1. CHAPTER 10
ENVIRONMENTAL IMPACT EVALUATION OF A CHEMICAL PROCESS FLOWSHEET – TIER 3

2. 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
Narration: This detailed environmental impact evaluation include all the information discussed in the chapters 1 to 9 that we need to perform an analysis of specific case studies, so the goal of the Tier 3.

3. 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

4. Introduction

5. 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
Narration : Once the established process flowsheet is obtained, and the mass and energy efficiencies have been maximized (as discussed in chapter 7), it is possible to use Tier 3 Environmental Performance Tools (Chapter 7, Section A). The end result of applying the Tier 3 tools will be a set of evaluation indexes that will represent the major environmental impacts and/or risks for the entire process or part of process analyzed.

6. 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
The environmental metrics are necessary to represent the major environmental impacts or risks of the entire process and account for potential health and environmental damages

7. 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

8. 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
Narration : As stated in the previous slide, some transport and fate models can be used to convert emissions into environmental concentrations. It is ideal to perfomr quantitative risk assessments to compare environmental performances of various chemical process designs when sources and receptors are defined and localized. If this is not the purpose, i.e. Industrial releases that may impact more then the local areas, then it may not be the best option. See next slide for more information

9. 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
Narration:
Potential Risk Assessment is an Alternative to the Quantitative Analyses of emissions -CAT
Calculations for the risk assessment can be review in detail on chapters 2 and 5 –CAT
The methodology to assess risk include: Hazard identification, exposure assessment, toxicity assessment and risk characterization.

10. 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)
Narration : The first time Impact Benchmarking was introduced was for the assessment of global warming and ozone-deleting potentials related to the use of refrigerants (in the early 90s). In this chapter, this concept will be adopted when assessments of the environmental and toxicological impact potentials of releases from chemical processes need to be performed. This concept will be incorporated into a multimedia model approach in order to determine fate and transport of chemical releases into the environment.

11. Boundaries for impact assessment
Narration: The goal of drawing boundaries is to determine where the impact assessment of the chemical processes should be applied and to define which kind of aspects need to be estimated. Thus, the boundaries specifications directly impacts the project of pollution prevention to be applied. The schema presented by Dr. Allen (Texas University) put in context all this aspects in a general way -CAT
It is also important to establish boundaries limited to the chemical processes to be analysed in order to better estimate the mass and energy flow and the appropriate pollution prevention project.
From Allen (2004) Design for the Environment -

12. Estimation of Environmental Fates and Emission Wastes

13. 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
Narration : There exist many models that can be used to quantify transport, fate and reaction processes after chemicals are released from a process. The transport, fate and reaction mechanisms also affect the substance’s concentrations in different medias (air, soil and water). In this section, we will discuss of different models.

14. 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
Narration : When trying to choose the appropriate type of model, two main issues should be raised. The accuracy of the results will obviously depend on the methods used in the model, and the ease of use will definitively impact the choice of model as it needs to fulfill the requirements of the user.

15. 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)
Narration : One approach to contaminant modelling for environmental applications is to focus on transport and fate processes that occur in one media (i.e. Air, land or water).
Examples :
Atmospheric dispersion models for predicting air concentrations from stationary sources
Groundwater dispersion models for predicting contaminant concentrations profiles in plumes

16. 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.
Narration : By taking different compartment models (for different medias) and linking them together, one may provide a multi-compartamental (multimedia) model with information concerning environmental fate and transport. This kind of models are used to predict chemical concentrations in several environmental compartments simultaneously.
Examples: The level III multimedia fugacity model (MacKay, D.(2001) Multimedia environmental models: the fugacity approach, CRC Press).

17. 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
Narration : This model used as example by Allen and Shonnard (2002) will be applied to the analysis of a case study at the end of this chapter.
The diagram represented here include the chemical processes occurring in the model domain and affecting the concentration in each of the four compartments by:
Emission (Ei) (mol/hr)
Advective inputs (GAiCBi) (mol/hr)
The transfer between compartments is ocurring by:
diffusive / non diffusive transport (intermediate transfer values Dij, mol/PA.hr)
Advective mechanisms (DAi)
Dissapear by chemical reaction (DRi)
The concepts used by this model are explained in the following slides. -CAT
Allen, A.T., D.R. Shonnard (2002) Green engineering, Prentice Hall
MacKay, D.(2001) Multimedia environmental models: the fugacity approach, CRC Press

18. Fugacity and Fugacity Capacity
Air Phase
Water Phase
Soil Phase
Fugacity Capacity Factors
CAT
Narration:
The MCMs use the concept of fugacity in describing mass transfer and reaction processes. Fugacity is defined as the “escaping tendency” of a chemical from a given environmental phase (soil organic matter, water, etc.). Thus, partitioning of a chemical between environmental phases can be described by the equilibrium criterion of equal fugacity ƒ(Pa) in all phases.
The fugacity is equal to partial pressure in the dilute limit typical of most environmental concentrations. Also, it is porportional to concentration ( C, mol/m3), C=ƒZ, where Z is the fugacity capacity (Pa/m3.mol)

19. Fugacity : Air Phase Defined as : Where : Concentration and Fugacity :
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 :
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))
Equations taken from green engineering p. 364
Narration : the first relationship shows the definition of fugacity in the air phase. At low pressures, in the environment (1 atm), the fugacity coefficient becomes 1 and therefore it can be assumed that at these pressures, fugacity is equal to the partial pressure of the contaminant in the air. By using this latter relationship, it is possible to relate fugacity to concentration by using the ideal gas law. The fugacity capacity is shown to be independent of the chemical’s nature and is only dependent of temperature.

20. 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 :
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)
Narration : the first relationship shows the definition of fugacity in the aqueous phase. The activity coefficient in the first equation can become constant at dilute concentrations, and therefore it is possible to develop a linear relationship between fugacity and concentration. The fugacity capacity for water therefore becomes the inverse of the Henry’s law constant.

21. 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 :
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
Narration : The first relationship shows the definition of fugacity in the soil phase. The second equation relates the equilibrium distribution constant with the fraction of organic carbon found in the soil (or sediment). This relationship is useful because the majority of natural organic matter is composed mainly of carbon. The last equation demonstrates the relationship between the substance’s concentration in the soil (or sediment) and fugacity. Koc can also be related to Kow by Koc = 0.41 Kow.
Koc = 0.41 Kow plus more information from chapter 4

22. 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 ,000
Soil Phase
Z3=(1/H)KOCΦ3ρ3/ ,400
Sediment Phase
Z4=(1/H)KOCΦ4ρ4/ ,400
Suspended Sediment Phase
Z5=(1/H)KOCΦ5ρ5/ ,400
Fish Phase
Z6=(1/H)0.048ρ6KOW ,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)
Narration : This table contains the equations needed to calculate the fugacity capacities of the different phases and compartments.
Source : Green Engineering, Allen and Shonnard, pp.367
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).

23. 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
Narration : The transport of chemicals between media is done via 2 mechanisms being diffusive and non-diffusive processes.
The parameters considered represent:
N = diffusive rate of transfer (mol/hr)
D = diffusion from one to another compartmen (mol/Pa.hr)
G = volumetric flow rate (m3/hr) of the transported material
f = fugacity
C = phase concentration of the material (mol/m3)
Z = fugacity capacity

24. Parameter Derivations : Air-Water Transports
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 :
Narration : Air-water transports include : Diffusion/absorption ; washout by rain and wet/dry depositions of aerosols.
The parameters considered represent:
DVW=1/(1/(u1AWZ1)+ 1/(u2AWZ2))
DRW=u3AWZ2
DQW=u4AWZ7
D12=Air Transport
D21=Water Transport
AW=Interfacial Area between the atmosphere and the surface water
Z1=Air phase; Z1=1/RT
Z2= Water Phase; Z2=1/H
Z7= Aerosol Phase; Z7=(1/RT)6x106/PSL
u1=kA
u2=kW
u3= rainfall rate (0.876 m/year)
u4= deposition velocity of aerosols (6x10-10 m/h)
k=Mass Transfer Coefficient in the interphase atmosphere-water
kA=Mass Transfer Coefficient using air-side (5 m/h)
kW=Mass Transfer Coefficient using water-side (0.05 m/h)

25. Parameter Derivations : Air-Soil Transports
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 :
Similar treatments of rain washout and wet/dry deposition are taken for the air to soil transport with the difference being in that the correct area term is in the air/soil interface area As. When examining the diffusion from air to soil, the chemical must pass through a thin mass-transfer resistance film at the atmosphere-soil interface before diffusing throught the soil air phase or the soil water phase, both of wich have resistances of their own. The mass transfer coefficient (into soil) is given by u5 at 5 m/h.

26. 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 :
u10 is the resuspension velocity (m/h)
Narration : These relationships are determined using main characteristics of diffusion. The first equation’s first term is characterized by the mass-transfer coefficient and the second term characterizes the sediment’s deposition velocity over a given area. Resuspension is assumed to happen at a rate of roughly 40% that of deposition, therefore the deposition velocity changes.

27. 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 :
uB is the sediment burial rate (m/h)
Narration : Soil to water transfer usually occurs via surface run-off. It is assumed that the rate of water run-off is assumed to occur at roughly 50% of the rate of rainfall.

28. 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 :
ZCi is the compartment i fugacity capacity
Narration : Chemicals enter and leave compartments (medias) by emissions and advective inputs from outside given models’ regions.

29. Reaction Loss Processes
Reaction loss processes occuring in the environment include :
Biodegradation
Photolysis
Hydrolysis
Oxidation
Narration : These are the general reaction mechanisms that occur in the environment

30. 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)
Narration: “There must be a balance between the rates of input from all emissions/bulk flow and the intermedia transport and the rates of output from intermedia transport, advection, and reaction loss processes within each compartment at steady state”.
Source : Green Engineering, Allen and Shonnard, pp.372

31. Metrics for environmental risk evaluation of process design

32. 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
Several methodologies exist for indexing environmental and health impacts. Many methods include metrics for abiotic as well as biotic impacts

33. Tier 3 Metrics for Environmental Risk Evaluation of Process Designs
Environmental Indexes
Global Warming
Ozone Depletion
Acid Rain
Smog Formation
Toxicity and Carcinogenity

34. Environmental indexes
Narration:
From a review of the existing methodology to determine environmental indexes, Allen and Shonnard (2002) have proposed nine abiotic (global warming, stratospheric ozone depletion, acidification, and smog formation), biotic (human health) and ecotoxicological (fish aquatic toxicity).

35. 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
Toxicity
Carcinogenicity
B stands for the benchmark compound and i is the chemical of interest.

36. 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 ...
Narration: The Global Warming Potential (GWP) is a common index for messuring the global warming.
Where:
ai=predicted radiative forcing of gas i (Wm-2), is a function of the chemical’s infrared absorbance properties and Ci
Ci is its predicted concentration in the atmosphere (ppm)
mi=mass emission rate of chemical i from the entire process (kg/h)
NC=Number of carbon atoms in the chemical i
MW=Molecular Weight

37. 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.
Narration: IOD: "The ozone depletion index for an entire chemical process is the sum of all contributions from emitted chemicals multiplied by the emission rates. The latter equation represents the equivalent emission of CFC-11"

38. 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.
ηi is the quantity of H+ created per mass of substance emitted.
ARPi is the Acid rain potential of any emitted acid-forming chemical related to SO2
IAR is the total acidification potential of an entire chemical process.
mi is mass emission rate of chemical i from the entire process (kg/h)
αi is a molar stoichiometric coefficient (mole H+/mole X)

39. 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.
ROG = Reactive Organic Gases
MIRi is the maximum incremental reactivity
MIRROG is the average value for background reactive organic gases
Smog Formation Potential
Process equivalent emission of ROG

40. 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
Ci,w and Ctoluene,w are the steady state concentrations of the chemical and benchmark compound (toluene) in the water compartment after release of 1000 kg/h of each into the water compartment.
Ci,a and Ctoluene,a are the concentrations of chemical i and of the benchmark compound (toluene) in the air compartment of the environment after release of 1000 kg/h of each into the air compartment.
2L/d and 70 kg are the standard ingestion rate and body weight used for risk assessment.
Non-carcinogenic toxicity index for the entire process (ingestion)
Non-carcinogenic toxicity index for the entire process (inhalation)

41. 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
Inhalation
SF is the cancer potency slope factor, (mg/kg/d)-1
Carcinogenic toxicity index for the entire process (ingestion)
Carcinogenic toxicity index for the entire process (inhalation)

42. Conceptual design of an environmental impact evaluation of a chemical process flowsheet

43. Conceptual design of an environmental impact evaluation of a process
Proposed by Allen (2004) Design for the Environment -