Construction And Mechanism Of Carbon Nanotubes-based Methane Hydrate Enhancement System

In recent years,natural gas as a safe,clean,efficient and economical energy source has become more and more favored by people.In order to more conveniently use natural gas energy derived from a variety of natural gas storage and transportation methods,such as LNG,PNG,CNG,and these traditional storage and transportation methods are each accompanied by corresponding shortcomings,so based on the reverse thinking of combustible ice mining,a safe and convenient storage and transportation mode-natural gas hydrate technology（NGH）has been derived.Although the storage and transportation conditions of natural gas hydrate technology are mild（3-6MPa,0-5°C）and the gas storage performance is good（180V/V）,the development and application of natural gas hydrate technology are hindered due to the long induction period,slow formation speed and low conversion rate of natural gas hydrate technology.Therefore,achieving rapid formation of natural gas hydrates and increasing conversion rates have become the key to using this technology.As a new type of carbon nanomaterial,Multi-walled carbon nanotubes（MWCNTs）have been shown to have a significant role in promoting the formation of natural gas hydrates,creating conditions for the industrial application of natural gas hydrate technology.At present,researchers have greatly improved the performance of CNTs in promoting the formation of natural gas hydrates by modifying CNTs（such as oxidation,sulfonation）or compounding with other accelerators,but the performance of the original CNTs in promoting the formation of hydrates and the relationship between the mechanism and the diameter of the pipe have not been studied,and only the excellent heat and mass transfer properties of CNTs are used as the promotion mechanism of natural gas hydrate generation.Therefore,based on the original CNTs,this paper studies the structure-activity relationship between the original CNTs and the promotion mechanism of natural gas hydrate generation.The main research contents are as follows:（1）Construct a hydrate generation enhancement system under the mass ratios of three MWCNTs/Water（1:1000（suspension）,1:10（slurry）,3:10（dry））using three MWCNTs with different pipe diameter distributions,and conduct experimental studies on the promotion performance and visualization of hydrate generation to analyze the promotion mechanism of enhanced natural gas hydrate generation of original MWCNTs.The results of the mixed blind kettle experiment show that with the increase of the mass ratio of MWCNTs/Water,the gas storage capacity of the same MWCNTs shows a downward trend,for example,the gas storage capacity of MWCNTs₁ drops from147V/V in suspension liquid to 136V/V in slurry to 83V/V in dry state,which is because with the viscosity of the reaction system increased and the mass transfer of the system was weakened under the high adsorption performance of MWCNTs,but the growth rate of methane hydrates in the slurry and dry components was higher than that of the suspension group,for example,the generation rate of MWCNTs₁ was 3.0（V/V）/min（slurry）and 1.5（V/V）/min（dry state）were greater than 1.25（V/V）/min（suspension）,because MWCNTs can promote the formation of hydrates by the methane molecules and water molecules adsorbed by MWCNTs.Under the same mass ratio,MWCNTs₁ with the largest specific surface area has strong adsorption capacity,and the gas storage capacity of 147V/V（suspension）,138V/V（slurry）and 84V/V（dry state）is higher than that of the other two MWCNTs with small surface area.In the visual experiments,it was found that natural gas hydrates began to grow into nuclei from the surface of the dry carbon nanotube bed layer or the part in contact with the inner wall/carrier that absorbed water,which is related to the fact that the adsorbed water molecules are more likely to form crystal nuclei.（2）In order to further study the mechanism of MWCNTs promoting the formation of natural gas hydrates,pure water is replaced with a small molecule accelerator-amino acid solution,and the above-mentioned performance and visualization of hydrate formation are also carried out.The results showed that the gas storage capacity and rate of the three MWCNTs suspension groups under suspension were not significantly different from those of the pure D-leucine group:the gas storage capacity of the MWCNTs suspension was slightly higher than that of the pure D-leucine solution at145V/V,and the hydrate generation rate of both MWCNTs suspension and pure D-leucine solution was about 3.8（V/V）/min.This is due to the fact that the D-leucine promoting effect is too strong to overwhelm the promoting effect of MWCNTs themselves with an absolute advantage.In the slurry and dry states,the gas storage capacity of MWCNTs₁（139V/V（slurry）,70V/V（dry state））and MWCNTs₃（118V/V（slurry）,79V/V（dry state））showing a higher specific surface area is higher than that of MWCNTs₂（116V/V（slurry）,63V/V（dry state））with a low specific surface area,and combined with simulation experiments by other researchers,it can be analyzed that small molecule accelerators can enter the tube of MWCNTs,induceing the growth of methane hydrates by water molecules and methane molecules adsorbed in the tube.In visualization studies,it was found that natural gas hydrates are still susceptible to nucleation on the surface of MWCNTs beds or in contact with the inner wall/carrier.（3）The mechanism of enhancing methane hydrate formation by means of cryogenic in-situ Raman analysis was explored.By analyzing and fitting the Raman displacement curve,it was found that the large cage:small cage=2.4 in pure D-Leucine group was lower than the theoretical value 3,so the d-Leucine solution could promote the growth of small cage more easily.However,in the group of MWCNTs and D-leucine compound,the large cage=3.2,indicating that the d-leucine entering the MWCNTs tube did not promote the formation of the cage shape according to the promotion effect of D-leucine,and D-Leucine only induced the adsorption of water molecules and methane molecules in the tube to form the hydrate crystal nucleus.Therefore,the cage formation of hydrate in dry MWCNTs was determined by the adsorption of MWCNTs.