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Environmental Evaluation in Process Synthesis

Learn to identify waste sources, estimate emissions, and apply pollution prevention in chemical processes. Explore case studies and methods for cleaner design.

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Environmental Evaluation in Process Synthesis

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  1. Module 4: Environmental Evaluation and Improvement During Process Synthesis - Chapters 8 and 9 David R. Shonnard Department of Chemical Engineering Michigan Technological University

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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,

  7. 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

  8. 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.

  9. Module 4: Emission Factors - major equipment

  10. Module 4: Emission factors - fugitive sources; minor equipment

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. Module 4: emissions from energy consumption:continued

  18. Module 4: Uncontrolled Air emission / releasesBenzene to MA Process (lb/103 lb MA)

  19. Module 4: Flowsheet evaluation - n-butane to maleic anhydride Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 15, pp. 893-927. 1991

  20. Module 4: Uncontrolled Air emission / releasesn-butane to MA Process (lb/103 lb MA)

  21. 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)]

  22. 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 -

  23. 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

  24. 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

  25. 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.

  26. Module 4: Pollution prevention through material selection - water pretreatment Reverse Osmosis to prevent 10 kg sludge/kg ppt RCRA waste

  27. 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

  28. 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

  29. 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

  30. 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

  31. 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

  32. 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

  33. 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

  34. 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

  35. 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

  36. 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)

  37. 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

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

  39. 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

  40. 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

  41. Module 4: Fugitive Sources -pollution prevention techniques

  42. Module 4: Flowsheet evaluation - maleic anhydride from n-butane Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 15, pp. 893-927. 1991

  43. 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

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

  45. Module 4: Case Study - reactor temperature:Maleic anhydride from n-butane

  46. 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

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