METHOD AND APPARATUS FOR MEASURING NITROGEN GAS CONTENT IN METHANE GAS

PROBLEM TO BE SOLVED: To suppress a nitrogen gas content in the case of liquefying and storing nitrogen gas to a preset range by measuring the content of the gas mixed within a methane gas real time. SOLUTION: A BOG 6 generated from an LNG storage tank 4 of an LNG storage device 3 is reliquefied, and mixed gas having large nitrogen gas content is separated by a nitrogen separator 9. The separated gas is measured at its ultrasonic velocity in real time by utilizing an ultrasonic velocity measuring means 19 for measuring the speed by utilizing an ultrasonic speedometer 15. The relationship index between the nitrogen gas content in the mixed gas and the speed is previously obtained as the index for a standard gas having nitrogen gas contents of different concentrations, and stored in a relationship index storage means 17. A nitrogen concentration finding means 20 finds a nitrogen concentration, a regulation control command generating means 21 generates a control signal for regulating a valve travel of a flow control valve 14. A portion having high nitrogen concentration of the gas 11 is exhausted out of a system from the valve 14, and treated.