Energy Fuels 2010, 24, 616–623 Published on Web 11/16/2009
: DOI:10.1021/ef900797e
Hydrate Precursor Test Method for the Laboratory Evaluation of Kinetic Hydrate Inhibitors Christophe Duchateau,† Philippe Glenat,‡ Tong-Eak Pou,§ Manuel Hidalgo,§ and Christophe Dicharry*,† † Thermodynamique et Energ etique des Fluides Complexes, UMR CNRS 5150, Universit e de Pau et des Pays de l’Adour, BP 1155, 64013 Pau Cedex, France, ‡Total CSTJF, Avenue Larribau, 64018 Pau Cedex, France, and §Arkema-Ceca, CRRA, Rue Henri Moissan, BP 63, 69493 Pierre-B enite, France
Received July 27, 2009. Revised Manuscript Received October 23, 2009
Because of the stochastic character of hydrate crystallization, the evaluation of kinetic hydrate inhibitor (KHI) performance, using high-pressure autoclaves, is subject to the substantial dispersion of experimental results. To overcome this problem, we proposed in a previous paper (Duchateau et al. Energy Fuels 2009, 23 (2), 962-966) a testing procedure based on “water memory”. The residual structures remaining in solution after a prior hydrate formation/dissociation cycle act as hydrate precursors in the subsequent formation, which yields less-scattered results. The present work shows that the comparison between the subcoolings ΔTsub,lim (difference between the hydrate equilibrium temperature and the crystallization temperature at hydrate reformation when a constant cooling rate is applied), measured with and without an additive at hydrate reformation is a convenient way to evaluate the potential of the additive to inhibit hydrate growth. It would also appear to be possible to rank the KHIs when the values of ΔTsub,lim are compared. On the other hand, the comparison of the KHI effectiveness in terms of hold time (difference between the onset of catastrophic hydrate growth and the time at which the test solution enters the hydrate stability domain) at subcoolings lower than ΔTsub,lim using the precursor test method should be made with caution.
possible extended field lifetime are economic drivers for choosing LDHIs instead of other hydrate prevention methods. These chemicals, usually injected at a concentration of less than 1-2 wt % of the water phase, are subdivided into two categories: (i) kinetic hydrate inhibitors (KHIs) and (ii) antiagglomerants (AAs). KHIs delay hydrate nucleation and/or hydrate growth, whereas AAs prevent hydrate particles from agglomerating and accumulating into large masses. Although the first successful field trials on LDHIs were reported in the mid 1990s,3 their presence in processing equipment has remained modest.4,5 If operators are more confident using THIs than LDHIs, it is primarily because thermodynamic models (such as the CSMGem and HWHyd programs) can quickly and accurately predict the hydrate equilibrium temperature depression in the presence of these additives. Once the amount of THI necessary to prevent hydrate formation is determined, it suffices to introduce it into the installation and the problem will be solved. In contrast, quantifying the effectiveness of chemicals that enable a production system to operate safely inside the hydrate stability region is a complex task. From an experimental point of view, the lack of consensus regarding an appropriate protocol for qualifying LDHIs does not encourage operators to use these chemicals. This is particularly the case when the use of KHIs is being considered because the stochastic nature of hydrate nucleation leads to scattered
1. Introduction Gas hydrates are nonstoichiometric crystalline compounds that can form at temperatures and pressures commonly encountered in oil and gas production. They are composed of host cages of hydrogen-bonded water molecules that have encaged nonpolar guest molecules of suitable size and shape, which contribute to stabilizing the cage-like crystal lattice. Their formation is associated with important industrial and environmental concerns for the oil and gas industry. Hydrates can block flow lines, valves, wellheads, and pipelines, causing huge production losses and posing serious safety problems.1 Nowadays, the most frequently used method for preventing hydrate plug formation consists in shifting the equilibrium for hydrate formation toward more severe P-T conditions (higher pressure and lower temperature) by adding chemicals called thermodynamic hydrate inhibitors (THIs).2 However, large quantities of these chemicals are necessary to prevent hydrate formation (20-60 wt % of the water phase), and using them involves processing wastewater. Over the last two decades, the combination of the high costs (storage, transportation, etc.) of these chemicals and the environmental concerns they inspire have sparked interest in another kind of chemicals called low dosage hydrate inhibitors (LDHIs).3 Significant savings in both operating expenditure (OPEX) and capital expenditure (CAPEX) and *To whom correspondence should be addressed. Telephone: þ33-559-40-76-82. Fax: þ33-5-59-40-76-95. E-mail: christophe.dicharry@ univ-pau.fr. (1) Hammerschmidt, E. G. Ind. Eng. Chem. 1934, 26, 851–855. (2) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases; 3rd ed., CRC Press, Taylor & Francis Group: New York, 2008. (3) Kelland, M. A. Energy Fuels 2006, 20 (3), 825–847. r 2009 American Chemical Society
(4) Mehta, A. P.; Herbert, P. B.; Cadena, E. R.; Weatherman, J. P. In Proceedings of the Offshore Technology Conference, Houston TX, 2002; OTC 14057. (5) Thieu, V.; Frostman, L. M. In Proceedings of the International Symposium on Oilfield Chemistry, Houston TX, 2005; SPE 93450.
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Energy Fuels 2010, 24, 616–623
: DOI:10.1021/ef900797e
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Figure 1. High-ressure cells apparatus.
results and can therefore mask the “true” performance of the inhibitor. In a previous work,6 we proposed an experimental procedure for the laboratory evaluation of KHIs that consists in quantifying their effectiveness on systems that have previously experienced hydrate formation and decomposition. We showed that water history can considerably enhance the reproducibility of results and facilitate clear discrimination between blank (uninhibited system) and KHI test results. This paper evaluates the potential of the proposed hydrate precursor test method. The effect of the conditions applied to decompose hydrates on the reproducibility of results was extensively investigated, and the conditions that enable the KHIs to be ranked for effectiveness were discussed.
2.2. Apparatus. The apparatus used was the same as in our previous work.6 It consisted basically of four high-pressure stirred cells immersed in a temperature-controlled, insulated bath (Figure 1). Cell temperature (Tcell) was measured using PT100 sensors with an accuracy of (0.2 °C. Cell pressure (Pcell) was measured with 250 bar full-scale transducers with a precision of 0.3% FSD. Tcell and Pcell were collected on a personal computer using a SpecView data acquisition program at intervals of 1 min. 2.3. Experimental Procedure. The cells were loaded with a mixture of 12 cm3 of the aqueous solution previously prepared at the selected concentration of KHI and 28 cm3 of condensate, and pressurized at 130 bar at 19.5 °C. The fluids were stirred at 550 rpm for at least 2 h to saturate the condensate with gas, while the gas pressure was regularly adjusted to remain at 130 bar. When the Pcell reached a steady-state condition, the cell valves were closed and the experiments were then conducted under isochoric conditions. A typical experiment was divided into three main stages as shown in Figure 2. The first stage consisted in inducing hydrate formation in a system that had never experienced hydrate formation. The Tcell was decreased from Tinit (outside the hydrate stability domain) to a targeted temperature Ttarg within the hydrate stability domain. For all the experiments, a time of 45 ( 5 min was set to reach Ttarg and the stirring speed was 550 rpm. The onset of hydrate formation (“catastrophic hydrate growth”) was indicated by a sudden temperature increase in Tcell due to the exothermic character of hydrate crystallization. Thirty minutes after hydrate formation commenced, fluid stirring was stopped, to maintain as far as possible the same experimental conditions in each cell. Without any agitation, hydrate growth stopped or radically slowed. Under these conditions, almost the same amount of hydrate was expected to
2. Experimental Section 2.1. Materials. The hydrate-forming gas was a binary mixture of 98 mol % methane and 2 mol % propane supplied by Linde. It was expected to form structure II (sII) hydrates. The oil phase was a degasified condensate from the Frigg field (North Sea) sampled during a production phase with no chemical treatment. Deionized water was produced by the Millipore Milli-Q 185 E system (conductivity PVCap > PVP > KHI B > KHI A > uninhibited system. Note that the ranking obtained for PVCap and PVP is consistent with results found in literature.2,22 Although the
dispersion of the ΔTsub,lim for KHI A increased, the same ranking can be considered obtained for the ΔTsup of 3.6 °C. For the ΔTsup of 5.6 °C, the increase of both ΔTsub,lim and its dispersion for some KHIs does not enable all the KHIs to be compared one to another, but some comparisons are still possible. For higher ΔTsup, no ranking is possible. High subcooling is known to reduce the scattering of results, and hydrate nucleation may then appear as less stochastic.23 To demonstrate that the good reproducibility of results at hydrate reformation was due to a great extent to water history and not only to the degree of subcooling applied to the systems, we plotted in Figure 7 the hold time, thold as a function of ΔTsub for uninhibited and inhibited systems. In the experiments, the superheating ΔTsup applied to decompose hydrates was 1.7 °C and the subcoolings investigated varied from 2.5 to 13.5 °C depending on the system. As expected, for a given system, thold was found to increase with decreasing ΔTsub. The dispersion of thold relative to its mean value was usually found to be lower than 40% and in most cases lower than 20%. Note that in our previous work6 we found a relative dispersion greater than 170% for thold measured at first hydrate formation for an inhibited system at a subcooling of 9.8 ( 0.3 °C. Although the water history can undoubtedly reduce the scattering of results, the decrease in the dispersion of thold observed at increased subcooling shows that a higher driving force has a positive influence on the reproducibility of results. It will be noticed that the KHIs (KHI C and PVCap) showing the best performance in Figure 7 gave the highest displacement of the final hydrate dissociation temperature (þ 5 and þ 4.5 °C, respectively). Although the latter probably testifies to tight interactions between KHI and hydrate
(22) Varma-Nair, M.; Costello, C. A.; Colle, K. S.; King, H. E. J. Appl. Polym. Sci. 2007, 103, 2642–2653.
(23) Bishnoi, P. R.; Natarajan, V. Fluid Phase Equilib. 1996, 117, 168– 177.
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Energy Fuels 2010, 24, 616–623
: DOI:10.1021/ef900797e
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Figure 7. Curves of performance (subcooling vs hold time) for inhibited and uninhibited systems.
Figure 9. (a) Comparison of the performance curves for the PVP and PVCap-inhibited systems obtained (a) after applying different superheating to decompose hydrates and (b) after applying similar “real” superheating to decompose hydrates. Figure 8. Effect of the superheating applied to decompose hydrates on the performance curve of the PVCap-inhibited system.
surface, no direct correlation was found between its degree and the level of protection against hydrate growth. The influence of the superheating applied to decompose hydrates on the “curve of performance” (ΔTsub vs thold) was also investigated (Figure 8). For a given system, the general trend observed is that the performance curves are almost superimposed when low ΔTsup are applied. For increasing ΔTsup, the curves progressively separate, first at low subcoolings and then at higher ones, and the reproducibility of tind decreases. From Table 1 and Figure 8, the performance curves do not depend on ΔTsup as long as the pressure difference, ΔPAD, is greater than 0.3 bar, that is, nonmelted hydrates are present at the end of the decomposition stage. Why the performance curves of the PVCap-inhibited systems are the same for a wide range of applied ΔTsup or, equivalently, for different amounts of nonmelted hydrates left in the system is not clear. It is possible that the PVCap molecules adsorbed on the surface of nonmelted hydrates prevent the immediate growth of hydrate particles when the system reenters the hydrate stability domain. This would be consistent
with the observations of Larsen et al.24 on inhibited crystals transferred back into uninhibited solutions. For low enough applied ΔTsup, the presence of such hydrate precursors (inhibited nonmelted hydrates) in large quantities would make crystallization almost deterministic. For higher applied ΔTsup, fewer hydrate precursors are present in the system and they are probably more inhibited due to the larger amount of KHI available in solution for adsorption at the hydrate surface. This may explain why in this case both tind and its dispersion increased. The question that arises from these results is: can the performance curves be used to compare and rank the KHIs at different subcoolings? From our point of view, the answer is yes, but the comparison should be made with caution. In fact, depending on the temperature chosen to decompose hydrates, some systems may contain nonmelted hydrates and others not. This difference can affect the relative position of the performance curves, which eventually may cross each other (Figure 9a), which is not satisfactory. This problem should be avoided if a low and positive real ΔTsup, which requires the knowledge of the hydrate equilibrium temperature in the presence of the tested KHIs, is applied to
(24) Larsen, R.; Knight, C. A.; Sloan, E. D. Fluid Phase Equilib. 1998, 150-151, 353–360.
(25) Gayet, P.; Dicharry, C.; Marion, G.; Graciaa, A.; Lachaise, J; Nesterov, A. N. Chem. Eng. Sci. 2005, 60, 5751–5758.
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: DOI:10.1021/ef900797e
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decompose hydrates, taking care in addition to reach almost the same small ΔPAD before applying the hydrate reformation stage. Figure 9b shows the curves of performance for the PVP and PVCap-inhibited systems, determined for a real ΔTsup of about 1 °C (more precisely 1.2 and 0.6 °C, respectively) and ΔPAD of 0.2-0.3 bar. Under these conditions, unambiguous discrimination between both inhibited systems is possible at all conditions of subcooling.
sensitive to variations in the procedure (changes of Td) than hold time measurements at a fixed degree of subcooling. Measuring ΔTsub,lim allows a rapid and unambiguous differentiation between blanks (uninhibited systems) and inhibited systems, and also enables the KHIs to be ranked according to their effectiveness in delaying hydrate growth. Comparisons of the KHIs’ performances at subcoolings lower than ΔTsub,lim should be undertaken with caution, especially for KHIs with relatively similar ΔTsub,lim. In addition to the possibility of evaluating the potential of a molecule to act as a KHI and/or comparing the performance of different KHIs, this protocol could be a useful tool for controlling the quality of KHI batches, a point that should be of interest to both suppliers and users of KHIs.
4. Conclusion Water memory can be used to obtain favorable conditions for evaluating KHI. Good reproducibility of results at hydrate reformation is obtained when the temperature Td, at which the samples are held to decompose hydrates after a prior hydrate formation, is in the vicinity of the hydrate equilibrium temperature, which is KHI-dependent for certain systems. The subcooling ΔTsub,lim, measured using a constant cooling rate, was found to be a variable less stochastic and less
Acknowledgment. The authors wish to thank Arkema/Ceca for their financial support and all the members of the KHI working group for fertile discussions.
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