Experimental Measurement and Thermodynamic Modeling of Methane Hydrate Dissociation Conditions in the Presence of Diglycolamine Aqueous Solution

Diglycolamine (DGA) aqueous solution is a sweetening agent that can be used in the natural gas processing plants to absorb acid gases. It can also be used for partial dehydration of natural gas. Since DGA has a hydroxyl functional group, it can connect with water molecules through hydrogen bonds, lowering the water activity and acting as a thermodynamic hydrate inhibitor. This work aims to investigate the impact of DGA aqueous solution on methane hydrate dissociation conditions. In this regard, a total of 39 dissociation data of methane hydrate in the presence of four DGA aqueous solutions (3.5, 10.0, 15.0, and 20.0 wt %) were experimentally measured in the pressure range of 3.49- 8.11 MPa and temperature range of (275.3 to 283.4) K. To do so, a stainless steel (SS-316) cell was utilized, and the isochoric pressure-search method was applied to measure the hydrate dissociation conditions. It was observed that by increasing the DGA concentration in aqueous solution, the inhibition effect of DGA is increased. The average hydrate dissociation temperature suppressions for 3.5, 10.0, 15.0, and 20.0 wt % DGA in aqueous solution are 0.5, 1.4, 2.2, and 3.5 K, respectively, which are very close to the inhibition strength of triethylene glycol (TEG), diethanolamine (DEA), and methyl diethanolamine (MDEA). In the thermodynamic modeling section, the van der Waals- Platteeuw (vdW-P) model is applied for the hydrate (solid) phase to calculate the methane hydrate dissociation conditions. Three activity coefficient models are employed for the aqueous phase: NRTL, UNIQUAC, and Wilson. Also, for the vapor/gas phase, three cubic equations of state (EoSs) are taken into account, namely, the Peng-Robinson (PR EoS), the Soave-Redlich-Kwong (SRK EoS), and the Valderrama-Patel-Teja (VPT EoS). All the models benefit from no parameter optimization. Among all the thermodynamic packages, the combinations of the (vdW-P + NRTL + VPT) and (vdW-P + UNIQUAC + PR) models are the best choices, with an average absolute deviation of 0.2 K in the determination of methane hydrate dissociation temperature.

Automated summary

This study investigates the impact of DGA aqueous solution on the dissociation conditions of methane hydrate. Experimental results show that increasing the concentration of DGA can enhance its inhibition effect, which is comparable to other common inhibitors. By thermodynamic modeling, the best combination of models is determined for calculating the dissociation conditions of methane hydrate.