Waste heat recovery

Indirect process heat integration

Indirect process heat integration differs from direct process heat integration in that an intermediate circuit is used for transferring heat between the two process streams. The transfer medium (water or thermal oil) absorbs heat in one part of the plant and releases it in another. This approach is used when:

  • Direct contact between heat source and heat sink is not allowed. The intermediate circuit works as a safety barrier and leakages can be detected in the loop, before the process fluids mix.
  • Long distances need to be covered.
  • Flexibility and reduced interdependence is required. Equipping the intermediate circuit with standby coolers and heaters makes it easier to disconnect a unit operation for maintenance, avoiding interdependence between plants.
  • One heat sink requires multiple heat sources.

Indirect process heat integration opens up a vast range of possibilities. Two common application examples are:

  • Sulphuric acid and mineral processing industries
  • Boiler feed-water heating where it is important to avoid interleakage and/or where there is a long distance between boiler and heat source
  • Ammonia still condenser systems with integrated mother liquor heating in the soda ash industry
  • Caustic soda pre-evaporation by recovery of electrolysis heat


Sulphuric acid plant

The following is a typical example from a metallurgical sulphuric acid plant. Here there are often opportunities to recover waste heat from the acid plant absorption towers and reuse it for heating in mineral processing steps. Examples where the heat can be reused are copper electrolyte heating, spent acid and zinc sulphate solution heating in zinc plants, boiler feed water heating, etc.

Before the technology change, the hot sulphuric acid from the absorption towers was cooled off in a shell-and-tube heat exchanger. Afterwards, the heat is transferred to a heat integration loop via a compact heat exchanger and released to an electrolyte heating bath. The result is 10 MW (34 MMBtu/h) of recovered energy.

Before                                                  After



  • 60 W/m heat loss in pipeline
  • Boiler efficiency 90% (fuel savings case)
  • Turbine isentropic efficiency 80% (electricity generation case)
  • Total system operating and capital cost estimation based on real quotations
  • Demineralized water used in intermediate loop
  • Installation factors 1.5-2 for heat exchangers
  • Installation factor 3 for pipeline construction (insulated pipes) 


Payback period

Fuel savings, revamp of existing plant

Payback period as a function of energy price when using the recovered energy for saving fuel

Payback period (years)

Electricity generation, revamp of existing plant

Payback period as a function of energy price when using the recovered energy for electricity generation

Payback period (years)

Fuel savings, new plant

Payback period for new plant (reduced utility investments)
1,000 m between unit operations

Payback period (years)