In an attempt to engage the engineering community more broadly, Paul Anastas teamed up with Julie Zimmerman and published the Twelve Principles of Green Engineering, shown in the accompanying box. As is readily apparent, there are some principles, as shown in Table 2.2, that are in part related to the 12 principles of green chemistry. It is interesting to see these repetitions between the principles of green chemistry and the principles of green engineering. One very apparent shortcoming of these two lists is that in a way they seem to have been published as if they were independent, but in reality the principles should not be considered separately. When designing products and processes, the chemistry should be designed with the real-life process in mind, which is beginning to be known asdesign for manufacturability. Green engineering should be able to feed the chemistry back to designers and provide ideas of what is feasible and the trade-offs between safety and toxicity. In addition, the 12 principles of green engineering take into account several additional concepts that are worth a moment’s consideration.
THE TWELVE PRINCIPLES OF GREEN ENGINEERING
1. Designers need to strive to ensure that all material and energy inputs and outputs are as inherently nonhazardous as possible.
2. It is better to prevent waste than to treat or clean up waste after it is formed.
3. Separation and purification operations should be designed to minimize energy consumption and materials use.
4. Products, processes, and systems should be designed to maximize mass, energy, space, and time efficiency.
5. Products, processes, and systems should be “output pulled” rather than “input pushed” through the use of energy and materials.
design choices on recycle, reuse, or beneficial disposition.
7. Targeted durability, not immortality, should be a design goal.
8. Design for unnecessary capacity or capability (e.g., “one size fits all”) solutions should be considered a design flaw.
9. Material diversity in multicomponent products should be minimized to promote disassembly and value retention.
10. Design of products, processes, and systems must include integration and intercon- nectivity with available energy and materials flows.
11. Products, processes, and systems should be designed for performance in a com- mercial “afterlife.”
12. Material and energy inputs should be renewable rather than depleting.
Source:Adapted from ref. 10.
2.3.1 Thermodynamics: Limits and Potential for Innovation See, for example, green engineering principle 6.
Although principle 6 is seen primarily as an engineering principle, it is unfortunate that it is not also a part of the green chemistry principles. It is quite clear that a good understanding of thermodynamics is required even of synthetic chemists if they are to be successful in green chemistry. An example would be designing a reaction to take advantage of phase differences for separations (gravity separation) rather than on distillations (energy inten- sive), or favoring a bias toward homogeneous reactions when heterogeneous reactions could work with better or different reactor design, or order of addition. Moreover, it is also clear that except for a few instances (e.g., Heusemann11), very few people in green chemistry and engineering consider thermodynamic limits to sustainable chemistry and engineering.
There are limits to what is possible under existing practice and state of the art.
Principle 6 is very useful in drawing attention to the tendency on the part of some people to assume that recycling/reuse is a preferred option in most cases. In actual fact, this is not always the case, at least with current chemistries and technologies. This principle reminds us to broaden our boundaries and look at the entire system or life cycle of a product to ensure that proposed processes and products achieve an appropriate consideration of thermodynamic opportunities and limits. In some instances, recycling to a certain point in the supply chain might be more effective than recycling the raw materials. From a chemist’s perspective, recent reconsideration of standard protection–deprotection schemes for making naturally occurring marine products resulted in the exploitation of cascade reactions to take advantage of thermodynamically favored reaction sequences.12
TABLE 2.2 Broad Themes in Green Chemistry and Green Engineering Principles
Principles
Chemistry and Technology
Innovation
Mass and Energy Efficiency
Toxicity and Persistence
Renewability of Feedstocks Green chemistry 2, 4, 5, 8, 9–12 1, 2, 5, 6, 8, 9 3, 4, 10 7, 10
Green engineering 3, 11 2–5, 10 1, 7 12
2.3.2 Complexity
See, for example, green engineering principles 6 and 9.
The degree of complexity embedded in some products and the processes used to make them is nothing less than astounding. There are few industries that embody this better than the semiconductor industry, where Moore’s law13has driven innovation ever closer to the fundamental limits of physics and the chemicals used (Si, Ge, In, etc.). Very large scale integration has worked to reduce the size and number of parts of modern electronics while increasing their capability dramatically. In this instance, complexity is generally considered to be a good thing in that electronics are doing more, with less embedded mass and energy to make them and use them. For example, Table 2.3 shows the mass intensity, packaging mass intensity, and idle energy consumption of an iMac.
In addition, in response to product-take-back legislation, many leaders in the industry are beginning to think about how they might simplify the overall design of a product (e.g., a photocopying machine) so that it might be disassembled easily and many of the parts either reused or easily recycled. In this instance, assembly, disassembly, and end-of-life con- siderations must be accounted for in the up-front design of a product if the overall complexity of the product is to be reduced.
2.3.3 Use, Reuse, and End-of-Life Considerations See, for example, general engineering principles 8, 9, and 11.
No industry better embodies a living example of principle 8 than the pharmaceutical industry. In this industry there is a considerable degree of structural and chemical complexity in the molecules that ultimately become products. However, although it is true that there is tremendous complexity in discovering and delivering drug candidates to the market, it is also true that most of the chemistries used to make these molecules, and the processes employed to synthesize and then formulate them into products, would be recognizable to anyone living 100 to 150 or more years ago.
Because of the phenomenal attrition rate of most drug candidates (i.e., 1 in 10,00015 candidates makes it to market), there is a tendency for manufacturing processes to be designed and implemented in a multipurpose batch chemical operation. Invariably volume estimates for drugs are very poor and either under- or overestimated, leading to “making do”
with existing equipment until additional capacity can be brought online. Then, because of short patent lives following compound registration, there is little appetite for optimizing processes that will be lost to manufacturers of generic drugs. The consequence of this is that the pharmaceutical industry has some of the worse mass and energy efficiencies of any industry.
TABLE 2.3 Mass and Energy Intensity of an iMac Year
Product Mass (kg/computer)
Packaging Mass (kg/computer)
Idle Power Consumption (W)
1998 18.3 3.7 93
2002 10.5 3.6 70
2006 7.0 1.9 55
Source:ref. 14.
Solution Applying life cycle techniques to electronics design can help engineers create features that enable the recovery of materials for reuse or recycling. Disassembly features allow for the quick sorting and removal of components and materials for servicing. For example, according to Sun Microelectronics,16some of the strategies that Sun incorporates in product design to enable ease of disassembly, reuse, and recycle are:
. Product upgrades are planned intentionally to prevent the premature retirement of materials.
. Many components, such as boards, memory, and disk drives, can be added or replaced by the latest technology improvements.
. Once recovered, these components can be refurbished and sold as re-marketed equipment, or can be disassembled to separate valuable components for reuse elsewhere.
. Instead of using permanent methods such as ultrasonic welding or spray coatings to unite components, engineers can design shields with the minimum number of heatstakes (bonding points), or they can snap-fit materials so that metal shields and plastic housings are easy to separate and recycle.
. Embedded ISO 11469 identification codes for plastic type on plastic parts increase the chances of reuse and make it easier to sort materials that are in demand.
. Thin-walled plastic design conserves the amount of material needed while maintaining strength requirements and yields extra environmental benefits by reducing the amount of fuel needed to transport new, lighter products.
. Nonpainted plastics make recycling and recovery easy.
Other computer manufacturers (e.g., Apple, Hewlett-Packard, Dell) have similar schemes and strategies that include end-of-life considerations.
Additional Points to Ponder Computers are complex machines and some substances might be released during the recycling process. For example, nickel–cadmium batteries, used previously for backup power, can release cadmium at the end of the useful life of the battery and as a result were phased out. When this is the case, substitution for these types of substances should be investigated.