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

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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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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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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

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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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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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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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(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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Let


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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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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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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

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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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11. Industrial Hygiene

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