μm lowered to 100 μm at 99% water cut and 184 μm lowered to G


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Energy & Fuels

interface, and then the lower temperature brought about by endothermic dissociation causes the formation of ice grains or film upon the remaining hydrates.25 Second, the process is determined by the diffusion of gas through the ice film to the gas phase, and the presence of surfactants would affect the structure of ice film.

containing system. The kinetics of dissociation given in Figures 8 and 9 also differed for TBAB and Lubrizol systems of 100% water. The dissociation ratio for the system with Lubrizol stabilized after 20 h seemed to be under control, while the ratio for the TBAB-containing system kept rising. These results showed that the imperfection of ice accompanied by Lubrizol addition seemed to be “temporary” and the initial prompt dissociation quickly led to a stop with annealing. The water produced by initial dissociation appeared to rapidly turn into ice, and imperfection generally disappeared. For the pure water system with TBAB, however, continued decomposition was observed to maintain over 20 h. This phenomenon was also reported by Masoudi and Tohidi64 in a pure water system with natural gas. The observed behavior may be a result of an imperfect ice coating discontinued by TBAB or formation of semi-clathrate hydrate that can incorporate methane.65−68 It is worth pointing out that, despite the initial limit in agglomeration in the above systems by surfactant addition, leading to the increase in the dissociation rate primarily in most cases, there is considerable unconverted water in high water cuts and water produced from dissociation that could gradually turn into ice and prevent further decomposition. The dissociation rate can be expected to eventually lower by reduced gas diffusion, corresponding to the heat-transfer rate and presumably flow ability. 3.5. Effect of the Dosage of Lubrizol in Pure Water Systems. Figure 10 shows the influence of different dosages of

4. CONCLUSION The self-preservation effect with respect to oil/water systems was investigated by monitoring the temperature and pressure data in the isothermal and isochoric dissociation process with PVM and FBRM sensors. Experiments were conducted in low and high water cuts with and without the addition of surfactants, and a comparative study has been made about the effect of the volume fraction of hydrates, mole conversion, and mean size of hydrate particles on the dissociation ratio and pressure variations. The following understandings were obtained: (1) For low water cut systems without surfactant, when almost all water has been converted to hydrates and oil is the continuous phase, it is demonstrated that, because of the increased surface/volume ratio as a result of hydrate particles dispersing in a continual oil phase, the self-preservation effect is undermined. With the increase in water cut or mean size of hydrate particles, the self-preservation effect can be enhanced. (2) For low water cut systems with the addition of a certain surfactant, the shapes of dissociation ratio curves were significantly altered by the surfactant type. With the addition of Lubrizol, the dissociation ratio promptly increased to close to 100% at 267.2 K upon venting and the system showed a weak self-preservation effect. The systems exhibited a slower dissociation rate with TBAB, but the effect is also weakened. Therefore, the addition of surfactants, especially those that are able to lower the size of hydrate particles in low water cut suspension systems could remarkably hinder the self-preservation effect by surface adsorption and alterations in structures and morphologies of ice film. (3) For high water cut systems, it displayed a lower conversion of water, higher hydrate volume fraction, and tendency to agglomerate for hydrate particles. The lower dissociation ratio (

Self-Preservation Effect for Hydrate Dissociation in Water + Diesel Oil

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