Many chemical plant operations require that vessels and piping be inerted or purged. If a vessel is.

Many chemical plant operations require that
vessels and piping be inerted or purged. If a vessel is to be opened for
maintenance or repair, for example, and if the vessel has contained either
toxic or flammable materials, purging is required before workers can enter the
vessel. For vessel entry, piping leading to or from the vessel will have to be
blanked off and at least the portions of the vessels that are open to the pipe
will have to be purged as well. Purging must be continued until the atmosphere
in the vessel is safe for entry. If the chemical in the vessel is flammable,
purging must

Many chemical plant operations require that
vessels and piping be inerted or purged. If a vessel is to be opened for
maintenance or repair, for example, and if the vessel has contained either
toxic or flammable materials, purging is required before workers can enter the
vessel. For vessel entry, piping leading to or from the vessel will have to be
blanked off and at least the portions of the vessels that are open to the pipe
will have to be purged as well. Purging must be continued until the atmosphere
in the vessel is safe for entry. If the chemical in the vessel is flammable,
purging must be accomplished in two steps: First, the flammable material is
purged from the vessel with an inert gas such as nitrogen, and the nitrogen is
then purged with air. When a vessel that is to be used for storing or
processing flammable chemicals is initially put into service, it must be purged
with an inert gas before flammable chemicals are put in the vessel. This step
is required to assure that a flammable mixture of the chemical and the air in
the tank does not form. Regardless of the method used to purge the equipment,
the final step should be to check the atmosphere in the equipment to make
certain that the concentration of the flammable or toxic material has been
reduced to safe levels. For tank entry, the oxygen concentration must also be
checked before the tank is entered. The measurement of residual concentrations
is sometimes required by law; it is always needed for good practice. Suppose
that a 150 ft 3 tank containing air is to be inerted to a 1% oxygen
concentration. Pure nitrogen is available for the job. The tank has a maximum
allowable working pressure of 150 psia, so either of two methods is possible.
In the first method, air is purged by a continuous sweep of nitrogen. The
nitrogen is simply allowed to flow into the tank at essentially atmospheric
pressure. It is assumed that the nitrogen mixes rapidly and completely with the
air in the tank, so the gas leaving the tank has the same concentration of
oxygen as the gas in the tank. In the second technique, the tank is
pressurized, the pure nitrogen inlet stream is turned off, and the gas mixture
in the tank is then exhausted, lowering the pressure in the tank to atmospheric
pressure. If the pressurization technique is used, multiple pressurization
cycles may be required, with the tank returned to atmospheric pressure at the
end of each cycle. Complete mixing is assumed for each cycle. In this problem,
you may assume that both nitrogen and air behave as ideal gases and that the
temperature remains constant at 80F throughout the process. Determine the
volume of nitrogen (measured at standard conditions of 1.0 atm and 0C)
required to purge the tank using each purging technique. For the pressurization
technique, assume the pressure in the tank is raised to 140 psig (a little
below its maximum working pressure) with nitrogen and then vented to 0 psig.
This problem was adapted from the publication Safety, Health, and Loss
Prevention in Chemical Processes, American Institute of Chemical Engineers, New
York (1990).