General Knowledge Question for Technical safety and Process Safety Engineer
Question 1: - National Fire Protection Association Standard No. 68 (1988 edition) defines an explosion as "The bursting or rupture of an enclosure or container due to the development of internal pressure from a deflagration." A deflagration is "Propagation of a combustion zone at a velocity that is less than the speed of sound in the unreacted medium." Detonation, which is "Propagation of a combustion zone at a velocity that is greater than the speed of sound in the unreacted medium," may also occur, and if the total energy release is the same, a detonation will be more damaging. Deflagrations may occur when three types of fuel-air mixtures are present: those with fuels in the form of gases, mists, and dusts.
The fuel must be within the flammable range for the deflagration to be initiated. The oxygen in the air serves as the oxidant, and a small spark or flame is all that is needed to ignite the mixture. If a deflagration occurs in a closed space, such as a building, an explosion may occur in which the building is destroyed. There are several methods available for preventing an explosion, including prevention of formation of flammable mixtures in the building. If, despite efforts at prevention, flammable mixtures do form and they are ignited, the damage to the building may be reduced by venting the deflagration. NFPA 68 provides a simple equation for determining the vent area required to prevent unusually large damage to a building if a deflagration occurs in the building. The equation is
A= CA/(P) 0.5
Where
Av = vent area (ft2 or m2 )
C = venting equation constant
As 2.2 = internal surface area of enclosure (ft or m)
Pred = maximum internal pressure that can be withstood by the weakest
structural element (psi or kPa)
The venting equation constant is given in NFPA 68 for various classes of gases and dusts. It is based on an extensive set of experimental tests using the gas or dust in question. The tests are difficult to perform, and the results vary somewhat from test to test, so the NFPA 68 values are meant to be used as a guide to establish conservative design bases. The units of the constant are (pressure)0.5 and it as always assumed that the outside of the enclosure is at atmospheric pressure, so the pressure used in the equation is the gauge pressure. The internal surface area includes the floor, the walls, and the ceiling. While a complete design will require close reference to NFPA 68, we can make a simple estimate of the vent area required from the venting equation for simple building layouts.
Question 2: - Trichloroethylene has a molecular weight of approximately 131.5, so the vapors are much more dense than air. As a first thought, one would not expect to find a high concentration of this material above an open tank because we would assume that the vapor, being dense, would sink to the floor. If this were so, then we would place the inlet of a local exhaust hood for such a tank near the floor. However, Industrial Ventilation* points out that toxic concentrations of many materials are not much more dense than the air itself, so where there can be mixing with the air we may not assume that all the vapors will go to the floor.
For the case of trichloroethylene OSHA has established a time-weighted average 8 hr PEL of 100 ppm; a 15-min ceiling of 200 ppm; and a 5-min peak of 300 ppm. Determine the density of a mixture of trichloroethylene in air at each of these limiting concentrations, as well as that of a saturated vapor at 2YC, and compare the values with that of pure air at the same temperature. That is, determine the specific gravity (relative to air) for each mixture. Which, if any, of these concentrations would you feel might readily sink to the floor, and which might circulate with the normal air currents which we would find in a room?
Question 3: - If it is assumed that loss of consciousness occurs when the average concentration of oxygen in the lungs and tracheobronchial tract drops below 11%, estimate how many breaths a worker will be able to take when he enters a vessel that contains 100% nitrogen before he loses consciousness. If help comes in time, he may recover if he gets into fresh air before the average concentration drops below about 6%. How much time is there to help him? (A person who is breathing normally will inhale about 30 L/min at 500 mll inhalation.) What might you conclude about using air-purifying respirators rather than air-supplying respirators for such a case?
Emergency plans are being formulated so that rapid action can be taken in the event of an equipment failure. It is predicted that if a particular pipeline were to rupture it would release ammonia at a rate of 100 lb l sec. Persons exposed to 500 parts per million (ppm) of ammonia will be endangered and anywhere that the concentration might be that high should be evacuated until repairs are made. What recommendation would you make as to how far from the rupture people should be evacuated.
The wind is 6 miles per hour and the sun is shining brightly.
- The night is overcast and the wind is 10 miles per hour.
Question 4: - 2-L grab sample of air (33"C, 99 kPa, and 70% relative humidity) was collected in a stainless steel container which had been evacuated to a hard vacuum. The sample was admitted to the container by opening a valve and allowing the air to enter until the pressures were equalized, whereupon, clean dry helium was admitted until the pressure was 500 kPa. The sample was taken to a gas chromatography laboratory where the temperature was 23°C.
The next day, a sample from the container was released to the chromatograph until the pressure in the container was reduced to 400 kPa. On analysis, the portion of the sample admitted to the instrument was found to contain 1.65 ng of benzene. What is the concentration of benzene in the original workroom air (in mg fm3), and is it in excess of the permissible exposure limit (PEL) of 1 part per million (ppm) on a mole basis?
NOTE: A grab sample as considered in this problem would not usually be used for determining compliance with the PEL, which is an 8-hr average limit.
Question 5: - Consider the following hypothetical accident: 1400 lb of PCB has just been spilled in a river because a rail car hauling a transformer in for replacement of the dielectric fluid was derailed while crossing a bridge. It appears that the chemical has collected in large pools on the bottom, over a combined area of about 150 ft2. Estimate the rate at which the chemical will be released into the water, and how long it would require for 1% of the chemical to be dissolved.
The following data may be assumed: Solubility of PCB in water: 0.25 mg/L Mass transfer coefficient, liquid pool to water: 0.5 Ib mole/ft2 hr PCB is a general term for a number of compounds with similar properties, and these were normally used as mixtures. Assume the average molecular weight in this case is 260.
Question 6: - Emergency plans are being formulated so that rapid action can be taken in the event of an accident. It is predicted that if a particular accident occurs, 1.0 kg of chlorine will be instantaneously released. There is a residential area 500 m away from the prospective release location. For a situation when there is a wind of 2 d s , blowing toward the residential area, estimate the time required for the gas cloud to arrive at the residences and the maximum concentration that would occur in the center of the cloud.
How does this concentration compare with 1 percent of the TLV? The TLV for chlorine gas is 1 ppm (a molar ratio). Determine the worst case situation, assuming the different stabilities presented above. Which case should we plan for?
Question 7: - A distillation column is separating a feed mixture of ethanol and water. The feed enters as a saturated liquid at an ethanol mole fraction of 0.035. The feed rate is 50,000 lblhr, and the overhead composition is 0.83 mole fraction ethanol. Assume that the bottom are about 0.99 mole fraction water.
The column normally operates at 1 atm as a nominal pressure. However, if the cooling to the condenser is lost, how long will it take for the pressure to rise to 1.5 atm ? If the pressure must not rise above 1.5 atm, what venting rate in pounds per second will be required? Note that there is a number of missing data items in the problem statement. These should be supplied by you if they are not supplied by your instructor. You are to make suitable engineering judgments regarding the height and diameter of the column, and such other items as might be required.