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Rheology of tetrahydrofuran hydrate slurries Paulo Henrique de Lima Silva, Mônica F. Naccache, Paulo R. de Souza Mendes, Flavio B. Campos, Adriana Teixeira, and Amadeu K. Sum Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02425 • Publication Date (Web): 20 Nov 2017 Downloaded from http://pubs.acs.org on November 27, 2017
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Energy & Fuels
Rheology of tetrahydrofuran hydrate slurries Paulo H. de Lima Silva,∗,† Mˆonica F. Naccache,∗,† Paulo R. de Souza Mendes,∗,† Fl´avio B. Campos,∗,‡ Adriana Teixeira,∗,‡ and Amadeu K. Sum∗,¶ †Department of Mechanical Engineering Pontif´ıcia Universidade Cat´ olica do Rio de Janeiro (PUC-Rio) Rua Marquˆes de S˜ao Vicente 225, Rio de Janeiro, RJ 22451-900, Brazil ‡Petrobras Research Center (CENPES), Petrobras Av. Hor´ acio Macedo 950, Cidade Universit´ aria, Q.7 Ilha do Fund˜ ao, Rio de Janeiro RJ 21941-598, Brazil ¶Chemical and Biological Engineering Department Colorado School of Mines, Golden, Colorado 80401, United States E-mail:
[email protected];
[email protected];
[email protected];
[email protected];
[email protected];
[email protected] 1
Abstract
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In this work we study the rheology of hydrate slurries in a mixture of water and
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THF (tetrahydrofuran, C4 H8 O). This hydrocarbon is miscible in water, and forms hy-
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drates at ambient pressure and temperatures above 0◦ C. Rheological tests—constant
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shear rate, flow curve, creep, and oscillatory—are carried out for different concentra-
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tions. Transient and steady state results are obtained, suggesting that the rheology
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is strongly affected by agglomeration and breakage of hydrate crystals that seems to
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happen simultaneously, when the slurry is sheared during hydrate formation.
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Introduction
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Blockage of pipelines due to hydrate formation is one of the big issues of the oil industry,
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especially in deep and ultra-deep water subsea environment. Hydrates of natural gas are
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crystalline solids resembling ice, usually formed in conditions of high pressure and low tem-
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perature, by the association of water and light hydrocarbon molecules in a given organized
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structure. 1
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Hydrate formation in the oil industry was first reported in 1934 by Hammerschmidt, 2
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who showed that clogging of gas pipes during winter months occurred due to hydrate ag-
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glomeration. Furthermore, Hammerschmidt studied the effects of temperature, pressure and
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composition of the water and gas mixture in the hydrate formation.
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Depending on the amount of hydrates formed, the pipeline can become fully blocked.
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Since the cost to remove hydrates is very high, an alternative commonly employed is the
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addition of an anti-agglomerant that allows the pipeline to keep to operate with a hydrate
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slurry.
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Some works in the literature have studied the effect of some parameters to control hydrate
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formation in pipelines. The use of hydrate formation control systems and its impact in
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deepwater wells has been studied by Davalath and Barker. 3 Kalbus et al. 4 used rheometry
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to analyze different chemical additives to inhibit hydrate formation. Two systems were
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analyzed, the first one using a mixture of tetrahydrofuran (THF) and water, and the other
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using gases dissolved in water. Wang et al. 5 also used a hydrate slurry obtained from a
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THF/water mixture to analyze the flow and blockage mechanism in tubes. A safe range
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of hydrate volume concentration in the suspension was suggested. Moreover, it was shown
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that turbulence can accelerate the hydrate formation. More recently, Delgado-Linares et al. 6
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investigated the effect of a surfactant mixture on hydrate formation in water-in-oil emulsions. 3 ACS Paragon Plus Environment
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They showed that the presence of surfactants leads to larger formation rates and to hydrate
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slurries with smaller hydrate crystals and lower viscosities.
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The process of hydrates formation and nucleation depends on composition and pres-
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sure/temperature conditions. The same parameters can affect the rheology of the resulting
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slurry. In order to understand the mechanical behavior of hydrates slurries, different systems
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have been used in the literature. Gas hydrates form naturally in conditions of high pressure
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and low temperature, but there are some systems that can form hydrates at ambient pres-
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sure. The rheological behavior of these systems is similar to the one of the natural systems. 7
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The experiments and measurements performed with these systems are less complex and ex-
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pensive, since no pressurization is required. Cyclopentane (CP) and tetrahydrofuran (THF)
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are examples of hydrate forming agents at ambient pressure that have been used in model
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systems to study hydrate-related phenomena and processes.
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Using a water/hydrochlorofluorocarbon model system, Ohmura et al. 8 showed the stochas-
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tic nature of hydrate nucleation in a system with a history of hydrate formation/dissociation.
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The authors also observed that the thermal history of the system affects the rate of nucle-
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ation. Yao et al. 9 also used a model system, formed by composed of condensate oil and
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THF with different water cuts, to analyze the rheology of a hydrate slurry. After nucleating,
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individual particles of hydrate form and become surrounded by free water. Then, liquid
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bridges are converted into hydrate that can form agglomerates that eventually break as the
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shear stress is increased. The viscosity is reduced and shows a shear thinning behavior. The
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hydrate slurry was observed to present a viscoelastic behavior, and the viscosity function
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was well described by the Herschel-Bulkley equation. It was also shown that the viscosity
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increases with the water cut.
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The process of hydrate formation in a water-in-oil emulsion with cyclopentane was studied
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by Nakajima et al.. 7 Temperature measurements were performed during hydrate formation,
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illustrating the exothermic nature of the process. Peixinho et al. 10 also performed an exper-
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imental study of the rheology of a hydrate formed at ambient pressure, using water-in-oil
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emulsions with cyclopentane. The authors analyzed the effects of temperature and shear
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on hydrate formation through rheology. It was shown that the viscosity and elastic moduli
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abruptly increase when hydrate forms. Also, the authors observed that hydrate formation
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is more sensitive to the sub-cooling temperature than to the shear rate.
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Hydrate formation at the cyclopentane/water interface was also studied by Leop´ercio
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et al., 11 using interfacial shear rheology. A double-wall-ring interfacial rheology cell was
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designed to provide the necessary temperature control. Strain sweeps of the interfacial elas-
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tic and viscous moduli were performed to observe the mechanical behavior of the hydrate
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films. The formation time as well as the shear modulus and the yield strain were observed
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to increase as temperature is increased. Ahuja et al. 12 performed yield strength measure-
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ments in hydrate slurries using the same model system, namely water-in-oil emulsions with
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cyclopentane. The slurries showed shear thinning and thixotropic behavior. The authors
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observed that the yield strength is strongly affected by the water cut and by the slurry rest
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time, increasing with the increase of both parameters. Wall slip effects were observed and
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demanded the use of rough walls, which were shown to accelerate hydrate formation.
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Ahuja et al. 13 obtained the yield strength and viscosity of hydrate slurries of water-
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in-kerosene emulsions with cyclopentane. The hydrate formation time was measured for
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different levels of subcooling, showing that larger subcoolings yield shorter hydrate formation
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times.
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Ahuja et al. 14 compared cyclopentane hydrates with ice forming emulsions, using rheology
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and visualization. Large differences in viscosity and yield strength of the final structure were
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observed between the hydrate- and ice- forming emulsions. Both emulsions exhibited non-
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thixotropic shear-thinning behavior, but the shear-thinning behavior of the hydrate slurry is
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more pronounced. Moreover, the evolution mechanism of the morphology of the two systems
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is different. The rheology of ice slurries formed in water-in-oil emulsions was also addressed
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by Rensing et al., 15 in order to understand hydrate rheology. Measurements of viscosity and
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yield strength were performed using different water cuts. The authors also obtained results
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using deionized water and brine in the emulsions. The emulsions using brine seem to be
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more stable, and hysteresis was observed in the rheological results.
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Studies have also been performed using high pressure systems. Rensing et al. 16 performed
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a rheological study in hydrate slurries formed under high pressure conditions. Transient tests
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revealed an abrupt viscosity increase due to hydrate nucleation. The effect on the storage
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and loss moduli is also strong, namely G′ increases while G′′ decays as hydrates form. The
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results showed a Bingham-type viscosity behavior, with the yield strength decreasing when
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the sample is pre-sheared, possibly due to breakage of the hydrate agglomerates. This result
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suggests that hydrate plugging may be prevented by flow. Webb et al. 17 also presented
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high-pressure rheological results of hydrate slurries formed from real water-in-oil emulsions
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showing the effects of time, shear rate, water cut and temperature. The slurries analyzed
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presented yield strength and shear thinning behavior. The trends of both the viscosity
99
and yield strength agree with the behavior of the model systems with cyclopentane and
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tetrahydrouran, namely they increase with the water cut.
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In this work we study the rheology of hydrate slurries formed at ambient pressure from
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mixtures of water and THF (tetrahydrofuran, C4 H8 O). In terms of hydrate properties,
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THF hydrate forms as Structure II, with THF filling only large cages. In contrast, methane
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hydrate most frequently occurs as Structure I, with methane filling both large and small
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cages. The main goal is to better understand the rheology of these hydrate slurries, in order
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to optimize the flow through oil & gas pipelines under severe conditions.
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Materials and methods
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The rheology of hydrate slurries is studied using a model system formed by a mixture of
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THF (from Sigma Aldrich, 99% purity) and deionized water. THF is soluble in water and
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forms hydrates at ambient pressure, and temperatures below 4◦ C.
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Dyadin et al. 18 showed that all THF and water are used up to form hydrates at a molar
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ratio of 1:17, equivalent to a THF weight concentration of 19%. In the present research,
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mixtures at three different THF weight concentrations above 19% were studied, namely
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30%, 35%, and 40%. Hence, for the three concentrations, after hydrate is formed all water is
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used up and some excess THF is expected, forming a slurry whose continuous phase is THF.
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The mixtures were prepared by adding THF to the deionized water and manually stirring
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to get homogenization. A semi-analytic mass balance (BG4400, from Gehaka) was used in the
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preparation of the mixtures. The solutions were stored in closed bottles to avoid evaporation
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before the beginning of the rheological tests. The mixtures were transferred to the rheometer
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with the aid of a glass syringe.
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Rheological characterization
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The rheology of the hydrate slurries was obtained using two rotational rheometers: the
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Physica MCR01 (Anton Paar), and the DHR-3 (TA Instruments). Preliminary tests were
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performed using the parallel plate geometry. It was observed however that hydrate formation
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in this geometry is very difficult, possibly due the small amount of fluid used and also to 7 ACS Paragon Plus Environment
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before reaching the 1-hour period, a small perturbation is introduced to the system to induce
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hydrate formation. This perturbation consists of rubbing the rim of the cup with a flexible
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swab partially saturated with a small amount of THF. The time of formation of the first
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hydrates crystals depends on temperature and THF concentration. During the tests, it is
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also important to avoid that water condensed on the bob shaft reaches the sample. To this
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end, a home-made acrylic protective case with silica gel droplets is used. The rheological
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tests were repeated at least twice, and a good repeatability was always observed.
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Results and discussion
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Formation and dissociation temperatures
6 5
Hydrate dissociation Hydrate formation
0
T ( C)
Dyadin et al. (1973)
4 3 2 1 0
10
20
30
40
50
60
70
wt% THF
Figure 2: Hydrate dissociation and formation temperatures as a function of THF weight concentration, at 1 atm.
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Figure 2 shows our measurements of hydrate formation and hydrate dissociation tem-
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peratures as a function of the THF weight concentration. We also plot the dissociation 9 ACS Paragon Plus Environment
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temperature results obtained by Dyadin et al., 18 for comparison purposes. The data per-
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taining to hydrate formation were obtained in the rheometer with the fluid at rest. For each
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concentration, the sample was loaded to the rheometer and the temperature was held con-
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stant at some value. If no hydrates were formed, the temperature was changed to a slightly
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lower value, and so on, until a temperature was reached at which hydrates were formed.
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The data pertaining to hydrate dissociation were also obtained in the rheometer, by
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simultaneously imposing to the sample a temperature ramp and small amplitude oscillations
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at constant frequency and amplitude (time sweep). A dramatic change in G′ and G′′ was
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observed when the dissociation temperature was reached.
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It can be noted that our dissociation temperature results are in fair agreement with the
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results obtained by Dyadin et al., 18 except at 19% THF concentration. Both the dissociation
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and the formation temperature increase with the THF concentration until a maximum is
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reached around 19% (when there is no excess of water or THF), beyond which the dissociation
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and formation temperatures decrease with the concentration. Another observation is that
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the formation temperature is always lower than the dissociation temperature, probably due
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to the existence of metastable states.
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In Fig. 3 we present results obtained with constant shear rate experiments, for 30% and
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40% THF weight concentrations. These experiments consisted of setting the temperature to
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1◦ C, loading the sample, and then starting the sample at a constant shear rate. Each curve
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pertains to a different shear rate, as indicated in the legend. Since the process of hydrate
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formation is exothermic, it can be observed that during hydrate formation the Peltier system
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is unable to maintain the temperature at 1◦ C. Therefore, the temperature peaks observed
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correspond to hydrate formation. Assuming that the maximum rate of heat removal of the
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Peltier system is fixed, then the height and width of the peaks should be proportional to
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2 (a)
. -1 !=10 s -1 . !=50 s -1 . !=100 s . -1 !=300 s . -1 !=500 s . -1 !=1000 s
0
T ( C) 1.5
1
0.5 1
10
100
1000
t (s) 2 (b) . -1 !=10 s . -1 !=100 s . -1 !=300 s . -1 !=500 s . -1 !=1000 s
0
T ( C) 1.5
1
0.5 1
10
100
1000
t (s)
Figure 3: Temperature peaks during hydrate formation. (a) 30 wt% THF, and (b) 40 wt% THF. 178
the amount of latent heat emitted per unit time, and hence to the rate of hydrate formation
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(mass of hydrates per unit time). Note that the total amount of latent heat is proportional
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to the total mass of hydrates formed. The rate of hydrate formation clearly increases as
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the shear rate is increased, indicating that the process is limited by heat and mass transfer
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which increase with the shear rate. Moreover, the higher the shear rate, the sooner the
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hydrates start to form. These trends are also illustrated in Fig. 4, which shows the peak
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temperature and the time that it occurs, for 30% and 40% THF concentrations. It also
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1000
1.6 Time
Temperature
30 wt% THF
30 wt% THF
40 wt% THF
40 wt% THF
1.5 0
T ( C)
800
t (s)
1.4 600 1.3 400 1.2 200
1.1
1
0 10
100
1000 .
-1
! (s )
Figure 4: Peak temperature (filled symbols) and time when it occurs (open symbols) as a function of the shear rate. 185
illustrates that at lower shear rates the hydrate formation process begins sooner for the 30%
186
THF concentration, but as shear rates increases this difference tends to disappear.
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It is also noted in Fig. 4 that the peak temperatures are higher for the 30% THF con-
188
centration than for the 40% THF concentration, indicating that more hydrates form at 30%
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THF concentration. This is so because, for THF concentrations above 19% (i. e. excess in
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THF), the total amount of hydrates formed is defined by the amount of water available in the
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mixture, because once all the water is consumed the hydrate formation process stops. Since
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there is proportionally more water in the 30% THF concentration mixture, more hydrate
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formation is expected at 30% THF concentration than at 40% concentration.
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Apparent wall slip
195
Apparent wall slip1 can be an issue in the rheometric measurements, as it is the case for
196
slurries in general. Figure 5 shows the results of tests performed to verify the presence of 1
when a layer of the continuous phase forms at the wall
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apparent wall slip.
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In these tests, the viscosity was measured using two different rheometers (DHR-3 and
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MCR01), and with smooth and rough Couette geometries (see Fig. 1). Figure 5(a), which
200
pertains to γ˙ = 10s−1 , shows that the smooth geometries give the same asymptotic value
201
of viscosity, which is much lower than the one obtained with the roughened geometry. This
202
indicates the presence of apparent wall slip at this small shear rate when the smooth ge-
203
ometry is employed. Figure 5(b) shows the results obtained for γ˙ = 100s−1 . Regarding
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the asymptotic viscosity values, the same trend observed for γ˙ = 10s−1 is also observed for
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γ˙ = 100s−1 , except that the difference in asymptotic values is much smaller. When the shear
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rate is further increased to γ˙ = 500s−1 (Fig. 5(c)), the asymptotic viscosity values obtained
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with smooth and roughened surfaces become essentially the same. This indicates that ap-
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parent wall slip occurs in the low shear rate range only, similarly to what is observed in other
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slurries, pastes and gels.
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It is interesting to note in Fig. 5 that at early times the viscosity evolution curves display
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a rather different behavior, indicating that the process of hydrate formation is different in
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the smooth and roughened surfaces. Our results suggest that with the roughened surfaces
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the hydrate formation is delayed, in contrast to the observations of Ahuja et al.. 12
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Another observation is that the data obtained with the smooth surfaces are more noisy
215
than the ones obtained with the rough surfaces. This may be related to the occurrence of
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the stick-slip phenomenon 19 during the formation process.
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Viscosity and THF concentration
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Figure 6 shows the viscosity time evolution during hydrate formation, for γ˙ = 500s−1 and
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three THF weight concentrations, namely 30%, 35% and 40%. These results are obtained
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1000 (a) 100
! (Pa.s) 10
1
30 wt% THF . "=10 s-1
0.1
DHR-3 rough DHR-3 smooth MCR01 smooth
0.01
0.001 0
500
1000
1500
2000
t (s) 1000 (b) DHR-3 rough DHR-3 smooth MCR01 smooth
30 wt% THF . "=100 s-1
100
! (Pa.s) 10
1
0.1
0.01
0.001 0
500
1000
1500
2000
t (s) 10 30% THF . -1 "=500 s 1
! (Pa.s) 0.1
MCR01 rough
0.01
MCR01 smooth
0.001 0
400
800
1200
1600
t (s)
Figure 5: Viscosity evolution for T = 1◦ C and 30% THF. (a) γ˙ = 10 s−1 , (b) γ˙ = 100 s−1 , and (c) γ˙ = 500 s−1 .
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% THF by wt 30% 35% 40%
% mass of hydrate 86.42% 80.25% 74.07%
% mass of excess THF 13.58% 19.75% 25.93%
Table 1: Percent masses of hydrates and excess THF. 226
is non-monotonic because the hydrate crystals tend to agglomerate on the one hand, but on
227
the other hand the agglomerates tend to be broken due to shearing, in a randomic process.
228
However, a steady state regime is eventually reached when hydrate formation ends and the
229
rates of buildup and breakdown become equal.
230
For these mixtures that possess excess THF (because the THF weight concentration is
231
above 19%), the hydrate formation ends when all water is consumed and a hydrate slurry is
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formed, whose continuous phase is THF. The percent mass of hydrate and excess THF are
233
given in Table 1 for the three THF weight concentrations analyzed. The viscosity is higher for
234
the 30% THF concentration—which presents the higher hydrate content—and decreases as
235
the hydrate content decreases (or equivalently, as the THF weight concentration increases).
236
The hydrate content dependence on the THF weight concentration can be directly ob-
237
served in Fig. 7, which provides a visualization of the hydrate slurries at different THF
238
concentrations right after the tests. For the 30% THF concentration, 54.73% of the initial
239
THF mass is used to form hydrates. The remaining THF mass (excess THF) represents only
240
13.58% of the total mass of the mixture, and the result is a solid-like, brittle slurry. On the
241
other hand, the slurries corresponding to the higher THF weight concentrations present a
242
gel-like behavior due to a larger excess THF, which is 19.75% and 25.93% of the total mass
243
for the 35% and 40% THF concentrations, respectively.
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Constant shear rate tests
245
The effect of shear rate on hydrate formation is presented in Fig. 8. All the results are
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obtained with the MCR01 with the smooth Couette geometry. The sample is loaded to the
247
rheometer at T = 1◦ C and kept at rest for 1 hr. A small perturbation is introduced to the
248
system to trigger hydrate formation 5 min before the end of this rest hour. Subsequently a
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constant shear rate is imposed.
250
It can be noted that, for all THF concentrations, a steady state regime is reached for
251
the higher shear rates, but not quite for the lower shear rates. It is well known from the
252
literature that the time to reach steady state increases as the shear rate decreases, due to
253
a slower rearrangement of the microscopic state. 20 This behavior is also observed for the
254
hydrate slurries, but in the present case it seems that it is more prominent since the steady
255
state regime is barely reached for γ˙ = 10 and 50 s−1 . This is probably due to the unstable
256
balance between breakdown and buildup of hydrate crystals, and/or due to wall slip effects.
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Flow curves
258
Flow curves were obtained with the MRC01 and the roughened Couette geometry. The tests
259
used to obtain these curves are similar to the ones described in the previous tests, namely the
260
sample is loaded to the rheometer at T = 1◦ C and kept at rest for 1 hr. A small perturbation
261
is introduced to the system to trigger hydrate formation 5 min before the end of this rest
262
hour. After the rest period the first (constant) shear rate is imposed for 30 min, when the
263
viscosity has already attained a steady value. To build the flow curve, the shear rate is then
264
changed (either increased or decreased) to the next neighboring value and kept for other
265
30 min to ensure that another steady viscosity value is attained, and so on until the desired
266
shear rate range is covered. These measurements with subsequent shear rate values are made 17 ACS Paragon Plus Environment
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100 (a)
30 wt% THF 0
T=1 C 10
" (Pa.s) 1
0.1
. -1 . -1 . -1 !=100 s . -1 !=300 s . -1 !=500 s . -1 !=10 s !=50 s
0.01
!=1000 s
0.001 0
500
1000
1500
2000
2500
t (s) 100 (b)
35 wt% THF 0
T=1 C 10
" (Pa.s) 1
0.1 .
-1
.
-1
.
-1
!=100 s !=300 s
0.01
!=500 s
.
!=1000 s
-1
0.001 0
500
1000
1500
2000
2500
t (s) 100 (c) 40 wt% THF 0
T=1 C
10
" (Pa.s) 1
0.1
.!=10 s-1 .!=100 s-1 .
-1
!=300 s
0.01
.!=500 s-1 .!=1000 s-1 0.001 0
500
1000
1500
2000
2500
t (s)
Figure 8: Viscosity evolution at constant shear rate, for T = 1◦ C. γ˙ = 10, 100, 300, 500, and 1000s−1 . (a) 30 wt% THF, (b) 35 wt% THF, and (c) 40 wt% THF.
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267
with the same sample. However, a different sample is employed for each “direction” (shear
268
rate up or shear rate down). For all three methods to obtain the flow curve, it is noted that
269
the repeatability is very good.
270
The flow curves thus obtained are presented in Fig. 9. The solid symbols correspond to
271
the results measured by increasing the shear rate values (from γ˙ = 10 s−1 to 1000 s−1 ). The
272
open symbols correspond to the results obtained by decreasing the shear rate values (from
273
γ˙ = 1000 s−1 to 10 s−1 ). The dotted line represents the results obtained with the smooth
274
Couette geometry for shear rates equal to γ˙ = 10; 100; 300; 500 and 1000 s−1 , with a fresh
275
sample for each shear rate (these steady state values are taken from the results shown in
276
Fig. 8).
277
For the low 30% THF weight concentration, the flow curves obtained with the three
278
different methods differ a great deal, the curve pertaining to the shear rate up procedure
279
falling below the two other curves. In addition, the shear rate up curve is non-monotonic
280
with the shear rate, in contrast to the two other curves. However, at the high shear rate end
281
the three flow curves merge. The shear rate down curve falls above the other two, and the
282
curve obtained by shearing the sample from rest is closer to the shear rate down curve, but
283
merges with the shear rate up curve in the low shear rate end.
284
These differences between the flow curves are clearly related to the different shear histories
285
to which the samples are subjected in each case. The shear history is expected to directly
286
affect the size and size distribution of the hydrates crystals and their aggregates, which in
287
turn have a direct impact on viscosity.
288
Recalling that the slurry changes from a brittle to a gel-like behavior as the THF con-
289
centration increases, it seems that the process of formation and breakage of aggregates has
290
less impact on viscosity for higher THF concentrations (35% and 40% wt). The qualitative
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1000 (a) 100
! (Pa.s) 10
1
0.1 increasing shear increasing shear increasing shear decreasing shear decreasing shear decreasing shear shear from rest
30 wt% THF 0.01
T=10C
0.001 10
100
1000
. -1 " (s )
1000 increasing shear increasing shear increasing shear decreasing shear decreasing shear decreasing shear shear from rest
100
! (Pa.s) 10
(b)
1
0.1 35 wt% THF 0.01
0
T=1 C
0.001 10
100
1000
. -1 " (s )
1000 (c) increasing shear increasing shear increasing shear decreasing shear decreasing shear decreasing shear shear from rest
100
! (Pa.s) 10
1
0.1 40 wt% THF 0.01
T=10C
0.001 10
100
. -1 " (s )
1000
Figure 9: Flow curve at T = 1◦ C. The solid symbols correspond to the shear rate up test, and the open symbols correspond to the shear rate down test. The dotted line follows data points (crosses) each of which was obtained with a fresh sample by applying the corresponding shear rate right after the one-hour rest period. (a) 30 wt% THF, (b) 35 wt% THF, and (c) 40 wt% THF. 20 ACS Paragon Plus Environment
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Energy & Fuels
1000 30 wt% THF
100
35 wt% THF
! (Pa.s)
40 wt% THF
10
1
0.1
0
T=1 C Decreasing shear rate
0.01
0.001 10
100
. -1 " (s )
1000
Figure 10: Shear rate down flow curve for the 30%, 35%, and 40% THF weight concentrations, at T = 1◦ C. 291
results are similar but the differences between the curves obtained with the three different
292
shear histories is smaller.
293
Figure 10 shows a comparison between the shear rate down flow curves for the three THF
294
weight concentrations. It is noted that as the THF concentration increases the viscosity
295
decreases and becomes less shear thinning, as expected when a slurry becomes more dilute.
296
Yield strength
297
To investigate the existence of a yield strength we perform creep tests for all three slurries,
298
using the MRC01 with the roughened Couette geometry. As in the other cases, the creep tests
299
begin with the sample left at rest for 1 hr at T = 1◦ C, and introducing a small perturbation
300
to the system to trigger hydrate formation t = 5 min before the end of the one hour rest
301
period. Subsequently a shear rate of 500 s−1 is imposed for 30 min, when a steady viscosity
302
is attained. A constant shear stress is then imposed. This test is repeated with fresh samples
303
each time for several shear stress values, and the shear rate evolution is observed. The curves
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Page 22 of 27
304
pertaining to stresses below the yield strength evolve to zero shear rates, while the curves
305
pertaining to stresses above the yield strength evolve to steady finite shear rates. Thus the
306
yield strength is taken as the average value between the two closest stresses whose shear rate
307
evolution is different, i. e. one evolves to zero and the other evolves to a finite value. The
308
creep curves obtained (given in the supplementary material) indicate that all slurries possess
309
a yield strength. The values obtained are equal to 415 Pa for the 30%, 398 Pa for the 35% and 16 Pa for the 40% of THF concentration. 1000
! (Pa) y
100
35 wt% THF 40 wt% THF
10 0
20
40
60
80
100
120
t (min)
Figure 11: Yield strength as a function of the rest time for the 35% and 40% THF weight concentrations, at T = 1◦ C. 310
311
We also investigated the effect of aging on yield strength. To this end, we performed creep
312
tests in which different rest times were observed between the end of the 500s−1 pre-shearing
313
and the imposition of the shear stress.
314
It can be observed from Fig. 11 that for the 40% THF concentration the yield strength
315
remains equal to 16 Pa, being unaffected by the rest time, indicating a stable microstructure.
316
However, for the 35% THF concentration, the yield strength clearly increases with the rest
317
time, indicating a continuous hardening of the microstructure.
318
We could not obtain results for the 30% THF concentration, because in this case the 22 ACS Paragon Plus Environment
Page 23 of 27
319
hardening process is fast and dramatic. For rest times as short as 5 min, the yield strength
320
already attains values beyond the measurement capability of the rheometer.
321
Stress sweep tests
106 105
T=10C f = 1 Hz
104
G', G'' (Pa)
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1000 100 10
G' (Pa) - 30% THF G'' (Pa) - 30% THF G' (Pa) - 35% THF
1
G'' (Pa) - 35% THF G' (Pa) - 40% THF G'' (Pa) - 40% THF
0.1 0.01 0.1
1
10
100
1000
Shear stress (Pa)
Figure 12: Storage and loss moduli for the 30%, 35% and 40% THF weight concentrations, at T = 1◦ C.
322
Finally, Fig. 12 shows the results of stress sweep tests. The storage and loss moduli
323
are given for the 30%, 35%, and 40% THF weight concentration, at T = 1◦ C. It can be
324
noted that the storage modulus is larger than the loss modulus for the 30% and 35% THF
325
concentrations, indicating that elasticity is present in these slurries while within the linear
326
viscoelastic limit (below the yield strength). However, the more diluted slurry, namely the
327
one with 40% THF concentration, possesses a lower level of elasticity, since G′ < G′′ .
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328
Final Remarks
329
In this work we performed a comprehensive experimental study to understand the rheology of
330
hydrate slurries formed at ambient pressure in mixtures of water and THF. Rheological tests
331
were carried out for three different THF concentrations above 19%, which is the concentration
332
at which all THF and water are used up to form hydrates.
333
Transient results show that hydrate formation depends on the rheometer geometry, shear
334
rate and shear history. The yield strength is also obtained via creep tests, and elasticity is
335
evaluated with oscillatory stress sweep tests. The steady state results show that the flow
336
curve is strongly affected by the shear history. A good repeatability is obtained for all
337
tests. The hydrate slurries are shown to be quite complex fluids, presenting a shear history
338
dependent elasto-viscoplastic behavior.
339
Acknowledgement
340
This work was possible due to the financial support of Petrobras, CNPq, CAPES, FAPERJ,
341
FINEP, and MCT.
342
References
343
344
345
346
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