Study On The Mechanism Of Cyclodextrins And Amino Acids To Promote Gas Hydrate Formation

Natural gas hydrates have attracted widespread attention from industry and academia around the world because of its huge energy potential,and its efficient and safe in transportation of natural gas.Reasonable addition of surfactants can effectively improve the generation rate of natural gas hydrates,which is conducive to the large-scale industrial application of hydrate storage and transportation technology.Nevertheless,the intrinsic mechanism of surfactants promoting hydrate formation has not been fully understood.Clarifying its mechanism is conducive to the research and development of new and efficient natural gas hydrate accelerators,and has important application value for the exploration and exploitation of marine natural gas hydrates and the safe transportation of deep-sea oil and gas in the petroleum industry.In this study,using quantitative Raman spectroscopy and high-pressure in-situ observation techniques,the mechanism of bio-friendlyβ-cyclodextrin（β-CD）and five amino acids（L-valine,L-leucine,L-methionine,L-histidine,L-phenylalanine）to promote hydrate formation was studied.The mechanism of surfactants promoting hydrate nucleation and the different ways of influencing hydrate growth were investigated at the molecular level.The following conclusions are reached:（1）Hydrate experiments at 275.15 K,6-20 MPa showed thatβ-CD had the self-aggregation trend at the gas/liquid interface,and theβ-CD aggregates could greatly increase the local CH4 concentration（23.08-43.56%）,and the hydrates would preferentially nucleate around theβ-CD aggregates.This phenomenon can be explained by the"hydrophobic effect of hydrate formation"by surfactants.On the one hand,β-CD enhances the orderliness of water molecular structure through"hydrophobic hydration"and reduces nucleation work.On the other hand,the"hydrophobic interaction"ofβ-CD and CH4 increases the local CH4 concentration.The highly ordered structure of water molecules and the locally increased concentration of guest molecules provide hot spots for hydrate nucleation.（2）Hydrate growth experiments at 275.15 K,6-20 MPa showed that the addition ofβ-CD can increase the growth rate of CH₄hydrate under diffusion flux control.Compared with pure water,the equivalent one-dimensional growth rate of hydrates（methane diffusion flux）increased by 18.78%and 2.77%in 0.5 wt%β-CD solutions and 1.0 wt%β-CD solutions,respectively.Althoughβ-CD aggregates increase local CH4 concentrations,they may hinder the diffusion of CH4.In 1.0 wt%β-CD solution,excessβ-CD aggregates form near the gas/liquid interface,blocking the gas/liquid interface,hindering the diffusion of CH4,and impairing the relative efficiency of the system.（3）Hydrate experiments at 275.15 K,10 MPa showed that compared with pure water,the addition of 0.05 mol L-valine,L-leucine,L-methionine,L-histidine,L-phenylalanine did not significantly increase the guest molecular supersaturation（-6.70–5.83%）at the gas/liquid interface of CH₄and CO₂hydrate systems,and the nucleation driving force of hydrates.The study of amino acid hydration shell spectroscopy showed that the addition of five amino acids all could enhance the local structure of the surrounding water molecules,and the amino acids with longer hydrophobic side chains increased the order of water molecules more obviously.The local structure of more tetrahedrold water molecules is conducive to nucleation of hydrates.（4）Diffusion experiments and hydrate experiments under 275.15 K,10 MPa showed that compared with pure water,the addition of 5 kinds of amino acids of 0.05mol reduced the diffusion coefficient of CH₄and CO₂in the liquid phase（-3.95--13.67%）and the equivalent growth rate of hydrates controlled by diffusion flux（-3.16--18.91%）.（5）In situ observations at 275.15 K,10 MPa showed that compared with pure water,the addition of 0.2 mol%L-valine,L-leucine,L-methionine reduced the perimeter angle of the three phases（methane,solution,quartz capillary）of the system,while the addition of L-histidine and L-phenylalanine increased the perimeter angle of the three phases of the system compared with pure water.And the angle of contact decreases with the decrease of amino acid molecular weight.The effect of amino acid addition on the gas/liquid contact area was not significant（-6.54-3.64%）.Through the review,it was found that there are many factors affecting the liquid phase surface tension,including:system temperature,pressure,solution p H,the type of ions in the solution,the type of surfactant,the concentration of surfactant,etc.The mechanism of promoting the formation of hydrate by surfactants may not be suitable to be explained only by the addition of surfactants to reduce the surface tension of the liquid phase.（6）Observations at 10 MPa pressure,variable temperature conditions（253.15 K to275.15 K）show that the interface instability of CH₄hydrate when nucleated in methionine solution is more severe than pure.Hydrate growth experiments at 10 MPa,275.15 K,in 0.2 mol%L-methionine solution showed that amino acid addition can increase the growth rate of hydrate under interface control.The spiral layered hydrate formed at the gas/liquid interface of L-methionine solution does not block the gas/liquid interface.Continuous dynamic contact between the gaseous CH₄and liquid phases at the gas/liquid interface accelerates the formation of hydrates.This may be related to the addition of amino acids reducing the adhesion of the hydrate,increasing the nucleation site,and changing the crystal habit of the hydrate.The above studies show that the mechanism ofβ-CD and amino acids to promote hydrate formation is not the same.The similarity is that the addition ofβ-CD and amino acids has"hydrophobic hydration"of water molecules,which enhances the orderliness of the molecular structure of the surrounding water and facilitates hydrate nucleation.The difference is that there is a clear"hydrophobic interaction"between theβ-CD and CH₄molecules,increasing the supersaturation of CH₄and the nucleation driver at the gas/liquid interface.At the same time,β-CD also increased the diffusion coefficient of CH₄in the homogeneous liquid phase,and increased the hydration growth rate under diffusion flux control.There is no obvious"hydrophobic interaction"between amino acid molecules and CH₄/CO₂.The addition of amino acids reduces the hydrate growth rate under diffusion flux control but has a significant increase in the hydrate growth rate under interface control.