Experimental Study Of Enhancing Methane Storage Via Gas Hydrate And Scale-up Of Continuous Gas Storage

Hydrate based gas storage is a newly-developed and promising technology for gas storage and transportation.However,its practical application is hindered by the slow formation kinetics and low gas storage capacity.To improve the feasibility of hydrate based gas storage by improving the formation kinetics and gas storage capacity,the promoting methods for hydrate formation in highly water-saturated fixed bed and bulk water were proposed.Based on the water-in-oil emulsion,we proposed a conceptual process for continuous hydrate production and built a pilot plant to verify the feasibility of this process.The main contents of this study are as follows:（1）A method that establishing gas channels in highly water-saturated porous bed by diesel oil was proposed to improve the gas storage performance.The results suggested that the sodium dodecyl sulfate（SDS）could not improve hydrate formation in the fine-particle beds,conversely,it suppressed the formation.By adding diesel oil,the storage capacity of hydrate and the storage density of the bed were increased from to 68.00 V/V_w and 51.93 V/V_bed to 180.40 V/V_w and 111.75 V/V_bed,and the time needed decreased to 3 hours from dozens of hours reported in literature.The storage density could be further improved by adjusting the proportions of water,oil and activated carbon.The storage density decreased with the increase of the recycle times of the bed.However,it could be simply renewed by manual stirring.（2）A physical method of reciprocating stirring/impact was proposed to improve hydrate formation in bulk water.The effect of the impactor configuration,temperature,pressure,water load,and the impact frequency on methane hydrate formation were systematically investigated.The results showed that the hydrate formation could be well improved by all the impactor configurations tested.The temperature and pressure had little effect on storage capacity but significantly affected the hydrate formation kinetics in the slurry stage.The multi-nucleation phenomenon was observed when reciprocating impact was adopted,and the hydrate growth rate in block stage depended on the conversion of the interstitial water.The highest storage capacity of 150.30 V/V_hwas acquired by the method,and the water conversion reached to 90%within 4 hours.Additionally,the smallest energy consumption of the impact device was found to be4.13×10^-3 k Wh/mol.（3）The mechanism of improving hydrate formation with reciprocating impact was further studied by investigating the morphology change and the kinetic characteristics of hydrate formation–dissociation–re-formation.In hydrate slurry stage,massive hydrate shells appeared at the interface between the gas and hydrate slurry,and with the ceaseless generation and crush,the shells provided excellent gas transfer and nucleation conditions for hydrate formation.In the fresh hydrate block,some of the interstitial water could be extruded out to convert to hydrate.With the conversion,the hydrate block got hard,and the hydrate shells enwrapped in it provided gas and space for the conversion of the rest interstitial water;thus the shells became the major factor for improving hydrate formation in this stage.Dissociation condition and the rest time of activated water had a certain effect on the storage capacity and formation kinetics of reformed hydrate.When at a large dissociation driving force or a short rest time of activated water,the formation kinetics and storage capacity of the re-formed hydrate were unsatisfactory.（4）A conceptual process for continuous production of gas hydrate was developed based on the water-in-oil emulsion.The hydrate formation in different emulsions and the filtration characteristics of hydrate slurry were investigated.The results showed that hydrate formation can be accomplished within half an hour in emulsion.When Span20was used as emulsifier individually,the highest gas storage capacity was achieved while the hydrate slurry was viscous.When Span20 was used along with TBAB or saponin,the gas storage capacity was lower while the flow performance was better than that when Span20 was used only.Based on the formation kinetics and fluidity of hydrate slurry,Span20（1.0wt%）+TBAB（4.9wt%）was considered as the most suitable emulsifier formula for industrial hydrate production.Similar formation kinetics and storage capacity were observed with this formula in the emulsion prepared under lower stirring speed.The results of hydrate filtration experiments suggested that the temperature higher than ice point was unfavourable to hydrate filtration.Bigger hydrate particle in the slurry was proved to be favourable to hydrate filtration;nevertheless,the formation rate was decreased by the large particle size.（5）The processing design of continuous hydrate production was accomplished and a pilot plant with a handling capacity of 1 m³ water per day was built.The batch and semi-continuous production were conducteded in the plant and the results showed that the hydrate formed fast even in pilot scale and no pipe blockage was observed.When a full closed sieve was used at 268.2 K,the hydrate slurry was filtered well;however,the filtration efficiency decreased rapidly after the filter cake got compacted.To address the problems appeared in production experiments,some measures were proposed,which include precooling the fresh water with reflux oil to avoid the temperature increase of hydrate slurry,and precooling the hydrate slurry to filtration temperature with a heat exchanger to avoid fast dissociation of hydrate in the filter.