Measurements of Thermodynamic Hydrate Inhibitor Methanol Partitioning in Methane at High Pressures

Methanol is one of the most common thermodynamic hydrate inhibitors (THIs) for offshore oil and gas production lines. Even with its widespread use, there are few data on methanol partitioning in the hydrocarbon phase at equilibrium due to the complexity of the measurements. Highly accurate data of methanol partitioning in hydrocarbons are crucial for various reasons. First, these data are needed to ensure that methanol is present in adequate amounts to be effective as a thermodynamic hydrate inhibitor. Methanol is effective at preventing hydrate formation only when it is present in the aqueous phase. Next, methanol portioning data are also needed to prevent the excessive injection of methanol in the flowline. Excess methanol in the flowline could cause corrosion to the pipeline and thus, reduce the integrity of the pipeline. Finally, the data are also needed to avoid large downstream processing issues. Specifically, oil/gas that is sent for processing needs to meet the downstream methanol specification. Contractors may be fined if the concentration of methanol is outside this specification. In this work, we performed measurements of the methanol portioning in the vapor methane phase using two different experimental apparatuses (high-pressure equilibrium flow and microfluidic). Each apparatus has its own advantages. The microfluidic system allows visual observation, and thus the first precipitation of methanol can be observed. However, the apparatus has a pressure limit of ∼11 MPa. On the other hand, the high-pressure equilibria flow system has a high-pressure limit (up to 34.5 MPa). This allows measurements to be performed at a high pressure, where currently there are limited reported values. In this work, methanol portioning measurements were conducted at various concentrations for methanol (90, 95, and 97 vol % methanol), a temperature of 40 °C, and pressures up to 34.5 MPa. In the microfluidic system, equilibrium compositions of methane, water, and methanol systems were measured at first liquid appearance using gas chromatography–mass spectrometry (GCMS). Results of these measurements showed that the compositions of methane, water, and methanol are consistent with the predicted values from simulation. Additionally, further analysis showed that in the vapor phase, the normalized methanol concentrations were consistent with the data reported by McGlashan and Williamson at both of the investigated experimental pressures. Similar investigations were also conducted in a high-pressure equilibrium flow system. Results of our measurements using the high-pressure equilibrium flow system showed that the mole fractions of water and methanol in the vapor phase were slightly higher than the predictions from simulation. However, the normalized methanol concentrations were consistent with the data reported by McGlashan and Williamson.

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