## 000u

3v,2

(0.001)(/2 - /,) + (0.47 x 10- if) = 0.30 (0.001)(15) + (0.47 x 10"3)(?2 - t,)(t2 + t,) = 0.30 (0.001)(15) + (0.47 x 10~3)(15)(i2 + h) = 0.30

The total time to reach the STEL of 0.02 ppm on a TWA basis is 15 +f,, or f2 = 27.7 min.

The time in minutes, t, to reach | LEL is given by

325,000 (ppm) = 0.001 (ppm) + 0.94 x 10"3? / = 3,460,000 min (6.6 yr)

The average concentration in terms of time may be expressed as follows: _ (0.001) + (0.001 + 0.94 x 10"3i)

Thus, for an 8-h averaging period,

Therefore, the TLV is exceeded by the mean TDI vapor concentration in the room over the 8-h averaging period by a factor of 45.4.

### TOX.9 IDLH AND LETHAL LEVEL

As described earlier, the immediately dangerous to life and health (IDLH) level is the maximum concentration of a substance to which one can be exposed for 30 min without irreversible health effects or death. A lethal level is the concentration at which death is almost certain to occur. The IDLH values were determined by the National Institute for Occupational Safety and Health (NIOSH) for the purpose of respirator selection. Respirators provide protection against the inhalation of toxic or harmful materials and may be necessary in certain hazardous situations.

Carbon dioxide is not normally considered to be a threat to human health. It is exhaled by humans and is found in the atmosphere at about 3000 parts per million (ppm). However, at high concentrations it can be a hazard and may cause headaches, dizziness, increased heart rate, asphyxiation, convulsions, or coma.

Two large bottles of flammable solvent were ignited by an undetermined ignition source after being knocked over and broken by a janitor while cleaning a 10 ft x 10 ft x 10 ft research laboratory. The laboratory ventilator was shut off and the fire was fought with a 10-lb C02 fire extinguisher. As the burning solvent had covered much of the floor area, the fire extinguisher was completely emptied in extinguishing the fire.

The IDLH level for C02 set by NIOSH is 50,000 ppm. At that level, vomiting, dizziness, disorientation, and breathing difficulties occur after a 30 min exposure. At a 10% level (100,000 ppm), death can occur after a few minutes even if the oxygen in the atmosphere would otherwise support life.

Calculate the concentration of C02 in the room after the fire extinguisher is emptied. Does it exceed the IDLH value? Assume that the gas mixture in the room is uniformly mixed, that the temperature in the room is 30°C (warmed by the fire above normal room temperature of 20°C), and that the ambient pressure is 1 atm.

Solution

First, calculate the number of moles of C02, «co,, discharged by the fire extinguisher:

nC02 = (10 lb C02)(454 g/lb)/(44 g/gmolC02) = 103 gmol of C02

The volume of the room, V, is

V = (10 ft)(10ft)(10 ft)(28.3 L/ft3) = 28,300 liters

The ideal gas law is used to calculate the total number of moles of gas in the room, n:

= 1138 gmol gas

The concentration or mole fraction of C02 in the room, xC0;, may now be calculated:

The IDLH level is 5.0% and the lethal level is 10.0%. Therefore, the level in the room of 9.05% does exceed the IDLH level for C02. It is also dangerously close to the lethal level. The person extinguishing the fire is in great danger and should take appropriate safety measures.

If a dangerous level is present, consideration must be given to using protective equipment such as a respirator. Respirators protect the individual from harmful materials in the air. Air-purifying respirators will clean the air but will not protect users against an oxygen-deficient atmosphere. Thus, air-purifying respirators are not used in IDLH applications. The only respirators that are recommended for fighting fires are self-contained breathing apparatuses with full facepieces. Recommendations for the selection of the proper respirator are based on the most restrictive of the occupational exposure limits.

If a toxic material is dispersed into the air, the engineer/scientist must know how high its concentration can be without causing danger to people. The Occupational Safety and Health Administration (OSHA) has set concentration levels for many substances. This concentration level is called the permissible exposure limit (PEL). The PEL is synonymous in most application with the TLV-TWA (threshold limit value-time-weighted average).

Most toxicity studies are performed by using test animals. Humans obviously cannot be exposed to lethal concentrations of toxic materials to determine toxicity. Please note that there are some differences in chemical tolerance levels for humans and animals. The differences include metabolism and other factors. Thus, toxicity tests are not always easy to interpret. However, the results of animal toxicity tests are used to guide the selection of acceptable exposure limits for humans.

### TOX. 10 ACCIDENTAL VAPOR EMISSION

Many industrial chemicals are toxic or flammable, or sometimes both. Regardless of whether the chemical is toxic or flammable, it can present a danger to plant operators and the public if it is released from its container. Substantial efforts are taken to assure that toxic or flammable materials are not spilled or released from containment. There is always a chance, however, that such materials might be released. Therefore, provisions must be made to protect the plant operators and anyone who lives or works in the vicinity.

A certain poorly ventilated chemical storage room (10 ft x 20 ft x 8 ft) has a ceiling fan but no air conditioner. The air in the room is at 51 °F and 1.0 atm pressure. Inside this room, a 1 -lb bottle of iron (III) sulfide (Fe2S3) sits next to a bottle of sulfuric acid containing 1 lb H2S04 in water. An earthquake (or perhaps the elbow of a passing technician) sends the bottles on the shelf crashing to the floor where the bottles break, and their contents mix and react to form iron (III) sulfate [Fe2(S03)] and hydrogen sulfide (H2S).

1. Calculate the maximum H2S concentration that could be reached in the room assuming rapid mixing by the ceiling fan with no addition of outside air (poor ventilation). Compare your result with the TLV (10 ppm) and IDLH (300 ppm) levels for H2S.

2. Later, when exhausted from the room, the H2S mixes with outside air. What will be the final volume of the H2S cloud when the concentration finally reaches the TLV?

Solution

1. Balance the chemical equation:

amount before

Fe2S:

+ 3H2S04

3H2S

lib lib

reaction 0.0048 lbmol 0.010 Ibmol

The molecular weights of Fe2S3 and H2S04 are 208 and 98, respectively.

The terms limiting reactant and excess reactant refer to the actual number of moles present in relation to the stoichiometric proportion required for the reaction to proceed to completion. See Problem STC.3 in Chapter 5. From the stoichiometry of the reaction, 3 lbmol of H2S04 are required to react with each lbmol of Fe2S3. The sulfuric acid is the limiting reactant and the iron (III) sulfide is the excess reactant. In other words, 0.0144 lbmol of H2S04 is required to react with each 0.0048 lbmol of Fe2S3, or 0.030 lbmol of Fe2S3 is required to react with 0.010 lbmol of H2S04.

Calculate the moles of H2S generated, nu s:

The final H2S concentration in the room in ppm, CH2S, can now be calculated. At 32°F and 1 atm, one lbmol of an ideal gas occupies 359 ft3; at 51°F, one lbmol occupies nHiS = (0.010 lbmol H2S04)(3H2S/3H2S04) = 0.010 lbmol

Next, convert the moles to mass:

Therefore,

1600 ft3

1600 ft3

This concentration of H2S far exceeds the TLV (10 ppm) as well as the IDLH (300 ppm).

2. Calculate the volume of H2S in the room at a room concentration of lOppm. Determine the dilution factor required to decrease the H2S concentration to the TVL.

Dilution factor = 2733ppm/lOppm = 273.3

Assuming that the outside air is also at 51°C and 1.0 atm, calculate the total gas volume required to reach the TVL.

Note that this volume includes the gas still in the room.

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