Chapter 11

Environmental Performance of a Flowsheet

Introduction

The end result of impact evaluation will be a set of environmental metrics (indices) which represent the major environmental impacts or risks of the entire process.

The indices can be used in several important engineering applications during process design, including (1) the ranking of technologies, (2) the optimizing of in-process waste recycle/recovery processes, and (3) the evaluation of the modes of reactor operation.

Introduction

Emissions from a process are the primary piece of information required for impact assessment.

A suitable fate and transport model can transform the emissions into environmental concentrations.

Information regarding toxicity or inherent impact is required to convert the concentration-dependent doses into probabilities of harm.

Steps for Environmental Impact Assessment

1. Estimates of the rates of releases for all chemicals in the process

2. Calculation of environmental fate and transport and environmental concentrations

3. The accounting for multiple measures of risk using toxicity and inherent environmental impact information

Level III Multimedia Fugacity Model (Mackay, 2001)

This model predicts the steady-state concentrations in four environmental compartments, i.e., (1) air, (2) surface water, (3) soil, and (4) sediment, in response to a constant emission into an environmental region of defined volume.

Model Domain2The surface area selected for the model is 10000 km , typically 10% water

and 90% land. The surface area of sediment is the same as water.

The atmosphere height is 1000 m. The depth of water is 20

-11

m and those of

the soil and sediment layers are assumed to be 10 cm and 1 cm respectively.

The atmospheric compartment contains a condensed (aerosol) phase having

a volume fraction of 2 10 or 3

-6

-6

about 30 g/m .

The water compartment contains suspended sediments of volume fraction 5 10

or 5 mg/L. Fish are included at a volume fraction of 10 and are assumed to

contain 5% lipid into whi

ch hydrophobic chemical can partition.

The soil compartment is assumed to contain 20% by volume of air, 30% water,

and the remainder solids. The organic carbon content of soils is 2%.

Intermedia Transport

1 2 3 4

1 1 2 2

12 21 13 31 32 24 42

1 2 4

, , , emissions, mole/hr

, advective input, mole/hr

, , , , , ,

intermedia transport parameter, mole/(Pa hr)

, ,

transport parameter (exit by advective mec

A B A B

A A A

E E E E

G C G C

D D D D D D D

D D D

1 2 3 4

hanism)

, , , loss parameter (exit by reaction)R R R RD D D D

Fugacity

• Fugacity (Pa) is a thermodynamic property of a chemical and is defined as the “escaping tendency” of the chemical fro a given phase.

• Partitioning of a chemical between environmental phases can be described by the equilibrium criterion of equal fugacity in all phases.

• The fugacity is equal to partial pressure in the dilute limit. It is generally proportional to concentration, i.e., C=fZ, where Z is termed the fugacity capacity.

Fugacity in Air Phase

where

mole fraction of the chemical in air

fugacity coefficient (dimentionless) which accounts for non-ideal behavior

total pressu

T

T

f y P P

y

P

re (Pa)

partial pressure (Pa)

At the relatively low atmospheric pressure (1 atm), 1, making the fugacity

equal to the partial pressure of the chemical in air.

P

Fugacity Capacity in Air Phase

1 1

3

3

1

4

1

where

number of moles

volume (m )

gas constant (8.31 Pa m / mole/K)

absolute temperature (K)

Z fugacity capacity

1/ 4.04 10 moles/

n PC f fZ

V RT RT

n

V

R

T

RT

3m / Pa (at 25 )C

Fugacity and Fugacity Capacity in Water Phase

where

mole fraction

activity coefficient (assume constant)

saturation vapor pressure of pure liquid chemical at the system

s

s

f x P

x

P

2 2

-5 3

3

2

temperature (Pa)

where

molar volume of solution (water, 1.8 10 m / mole)

Henry's Law constant (Pa m /mole)

water fugacity ca

sw w

w

x f fC fZ

P H

H

Z

pacity

Fugacity in Solid Phase

2

2

where

sobred concentration in the natrual organic mater in soil or sediment

(moles/kg soil or sediment)

aqueous concentration (m

s d

s

C K C

C

C

3

oles/L solution)

equilibrium distribution coefficient (L solution/kg solids)

where

organic carbon-based distribution coefficient (L/kg)

d

d OC

OC

K

K K

K

3 mass fraction of organic carbon in the soil phase

Fugacity in Solid Phase

3 3 3 2 3 3 2

3 3 3

33

0.41

where

octanol-water partition coefficient

( )

( )1000

where

phase density (kg solid/m solid)

OC OW

OW

s d OC

OC

K K

K

C C K C K C

fK Z f

H

Diffusive Intermedia Transport

The diffusive rate of transfer from compartment i to compartment j is defined by

A comparable expression exists for the transfer rate from compartment j to i. The difference between the two is the net transfer rate.

where

diffusive rate from compartment to compartment (moles/hr)

intermedia transport parameter (mole/Pa/hr)

fugacity in compartm

ij ij i

ij

ij

i

N D f

N i j

D

f

ent i

Non-Diffusive Intermedia Transport

where

transfer rate (mole/hr)

volumetric flow rate of the transported material

(rainwater, suspended sediment, etc.)

phase concentration

N GC GZf Df

N

G

C

Air/Water Transports

Three processes are involved in air-to-water transport: diffusion (absorption), washout by rain and wet/dry deposition of aerosols.

The conventional 2-film approach is taken for absorption using the following air-side and water-side mass transfer coefficients:

5 m/h

0.05 m/hA

W

k

k

Air/Water Transport Parameters

1 2

1 1 2 2

Let

For absorption

11 1

where is the interfacial area

between air and water

A W

VW

W W

W

u k u k

D

u A Z u A Z

A

Air/Water Transport Parameters

3 2

-43

4 7

4

For rain washout

where is the rainfall rate (0.876 m/yr or 10 m/hr)

For wet and dry deposition of aerosols

where is the t

RW W

QW W

D u A Z

u

D u A Z

u

-10

he sum of these two parallel mechanisms

(6 10 m/hr)

Air/Water Transport Parameters

12

21

Culmulative D value for air-to-water transfer

For water-to-air trasnport

VW RW QW

VW

D D D D

D D

Air/Soil Transport Parameters

5 1 6 2 7 1

5 1

56

For absorption

1

1 1

where

the air/soil interfacial area

5 m/h air-side mass transfer coefficient

10 m/h mass transfe

VS

S S S

S

D

u A Z u A Z u A Z

A

u u

u

7

r coefficient in soil water

0.02 m/h mass transfer coefficient in soil airu

Air/Soil Transport Parameters

3 2

-43

4 7

4

For rain washout

where is the rainfall rate (0.876 m/yr or 10 m/hr)

For wet and dry deposition of aerosols

where is the t

RS S

QS S

D u A Z

u

D u A Z

u

-10

he sum of these two parallel mechanisms

(6 10 m/hr)

Air/Soil Transport Parameters

13

31

Culmulative D value for air-to-soil transfer

For soil-to-air trasnport

VS RS QS

VS

D D D D

D D

Water/Sediment Transport Parameters

24 8 2 9 5

48

9

Culmulative D value for water-to-sediment transfer

where

water-to-sediment MT coefficient 10 m/h

the suspended sediment deposition rate 5

W WD u A Z u A Z

u

u

7

10 2

5

42 8 2 10 4

10

10 m/h

water/sediment area 10 m

fugacity capacity for sediment

For sediment-to-water trasnport

where

resuspension v

W

W W

A

Z suspended

D u A Z u A Z

u

-7elocity = 2 10 m/h

Soil-to-Water Transport Parameters32 11 2 12 3

11

53

12

-611

where

run-off velocity of surface water

0.5 5 10 m/h

run-off solid velocity (assume 200ppm)

200 10 1.0

S SD u A Z u A Z

u

u

u

u

8

4 4

7

10 m/h

soil surface area

An additional non-diffusive transport mechanism which

removes chemical from sediment is burial.

where 2 10 m/h sediment burial r

S

A B W

B

A

D u A Z

u

ate

Advective Transport

Chemical may directly enter into compartments by emissions and advective inputs from outside the model region. The total rate of inputs for each compartment i is

3

3

where

(mole/h) is the emission rate

(m /h) is the advective flow rate

(mole/m ) is the background concentration

external to compartment i

Chemical may also

i i Ai Bi

i

Ai

Bi

I E G C

E

G

C

exit the model domain by advective

processes having transfer values

Ai Ai CiD G Z

Reaction Loss

Reaction processes occurring in the environment include biodegradation, photolysis, hydrolysis, and oxidation. A good approximation for reaction processes in the dilute limit commonly found in the environment is to express them as first order.

Reaction Loss

-1

1/ 2

1/

where

rate of reaction loss in compartment i (mole/hr)

ln(0.5) rate constant (hr )

compartment volume

molar concentration of the chemical

t

Ri Ri i i Ri i Ci Ri

Ri

Ri

i

i

N k V C k V Z f D f

N

kt

V

C

2 reaction half life

ExampleBenzene, ethanol, and pentachlorophenol (PCP) are examples

of organic pollutants with very different environmental properties. Benzene and ethanol are volatile and have comparatively short reaction time half-lives. PCP has a long reaction half life, low volatility and water solubility, and strong sorptive properties. Benzene is the most reactive in air and ethanol is the most reactive in water, soil and sediment.

Use the Mackay level III spreadsheet to determine the amounts of each chemical in the 4 compartments at steady state for

1. 1000 kg/hr emitted into air only.2. 1000 kg/hr emitted into water only.3. 1000 kg/hr emitted into soil only.