Mechanisms of gas hydrate formation and inhibition

The formation of gas hydrates in gas and oil subsea pipelines often results in blockage and shutdown of these pipelines. Modem control methods depend on understanding the mechanisms through which gas hydrates form. This paper reviews our recent studies of clathrate hydrate formation and inhibition mechanisms using neutron diffraction, differential scanning calorimetry (DSC) and a multiple cell photo-sensing instrument. The structural transformations of water around methane during methane hydrate formation have been studied using neutron diffraction with isotope substitution over the temperature range 4-18 degreesC and at pressures of 3.4-14.5 MPa. The hydration sphere around methane in the liquid only changes significantly when methane hydrate is formed, with the water shell in the crystalline hydrate being about I A larger than the shell in the liquid. The hydration shell is disordered during methane hydrate formation, with ordering of solvent separated methane molecules occurring only when hydrate has formed. The effects of the addition of three low dosage hydrate inhibitors, PVP, VC-713 and QAB on THF hydrate formation at the surface and in bulk solution have been examined. The QAB inhibitor exhibits the greatest hydrate crystal growth control, while VC-713 is most effective at inhibiting hydrate nucleation. Insight into the perturbations on host and guest molecules due to the presence of these inhibitor molecules has been obtained. (C) 2002 Elsevier Science B.V. All rights reserved.