Molecular Simulation Study On Kinetic Hydrate Inhibitors Containing Amide Group

Hydrate blockage is considered as one of the concerned problems in oil and gasproduction and transportation. One of the common solutions is to add low dose hydrateinhibitors (such as kinetic inhibitors, known as KHIs), which has attracted arising attentionfor the advantages of low cost and environmental friendliness. So far, the founded effectiveKHIs are polymers with amide group, which lead to ponder whether amide group is the activegroup for inhibitors and in which kind of chemical environment it can perform excellentinhibition. Finding out the main factors that influence KHIs inhibitory effect has importantimplications both for the development of new KHIs and oil and gas industrial safety.In this thesis, molecular dynamics (MD) simulation was used to investigate the influenceof inhibitors to methane hydrate. These inhibitors contained amide group in different chemicalenvironments (such as in the ring, in the main chain, branched-chain and with hydroxyl whichmay bring synergies effect). Amide polymers with different number of-CH2-number wereselected to study whether there exist inhibition ability difference. A further investigation onthe inhibition performance of amide polymers containing different plain rings such aspyridine ring, benzene, phenol structure was followed. Then MD simulation of amidepolymers with hydroxyl groups was carried out. Finally, the binding energy that polymerabsorbs onto hydrate surface and the charge of oxygen atom in amide group were calculatedto discuss inhibition mechanism.-CH2-number affects the inhibition ability of lactam polymers with a similar structure,i.e., polyvinylpyrrolidone (PVP), poly(N-vinyl piperidone (PVPip)and poly(N-vinylcaprolactam)(PVCap). The bigger number-CH2-in the ring, the stronger the inhibitionperformance is. Seven-membered ring (PVCap) showed the best inhibition ability, followedby a six-membered ring (PVPip), the worst was five-membered ring (PVP).Plain ring structure (pyridine ring, benzene ring, phenol) were confirmed to impact theinhibition strength of amide polymers, and the inserted ring on N-vinyl caprolactam, with theinteraction of their dimer, the diffusion coefficient of water molecules were1.446×10-9,1.530×10-9,1.468×10-9m2/s, respectively. A larger diffusion coefficient usually indicates stronger polymer inhibition ability. The benzene ring favors amide groups to performinhibition ability the most, then phenol was followed, and pyridine ring ranked thirdly whichmight owe to its strong electron attracting ability.Poly(2-ethyl-2-oxazoline)(PEtO) with introduction of hydroxyl groups, the diffusioncoefficient of water was1.591×10-9m2/s, whose inhibitory effect was enhanced, so thehydroxyl group show synergistic effect with amide group. PVCap with introduction ofhydroxyl groups, the diffusion coefficient (the main chain, ring) were1.391×10-9m2/s,1.285×10-9m2/s, respectively, so the hydroxyl group reduced polymer inhibition ability. Anintroduction of hydroxyl groups to amide polymers promotes the interactivity with watersolution, therefore improves the biodegradation performance.The amide polymers disrupt local water clusters structure, make water clusters unstableand cannot grow to their crystal critical size. When hydrate nucli is formed, amide polymersabsorb onto hydrate surface to surpress further growth. The larger polymer surface bindingenergy is, the easier for it to adsorb onto hydrate surface and inhibit hydrate growth. O atomsin amide group or hydroxyl group forming hydrogen bonds with water molecules in hydrate,disrupts cage structure and suppresses hydrate nucleation and growth. The stronger negativecharge O atom is, the easier hydrogen bonds form.