Presentation on theme: "Green Engineering, Process Safety and Inherent Safety: A New Paradigm"— Presentation transcript:
1Green Engineering, Process Safety and Inherent Safety: A New Paradigm David R. Shonnard, Ph.D.Department of Chemical EngineeringMichigan Technological UniversityHui Chen, Ph.D.Chemical and Materials EngineeringArizona State UniversitySACHE Faculty WorkshopSheraton Hotel and ExxonMobil, Baton Rouge, LA USASeptember 28 – October 1, 2003
2Presentation OutlineIntroduction to Green Engineering (GE) and Inherent Safety (IS)GE definition, concepts, principles, and toolsIS concepts and toolsSimilarities and differences between GE and ISEnvironmentally-Conscious Process Design MethodologyA hierarchical approach with three “tiers” of impact assessmentA case study for maleic anhydride (MA) process designEarly design methods and software toolsFlowsheet synthesis, assessment, and software toolsFlowsheet optimization - comparison of process improvementSummary of environmentally-conscious design methodsThis presentation is divided into two main parts. The first is an introduction to the concepts and tools of Green Engineering (GE) and of Inherent Safety (IS). Comparisons and differences between GE and IS will also be discussed. The second part describes in some detail a systematic and hierarchical framework for the design, assessment, and improvement of chemical processes using environmental impacts as design objectives.
3What is Green Engineering? Design, commercialization and use of processes and products that are feasible and economical while minimizing:+ Risk to human health and the environment+ Generation of pollution at the sourceThis is a working definition of Green Engineering as given by the US EPA, Office of Pollution Prevention and Toxics. The key words to notice here are “risk to human health and the environment” and “feasible and economical”. In Green Engineering it is not sufficient to be only green, but other design considerations must also be included, like economics, safety, and others.US EPA, OPPT, Chemical Engineering Branch, Green Engineering Program
4Why Chemical Processes (USA)? Positives1 million jobs$477.8 billion to the US economy5% of US GDP+ trade balance (in the recent past)57% reduction in toxic releases (`88-`00)It is worth some time to discuss the motivation for Green Enineering for chemical processes. First, let us examine some positive aspects of the chemical industry and then cover some environmental challenges for chemical manufacturing. Although the chemical manufacturing industry contributes greatly to a high standard of living, through job and wealth creation,• Chemical and Engineering News, Vol. 80, No. 25, pp , June 24, 2002• US EPA, Toxics Release Inventory (TRI) Public Data Release, 2000
5Why Chemical Processes (USA)? Environmental ChallengesManufacturing industries in the US (SIC codes 20-39) 1/3 of all TRI releasesChemical/Petroleum industries about 10% of all TRI releasesIncrease of 148% of TRI wastes managed on-site (`91-`00)Chemical products harm the environment during their useEnergy Utilization – ~15% of US consumptionthere still are many environmental challenges to be met through pollution prevention, risk reduction, and efficient materials utilization. Some of these challenges include toxicity of chemical products, life-cycle issues, and energy consumption.• US EPA, Toxics Release Inventory (TRI) Public Data Release, 2000• US DOE, Annual Energy Review, 1997.
6Energy Use: U.S. Industry SIC Code BTU/yr29 Petroleum/Coal Products28 Chemicals / Allied Products26 Paper / Allied Products33 Primary Metals Industries20 Food / Kindred Products32 Stone,Clay and Glass24 Lumber / Wood ProductsNumbers represent roughly the % of US annual energy consumptionChemical Manufacturing, allied products, petroleum and coal products consume for about 12% of the annual US energy consumption. When paper products is added, about 14%.Annual Energy Review 1997, U.S. DOE, Energy Information Administration, Washington, DC, DOE/EIA-0384(97)
7Pollution Prevention (P2) vs. Pollution Control (PC) Traditional ProcessProductsRaw Materials,EnergyChemicalProcessPollutionControlWastesGreener ProcessHigher income,Higher operating costsProductsRaw Materials,EnergyModifiedChemicalProcessGreen Engineering emphasizes pollution prevention and risk reduction over pollution control at the end of pipe. Pollution prevention can be accomplished through more efficient utilization of mass and energy, generation of fewer and less toxic pollutants, and reuse and recycle when pollutants are generated or when unconverted raw materials are present. Process modification to accomplish pollution prevention is one major approach in designing cleaner processes and in retrofitting existing processes. Pollution prevention can lead to higher production efficiencies and fewer wastes to treat using pollution control.PollutionControlLower PC costsWastesRecycle
8Examples of Green Engineering Chemical reactions using environmentally-benign solventsImproved catalyststhat increase selectivity and reduce wastesthat improve product quality and reduce environmental impactsthat process wastes into valuable productsSeparations using supercritical CO2 rather than R-Cl solventsSeparative reactors that boost yield and selectivityFuel cells in transportation and electricity generationCO2 sequestrationNew designs that integrate mass and energy more efficientlyProcess modifications that reduce emissionsEnvironmentally-conscious design methods and software tools.Green Engineering has many aspects that link well with traditional chemical engineering specialties, such as reaction engineering, separations engineering, and process design. This list shows some current GE research areas within Chemical Engineering. Green Engineering for chemical engineers seeks to develop advances in chemical process technologies that result in lower environmental impacts while improving profitability.
9Environmentally-Conscious Design Methods and tools to evaluate environmental consequences of chemical processes and products are needed.quantify multiple environmental impacts,guide process and product design activitiesimprove environmental performance of chemical processes and productsEnvironmental impactsenergy consumption - raw materials consumptionimpacts to air, water - solid wasteshuman health impacts - toxic effects to ecosystemsEconomic Performancecosts, profitabilityAlthough Green Engineering involves advances in chemical process technologies, ultimately methods are needed to quantify improvements to the environment as a results of incorporating these technologies into chemical processes. Environmentally-conscious design methodologies are thus required to guide the design process and to provide indicators of environmental performance for decision-making.
10Tools of Environmentally-Conscious Chemical Process Design and Analysis E-CDEnvironmental Fate Properties• databases• estimationChemical Process Properties• thermodynamics• reactions• transportChemical Process Models• simulation• waste generation and releaseEnvironmental Fate Models• single compartment• multi-mediaEnvironmental Impacts Models• midpoint vs endpoint• normalization• valuationProcess Integration• mass integration• heat integrationProcess Optimization• multi-objective• mixed integer• non-linearHierarchical DesignComputer-aided tools for E-CD can help meet environmental challenges by putting in place environmental impact assessment methodologies and tools that inform decision-making and lead to pollution prevention and environmental risk reduction. These tools fit into several categories: Property estimation or databases, models for process and environmental performance, and process integration and optimization. These tools are being developed by industry, government, and academia.
11Principles of Green Engineering The Sandestin Declaration onGreen Engineering PrinciplesGreen Engineering transforms existing engineering disciplines and practices to those that lead to sustainability. Green Engineering incorporates development and implementation of products, processes, and systems that meet technical and cost objectives while protecting human health and welfare and elevates the protection of the biosphere as a criterion in engineering solutions.A recent multi-disciplinary engineering conference was held to achieve a consensus on guiding principles for Green Engineering. There are many complimentary ideas in both this and the prior EPA definition.Green Engineering: Defining the Principles, Engineering Conferences International, Sandestin, FL, USA, May 17-22, 2003.
12Principles of Green Engineering The Sandestin GE PrinciplesEngineer processes and products holistically, use systems analysis, and integrate environmental impact assessment tools.Conserve and improve natural ecosystems while protecting human health and well-beingUse life-cycle thinking in all engineering activitiesEnsure that all material and energy inputs and outputs are as inherently safe and benign as possibleMinimize depletion of natural resourcesStrive to prevent wasteDevelop and apply engineering solutions, while being cognizant of local geography, aspirations, and culturesCreate engineering solutions beyond current or dominant technologies; improve, innovate and invent (technologies) to achieve sustainabilityActively engage communities and stakeholders in development of engineering solutionsHere are the 9 GE principles from the Sandenstin Conference. These guiding principles stress pollution prevention, life-cycle thinking, efficient utilization of resources as approaches in the design and development of technologies, as well as minmizing ecosystem impacts as design objectives.Green Engineering: Defining the Principles, Engineering Conferences International, Sandestin, FL, USA, May 17-22, 2003.
13Definition of Inherent Safety (IS) A chemical manufacturing process is described as inherently safer if it reduces or eliminates hazards associated with materials used and operations, and this reduction or elimination is a permanent and inseparable part of the process technology. (Kletz, 1991; Hendershot, 1997a, b)The definition of Inherent Safety emphasizes hazards elimination and reduction, in much the same way that GE stresses risk reduction for human health effects and for the environment. The approach is much the same, yet the targets are different. In IS, the target is protection of workers at the facility while GE targets the protection of the environment and general population.
14IS Concepts Intensification - using less of a hazardous material Example: improved catalysts can reduce the size of equipment and minimize consequences of accidents.Attenuation - using a hazardous material in a less hazardous form. Example: larger size of particle for flammable dust or a diluted form of hazardous material like aqueous acid rather than anhydrous acid.Substitution - using a safer material or production of a safer product. Example: substituting water for a flammable solvent in latex paints compared to oil base paints.This slide and the next show the main concepts of IS.
15IS Concepts (cont.)limitation - minimizing the effect of an incident. Example: smaller diameter of pipe for transport of toxic gases and liquids will minimize the dispersion of the material when an accident does occurSimplification - reducing the opportunities for error and malfunction. Example: easier-to-understand instructions to operators.
16Comparison between IS and (GE) Strategy/Tenet (Based on IS)Example ConceptsInherent Safety (IS)Green Engineering (GE)SubstitutionReaction chemistry, Feedstocks, Catalysts, Solvents, Fuel selection√√√√MinimizationProcess Intensification, Recycle, Inventory reduction, Energy efficiency, Plant location√√√SimplificationNumber of unit operations, DCS configuration, Raw material quality, Equipment designModeration (1) [Basic Process]Conversion conditions, Storage conditions, Dilution, Equipment overdesignModeration (2) [Overall Plant]Offsite reuse, Advanced waste treatment, Plant location, Beneficial co-disposal√√√√ = Primary tenet/concepts √√√ = Strongly related tenet/concepts√√ = Some aspects addressed √ = Little relationshipThis table shows strong overlap in concepts of IS and GE. Substitution is a clear example of a concept that overlaps IS and GE. Moderation has a greater relevance to IS than to GE, yet when the process conditions that achieve moderation also improve reaction conversions or reduce process emissions, then there will be a strong GE overlap.
17Similarities between (GE) and (IS) Benign and less hazardous materials.Both focus on process changes.Improving either one often results in improving the other.Both use a life-cycle approach.Both are best considered in the initial stages of the design.Inherent safer design and green engineering design both encourage the use of benign and less hazardous materials in the process. Examples include the use of environmentally benign and less toxic solvents, fuels, and other raw materials. Both focus on in-process reduction, rather than accepting the hazards/pollution and implementing layers of protection/treatment to control them. The resulting process and product improvements are often beneficial from both environmental and process safety perspectives. Both inherent safer design and green engineering design adopt life-cycle approach, although its use in GE is perhaps more common. When implemented early in design, both GE and IS are the most effective.
18Differences between GE & IS Focus on a different parts of the product life cycle.Focus on different aspect of EHS (environmental, health and safety) field and may conflict in application.Environmental impacts are more numerous than safety impacts.Inherently Safety tends to focus only on the chemical processing stage of the life cycle of the chemical product while Green Engineering attempts to minimize environmental impacts throughout the life cycle. GE and IS may conflict. Examples include process simplification versus internal recycle (which in itself adds complexity to a process). The primary concern in process safety is eliminating accidents, both catastrophic failures of process equipment and smaller accidents to workers, as well as minimizing risks to chronic releases to toxic chemicals. On the other hand, the number of environmental impacts that a chemical process or product can have is much larger in comparison.
19Presentation OutlineIntroduction to Green Engineering (GE) and Inherent Safety (IS)GE definition, concepts, principles, and toolsIS concepts and toolsSimilarities and differences between GE and ISEnvironmentally-Conscious Process Design MethodologyA hierarchical approach with three “tiers” of impact assessmentA case study for maleic anhydride (MA) process designEarly design methods and software toolsFlowsheet synthesis, assessment, and software toolsFlowsheet optimization - comparison of process improvementSummary of environmentally-conscious design (ECD) methodsHaving covered some introductory concepts of GE in the previous slides, the purpose of this next section is to introduce a hierarchical approach for incorporating environmental impacts in the design of chemical processes. This will be accomplished using a case study design of maleic anhydride production from either benzene or n-butane. Our goal will be to determine which of these routes for production of MA is the least harmful to the environment and the most economical. As we proceed, several topics will be covered, including the hierarchical approach for environmentally-conscious design (ECD) with three “tiers” of environmental impact assessment, a framework for calculating impacts and the models used for this purpose, and key results from the case study.
20Scope of environmental impacts Pre-Chemical Manufacturing Stages• Extraction from the environment • Transportation of materials • Refining of raw materials • Storage and transportation • Loading and unloadingChemical Manufacturing Process• Chemical reactions • Separation operations • Material storage • Loading and unloading • Material conveyance • Waste treatment processesPost-Chemical Manufacturing Stages• Final product manufacture• Product usage in commerce• Reuse/recycle• Treatment/destruction• Disposal• Environmental release• Airborne releases wastewater releases • Solid/hazardous waste • Toxic chemical releases• Energy consumption • Resource depletionEnvironmental/Health Impacts• Global warming • Ozone layer depletion • Air quality – smog • Acidification • Ecotoxicity• Human health effects, carcinogenic and non carcinogenic • Resource depletionIn the design of chemical products and processes, environmental impacts must include all stages in the life cycle. Premaunfacturing impacts include all environmental burdens required to extract materials from the environment, refine, and transport them to the manufacturing facility. Impacts of chemical manufacturing include additional energy consumption and emissions from all activities at the manufacturing facilities. Post chemical manufacturing impacts involve the use of the chemical product, its possible reuse or recycle, treatment, and disposal. Pollutant releases from all of these activities will have impacts on multiple environmental media and and on human health. The main focus of this presentation is the assessment of environmental impacts during the chemical manufacturing stage of the life-cycle.
21Tools of Environmentally-Conscious Chemical Process Design and Analysis E-CDEnvironmental Fate Properties• databases• estimationChemical Process Properties• thermodynamics• reactions• transportChemical Process Models• simulation• waste generation and releaseEnvironmental Fate Models• single compartment• multi-mediaEnvironmental Impacts Models• midpoint vs endpoint• normalization• valuationProcess Integration• mass integration• heat integrationProcess Optimization• multi-objective• mixed integer• non-linearHierarchical DesignECD requires a core set of process design and analysis tools and a framework within which to apply them. The computer-aided tools shown in this slide are used in a sequential fashion as the design progresses, starting with property estimation software and databases. With a known set of chemicals and properties, the process is simulated to obtain a mass and energy balance for the process. Emissions to the environment from process units are estimated using information from the process simulation. The environmental fate of these emitted pollutants is calculated using a multimedia compartment model. Information on emissions and fate are combined to create a set of environmental impact indices which are used to judge and compare competing process designs. After implementing improvement steps through process integration, the improved process flowsheet is optimized using environmental impacts as objectives in the optimization. All of these environmental impact assessment steps are implemented using a hierarchical approach.
22Hierarchical Approach to E-CD Process Design StagesEnvironmental AssessmentsLevel 1. Input Informationproblem definitionSimple (“tier 1”)toxicity potential, costsLevel 2. Input-Output Structurematerial selection reaction pathwaysLevels 3 & 4.recycle separation system“tier 2” – material/energy intensity, emissions, costsHierarchical environmentally-conscious design approaches can aid in systematically evaluating the impacts at several stages in the design process. In this slide are shown various process design stages, design tasks, and three tiers of environmental impact assessment. This tiered assessment approach is intended to introduce environenmental concerns at all important stages of the design.Levelsenergy integration detailed evaluationcontrol safety“tier 3” – emissions, environmental fate, riskAllen, D.T. and Shonnard, D.R.Green Engineering : EnvironmentallyConscious Design of Chemical Processes,Prentice Hall, pg. 552, 2002.Douglas, J.M., Ind. Eng. Chem. Res.,Vol. 41, No. 25, pp. 2522, 1992
23Early vs Detailed Design Tasks Chen, H., Rogers, T.N., Barna, B.A., Shonnard, D.R.,, Environmental Progress, in press April, 2003.The general structure of hierarchical design can be divided further into a number if design tasks. In early design, the purpose is to define the initial structure of the process flowsheet and to eliminate undesirable alternative designs. In detailed design, the objective is to optimize the flowsheet for minimum impacts and maximum profit. Many of these tasks will be covered in this presentation through a case study.Early DesignDetailed Design
24Hierarchical Approach to E-CD Process Design StagesEnvironmental AssessmentsLevel 1. Input Informationproblem definitionSimple (“tier 1”)toxicity potential, costsLevel 2. Input-Output Structurematerial selection reaction pathwaysLevels 3 & 4.recycle separation system“tier 2” – material/energy intensity, emissions, costsWe begin with considerations of input and outputs from the process, with a focus on the reactions.Levelsenergy integration detailed evaluationcontrol safety“tier 3” – emissions, environmental fate, riskAllen, D.T. and Shonnard, D.R.Green Engineering : EnvironmentallyConscious Design of Chemical Processes,Prentice Hall, pg. 552, 2002.Douglas, J.M., Ind. Eng. Chem. Res.,Vol. 41, No. 25, pp. 2522, 1992
25Case Study: Maleic Anhydride (MA) Production Level 1. Input / Output InformationBenzene Processn-Butane ProcessV2O5-MoO3VPOShown here are the major reactions for product and byproduct generation, with expected conversions, yields, and reaction conditions (temperature, pressure, etc.) for MA production from either benzene or n-butane. Most MA production has migrated from benzene to n-butane, and this represents one of the earliest examples of Green Chemistry making a significant change in chemical production.Benzene conversion, 95%MA Yield, 70%Air/Benzene, ~ 66 (moles)Temperature, 375°CPressure, 150 kPan-butane conversion, 85%MA Yield, 60%Air/n-butane, ~ 62 (moles)Temperature, 400°CPressure, 150 kPa
26MA Production: Early Design Costs Level 1. Input / Output Information“Tier 1” Economic analysis (raw materials costs only)Benzene Process(1 mole/0.70 mole) (78 g/mole) ( $/g) = $/mole of MAMA YieldBz MWBenzene costN-butane processhas lower costUsing this input / output information for the reaction step, we can compute raw material costs for each reaction route for MA production. n-butane is superior to benzene with regard to raw material costs, even when the benzene route exhibits higher conversions and yields compared to n-butane. One reason is because 2 of the carbons on the benzene molecule are not incorporated into the final product, MA. The Green Chemistry concept of “atom economy” relates the efficiency by which atoms of the reactants get incorporated into final product. The higher atom economy of the n-butane route coupled with its lower cost result in lower raw material costs even though the benzene route has a higher yield and selectivity.n-Butane Process(1mole/0.60 mole) (58 g/mole) ( $/g) = $/mole of MAMA YieldnC4 MWnC4 costAssumption: raw material costs dominate total cost of the process
27MA Production: Environmental Impacts Level 1. Input / Output Information“Tier 1” Environmental Impact AnalysisBased on Products and Byproducts from the ReactorAlternative “tier 1” assessment approachesToxicity and stoichiometryToxicity, other impact potentials, and stoichiometryToxicity, other impact potentials, stoichiometry, and environmental fateToxicity, other impact potentials, stoichiometry, environmental fate, and pollution control.There are a number of “tier 1” impact assessment approaches that have appeared in the literature, each varying in complexity. For this case study, we choose to use a more complex approach that includes toxicity of emitted chemicals, other impact potentials (GW, OD, etc.), stoichiometry of reactions, environmental fate of emitted pollutants, and effects of pollution control. The main reason for choosing a more complex impact assessment approach is so we can compare environmental performance at various stages of the design process using an identical assessment approach. Thus, we should be able to identify at what stage in the design, early or late, where most of the improvement in process design occurs.
28MA Production: IO Assumptions Level 1. Input / Output Information“Tier 1” Environmental Impact AnalysisCO2, H2O, air,traces of CO, MA benzene, n-butanePollutionControlUnreactedBenzeneorn-butane99% controlBenzeneorn-butaneCO, CO2 , H2O, air, MAA production rate of 50x106 lb per year is chosen as the basis for this design and impact assessment. Shown also are estimated recoveries and pollution control efficiency for the chemicals in the process. Using the reaction conversions and yields and these assumed recovery and pollution control efficiencies, emissions can be calculated, as shown in the following slide.ReactorProductRecoveryAirMA, CO,CO2 , H2OairMA50x106lb/yr99% MA recovery
29Emission Estimation Emissions to Air Level 1. Input / Output Information“Tier 1” Environmental Impact AnalysisEmissions to AirEmission factors from US EPAReactors, separation devicesAir ClearingHouse for Inventories and Emission FactorsAir CHIEFCO, CO2 generation from the reactorBenzene processBenzene: 0.07 moles benzene / mole MACO + CO2: 4.1 moles / mole MAn-butane processn-butane: 0.25 moles benzene / mole MACO + CO2: 1.7 moles / mole MAEmissions are estimated from major process units (reactors, separation devices) using emission factors obtained from the US EPA. These emission factors are average values obtained from studies conducted on many industrial processes. A table of these average values is found in Chapter 8 of the Green Engineering textbook. Accuracy is order of magnitude. Emissions of CO2 and CO from the reactor were calculated using conversions and yields and are equally divided among these two species (reaction kinetics parameters and model support this assumption). Pollution control eliminates 99% of the CO, unrecovered MA, benzene, and n-butane.Conversions,Yields
30Environmental / Toxicity Properties Level 1. Input / Output Information“Tier 1” Environmental Impact AnalysisEnvironmental/Toxicological PropertiesEstimation SoftwareEPI (Estimation Program Interface) SuiteHenry‘s constant, partitioning, degradation, toxicityOnline DatabaseEnvironmental Fate DatabaseEnvironmental and toxicological properties for the emitted chemicals can be estimated using software based on structure activity relationships (SAR) or may be found in databases, some of which are available on-line.Compilation in: Appendix F.Allen, D.T. and Shonnard, D.R., Green Engineering : Environmentally- Conscious Design of Chemical Processes, Prentice Hall, pg. 552, 2002
31Environmental Fate Calculations Level 1. Input / Output Information“Tier 1” Environmental Impact AnalysisMultimediacompartmentmodelProcesses modeled• emission inputs, E• advection in and out, DA• intercompartment mass transfer,Di,j• reaction loss, DRA multimedia compartment model is often used to estimate environmental fate of emitted pollutants. This calculation aids in evaluating potential exposure to the emitted chemicals by affected systems (either human exposure or ecosystem).Model Domain Parameters• surface area km2• 90% land area, 10% water• height of atmosphere - 1 km• soil depth - 10 cm• depth of sediment layer - 1 cm• multiphase compartmentsMackay, D. 1991, ”Multimedia Environmental Models", 1st edition,, Lewis Publishers, Chelsea, MI
32Impact Indicator Calculation Level 1. Input / Output Information“Tier 1” Environmental Impact AnalysisCarcinogenic Risk Example (inhalation route)ExposureFactorsA dimensionless relative risk index can be derived using environmental fate predictions and impact potential (toxicological data in this example) information. The impact of a chemical emissions is compared in a relative fashion to the impacts of a „benchmark“ compound, in this example, benzene for carcinogenic effects. Other categories if impact are calculated in a similar fashion.Multimedia compartment modelconcentration in airCarcinogenic Slope Factor, SF(toxicological property)
33Indicators for the Ambient Environment Level 1. Input / Output Information“Tier 1” Environmental Impact AnalysisThe TRACI method and softwarecontains a comprehensive listingof impact categories and indicators.A number of abiotic indicators of impact are shown here. The TRACI method and software (from the US EPA) contains a comprehensive list of impact categories and indicators. However, other compilations are available (EcoIndicator 99, Netherlands, etc.)Compilation impact parameters in: Appendix D.Allen, D.T. and Shonnard, D.R., Green Engineering : Environmentally- Conscious Design of Chemical Processes, Prentice Hall, pg. 552, 2002
34Indicators of Toxicity Level 1. Input / Output Information“Tier 1” Environmental Impact AnalysisThe TRACI method and softwarecontains a comprehensive listingof impact categories and indicators.A number of health related indicators are shown here. The TRACI method and software contains a comprehensive list of impact categories and indicators. However, other compilations are available (EcoIndicator 99, Netherlands, etc.)
35Indicators for MA Production Level 1. Input / Output Information“Tier 1” Environmental Impact AnalysisChemicalBenzenen-butaneIFT (kg/mole MA)5.39x10-62.19x10-6IING “3.32x10-33.11x10-3IINH “8.88x10-23.93x10-2ICING “1.43x10-40.00ICINH “IOD “IGW “2.01x10-11.17x10-1ISF “3.04x10-54.55x10-6IAR “n-butaneprocesshas lowerenviron-mentalimpactsA process index is then calculated by summing the products of each dimensionless risk index with each chemical‘s emission rate. The process index in each category represents the output of environmental impact for the process in terms of equivalents of the benchmark compounds from each category (e.g. CO2 for GW). n-butane process is superior in each category of impact, so this process is superior in both costs and environmental impacts compared to benzene process.
36Hierarchical Approach to E-CD Process Design StagesEnvironmental AssessmentsLevel 1. Input Informationproblem definitionSimple (“tier 1”)toxicity potential, costsLevel 2. Input-Output Structurematerial selection reaction pathwaysLevels 3 & 4.recycle separation system“tier 2” – material/energy intensity, emissions, costsAfter screening a number of alternative reaction routes for MA production, and deciding upon a smaller number of alternatives to evaluate further, we can progress to the next step in EC-D. A second tier of environmental assessment having an intermediate level of complexity is sometimes used. For this presentation, we proceed to the third tier of environmental impact assessment which is used at the stage of flowsheet simulation.Levelsenergy integration detailed evaluationcontrol safety“tier 3” – emissions, environmental fate, riskAllen, D.T. and Shonnard, D.R.Green Engineering : EnvironmentallyConscious Design of Chemical Processes,Prentice Hall, pg. 552, 2002.Douglas, J.M., Ind. Eng. Chem. Res.,Vol. 41, No. 25, pp. 2522, 1992
37Case Study: MA Production Level 3-8. Flowsheet Synthesis and Evaluation“Tier 3” Environmental Impact AnalysisBased on an initial process flowsheet created using “traditional“ economic-based design heuristics.“tier 3” assessmentEmissions estimation from units and fugitive sourcesEnvironmental fate and transport calculationToxicity, other impact potentials, environmental fate and transport, and pollution control.This begins with initial flowsheet generation using traditional economic-based design rules (heuristics), process simulation and impact assessment. The tier 3 assessment includes emission estimation, environmental fate and transport calculation, and impact indicator calclulation.
38Integrated Process Simulation and Assessment Method and Software It is critically important at this stage of the design synthesis to have access to automated process evaluation software that are linked to commercially-available process simulation software. Then, changes to the design can be immediately translated into changes in environmental impacts and costs. SCENE is such a collection of software tools for integrate assessments of process designs, although other groups (US EPA NRMRL – WAR algorithm, etc.) have assembled similar assessment systems for EC-D.HYSYS – a commercial chemical process simulator software, EFRAT – a software for calculating environmental impacts, DORT - a software to estimate equipment costs and operating costs, AHP (Analytic Hierarchy Process) – multi-objective decision analysis, PDS – Process Diagnostic Summary Tables, SGA – Scaled Gradient Analysis
39Initial Flowsheet for MA from n-C4 CompressorReactorsAirPumpn-ButaneVaporizerOff-gasOff-gasMAAn initial flowsheet for MA production from n-butane is shown here. The major components of this flowsheet are the reactors, absorber column for MA recovery from the reactor off-gas, and a distillation column for MA purification. Most energy requirements are satisfied using external utilities (steam for heating streams, and cooling water for cooling of streams).Distillation columnAbsorberSolvent
40Process Diagnostic Summary Tables: Energy Input/Output for nC4 Process The energy input/output table shows that the Reactor Feed Heater requires the most input energy, while Reactors 1-3 and Reactor off-gas cooler dominate the process cooling requirements. Furthermore, the energy duty from the Reactor off-gas cooler is almost sufficient to satisfy the energy input requirements of the Reactor feed heater, and the temperature ranges are a close match. Matching these stream energy requirements is a logical choice for heat integration.
41Process Diagnostic Summary Tables: Manufacturing Profit and Loss, nC4 The highest cost element is the cost for raw materials (n-butane) for the reaction, and the second is the natural gas for process heating (heat exchangers and distillation column reboiler). The use of raw material cost in the “tire 1“ economic assessment of the benzene and n-butane processes is justified using these process-level results.
42Process Diagnostic Summary Tables: Environmental Impacts, nC4 Process IndexNormalizationNational IndexThis table lists the impact indicators for each emitted chemical from the C4 process. The Process Index in each category of impact is divided by the national output of impact for that category to yield a Normalized Index. Each Normalized Index is multiplied by a Weighting Factor and summed over all categories to yield a Process Composite Index for environmental impacts. The largest contributor to IPC is IINH, and CO is the dominant pollutant for IINH. Alterations to Reactor conditions to increase MA selectivity will reduce IINH and therefore, IPC. Heat integration will help lower IPC by reducing IGW and IAR.Weighting Factorsglobal warming 2.5ozone depletion 100smog formation 2.5acid rain 10carcinogenic 5noncarcinogenic 5ecotoxicity 10Process composite indexSource: Eco-Indicator 95 framework for life cycle assessment,Pre Consultants,
43Flowsheet for MA Production from n-C4: with Heat Integration. CompressorAirReactorsPumpn-ButaneVaporizerOff-gasOff-gasMAThe addition of heat integration using heat exchangers between the reactor feed stream and the reactor off-gas stream will reduce energy consumption by about 75%. Using steam generated from the cooling of the Reactors in the Distillation Column reboiler will reduce energy consumption another 12%. These changes will also reduce the indices for global warming and acifidication.Distillation columnAbsorberSolvent
44Scaled Gradient Analysis Flowsheet Optimization:Scaled Gradient Analysis (SGA):Rank Order Parameter, rji = i-th unit operationj = j-th design variableProximity Parameter, pji = i-th unit operationj = j-th design variableAfter making structural changes to the initial flowsheet design using heat integration, the next step in environmentally-conscious design is to optimize the operating parameters. This is accomplished by identifying the most important parameters using Scaled Gradient Analysis (SGA). Design variables are identified according to the information gained from the PDS. Each design variable is changed slightly, increasing and decreasing relative to the base case values. Incremental values for each economic and environmental indicator are calculated for each unit operation or each emission source (dI). These incremental values are further divided by the incremental change of that design variable (dx), and multiplied by the scale factor (delta x). The rank-order parameter is then the sum of the absolute values of each.Douglas, J. M., “Conceptual Design of Chemical Process,” McGraw-Hill, New York (1988).
45SGA: variable changes and scale factors Flowsheet Optimization:Scaled Gradient Analysis (SGA):This table shows the design variables and their changes and the scale factors. Results from SGA is a list of the most important optimization variables. These were identified for both the n-butane and benzene MA production processes, and will be shown with the optimization results.
46Optimization using the Genetic Algorithm Chen, H., Rogers, T.N., Barna, B.A., Shonnard, D.R.,, Environmental Progress, in press April, 2003.Flowsheet Optimization:Genetic AlgorithmGenerations, 100Population Size, 100Mutation Probability, 0.04This diagram shows the major steps in the Genetic Algorithm-based optimization. Each optimization variable is divided into 10 nodes over it range of possible values. For 6 optimization variables, there are 106 unique combinations, from which 100 are chosen for the initial population. For each of these 100 members of the population, the parameter values are input to the process simulator and afterwards economic and environmental indices are generated using the integrated software. The remainder of the Genetic Algorithm is design to make changes to the population and continue until a desired convergence is achieved. The end result is the optimum population (operating parameter values) that minimizes environmental impacts, maximizes profit, or maximizes the AHP ranking. The Genetic Algorithm can often find the global optimum, but is not guaranteed.
47Optimization Results: n-butane Process AHP Ranking is the Objective FunctionThis table shows the optimization results, the list of optimization parameters, their ranges, and optimum values. Included are the values of each economic or environmental impact index. After optimization, Net Present Value (NPV) increases about 69% compared to the improved flowsheet realized after heat integration. IPC for this process is improved about only 0.4% compared to the improved flowsheet. Similar tables are obtained when NPV or IPC are the objective functions, for both the n-butane and benzene processes.
48Optimization Results: n-butane Process NPV is the Objective FunctionThis table shows the optimization results when NPV is the objective function. The values in this table are almost identical to the previous table because economic performance dominates compared to environmental impacts in the AHP weighting scheme.
49Optimization Results: n-butane Process IPC is the Objective FunctionThis table shows the optimization results when IPC is the objective function. NPV decreases about 8% compared to the value when NPV is the objective function, but IPC decreases only 0.4%. Thus, a large trade-off in economic performance occurs for a negligible improvement in environmental performance. Reaction inlet temperature changes from about 400 C to 390 C when IPC is the objective function. IPC is dominated by IINH due to the emission of CO from reactors and incineration of absorber off-gas. Therefore, when decreasing reaction temperature, the selectivity increases and less CO is produced in the reactor.
50Optimization Results: Benzene Process AHP Ranking is the Objective FunctionFor the benene process, this table shows the optimization results with AHP as the objective function. NPV is improved 34% over the improved flowsheet after heat integration and IPC is reduced by 9%.
51Optimization Results: Benzene Process NPV is the Objective FunctionThis table shows the optimization results when NPV is the objective function. As in the n-butane case, values in this table are almost identical to the AHP table because economic performance dominates compared to environmental impacts in the AHP weighting scheme.
52Optimization Results: Benzene Process IPC is the Objective FunctionThis table shows the optimization results when IPC is the objective function. IPC is reduced about 44% relative to the improved flowsheet after heat integration, a significant improvement compared to the AHP result, however, at the same time, NPV is decreased to -$2.13 MM. Again, as in the n-butane case, a large trade-off in economic performance occurs for an improvement in environmental performance. Reaction inlet temperature changes from about 375 C to 395 C when IPC is the objective function. IPC is dominated by ICIHG and ICINH and these two indices are closely tied to the fugitive release of benzene from the reactors and the uncontrolled benzene from incineration of absorber off-gas. When increasing reaction temperature and pressure, the conversion of benzene increases (lowering IPC); however, the selectivity towards maleic anhydride decreases (thus decreasing NPV).
53Continuous Improvement of Design Performance benzeneprocessdesignThese figures show a continuous improvement in the process economic and environmental performance as the design progresses from original base case flowsheet to modified (improved) base case flowsheet and ultimately to optimum flowsheet. The n-butane process is about 50% higher in NPV compared to the benzene process, which is the same degree of superiority shown in the “tier 1” economic assessment for screening reaction pathways. This agreement is a validation of the more simplistic economic assessment approach using only raw materials costs. The environmental impacts of the n-butane process are a factor of about 200 lower than the benzene process based on IPC, whereas the “tier 1” impact assessment shows a factor of 7. These optimization results appear to validate both the early design economic and environmental assessment methods in this case study.n-butaneprocessdesign
54Summary / ConclusionsA systematic and hierarchical approach for EC-D of chemical processes is shown.The EC-D approach is applied to a case study design for MA production from either benzene or n-butane.A number of computer-aided tools are available to facilitate EC-D.This approach yields a continuous improvement in both economic and environmental performance through the designs process.Early design assessment methods are validated using detailed design and optimization results.
55AcknowledgementsEPA Contract 3W-0500-NATA – OPPT, Green Engineering ProgramNSF/Lucent Technologies Industrial Ecology Research Fellowship (BES )National Center for Clean Industrial and Treatment Technologies (CenCITT)