Michigan Technological University 1 David R. Shonnard Department of Chemical Engineering Michigan Technological University Detailed Environmental Assessment of Chemical Process Flowsheets - Chapter 11 Michigan Technological University 2 Outline ! Educational goals and topics covered in the module ! Review of risk assessment concepts ! Introduction to environmental multimedia models ! Tier III environmental impact assessment for chemical process flowsheets After the completing flowsheet input output structure, unit operation designations, and mass/heat integration, the last step for improving the environmental performance of a chemical process design is a detailed environmental impact assessment of a process flowsheet
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Michigan Technological University1
David R. ShonnardDepartment of Chemical Engineering
Michigan Technological University
Detailed Environmental Assessment of Chemical Process Flowsheets - Chapter 11
Michigan Technological University2
Outline
! Educational goals and topics covered in the module
! Review of risk assessment concepts
! Introduction to environmental multimedia models
! Tier III environmental impact assessment for chemical process flowsheets
After the completing flowsheet input output structure, unit operation designations, and mass/heat integration, the last step for improving the environmental performance of a chemical process design is a detailed
environmental impact assessment of a process flowsheet
Michigan Technological University3
Educational goals and topics covered in the module
Students will:! learn to apply a systematic risk assessment methodology to the
evaluation of chemical process designs
! integrate emission estimation, environmental fate and transport calculation, and relative risk assessment to rank process designalternatives
Michigan Technological University4
Design Stage P2 Tools Environmental Evaluation
Book Chapter
1. Earliest Design Stage
• Green Chemistry • atom efficiency
Tier 1 (persistence, bioaccumulation, toxicity)
7, 8
2. Preliminary Design Stage
Release estimation, optimum choice of • mass separating agents • process units • processing conditions
Tier 2 (material usage, energy consumption, emission of targeted pollutants)
toxic chemical releases energy consumption resource depletion
Environmental Impactsglobal warming ozone layer depletion air quality – smog acidification ecotoxicityhuman health effects, carcinogenic and non carcinogenic resource depletion
Chapter 11: chemical manufacturing stage only - Chapter 13: all stages
Michigan Technological University6
Essential features of environmental impact assessment for chemical process design
Computationally efficient→ Environmental performance indices to be quickly calculated
using output from commercial process simulators
→ Multiple environmental impacts considered
Link waste generation and release to environmental impacts→ Environmental indices linked to process parameters
Impacts based on a systematic risk assessment methodologyRelease estimates → fate and transport → exposure → risk
Michigan Technological University7
Systematic risk assessment methodology
National Academy of Sciences, 1983
1. Hazard Identification (which chemicals are important?)
2. Exposure assessment (release estimation, fate and transport, dose assessment)
CR - contact rate (m3 air inhaled / day)EF - exposure frequency (days exposed / yr)ED - exposure duration (yr)BW - body weight (kg)AT - averaging time (number of days in a lifetime)
Result: # excess cancers per 106 cases in the population; 10-4 to 10-6 acceptable
Disadvantage: Only a single compartment is modeled / Computationally inefficientHighly uncertain prediction of risk
Michigan Technological University9
Carcinogenic Risk Example (inhalation route)
Relative risk calculation(what is the relative toxic potency?)
Relative Risk =
(Ca × CR × EF × ED)
(BW × AT)× SF
i
(Ca × CR × EF × ED)(BW × AT)
× SF
Benchmark
= Ca × SF[ ]i
Ca × SF[ ]Benchmark
Result: Risk of a chemical relative to a well-studied benchmark compound
Advantage: If C is calculated for all compartments using a multimedia compartment model, computationally efficient
Michigan Technological University10
Airborne emissions estimation - chapter 8
"" Unit Specific EPA Emission Factors## Distillation/stripping column vents# Reactor vents# Fugitive sources
" Criteria Pollutants from Utility Consumption# Factors for CO2, CO, SO2, NOx, # AP- 42 (EPA) factors
" Process Simulators (e.g. HYSYS)
Michigan Technological University11
Model Domain Parameters• surface area - 104 -105 km2
• 90% land area, 10% water• height of atmosphere - 1 km• soil depth - 10 cm• depth of sediment layer - 1 cm• multiphase compartments
Multimedia compartment model Processes modeled• emission inputs, E• advection in and out, DA• intercompartment mass transfer,Di,j
• reaction loss, DR
Multimedia compartment model formulation -Chapter 11.2
Mackay, D. 1991, ”Multimedia Environmental Models", 1st edition,, Lewis Publishers, Chelsea, MI
Michigan Technological University12
Fugacity - a thermodynamic property of a chemical and represents the “escaping tendency” of the chemical from an environmental phase (air, water, or solid) and has units of pressure (Pa).
At equilibrium, the fugacity of a chemical in one phase is equalto the fugacity of the chemical (i) in the adjoining phasefor example:
fi (air) = fi (water)
Also, the fugacity is related to the molar concentration using the fugacity capacity, Z
advection (bulk flow) emissions andbulk flow in Ii = Ei + GAiCBi
bulk flow out DAi = GAiZCi
reaction DRi = kRi Vi ZCi
Michigan Technological University17
Table 11.2-4. Mole Balance Equations for the Mackay Level III Fugacity Model
Air I1 + f2D21 + f3D31 = f1DT1
Water I2 + f1D12 + f3D32 + f4D42 = f2DT2
Soil I3 + f1D13 = f3DT3
Sediment I4 + f2D24 = f4DT4
where the left hand side is the sum of all gains and the right hand side is the sum of alllosses, Ii = Ei + GaiCCi, I4 usually being zero. The D values on the right hand side are;
DT1 = DR1 + DA1 + D12 + D13
DT2 = DR2 + DA2 + D21 + D24
DT3 = DR3 + DA3 + D31 + D32
DT4 = DR4 + DA4 + D42
The solution for the unknown fugacities in each compartment is;
(a) 1000 kg/hr emitted into the air compartment(b) 1000 kg/hr emitted into the water compartment(c) 1000 kg/hr emitted into the soil compartment
Michigan Technological University21
1. The percentages in each environmental compartment depend upon the emission scenarioa) the highest air concentrations result from emission into the air
b) the highest water concentrations are from emission into water
c) the highest soil concentrations are from emission into soild) highest sediment concentrations are from emission into water
2. Chemical properties dictate percentages and amounts
a) high KH results in high air concentrations
b) high KOW results in high soil concentrationsc) high reactions half lives results in highest pollutant amounts
Multimedia compartment model typical results - interpretations
Michigan Technological University22
Tier 3 Relative risk index formulation for one environmental impact category - Ch 11.3
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IGW ,i* = GWPi
Global Warming
IGW ,i* = NC
MWCO2
MWi
Ozone Depletion
IOD,i* = ODPi
Smog Formation
ISF,i* =
MIRi
MIRROG
Acid Rain
IAR ,i* =
ARPi
ARPSO2
GWP = global warming potential, NC = number of carbons atoms, ODP = ozone depletion potental, MIR = maximum incremental reactivity, ARP = acid rain potential.
Compilation impact parameters in: Appendix D. Allen, D.T. and Shonnard, D.R., Green Engineering : Environmentally-Conscious Design of Chemical Processes, Prentice Hall, pg. 552, 2002
Michigan Technological University24
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