Patent ID: 9671161
Date: 2017-06-06
CPC Classifications: F25J

Claim:
1. A method of maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of: comparing said predetermined LNG flow rate value with the actual LNG flow rate and generating a signal proportionate to the difference between the actual and predetermined LNG flow rates; comparing said predetermined LNG temperature value with the actual LNG temperature and generating a signal proportionate to the difference between the actual and predetermined LNG temperatures; (i) comparing a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference”) with the actual warm end temperature difference and generating a signal proportionate to the difference between the actual and predetermined warm end temperature differences and/or (ii) comparing a predetermined value of the temperature (“mid-point temperature”) of a stream at a location between the liquefying and subcooling sections of the heat exchanger means with the actual mid-point temperature and generating a signal proportionate to the difference between the actual and predetermined mid-point temperatures; varying, in response the signal proportionate to the difference between the actual and predetermined LNG flow rates and by an amount corresponding to the difference between the actual and predetermined LNG flow rates, one of the MRL and MRV flow rates; and either (i) varying, in response to the signal proportionate to the difference between the actual and predetermined LNG temperatures, the other of the MRV and MRL flow rates to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and predetermined LNG temperatures, and (ii) varying, in response to the signal proportionate to the difference between the actual and predetermined warm end temperature differences or mid-point temperatures and by an amount corresponding to the difference between the actual and predetermined warm end temperature differences or mid-point temperatures, the actual LNG flow rate; or (i) varying, in response to the signal proportionate to the difference between the actual and predetermined warm end temperature differences or mid-point temperatures, the other of the MRV and MRL flow rates to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and predetermined warm end temperature differences or mid-point temperatures, and (ii) varying, in response to the signal proportionate to the difference between the actual and predetermined LNG temperatures and by an amount corresponding to the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.