Waste heat recovery

Direct process heat integration

Process heat integration means heat that was previously cooled off is recovered and reused in another unit operation.. With direct process heat integration, heat is transferred directly from one process stream to the other in a single heat exchanger.

The two streams need to be fairly close to each other, and there should not be any dangers involved if the streams mix in case of a leak.

The result is a reduced load on both the heating and cooling utility systems.

There are numerous examples in chemical plants where direct process heat integration can be utilized, for example:

  • In refineries, the heat from several waste streams can be used to preheat crude oil upstream of a fired heater.
  • Preheating of naphtha or feed gas in steam reformers.
  • Preheating of the feed in fired heaters.
  • Preheating of mill water and chlorine dioxide in chemical pulp mills.
  • Column integration in multistage distillation.


Feed preheating in direct fired heater/furnace

In this example heat from a process gas stream is recovered and used for preheating the feed gas of a direct fired heater. A shell-and-tube is replaced by a compact heat exchanger, increasing heat recovery and reducing gas consumption. The result is an increase in heat recovery by 1.7 MW (5.814 MMBtu/h).

 Before                                             After




  • Operating hours: 8,400 hours/year
  • Installation factor: 3
  • Heat recovered before/after: 4.3/6 MW
  • Fired heater efficiency: 80%
  • Gas pressures 20-30 barg – only centrifugal compressors used
  • Physical properties similar to syngas on process side and natural gas on feed side
  • ∆P: same before and after technology change 

Direct process heat integration

Payback period

Fuels savings in direct fired heater/furnace

Payback period for heat recovery system as a function of fuel price. 

Payback period (years)