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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 After the Input-Output structure is established, an environmental

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module 4 environmental evaluation and improvement during process synthesis chapters 8 and 9

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
Module 4: Outline

After the Input-Output structure is established, an environmental

evaluation 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

  • 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
module 4 educational goals and topics covered in the module
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
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

chapter 8
Chapter 8

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

module 4 typical waste emission sources from chemical processes ch 8
Module 4: Typical waste emission sources from chemical processes - Ch 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 process release estimation methods
Module 4: Process release estimation methods

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 emission estimation methods based on surrogate processes
Module 4: Emission estimation methods: based on surrogate processes

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 factors criteria pollutants from energy consumption
Module 4: Emission factors - criteria pollutants from energy consumption

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

module 4 emission factors co 2 from energy consumption
Module 4: Emission factors - CO2 from energy consumption

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

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

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 benzene to ma process
Module 4: Benzene to MA Process

V2O5

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

MoO3

AP-42, Chapter 6, section 6.14, Air CHIEF CD, www.epa.gov/ttn/chief/airchief.htm

module 4 air emission and releases sources benzene to ma process
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

module 4 emissions from energy consumption criteria pollutants so 2 so 3 nox co pm
Module 4: emissions from energy consumption:Criteria pollutants (SO2, SO3, NOx, CO, PM)

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

AirCHIEF Demonstration

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

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

module 4 tier 2 environmental assessment indexes
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
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
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
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

module 4 pollution prevention through material selection fuel type
Module 4: Pollution prevention through material selection - fuel type

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 water pretreatment
Module 4: Pollution prevention through material selection - water pretreatment

Reverse Osmosis

to prevent

10 kg sludge/kg ppt

RCRA waste

module 4 pollution prevention through material selection reactor applications
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
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

module 4 pollution prevention for chemical reactions
Module 4: Pollution prevention for chemical reactions

1st Order Irreversible Parallel Reactions

High

Conversion

t > 5(kp+ kw)-1

High Selectivity

kp >> kw

Selectivity

Independent

of residence

time

module 4 pollution prevention for chemical reactions30
Module 4: Pollution prevention for chemical reactions

1st Order Irreversible Series Reactions

High

Conversion

t > 5 kp-1

High Selectivity

kp >> kw

Selectivity

dependent

on residence

time

module 4 pollution prevention for chemical reactions31
Module 4: Pollution prevention for chemical reactions

Reversible Series ReactionsCH4 + H2O  CO + 3H2

Steam reforming of CH4 CO + H2O  CO2 + H2

R = CH4

P = CO

W = CO2

Separate and recycle waste to extinction

module 4 pollution prevention reactor types
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
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.

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
Module 4: Pollution prevention - mixing effects

CSTR

Bo

Ao

Irreversible 2nd order competitive-consecutive reactions

Y = yield

= P/Ao

Yexp = expected

yield

t = mixing time

scale

Increased

mixing will increase observed yield

module 4 pollution prevention other reactor modifications
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
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

Absorber Vent; 90% recovery of VOC

Absorber oil

recycle

HYSYS Flowsheet

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

MSA Screening

1. 857 chemicals

2. Hansen Sol. Par.

11.8  dd  22

0  dp  9.3

0  dh  11.2

3. Tbp > 220 ˚C

4. Tmp < 26 ˚C

5. 23 chemicals

remain

Conditions for simulations

1. 10-stage columns,

2. 10 ˚C approach temperature for heat integration,

3. absorber temperature = 32 ˚C

module 4 pollution prevention storage tanks
Module 4: Pollution prevention - Storage Tanks

Emission Mechanisms; Fixed Roof Tank

LTOTAL = LSTANDING + LWORKING

Vent

Vapor pressure of liquid drives emissions

DT

DP

Liquid

Level

- Weather, paint color/quality

- Weather

- liquid throughput, volume of

tank

Roof Column

module 4 storage tank comparison tanks 4 0 demonstration
Module 4: Storage tank comparison -TANKS 4.0 Demonstration

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

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

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

Principal Reaction

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

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

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

Activation Energies Rate Equations

E1/R = 8,677 K

E2/R = 8,663 K

E3/R = 8,940 K

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

module 4 summary conclusions
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