for an ethylene plant with a low pressure demethanizer would be 10 methane 10 ethylene and 80 propylene the use of the tertiary refrigerant provides all the refrigeration loads and temperatures required for an ethylene plant while obviating the need for two or three separate refrigerant systems the purpose of the present invention is to provide the necessary refrigeration to separate the hydrogen and methane from the charge gas and provide the feed for the demethanizer as well as provide for the other refrigeration requirements of the entire plant referring to the specific embodiment of the invention shown in the drawing which is for a low pressure demethanizer the tertiary refrigeration system is arranged to provide all of the required levels of refrigeration for an ethylene plant in the series of heat exchangers 10 12 14 16 18 and 20 these heat exchangers can be combined as fewer units or expanded into a greater number of units depending on the particular needs for any particular ethylene process and in particular on the specific charge gas composition they are typically plate fin type heat exchangers and are preferably packed inside of a heavily insulated structure referred to as a cold box to prevent heat gain and to localize the low.