Page 121 - Chemical engineering design

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FUNDAMENTALS OF ENERGY BALANCES
3.16. ENERGY RECOVERY
Process streams at high pressure or temperature, and those containing combustible
material, contain energy that can be usefully recovered. Whether it is economic to recover
the energy content of a particular stream will depend on the value of the energy that can
be usefully extracted and the cost of recovery. The value of the energy will depend on
the primary cost of energy at the site. It may be worth while recovering energy from a
process stream at a site where energy costs are high but not where the primary energy
costs are low. The cost of recovery will be the capital and operating cost of any additional
equipment required. If the savings exceed the operating cost, including capital charges,
then the energy recovery will usually be worthwhile. Maintenance costs should be included
in the operating cost (see Chapter 6).
Some processes, such as air separation, depend on efficient energy recovery for
economic operation, and in all processes the efficient utilisation of energy recovery
techniques will reduce product cost.
Some of the techniques used for energy recovery in chemical process plants are
described briefly in the following sections. The references cited give fuller details of
each technique. Miller (1968) gives a comprehensive review of process energy systems;
including heat exchange, and power recover from high-pressure fluid streams.
Kenney (1984) reviews the application of thermodynamic principles to energy recovery
in the process industries.

3.16.1. Heat exchange

The most common energy-recovery technique is to utilise the heat in a high-temperature
process stream to heat a colder stream: saving steam costs; and also cooling water, if
the hot stream requires cooling. Conventional shell and tube exchangers are normally
used. More total heat-transfer area will be needed, over that for steam heating and water
cooling, as the overall driving forces will be smaller.
The cost of recovery will be reduced if the streams are located conveniently close.
The amount of energy that can be recovered will depend on the temperature, flow,
heat capacity, and temperature change possible, in each stream. A reasonable temper-
ature driving force must be maintained to keep the exchanger area to a practical size.
The most efficient exchanger will be the one in which the shell and tube flows are
truly countercurrent. Multiple tube pass exchangers are usually used for practical reasons.
With multiple tube passes the flow will be part counter-current and part co-current and
temperature crosses can occur, which will reduce the efficiency of heat recovery (see
Chapter 12).
The hot process streams leaving a reactor or a distillation column are frequently used
to preheat the feedstreams.

3.16.2. Heat-exchanger networks
In an industrial process there will be many hot and cold streams and there will be
an optimum arrangement of the streams for energy recovery by heat exchange. The
problem of synthesising a network of heat exchangers has been studied by many workers,