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

Abstract

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.

Impacts of Triethylene Glycol and Polyvinyl Caprolactam on Dissociation Conditions and Complete Inhibition Region of Methane Hydrate

“…Comparison between the experimental water activity data in the presence of TEG, methanol, and MEG and water activity results calculated with the FH model. ,− …”

“…Then, in the decomposition step, the temperature of the system was increased at a rate of 0.1 K.hr –1 . Dissociation points were determined by monitoring of pressure-temperature ( P - T ) diagrams as well as the visual observation through sight glass. ,,, …”

A Precise Thermodynamic Approach for Calculating Activity of Water in the Presence of Aqueous Alcohol + Salt Solutions and its Application to Gas Hydrate Dissociation Modeling

“…The UNIQUAC model was explained in details elsewhere. , The FH model assumes a rigid lattice frame, in which the molecules in the pure liquids or solutions are placed in the lattice sites. In this theory, after mixing, the overall volume remains constant, and a homogeneous mixture is formed. ,, The activity of water can be calculated from ,− ln ( a normalw ) = ln ( 1 − ϕ ) + ( 1 − 1 r ) ϕ + χ ϕ 2 …”

“…In eq , ϕ stands for the volume fraction of the inhibitor (alcohol), r is the ratio of inhibitor molar volume to the water molar volume, and χ represents the FH interaction parameter indicating a quantitative measure of interactions between water and the inhibitor. ,− For athermal solutions in which molecules of one component are much larger than the molecules of another component, the value of χ is zero . In this work, χ is calculated from the general formula provided by van der Sman − : χ = χ 0 + ( χ 1 − χ 0 ) ϕ 2 where χ 0 is equal to 0.5 and χ 1 is a function of the additive molecular weight − : χ 1 = ( 0.0005 × MW additive ) + 0.1807 …”

“…The majority of THIs do not participate in the hydrate structure. The ability to reduce water activity is the primary factor used to evaluate their strength. ,− The hydroxy functional group of alcohol molecules has the ability to establish a new hydrogen bond with water molecules. Additionally, hydrogen bonding between water–alcohol molecules causes a decrease in the water chemical potential.…”