METHOD OF PREPARING A CLATHRATE OF A GAS-HYDRATE-FORMING MATERIAL AND WATER

Abstract: A method of preparing a clathrate of a gas-hydrate-­ forming material and water by preparing a dispersion of liquid water in a continuous fluid phase comprising the gas-hydrate-forming material and controlling the temperature and the pressure such that a clathrate is formed.

Description: The invention relates to a method of preparing a clathrate of at least one gas-hydrate-forming material and water, possibly containing small amounts of other compatible compounds, wherein the gas-hydrate-forming material is dissolved into liquid water, possibly containing the other compounds, after which the product formed thereby is solidified by lowering the temperature. A method as described above is known from U.S. specification 4 393 660 disclosing dissolving a gas-hydrate-forming compound, such as nitrous oxide, chlorine and carbon dioxide, into an aqueous liquid under conditions of temperature and pressure suitable to form a stable hydrate and freezing the resulting product. For obtaining a particulate product the resulting product may be granulated, such as by crushing. U.S. 3 255 600 discloses preparing a carbon dioxide hydrate clathrate by reducing the pressure of a continuously stirred dispersion of liquid water and liquid carbon dioxide. First a slush is formed and thereafter a snow-like product. Another process for obtaining gas-hydrate clathrate using the method described above is disclosed by EP 0 330 578. In this process the temperature of liquid CO₂ in a pressurized reaction vessel is set between -30°C and -10°C. Thereafter liquid water containing dissolved CO₂ is dispersed in the continuous phase. As soon as the water droplets enter the reaction vessel they will freeze on their outside due to the low temperature of the continuous phase. An object of the invention is the provision of a well controllable method with regard to both composition and particle size, which may be used for continuous production and which may be used for producing high gas content clathrated products. In accordance with the present invention it has been found that these objects can be met if an emulsion of liquid water in a continuous fluid phase comprising the gas-hydrate-forming material is prepared while using a water-in-oil emulsifier and then the temperature and pressure of this emulsion are controlled such that a clathrate is formed. Preferably, the continuous phase of the emulsion consists at least predominantly of gas-hydrate-forming material, which is in an even more preferred embodiment liquid. This may be practically effected by choosing suitable conditions of temperature and pressure from the phase diagram H₂O-CO₂ and incorporating liquid water in liquid carbon dioxide as the disperse phase. In this case too, the formation of the emulsion is assisted by the incorporation of a suitable water-in-oil emulsifier of low HLB value, such as "Admul W.O.L., a trade-name for an emulsifier, obtainable from Quest International, Ashford, England, or sorbitan monooleate and analogous esters. More particularly, this invention is directed to a method as referred to hereinbefore, wherein an emulsion of liquid water in a continuous phase comprising the gas-hydrate forming material is prepared while using a water-in-oil emulsifier and then the temperature and pressure of this emulsion are controlled such that a clathrate is formed. In a preferred embodiment of the invented method wherein the continuous phase has a lower solidification point than the clathrate-forming temperature under the same conditions, the temperature is controlled such that the continuous phase does not solidify and thereafter the clathrate particles are separated from said continuous phase. For this separation step any suitable known method may be used, such as filtering and centrifuging. Alternatively, the temperature of the product obtained may be lowered after the clathrate formation so as to freeze at least some of the continuous phase containing the clathrate particles. This method may be applied if the material forming the continuous phase is not interfering with the expected future use of the clathrate. Although a dispersion of solid aqueous particles in a liquid continuous phase may be used with acceptable results, preferably an emulsion of liquid aqueous droplets emulsified in the continuous phase is used since the size control of the droplets is very easy, and the size distribution of the droplets may be made relatively monodispersed. In case carbon dioxide is the gas-hydrate-forming material, oil is a preferred main constituent of the continuous phase. The above described methods are particularly well-suited for preparing H₂O-CO₂ clathrates. The invention will be exemplified in the following description of two embodiments of the invented method, the second of which is preferred at present. EXAMPLE I AN EMULSION OF 20 % water in an edible, vegetable oil (for example bean oil) is prepared by emulsification with a Silveson stirrer in the presence of a suitable w/o emulsifier (e.g. 3% Admul W.O.L. calculated on the weight of oil) with stirring to give an average water droplet size of between 50 and 300 µm. The EMULSION is exposed to an atmosphere of 2942 kPa (30 atm.) of CO₂ at a temperature (suitably about 15°C), at which temperature no CO₂-gas hydrate forms. The temperature is then lowered to 10°C or lower, depending on the nucleation of the gas hydrate or ice crystals. After complete formation of the gas hydrate or-freezing of the residual water (if the temperature is lowered sufficiently), the finely divided gas hydrate/ice mixture is physically removed from the suspending oil, such as by filtration. The carrying liquid oil is then re-used in a next cycle. EXAMPLE II AN EMULSION OF 10 % water in liquid CO₂, with water droplets in the range of 50 to 1000 µm, is formed by vigorously agitating a mixture of the two liquid phases in the presence of 0.5% w/w of a commercial polyglycerol ester of a fatty acid as emulsifying agent. The temperature and pressure should be in the liquidus region for both water and CO₂ in the phase diagram. The CO₂ is admitted to the reaction vessel through a mass detector to monitor the CO₂ content of the mixture. Continuous gentle agitation is used to ensure homogeneous distribution of the water droplets. At the same time the temperature is lowered to bring conditions progressively into the hydrate-forming region, for that particular gas hydrate, until gas hydrate crystals are nucleated. The temperature is then progressively lowered, typically to -20°C, at which time the unbound CO₂ can be removed by any classical physical separation technique, such as filtration. The product at -20°C is then returned to atmospheric pressure and stored under conditions usually employed by those experienced in handling gas hydrates.

Claims: 1. Method of preparing a clathrate of a gas-hydrate-forming material and water, possibly containing small amounts of other compatible compounds, wherein the gas-hydrate-forming material is dissolved into liquid water, possibly containing the other compounds, after which the product formed thereby is solidified by lowering the temperature, characterised in preparing an emulsion of liquid water in a continuous fluid phase comprising the gas-hydrate-forming material while using a water-in-oil emulsifier and then controlling the temperature and pressure of this EMULSION such that a clathrate is formed. 2. Method according to claim 1, wherein the continuous phase at least predominantly consists of liquid gas-hydrate-forming material. 3. Method according to claim 1, wherein the continuous phase is liquid and is selected such that the gas-hydrate-forming material is more soluble in the continuous liquid phase than in the water phase. 4. Method according to claim 1, 2 or 3, wherein the continuous phase has a lower solidification point than the clathrate-forming temperature under the same conditions, and the temperature is controlled such that the continuous phase does not solidify and thereafter the clathrate particles are separated therefrom. 5. Method according to claim 1 or 2, wherein the temperature after the clathrate formation is lowered to freeze at least some of the continuous phase containing the clathrate particles. 6. Method according to claim 1, wherein the gas-hydrate-forming material is carbon dioxide and the continuous phase mainly comprises an oil. 7. Method according to claim 1 or 2,wherein the gas-hydrate-forming material is CO₂.

Inventors: BEE, RODNEY DAVID
Applicants: UNILEVER N.V., UNILEVER PLC
Application year: 1989
Main classification: C01B31
IPC codes: B01J13, F25J1, B01J3, C01B31