Page 130 - Chemical engineering design

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CHEMICAL ENGINEERING
3.16.7. Heat pumps
A heat pump is a device for raising low grade heat to a temperature at which the heat can
be utilised. It pumps the heat from a low temperature source to the higher temperature
sink, using a small amount of energy relative to the heat energy recovered.
Heat pumps are increasingly finding applications in the process industries. A typical
application is the use of the low grade heat from the condenser of a distillation column
to provide heat for the reboiler; see Barnwell and Morris (1982) and Meili (1990). Heat
pumps are also used with dryers, heat being abstracted from the exhaust air and used
to preheat the incoming air. The use of a heat pump with an evaporator is described in
Volume 2, Chapter 14.
Details of the thermodynamic cycles used for heat pumps can be found in most
textbooks on Engineering Thermodynamics, and in Reay and MacMichael (1988). In
the process industries heat pumps operating on the mechanical vapour compression cycle
would normally be used. A vapour compression heat pump applied to a distillation column
is shown in Figure 3.17a. The working fluid, usually a commercial refrigerant, is fed to
the reboiler as a vapour at high pressure and condenses, giving up heat to vaporise the
process fluid. The liquid refrigerant from the reboiler is then expanded over a throttle
valve and the resulting wet vapour fed to the column condenser. In the condenser the
wet refrigerant is dried, taking heat from the condensing process vapour. The refrigerant
vapour is then compressed and recycled to the reboiler, completing the working cycle.
If the conditions are suitable the process fluid can be used as the working fluid for the
heat pump. This arrangement is shown in Figure 3.17b. The hot process liquid at high

Condenser

Vapour Low

press
Expansion Compressor
Feed valve
High
press

Reboiler
Liquid

(a) (b)
Figure 3.17. Distillation column with heat pump (a) Separate refrigerant circuit (b) Using column fluid as the
refrigerant