1 Pollution Prevention & Environmental Essentials Conference Paul Haas CSP, CIHUniversity of South FloridaSafetyFlorida Consultation Program
2 Emergency Planning and Disaster Control Safety and Health Planning for the Trades
3 Exposure Modeling (Spill Models) Exposure Assessment Using Modeling To Determine Air Concentration After Chemical Releases
4 Exposure Modeling (Spill Models) The use of models to describe employee exposures is not new, but the Occupational Safety and Health Administration (OSHA) has proposed a simple methodology to use for calculation of air concentrations from spills.OSHA has not determined if these will apply in all cases where employees may be exposed.Chemical reactivity is not considered in these models.
5 Exposure Modeling (Spill Models) More than 20 chemical accidents are reported each day in the U.S, according to data collected by the U.S. Environmental Protection Agency. Responding to these accidents is a dangerous but essential job. In the U.S., this job is usually handled by firefighters from local fire departments.
6 Exposure Modeling (Spill Models) RIO NEUQUEN chemical incident, Houston, Texas, July One of many containers of the culprit substance is pictured.Models can be used for release of chemicals.
8 EXPOSURE MODELING Principles from physical chemistry are applied Applications will be presented in examplesGraphical presentations from Dr. Mark Nicas – UC Berkeley are provided
9 Elements of the Exposure Model A chemical substance releaseDetermine the air concentration from a release into a roomAndEstimate if the release may pose a probable risk inhalation health hazard.
10 Elements of the Exposure Model An airborne exposure model uses the following elements:*The contaminant mass emission rateThe contaminant dispersion in a roomThe employee exposure pattern*Source – OSHA TECHNICAL MANUAL ON PHYSICAL – CHEMICAL MATHEMATICAL EXPOSURE MODELS
11 EXPOSURE ASSESSMENTWhen must an employer conduct an exposure assessment?When there is a substance specific standard (e.g. lead, methylene chloride)When employees notice symptoms or complain of respiratory effectsWhen the workplace contains visible emissions (e.g. fumes, dust aerosols)
12 EXPOSURE ASSESSMENTOSHA Regulations for methods to determine employee exposure can be found for the following:HAZWOPER – 29 CFRRESPIRATORS – 29 CFRSUBSTANCE SPECIFIC STANDARDS – 29 CFR – 1052 (e.g. Formaldehyde)
13 EXPOSURE ASSESSMENTOSHA Regulations do not specify how the employer is to make a reasonable estimation for the purposes of selecting respirators for example (osha.gov/SLTC/respiratory_advisor)OSHA Substance Specific Standards allow for the use of ‘objective evidence’ to estimate exposure.
14 EXPOSURE ASSESSMENTWhen? What? How much employee exposure is there in the workplace?Sampling – Personal exposure monitoringObjective information – DataVariation – Sampling + Data + Safety Factors
15 Air Monitoring Equipment What To Do:1. N/ADiscussion:1. Air monitoring equipment should be employed whenever work processes or response processes change. Other areas where air monitoring are important are:a) change in job task(s)b) changes in weather or temperaturec) anticipated changes in ambient air levels ofcontaminants2. Air monitoring can be either active, with meters or similar devices, or passive using badges.
16 Personal Samplers What To Do: 1. N/A Discussion: 1. For personal sampling, should be accomplished using the “Gold” standard and it is the best method to determine an individual’s exposure.
17 Personal Samplers- Media What To Do:1. N/ADiscussion:1.Various sample media are used to collect mists, gases and vapors such as these shown on the slide.
18 Other Types Site Monitoring ofSite MonitoringHAZCAT KitGeiger Counter- radiationSpecialty Monitors- Passive monitoring badges- TIFF 5000What To Do:1. N/ADiscussion:1. There are other types of equipment that can be used to conduct site assessment and monitoring. A few examples are:HAZCAT Kit -Identifies or categorizes over 1,000 hazardous and non-hazardous substances in 3 to 9 steps or approximately 10 minutes Contains everything needed for efficient field testing of both liquids and solids.Geiger Counter - used to determine if there is a radiation hazardPassive Monitoring Devices - there are various types on the market such as organic vapor and mercuryTIFF “sniffs” the air for Halogenated organic materials
19 EXPOSURE ASSESSMENT BASIC TERMS Exposure Model Air Contaminants Parts Per MillionMilligrams Per Cubic MeterEmployee Exposure
20 EXPOSURE ASSESSMENT BASIC TERMS Exposure Model An exposure model is the description of a:Air contaminantRoom or space volumeEmployee exposure
21 EXPOSURE ASSESSMENT BASIC TERMS Air Contaminants Parts Per Million (PPM)PPM is a ‘dimensionless number’A 1 PPM concentration is $1 in a $1,000,000Milligrams Per Cubic MeterIs in weight per unit volumeExpressed in milligrams per cubic meter for gases, mists, vapors
22 EXPOSURE ASSESSMENT BASIC TERMS Employee Exposure Who, What, When, Why, How Exposed?‘Typical’ or ‘Emergency’ releaseWhat is an ‘Incidental’ release as defined in the HAZWOPER regulation?
23 Chemical Identity and Form · If the chemical is a gas – The molecular weight and the gas density· If the chemical is a liquid – The molecular weight and the vapor pressure· If the chemical is a solid – The molecular weight
24 Material Release Parameters · The room or space volume (V) in cubic meters (m3)· The room supply/exhaust air rate (Q) in cubic meters per minute (m3/min)· The contaminant emission rate function (G) in milligrams per minute (mg/min)
25 Room Ventilation and Volume V = Room volume determined by (Length X Width X Height)Q = Air supply. It is assumed to be the room’s entire supply/exhaust air exchange rate from a mechanically – driven system. note: If room air supply is not known use the following assumptions****Air speed (s) = m/min in a room with no strong air motionAir speed (s) = 7.6 m/min in a room with strong air currents
26 Mass EmissionQ = The product of the air speed times the room area (Speed X Length X Width)Gt = The emission rate function (Gt) is expressed in a release rate of mass-per-time or milligrams per minute (mg/min).Air Concentration in a room after a release (C0) is a ‘worst case’ scenario of the Emission Rate Function over the Room Exhaust Rate or (Gt/Q)
27 ‘Worst Case Scenerio’The air concentration (C0) of a release of a material in a room using the Exposure Model Gt/Q is a ‘worst case’ scenario modelUse the ideal gas law equation – (PV/nRT) to determine the concentration C0
28 ‘Worst Case Scenerio’ The chemical is continually exposed to room air There is no initial air dispersalRoom temperature is constantThere is sufficient time to reach equilibriumEnough chemical mass existsThe ideal gas law holds
29 Ideal Gas Law Background A variation of Boyle’s and Dalton’s lawsP1 V1 = P2 V2T T2PTOTAL = P1 + P2 + … + Pk, for k constituentsApplication includes converting a mass of liquid evaporating per minute to the vapor volume evaporating per minute
30 Dalton’s LawThe total pressure of a gaseous mixture is the sum of the partial pressures exerted by each constituent of the mixturePTOTAL = P1 + P2 + … + Pk, for k constituentsAccording to the ideal gas law, the mole fraction (Yi) of a gas constituent isYi = Pi / PTOTAL , expressed in ppm (parts per million)
31 Ideal Gas Law Constants P = Pressure in mm HgV = Volume in M3T = Temperature in KR = Gas Constant0.623 mm Hg M3MOL-1K-1n = number of moles of gas
32 Ideal Gas LawPV = nRTAt NTP (298.3 K and 760 mm Hg), one mole of gas generates a gas volume V = M3 (24.45 Liters)What about gas in containers?One mole of gas introduced into a rigid container = ? V
33 Ideal Gas LawOne mole of gas introduced into a 1 M3 container will occupy 1 M3 not M3However, a constant temperature will yield a gas partial pressure of 18.6 mmP = nRT/V = (1 mol)(0.623 mm Hg M3MOL-1K-1)(298.3 k)/1 M3 = 18.6 mm
34 G(t), M3/min = (G(t), mg/min)(0.001 g/mg) Vapor VolumeMass per time is converted to volume per time using the following equation:G(t), M3/min = (G(t), mg/min)(0.001 g/mg)XRTA/Mol. Wt. PAThis equation will be explained in examples to illustrate the gas conversion relationship
35 Vapor Pressure (Eq)When the rate of evaporation = rate of condensation in the headspace of containersIf the system is at equilibrium and the headspace air is saturated with chemical vaporThe partial pressure (Pv) of a chemical is related to the temperature, e.g.Pv of benzene at 20 C is 75 mm Hg and 96 mm Hg at 25 C
36 Saturation Concentration (Csat) Csat in ppm= PV in mm Hg X 106760 mm HgThis unifies Boyle’s and Dalton’s Law into the ideal gas law
37 Saturation Concentration (Csat) Csat in mg/M3= (Csat in ppm) X Mol. Wt.24.45This is expressed at NTP conditions (298.3 K, 760 mm Hg)
38 Saturation Concentration (Csat) Csat in mg/M3Can also be determined by the following:= PV X Mol. Wt. X 1000RTThis is the product of the vapor pressure of n moles of gas and the chemical’s molecular weight
39 Spill ModelingExample 1 – Estimate the air concentration using a ‘worse case scenario using an identified chemical and mass emission rateExample 2 – Determine the air concentration of a spill after one minute knowing the room volume and dispersal rate
40 Example #1 (C2Cl4 Release) A container of perchloroethylene (C2Cl4) is left open in a unventilated cabinet.An individual opens the door and is exposedWhat is the perchloroethylene concentration in ppm that the employee is exposed to?
41 Example (C2Cl4 Release) Use the following values in the Csat equation: T = 20 C; Perchloroethylene PV = 14 mm Hg at 20 C; Cabinet Pressure = 760 mm HgCsat = 14 mm Hg X 106760 mm HgCsat = 18,421 ppm
42 Dalton’s Law (Remember!!!) To be rigorous, one must add the partial pressure of the perchloroethylene to the total pressure in the cabinet.This yields a Csat of 18,090 ppm (A 2% difference)This estimate is based on a Qintial of 0, any air movement (Q) > 0 would revise the estimate of concentration downward.
43 Exposure Limits for C2Cl4 For perchloroethylene (C2Cl4 ) the acceptable OSHA maximum peak is 300 ppm** 29 CFR , Table Z-2The initial exposure concentration of 18,090 ppm C2Cl4 is 60 times greater than the acceptable ceiling limit.
44 Example #2 (Gas Release) Carbon Dioxide (CO2) is released into a 10 X 20 square meter (33 X 65 foot2) room from a 10 - liter container. How many parts per million (ppm) of CO2 are released?Assume that the air speed in the room is 3 meters/minute.
45 SolutionFirst, determine the molecular weight of Carbon Dioxide (CO2) in a molar unit (mole) of gas. It is the product of the gas molecules of carbon (C) and oxygen (O):C + O + O2 = or44 milligrams CO2
46 SolutionThen, assume that carbon dioxide gas occupies all of the container and all of the gas is released at once. The total volume of the container is 10 liters (L). The amount of gas released in cubic meters is as follows:10 liters X cubic meter (M3)/ 1000 liters or 0.01 m3
47 SolutionIn a ‘normal’ temperature (T) and pressure (P) environment, a mole of gas occupies a volume (V) 0f M3 (24.45 L) The number (n) of moles of CO2 released is:0.01 M3 CO2 / M3/mole= 0.4 moles CO2
48 AnswerTo determine a ‘worst-case’ scenario, we first determine the partial pressure of the gas release in the room air.
49 AnswerThe partial pressure of the CO2 released is determined using the ideal gas equation:P = nRT/V
50 Temperature & Pressure Room temperatures must be converted to degrees Kelvin (K) using the ideal gas law, so room temperature in C is added to 273.3Pressure is expressed in milliliters of mercury (mm Hg)
51 AnswerPCO2 = nRT/VP = (0.4 mole CO2)( mm Hg . m3 . mole-1 . K-1)(298.3 K)/ 0.01 M3PCO2 = / 0.01 mm Hg = 2.55 mm Hg
52 Answer P1 nRT/V/ P2 nRT/V Partial Pressure = P1/ P2 Concentration is then expressed as the vapor volume of a chemical in its ratio to the vapor volume of air at atmospheric pressure (V/V).
53 V/V – vapor pressure of contaminant X 106 AnswerThe V/V application model is a ‘best-guess’ assumption of exposure. This is the estimated vapor volume of a contaminant in a million parts of air (ppm).V/V – vapor pressure of contaminant X 106vapor pressure of air
54 Concentration (C0)The ‘worst-case concentration (C0) of carbon dioxide released into the room is determined by the following equation:(C0) = Partial Pressure/ Total Atmospheric Pressure in a million parts of air (ppm)
55 Concentration (C0)(C0) = 2.5 mm Hg Carbon Dioxide / 760 mm Hg X 106 = X 106(C0) is 3279 ppm of carbon dioxide from the release of the 10 liter container.
56 Concentration (Ct)(Ct) is the concentration at a time from the initial release C0 to a time tThis takes into account diffusion of the release into a space by airflow
57 Concentration (Ct)What is the expected air concentration of CO2 in the room after one minute or (Gt/Q)?Assume that in one minute the entire container is released or that 3279 ppm exists from the release. We need to determine the concentration of CO2 throughout the room.
58 Concentration (Ct)The room floor area is 200 meters square and the air speed is assumed to be 3 meters/minute so Q is expected to be 3 M/min X 200 M or 600 cubic meters/ minute ( 600 M3/min)Gt?
59 Concentration (Ct)Convert 3279 ppm CO2 to milligrams per cubic meter (mg/M3)C0 of carbon dioxide in mg/m3= 3279 ppm X 44 mg/mole / M3/mole= 5900 mg/M3 of carbon dioxide
60 Concentration (Ct)Answer: After one minute there are mg/M3 / 600 M3 or 9.83 mg/ M3 of carbon dioxide remaining in the room.The concentration is then reduced by a factor of 5900/10 or 59 times less
61 Constant Decay Theory by Dr. Mark Nicas Applications:Objective Exposure AssessmentSpill models from container fillingConfined Spaces?Exponential decay model with an initial concentration (Co)
62 Constant Decay Theory by Dr. Mark Nicas CIN -- mg/m V -- m3G -- mg/min kL -- per minQ -- m3/min C0 -- mg/m3
64 Backpressure Effect on Contaminant Emission Applications:Low vapor pressure chemicals < 1 mm HgPesticidesNerve AgentsNet rate model with a steady state factor for chemicals with a low vapor pressure at equilibrium
65 Backpressure Effect on Contaminant Emission Ct -- mg/m V -- m3G0 -- mg/min t -- minQ -- m3/min Csat -- mg/m3
67 NEAR / FAR FIELD (NF/ FF) with constant contaminant emission rate Applications:Operations with Dilution VentilationAssume a well mixed room dispersion patternModel describes zonal concentration in the near hemispherical free surface area and an air flow rate to a far field
68 Near Field / Far Field Model G – mg /min Q -- m3/minNF -- Volume m3 -- m3/minFF -- Volume m3 *( -b + SQRT((b2) - 4*cc))Dr.Nicas
70 Spherical Turbulent Diffusion – Pulse Release Applications:Operations with Local Exhaust VentilationStorage tank filling operations in buildingsModels apply turbulent eddy diffusion for a continuous concentration near an emission source
71 The Spherical Turbulent Diffusion Model without Advection - Pulse Release M0 -- mg Y -- mDT -- m2/min X -- mr -- SQRT(X2 + Y2 + Z2) -- m Z -- m