Method to control the cooldown of main heat exchangers in liquefied natural gas plant

A method for controlling start-up of a heat exchange system having a main heat exchanger comprising a warm end, a cold end, and an intermediate zone, at least one feed stream, and at least one refrigerant stream, the method comprising the steps of:

• (a) cooling the main heat exchanger from a first temperature profile at a first time to a second temperature profile at a second time, the first temperature profile having a first average temperature that is greater than a second average temperature of the second temperature profile;
• and
• (b) executing the following steps, in parallel during the performance of step (a):
• (i) measuring a cold end temperature at the cold end of the main heat exchanger;
• (ii) calculating a first value comprising a rate of change of the first cold end temperature;
• (iii) providing a cold end set point representing a preferred rate of change of the cold end temperature;
• (iv) controlling a flow rate of the at least one feed stream through the main heat exchanger based on the first value and the cold end set point;
• (v) measuring a first intermediate zone temperature at a first location in the intermediate zone of the main heat exchanger;
• (vi) calculating a second value comprising a rate of change of the first intermediate zone temperature;
• (vii) providing a first intermediate zone set point representing a preferred rate of change of the first intermediate zone temperature;
• and
• (viii) controlling a flow rate of a first stream of the at least one refrigerant stream through the main heat exchanger based on the second value and the first intermediate zone set point.