Preheating in interchangers

Feed/effluent heat exchangers, lean/rich interchangers, in-and-out heat exchangers, and feed/bottoms heat exchangers are all different names for the same concept: preheating of an ingoing stream of a unit operation using heat from the outgoing stream.

This is commonly done in:

• Reactors
• Electrolytic cells
• Absorption stripping systems
• Leaching tanks
• Stripper- or distillation columns
• Evaporation systems

Changing to plate heat exchangers is a straightforward way to improve heat recovery levels. This reduces the heat/steam consumption, resulting in either fuel savings and reduced emissions, or increased electricity generation. Cooling needs are also often reduced, which is valuable in many situations.

Example

Feed/effluent heat exchanger – condensate stripper

The following example is based on a heat exchanger specification from a condensate treatment stripper. An existing shell-and-tube heat exchanger is replaced with a Compabloc welded plate heat exchanger, which increases heat recovery by 3.8 MW (13 MMBtu/h).  It is assumed that 85% of the recovered heat translates into saved steam and, in turn, fuel savings or increased electricity generation.

Before                                                 After

 Results 

Assumptions

• 8,400 operating hours per year
• Overall pressure drop increases by approximately 1 bar, but no new pump is needed only operating costs increase)
• Physical properties on both sides are similar to water
• Installation factor = 3, i.e. the heat exchanger represents 33 % of the total project cost
• Cold side flow rate 75 m³/h and hot side 90 m³/h
• Boiler efficiency 90% (boiler case)
• Turbine isentropic efficiency 80% (electricity case)