Page 125 - Chemical engineering design

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FUNDAMENTALS OF ENERGY BALANCES
2. Figure 3.15b. Similar to the first arrangement but with the water kept at high pressure
to prevent vaporisation. The high-pressure water is flashed to steam at lower pressure
in a flash drum. This system would give more responsive control of the reactor
temperature.
3. Figure 3.15c. In this system a heat-transfer fluid, such as Dowtherm (see Perry and
Green (1984) and Singh (1985) for details of heat-transfer fluids), is used to avoid
the need for high-pressure tubes. The steam is raised in an external boiler.

Steam
Steam
Flash drum
Steam drum

Feed pump
Reactor Reactor
(a) (b)

Steam
Boiler
Feed
water
Reactor
(c)
Figure 3.15. Steam generation

3.16.5. Low-grade fuels
The waste products from any process (gases, liquids and solids) which contain significant
quantities of combustible material can be used as low-grade fuels; for raising steam or
direct process heating. Their use will only be economic if the intrinsic value of the fuel
justifies the cost of special burners and other equipment needed to burn the waste. If the
combustible content of the waste is too low to support combustion, the waste will have
to be supplemented with higher calorific value primary fuels.

Reactor off-gases
The off-gases (vent gas) from reactors, and recycle stream purges are often of high enough
calorific value to be used as fuels.
The calorific value of a gas can be calculated from the heats of combustion of its
constituents; the method is illustrated in Example 3.14.
Other factors which, together with the calorific value, will determine the economic
value of an off-gas as a fuel are the quantity available and the continuity of supply.
Waste gases are best used for steam raising, rather than for direct process heating, as this
decouples the source from the use and gives greater flexibility.