Feature pubs.acs.org/journal/ascecg
Quantitative Sustainability Analysis: A Powerful Tool to Develop Resource-Efficient Catalytic Technologies Bala Subramaniam,*,†,‡ Richard K. Helling,§ and Claudia J. Bode‡ †
Department of Chemical and Petroleum Engineering and ‡Center for Environmentally Beneficial Catalysis, University of Kansas, 1501 Wakarusa Drive, Lawrence, Kansas 66047, United States § The Dow Chemical Company, 715 East Main Street, Midland, Michigan 48674, United States ABSTRACT: Demand for chemicals is growing. In fact, the chemical industry’s global output is expected to nearly double its 2010 volume by 2020. Lessening the collateral environmental burden of such growth requires sustainable alternative technologies. Fortunately, green chemistry and engineering research has made remarkable progress, laying the foundation for developing efficient processes that conserve feedstock and energy. Life cycle assessment (LCA) also plays a key role by identifying environmental hotspots along the supply chain, either within the manufacturing plant (catalysts, solvents, reactors, separators) or upstream during raw material extraction or during the generation of fossil-based energy at any stage. We present several examples that illustrate how important it is to incorporate LCA with techno-economic analysisespecially during the early stages of process development. This approach is not just useful, it is an essential tool for the rational development of sustainable chemical processes. KEYWORDS: Life cycle assessment, Sustainable catalytic processes, Energy intensity, Commodity chemicals
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he global chemical industry output is estimated to reach 3.4 trillion US$ by 2020, nearly double the 2010 output.1 A majority of this growth will occur in developing economies such as China, India, and South America. This is because of the rapidly improving economic climate in these populous regions and the increased affordability of everyday items, such as shampoos, diapers, synthetic clothing, and plastic containers. Meeting the growing demand for man-made products will require new chemical manufacturing facilities. Future ecological burdens would be especially pronounced if today’s energyintensive chemical technologies are continued, since generating energy by traditional routes contributes greatly to adverse effects on the environment and human health. Chemical and petrochemical manufacturing currently requires copious amounts of energy. It consumes an estimated 15 exajoules per year (EJ/y) worldwide (considering only process-related energy).2 Roughly half of this energy, 7.1 EJ/y, goes into making just four groups of commodity chemical precursors: olefins (ethylene and propylene), ammonia, BTX aromatics (benzene, toluene, xylenes), and methanol.3 Energy demand goes up to 9.4 EJ/y or 63% of the overall consumption for this sector, when considering the top 18 large-volume chemicals shown in Figure 1. Energy generation goes hand-in-hand with greenhouse gas (GHG) emissions. In 2010, chemical and petrochemical processes emitted 1.24 Gt CO2 equiv.3 In comparison, the transportation sector emitted roughly 7.2 Gt CO2 equiv that same year.4 Figure 1 shows the GHG emissions for the top 18 © XXXX American Chemical Society
Figure 1. Global GHG emissions of top 18 large-volume chemicals, 2010.3
large-volume chemicals. These chemical precursors cumulatively account for nearly 75% of the overall GHG emissions attributed to the chemical industry (0.96 Gt CO2 equiv).3 Special Issue: Building on 25 Years of Green Chemistry and Engineering for a Sustainable Future Received: July 7, 2016 Revised: September 1, 2016
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DOI: 10.1021/acssuschemeng.6b01571 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Figure 2. Cumulative energy demand for plastics and petrochemicals.
In light of these guiding principles, a significant effort was focused on using benign solvents in chemical reactions to minimize waste and toxicity.12−14 Several types of benign alternative solvents have been proposed for chemical processing, 15−18 including supercritical carbon dioxide (scCO2),19−24 water,25−28 gas-expanded liquids (GXLs),29−31 room temperature ionic liquids (ILs),32−35 and switchable solvents.36 The decaffeination of coffee beans using scCO2 is an often cited example of how a benign solvent replaced a toxic solvent, methylene chloride, in industrial processing.37 In 1995, the EPA established the annual Presidential Green Chemistry Challenge Awards to highlight scientific advances in green chemistry. Winners of this prestigious award serve as exemplars of how this field has impacted a broad range of everyday products, including pharmaceuticals, foods, packaging, cosmetics, clothing, and electronics.38 They range from the development and demonstration of renewable feedstocks, renewable fuels and materials that are less toxic than their functional counterparts. In all, the 109 technologies recognized by this award are reported to eliminate nearly 800 million pounds of hazardous chemicals and solvents each year, save 21 billion gallons of water and eliminate 3.55 million metric tons of CO2 equivalent GHG emissions.39 While these results are significant, there is still a long way to go to shrink the sector’s footprint, such as the 1.2 billion metric tons of CO2 equivalent emitted by the entire chemical industry.1 Other reported technologies deserve mention. For example, refrigerants made from mixtures of room temperature ionic liquids and hydrofluorocarbons prevent ozone depletion in the stratosphere.40 Additionally, chlorocuprate(II)-based supported ionic liquid phases (SILPs) absorb mercury traces from natural gas.41 Despite questions about the environmental impacts of natural gas extraction and ionic liquids, the removal and safe disposal of mercury eliminate its harmful effects on humans and the environment.
Ethylene and propylene are precursors for a number of the other chemicals listed, including ethylene oxide, ethylene glycol, and propylene oxide, thus adding to the overall energy consumption for manufacturing these chemicals. Similarly, pxylene is used to make terephthalic acid (TPA), a major precursor for polyethylene terephthalate (PET) plastic. The cumulative energy demand for a variety of plastics and petrochemicals is shown in Figure 2. It compares the relative contributions of process and transport energy (denoted as “total fuels”) as well as the energy value of materials used as feedstocks (denoted as “fossil feedstocks”), commonly referred to as “upstream” energy.5 The variations in these two contributions show how critical it is to consider both the feedstock energy and other energy inputs−neither can be ignored. Materials shown with a large fraction of red (total fuels) present a greater opportunity to use new energy sources or processes. The expansion of the global commodity chemicals industry presents a significant opportunity to promote sustainability; in other words, enable our current generation to meet its needs without compromising the ability of future generations to meet theirs.6 Resource-efficient technologies are urgently needed to save energy, conserve raw materials, and minimize harm to the environment and human health. While technologies exist for crude oil and coal, new or reoptimized conversion technologies are needed to produce petrochemical equivalents from biomass and shale gas. Indeed, there have been major efforts to develop catalytic technologies for making renewable chemical precursors from biomass.7,8 Green chemistry/engineering principles, life cycle assessment (LCA), and economic analysis play a central role in developing more sustainable alternatives as illustrated below with three examples: hydroformylated olefins, terephthalic acid, and ethylene oxide. Consistent with this scope, no attempt has been made to present an exhaustive review of the field of green chemistry/engineering or LCA methodology.
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QUANTITATIVE SUSTAINABILITY ASSESSMENT The sustainability of a chemical process or product is dictated by both economic and ecological considerations. While technoeconomic modeling is fairly well established in the chemical industry,42 it is more challenging to reliably predict environmental impacts, although many advances are being made in this area as noted here. Today, LCA is the most commonly used tool for quantifying ecological burdens associated with making a chemical product, its subsequent use, and disposal, including recycling.43−45 Such an analysis is often referred to as cradle-to-grave LCA.
TWENTY-FIVE YEARS OF ACHIEVEMENTS IN GREEN CHEMISTRY AND GREEN ENGINEERING For nearly two decades, the qualitative principles of green chemistry9,10 and green engineering11 have guided research to develop sustainable chemical pathways and processes. They suggest, for example, that renewable materials should be considered as feedstocks when feasible. Nonhazardous reagents should be used as reaction and separation media. And, processes should be inherently safe, intensified at mild conditions to conserve energy. B
DOI: 10.1021/acssuschemeng.6b01571 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Figure 3. Stackable metrics for the PVC supply chain.46
power generation and supply, etc.58,59 The purchase costs of a certain product, process or service associated with the appropriate sector are used as the starting points for the EIOLCA model. Numerous examples of how the foregoing metrics and tools have been applied to promote sustainability in the corporate sector may be found in the literature.60−67 Recently, the American Chemical Society’s Green Chemistry Institute reported a survey on how the qualitative principles of green chemistry and quantitative sustainability metrics are used in chemical manufacturing as well as future needs to translate promising green chemistry ideas into industrial technologies.68 In addition, major chemical and product manufacturers such as Dow,69 BASF,70 DuPont,71 and Procter & Gamble72 have used metrics and tools developed within their organizations to identify hotspots and dramatically reduce the environmental footprints of their existing products and processes. The extent of these reductions may be seen in the annual sustainability reports posted at the referenced company websites. While such advances are clear examples of the industry’s commitment to sustainability, opportunities still exist to further reduce the overall energy intensity and associated environmental footprints of the growing industry. The development of novel catalysts and catalytic processes is essential to reduce the mass and energy intensities. In the following section, we discuss the application of LCA to guide the rational development of such technologies.
Important indicators of sustainability include material intensity, energy intensity, water consumption, toxicity, and pollutant emissions.46 Appropriate metrics are used to quantify these indicators in the various phases of a chemical product’s life cycle. An important characteristic of the metrics is that they can be stacked or combined to estimate the environmental impact per pound of product over the series of processes that comprise a supply chain. Figure 3 illustrates how metrics for ethylene, chlorine, vinyl chloride, and PVC can be stacked to obtain metrics during the production phase of the PVC life cycle, beginning with naphtha and brine. Thus, by organizing the input/output flows in the various phases under the impact categories, LCA helps to identify the dominant impacts in the life cycle of a product. Clearly, identifying the environmental hotspots to be mitigated provides valuable guidance for sustainable process and product development. During the early stages of process or product development, there are several ways to assess environmental impacts when detailed information to perform a reliable LCA is not available. For example, impacts can be described qualitatively based on the principles of green chemistry10 and green engineering.11 Other quantitative metrics can be useful as well, such as atom economy,47 process mass intensity (PMI, a measure of the total mass usage in a process relative to the mass of the product),48,49 environmental factor50 (the so-called E factor, a measure of waste generated relative to desired product formed), carbon utilization efficiency,51 emission rates, and the like.52 As more data become available, however, detailed LCAs must be employed to quantify impacts along the supply chain, including those stemming from energy usage. Multiple software tools are available for quantifying environmental impact metrics. For example, the free tool iSUSTAIN quantifies progress in the application of the principles of green chemistry.53 The EPA has developed online tools that can be used to assess the environmental fate, bioaccumulation, and toxicity of chemicals, using structure−activity relationships and compilations of available property measurements.54 In addition, new computational methods for property estimation, relevant to sustainability assessments, are being continually developed by the EPA. LCA software tools such as SimaPro,55 GaBi,56 and GREET57 are available to benchmark novel process concepts with current supply chains. The environmental input/output life cycle assessment (EIO-LCA) is a complementary tool that estimates the material and energy requirements and associated environmental impacts over the entire supply chain in various economic sectors. For example, the goods and services of the U.S. economy are divided into distinct aggregate sectors such as petrochemical manufacturing, industrial gas manufacturing,
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LCA GUIDED DEVELOPMENT OF CATALYTIC PROCESSES LCA results combined with economic analysis are powerful tools for researchers. They define catalyst performance targets, solvent choice, and operating conditions that reduce adverse environmental impacts while maintaining economic viability. Such metrics may then be used to rationally guide the development of sustainable catalytic process concepts, as is illustrated schematically in Figure 4. Methods already exist for the optimal synthesis of process flowsheet configurations and task integration, synthesis of reactor and separation systems, mass and heat exchange networks, and process water networks.73 These techniques rely on novel ways to represent conceptual design and powerful large-scale techniques to optimize superstructures of the various process systems. Multiscale process models74,75 that integrate high-level design decisions on process configurations with detailed simulations can be made more sophisticated by also factoring in environmental sustainability considerations. C
DOI: 10.1021/acssuschemeng.6b01571 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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dehyde (4-CBA), a troublesome impurity (see Figure 5). To remove the 4-CBA from the crude TPA, a subsequent
Figure 5. Sequential reaction network for pX oxidation to TPA.
Figure 4. LCA as a tool to set performance targets for sustainable catalytic processes.
hydrogenation stage is needed.81 This step requires rather harsh reaction conditions (275−290 °C and 70−90 bar) and uses carbon-supported palladium catalyst. The hydrogenation unit accounts for nearly 50% of the capital cost and 15% of the operating costs. Further, nearly 5% of the acetic acid entering the reactor is burned to CO2. To address these sustainability challenges, a spray process concept was reported for TPA.82 The reaction mixture is dispersed as fine droplets into a continuous vapor phase containing stoichiometric excess of O2, CO2, and saturated acetic acid vapor. The result is enhanced gas/liquid interfacial mass transfer. This allows O2 saturation of the liquid phase and promotes more complete oxidation to TPA. Consequently, high-purity TPA (
