Structural Effects of Gas Hydrate Antiagglomerant Molecules on Interfacial Interparticle Force Interactions

Abstract

Gas hydrate interparticle cohesive forces are important to determine the hydrate crystal particle agglomeration behavior and subsequent hydrate slurry transport that is critical to preventing potentially catastrophic consequences of subsea oil/gas pipeline blockages. A unique high-pressure micromechanical force apparatus has been employed to investigate the effect of the molecular structure of industrially relevant hydrate antiagglomerant (AA) inhibitors on gas hydrate crystal interparticle interactions. Four AA molecules with known detailed structures [quaternary ammonium salts with two long tails (R1) and one short tail (R2)] in which the R1 has 12 carbon (C12) and 8 carbon (C8) and saturated (C–C) versus unsaturated (CC) bonding are used in this work to investigate their interfacial activity to suppress hydrate crystal interparticle interactions in the presence of two liquid hydrocarbons (n-dodecane and n-heptane). All AAs were able to reduce the interparticle cohesive force from the baseline (23.5 ± 2.5 mN m–1), but AA-C12 shows superior performance in both liquid hydrocarbons compared to the other AAs. The interfacial measurements indicate that the AA with an R1 longer alkyl chain length can provide a denser barrier, and the AA molecules may have higher packing density when the AA R1 alkyl tail length is comparable to that of the liquid hydrocarbon chain on the gas hydrate crystal surface. Increasing the salinity can promote the effectiveness of an AA molecule and can also eliminate the effect of longer particle contact times, which typically increases the interparticle cohesive force. This work reports the first experimental investigation of high-performance known molecular structure AAs under industrially relevant conditions, showing that these molecules can reduce the interfacial tension and increase the gas hydrate–water contact angle, thereby minimizing the gas hydrate interparticle interactions. The structure–performance relation reported in this work can be used to help in the design of improved AA inhibitor molecules that will be critical to industrial hydrate crystal slurry transport.

Correlating Antiagglomerant Performance with Gas Hydrate Cohesion

“…To probe realistic conditions, the high-pressure micromechanical force (HP-MMF) 44 apparatus was designed to quantify cohesive forces between ice particles and natural gas hydrate particles in both gas 44,45 and liquid hydrocarbons. 46−48 Recently, Koh and coworkers 48 employed the HP-MMF apparatus to measure CH 4 /C 2 H 6 hydrate cohesive forces in the presence of AAs. The flow assurance performance of those AAs was assessed using a rocking cell apparatus, as frequently done in industry.…”

“…Comparing the data sets obtained, it was found that those AAs that substantially decrease the cohesive forces can show good overall performance in the rocking cell. 48 Of note, the molecular structure of the AAs used in these experiments was known, and in fact, it had been used in prior atomistic molecular dynamics (MD) simulations by Striolo and his collaborators. 31,33,49,50 These simulations suggested that AAs that yield an ordered structure at the hydrate−oil interface can exhibit good practical performance.…”

“…The goal of this manuscript is to test whether such correlations are possible. While new MD simulation results are presented in this article, the experimental data are taken from the study by Koh et al 48 ■ SIMULATION METHODOLOGY Model Setup. One representative simulation setup (see Figure 1A) mimics the measurements of hydrate particle cohesive forces conducted in experiments.…”

“…One representative simulation setup (see Figure 1A) mimics the measurements of hydrate particle cohesive forces conducted in experiments. 48 Each simulated system contains two hydrate nanoparticles immersed in a hydrocarbon mixture. Each hydrate nanoparticle was carved out of the bulk structure of sII CH 4 /C 2 H 6 hydrates.…”

“…Each hydrate nanoparticle was carved out of the bulk structure of sII CH 4 /C 2 H 6 hydrates. 51 These hydrates were used in the HP-MMF apparatus and are expected to form within rocking cell experiments 48 to test the AA performance . The X, Y, and Z dimensions of each hydrate nanoparticle were 5.193, 3.462, and 1.731 nm, respectively.…”