TABLE 52 Twelve Principles of Green Chemistry

1. Prevent waste

2. Maximize the integration of all process materials into the finished product

3. Use and generate substances with little or no toxicity

4. Design chemical products with less toxicity while preserving the desired functions

5. Minimize auxiliary substances (e.g., solvents, separation agents)

6. Minimize energy inputs

7. Prefer renewable feedstocks over nonrenewable ones

8. Avoid unnecessary derivations and minimize synthesis steps

9. Prefer selective catalytic reagents over stoichiometric reagents

10. Design products for post-use decomposition and no persistence in the environment

11. Use in-process monitoring and control to prevent formation of hazardous substances

12. Use inherently safer chemistry that minimizes the potential for accidents

Source: Anastas and Warner (1998).

designing for sustainability

Hierarchical design methodology informs much of the efforts in sustainable design of chemical processes. Proposed by Douglas (1988), the methodology recognizes a hierarchy of decisions in the design of chemical processes. One begins with a simple, conceptual design. successive layers of detail are then added to the process flowsheet with each design iteration. The hierarchical method has been adapted for the purposes of waste minimization and pollution prevention by various authors (e.g., Alva-Argaez et al., 2001; Chen et al., 2003).

Chemistry generally constitutes the first step in hierarchical process design. This relates to process synthesis, specifically the selection of reaction routes and separation agents (Chen et al., 2003). Anastas and Allen (2002) proposed a set of strategies based on the Green Chemistry principles discussed above, which include:

• Identifying synthesis pathways with superior environmental performance;

• Selecting alternative feedstocks that are benign and leads to higher selectivity and yield; and

• Selecting solvents that are less hazardous, safe, with less environmental impacts.

The strategies also seek to come up with chemistries that are inherently safe. The considerations of alternative feedstocks and catalysts as well as new technologies such as biotechnology are necessary in identifying the superior synthesis route. Safety and environmental performance of the reaction byproducts are also important.

The EPA Presidential Green Chemistry Challenge also provides some illustrations on how Green Chemistry contributes to sustainability of chemical processes. The 2004 awards recognized, among others, the development of an alternative synthesis pathway for Taxol®, the anticancer drug. Previous technologies require pacli-taxel, the active ingredient in Taxol®, to be isolated from the barks of the Pacific yew trees - plants that take 200 years to mature and are part of sensitive ecosystems - or produced semisynthetically from the twigs and leaves of European yew. Scientists and engineers at Bristol-Myers Squibb succeeded in developing an alternative pathway through the latest plant cell fermentation technology. This biotechnology alternative significantly simplifies the synthesis route, with no chemical transformation required beyond fermentation, and thus reducing the use of solvents and energy. Not only does the alternative improve environmental performance, it is also good business, ensuring yearlong harvests and a sustainable supply of paclitaxel.

Once the chemistry is known, one must then select the technologies and equipments for the various unit operations that make up the chemical process. Guidelines and heuristics have been developed to improve the environmental performance from this design step. Many have been summarized by Allen and Rosselot (1997) and more recently by Shonnard (2002). Choices of catalyst technology (e.g. homogeneous versus heterogeneous catalysts) and reactor technology (e.g., fixed bed versus fluid bed) can have a tremendous effect on efficiency and environmental performance. In designing the reactor, one may further optimize the reaction conditions, mixing conditions, and reactant concentration to maximize conversion, yield, and selectivity toward the intended product. The choice of separation technology is also important, as separations typically constitute the most energy- and resource-intensive unit operations in a chemical process. While distillation remains the most widely used separation technology in the chemical industry today based on volume of materials separated, numerous alternative separation technologies have evolved over the past century (Fig. 5.3). New separation technologies, such as dilute solution separation and simulated moving bed chromatography, tend to be more efficient, benign, but costly. However, these technologies are increasingly used when very high value-added materials are involved, such as in the separation of individual enantiomers of pharmaceuticals.

Another important step in improving environmental performance in chemical process design is process integration, which is "a holistic approach to process design and operation that emphasizes the unity of the process" (Dunn and El-Halwagi, 2003). Design of "heat exchanger networks" (HENs) is perhaps the most familiar example of process integration. To optimize energy use, heating and cooling requirements of the process are analyzed systematically using a method called "pinch analysis" (e.g., Shenoy, 1995). The term "pinch" refers to a key system temperature constraint that thermodynamically limits heat recovery and thermal energy efficiency. Pinch analysis enables the matching of cold streams (i.e., streams that need heating) with hot streams (i.e., streams that need cooling) in the most effective way, hence minimizing the need for additional process heating and cooling. Similar approaches have been developed to optimize other aspects of a chemical process, such as the "mass exchange networks" (MENs) that optimize

100 Years

First Use

Filtration

Distillation

Decantation

Solution Crystallization Centrifugation Azeotropic Distillation

Froth Flotation Extraction

Melt Crystallization Adsorption: Gas

Ion Exchange Adsorption: Liquid Membranes: Gas

Leaching

Membranes: Liquid

Liquid Membranes

Affinity Separations Dilute Solutions

Simulated Moving Bed Chromatography

Invention

Technical Sophistication (Time)

^^^ Patent Activity Wanes Figure 5.3. Maturity of separation processes (adapted from George E Keller II).

the design of end-of-pipe separation and recycle structure and the water conservation networks that optimize water recycle and reuse (Dunn and El-Halwagi, 2003).

Lifecycle perspective is also increasingly used in chemical process design, especially in evaluating processes involving different feedstocks or auxiliary materials, or significantly different quantities of the same materials. Even when one focuses only within the boundaries of the chemical process, various impact assessment categories in LCIA are frequently used to assess its environmental performance. Computational tools such as the Waste Reduction algorithm (WAR) (Young and Cabezas, 1999) and Environmental Fate and Risk Assessment Tool (EFRAT) (Shonnard and Hiew, 2000) automate the impact assessment process. Midpoint analysis (described in Section 5.3.2) is used in these automated tools, where impacts are categorized in terms of human and ecosystem toxicity as well as pollutant impacts such as global warming potential, acidification, and water eutro-phication. Integration of these tools into chemical process simulators such as Chem-Cad (ChemStations, Inc., Houston, TX), Aspen Plus and HYSIS (both of Aspen Tech, Cambridge, MA) significantly simplifies the consideration of environmental impacts in chemical process design.

Figure 5.4 illustrates the use of some of the above considerations in the production of maleic anhydride from n-hexane. The process comprises a reactor and a separation system that includes an organic solvent absorber and a series of distillation columns (Lagace, 1995; Chong, 1995). Energy use shown for the chemical processes in Figure 5.4 includes the feedstock energy consumed by the process (i.e., the difference between the heating values of the product and the feedstock, DHc). Using pinch analysis, an optimum heat exchange network (HEN) can be constructed, which significantly reduces the energy consumption by the chemical process. Energy consumption is further reduced when a different reactor technology is employed (fluid bed instead of fixed bed). This, obviously, represents a fundamental redesign of the process. Despite the lower energy consumption, the fluid-bed

Figure 5.4. Comparison of energy use among alternatives for the production of maleic anhydride from n-butane. (Source: BRIDGES, 2001.)

technology consumes more of the «-butane feedstock compared to the fixed-bed technology. This calls for the integration of lifecycle thinking: the consideration of energy use required to provide the «-butane. As shown in Figure 5.4, the fluid-bed process indeed requires more energy in the upstream processes to produce «-butane. However, the lifecycle consideration also shows that the fluid-bed technology still consumes less energy overall. Other impacts can also be calculated for the three alternatives (BRIDGES, 2001).

5.1.3.3 Facility Design. Industries' quest for greater sustainability has fueled the growth of industrial ecology, which builds on the notion that nature "can serve as a useful metaphor for industrial systems, which can be used to help industry become more efficient and more sustainable" (Allen and Butner, 2002). In nature, there exists an optimal network of nutrient exchanges as the waste from one organism becomes food to others. With nature as a source for inspiration, a similar network of material and energy flows may be created where the waste from one process serves as feedstock or energy source for other processes, hence increasing resource efficiency and reducing releases to the environment.

An industrial network in the small Danish town of Kalundborg is an example of the efficient integration of material and energy flows. For more than 30 years, this multi-industry "eco-park" has been swapping material and energy between processes (Garner and Keoleian, 1995; Kaiser, 1999; Allen and Butner, 2002). For instance, steam and waste heat from power generation are transferred for industrial and municipal heating such that up to 90 percent of the heat from the coal-burning power plant can be utilized (Allen and Butner, 2002), as opposed to the industry average of approximately 34 percent efficiency. Further, gypsum from the power plant is utilized by a drywall factory, a biotech's fermentation waste is shipped to farmers for fertilizing fields, and cooling water from a refinery is used as boiler water by the power plant (Kaiser, 1999).

These examples may not be directly applicable in the U.S. settings. Legal issues related to the transfer of liability often hamper such efforts. However, it illustrates that, ideally, a well-designed industrial network would allow not only very high material and energy efficiency, but also minimal environmental costs as only limited waste is generated. This concept of industrial ecology also underlies the "cradle-to-cradle" design strategy discussed in Section 5.2.

5.1.3.4 Further Considerations. Principles and strategies that have been developed to date for more sustainable design have largely focused on reducing the environmental impacts of the industrial products and processes. However, design of truly sustainable products and processes also requires the consideration of societal impacts beyond the impacts created through resource use and environmental degradation. This has been recognized by many engineers and scientists, including those that gathered at the 2003 Green Engineering conference (Nguyen and Abraham, 2003). Described by Dr. Martin Abraham in Section 5.3, the conference participants produced a preliminary set of principles of green engineering that recognizes the needs to "develop and apply engineering solutions, while being cognizant of local geography, aspirations, and cultures" and to "actively engage communities and stakeholders in development of engineering solutions." Such considerations will ensure the design of chemical products and processes to optimally meet society's needs.

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