Page 95 - Dust Explosions in the Process Industries

68 Dust Explosions in the Process industries

The choice of inert gas depends on several considerations, such as availability and cost,
possible contaminating effects on products,and effectiveness.In the case of dusts of light
metals, such as aluminum and magnesium, exothermic reactions with C02and also in
some situations with N2are known, and the use of rare gases may have to be considered
in certain cases.
TableA.2 in the Appendix gives some data for the maximum permissibleoxygen con-
centration in the gas for inerting clouds of various dusts.
The design of gas inerting systems depends on whether the process is continuous or
of the batch type, the strength of the process equipment, and the type and source of inert
gas. Two main principles are used to establish the desired atmosphere in the process:
0 Pressure variation method.
Flushing method.
The pressure variation method operates either above or below atmospheric pressure.
In the former case, the process equipment, initially filled with air at atmospheric pres-
sure, is pressurized to a given overpressure by inert gas. When good mixing of air and
inert gas has been obtained, the process equipment is vented to the atmosphere and the
cycle repeated until a sufficiently low oxygen content has been reached. The alternative
is to first evacuate the process equipment to a certain underpressure,then fill with inert
gas to atmospheric pressure, and repeat the cycle the required number of times. By
assuming ideal gases, as shown by Wiemann (1989), there is a simple relationship
between the oxygen content c2(~01%)at the end of a cycle and the content c1at the begin-
ning, as a function of the ratio of the highest and lowest absolutepressures of the cycle.
c2 = C1(Pma/P~J1’~ (1.13)
where n = 1for isothermal and n = C,/C,, for adiabatic conditions.
The flushing method is used if the process equipment has not been designed for the
significant pressure increase or vacuum demanded by the pressure variation method. It
is useful to distinguish between two extreme cases of the flushing method, the replace-
ment method (plug flow) and the through-mixing method (stirred tank). To maintain plug
flow, the flow velocity of inert gas into the system must be low (4ds) and the geom-
etry must be favorable for avoiding mixing. In practice, this is very difficult to achieve;
and the stirred tank method, using high gas velocities and turbulent mixing, is normally
employed. It is essential that the instantaneous through mixing is complete over the
entire volume; otherwise, pockets of unacceptably high, hazardous oxygen concentra-
tions may form. Wiemann (1989) referred to the following equation relating the oxygen
content c2(~01%)in the gas after flushing and the oxygen content c1before flushing:
c2 = (cl -c,)P +c, (1.14)

where c, is the content of oxygen, if any, in the inert gas used, and v is the ratio of the
volume of inert gas used in the flushing process, and the process volume flushed. Leaks
in the process equipment may cause air to enter the inerted zone. Air may also be intro-
duced when powders are charged into the process. It is important, therefore, to control
the oxygen content in the inerted region, at given intervals or sporadically, depending
on the size and complexity of the plant. The supply of inert gas must also be controlled.
Oxygen sensors must be located in regions where the probability of hitting the highest
oxygen concentrationsin the system is high. A sensor located close to the inert gas inlet is