962
Energy & Fuels 2009, 23, 962–966
Laboratory Evaluation of Kinetic Hydrate Inhibitors: A Procedure for Enhancing the Repeatability of Test Results Christophe Duchateau,*,† Jean-Louis Peytavy,‡ Philippe Gle´nat,‡ Tong-Eak Pou,§ Manuel Hidalgo,§ and Christophe Dicharry† Laboratoire des Fluides Complexes, UMR CNRS 5150, UniVersite´ 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-Be´nite Cedex, France ReceiVed August 27, 2008. ReVised Manuscript ReceiVed NoVember 28, 2008
Evaluation of kinetic hydrate inhibitors (KHIs) in laboratory high-pressure cells generally yields scattered results. In this work we propose a procedure that consists of characterizing the effectiveness of KHI on second hydrate formation. This procedure limits the stochastic character of hydrate crystallization using the persistence of precursory hydrate structures in water that has previously experienced hydrate formation and decomposition. It is shown that the presence of these precursors strongly increases the repeatability of results compared with systems containing “fresh water” and allows unambiguous discrimination between blank (uninhibited system) and KHI tests.
1. Introduction Gas hydrates can form at pressures and temperatures commonly encountered in oil and gas production systems.1 They can block flow lines, valves, wellheads, and pipelines, causing huge production losses and posing safety problems. The conventional treatment for prevention of hydrate plug formation is based on injection of thermodynamic hydrate inhibitors (THIs) such as methanol or ethylene glycol, which shift the hydrate equilibrium conditions to higher pressures and lower temperatures. As the search for oil and gas goes into colder and/or deeper regions, the quantity of THI required to prevent installations from plugging increases, thereby inducing prohibitive expenses.2 Injection of low-dosage hydrate inhibitors can be an alternative and cost-effective method of hydrate prevention.3,4 This class of molecules can be separated into two groups: antiagglomerants (AAs) and kinetic inhibitors (KHIs). AAs are surfactants that allow formation of transportable hydrate slurry, whereas KHIs are polymers with surfactant properties which delay hydrate nucleation and/or crystal growth. Only KHIs are discussed in this paper. Before being used on production fields, KHIs have to be evaluated in the laboratory.5 Generally, oil companies use flow * To whom correspondence should be addressed. Phone: +33-5-59-4076-86. Fax: +33-5-59-40-76-95. E-mail:
[email protected]. † Universite ´ de Pau et des Pays de l’Adour. ‡ TOTAL. § Arkema-Ceca. (1) Sloan, E. D. Clathrate hydrates of natural gases, 2nd ed.; Marcel Dekker: New York, 1998. (2) Sloan, E. D. Fluid Phase Equilib. 2005, 228-229, 67–74. (3) Phillips, N. J.; Kelland, M. A. The application of surfactants in preventing gas hydrate formation. In Industrial application of surfactants IV; Proceedings of the 4th Congress on Industrial Application of Surfactants, Cambridge, 1998. (4) Varma-Nair, M.; Costello, C. A.; Colle, K. S.; King, H. E. J. Appl. Polym. Sci. 2007, 103 (4), 2642–2653. (5) Leporcher, E. M.; Fourest, J. M.; Labes-Carrier, C.; Lompre, M. Multiphase transportation: a kinetic inhibitor replaces methanol to prevent hydrates in a 12-inc. pipeline. In Proceedings of the European Petroleum Conference, The Hague, 1998.
loops to simulate field conditions.6 Because such equipment is rather expensive to run, KHI suppliers prefer using laboratory high-pressure cells to evaluate the effectiveness of their additives. However, this more convenient and cost-effective way for testing KHIs generally yields scattered results due to the stochastic nature of hydrate crystallization.7 Improving the repeatability of high-pressure cell test results is necessary to increase confidence in the evaluation of KHIs and develop better KHIs. In this paper, we propose an experimental procedure for laboratory evaluation of KHIs that allows producing repeatable results. This procedure is based on use of the residual hydrate structures remaining in a water phase that has previously experienced a hydrate formation/decomposition cycle.8-10 The procedure is derived from the protocol that has been implemented by Total for 15 years in their flow loop.11 2. Experimental Section 2.1. Chemicals. The feed gas was a mixture of 98 mol % methane and 2 mol % propane supplied by Linde. This binary gas mixture is predicted to form structure II hydrates. The hydrate phase diagram12 for this gas mixture and for the range of pressures and temperatures of interest for this study is shown in Figure 1. The oil phase was a degasified condensate from the Frigg field (North Sea) sampled during a no treatment production phase. (6) Peytavy, J. L.; Gle´nat, P.; Bourg, P. IPTC 11233. International Petroleum Technology Conference, Dubai, Dec 4-6, 2007. (7) Kashchiev, D.; Firoozabadi, A. J. Cryst. Growth 2002, 243 (3-4), 476–489. (8) Vysniauskas, A.; Bishnoi, P. R. Chem. Eng. Sci. 1983, 38 (7), 1061– 1072. (9) Herri, J. M.; Gruy, F.; Pic, J. S.; Cournil, M.; Cingotti, B.; Sinquin, A. Chem. Eng. Sci. 1999, 54 (12), 1849–1858. (10) Lee, J. D.; Susilo, R.; Englezos, P. Chem. Eng. Sci. 2005, 60 (15), 4203–4212. (11) Peytavy, J. L.; Gle´nat, P.; Bourg, P. Qualification of low dose hydrate inhibitors (LDHIS): Field cases studies demonstrate the good reproducibility of the results obtained from flow loops. 6th International Conference on Gas Hydrates, Vancouver, 2008. (12) Tohidi, B.; Danesh, A.; Todd, A. C. Chem. Eng. Res. Des. 1995, 73 (A4), 464–472.
10.1021/ef800710x CCC: $40.75 2009 American Chemical Society Published on Web 01/13/2009
EValuation of Kinetic Hydrate Inhibitors
Figure 1. CH4 and (98 mol % CH4/2 mol % C3H8)-hydrate equilibrium curves (without condensate) calculated with the HWHYD model,12 and experimental hysteresis curve for the (98 mol % CH4/2 mol % C3H8)/ water/condensate system.
Energy & Fuels, Vol. 23, 2009 963 solutions were sometimes colored but always had a homogeneous aspect. The cells were pressurized to 13 MPa, Tinit ) 19.5 °C, and the fluids were stirred at 550 rpm for at least 2 h to saturate the condensate with gas. When the cell pressure, Pcell, reached a steadystate condition, the gas pressure was readjusted to 13.0 ( 0.1 MPa. The gas inlet valves between the cells and the reservoirs were then closed, and all experiments were done under isochoric conditions. In a typical hydrate formation experiment Tcell was decreased from Tinit (outside of the hydrate stability domain) to a targeted temperature, Ttarg, located within the hydrate stability domain. For all experiments the time required to reach Ttarg was set to 45 ( 5 min. The stirring speed was maintained at 550 rpm during the experiments. Hydrate formation was detected by a sharp increase in Tcell due to the exothermic character of hydrate crystallization. Thirty minutes after hydrate formation was detected, fluid stirring was stopped. Without any agitation hydrate growth stopped or slowed down radically. Under these conditions almost the same amount of hydrate was expected to form in each cell. This precaution was taken to maintain quite the same experimental conditions in each cell and therefore obtain more comparable results. Thirty minutes after hydrate formation was detected in the last cell stirring was restarted in all cells and the hydrate decomposition procedure was started. To decompose hydrates the temperature of the system was increased from Ttarg to a so-called “decomposition temperature” Td located outside of the hydrate stability domain at a heating rate of 0.4 °C/min.
3. Results and Discussion
Figure 2. High-pressure cells apparatus.
Therefore, the condensate was not contaminated with common oilphase chemicals. The aqueous phase was deionized water produced by a Millipore Milli-Q 185 E system. KHIs were poly(Nvinylpyrrolidone) (PVP-K30) purchased from Fluka, a commercial KHI referred to as KHI “A”, and an experimental KHI referred to as KHI “B” designed by Arkema/Ceca. 2.2. Experimental Equipment. The high-pressure apparatus (Figure 2) consisted of four identical stirred cells from Parr Instrument Co. with an inner diameter of 33 mm, a volume of 100 cm3, and a maximum working pressure of 20 MPa. Each cell contained an extractable glass liner in which the fluids were placed for the experiments. The thermal link between the cell wall and the glass liner was ensured by silicon oil. The cells were immersed in a temperature-controlled insulated bath agitated with two impellers to provide homogeneous temperature control during tests. The bath temperature was regulated by two refrigerating/heating circulators (Julabo F32-HE and Huber ICO45). Cell and bath temperatures were measured using PT-100 sensors with an accuracy of (0.2 °C. Cell pressures were measured with 25 MPa full-scale transducers with a precision of 0.3% FSD. The gas mixture was supplied to each cell by external stainless steel reservoirs. A gas booster pump (Haskel AGT 32/152) was used to pressurize the reservoirs. Cell and bath temperatures and cell pressures were continuously monitored by a personal computer. 2.3. Procedures. Before being placed in the cells each glass liner was filled with 12 cm3 of deionized water and 28 cm3 of condensate. In the experiments with the inhibitors the KHIs were previously dissolved in water at the required concentration using an adequate vessel and stirring for at least 15 min. The KHI aqueous
3.1. Hydrate Equilibrium Conditions for the CH4/C3H8/ Water/Condensate System. The presence of liquid hydrocarbons in a gas mixture/water system is known to cause a depression in hydrate equilibrium temperature.13 The selective solubility of the preferred hydrate former in the oil phase changes the gas-phase composition and therefore modifies the hydrate equilibrium conditions. To experimentally evaluate the hydrate equilibrium temperature depression in the presence of Frigg condensate the hydrate equilibrium temperature Teq of the uninhibited system was determined for different Pcell using the continuous heating method.14 In these experiments hydrate formation was achieved by applying the same procedure as that described before but without stopping the stirrers at any time. The system containing hydrates was then heated continuously at a rate of 0.05 °C/h. This rate was low enough to assume equilibrium conditions for the system at each instant.15 Figure 1 shows the variation in Pcell as a function of Tcell for a hydrate formation/decomposition cycle of the uninhibited CH4/ C3H8/water/condensate system. The interesting part of this “hysteresis curve” is the heating part limited by points A and B. Before point A the increase in Pcell was due to mainly expansion of the gas with temperature. Hydrates started decomposing as from point A, which coincides with the pure methane-hydrate equilibrium curve. Because of the isochoric conditions of our experiments and the presence of a gas mixture in nonstoichiometric proportions hydrates of different compositions were expected to be present. Consequently, heating the system from point A led to decomposition of hydrates with increasing propane content. Hydrate decomposition was complete at point C, where the heating curve meets the cooling (13) Becke, P.; Kessel, D.; Rahimian, I. Influence of liquid hydrocarbons on gas hydrate equilibrium. SPE 25032. European Petroleum Conference, Cannes, 1992. (14) Dicharry, C.; Gayet, P.; Marion, G.; Graciaa, A.; Nesterov, A. J. Phys. Chem. B 2005, 109 (36), 17205–17211. (15) Svartaas, T. M.; Gulbrandsen, A. C.; Fjelldal, T. H.; Gjeessen, M. An experimental study on the effect of the KHI polymer size on sII hydrate formation and dissociation in an isochoric cell system. 6th International Conference on Gas Hydrates, Vancouver, 2008.
964 Energy & Fuels, Vol. 23, 2009
Duchateau et al.
Table 1. Depression of Hydrate Equilibrium Temperature in the Presence of Frigg Condensate Peq (MPa)
Teq,expa (°C)
Teq,HWHYDb (°C)
temperature depression (Teq,HWHYD - Teq,exp) (°C)
9.93 11.59 12.76
16.2 17.3 17.8
17.6 18.4 18.9
1.4 1.1 1.1
a Determined from the experimental hysteresis curves in the presence of Frigg condensate. b Calculated from the HWHYD model in the absence of oil phase.
curve. The temperature and pressure at point C were defined as the final hydrate equilibrium conditions (Teq, Peq). When further heating, the heating curve followed the initial path back to the starting conditions (point B). Table 1 shows the final hydrate equilibrium conditions measured for three different cell pressurizations, the corresponding predictions from HWHYD model without oil phase,12 and the temperature depressions induced by the presence of the Frigg condensate. The temperature depression of the hydrate equilibrium curve was found to be almost constant and on average equal to 1.2 ( 0.2 °C in the measured pressure range. The independency of the temperature depression with pressure is consistent with the literature.13 Teq (°C) was then defined as the prediction of HWHYD minus 1.2 °C and can be expressed in the form of a regression equation in the studied pressure range as follows Teq ) -0.0198P2cell + 0.908Pcell + 9.33 (R2 ) 1)
(1)
for 10 MPa e Pcell e 13 MPa. Different authors have reported that the presence of a KHI could result in a shift of the hydrate equilibrium conditions toward higher temperatures.15-19 This effect does not seem to occur for PVP-K3017 and has not been investigated yet for KHI “A” and “B” but will be studied in detail in a future article. 3.2. First Hydrate Formation. The purpose of these experiments was to illustrate the well-known fact that hydrate crystallization in systems containing “fresh water” (water that has never experienced hydrate formation/decomposition) leads to scattered results20 and thus to the impossibility to discriminate between blank (uninhibited system) and KHI test results. Figure 3 shows the maximum subcooling ∆Tmax defined as Teq - Tform, where Teq is the hydrate equilibrium temperature at Pcell calculated from eq 1 and Tform is the temperature at which hydrate formation was detected. In these experiments Ttarg was set to 2 °C. It is shown that the maximum subcooling difference between two cells containing the same components could reach 4.6 °C, which corresponds to a relative dispersion of about 20% (calculated as (Max|∆Tmax,mean - ∆Tmax|)/∆Tmax,mean). The induction times, tind, defined as the time elapsed from when the system enters the hydrate stability domain to the first indication of hydrate formation, measured for 1 and 2 wt % (16) Lee, J. D.; Englezos, P. Chem. Eng. Sci. 2006, 61, 1368–1376. (17) Habetinova, E.; Lund, A.; Larsen, R. Hydrate dissociation under the influence of low-dosage kinetic inhibitors. In Proceedings of the Fourth International Conference on Gas Hydrates, Yokohama, 2002. (18) Svartaas, T. M.; Gulbrandsen, A. C.; Huseboe, S. B. R.; Sandved, O. An experimental study on “un-normal” dissociation properties of structure II hydrates formed in presence of PVCAP at pressures in the region 30 to 175 bar - dissociation by temperature increase. 6th International Conference on Gas Hydrates, Vancouver, 2008. (19) Malaret, F.; Dalmazzone, C.; Sinquin, A. Study of the effect of commercial kinetic inhibitors on gas-hydrate formation by DSC: nonclassical structures. 6th International Conference on Gas Hydrates, Vancouver, 2008. (20) Natarajan, V.; Bishnoi, P. R.; Kalogerakis, N. Chem. Eng. Sci. 1994, 49 (13), 2075–2087.
Figure 3. Maximum subcooling on first hydrate formation.
Figure 4. Induction times measured at a subcooling of 9.8 ( 0.3 °C and on first hydrate formation for KHI “A”.
KHI “A” in water at a subcooling of 9.8 ( 0.3 °C are shown Figure 4. tind was found to range from 50 to 1440 min with a mean value of 530 min, which corresponds to a relative dispersion of 172%. As expected, the stochastic nature of hydrate crystallization led to poorly reproducible results. Under these conditions comparison of KHI effectiveness is not possible and discrimination between blank and KHI test results is unavailing. 3.3. Second Hydrate formation. The aim of these experiments was to use the residual hydrate structures left in water that had just experienced hydrate formation and decomposition in order to reduce the stochastic character of hydrate crystallization.8-10,21-23 The effect of this preliminary cycle on the reproducibility of ∆T max and tind measured on second hydrate formation was investigated. For preliminary hydrate formation the conditions were the same as those described for first hydrate formation. In each cell the stirrer was stopped 30 mins after hydrate formation was detected to limit the amount of hydrates formed. Thirty minutes after hydrate formation was detected in the last cell the stirrers were restarted and the temperature was increased to Td ) 29.5 °C at a heating rate of 0.4 °C/min. The system was maintained at this temperature for 2 h. The same procedure was then repeated for second hydrate formation. As Figure 5 shows, the ∆T max values measured on second hydrate formation were not significantly different from those measured on first hydrate formation and their dispersion was roughly the same. This result suggests that no (or not enough) residual hydrate structures remained in the water phase at the end of the decomposition step. Consequently, the systems behaved as if they contained fresh water. To preserve hydrate precursors in the water phase we softened the thermal conditions applied for hydrate decomposition. To this aim we set Td ) 19.5 °C for 2 h. At the end of this decomposition stage Pcell was recovered within the uncertainty (21) Song, K. Y.; Feneyrou, G.; Fleyfel, F.; Martin, R.; Lievois, J.; Kobayashi, R. Fluid Phase Equilib. 1997, 128 (1-2), 249–259. (22) Sloan, E. D.; Subramanian, S.; Matthews, P. N.; Lederhos, J. P.; Khokhar, A. A. Ind. Eng. Chem. Res. 1998, 37 (8), 3124–3132. (23) Makogon, Y. F.; Holditch, S. A.; Makogon, T. Y. Kinetics and morphology of secondary gas hydrates - experimental results. In Proceedings of the Fifth International Conference on Gas Hydrates, Trondheim, 2005.
EValuation of Kinetic Hydrate Inhibitors
Figure 5. Maximum subcoolings measured on first and second hydrate formations (after two hours at Td of 29.5 and 19.5 °C).
Figure 6. Curves of effectiveness: subcooling versus induction time.
of the pressure transducer, suggesting that no hydrates remained in the system prior to second hydrate formation experiment. Under these conditions both the values of ∆T max obtained on second hydrate formation and their dispersion strongly decreased (Figure 5). The presence of residual hydrate structures in the water phase provided templates for faster hydrate formation and therefore reduced the stochastic character of hydrate crystallization. In this case the test conditions were harder and thus the differences between systems inhibited with effective KHI and uninhibited systems were easier to appraise. Note that as the nature and proportion of the active antihydrate components in KHIs “A” and “B” are unknown parameters, no conclusion on the relative effectiveness of KHIs “A”, “B”, and PVP can be drawn from Figure 5. The induction times on second hydrate formation were measured at different subcoolings to determine the “curve of effectiveness” of inhibited and uninhibited systems (Figure 6). It can be seen that both the values of tind and their dispersion (error bars in Figure 6) strongly decreased when compared to those obtained on first hydrate formation (Figure 4). For the system inhibited with 2 wt % KHI “A” the median value of tind measured at a subcooling of about 9.8 °C decreased from 530 min on first hydrate formation (Figure 4) to about 40 min on second hydrate formation. The relative dispersion of tind was lower than 40% for all experiments and generally lower than 20%. This should be compared against the 172% relative dispersion measured on first hydrate formation (Figure 4). However, it can be observed that dispersion of tind increased as subcooling decreased. Subcooling is usually considered as the driving force for hydrate formation:24 the lower the subcooling, the lower the driving force and thus the higher the dispersion of induction times. (24) Arjmandi, M.; Tohidi, B.; Danesh, A.; Todd, A. C. Chem. Eng. Sci. 2005, 60 (5), 1313–1321.
Energy & Fuels, Vol. 23, 2009 965
Figure 7. Effect of the temperature applied to decompose hydrate (Td) on the maximum subcooling and comparison with a first formation (New).
Figure 8. Effect of the temperature applied to decompose hydrate (Td) on the curve of effectiveness of the uninhibited system.
Figure 9. Effect of the temperature applied to decompose hydrate (Td) on the curve of effectiveness of the 2 wt % KHI “B” inhibited system.
As expected, inhibited systems exhibited higher protection against hydrate formation and increasing KHI concentration improved hydrate prevention. The protocol is therefore discriminating with regard to these parameters (presence and concentration of KHI). Because the temperature Td applied to decompose hydrates appeared to have a great influence on the conditions of second hydrate formation we evaluated the effect of Td on ∆T max by varying Td from 19.5 to 29.5 °C for the uninhibited system and two inhibited systems (Figure 7). The values of ∆Tmax were almost constant as long as Td was lower than 21.5 °C. For higher values of Td both the values of ∆Tmax and their dispersion were found to increase. These results suggest that though good repeatability of results requires the presence of precursory hydrate structures in the system the precise amount of these materials does not significantly change the value of subcooling above which immediate hydrate formation occurs, at least to a certain extent.
966 Energy & Fuels, Vol. 23, 2009
The influence of Td on tind measured at different subcoolings for uninhibited and inhibited systems was also investigated. The results are shown in Figures 8 and 9, respectively. Figure 8 shows that although increasing Td from 19.5 to 21.5 °C did not modify ∆T max it shifted the measured induction times toward higher values at lower subcoolings; the lower the subcooling, the higher the shift for tind. However, one can observe that the values of tind were near or within the error bars at a given subcooling. Moreover, these error bars had the same width, showing that the increase of Td did not drastically change the dispersion of the induction times. The same behavior was observed for the system inhibited with 2 wt % KHI “B” (Figure 9). These results seem to support the hypothesis that some variations in the amount of residual hydrate structures left in the system after hydrate decomposition do not have a major impact on the results of KHI evaluation. However, further experiments with systems containing other KHIs should be made to confirm or refute this point. 4. Conclusions Hydrate formation in a system containing fresh water leads to highly scattered results due to the stochastic nature of
Duchateau et al.
hydrate crystallization. On the other hand, hydrate formation in a system containing water that has previously experienced hydrate formation and decomposition can outstandingly improve the repeatability of results and thus allow unambiguous discrimination between blank (uninhibited system) and KHIs tests. The temperature applied to decompose hydrates after first hydrate formation is a key parameter of such a procedure: too high a temperature leads to destruction of too many hydrate precursory structures, making the results poorly repeatable (just as they were with fresh water). However, a critical temperature below which the repeatability of results remains acceptable seems to exist, which gives hope that some flexibility may be tolerated in the use of the presented procedure. Further experiments with other KHIs would be necessary to confirm that point. Acknowledgment. Authors thank Total and Arkema/Ceca for financial support, Arkema/Ceca for KHI furniture and all the members of the KHI working group for fruitful discussions.
EF800710X
