Page 108 - Chemical process engineering design and economics

Process Circuit Analysis 93

freedom or the number of variables that we must specify before attempting to
solve the relations. Following this, to determine which variables to specify, thus
completing the formulation of the problem. The problem then reduces to a
mathematical problem of determining a solution procedure and "grinding" out an
answer, which are not trivial steps.

Example 3.1 Purging Air from a Tank______________________

For the first example, consider the operation of purging a storage tank of air before
filling it with a flammable liquid. Purging has two meanings. One meaning is
purging a process unit by displacing the air with an inert gas to conduct safe plant
operations and maintenance. Another meaning is withdrawing a stream to limit the
concentration of contaminants within a process. Later, we will examine the latter
application of the purge. When plants are shut down for routine maintenance,
workers must frequently enter vessels - used to process or store flammable or
toxic chemicals - for cleaning or repairs. In many cases vessels require welding.
For safety reasons, it is essential to remove all traces of a chemical before allowing
workers to enter a vessel. Explosions triggered by a welder's torch occur fre-
quently. The New York Times [5] reported that an explosion killed a welder who
entered an "empty" compartment of a barge used to transport oil. Apparently, his
welding torch ignited residual fumes left by the oil.
Organic vapors, and some inorganic gases, have flammability limits when
mixed with air. To burn these gases requires a mixture composition between a
minimum and maximum fuel concentration. The fuel concentration from the mini-
mum to the maximum is the flammability range for the gas. Figure 3.1.1 shows the
range for a number of gases. Outside the flammability range, the mixture is too
diluted with either air or fuel to sustain combustion. For example, when a car
"floods" and will not start, the gasoline is in excess, and the air-fuel ratio is out-
side the flammability range. Figure 3.1.1 shows that the flammability range is very
narrow for gasoline. We can obtain the flammability limits for many more chemi-
cals from the Chemical Engineering Handbook [7] and chemical manufacturers.
Usually, the wider the flammability range, the more unsafe the gas is. Other fac-
tors influence the manageability of a gas, such as the minimum energy required to
ignite an air-gas mixture. Thus, for a gas to burn, the air-gas mixture must be
within the flammable range and must have an ignition source. The source, such as
a flame, a spark or a hot metal, must be capable of supplying sufficient energy for
ignition. A good rule to follow is that if a gas is within the flamability range, igni-
tion is inevitable, and if repairing a tank requires welding, a welder's torch is cer-
tainly sufficient.
If a vessel contained a flammable gas or if it will contain a flammable gas,
then we must purge the vessel with a gas. Purging is dilution of a flammable gas

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