Waste heat recovery process and apparatus

In a process for recovering heat from an exhaust gas having a temperature of from 500.degree. to 2500.degree. F. generated in a unit operation effecting an exothermic reaction and cycling between an operational mode and an idling mode, the improvement comprising:

• (a) passing said exhaust gas during said operational mode in indirect heat transfer relationship to at least two intermediate heat transfer media in at least two successive heat exchange zones operating at different temperature levels;
• (b) recovering heat from said intermediate heat transfer media at different temperature levels;
• (c) passing one intermediate heat transfer media through a first heat transfer zone;
• (d) passing another intermediate heat transfer medium having a lower operational temperature level through a succeeding heat transfer zone;
• (e) passing an air stream sequentially through said succeeding heat transfer zone and said first heat transfer zone to preheat said air stream;
• (f) introducing said preheated air into said unit operation;
• (g) passing said intermediate heat transfer media to respective storage zones
• (h) during the idling mode passing said intermediate heat transfer media from said storage zones to a user other than the unit operation;
• and
• (i) passing to step (a) said intermediate heat transfer media from said storage zones.