Studies Of Tetrahydrofuran Hydrate Adhesion And Hydrate Growth In The Presence Of Inhibitors

The formation of gas hydrates in natural gas or oil pipelines has long been a serious problem in gas and oil industries. Once the formation starts, the hydrates crystal nucleus would grow up to be large hydrate masses, causing an increase in the slurry viscosity and eventually lead to a plug and hydrate blockages. The particle-particle adhesion force is the key factor that causes the aggregation of hydrate particles and hydrate blockages. Adding inhibitors is the most effective way to solve the hydrate plugging. Reliable mechanism of hydrate particles adhesion and growth plays an important role in developing inhibitors. In this thesis, the micro-manipulation and observation method has been used to study the mechanism of various of inhibitors working on hydrate adhesion, morphology and hydrate crystal growth, and the main research achievements are summarized as follows:1. The adhesion forces of tetrahydrofuran (THF) hydrate particles in different immersion time, contact force, temperature, NaCl concentration and medium were measured. Results showed that immersion time had no significant effects on adhesion force. Adhesion force increased as the contact pressure increased. Adhesion force increased linearly with the increase of temperature. When the medium was decane, adhesion force increases with the increase of NaCl concentration as a logarithmic type. When the medium was1%span85+99%decane or1%span80+99%decane and the NaCl concentration was1×10-1mol/L, the adhesion force measured was close to zero. This indicated that the combination of NaCl and span85or span80could reduce the adhesion force significantly. When NaCl was used alone, it could not change the morphology of hydrate particle, but when NaCl and span85and span80were used in combination, it could make hydrate particle surface become rough.2. Effects of cyclic structure inhibitors on adhesion force was investigated, the cyclic structure inhibitors used in the experiment include polyvinylpyrrolidone (PVP), poly(N-vinyl-2-pyrrolidone-co-2-vinyl pyridine)(PVPP), poly(2-vinyl pyridine-co-N-vinylcaprolactam)(PVPC) and poly(N-vinylcaprolactam)(PVCap). The results show that PVP could increase adhesion force, PVPP and PVPC could reduce adhesion force. When the concentration of PVPP was greater than2.0wt%, the adhesion force was decrease by more than70%. When using concentration was0.5wt%-3.0wt%, PVPC could reduce adhesion force of more than40%. PVCap could reduce adhesion force in low concentration, but viscosity reduction effect was not obvious with the increase of concentration. Morphology of the surface THF hydrate particle was smooth without inhibitors, PVPP could make particle surface rough. And the roughness surface might reduce the real effective contact area between hydrate particles, thus reducing adhesion force.3. Effects of linear inhibitors on adhesion force was investigated, the linear inhibitors used in the experiment was a series of Alkyl Trimethyl Ammonium Chloride. Results showed that linear inhibitors can reduce adhesion force, and the adhesion force got smaller and smaller as concentration of linear inhibitors rose. When the concentration was2.0wt%, the adhesion force measured wsa about0.006N/m, nearly close to zero, this indicated that linear inhibitors had good effect on viscosity reducing. Linear inhibitors could make hydrate particles surface become rough, reduce the area of effective contact area between hydrate particles, thus reducing adhesion force.4. Morphology and growth of hydrate crystals with cyclic structure inhibitors at a hydrate-liquid interface were directly observed throught a microscopic manipulating apparatus. Morphological patterns between each hydrate crystal growth from hydrate-liquid interface into droplet were found different significantly. Lamellar structure growth of hydrate crystal was observed without inhibitor, while with PVP was featheriness-like, PVPP was like long dendritic crystal, PVPC was mimosa pudica leaf-like and PVCap was like weeds. Hydrate crystal just had one growth site on the interface in the presence of our synthetized inhibitors PVPP and PVPC, while countless growth sites bestrewed on the interface in the presence of commercial inhibitors PVP and PVCap. This indicated that inhibitors synthetized by our lab could inhibit the hydrate growth more effectively compared to commercial inhibitors. The growth rate of hydrate crystal without inhibitor was0.00498mm3/s, while with PVPP, PVPC and PVCap, were0.00339mm3/s,0.00350mm3/s,0.00386mm3/s and0.00426mm/s, respectively. Cyclic structure inhibitors can decrease the growth rate, degree of reduction in growth rate of hydrate crystals decrease with the increase of cylinder number.