| abstract |
(57) [Summary] [PROBLEMS] To measure the content of nitrogen gas mixed in methane gas in real time, and to suppress the nitrogen gas content when liquefied and stored within a predetermined range. SOLUTION: BOG 6 generated from an LNG storage tank 4 of an LNG storage device 3 is reliquefied, and a nitrogen separator 9 separates a mixed gas having a large nitrogen gas content. The separation gas is The sound velocity is measured in real time by the sound velocity measuring means 19 for measuring the sound velocity using the ultrasonic current meter 15. The relationship index between the nitrogen gas content in the mixed gas and the sound velocity is obtained in advance as a relationship index for standard gases having different concentrations of nitrogen gas in different concentrations, and is stored in the relationship index storage unit 17. The nitrogen concentration deriving means 20 derives the nitrogen concentration, and the adjustment control command generating means 21 generates a control signal for adjusting the valve opening of the flow control valve 14. The portion of the mixed gas 11 where the nitrogen concentration is high is discharged out of the system from the flow control valve 14 and processed. |