Page 390 - Chemical engineering design

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CHEMICAL ENGINEERING
1. The flash-point of the material.
2. The autoignition temperature of the material.
3. The flammability limits of the material.
4. The energy released in combustion.
Flash-point
The flash-point is a measure of the ease of ignition of the liquid. It is the lowest temper-
ature at which the material will ignite from an open flame. The flash-point is a function of
the vapour pressure and the flammability limits of the material. It is measured in standard
apparatus, following standard procedures (BS 2000). Both open- and closed-cup apparatus
is used. Closed-cup flash-points are lower than open cup, and the type of apparatus used
should be stated clearly when reporting measurements. Flash-points are given in Sax’s
handbook, Lewis (2004). The flash-points of many volatile materials are below normal
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ambient temperature; for example, ether 45 C, petrol (gasoline) 43 C (open cup).
Autoignition temperature
The autoignition temperature of a substance is the temperature at which it will ignite
spontaneously in air, without any external source of ignition. It is an indication of the
maximum temperature to which a material can be heated in air; for example, in drying
operations.
Flammability limits
The flammability limits of a material are the lowest and highest concentrations in air, at
normal pressure and temperature, at which a flame will propagate through the mixture.
They show the range of concentration over which the material will burn in air, if ignited.
Flammability limits are characteristic of the particular material, and differ widely for
different materials. For example, hydrogen has a lower limit of 4.1 and an upper limit of
74.2 per cent by volume, whereas for petrol (gasoline) the range is only from 1.3 to 7.0
per cent.
The Flammability limits for a number of materials are given in Table 9.2.
The limits for a wider range of materials are given in Sax’s handbook, Lewis (2004).
A flammable mixture may exist in the space above the liquid surface in a storage
tank. The vapour space above highly flammable liquids is usually purged with inert gas
(nitrogen) or floating-head tanks are used. In a floating-head tank a “piston” floats on top
of the liquid, eliminating the vapour space.

Flame traps
Flame arresters are fitted in the vent lines of equipment that contains flammable material
to prevent the propagation of flame through the vents. Various types of proprietary flame
arresters are used. In general, they work on the principle of providing a heat sink,
usually expanded metal grids or plates, to dissipate the heat of the flame. Flame arrestors
and their applications are discussed by Rogowski (1980), Howard (1992) and Mendoza
et al. (1988).
Traps should also be installed in plant ditches to prevent the spread of flame. These
are normally liquid U-legs, which block the spread of flammable liquid along ditches.