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c hapter 10




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
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
what information is needed to perform a tier 3 environmental assessment
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
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
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

emission assessment quantitative analyses continued
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
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
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
Boundaries for impact assessment

From Allen (2004) Design for the Environment - http://www.utexas.edu/research/ceer/che341


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
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)

One Compartment Models
  • 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 Compartment Models (MCMs)
multimedia models example level iii multimedia fugacity model
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
Fugacity and Fugacity Capacity
  • Air Phase
  • Water Phase
  • Soil Phase
  • Fugacity Capacity Factors
fugacity air phase
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))
fugacity water phase
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)
fugacity soil phase
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
fugacity capacities for compartments and phases in the environment
Fugacity Capacities for Compartments and Phases in the Environment

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
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 :
  • 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 :

Transport between interfaces... continued

Parameter Derivations : Air-Soil Transports

  • 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 :
transport between interfaces continued
Transport between interfaces... continued

Parameter Derivations : Water-Sediment Transports

  • 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)
transport between interfaces continued1
Transport between interfaces... continued

Parameter Derivations : Soil-Water Transports

  • 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)
transport between interfaces continued2
Transport between interfaces... continued

Parameter Derivations : Advective Transports

  • 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
reaction loss processes
Reaction Loss Processes

Reaction loss processes occuring in the environment include :

  • Biodegradation
  • Photolysis
  • Hydrolysis
  • Oxidation
balance equations
Balance Equations

Mole Balance Equations for the Mackay Level III Fugacity Model.


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
Tier 3 Metrics for Environmental Risk Evaluation of Process Designs
  • Environmental Indexes
  • Global Warming
  • Ozone Depletion
  • Acid Rain
  • Smog Formation
  • Toxicity and Carcinogenity
dimensionless risk index
Dimensionless Risk Index
  • Global Implications
    • Global Warming
    • Stratospheric Ozone Depletion
  • Regional Implications
    • Smog Formation
    • Acid Deposition
  • Local Implications
    • Toxicity
    • Carcinogenicity
  • Abiotic Impacts :
    • Global Warming
    • Stratospheric Ozone Depletion
    • Acidification
    • Eutrofiaction
    • Smog formation

B stands for the benchmark compound and i is the chemical of interest.

global warming
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 ...
ozone depletion
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.

acid rain
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.

smog formation
Smog Formation

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


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

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.

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)

conceptual design of an environmental impact evaluation of a process
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