Download - C HAPTER 10
CHAPTER 10ENVIRONMENTAL 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 metricsCan 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 :
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
)))((( rateemissionionconcentratdosefimpact
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
GoalTo 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-of-magnitude 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
Allen, A.T., D.R. Shonnard (2002) Green engineering, Prentice HallMacKay, D.(2001) Multimedia environmental models: the fugacity approach, CRC Press
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
Fugacity and Fugacity Capacity
• Air Phase• Water Phase• Soil Phase• Fugacity Capacity Factors
Fugacity : Air Phase• Defined as : Where :
– y is the mole fraction of the chemical in the air phase– Ф is the dimensionless fugacity coefficient which accounts for non-ideal
behaviour– PT is the total pressure (Pa)– P is the partial pressure of the chemical in the air phase
• Concentration and Fugacity :
Where : – 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))
PPyf T
11 )/()/(/ ZfRTfRTPVnC
Fugacity : Water Phase• Defined as : Where :
– 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)
• Concentration and Fugacity : • 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)
SPxf
22 /)/(/ fZHfPvfvxC SWW
Fugacity : Soil Phase• Defined as :• Where :
– 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:
• Concentration and Fugacity :
• 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
2CKC dS
3/dOC KK
fZfKHC OCS 3333 1000//1
Fugacity Capacities for Compartments and Phases in the Environment
Environmental Phases Phase Densities (kg/m3)
Air Phase Z1=1/RT 1.2
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 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=PS
S/exp{6.79(1-TM/T)} where TM is the melting point (K). Adapted from Mackay et. Al. (1992).
Transport between interfaces
• 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
Diffusive and Non-Diffusive Processes
• 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 :
• The D-value for rain washout can be given as :
• The D-value for wet/dry deposition is given as :
• The cumulative D-value for air to water tranfer :
• The D-value for water to air transfer is :
Parameter Derivations : Air-Water Transports
))/(1)/1/(1 2211 ZAuZAuD WWVW
23 ZAuD WRW
74 ZAuD WQW
VWDD 21
RWQWVW DDDD 12
Transport between interfaces... continued
• After development, the d-value equation for air to soil diffusion is given as :
• With :
• The cumulative D-value for all air-to-soil processes is given by :
• And the soil-to-air diffusion transport is :
17 ZAuD SSA
Parameter Derivations : Air-Soil Transports
))/(1/1/(1 SASWSVS DDDD
15 ZAuD SS 26 ZAuD SSW
RSQSVS DDDD 13
Transport between interfaces... continued
VSDD 31
• Water to sediment D-value can be estimated by :
Where :– 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 : – u10 is the resuspension velocity (m/h)
Parameter Derivations : Water-Sediment Transports
592824 ZAuZAuD WW
4102842 ZAuZAuD WW
Transport between interfaces... continued
• The D-value for soil to water transfer is :
• Where : – 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 :– uB is the sediment burial rate (m/h)
Parameter Derivations : Soil-Water Transports
31221132 ZAuZAuD SS
44 ZAuD WBA
Transport between interfaces... continued
• The total rate of inputs for each media is :
• 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 :
• Where : – ZCi is the compartment i fugacity capacity
Parameter Derivations : Advective Transports
BiAiii CGEI
CiAiAi ZGD
Transport between interfaces... continued
Reaction Loss Processes
Reaction loss processes occuring in the environment include :
– Biodegradation– Photolysis– Hydrolysis– Oxidation
Balance Equations
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)
Mole Balance Equations for the Mackay Level III Fugacity Model.
Metrics for environmental risk evaluation of process design
• 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
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
• 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– Toxicity– Carcinogenicity
B
i
IIPEPIIPEP
)])([()])([(Index)Risk less(Dimension i
Dimensionless Risk Index
B stands for the benchmark compound and i is the chemical of interest.
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
• Index for GW can be estimated using the GWP with :
• Using organic compound effects ...
n
COCO
n
iii
dtCa
dtCaGWP
0
0
22
i
iiGW mGWPI )(
i
COCi MW
MWNindirectGWP 2)(
i
iiOD mODPI )(
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.
Ozone Depletion
113
3
CFC
ii O
OODP
Acid Rain
HX
i
ii MW
2SO
iiARP
i
iiAR mARPI )(
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.
Smog Formation
223
323
32
)(
)(
ONONOOMOMOPO
NOPOhvNO
productsoxidationotherOHradicalsradicalsNONORO
productsoxidationotherROOHVOC
22
2
ROG
ii MIR
MIRSFP i
iiSF mSFPI )(
The following equations represent the most important process for ozone formation in the lower atmosphere (photo-dissociation of NO2)
VOC's do not destroy O3 but they form radicals which convert NO to NO2.
Smog Formation Potential Process equivalent emission of ROG
ToxicityNon-Carcinogenic Toxicity
)/()70/()/2)((
)/()70/()/2)((
,
,
toluenewtoluene
iwi
RfDkgdLCRfDkgdLC
INGTPi
tolueneatoluene
iaii RFCC
RFCCINHTP
//
,
,
)( iiING mINGTPI
)( iiINH mINHTPI
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
Toxicity potential for inhalation exposure
Non-carcinogenic toxicity index for the entire process (ingestion)
Non-carcinogenic toxicity index for the entire process (inhalation)
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.
ToxicityCarcinogenicity
))(,(
))(,(
benzeneSFwbenzeneCiSFwiC
INGCPi ))(,(
))(,(
benzeneSFabenzeneCiSFaiC
INHCPi
)( ii
iCING mINGCPI
)( ii
iCINH mINHCPI
Carcinogenic potential of a chemical determinated by the ratio of the chemicals risk to that for the benchmark compound.
Carcinogenic toxicity index for the entire process (ingestion)
Carcinogenic toxicity index for the entire process (inhalation)
Ingestion Inhalation
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