Method and device for liquefying a natural gas without phase separation of the coolant mixtures

A method of liquefying a natural gas, comprising the steps of:

• (a) subjecting the natural gas to a first cooling cycle in which the natural gas is cooled to a temperature at least as low as -30.degree. C. by a first coolant mixture that has been compressed, at least partially condensed by cooling with a first external coolant fluid, subcooled, expanded, and vaporized;
• (b) after step (a), subjecting the natural gas to a second cooling cycle in which the natural gas is condensed and subcooled by a second coolant mixture that has been compressed, cooled with a second external coolant fluid, cooled by heat exchange with the first coolant mixture during the first cooling cycle, to bring the second coolant mixture to an at least partially condensed state, and subjected without phase separation to the second cooling step, to cause the second coolant mixture to be totally condensed, expanded, and evaporated at at least two pressure levels;
• and
• (c) after step (b), expanding the natural gas to form liquefied natural gas.