CHAPTER 10
ENVIRONMENTAL IMPACT
EVALUATION OF A CHEMICAL
PROCESS FLOWSHEET – TIER 3
Goal
To perform a detailed environmental impact
evaluation of a chemical process flowsheet in
order to identify a set of environmental
indexes (metrics) and evaluate the impact o
risk of the entire process to the human
health or to the environmental media
Order of topics :
• Introduction
• Estimation of environmental fates of
emissions and wastes
• Tier 3 metrics for environmental risk
evaluation of process designs
• Conceptual design of an environmental
impact assessment of a chemical process
flowsheet
Introduction
What Information is Needed
to Perform a Tier 3
Environmental Assessment?
• To establish a Process Flowsheet
• To define the boundaries around the
environmental assessment
• To formulate environmental impact indicators
(indexes or metrics)
• To maximize the Mass Efficiency
• To maximize the Energy Efficiency
Indexes or environmental metrics
Can be used for several important engineering
applications related to process designs,
including :
– Ranking of technologies
– Optimizing of in-process waste recycle/recovery
processes
– Evaluation of the modes of reactor operation
Emission assessment: Quantitative
Analyses
EMISSIONS are the most important and basic
information regarding process design flowsheets
because :
impact  f (dose(concentration (emission rate)))
Concentration and location are a (emissions,
chemical properties and physical properties)
Transport and fate models can be used to transform
emission values into their related environmental
concentrations
Emission assessment: Quantitative Analyses ... continued
Toxicity and/or inherent impact information is
required to convert concentration-dependent doses
into probabilites of risk
Categories of environmental impact assessment steps :
– Estimates of the rates of release for all chemicals in the
process
– Calculation of environmental fate and transport and
environmental concentration
– Accounting for multiple measures of risk using
toxicology and inherent environmental impact
information
Potential Risk Assessment
...suitable for large scale applications where
potential environmental and health risk
assessment should be follow by quantitative
analysis.
...better suited to compare the environmental risks
of chemical process designs
...of chemical process and their design can be
evaluated by impact benchmarking
Impact Benchmarking
• Is a dimensionless ratio of the environmental impact
caused by a chemical’s release in comparison of the
identical release of a well-studied (benchmark)
compound
• If the benchmark value is greater then 1, then the
chemical has a greater potential for environmental
impact then the benchmarked compound
• Equivalent emission of the benchmark compound (in
terms of environmental impact) = (Benchmarked
enviromental impact potential) * (process emission
rate)
Boundaries for impact assessment
From Allen (2004) Design for the Environment - http://www.utexas.edu/research/ceer/che341
Estimation of Environmental
Fates and Emission Wastes
Goal
To determine the transport and reaction
processes that affect the ultimate
concentration of a chemical released to
the environment (water, air and soil)
The evaluation is done by using
environmental fate and transport models:
-One compartment
- Multimedia compartment
Choosing Types of Models
• Accuracy :
– This parameter varies according to the model’s
method of incorporating environmental
processes in it’s description of mass transfers
and reactions
• Ease of Use :
– This parameter reflects the data and
computational requirements which the model
places on the environmental assessment
One Compartment Models
• Advantages :
– Little chemical and/or
environmentally
specific data required
– Relatively accurate
results using modest
computer resources
• Disadvantages :
– Information is for only
one media (severe
limitation when
multiple environmental
impacts are being
considered)
•Examples :
– Atmospheric dispersion models for predicting air concentrations from
stationary sources
– Groundwater dispersion models for predicting contaminant concentrations
profiles in plumes
Multimedia Compartment Models
(MCMs)
• Advantages :
– Information on transport
and fate in more than one
media
– Minimal data input required
– Relatively simple and
computationally efficient
– Accounts for several
intermediate transport
mechanisms and
degradations
• Disadvantages :
– Lack of experimental data
can be used to verify the
model’s accuracy
– General belief that they
only provide order-ofmagnitude estimates of the
environmental
concentrations
– Large computational
requirements can result in
difficult practical
implementations for routine
chemical process
evaluations.
Multimedia Models Example: Level
III Multimedia Fugacity Model
The model predicts steady-state
concentrations of a chemical in four
environmental compartments (1) air,
(2) surface water, (3) soil, (4) sediment
in response to a constant emission into
an environmental region of defined
volume
Allen, A.T., D.R. Shonnard (2002) Green engineering, Prentice Hall
MacKay, D.(2001) Multimedia environmental models: the fugacity approach, CRC Press
Fugacity and Fugacity Capacity
•
•
•
•
Air Phase
Water Phase
Soil Phase
Fugacity Capacity Factors
Fugacity : Air Phase
• Defined as :
f  y  PT  P
Where :
– y is the mole fraction of the chemical in the air phase
– Ф is the dimensionless fugacity coefficient which accounts for nonideal behaviour
– PT is the total pressure (Pa)
– P is the partial pressure of the chemical in the air phase
• Concentration and Fugacity :
Where :
–
–
–
–
–
C1  n / V  P /( RT )  f /( RT )  f Z1
n is the number of moles of the chemical in a given volume V (mol)
V is the given volume (m3)
R is the gas constant (8.312 (Pa m3)/(mole K))
T is the absolute temperature (K)
Z1 is the fugacity capacity (=1/(RT))
Fugacity : Water Phase
• Defined as :
Where :
f  x P
S
– x is the mole fraction
– y is the activity coefficient in the Raoult’s law convention
– PS is the saturation vapor pressure of pure liquid chemical at the system
temperature (Pa)
S
C

x
/
v

f
/(
v

P
)  f / H  fZ 2
W
W
• Concentration and Fugacity : 2
• Where :
–
–
–
–
vw is the molar volume of solution (water, 1.8x10-5m3/mole)
H is the Henry’s law constant for the chemical (Pa.*m3/mole)
Z2 is the water fugacity capacity for each chemical (=1/H)
C2 is the concentration in aqueous solution (moles/m3)
Fugacity : Soil Phase
• Defined as :
• Where :
C S  K d C2
– Cs is the sorbed concentration (moles/kg soil or sediment)
– C2 is the aqueous concentration (moles/L solution)
– Kd is the equilibrium distribution coefficient (L solution/kg solids)
• Distribution coefficient related to organic content:
K OC  K d / 3
• Concentration and Fugacity :
3 CS  1 / H KOC3 3 f / 1000  Z 3 f
• Where :
– р3 is the phase density (kg solid/m3 solid)
– Ф3 is the mass fraction of organic carbon in teh soil phase (g organic
carbon/g soil solids)
– Koc is the organic carbon-based distribution coefficient (L/kg)
– Z3 is the fugacity capacity
Fugacity Capacities for Compartments
and Phases in the Environment
Environmental Phases
Phase Densities (kg/m3)
Air Phase
Z1=1/RT
Water Phase
Z2=1/H
1,000
Soil Phase
Z3=(1/H)KOCΦ3ρ3/1000
2,400
Sediment Phase
Z4=(1/H)KOCΦ4ρ4/1000
2,400
Suspended Sediment Phase
Z5=(1/H)KOCΦ5ρ5/1000
2,400
Fish Phase
Z6=(1/H)0.048ρ6KOW
1,000
Aerosol Phase
Z7=(1/RT)6x106/PSL
Where
1.2
R=Gas constant (8.314Pa*m3/mole*K)
T= Absolute Temperatura (K)
H=Henry’s Law constant (Pa*m3/mole)
KOC=Organic-carbon partition coefficient (=0.41KOW)
KOW=Octanol-water partition coefficient
ρi=phase density for phase i (kg/m3)
Φi=Mass fraction of organic carbon in phase i (g/g)
Environmental Compartments
Air comparment (1)
ZC1=Z1+2x10-11Z7 (Approximately 30 μg/m3 aerosols)
Water comparment (2)
ZC2=Z2+5x10-6Z5+10-6Z6 (5 ppm solids, 1 ppm fish by vol.)
Solid compartment (3)
ZC3=0.2Z1+0.3Z2+0.5Z3 (20% air, 30% water, 50% solids)
Sediment compartment (4)
ZC4=0.8Z2+0.2Z4 (80% water, 20% solids)
Note: For solid aerosols PSL=PSS/exp{6.79(1-TM/T)} where TM is the melting point (K). Adapted from Mackay et. Al. (1992).
Transport between interfaces
Diffusive and Non-Diffusive Processes
• Diffusive Processes
– Can occur in more then one direction, depending on
the fugacity signs of the different compartments
– Rate of transfer : N = D(f)
– Ex. Volatilization from water to air or soil to air
• Non-Diffusive Processes
– Is a one-way transport between compartments
– Rate of transfer : N = GC = GZf = Df
– Ex. Rain washout, wet/dry depositions to water and
soil, sediment depositions and resuspensions
Transport between interfaces... continued
Parameter Derivations : Air-Water Transports
• A two film approach is used with mass transfer coefficients for air
(u1 = 5m/h) and water (u2 = 0.05 m/h). The intermediate transport
parameter for absorption is given as :
DVW 1 /(1 / u1 AW Z1 )  1 /(u2 AW Z 2 ))
• The D-value for rain washout can be given as :
DRW  u3 AW Z 2
• The D-value for wet/dry deposition is given as :
DQW  u 4 AW Z 7
• The cumulative D-value for air to water tranfer :
D12  DVW  DQW  DRW
• The D-value for water to air transfer is :
D21  DVW
Transport between interfaces... continued
Parameter Derivations : Air-Soil Transports
• After development, the d-value equation for air
to soil diffusion is given as :
DVS 1 /(1 / DS  1 /( DSW  DSA ))
• With :
DS  u5 AS Z1
DSW  u6 AS Z 2
DSA  u7 AS Z1
• The cumulative D-value for all air-to-soil
processes is given by :
D13  DVS  DQS  DRS
• And the soil-to-air diffusion transport is :
D31  DVS
Transport between interfaces... continued
Parameter Derivations : Water-Sediment Transports
• Water to sediment D-value can be estimated by :
Where :
D24  u8 AW Z 2  u9 AW Z 5
– u8 is the mass transfer coefficient (m/h)
– AW is the area (m2)
– u9 is the sediment deposition velocity (m/h)
• Sediment to water D-value can be estimated by :
• Where :
D42  u 8 AW Z 2  u10 AW Z 4
– u10 is the resuspension velocity (m/h)
Transport between interfaces... continued
Parameter Derivations : Soil-Water Transports
• The D-value for soil to water transfer is :
• Where :
D32  u11 AS Z 2  u12 AS Z 3
– u11 is the run-off water velocity (m/h)
– u12 is the run-off solid’s velocity (m/h)
• The non-diffusive transport mechanism’s D-value
used to describe the removal of chemical from the
sediment via burial is :
• Where :
DA4  uB AW Z 4
– uB is the sediment burial rate (m/h)
Transport between interfaces... continued
Parameter Derivations : Advective Transports
• The total rate of inputs for each media is :
I i  Ei  G AiC Bi
• Where :
– Ei is the emission rate (moles/h)
– GAi is the advective flow rate (m3/h)
– CBi is the background concentration external to
compartment i (moles/m3)
• The total rate of bulk flow outputs for each media is :
DAi  GAi Z Ci
• Where :
– ZCi is the compartment i fugacity capacity
Reaction Loss Processes
Reaction loss processes occuring in the
environment include :
–
–
–
–
Biodegradation
Photolysis
Hydrolysis
Oxidation
Balance Equations
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 lefthand side is the sum of all gains and the righthand side is the sum of all losses, II=EI+GAICBI, I4 usually being zero. The
D values on the righthand 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:
f2 = (I2+ J1J4/J3 + I3D32/DT3 + I4D42/DT4)/(DT2 - J2J4/J3- D24D42/ DT4)
f1 = (J1+ f2J2) /J3
f3 = (I3+ f1D13) /DT3
f4 = (I4+ f2D42)/DT4
Where
J1 = I1 / DT1 + I3D31/(DT3DT1)
J2 = D21/ DT1
J3 = 1 – D31D13/(DT1DT3)
J4 = D12 + D32D13/DT3)
Metrics for environmental risk
evaluation of process design
Tier 3 Metrics for Environmental
Risk Evaluation of Process Designs
• This tier will discuss how to combine data
concerning emission estimation, environmental
fate and transport information and environmental
impact data in order to develop an assessment of
the potential risks caused by the releases of
substances from chemical process designs
• Indices will be used and the multimedia
compartment model example will be source of
environmental concentrations that will be used in
INDEXES
Tier 3 Metrics for Environmental
Risk Evaluation of Process Designs
•
•
•
•
•
•
Environmental Indexes
Global Warming
Ozone Depletion
Acid Rain
Smog Formation
Toxicity and Carcinogenity
Environmental
indexes
Dimensionless Risk Index
• Abiotic Impacts :
– Global Warming
– Stratospheric Ozone
Depletion
– Acidification
– Eutrofiaction
– Smog formation
• Global Implications
– Global Warming
– Stratospheric Ozone
Depletion
• Regional Implications
– Smog Formation
– Acid Deposition
• Local Implications
[( EP)( IIP )]i
(Dimension less Risk Index) i 
[( EP)( IIP )] B
B stands for the benchmark compound and i is
the chemical of interest.
– Toxicity
– Carcinogenicity
Global Warming
• GWP is a common index and is the
cumulative infrared energy captured from
the release of 1 kg of greenhouse gas
relative to that from 1 kg of carbon dioxide
GWPi 

n
0

n
0
ai Ci dt
aCO2 CCO2 dt
• Index for GW can be estimated using the
I   (GWP  m )
GWP with :
• Using organic compound effects ...
GW
i
i
i
GWPi (indirect )  N C
MWCO 2
MWi
Ozone Depletion
The Ozone Depletion Potential (ODP) is an integrated
change of the stratospheric ozone caused by a specific
quantity of a chemical.
It is a comparison between the damage caused by a
specific quantity of given chemical and the damage
caused by the same quantity of a benchmark compound.
ODPi 
 O3 i
 O3 CFC 11
I OD   (ODPi  mi )
i
Acid Rain
The relation between the number of moles of H+ created per
number of moles emitted is called potential of acidification.
The following equation (balance) provides this relationship.
X        H      

i 
i
MWi
i
ARPi 
 SO
2
I AR   ( ARPi  mi )
i
Smog Formation
The following equations represent the most important process for
ozone formation in the lower atmosphere (photo-dissociation of NO2)
NO2  hv  O(3P)  NO
O(3P)  O2  M  O3  M
O3  NO  NO2  O2
VOC's do not destroy O3 but they form radicals which convert NO to
NO2.
VOC   OH   RO2  other oxidation products
 RO2  NO  NO2  radicals
radicals   OH  other oxidation products
Smog Formation Potential
MIRi
SFPi 
MIR ROG
Process equivalent emission of ROG
I SF   ( SFPi  mi )
i
Toxicity
Non-Carcinogenic Toxicity
Non carcinogenic toxicity is controlled by established exposure thresholds.
Above this values a toxic response is manifested. The key parameters for
these chemicals are the reference dose (RfD [mg/kg/d]) or reference
concentration (RfC [mg/m3]).
Toxicity potential for
ingestion route exposure
(C
)( 2 L / d ) /( 70kg)/( RfDi )
INGTPi 
(Ctoluene,w )(2L / d ) /(70kg)/( RfDtoluene)
Toxicity potential for
inhalation exposure
Ci ,a / RFC i
INHTPi 
Ctoluene,a / RFC toluene
i ,w
Non-carcinogenic toxicity index
for the entire process (ingestion)
I ING   ( INGTPi  mi )
Non-carcinogenic toxicity index
for the entire process (inhalation)
I INH   ( INHTPi  mi )
Toxicity
Carcinogenicity
A method similar to the non-carcinogenicity toxicity is used for
measuring cancer related risk; it is based on predicted concentrations of
chemicals in the air and water from a release of 1000 kg/h.
Carcinogenic potential of a chemical determinated by the ratio
of the chemicals risk to that for the benchmark compound.
Ingestion
(C )( SF )
i, w
i
INGCPi 
(C
)( SF
)
benzene, w
benzene
Carcinogenic toxicity index for
the entire process (ingestion)
Carcinogenic toxicity index for
the entire process (inhalation)
Inhalation
(C )( SF )
i, a
i
INHCPi 
(C
)( SF
)
benzene, a
benzene
I CING   ( INGCPi  mi )
i
I CINH   ( INHCPi  mi )
i
Conceptual design of an
environmental impact evaluation
of a chemical process flowsheet
Conceptual design of an environmental
impact evaluation of a process
Proposed by Allen (2004) Design for the Environment - http://www.utexas.edu/research/ceer/che341