University of Texas at AustinMichigan Technological University 1 Module 4: Environmental Evaluation and Improvement During Process Synthesis - Chapters.

University of Texas at Austin Michigan Technological University1

Module 4: Environmental Evaluation and

Improvement During Process Synthesis - Chapters 8 and 9

David R. Shonnard

Department of Chemical Engineering

Michigan Technological University


Module 4: Outline

Educational goals and topics covered in the module

Potential uses of the module in chemical engineering courses

Identify and estimate emissions from process units - Chapter 8

Pollution prevention strategies for process units - Chapter 9

After the Input-Output structure is established, an environmentalevaluation during process synthesis can identify large sources of waste generation and release; directing the attention of the

designer to pollution prevention options within the process


Module 4: Educational goals and topics covered in the module

Students will: estimate air emissions and other releases from process units

after developing a preliminary process flowsheet, using software and hand calculations

have a better understanding of the mechanisms for pollutant generation and release from process units

become familiar with practical pollution prevention strategies for process units


Module 4: Potential uses of the module in chemical engineering courses

Mass/energy balance course: • criteria pollutant emissions from energy consumption

• emission of global change gases from energy consumption

• calculate emission factors from combustion stoichiometry

Continuous/stagewise separations course:• evaluate environmental aspects of mass separating agents

Design course:• pollution prevention strategies for unit operations

Reactor design course:• environmental aspects of chemical reactions and reactors

• pollution prevention strategies for chemical reactors


Identifying and estimating air emissions and other releases from process units

1. Identify waste release sources in process flowsheets

2. Methods for estimating emissions from chemical processes

3. Case study - Benzene to Maleic Anhydride process evaluation

Chapter 8


1. Waste streams from process units

2. Major equipment - vents on reactors, column separators, storage tanks, vacuum systems, ..

3. Fugitive sources - large number of small releases from pumps, valves, fittings, flanges, open pipes, ..

4. Loading/unloading operations

5. Vessel clean out, residuals in drums and tanks

6. Secondary sources - emissions from wastewater treatment, other waste treatment operations, on-site land applications of waste, ..

7. Spent catalyst residues, column residues and tars, sludges from tanks, columns, and wastewater treatment, …

8. Energy consumption - criteria air pollutants, traces of hazardous air pollutants, global warming gases,

Module 4: Typical waste emission sources

from chemical processes - Ch 8


1. Actual measurements of process waste stream contents and flow rates or indirectly estimated based on mass balance and stoichiometry. (most preferred but not always available at design stage)

2. Release data for a surrogate chemical or process or emission factors based on measured data

3. Mathematical models of emissions (emission correlations, mass transfer theory, process design software, etc.)

4. Estimates based on best engineering judgement or rules of thumb

Module 4: Process release estimation methods


Waste stream summaries based on past experience

1. Hedley, W.H. et al. 1975, “Potential Pollutants from Petrochemical

Processes”, Technomics, Westport, CT

2. AP-42 Document, Chapters 5 and 6 on petroleum and chemical industries,

Air CHIEF CD, www.epa.gov/ttn/chief/airchief.htm

3. Other sources

i. Kirk-Othmer Encyclopedia of Chemical Technology, 1991-

ii. Hydrocarbon Processing, “Petrochemical Processes ‘99”, March 1999.

Module 4: Emission estimation methods: based on surrogate processes


Module 4: Emission Factors - major equipment

Table 8.3.2 Average Emission Factors for Chemical Process UnitsCalculated from the US EPA L&E Database

Process Unit EFav ; (kg emitted/103 kg throughput) Reactor Vents 1.50 Distillation Columns Vents 0.70 Absorber Units 2.20 Strippers 0.20 Sumps/Decanters 0.02 Dryers 0.70 Cooling Towers 0.10


Module 4: Emission factors - fugitive sources; minor equipment

Ei (kgi / yr) mi EFav N s 24 365


Module 4: Emission factors - criteria pollutants from energy

consumption

Ei (lb i / yr)EFav(lb i / 10

3gal)ED(Btu / yr)HV(Btu / 103gal) BE

AP-42, Chapter 1, section 1.3, Air CHIEF CD, www.epa.gov/ttn/chief/airchief.htm


Module 4: Emission factors - CO2 from energy consumption





Software Tools

Storage tanks

TANKS 4.0 - program from EPA - www.epa.gov/ttn/chief/tanks.html

Wastewater treatment

WATER8 - on Air CHIEF CD - www.epa.gov/ttn/chief/airchief.html

Treatment storage and disposal facility (TSDF) processes

CHEMDAT8 - on Air CHIEF CD

Module 4: Emission correlations/models - storage tanks and waste treatment


Module 4: Benzene to MA Process


V2O5

2 C6H6 + 9 O2 ----------> 2 C4H2O3 + H2O + 4 CO2

MoO3


Module 4: Air emission and releases sources:

Benzene to MA Process

Source Identification

1. Product recovery absorber vent

2. Vacuum system vent

3. Storage and handling emissions

4. Secondary emissions from water out, spent catalyst, fractionation column residues

5. Fugitive sources (pumps, valves, fittings, ..)

6. Energy consumption


Process data for energy consumption• 0.15 lb fuel oil equivalent per lb Maleic Anhydride product• fuel oil #6 in a Normally Fired Utility Boiler• 1% sulfur• Boiler efficiency included in the energy usage data

Module 4: emissions from energy consumption:

Criteria pollutants (SO2, SO3, NOx, CO, PM)

AirCHIEF Demonstration


Module 4: emissions from energy consumption:

continued

SO2

157 lb SO2

%S 103 gal #6

1%S 1 gal #6

6.68 lb #6

0.15 lb #6lb MA

= 3.53x10-3

lb SO2

lb MA = 3.53

lb SO2

103 lb MA

SO3

5.7 lb SO3

%S 103 gal #6

1%S 1 gal #6

6.68 lb #6

0.15 lb #6lb MA

= 1.28x10-4

lb SO3

lb MA = 0.13

lb SO3

103 lb MA

NOx

67 lb NOx

103 gal #6

1 gal #66.68 lb #6

0.15 lb #6lb MA

= 1.50x10-3

lb NOxlb MA

= 1.50 lb NOx

103 lb MA

CO

5 lb CO

103 gal #6

1 gal #66.68 lb #6

0.15 lb #6lb MA

= 1.12x10-4

lb COlb MA

= 0.11 lb CO

103 lb MA

PM

9.19 lb PM

%S 103 gal #6

1%S 1 gal #66.68 lb #6

0.15 lb #6lb MA

= 2.06x10-4

lb PMlb MA

= 0.21 lb PM

103 lb MA


Module 4: Uncontrolled Air emission / releases

Benzene to MA Process (lb/103 lb MA)Release Source

i Methods used

MaleicAnhydride(MA)

Benzene Xylene CriteriaPollutants

CO2 Tars andoxygenates

Venting fromstorage tanks 1 0.03 0.14

Absorber columnvent 2 100 700 (CO) 972 20

Vacuum systemvent 2 0.02 0.02

FugitiveEmissions 3 0.2 0.2 0.1

Loading/unloadingoperations 4

0.2 2.0

Wastes fromvacuum columns 5

3.8 26.5

Energy useemissions 6 5.5 562

Total 4.2 102.3 0.1 705 1,534 46.51 Vertical fixed-roof tank; surrogate for MA is perchloroethylene; Tanks 4.0 program from EPA – www.epa.gov/ttn/chief/tanks/html2 Hedley et al. 1975. Potential Pollutants from Petrochemical Processes. Technomic, West Port, CT AP-42 chapter 6 section 6.14, Air CHIEF CD, www.epa.gov/ttn/chief/airchief.htm3 Typical chemical industry emission factor from Berglund and Hansen, 1990.4 Equation 8.3-4, chapter 8, Green Engineering textbook.5 Hedley et al. 1975. Potential Pollutants from Petrochemical Processes. Technomic, West Port, CT6 AP-42, Chapter 1, section 1.3, Table 1.3-11, Air CHIEF CD, www.epa.gov/ttn/chief/airchief.htm


Module 4: Flowsheet evaluation - n-butane to maleic anhydride

Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 15, pp. 893-927. 1991


Module 4: Uncontrolled Air emission / releases

n-butane to MA Process (lb/103 lb MA)Release Source

i Methods used

MaleicAnhydride(MA)

n-butane CriteriaPollutants

CO2 Tars andoxygenates

Venting fromstorage tanks 1 1.0

Absorber columnvent 2 8.0 251.4 761 (CO) 1033

Vacuum systemvent 3 9.6

FugitiveEmissions 4 0.2 0.2

Loading/unloadingoperations 5

0.2

Wastes fromvacuum columns 6 3.8

Energy useemissions 7 2.9 600

Total 22.6 251.6 764 1,6331 Vertical fixed-roof tank; Tanks 4.0 program within the Environmental Fate and Risk Assessment Tool (EFRAT) (see module 6)2 from stream information using a commercial process simulator, HYSYS3 from emission estimation program in EFRAT using the emission factor for a vacuum distillation column.4 Typical chemical industry emission factor from Berglund and Hansen, 1990.5 Equation 8.3-4, chapter 8, Green Engineering textbook.6 Hedley et al. 1975. Potential Pollutants from Petrochemical Processes. Technomic, West Port, CT7 from emission estimation program in EFRAT using emission factors for energy consumption, AP-42, Chapter 1, Air CHIEF CD, www.epa.gov/ttn/chief/airchief.htm , bituminous coal for electricity demand only


Module 4: Tier 2 environmental assessment indexes

1. Energy: [total energy (Btu/yr)] / [production rate (MM lb/yr)]

2. Materials: [raw materials (MM lb/yr)] / [production rate (MM lb/yr)]

3. Water: [process water (MM lb/yr)] / [production rate (MM lb/yr)]

4. Emissions: [total emissions and wastes (MM lb/yr)] / [production rate (MM lb/yr)]

5. Targeted emissions: [total targeted emissions and wastes (MM lb/yr)] / [production rate (MM lb/yr)]


Module 4: Benzene to MA Process Conclusions from emissions summary

1. Chemical profile:

CO2 > CO > benzene > tars-oxygenates > MA

2. Toxicity profile:

Benzene > MA > CO > tars-oxygenates > CO2

3. Unit operations profile:

Absorber vent > energy consumption > vacuum system vent

- Pollution prevention and control opportunities are centered

on benzene, the absorber unit, and energy consumption -


Module 4: Chapter 9

Pollution prevention strategies for process units

1. Material choices for unit operations

2. Pollution prevention for chemical reactions and reactors

3. Separation units: reducing energy consumption and wastes

4. Preventing pollution for storage tanks and fugitive sources

5. Case study applications - • VOC recovery/recycle: effect of MSA choice on energy consumption

• Maleic anhydride from n-butane: MA yield vs reaction temperature


Module 4: Important issues regarding pollution prevention for unit operations

1. Material selection: fuel type, mass separating agents (MSAs), air, water, diluents, heat transfer fluids

2. Operating conditions: temperature, pressure, mixing intensity

3. Energy consumption: high efficiency boilers, operation of units to minimize energy usage

4. Material storage and fugitive sources: storage tank choices and equipment monitoring and maintenance

5. Waste generation mechanisms: understanding this will lead to pollution prevention strategies


Example Problem:

Calculate the annual uncontrolled SO2 emissions to satisfy a steam energy demand of 108

Btu/yr with a boiler efficiency of .85 assuming Fuel Oil #6, #2, and Natural Gas.

Module 4: Pollution prevention through material selection - fuel type

#6 Fuel Oil #2 Fuel Oil Natural Gas

Emission Factor, EF(lb/103 gal) 157S 143S 0.6 lb/106 scf

Sulfur Content, S % 0.84 0.24 ----

Heating Value, HV(Btu/103 gal) 1.48x108 1.30x108 1050x106

Btu/106scf

Annual Emission, E(lb SO2/yr) 105 31 .07



Ei (lb i / yr)EF(lb i / 106scf )ED(Btu / yr)

HV(Btu / 106scf)BE


Module 4: Pollution prevention through material selection - water pretreatment

to prevent10 kg sludge/kg pptRCRA waste

Reverse Osmosis


Module 4: Pollution prevention through material selection - reactor applications

1. Catalysts:

• that allow the use of more environmentally benign raw materials

• that convert wastes to usable products and feedstocks

• products more environmentally friendly - e.g. RFG / low S diesel fuel

2. Oxidants: in partial oxidation reactions

• replace air with pure O2 or enriched air to reduce NOx emissions

3. Solvents and diluents :

• replace toxic solvents with benign alternatives for polymer synthesis

• replace air with CO2 as heat sinks in exothermic gas phase reactions


Module 4: Pollution prevention for chemical reactors

1. Reaction type:

• series versus parallel pathways

• irreversible versus reversible

• competitive-consecutive reaction pathway

2. Reactor type:

• issues of residence time, mixing, heat transfer

3. Reaction conditions:

• effects of temperature on product selectivity

• effect of mixing on yield and selectivity


1st Order Irreversible Parallel Reactions

Module 4: Pollution prevention for chemical

reactions

Rk p P

R kw W

High Conversiont > 5(kp+ kw)-1

High Selectivitykp >> kw

SelectivityIndependent of residencetime

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 1 2 3 4 5

= (k p+kw) t

kp/kw = 1

kp/kw = 10

kp/kw = 100

kp/kw = 10

kp/kw = 100

[P]/[R]o

[W]/[R] o



reactions

1st Order Irreversible Series Reactions

0.0

0.2

0.4

0.6

0.8

1.0

0 1 2 3 4 5= kp t

[R]/[R]o[P]/[R]o[W]/[R]o

kp/kw = 1

kp/kw = 2

kp/kw = 10

kp/kw = 10

kp/kw = 100

kp/kw = 100

kp/kw = 1

kp/kw = 2

High Conversiont > 5 kp

-1

High Selectivitykp >> kw

Selectivitydependent on residencetime

Rk p P kw W


Reversible Series Reactions CH4 + H2O CO + 3H2

Steam reforming of CH4 CO + H2O CO2 + H2

R = CH4

P = CO

W = CO2

Separate and recycle waste to extinction


reactions


Module 4: Pollution prevention - reactor types

1. CSTR:

• not always the best choice if residence time is critical

2. Plug flow reactor:

• better control over residence time

• temperature control may be a problem for highly exothermic reactions

3. Fluidized bed reactor :

• if selectivity is affected by temperature, tighter control possible

4. Separative reactors:

• remove product before byproduct formation can occur: series reactions


Module 4: Pollution prevention - reaction

temperature

1st Order Irreversible Parallel Reactions

For Ep > Ew, Ep was set to 20 kcal/mole and Ew to 10 kcal/mole. R

k p P

R kw W

0

2

4

6

8

10

12

14

-100 -80 -60 -40 -20 0 20 40 60 80 100T (K)

(kp/kw)

Ep>Ew

Ew>Ep

for Ep > Ew,

Ep = 20 kcal/mole

Ew to 10 kcal/mole

for Ew > Ep,

Ep = 10 kcal/mole

Ew to 20 kcal/mole

E = activation energy


Module 4: Pollution prevention - mixing effects

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

1.E-05 1.E-04 1.E-03 1.E-02 1.E-01

(k1 Bo )(Ao/Bo)

Y/Yexp

Irreversible 2nd order competitive-consecutive reactions A B k1 P

P B k2 W

Y = yield = P/Ao

Yexp = expected yield = mixing time scale

Increasedmixing will increase observed yield

Ao

BoCSTR


Module 4: Pollution prevention - other reactor modifications

1. Improve Reactant Addition:

• premix reactants and catalysts prior to reactor addition

• add low density materials at reactor bottom to ensure effective mixing

2. Catalysts:

• use a heterogeneous catalyst to avoid heavy metal waste streams

• select catalysts with higher selectivity and physical characteristics

(size, porosity, shape, etc.)

3. Distribute flow in fixed-bed reactors

4. Heating/Cooling:

• use co-current coolant flow for better temperature control

• use inert diluents (CO2) to control temperature in gas phase reactions

5. Improve reactor monitoring and control


Module 4: Pollution prevention - for separation devices

1. Choose the best technology:

• take advantage of key property differences (e.g. volatility for distillation)

2. Choose the best mass separating agent:

• consider operability, environmental impacts, energy usage, and safety

3. Separation Heuristics

• combine similar streams to minimize the number of separation units

• separate highest-volume components first

• remove corrosive and unstable materials early

• do the most difficult separations last

• do high-purity recovery fraction separations last

• avoid adding new components to the separation sequence

• avoid extreme operating conditions (temperature, pressure)


Module 4: Pollution prevention - example of mass separating agent choice

Gaseous Waste StreamToluene & Ethyl Acetate193.5 kg/h each; 12,000scfm, balance N2

Vent

Vent ; 21 - 99.8 % recoveryof Toluene and Ethyl Acetate

Make-up oilAbsorption oil (C-14)10 – 800 kgmole/h

50/50 MassMixed Product

AbsorptionColumn

DistillationColumn

HYSYS Flowsheet

Absorber oilrecycle

Absorber Vent; 90% recovery of VOC

Conditions for simulations1. 10-stage columns, 2. 10 ˚C approach temperature for heat integration, 3. absorber temperature = 32 ˚C

MSA Screening1. 857 chemicals2. Hansen Sol. Par. 11.8 d 22

0 p 9.3

0 h 11.2

3. Tbp > 220 ˚C

4. Tmp < 26 ˚C

5. 23 chemicals remain


Module 4: Pollution prevention - results of mass separating agent choice

Chemical Utility(Btu/hr)

Rank

o-Dibromobenzene 1.37x106 1

Butyl benzoate 1.39x106 2

Nitrobenzene 1.41x106 3

o-Bromoanisole 1.42x106 4

Dibenzyl ether 1.42x106 5

Diethylene glycol dibutyl ether 1.46x106 6

Diethylene glycol butyl ether acetate 1.46x106 7

Octanioc acid 1.47x106 8

Ethyl cinnamate 1.48x106 9

1-Bromo-4-ethoxy benzene 1.49x106 10

trans-Anethole 1.62x106 11

Diethylene glycol monobutyl ether 1.68x106 12

1-Methyl naphthalene 1.70x106 13

p-Chlorobenzoyl chloride 1.75x106 14

4-Chlorobenzotrichloride 1.75x106 15

Diethylene glycol monoethyl ether acetate 1.83x106 16

Quinoline 2.30x106 17

1-Decanol 2.37x106 18

2-Decanol 2.55x106 19

Hexadecane 3.39x106 20

Tetradecane 3.94x106 21

1,2,4-Trichlorobenzene 3.95x106 22

Dodecane 5.35x106 23


Emission Mechanisms; Fixed Roof Tank

LTOTAL = LSTANDING + LWORKING

Roof Column

Vent

T

P

LiquidLevel

- Weather, paint color/quality

- Weather

- liquid throughput, volume of tank

Vapor pressure of liquid drives emissions

Module 4: Pollution prevention - Storage Tanks


Module 4: Storage tank comparison -TANKS 4.0 Demonstration

Storage Tank Type Vertical Internal Domed External

Fixed Roof Floating Roof Floating Roof

Annual Emissions (lb)

White Paint 337.6 66.2 42.8

Grey (Medium) Paint 489.1 85.1 52.4

Heated (White) 313.5

Poor (Grey/Medium) 509.7 81.0 51.5

Gaseous waste stream flowsheet ; pg 37 • Toluene emissions only • 516,600 gal/yr flowrate of toluene • 15,228.5 gallon tank for each comparison

Pollution prevention strategies • replace fixed-roof with floating-roof tank • maintain light-colored paint in good condition • heat tank to reduce temperature fluctuations


Module 4: Fugitive Sources -pollution prevention techniques


Module 4: Flowsheet evaluation - maleic anhydride from n-butane

Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 15, pp. 893-927. 1991


Principal Reaction

1. C4H10 + 3.5O2 C4H2O3 + 4H2O -HR,1 = 1.26x106 kJ/kmole

2. C4H10 + 4.5O2 4CO + 5H2O -HR,2 = 1.53x106 kJ/kmole

3. C4H10 + 6.5O2 4CO 2 + 5H2O -HR,3 = 2.66x106 kJ/kmole

Activation Energies Rate Equations

E1/R = 8,677 K

E2/R = 8,663 K

E3/R = 8,940 K

Module 4: Reaction rate parameters - Maleic anhydride from n-butane

r1 k1(KDiss pO2 )

1/2

1 (KDiss pO2 )1/ 2 pBut.

r2 k2KSorp pO21 KSorp pO2

pBut.

r3 k3KSorppO21KSorp pO2

pBut.

Schneider et al. 1987, “Kinetic investigation and reactor simulation…”, Ind. Eng. Chem. Res., Vol. 26, 2236-2241


Module 4: Fixed-bed reactor section - 100 MM tons/yr maleic anhydride process


Module 4: Case Study - reactor temperature:

Maleic anhydride from n-butane

0

10

20

30

40

50

60

70

80

90

100

370 380 390 400 410 420 430

Reactor Temperature (C).

(%)

n-butane conversion MA Yield MA-CO2 selectivity MA-CO selectivity


Module 4: Summary/Conclusions

1. Methodologies/software tools - process synthesis:

• emission factors

• surrogate process information from historical sources

• emission estimation software: TANKS 4.0, AirCHIEF 7.0, process

simulator packages,

• Tier 2 environmental assessment

2. Case studies:• VOC recovery/recycle from a gaseous waste stream - effects of MSA

choice on energy consumption

• Maleic anhydride from n-butane - effect of reaction temperature on

conversion, MA yield, MA selectivity

University of Texas at AustinMichigan Technological University 1 Module 4: Environmental Evaluation and Improvement During Process Synthesis - Chapters.

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