2064
Ind. Eng. Chem. Res. 2002, 41, 2064-2068
RESEARCH NOTES Can 2-Propanol Form Clathrate Hydrates? Kasper K. Østergaard, Bahman Tohidi,* Ross Anderson, Adrian C. Todd, and Ali Danesh Department of Petroleum Engineering, Heriot-Watt University, Edinburgh EH14 4AS, U.K.
2-Propanol is an alcohol commonly used in petroleum exploration and production operations. However, there are at present no data concerning its effects on hydrate stability available in the open literature. Here, we present experimental hydrate equilibrium data for methane with 10.0, 16.4, and 25.0 mass % 2-propanol aqueous solutions. The results show that 2-propanol does not have the inhibition effect which would be expected from an alcohol and may, in fact, take part in clathrate formation. Consequently, 2-propanol has been modeled as a hydrateforming compound using a thermodynamic model. Comparisons between experimental hydrate dissociation data and model predictions indicate that 2-propanol may take part in structure II hydrate formation, occupying the large cavity of the hydrate structure. The hydrate phase boundary for a natural gas system with a 25 mass % 2-propanol aqueous solution has also been measured. The predicted hydrate stability zone, considering 2-propanol as a structure II hydrateforming compound, is in good agreement with the experimental data, confirming the results obtained for methane/2-propanol hydrates. Introduction Clathrate hydrates are icelike crystalline inclusion compounds composed of water and suitably sized guest molecules. In the hydrate structure, water molecules form different-sized cavities, some of which are occupied and stabilized by the guest molecules. Hydrates are a serious concern in petroleum exploration and production operations because their formation can result in the blockage of well-tubing, flowlines, and processing facilities. Three clathrate hydrate structures are currently known to the petroleum industry: structure I (sI), structure II (sII), and structure H (sH). Hydrate structure II is believed to be the most common structure encountered in the petroleum industry.1 Clathrate hydrates of natural gases have been reviewed in depth by Sloan.2 In petroleum exploration and production, 2-propanol is often used during stimulation and workover to aid in the rapid recovery of injected fluids.3,4 In recent years, 2-propanol has also found use in the prevention and remediation of hydrate-related problems in offshore operations. Peavy and Cayias5 report the combined use of 2-propanol and hydrochloric acid for hydrate inhibition during gravel packing in the Gulf of Mexico, and Leporcher et al.6 document that 2-propanol acts as a solvent additive in THI 178D, a low-dosage kinetic hydrate inhibitor used in North Sea operations. Within the petroleum industry, 2-propanol is currently viewed as a hydrate inhibitor, alongside other alcohols and glycols such as methanol, ethanol, and ethylene glycol. However, this view is based purely on assumption because no data exist within the open literature concerning the effects of 2-propanol on hydrate stability. * Corresponding author. Phone: +44 (0)131 451 3672. Fax: +44 (0)131 3127. E-mail:
[email protected].
Table 1. Purity of Test Fluids and Suppliers chemical
purity
supplier
methane 2-propanol water
99.9% 99.7% (min) distilled
Air Products Aldrich Chemical Co.
In this paper we demonstrate that, contrary to these assumptions, 2-propanol does not act solely as an inhibitor but, in fact, appears to take part in the hydrate crystal structure and, under certain conditions, acts to increase hydrate stability to higher temperatures and lower pressures. We present hydrate dissociation data for three methane-2-propanol-water systems at different concentrations of 2-propanol and a natural gas-2-propanolwater system. The phase relations of hydrate-forming systems have been modeled thermodynamically, and predictions are compared with the experimental results, with the overall aim being to determine the prevailing hydrate structure of systems containing 2-propanol. Experimental Section Measurements were performed in a high-pressure, temperature-controlled hydrate equilibrium cell. Mixing was achieved by rocking of the cell, allowing injected mercury to flush through test samples. A nonvisual isochoric method was used to determine the hydrate dissociation conditions. A detailed description of the experimental setup and the test procedures is given in Østergaard et al.7 Test fluid purities and material suppliers are provided in Table 1. The natural gas used in experiments was supplied by BOC (British Oxygen Company), with its composition being determined by gas chromatography, as presented in Table 2. Modeling An in-house thermodynamic model was employed to predict hydrate phase behavior at test conditions. The
10.1021/ie010833d CCC: $22.00 © 2002 American Chemical Society Published on Web 03/20/2002
Ind. Eng. Chem. Res., Vol. 41, No. 8, 2002 2065 Table 2. Natural Gas Composition (from GC Analysis)
component
natural gas composition (mol %)
component
natural gas composition (mol %)
N2 CO2 C1 C2 C3
4.99 1.12 86.36 5.43 1.49
i-C4 n-C4 i-C5 n-C5 C6+
0.18 0.31 0.06 0.06
