the filtration layer due to volume expansion further the ice particles may clog the filtration layer according to the inventive process it is necessary to first introduce cold nitrogen gas into the filter device to avoid the foregoing defects alternatively a cross flow filtration system such as that shown in u s pat no 4 863 617 can be used instead of the whole amount filtration system described above in this embodiment liquid nitrogen was supplied from bomb 1 to a vaporizer not shown which takes the place of the refrigerator 5 further the filter device 13 is omitted in order to determine the amount of particles contained in crude unfiltered liquid nitrogen since the flow rate of liquid nitrogen is difficult to measure the flow rate of the nitrogen gas is measured after the liquid has vaporized in the vaporizer the measurement takes place at 21 u00b0 c under 1 atmosphere pressure in example 1 the distribution of particles the assumed amount weight of liquid nitrogen and the average pressure values p1 at pressure regulator 11 and p2 at regulator 15 are measured each 5 minutes in sequence with the indicated flow rate the results are shown in table 1 the assumed amount weight was measured by calculating the difference in weight of the bomb 1 just before and after each 5 minutes the particle counter 23 measures the number of particles by taking samples of liquid nitrogen each sample being 0 228 nm 3 per hour nm 3 h this particle counter 23.