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

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 -

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-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 Hall MacKay, 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 –P T 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 (m 3 ) –R is the gas constant (8.312 (Pa m 3 )/(mole K)) –T is the absolute temperature (K) –Z1 is the fugacity capacity (=1/(RT))

Fugacity : Water Phase Defined as : Where : –x is the mole fraction –y is the activity coefficient in the Raoult’s law convention –P S is the saturation vapor pressure of pure liquid chemical at the system temperature (Pa) Concentration and Fugacity : Where : –v w is the molar volume of solution (water, 1.8x10 -5 m 3 /mole) –H is the Henry’s law constant for the chemical (Pa.*m 3 /mole) –Z 2 is the water fugacity capacity for each chemical (=1/H) –C 2 is the concentration in aqueous solution (moles/m 3 )

Fugacity : Soil Phase Defined as : Where : –C s is the sorbed concentration (moles/kg soil or sediment) –C 2 is the aqueous concentration (moles/L solution) –K d is the equilibrium distribution coefficient (L solution/kg solids) Distribution coefficient related to organic content: Concentration and Fugacity : Where : –р 3 is the phase density (kg solid/m 3 solid) –Ф 3 is the mass fraction of organic carbon in teh soil phase (g organic carbon/g soil solids) –K oc is the organic carbon-based distribution coefficient (L/kg) –Z 3 is the fugacity capacity

Fugacity Capacities for Compartments and Phases in the Environment Environmental PhasesPhase Densities (kg/m 3 ) Air PhaseZ 1 =1/RT 1.2 Water PhaseZ 2 =1/H 1,000 Soil PhaseZ 3 =(1/H)K OC Φ 3 ρ 3 /1000 2,400 Sediment PhaseZ 4 =(1/H)K OC Φ 4 ρ 4 /1000 2,400 Suspended Sediment PhaseZ 5 =(1/H)K OC Φ 5 ρ 5 /1000 2,400 Fish PhaseZ 6 =(1/H)0.048ρ 6 K OW 1,000 Aerosol PhaseZ7=(1/RT)6x10 6 /P S L WhereR=Gas constant (8.314Pa*m 3 /mole*K) T= Absolute Temperatura (K) H=Henry’s Law constant (Pa*m 3 /mole) K OC =Organic-carbon partition coefficient (=0.41K OW ) K OW =Octanol-water partition coefficient ρ i =phase density for phase i (kg/m 3 ) Φ i =Mass fraction of organic carbon in phase i (g/g) Environmental Compartments Air comparment (1)Z C1 =Z 1 +2x Z 7 (Approximately 30 μg/m3 aerosols) Water comparment (2)Z C2 =Z 2 +5x10 -6 Z Z 6 (5 ppm solids, 1 ppm fish by vol.) Solid compartment (3)Z C3 =0.2Z Z Z 3 (20% air, 30% water, 50% solids) Sediment compartment (4)Z C4 =0.8Z Z 4 (80% water, 20% solids) Note: For solid aerosols P S L =P S S /exp{6.79(1-T M /T)} where T M 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 (u 1 = 5m/h) and water (u 2 = 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 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 : Parameter Derivations : Air-Soil Transports Transport between interfaces... continued

Water to sediment D-value can be estimated by : Where : –u 8 is the mass transfer coefficient (m/h) –A W is the area (m 2 ) –u 9 is the sediment deposition velocity (m/h) Sediment to water D-value can be estimated by : Where : –u 10 is the resuspension velocity (m/h) Parameter Derivations : Water-Sediment Transports Transport between interfaces... continued

The D-value for soil to water transfer is : Where : –u 11 is the run-off water velocity (m/h) –u 12 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 : –u B is the sediment burial rate (m/h) Parameter Derivations : Soil-Water Transports Transport between interfaces... continued

The total rate of inputs for each media is : Where : –E i is the emission rate (moles/h) –G Ai is the advective flow rate (m 3 /h) –C Bi is the background concentration external to compartment i (moles/m 3 ) The total rate of bulk flow outputs for each media is : Where : –Z Ci is the compartment i fugacity capacity Parameter Derivations : Advective Transports Transport between interfaces... continued

Reaction Loss Processes Reaction loss processes occuring in the environment include : –Biodegradation –Photolysis –Hydrolysis –Oxidation

Balance Equations AirI 1 +f 2 D 21 +f 3 D 31 =f 1 D T1 WaterI 2 +f 1 D 12 +f 3 D 32 +f 4 D 42 =f 2 D T2 SoilI 3 +f 1 D 13 =f 3 D T3 SedimentI 4 +f 2 D 24 =f 4 D T4 Where the lefthand side is the sum of all gains and the righthand side is the sum of all losses, I I =E I +G AI C BI, I 4 usually being zero. The D values on the righthand side are: D T1 =D R1 +D A1 +D 12 +D 13 D T2 =D R2 +D A2 +D 21 +D 24 D T3 =D R3 +D A3 +D 31 +D 32 D T4 =D R4 +D A4 +D 42 The solution for the unknown fugacities in each compartment is: f 2 = (I 2 + J 1 J 4 /J 3 + I 3 D 32 /D T3 + I 4 D 42 /D T4 )/(D T2 - J 2 J 4 /J 3 - D 24 D 42 / D T4 ) f 1 = (J 1 + f 2 J 2 ) /J 3 f 3 = (I 3 + f 1 D 13 ) /D T3 f 4 = (I 4 + f 2 D 42 )/D T4 Where J 1 = I 1 / D T1 + I 3 D 31 /(D T3 D T1 ) J 2 = D 21 / D T1 J 3 = 1 – D 31 D 13 /(D T1 D T3 ) J 4 = D 12 + D 32 D 13 /D T3 ) 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

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 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...

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

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 The following equations represent the most important process for ozone formation in the lower atmosphere (photo-dissociation of NO 2 ) VOC's do not destroy O 3 but they form radicals which convert NO to NO 2. Smog Formation PotentialProcess equivalent emission of ROG

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/m 3 ]). 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. Toxicity Carcinogenicity 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) IngestionInhalation

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 -