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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 :

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)


• 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:


• 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


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


• 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


• 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


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

– CarcinogenicityB

i

IIPEP

IIPEP

)])([(

)])([(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

ii

i

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

)(

)(

ONONOO

MOMOPO

NOPOhvNO

productsoxidationotherOHradicals

radicalsNONORO

productsoxidationotherROOHVOC

22

2

ROG

ii MIR

MIRSFP

iiiSF 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

RfDkgdLC

RfDkgdLCINGTPi

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

))(,

(

))(,

(

benzeneSF

wbenzeneC

iSF

wiC

INGCPi ))(,

(

))(,

(

benzeneSF

abenzeneC

iSF

aiC

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

c hapter 10

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