Green Chemistry and Engineering: A Practical Design Approach

Part I

Green Chemistry and Green Engineering in the Movement Toward Sustainability

Chapter 1

Green Chemistry and Engineering in the Context of Sustainability

What This Chapter Is About

Green chemistry and green engineering need to be seen as an integral part of the wider context of sustainability. In this chapter we explore green chemistry and green engineering as tools to drive sustainability from a triple-bottom-line perspective with influences on the social and economic aspects of sustainability.

Learning Objectives

At the end of this chapter, the student will be able to:

Understand the need for the development of greener chemistries and chemical processes.

Identify sustainability principles and associate standard chemical processes with the three areas of sustainability: social, economic, and environmental.

Identify green chemistry and green engineering as part of the tools used to drive sustainability through innovation.

Understand the need for an integrated approach to green chemistry and engineering.

1.1 Why Green Chemistry?

(1.1) equation

Reactant A plus reactant B gives product C. No by-products, no waste, at ambient temperature, no need for separation. Is it really that easy?

If industrial chemical reactions were that straightforward, chemists and engineers would have significantly more time on their hands and significantly less excitement and fewer long hours at work. Chemists know that this hypothetical reaction is not the case in real life, as they have less-than-perfect chemical conversions, competing reactions to avoid, hazardous materials to manage, impurities in raw materials, and the final product to reduce. Engineers know that in addition to conquering chemistry, there are by-products to separate, waste to treat, energy transfer to optimize, solvent to purify and recover, and hazardous reaction conditions to control. At the end of this first reality check, we see that our initial reaction is a much more complicated network of inputs and outputs, something that looks more like Figure 1.1.

Figure 1.1 Simplified vision of some of the challenges and realities of designing a chemical synthesis and process.

Green chemistry and green engineering are, in a very simplified way, the tools and principles that we use to ensure that our processes and chemical reactions are more efficient, safer, cleaner, and produce less waste by design. In other words, green chemistry and green engineering assist us in first thinking about and then designing synthetic routes and processes that are more similar to the hypothetical reaction depicted in equation (1.1) than to the more accurate reflection of current reality shown in, Figure 1.1.

What are the drivers in the search for greener chemistries and processes? Engineers and scientists have in their capable hands the possibility of transforming the world by modifying the materials and the processes that we use every day to manufacture the products we buy and the way we conduct business. However, innovation and progress need to be set in the context of their implications beyond the laboratory or the manufacturing plant. With the ability to effect change comes the responsibility to ensure that the new materials, processes, and designs have a minimum (or positive) overall environmental impact. In addition, common sense suggests that there is a strong business case for green chemistry and engineering: linked primarily to higher efficiencies, better utilization of resources, use of less hazardous chemicals, lower waste treatment costs, and fewer accidents.

Example 1.1

Potassium hydroxide is manufactured by electrolysis of aqueous potassium chloride brine,¹ as illustrated by the following net reaction:

How is this simple inorganic reaction different from the more complex challenges of the real world? Identify some of the green chemistry/green engineering challenges.

Solution The electrolysis reaction can be carried out in diaphragm, membrane, or mercury cell processes. The complexity of the reactions depend on the process that is used. Let's explore the mercury cell process, which has, historically, been the most commonly used method to produce chlorine.¹² In this case, potassium chloride is converted to a mercury amalgam in a mercury cell evolving chlorine gas. The depleted brine is recycled to dissolve the input KCl. The mercury amalgam passes from the mercury cell to the denuder. In the denuder, fresh water is added for the reaction and as a solvent for the KOH. Hydrogen gas is evolved from the reaction and mercury is recycled to the electrolysis cell:

Our simple net reaction has become a bit more complex, but it does not end there. We've not talked about a key input— energy. Electricity is required to drive the reaction forward; it represents the major part of the energy requirement for these types of reactions, and there is a need to optimize it. As a matter of fact, as of 2006 the chlor-alkali sector was the largest user of electricity in the chemical industry.²

But energy is not the only thing that we need to worry about. In addition to energy inputs, there is a need to eliminate impurities. To do that, the brine can be treated with potassium carbonate³ to precipitate magnesium and heavy metals, and barium carbonate is often used to precipitate sulfates.⁴ Also, hydrochloric acid needs to be added, as an acidic pH is required to drive the reaction to produce the desired chlorine gas, which can then be recovered from the solution, as shown in the following equilibrium reaction:

Besides using a large quantity of electricity, we have to worry about potential emissions from the reaction. Mercury is present in the reaction cell and the purged brine. Mercury emissions from the cell and the brine have long been a target for significant reduction. The purged brine is typically treated with sodium hydrosulfide to precipitate mercury sulfide, and the mercury-containing solid wastes need to be sent for mercury recovery. Other emission concerns include management of the environmental, health, and safety (EHS) challenges related to the gases in the reactions. Both the chlorine and hydrogen gas streams must be processed further. Chlorine is cooled and scrubbed with sulfuric acid to remove water, followed by compression and refrigeration. The hydrogen gas is cooled to remove water, impurities, and mercury, followed by further cooling or treatment with activated carbon for more complete mercury removal.⁵ In addition, hydrogen is often burned as fuel at chlor-alkali plants.

The membrane process was introduced in the 1970s and it is more energy efficient and more environmentally sustainable, which is making it the technology of choice. However, a typical mercury-based plant can contain up to 100 cells and has an economic life span of 40 to 60 years. A long phase-out is required to convert an existing mercury plant. For example, as of 2005, 48% of the European chlor-alkali capacity was mercury cell–based.²

Additional Point to Ponder Chemistries and processes described in most textbooks normally don't give you all the information you need to consider the mass and energy inputs and outputs associated with a given reaction. In reality you won't always have the data you need and will have to use estimations to generate data, run experiments, perhaps use nearest neighbor approaches and/or make assumptions based on your experience. Sometimes, you will just have to use simple common sense.

1.2 Green Chemistry, Green Engineering, and Sustainability

The modern understanding of sustainability began with the United Nations World Commission on Environment and Development's report Our Common Future,⁶ also known as the Brundtland Report. The Brundtland Commission described sustainable development as development that meets the needs of the present without compromising the ability of future generations to meet their own needs. What does this actually mean? This definition doesn't give us many clues or supply much practical guidance as to how to implement sustainable development or move toward more sustainable activities, but it does provide us with a powerful aspiration. It has been up to society collectively and up to us as individuals to develop guidance and tools that will help us to design systems and processes that have the potential to achieve the type of development described in the definition.

The first thing to remember is that sustainability or sustainable development is a complex concept with which many people are still attempting to come to terms. In 1998, John Elkington, one of the early innovators of sustainable development, coined the phrase triple bottom line.⁷ Elkington did this in an attempt to make sustainable development more understandable and palatable to business people, to encourage them to see it as a logical extension of the traditional business focus on economic performance. By using this term, Elkington was trying to highlight the need to consider the intricate nterrelationships among environmental, social, and economic aspects of human society and the world. In a way, sustainability can be seen as a very delicate balancing act among these three factors, and not always with a strong one-to-one relationship. Table 1.1 provides a summary of several approaches to sustainable development principles. It should be noted that the Carnoules statement includes an organizational principle framework, in addition to the overarching social aspects widely recognized to be an integral part of sustainability. This organizational principle is useful when relating the operational aspects of sustainability within the sphere of controls defined by company culture and policy.

Table 1.1 Summary of Several Approaches to Sustainable Development Principles

When talking about sustainability, one cannot focus on only a single aspect, as this necessarily limits and biases one's view. For a system to be sustainable, there is the need to balance, insofar as possible, social, economic, and environmental aspects, ideally having each area in the black, that is, with no single aspect optimized to the detriment of the others. One of the most puzzling, challenging, and exciting characteristics in the study of sustainability is the inherent complexity of the concept. There are synergies, trade-offs, a variety of shared values of what constitutes a sustainable practice, and so on. Figure 1.2 displays those interrelations graphically.

Figure 1.2 Spheres of action of sustainability.

Green chemistry and green engineering represent some of the many concepts, tools, and disciplines that come into play in helping to move society toward more sustainable practices. They do this by focusing scientists and engineers on how to design more environmentally friendly, more efficient, and inherently safer chemistries and manufacturing processes. However, some might suggest that when talking about green chemistry and green engineering in the context of sustainable development, we can honestly say simply that the primary focus area is what has come to be known as environmental sustainability. Is this really true? Whereas green chemistry and green engineering may be seen as being related primarily to the environmental aspects of sustainability, they also have strong ties to the eco-environmental (or eco-efficiency) sub-area of sustainability by virtue of the fact that they include resource conservation and efficiency. By the same token, green chemistry and green engineering are related to the social aspects of sustainability because they promote the design of manufacturing processes that are inherently safer, thereby ensuring that workers and residential neighborhoods close to manufacturing sites are protected.

Example 1.2

Explain how reaction (1.1) relates to the three aspects of sustainability.

Solution Several of the issues related to green chemistry and green engineering were highlighted in the solution to Example 1.1. Table 1.2 provides examples of how they relate to the three aspects of sustainability.

Table 1.2 Issues Related to Sustainability

Additional Point to Ponder Most textbook examples and problems have only one correct answer, although many examples have several possible answers. In real-world manufacturing processes, it is common to have difficulties in defining what the true problem is—and when this is defined, several not-quite-optimal answers may be found. When this happens, a decision must be made that accounts for or balances all the important factors and, hopefully, leads to the optimal or best answer.

1.3 Until Death Do Us Part: A Marriage of Disciplines

What does it mean to have an integrated perspective between green chemistry and green engineering? Just imagine the following not-so-hypothetical scenario. A chemist works at a large company and after years of hard work discovers a novel synthesis to produce a valuable material. At this point, hundreds of engineering questions are formulated and need to be addressed, such as:

What is the best design for the reactor? Which material?

Does the reaction need to be heated? Cooled? How fast are heating and cooling transferred?

What types of separation processes are needed?

How could the desired purity be achieved?

How fast is the reaction? Is there a risk of an exothermic runaway?

What can possibly go wrong? How can we prepare for problems?

Are there inherent hazards in the materials?

Are there any incompatibilities with materials?

How much waste is produced? How toxic is it? Can it be avoided?

Where should the reactants be procured? Is it more efficient to make them or to buy them?

How much would this process cost?

What types of preparations and skills would future operators need?

Imagine how difficult it would be to answer these and other questions if the chemist doesn't work closely with a chemical engineer. How efficient would the final process be? To truly understand the impacts of this novel chemistry in the real-world manufacturing environment, the chemist will need to involve engineers beginning at the earliest stages of development.

Similarly, a chemical engineer working on transforming a laboratory synthesis into a scalable, effective production process will need to collaborate closely with a chemist to understand how the chemical synthesis might be changed. A myriad of chemically related questions must be answered to design and scale-up a good manufacturing process:

What function is the solvent performing in the reaction?

Are there alternative reaction pathways that can be used to:

Avoid uncontrollable exotherms?

Substitute reactant A for B to avoid safety issues?

Eliminate hazardous reagents?

If we recirculate part or all of the reaction mother liquors, how much of material X can be tolerated by the reaction system before we are not able to do this?

Are there any reactivity issues by introducing solvent Y as a mass separating agent?

What are the potential side reactions?

Are there any alternative catalytic methods that we might be able to use?

The decisions that are made in the design of synthetic chemistry pathways affect and either enable or restrict the engineering opportunities, and vice versa. Chemists and chemical engineers should operate in an integrated fashion if the goal is to design an efficient process, in the widest sense of the term and in the context of green chemistry and engineering.

Hopefully, we have made a good case for integrating green chemistry and green engineering, but our effort to integrate disciplines is not over. Carrying on with our original scenario, the chemist and engineer have successfully identified a chemical they want to make and the synthetic route or pathway to be used to make it, and have some idea of the critical process parameters that they need to focus on if they are to optimize the process from a green chemistry and green engineering perspective. So, is anything missing? What about knowledge of how the various reactants, reagents, catalysts, solvents, by-products, and so on, used in the process affect living organisms and the environment? One might be tempted to ask who really cares about such things, since most of the materials may be consumed in the process and the product we are making is a valuable material that others need or want.

These questions are not merely rhetorical; the answers are very important for current and future generations. Human beings have and continue to affect the world in very significant ways, and it is critical that all chemists and engineers understand how material choices, process designs, energy use, and so on, affect the world. Chemists and engineers need to design and choose synthetic strategies that minimize the potential for causing short-, medium-, and long-term harm not only to humans, but to other environmental organisms as well. To do this correctly, they need to collaborate with toxicologists and environmental, health, and safety professionals to discuss and develop appropriate options for syntheses. In short, a host of disciplines are required to bring a product to market appropriately and successfully and to ensure that this is done in a sustainable fashion. It is no longer acceptable practice for chemists to isolate themselves in a laboratory and design reactions that are chemically interesting but, because it is expedient to do so, utilize reagents, reactants, and solvents that are inherently hazardous.

Problems

1.1 How do green chemistry and green engineering differ from chemistry and engineering?

1.2 Examples 1.1 and 1.2 refer to the environmental, health, and safety challenges related to mercury, chlorine, and hydrogen. What are those challenges?

1.3 The primary route for making copper iodide is by reacting potassium iodide with copper sulfate:

Identify potential green chemistry and green engineering challenges of the reaction.

1.4 From a sustainability framework, identify environmental, social, and economic impacts derived from the chemistry shown in Problem 1.3.

1.5 Using reaction system of example 1.1, provide some examples of how the chemistry can affect decisions made in engineering.

1.6 What are some potential barriers for an effective, close collaboration between a chemist and an engineer when designing a novel process. Provide some ideas on how to circumvent these obstacles.

References

1. Schultz, H., Günter Bauer, G., Schachl, E., Hagedorn, F. Schmittinger, P. Potassium compounds. In Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH, New York, 2000.

2. Chlorine Industry Report: 2005–2006. Euro Chlor, Brussels, Belgium, 2006.

3. McKetta, J. Potash, caustic. In Kirk–Othmer Encyclopedia of Chemical Technology, 2nd ed. Wiley, New York, 1970.

4. U.S. Environmental Protection Agency. Profile of the Inorganic Chemical Industry. EPA Office of Compliance Sector Notebook Project. EPA 310-R-95-004. U.S. EPA, Washington, DC, 1995.

5. European Commission, 2001. Integrated Pollution Prevention and Control (IPPC). Reference document on best available techniques in the chlor-alkali manufacturing industry. BREF 12.2001. ftp://ftp.jrc.es/pub/eippcb/doc/cak_bref_1201.pdf.

6. World Commission on Environment and, Development. Our Common Future. Oxford University Press, Oxford, UK, 1987, p. 43.

7. Elkington, J. Cannibals with Forks: The Triple Bottom Line of 21st Century Business. New Society Publishers, Gabriola Island, New Brunswick, Canada, 1998, p. 416.

8. Alcoa. 2020 Framework. http://www.alcoa.com/global/en/about_alcoa/sustainability/2020_Framework.asp.

9. International Chamber of Commerce. The Business Charter for Sustainable Development: 16 Principles. http://www.iccwbo.org/policy/environment/id1309/index.html.

10. International Council of Chemical Associations. Responsible Care Web site. http://www.responsiblecare.org/page.asp?p=6341&l=1, accessed Sept. 27, 2009.

11. Bartz, P., et al. Pignans Set of Indicators Statement: Carnoules Statement on Objectives and Indicators for Sustainable Development. Governance for Sustainable Development, Carnoules/Pignans, Provence, France, May 1–4, 2003.

12. McDonough and Partners. The Hanover Principles. McDonough and Partners, Charlottesville, VA, 1992.

13. The Natural Step Web site. http://www.naturalstep.org/, accessed Sept. 27, 2009.

14. United Nations Global Compact. http://www.unglobalcompact.org/, accessed Sept. 27, 2009.

Chapter 2

Green Chemistry and Green Engineering Principles

What This Chapter Is About

Following several decades of increased awareness of the human impact on the environment, there was a need to spur chemists and engineers on toward a deeper consideration of how they might facilitate pollution prevention beyond end-of-pipe solutions. True pollution reduction at its source would require a revised approach that emphasized new chemistries and technologies. There have been a number of notable attempts to define what it means for scientists and engineers to be green, and in this chapter we outline the major contributions to the discussion.

Learning Objectives

At the end of this chapter, the student will be able to:

Identify the principles of green chemistry and green engineering.

Understand the interrelationships between the principles of green chemistry and green engineering.

Contrast the differences between some the principles postulated by Anastas and Warner, Anastas and Zimmerman, Winterton, and the San Destin declaration.

Critique and analyze chemical reactions as related to the principles of green chemistry.

2.1 Green Chemistry Principles

What is a principle, and why do we develop principles? Merriam-Websters, Collegiate Dictionary defines a principle as 1a: a comprehensive and fundamental law, doctrine, or assumption; b(1): a rule or code of conduct; (2): habitual devotion to right principles ; c: the laws or facts of nature underlying the working of an artificial device.¹ Now that we know what a principle is, why would someone want to have principles for green chemistry and/or green engineering? To answer that question, it may be valuable to begin by providing just a bit of historical context. As the story goes, John Warner, formerly on the staff of the research and development department at the Polaroid Corporation, was working on novel chemistries related to dyes used in photographic films. John is not your usual chemist and was aware of many environmental regulations that might stand in the way of getting a new product to market (see Figure 2.1). In addition to being a great chemist, John is a very creative person, so he began to wonder how he might design novel molecules and chemical synthetic processes to make them in a way that would avoid creating and/or using toxic and/or regulated materials along the way. With this simple thought in mind, he contacted Paul Anastas, formerly a division head in the Office of Pollution Prevention and Toxics at the U.S. Environmental Protection Agency, to discuss what would now seem to be obvious to many, but at that time, was quite revolutionary: What might the average synthetic chemist do to make molecules that do not harm the environment or people? Thus began a continuing dialogue and fruitful collaboration between John and Paul that resulted in the publication of the Twelve Principles of Green Chemistry, first published in 1998.² Let's look at these principles for a moment and think about some of the broader issues and implications that they present. We should also ask ourselves whether or not they promote movement toward more sustainable behaviors and actions.

Figure 2.1 Chronological representation of environmental laws.

The Twelve Principles of Green Chemistry

1. It is better to prevent waste than to treat or clean up waste after it is formed.

2. Synthetic methods should be designed to maximize the incorporation into the final product of all materials used in the process.

3. Wherever practicable, synthetic methodologies should be designed to use and generate substances that possess little or no toxicity to human health and the environment.

4. Chemical products should be designed to preserve efficacy of function while reducing toxicity.

5. The use of auxiliary substances (e.g., solvents, separation agents) should be made unnecessary whenever possible and innocuous when used.

6. Energy requirements should be recognized for their environmental and economic impacts and should be minimized. Synthetic methods should be conducted at ambient temperature and pressure.

7. A raw material feedstock should be renewable rather than depleting whenever technically and economically practical.

8. Unnecessary derivatization (blocking group, protection–deprotection, temporary modification of physical/chemical processes) should be avoided whenever possible.

9. Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.

10. Chemical products should be designed so that at the end of their function they do not persist in the environment and break down into innocuous degradation products.

11. Analytical methodologies need to be further developed to allow for real-time in-process monitoring and control prior to the formation of hazardous substances.

12. Substances and the form of a substance used in a chemical process should be chosen so as to minimize the potential for chemical accidents, including releases, explosions, and fires.

Source: Adapted from ref. 2.

2.1.1 Chemistry and Chemical Technology Innovation

See, for example, green chemistry principles 1, 2, 4, 5, 8, and 9 through 12. It should be noted that chemistry and chemical technology innovations will be required to foster the aims of each principle.

The first broad implication is that we cannot achieve the aims of these principles if we are not constantly striving for innovation in chemistry and chemical technology innovation. Innovation is at the end of the day what has given us a wide range of materials and products that have made our lives more comfortable, and many of those products derive from chemistry and engineering innovation (Figure 2.2). There are many in the synthetic chemistry and engineering community who believe green chemistry and/or green engineering to be a soft science: that is, not a hard physical scientific or engineering discipline and not quite worthy of real academic consideration. In actual fact, success in green chemistry and engineering presents more difficult challenges and opportunities for innovation than does much of synthetic organic chemistry. It can be argued that green chemistry and green engineering are indeed smart chemistry and engineering, insofar as their practitioners attempt to design more mass- and energy-efficient processes and to avoid design concerns and problems that have plagued chemical processes for decades.

Figure 2.2 Some of the many products in use.

Think about it for a moment. How difficult is it to design a synthesis with a very reactive molecule such as an azide or acetylene, where the reaction is overwhelmingly thermodynamically and kinetically favorable, using whatever solvent you want, in as dilute a solution as you like, when you don't care how you'll separate the product from the reaction mixture? This is approximately equivalent to being proud of hitting the ocean when you throw a stone into it.

In contrast to the above, think for a moment about the green chemistry design challenge. You are being asked to make a complex chemical such as a drug or an advanced liquid crystal with the following design constraints: Use as little extra chemical material as possible, with as little energy as possible, using compounds that are nontoxic and safe to handle, that are either biodegradable or recoverable and reusable; and extract the desired product without resorting to a large amount of solvent or energy, all the while causing no long-term impacts to people and or to the environment as you do it.

Green chemistry and green technology therefore require the best and the brightest to rethink and challenge existing paradigms and push the limits of our knowledge. This require people who understand and embrace different academic disciplines within chemistry, engineering, mathematics, and interrelated sciences (e.g., toxicology, biology, biochemistry). Imagine, for example, a synthetic organic chemist who understands thermodynamics and kinetics (largely the domain of the physical chemist), but who knows enough biochemistry to use enzymatic transformations while making use of process analytical technologies to develop reaction and process understanding and control. But this is not enough. Chemists and engineers also need to understand enough about other disciplines, such as biology, toxicology, engineering, and geology, that they are able to use chemistry more knowledgeably and design products, processes, separation technologies, and manufacturing plants based on greener, safer principles. Above all, it requires intellectual flexibility to provide continual innovation and change on a rapid scale.

2.1.2 Mass and Energy Efficiency

See, for example, green chemistry principles 1, 2, 5, 6, 8, and 9.

Only fairly recently has society become more aware of its impact on the global environment. Although it is true that different societies have become more or less aware of local or regional impacts on the environment (it is, after all, somewhat difficult to ignore a burning river, deforestation in parts of the northeastern United States, a large explosion, etc.), society is only beginning to become aware that human beings are engaged in earth systems engineering on a grand scale.³ This has been spurred on, perhaps, by publication of a report by the UN International Panel on Climate Change, which has amassed sufficient and conclusive evidence for the impact on the climate of the increase in carbon dioxide (and other greenhouse gas) concentrations in Earth's atmosphere.⁴

If one thinks on a global scale and begins to ask where materials come from to make the products that society uses on a daily basis, it is not difficult to see evidence of our insatiable need for materials of commerce, such as plastics, electronics, clothing, food, and housing. Producing these materials requires increasingly complex global supply chains to meet the needs and wants of developed and developing nations. Increases in the costs of a variety of key materials, including fossil fuels for energy and petrochemical feedstocks, are a reflection of the demands being placed on supplying chemicals that are increasingly more difficult to find and transform into the desired materials. In addition, the production of materials is intimately related to emissions and resource depletion; in general, the more materials are needed to produce a good, the more resources that will be needed along the supply chain and the more emissions to the air, water, and land that will need to be controlled.

It could be argued that these trends are pushing society toward increasing material and energy efficiencies in relation to the material and energy utilized for every product produced. The consequence of low material and energy efficiencies, is, of course, the production of waste. Roger Sheldon pointed out the relative waste of different industrial sectors and coined the phrase E-factor.⁵ The E-factor is related to the mass intensity (MI) as follows:

(2.1) equation

where

As can be seen from Table 2.1, the farther one is from raw material extraction, as is true in the pharmaceutical sector, the greater the waste that is produced. The challenge for green chemistry and green engineering is to decrease significantly the material intensities observed in all industrial sectors: for example, decrease the mass intensity by at least an order of magnitude, if not more. It is interesting to note that in many cases, material and energy intensity are very highly correlated. If one thinks about this for a moment, it makes a certain intuitive sense that if I decrease the volume of material I am handling, I should use less energy to produce, use, reuse, and hopefully, dispose of it.

Table 2.1 Mass Intensity of Various Sectors of the Chemical Industry

Historically, in response to regulations, industry has focused on waste (E-factor) and its elimination, as opposed to preventing waste generation through innovations in chemistry and chemical technology (principle 1). As the U.S. Congress opined in 1986, the major obstacles to increased waste reduction are institutional and behavioral rather than technical. Although this is perhaps understandable, in many respects it is unfortunate because a focus on end-of-pipe solutions is generally costly and only increases the overall mass and energy intensity associated with the production of any product. Looking at mass and energy efficiency instead, we can shift our mindset from a treatment, end-of-pipe viewpoint, to a efficiency-increasing, revenue-generating solution.

2.1.3 Toxicity and Persistence

See, for example, green chemistry principles 3, 4, and 10.

Although a decrease in the amount of energy and materials used for our products is critical, it is important to understand that the nature of the materials we use is also critically important. Once again, there is a certain intuitive sense in this, as we might ask why society would want to use a chemical that might render us sterile or incapacitated while consigning us at some point in the future to a slow painful death by cancer or some other chronic illness (e.g., emphysema, heart disease). Most of us would, of course, say that this is probably not a good thing, yet this is exactly what is done, and generally done safely, on a daily basis. We use a large number of materials that are extremely toxic and difficult to handle because they happen to be extremely useful to us chemically. However, one might ask if this is a practice that we wish to continue if we can devise a way to avoid these inherently hazardous chemicals.

Legislation such as the Regulation, Evaluation, and Authorization of Chemicals Act (REACH)⁶ approved by the European Commission suggests that at least some societies are interested in obtaining a better understanding of the environmental, health, and safety (EHS) hazards associated with existing and new chemicals. It may be surprising to many readers that for a large number of chemicals, despite a long history of use in a range of industries, an understanding of the EHS hazards associated with many compounds is not sufficient. The long-term objective of REACH is to obtain that EHS understanding, and once this better understanding is obtained, it is likely that certain chemicals will be banned if the risk associated with their use is deemed to be too great.

In addition to legislative restriction, to operate processes safely with chemicals that are highly toxic, the appropriate controls should be in place, and the more toxic a material is, the cost to design, set, validate, and maintain the appropriate controls normally increases. Thus, eliminating, substituting, or reducing the amounts of toxic chemicals is also tied to economic engineering and the economic bottom line of processes.

Green chemistry principles 3, 4, and 10 anticipated chemicals legislation and challenge chemists to design molecules and their basic building blocks in such a way that toxicity is eliminated or reduced sufficiently to eliminate high risk. These principles are arguably among the most difficult for chemists to address, for two reasons. First, synthetic chemists generally lack any understanding of toxicity, and for the most part, the relationship of molecular structure to toxicity is not well known for many chemicals represented by the myriad of potential combinations of the usual elements (i.e., C, H, O, N, S, Cl). A second thorny issue is that the efficacy of a molecule, as in pesticides, herbicides, and drug substances, among others, is related directly to their ability to exert a toxic effect on a target organism. Indeed, it is a tall order just to find and then make a compound of interest that works as intended without adding additional design constraints related to reducing potential toxicity!

Finally, after discovering an efficacious molecule of interest with no or minimal associated toxicity hazards, it must be designed either for reuse or for biodegradation. Implicit in any consideration of biodegradation is an aspect of risk management that is often poorly understood: chemical fate. Chemical fate concerns itself with where a molecule ends up once it is released to the environment, either in air, water, or on land, and will have a different degradation pathway depending on where it is distributed to, as is shown in Figure 2.3 for a household detergent. If the compound is chemically degradable or biodegradable, the degradation by-products must themselves be nontoxic. All of this emphasis on fate and toxicity should drive anyone who wants to introduce a new chemical, to EHS hazard testing on a very large scale unless the science to model fate and environmental effects, explosivity, flammability, and so on, in silico improves dramatically. For the time being, however, EHS hazard testing is generally the only way that we can adequately assess potential risks, and this will necessarily increase the cost and complexity of bringing new products to society. Fate and effects are covered in more detail in Chapter 3.

Figure 2.3 Fate and effects of a common household detergent.

2.1.4 Renewability of Feedstocks

See, for example, green chemistry principles 7 and 10.

One of the most exciting areas to think about is how to change the way we make and use the items we need and want in such a way that all Earth's species can continue to live at as good or at a better standard of living. Although very exciting for some to think about, this is still largely simply a nice thought. In actual fact, we are living in ways that are not sustainable. Human beings are depleting raw materials at an alarming rate or are having to expend more energy and to inflict greater environmental damage to obtain many of the key minerals, raw materials, and energy that we require to maintain a Western standard of living. Stated differently, that is a high standard of living for only about one-fourth of Earth's population. What about the rest of the human world and of all species that live in what are arguably less than ideal conditions?

In a sense, principle 7 draws a line in the sand and asks chemists and engineers to find, develop, and provide the materials and energy that we need and want in ways that reverse current trends. This is a very tall order. Think about the petrochemical industry for a moment; it did not start out being a highly efficient industry, but has developed and evolved over the course of more than 100 years. Chemists have to replicate for a biologically derived supply chain what took 100 years (not counting a few hundred millions of years to form) to optimize using a completely different type of feedstock. And they must do this in a shorter time frame and without major environmental damage if we are to preserve Earth's biodiversity and ability to maintain large human and nonhuman populations.

Some surprise might be registered by seeing that principle 10 is included here, and one may think that it does not belong in a category about renewability, so some explanation is warranted. This principle is, after all, about biodegradability or persistence. Some have argued, most notably Bill McDonough and Michael Braumgartner in their book Cradle to Cradle,⁷ that society could do with having some materials considered to be technical nutrients. By technical nutrients they mean materials that are used for a certain period of time, then after a given service life can be collected and returned to their original state and formed into new products. An example they give is replacing paper with a polymeric substance that can be reused repeatedly without a considerable amount of energy or loss.

The point here is that persistence is sometimes a useful characteristic if it is possible to have a closed-loop recycling system. The problem is, of course, that it is very likely that there will never be a completely closed loop, so some of this material will end up in the environment. In that case, such materials would have to be either biodegradable or completely nontoxic to all organisms. In either case, there is still a need to develop materials that are renewable and which do not cause environmental degradation. A lot of work needs to be done.

Example 2.1

Dimethyl carbonate can be produced by the following reaction⁸:

Describe which of the green chemistry principles postulated by Anastas and Warner you could apply to this reaction to improve its greenness given the information provided.

Solution

Principle 1. It is better to prevent waste than to treat it or clean up after it is formed. In the reaction above, looking at the stoichiometry, there will be an aqueous waste stream with sodium hydroxide and sodium chloride in significant concentrations. The sodium hydroxide being formed is corrosive and will need to be neutralized and treated. Is there a way to produce the desired carbonate while avoiding the generation of this waste stream? How about separating and purifying the final product and the related waste? Is there a way to obtain a final product that is close to being pure?

Principle 2. Synthetic methods should be designed to maximize the incorporation into the final product of all materials used in the process. This is the concept of atom economy. In reactions with 100% atom economy, all the materials added to the chemistry are incorporated into the final product. Can we design an addition reaction that can produce the carbonate with no by-products?

Principle 3. Wherever practicable, synthetic methodologies should be designed to use and generate substances that possess little or no toxicity to human health and the environment. This reaction requires phosgene, a highly acute toxicant. Can we devise a different synthetic pathway that avoids the use of phosgene and doesn't replace it with another toxic material?

Principle 9. Catalytic reagents (as selective as possible) are superior to stoichiometric reagents. This reaction is stoichiometric. Is there a way that this chemical can be produced by catalytic means?

Principle 12. Substances and the form of a substance used in a chemical process should be chosen so as to minimize the potential for chemical accidents, including releases, explosions, and fires.

Additional Points to Ponder Although no information was provided on yields and conversion, mass efficiency is another factor to consider in improving the greenness of this reaction. Also, what about separation processes needed to purify the product which are not included in the information? What about energy requirements to run the reaction and to purify and separate the product?

2.2 Twelve More Green Chemistry Principles

Since the publication of the Twelve Principles of Green Chemistry, there have been a variety of publications, symposia, and conferences dedicated to increasing our understanding of green chemistry and how it might be advanced. Reflecting on many of these presentations and publications, in 2001 Neil Winterton published 12 More Principles of Green Chemistry.⁹ Critics of these additional principles have argued that it was not necessary to add to the Anastas and Warner list, as the principles outlined by Winterton can, in their minds, be subsumed within the original 12 principles. Supporters have argued that while the original 12 principles are useful, the Winterton list represents a practical, pragmatic, and industry-driven expansion of great value because the principles are not well understood or appreciated by the academic synthetic chemistry community. They are also an excellent bridge to chemical engineers, as they highlight the tight relationship between green chemistry and green engineering and how some of the most elementary principles that chemical engineers have learned and applied routinely for generations are also fundamental in designing safer, greener chemical processes (e.g., performing full mass balances, quantifying utilities, measuring losses, investigating heat and mass transfer limitations).

Twelve More Principles of Green Chemistry

1. Identify and quantify by-products.

2. Report conversions, selectivities, and productivities.

3. Establish full mass balances for a process.

4. Measure catalyst and solvent losses in aqueous effluent.

5. Investigate basic thermochemistry.

6. Anticipate heat and mass transfer limitations.

7. Consult a chemical or process engineer.

8. Consider the effect of the overall process on the choice of chemistry.

9. Help develop and apply sustainability measures.

10. Quantify and minimize the use of utilities.

11. Recognize where safety and waste minimization are incompatible.

12. Monitor, report, and minimize laboratory waste emitted.

Source: Adapted from ref. 9.

Example 2.2

How do the Anastas and Warner green chemistry principles relate to Winterton's additional green chemistry principles?

Solution To illustrate the point of how the Winterton principles might be subsumed in the Anastas and Warner principles, the accompanying box contains a combination of the two sets. These lists are complementary to one another and are useful for focusing on some important practical aspects of the principles as they apply to industry.

The Twelve Principles of Green Chemistry (Combined)

1. It is better to prevent waste than to treat or clean up waste after it is formed.

a. Consider the effect of the overall process on the choice of chemistry.

b. Recognize where safety and waste minimization are incompatible.

c. Monitor, report, and minimize laboratory waste emitted.

d. Consult a chemical or process engineer.

2. Synthetic methods should be designed to maximize the incorporation into the final product of all materials used in the process.

a. Identify and quantify by-products.

b. Report conversions, selectivities, and productivities.

c. Establish full mass balances for a process.

d. Measure catalyst and solvent losses in aqueous effluent.

e. Consult a chemical or process engineer.

f. Help develop and apply sustainability measures.

3. Wherever practicable, synthetic methodologies should be designed to use and generate substances that possess little or no toxicity to human health and the environment.

a. Help develop and apply sustainability measures.

b. Consult a chemical or process engineer.

4. Chemical products should be designed to preserve efficacy of function while reducing toxicity.

5. The use of auxiliary substances (e.g., solvents, separation agents) should be made unnecessary whenever possible and, innocuous when used.

6. Energy requirements should be recognized for their environmental and economic impacts and should be minimized. Synthetic methods should be conducted at ambient temperature and pressure.

a. Investigate basic thermochemistry.

b. Quantify and minimize use of utilities.

c. Consult a chemical or process engineer.

7. A raw material feedstock should be renewable rather than depleting whenever technically and economically practical.

a. Help develop and apply sustainability measures.

b. Consult a chemical or process engineer.

8. Unnecessary derivatization (blocking group, protection/deprotection, temporary modification of physical/chemical processes) should be avoided whenever possible.

9. Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.

10. Chemical products should be designed so that at the end of their function they do not persist in the environment and break down into innocuous degradation products.

a. Help develop and apply sustainability measures.

11. Analytical methodologies need to be developed further to allow for real-time in-process monitoring and control prior to the formation of hazardous substances.

a. Consult a chemical or engineer.

12. Substances and the form of a substance used in a chemical process should be chosen so as to minimize the potential for chemical accidents, including releases, explosions, and fires.

a. Investigate basic thermochemistry.

b. Recognize where safety and waste minimization are incompatible.

c. Consult a chemical or process engineer.

2.3 Twelve Principles of Green Engineering

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 as design 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.

Table 2.2 Broad Themes in Green Chemistry and Green Engineering Principles

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.

6. Embedded entropy and complexity must be viewed as an investment when making 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 interconnectivity with available energy and materials flows.

11. Products, processes, and systems should be designed for performance in a commercial 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 intensive), 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

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 law¹³ has 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.

Table 2.3 Mass and Energy Intensity 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 considerations 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.

Example 2.3

How can an electronic product can be designed for ease of disassembly?

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,¹⁶ some 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.

2.4 The San Destin Declaration: Principles of Green Engineering

In the spring of 2003, a very heterogeneous mix of chemists and engineers from industry, government and academia met in San Destin, Florida to discuss principles of green engineering. This group was intending to appeal to a slightly larger engineering audience beyond that generally associated with the chemical industry, in addition to potentially broadening the scope of previous work to incorporate principles of sustainability. The output is shown in the accompanying box.

The San Destin Declaration: Principles of Green Engineering

1. Engineer processes and products holistically, use systems analysis, and integrate environmental impact assessment tools.

2. Conserve and improve natural ecosystems while protecting human health and well-being.

3. Use life cycle thinking in all engineering activities.

4. Ensure that all material and energy inputs and outputs are as inherently safe and benign as possible.

5. Minimize depletion of natural resources.

6. Strive to prevent waste.

7. Develop and apply engineering solutions while being cognizant of local geography, aspirations, and cultures.

8. Create engineering solutions beyond current or dominant technologies; improve, innovate and invent (technologies) to achieve sustainability.

9. Actively engage communities and stakeholders in development of engineering solutions.

Source: Adapted from ref. 17.

As with the previous green chemistry and green engineering principles, several of the San Destin principles are similar to previous approaches; for example, principles 6 and 8, which fits nicely with the theme of chemical and chemical technology innovation. There was also an element of pragmatism, or perhaps pessimism, as in principle 5, which asserts that natural resource depletion should be minimized, the implicit assumption being that we will never attain a situation where society will not deplete natural resources and achieve a cradle-to-cradle sustainable society.

Apart from these three principles, which were discussed previously, there are several concepts that are brought out in this declaration that are worth a moment's consideration.

2.4.1 Systems and Life Cycle Thinking

See, for example, principles 1 and 3.

In general, most human beings are reductionist thinkers; that is, we cut things down into small bits that are easily grasped or understood so that we are not overwhelmed by the considerable complexity that attends most things in our world. However, in our attempts to reduce complexity, we are sometimes guilty of not seeing the bigger picture, or optimizing one small corner of our universe to the detriment of a broader part or another aspect of the system. This is where systems and life cycle thinking come into play. Certainly, biologists and environmentalists are more attuned than most chemists and engineers to systems thinking, given their attempts to understand entire ecosystems: for example, the interplay of microorganisms, plants, invertebrates, vertebrates (animals), and humans across space and time. It is this ability to look at the big picture and discern the key interactions, responses, and impacts that is so important in green chemistry and green engineering.

One of the disciplines that helps us to better understand some of the interactions and impacts from chemicals and the processes used to make them is life cycle inventory and assessment (LCI/A). Although LCI/A is covered in more detail in Chapter 16, it is worth just a moment's explanation. LCI/A is a rigorous analytical methodology developed to evaluate the environmental impacts of a product or activity, starting with the product or functional unit (e.g., a car, a single dose of a drug, a can of paint, a service) and works back through all the unit operations and materials that are used to make the product, all the way back to raw material extraction. A person does this by performing an input/output inventory or an accounting of all the mass and energy used for each unit operation or production process. It also includes a look at the product in use and its ultimate disposition (i.e., what happens to it when it no longer performs the function originally intended.) Such an exercise generally forces one to look across entire systems because most products are not simple extractions of raw materials followed by immediate use with no emissions. LCI/A broadens our perspective as we try to understand the material and energy flows and their impacts.

The tie-in of these principles to green chemistry and green engineering is hopefully apparent. If one considers or optimizes only one reaction or one part of a process, it is easy to miss the rest of the process, or perhaps the use of a very nasty chemical in another part of the supply chain. By looking across the entire supply chain, one can optimize material use so that only the best materials, the appropriate chemistries, and the best processes