A low-temperature device for separating and purifying gas based on a small-sized low-temperature refrigerating machine includes a primary, secondary and quaternary heat exchanger, at least one small-sized low-temperature refrigerating machine, and at least one liquid collecting tank. The small-sized low-temperature refrigerating machine includes a first cold head and a second cold head, the secondary heat exchanger is provided on the first cold head to form a primary cold head heat exchanger, the quaternary heat exchanger is provided on the second cold head to form a secondary cold head heat exchanger, a mixed gas outlet is connected to an inlet of the primary cold head heat exchanger. By using primary and secondary cold heads of the small-sized low-temperature refrigerating machine as cold sources, gases having different condensing temperature are liquefied and solidified separately, and two or more gases can be separated and purified at a lower cost.

A system for purifying helium gas, a method, and an application. The system includes a first gas-liquid separation device, a primary helium extraction tower, a second gas-liquid separation device, a secondary helium extraction tower, and a nitrogen removal tower, which are in sequential communication. The first gas-liquid separation device performs first treatment to convert helium-containing natural gas into a first gas and first liquid phases; the primary helium extraction tower performs first distillation on the first gas and first liquid phases to obtain a second gas and second liquid phases; the second gas-liquid separation device performs second treatment to convert the second gas phase to a third gas and third liquid phase; the secondary helium extraction tower performs second distillation on the third gas and third liquid phases to obtain crude helium and a fourth liquid phase form which nitrogen is removed by the nitrogen removal tower.