Page 127 - Plant design and economics for chemical engineers

GENERAL DESIGN CONSIDERATIONS 103

PROBLEMS?
1. Identify some of the principal events leading to spillage and leakage in petroleum
refineries and/or chemical plants. Provide specific examples, if possible, to support
the selections.
2. Approximately 25 kg/s of vapor are released to the atmosphere. The lower explosive
limit C, for the vapor is 0.010 m3 vapor/m’ air. The vapor density of the vapor is
2 kg/m3. If the wind speed U, is 3 m/s, what is the distance d required for safe
disposal of the vapor in the air as evaluated from the relation

where Q is the escape rate in m3/s?
3. A mercury spill occurs in an unventilated stockroom with some of the mercury
trapped in several floor cracks. What is the maximum concentration of mercury that
can be attained in the storeroom when the temperature is 23”C? If the Federal
standard permissible exposure limit for mercury in the air is 0.1 mg/m3, is the
maximum concentration of mercury in the air acceptable?
4. Benzene has a PEL of 1 ppm for an 8-h exposure. If liquid benzene is evaporating at
a rate of 2 ml/min in air whose temperature and pressure are 72°F and 12.8 psia,
respectively, what must the ventilation rate of the air be to keep the benzene
concentration below the PEL value?
5. Oxygen-deficient atmospheres can develop in closed containers by a number of
seemingly harmless mechanisms. Corrosion inside of a closed container is such a
mechanism. The typical corrosion rate for carbon steel in very moist air is about
0.005 in./yr and is about first order with respect to the oxygen concentration. If the
corrosion reaction can be approximated by the reaction,

2Fe + +O, = Fe,O,

how long will it take to reduce the oxygen content in a closed 25-ft-diameter
spherical container from 21 to 19.5 mol%?
6. Vinyl chloride, a suspected carcinogen, evaporates into the air at a rate of 7.5 g/min.
If the permissible exposure limit (PEL) is 1 ppm, what flow rate of air will be
necessary to achieve the PEL utilizing dilution ventilation? Assuming there is
insufficient mixing of the vinyl chloride and the dilution air, a safety factor of 5 or
more needs to be used to assure proper dilution. Under these conditions what flow
rate is necessary?
7. Chemical cartridge respirators are personal protective devices used to adsorb harm-
ful vapors and gases. A performance analysis of such respirators is similar to that
used in a fixed bed adsorber. The generalized correlation of adsorption potential

tA number of the problems are adapted from Safety, Health, and Loss Prevention in Chemical
Processes: Problems for Undergraduate Engineering Curricula, 0 1990 by the American Institute of
Chemical Engineers, with permission from the Center for Chemical Process Safety/AIChE.