lower closure 26 closures 24 and 26 are adapted to tightly seal openings on the top and bottom respectively of chamber 22 inert gas line 28 connects the top of impregnation chamber 22 to branched line 30 one branch of line 30 is connected through valve 32 to atmospheric vent 34 the other branch of line 30 is connected through valve 36 to lines 40 52 and 58 valve 38 controls the flow of fluid in line 40 which is connected at its lower end to the top of process vessel 12 valve 42 controls the flow of gas through line 40 line 44 branches off line 40 between valves 38 and 42 and it joins line 18 at the lower end of impregnation chamber 22 flow through line 44 is controlled by valve 46 a liquefied gas return line 50 connects the bottom of impregnation chamber 22 with process vessel 12 flow through line 50 is controlled by valve 48 line 52 controlled by valve 54 connects inert gas line 30 with low pressure inert gas accumulator 56 and line 58 controlled by valve 60 connects line 30 with high pressure inert gas accumulator 62 line 64 connects low pressure accumulator 56 with the low pressure side of compressor 66 the high pressure side of compressor 66 is connected to high pressure inert gas accumulator 62 process vessel 12 may be additionally provided with a liquefied gas heater 68 and a condenser 70 storage vessel 2 which may take the form of a conventional.