Industrial HygieneERT 312

Lecture 7 – Identification, Evaluation and Control

Identification Able to identify the hazard from single

exposure or potential combined effects from multiple exposures

Require deep study on the chemical process, operating conditions and operating procedures

Source of information; Process design descriptions Operating instructions Safety reviews Equipment specs Etc.

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3

4

Material Safety Data Sheets (MSDS) Chemical Safety Data Sheets (CSDS) MSDS lists the physical properties of a

substance that may be required to determine the potential hazards of the substance

Manufacturer/supplier is responsible to provide the MSDS to their customers

* Example of MSDS

5

Evaluation To determine the extent and degree of

employee exposure to toxicants and physical hazards in the workplace

Once exposure data obtained, comparison is being made to acceptable occupational health standards eg: TLVs, PELs and IDLH concentrations (page 56)

Then, the decision on proper control measure can be made accordingly in order to reduce the risk

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Threshold Limit Value (TLV) of a chemical substance is a level to which it is believed a worker can be exposed day after day for a working lifetime without adverse health effects

The Permissible Exposure Limit (PEL or OSHA PEL) is a legal limit for exposure of an employee to a substance or physical agent. For substances it is usually expressed in parts per million (ppm), or sometimes in milligrams per cubic metre (mg/m3)

IDLH is an initials for Immediately Dangerous to Life and Health, and is defined by the NIOSH as exposure to airborne contaminants that is "likely to cause death or immediate or delayed permanent adverse health effects or prevent escape from such an environment”

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Table 2.7 – established by ACGIH

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TLVs units – ppm, mg/m3, For dust – mg/m3 or mppcf For vapors, concentration in ppm;

Cppm =

=

T (temperature, Kelvin), P (absolute pressure, atm) M (molecular weight, g/g-mol)

)/)(1

)(273

(4.22 3mmg

P

T

M

)/)((08205.0 3mmgPM

T

Equation 1

Equation 2

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Problem 2.7 (Crowl & Louvar, 2002) How much acetone liquid (ml) required to

produce a vapor concentration of 200 ppm in a room of dimension 3 x 4 x 10 m? Given T is 25°C, P is 1 atm, molecular weight is 58.1 and specific gravity is 0.7899.

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Evaluation Exposure of Organic Toxicants The simplest way to determine worker exposures

is through continuous monitoring of the air concentrations.

For computation of continuous concentration data C(t) the TWA concentration,

C(t) the concentration of the toxicant in the air, ppm @ mg/m3

tw the worker shift time in hours

wt

dttCTWA0

)(8

1Equation 3

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Sometimes, continuous monitoring is not feasible. Therefore, intermittent samples representing worker exposure at fixed points of time are obtained.

8

...2211 nn

TCTCTCTWA

Single Component Exposure, workers are overexposed if the sum of conc. >

permitted TWA

Equation 4

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Example 3.3 (Crowl & Louvar, 2002) Determine the 8-hr TWA worker exposure if

the worker is exposed to toluene vapors as follows;

Solution:

Answer: 155 ppm

Duration (h) Concentration (ppm)

2 110

2 330

4 90

8

...2211 nn

TCTCTCTWA

Equation 5

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For a case of more than 1 toxicant is present in the workplace; the combined exposures from multiple toxicants with different TLV-TWAs is determined by;

n the total number of toxicants Ci the conc. of toxicant i with

respect to the other toxicants (TLV-TWA)ithe TLV-TWA for toxicant sp. i

n

ii

i

TWATLV

C1 )(

If the sum of the equation > 1, workers

are overexposedEquation 6

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The mixture also TLV-TWA can be computed using equation below;

n

ii

i

n

ii

mix

TWATLVC

CTWATLV

1

1

)(

)(

If the total mixture conc. > (TLV-TWA)mix , workers are overexposed

Equation 7

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Example 3.2 (Crowl & Louvar, 2002)Air contains 5 ppm of diethylamine (TLV-TWA = 10 ppm), 20 ppm cyclohexanol (TLV-TWA = 50 ppm) and 10 ppm of propylene oxide (TLV-TWA = 20 ppm). What is the mixture TLV-TWA and has this level been exceeded?

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Evaluation of exposure to dusts Dusts particle size range of 0.2-0.5 µm Particles > 0.5 µm unable to penetrate the

lungs Particle < 0.2 µm settle out too slowly, most

exhaled with the air Units: mg/m3 @ mg/mppcf

n

ii

i

n

ii

mix

TLVC

CTLV

1

1Equation 8

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Example 3-5 (Crowl & Louvar, 2002) Determine the TLV for a uniform mixture of

dusts containing the following particles;

Solution:

Answer: 6.8 mppcf

Type of dust Concentration (wt.%)

TLV (mppcf)

Nonasbestiform 70 20

Quartz 30 2.7

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Evaluation of exposure to Noise Noise levels are measured in decibels (dB) A dB is a relative logarithmic scale used to

compare the intensities of two sounds. If one sound is at intensity I and another sound is at intensity Io, then the difference in intensity levels in dB is given;

Noise intensity (dB) = - 10 log10(I/Io)

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Example 3.6 (Crowl & Louvar, 2002)Determine whether the following noise level is permissible with no additional control features:

Noise Level (dBa) Duration (hr) Max. allowed (hr)

85 3.6 No limit

95 3.0 4

110 0.5 0.5

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Solution:

(TLV – TWA)mix, noise =

The sum > 1.0, workers are immediately required to wear ear protection. For long term plan, noise reduction control should be applied.

n

ii

i

TWATLV

C1 )(

75.15.0

5.0

4

3

limit no

6.3

)(1

n

ii

i

TWATLV

C

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Evaluation of exposure to Toxic Vapors

Ventilation rate, Qv

(volume/time)Volatile rate out, kQvC

(mass/time)

Evolution rate of volatile, Qm

(mass/time)

Enclosure volume, V Volatile concentration, C(mass/volume)

6

v

gmppm 10

PMkQ

TRQC

Equation 9

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Assumptions The calculated concentration is an average

concentration in the enclosure. Localized conditions could result in significantly higher concentrations; workers directly above an open container might be exposed to higher concentrations

A steady-state condition is assumed; that is, the accumulation term in the mass balance is 0

The non-ideal mixing factor, k varies from 0.1 – 0.5 for most practical situations. For perfect mixing, k = 1

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Example 3.7 (Crowl & Louvar, 2002)An open toluene container in an enclosure is weighed 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 80oF and the pressure is 1 atm. Estimate the concentration of toluene vapor in the enclosure, and compare your answer to the TLV for toluene of 50 ppm.

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Solution Use equation 9 to solve the problem

From data given;

Qm 0.1 g/min

Rg 0.7302 ft3.atm/lb-mol.oR

T 80oF = 540oR

Qv 100 ft3/min

M 92 lbm/lb-mol

P 1 atm

k ?

Answer: kCppm = 9.43 ppm

K varies from 0.1 – 0.5, therefore Cppm may vary from 18.9 – 94.3 ppm. Actual vapor sampling is recommended to ensure that TLV is not exceeded

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v

gmppm 10

PMkQ

TRQC

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Estimating the vaporization rate of a liquid

Qm Qm

Volatile Substances

Open Vessel Chemical Spill

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General expression for vaporization rate, Qm (mass/time):

M Molecular weight of volatile substance K mass transfer coefficient (length/time) for

an area A Rg ideal gas constant TL absolute temperature of the liquid

Lg

sat

m TR

pPMKAQ

)( Equation 10

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For most cases, Psat >> p;

The equation is used to estimate the evaporation rate of volatile from an open vessel or a spill of liquid

Lg

sat

m TR

MKAPQ Equation 11

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To estimate the concentration of volatile in enclosure resulting from evaporation of a liquid;

K gas mass transfer coefficient

610Lv

sat

ppm PTkQ

KATPC

610PkQ

KAPC

v

sat

ppm

Most events, T =

TL

Equation 12

Equation 13

Equa. 11 used in Equa .9

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Estimation of K, gas mass transfer coefficient;

a constant D gas-phase diffusion coeeficient

3/2aDK Equation 14

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To determine the ratio of the mass transfer coefficient between species K and a reference species Ko;

The gas-phase diffusion coefficients are estimated from the molecular weight, M of the species;

3/2

ooD

D

K

K

M

M

D

Do

o

Equation 15

Equation 16

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Combined equation 15 & 16, simplified;

Kwater 0.83 cm/s

3/1

M

MKK o

oEquation 17

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Example 3.8 (Crowl & Louvar, 2002) A large open tank with a 5-ft diameter

contains toluene. Estimate the evaporation rate from this tank assuming a temperature of 77oF and a pressure of 1 atm. If the ventilation rate is 3000 ft3/min, estimate the concentration of toluene in this workplace enclosure.

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Evaluation of exposure during vessel filling operations For this case, volatile emissions are generated

from 2 sources: Evaporation of a liquid, (Qm)1

Displacement of the vapor in the vapor space by the liquid filling the vessel, (Qm)2

Therefore, the net generation of volatile;

(Qm) = (Qm)1 + (Qm)2

Equation 18

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Vapor

Liquid

Volatile in

Vessel

Total Source = Evaporation + Displaced Air

Evaporation

rf constant filling rate of the vessel (time-1)

v density of the volatile vapor

Lg

sat

m TR

MKAPQ

1)(

vcfmVrQ

2)(

Equation 19

Equation 20

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Hence, the net source term;

)()()(21

KAVrTR

MPQQQ

cf

Lg

sat

mmm

Equation 20

Equation 21

610)( KAVrPkQ

PC

cf

v

sat

ppm

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Problem 3.24 (Crowl & Louvar, 2002)55-gallon drums are being filled with 2-butoxyethanol. The drums are being splash-filled at the rate of 30 drums per hour. The bung opening through which the drums are being filled has an area of 8 cm2. estimate the ambient vapor concentration if the ventilation rate is 3000 ft3/min. the vapor pressure for 2-butoxyethanol is 0.6 mm Hg under these conditions.

2-butoxyethanol chemical formula: HOCH2C2HOC4H9

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Solution:

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Appendix A

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Appendix B Conversion of Fahrenheit (°F) to Rankine (°R)

1st stepConvert Fahrenheit to Celcius2nd stepConvert Celcius to Kelvin3rd stepConvert Kelvin to Rankine

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8.1

32 F

C

TT

15.273CKTT

KRTT 8.1