Molecular Simulation Study On Kinetic Hydrate Inhibitors With Natural Products

The mainly used kinetic hydrate inhibitors(KHIs) are amide polymers, which are active in low dosage, but their effective subcooling is low and their degradability is poor. Therefore, it is necessary to develop new KHIs, which are efficient at higher subcooling and being environmental friendly. The natural products have been widely researched due to their high degradability and environmental friendly. These natural products, such as chitosan, starch, amino acid and pectin, have some effects on the hydrate inhibitions, but few researches investigated the inhibition mechanism of these natural products. However, in this paper, we studied the effects of polysaccharides on the growth of CH4 hydrate by molecular dynamics simulations,the natural products polysaccharides included chitosan, starch, pectin. From the microscopic, we analyzed the inhibitory effect of the different functional groups in polysaccharides. And we also explored the inhibition mechanism of monosaccharides, such as six-member ring of glucose, five-member ring of fructose and the linear of ribose. All these studies provided a theoretical basis for the wide application of natural products inhibitors..All the molecular dynamic simulations were performed using the Gromcas software, NPT run for 20 ns at pressure P=15 MPa and temperature T=260 K. Snapshots of the system configurations with time, radial distribution functions of the carbon atoms and the total energy of the system were employed to characterize the effects of natural products on methane hydrate growth.First of all, we investigated the pure methane hydrate growth system. There is no methane bubbles, the hydrate began to grow at the interface of hydrate crystal. The system energy reached to the equilibrium state at 3 ns.Then, we investigated the growth of methane hydrate with chitosan, the 2.26 wt % chitosan system energy reached to the equilibrium state at 5.0 ns, the 3.53 wt % chitosan system energy reached to the equilibrium state at 6.5 ns. In a certain concentration range, the higher the concentration of chitosan is, the better the effect of inhibition is. The active groups of amino, hydroxyl in chitosan combine with water through hydrogen bonds, which disturbed the further growth of the methane hydrate. There appeared the methane gas gathered in the system which increased the molecular mass transfer resistance.In the the growth of methane hydrate with starch system, the 2.09 wt % starch system energy reached to the equilibrium state at 5.0 ns, the 3.09 wt % starch system energy reached to the equilibrium state at 6.0 ns. The active groups of hydroxyl in starch interacted with water molecules, which disrupted the neat H-bonds network between H2 O molecules. And steric hindrance of ring structures blocked the molecular moved.For the growth of methane hydrate with pectin, the 2.46 wt % pectin system energy reached to the equilibrium state at 6 ns, the 3.62 wt % pectin system energy reached to the equilibrium state at 12 ns. The pectin molecules were more likely to stay at the interface between water and gas molecules, and active groups of pectin were easily to form hydrogen bonds with water in hydrate and solution under appropriate conditions. The pectin layer in the interface of gas-liquid increased the mass transfer resistance.Compared to the results, the inhibition performance of pectin is better than chitosan and starch. The double bond oxygen atom of pectin, the nitrogen atom of chitosan, the oxygen atom of starch interacted with water molecules can be-40.30 KJ/mol、-31.60 KJ/mol、-33.80 KJ/mol. The pectins have the biggest interaction energy and strongest ability to form hydrogen bonding, So the pectin have good inhibition performance.We also investigated the effects of ring structures on methane hydrate, the monosaccharides inclued six-member ring of glucose, five-member ring of fructose and the linear of ribose. The results showed the glucose, fructose ribose systems energy reached to the equilibrium state at 5.5 ns、5.5 ns、5.0 ns. Theses ring structures blocked the moleculars moved by the space steric hindrance.Through the analysis of the H-bonds numbers between polysaccharide and H2 O molecules, the interaction energy between active functional groups and H2 O molecules. We proposed the layer resistance disturbance mechanism. The polysaccharides stayed at the interface between water and gas molecules, the layer in the interface of gas-liquid increased the mass transfer resistance. At the same time, active groups of polysaccharides formed hydrogen bonds with water and disrupted the order between water molecules.