Abstract: In a process for cooling gas fluid feed material from an initial temperature to a second, cryogenic temperature which utilizes a single mixed refrigerant composition whose respective constituents possess a range of successively lower boiling points, wherein said refrigerant composition is passed through a single closed loop refrigeration cycle involving compression, partial condensation in an aftercooler phase separation, heat exchange and expansion, wherein heat exchange occurs in a cold box containing a warm end and a cold end heat exchanger, said exchangers being thermally connected in series relationship, wherein the improvement comprises regulating the flow rate of the vapor-containing refrigerant stream passing from said partial condensation stage by diverting predetermined amounts of liquid refrigerant from the liquid-containing refrigerant stream, also passing from said partial condensation stage, into said vapor-containing stream, said diverting occurring at a point inside the cold box after said vap

Abstract: In the recovery of waste heat from flue gas containing particulate solids, CO.sub.2, SO.sub.2, SO.sub.3 and H.sub.2 O, several improvements are provided, e.g., passing hot flue gas, e.g., from a power plant, over heat exchange surface attackable by sulfuric acid, e.g., in a combustion air preheater, so as to release heat to said heat exchange surface and to partially cool said flue gas to a temperature within the range of about 350.degree. F.