Ternary-cycle cascade refrigeration natural gas liquefaction system and method thereof

The invention discloses a ternary-cycle cascade refrigeration natural gas liquefaction system and a method thereof. Heat is transferred from a relatively low temperature level cycle to an adjacent relatively high temperature level cycle: a first stage propylene refrigeration cycle provides cold energy for natural gas, refrigerant ethene, and formulated refrigerant; a second stage ethene refrigeration cycle provides cold energy for the natural gas and the formulated refrigerant; and a third stage formulated refrigerant refrigeration cycle provides cold energy for the natural gas and the formulated refrigerant itself. Through the cooling by seven heat exchangers and one plate-fin heat exchanger, temperature of the natural gas is reduced gradually till the natural gas is liquefied. In the invention, temperature of purified natural gas is reduced gradually in seven heat exchangers and one plate-fin heat exchanger till the purified natural gas is liquefied. The cold energy needed in a liquefaction process is provided by the three systems of propylene refrigerant, ethene refrigerant, and formulated refrigerant. The systems have abundant adjusting means, and can provide matching cold energy for the natural gas liquefaction process, thereby showing operation flexibility and adaptability to raw materials of the method.