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5/23/2018 11. Industrial Hygiene
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Industrial Hygiene
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Estimating Worker Exposures to
Toxic Vapors
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Enclosure volume, V Concentration of volatile, C(Mass/ Volume)
Ventilation rate, Qv
(Volume/ Time)
Evolution rate of volatile, Qm
(Mass/ Time)
Volatile rate out, kQvC
(Mass/ Time)
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kvaries from 0.1 to 0.5
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Example
4
An open toluene container in an enclosure isweighed as a function of time, and it is determined
that the average evaporation rate is 0.1 g/min. The
ventilation rate is 100 ft3/min. The temperature is
80F and the pressure is 1 atm. Estimate theconcentration of toluene vapor in the enclosure,
and compare your answer to the TLV for toluene of
50 ppm.
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Solution
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Because the value of k is not known directly, it mustbe used as a parameter.
Because k varies from 0.1 to 0.5, the concentration isexpected to vary from 18.9 ppm to 94.3 ppm.
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Estimating the Vaporization Rate
of a Liquid
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Liquids with high saturation vapor
pressures evaporate faster.
Hence, the evaporation rate (mass/time) is a
function of the saturation vapor pressure.
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Estimating the Vaporization Rate
of a Liquid
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For vaporization into stagnant air, thevaporization rate is proportional to the
difference between the saturation and partial
pressure of the vapor.
PSatis the saturation vapor pressure of the
pure liquid at the temperature of the liquid and
p is the partial pressure of the vapor in the
bulk stagnant gas above the liquid.
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Estimating the Vaporization Rate
of a Liquid
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Above equation is used to estimate the vaporization rate of volatilefrom an open vessel or from a spill of liquid.
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The gas mass transfer coefficient is estimated using the relationship
whereais a constant and
Dis the gas-phase diffusion coefficient
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It is used to determine the ratio of the mass transfer coefficients
between the species of interest Kand a reference species KO
The gas-phase diffusion coefficients are estimated from the
molecular weights Mof the species
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Example
12
A large open tank with a 5-ft diameter containstoluene. Estimate the evaporation rate from this
tank assuming a temperature of 77F and a
pressure of 1 atm. If the ventilation rate is 3000
ft3/min, estimate the concentration of toluene in
this workplace enclosure. TLV for toluene is 50
ppm.
For water
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Solution
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The molecular weight of toluene is 92. The masstransfer coefficient is estimated from following
Equation, using water as a reference:
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The pool area is
The evaporation rate is
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The concentration is estimated using Equation
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The concentration will range from 460 ppm to2300 ppm, depending on the value of k.
Because the TLV for toluene is 50 ppm,additional ventilation is recommended, or the
amount of exposed surface area should be
reduced.
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The amount of ventilation required to reducethe concentration from 2300 ppm to 50 ppm is
This represents an impractical level of general
ventilation.
Potential solutions:
containing the toluene in a closed vessel or
using local ventilation at the vessel opening.
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Estimating Worker Exposures
during Vessel Filling Operations
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Estimating Worker Exposures
during Vessel Filling Operations
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For vessels being filled with liquid, volatile
emissions are generated from two sources as
shown in Figure.
1. Evaporation of the liquid, represented by
Equation and
2. Displacement of the vapor in the vapor space
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Estimating Worker Exposures
during Vessel Filling Operations
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The net generation of volatile is the sum of thetwo sources:
(Qm)1= source resulting from evaporation and (Qm)2= source resulting from displacement
The source term (Q
m)1 is computed usingEquation
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Estimating Worker Exposures
during Vessel Filling Operations
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(Qm)2 is determined by assuming that the
vapor is completely saturated with the volatile.
Generally, an adjustment is introduced for less
than saturated conditions.
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Estimating Worker Exposures
during Vessel Filling Operations
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Let
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Estimating Worker Exposures
during Vessel Filling Operations
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rfVc = volumetric rate of bulk vapor being
displaced from the drum (volume/time).
If vis the density of the volatile vapor, then
rfVc
v
= mass rate of volatile displaced from
the container (mass/time).
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and it follows that
It can be modified for container vapors that are not saturated with
the volatile.
Let represent this adjustment factor; then,
Using the ideal gas law,
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Estimating Worker Exposures
during Vessel Filling Operations
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For splash filling (filling from the top of acontainer with the liquid splashing to the
bottom), = 1.
For subsurface filling (by a dip leg to the
bottom of the tank),= 0.5.
The net source term resulting from filling:
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Estimating Worker Exposures
during Vessel Filling Operations
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Vapor concentration (in ppm) in an enclosureresulting from a filling operation assuming T =
TL:
In practical situations, the evaporation term
KA is much smaller than the displacement
term and can be neglected.
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Example
27
Railroad cars are being splash-filled withtoluene. The 10,000-gal cars are being filled at
the rate of one every 8 hr. The filling hole in the
tank car is 4 in. in diameter. Estimate the
concentration of toluene vapor as a result of this
filling operation. The ventilation rate is
estimated at 3000 ft3/min.
The temperature is 77F and the pressure is 1atm.
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Solution
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As expected, the evaporation term is small compared to the
displacement term.
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The actual concentration could range from 69 ppm to 344 ppm,
depending on the value of k.
Sampling to ensure that the concentration is below 50 ppm is
recommended.
For subsurface filling, = 0.5, and the concentration range is
reduced to 35-172 ppm.
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Ventilation
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