TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Apparatus: The apparatus includes valve (15) for withdrawing a portion of the expansion agent from the low pressure gas tank; valve (27) for transmitting the compressed expansion agent to a high-pressure gas tank; valve (19) for withdrawing a portion of the compressed expansion agent from the high pressure gas tank; high-pressure gas compressor (24) for further compressing the compressed expansion agent withdrawn from the high-pressure gas tank; condenser; storage tank; differential flow metering device in communication with the flow control valve and the low-pressure gas compressor; and set-point controller in communication with the flow control valve and the flow metering device.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Agent: The expansion agent is carbon dioxide, chlorine, nitrogen, bromine, oxygen, hydrogen sulfide, argon, arsine, bromine chloride, chlorine dioxide, hydrogen selenide, krypton, nitrous oxide, phosphine, sulfur dioxide, sulfur hexafluoride, sulfuryl chloride, stibine or xenon.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Agent: The expansion agent can also be ethylene, propylene, cyclopropane, isobutane, methane, acetylene, ethane, methyl iodide, methyl hydro sulfide or a refrigerant e.g. trichlorofluoromethane, dichlorodifluoromethane, bromotrifluoromethane, trichloromethane, dichlorofluoromethane, chlorodifluoromethane, dichloromethane, chlorofluoromethane, difluoromethane, chloromethane, fluoromethane, chloroethane, methyl bromide or fluoroethane. EXAMPLE - In an EMBODIMENT of the method, the expansion agent withdrawn from the low pressure gas tank is compressed in a low-pressure gas tank. A portion of the compressed expansion agent from the high-pressure gas tank is withdrawn via valve (19), compressed in the high-pressure gas compressor, condensed, and then goes to the storage tank. To determine the optimum depressurization mass flow rate for maximum hydrate formation over a range of depressurization pressures (from an initial impregnation pressure to a pressure where the expansion agent ceases to form water hydrate), the following steps are performed:    (i) the mass flow rate of the expansion agent is set,    (ii) the amount of expanding agent present in an impregnated product at the end of an impregnation cycle is determined,    (iii) the incremental amount of the expansion agent mass flow rate is adjusted, and    (iv) steps (i)-(iii) are repeated until the maximum amount of expanding agent is determined to be present in the impregnated product.