By David Constable, Director, ACS Green Chemistry Institute®
Back in October, I wrote about systems thinking: what it is and why we need to think about it more in chemistry. This month, I’d like to talk about a related idea, and that is life cycle thinking. Most people, when they hear “life cycle,” think about a product they know and perhaps how long that product has been on the market. Or, they think about all the activities to bring a product to market, and how long it takes to do that. One obvious example of this is how people think about an automobile. Large auto companies (Ford, GM, Volkswagen, Toyota, etc.) have been around for the better part of 100 years, they have multiple brands (Lincoln, Cadillac, Buick, etc.), and multiple cars that are part of those brands. For any line of cars, there is a design stage, a pre-production stage, a production stage, market entry, regular major redesign stages, etc., and sometimes, a line brand and line of cars is halted (e.g., GM – Saturn, multiple car lines). We can then talk about the life cycle of a car either from the development of a line of cars to the retirement of that line of cars, or we can think about the life cycle of the car from the perspective of the time I bought the car, sold the car, someone else buys it, etc. until it ends up as scrap.
The above example illustrates two important points. First, this is how most business people think about life cycle. Second, a fundamental idea in life cycle thinking and systems thinking is the idea of scope and “boundary conditions.” In other words, I can talk about a line of cars from the broader perspective of when a company first conceives and develops a brand, a line within that brand and the retirement of the brand from the market. Or, I can talk about how I bought my Prius in 2010, and I’ll keep it until it dies, and I have to buy a new one. In either case, stating my boundary conditions is extremely important in order to understand the context of a business case, say the return on investment (ROI) of a car. A life cycle ROI will obviously mean something different to a car manufacturer as opposed to an individual buyer.
An important tool that is used to better understand systems or systems-level impacts is life cycle inventory and assessment (LCI and LCA). This kind of life cycle is different than the business person’s idea of life cycle but let’s return to the example of an automobile. An automobile is a very complex assembly of smaller parts that we use to transport ourselves from point A to point B. Each of those assemblies can be broken into parts and individual parts can be broken down into their component pieces, and those component pieces can be further delineated into individual materials. So, the body of the car has bumpers, a hood, doors, etc. A door will have a window, interior panels, some electronics and mechanicals, multiple coatings, padding, etc. Each of these individual components can be broken down until you get to the smallest, indivisible part, and each part has a supply chain associated with it that extends backwards to raw material extraction (e.g., an ore, or petroleum, etc.), and extends forward to when that part is no longer usable and it is recycled or disposed of in some fashion.
What the Life Cycle Inventory Assessment does is take an inventory of the inputs and outputs of each manufacturing process for each part as it moves through its series of manufacturing steps from the extraction of the ore (raw material) to where it finally ends up in the environment. Those inputs (fossil fuels (mass and energy), ores, oxygen, etc.) and outputs (products, by-products, emissions, etc.) are generally grouped into impact categories like ozone depleting substances, greenhouse gas equivalents, etc. and added up across the entire life cycle. For a product like a car, you can probably imagine that to do a full life cycle inventory, you have thousands of parts for which detailed input and output information is collected. To say the least, it’s pretty complex, and I haven’t begun to talk about the assessment phase; i.e., once I have all those impact categories, what does that mean for human and environmental health and safety?
But chemists don’t make cars, so what does life cycle have to do with chemistry? Well, chemists make chemicals and materials, many of which do go into making an automobile, and each of those chemicals and materials have a life cycle. Let’s take another example closer to what a chemist may encounter and say we make a chemical for a crop protection agent like a pesticide. Apart from looking at how the pesticide might affect the target pest, e.g., a leaf borer through a mechanism related to one part of its growth phase and therefore non-toxic to other organisms, making that pesticide, applying it, and thinking about how it degrades in the environment are all part of its life cycle considerations. In making that compound, you are likely going to have a synthetic route characterized by a set of complex intermediates, and a process that employs a variety of reagents, catalysts, solvents, etc. to make each intermediate and final product, after which there is a formulation step where the pesticide is mixed with other solvents, and chemicals so it may be applied. There’s also going to be a lot of energy associated with the production of the pesticide, and energy use in its application. How much of the pesticide is manufactured is directly related to how much needs to be applied per acre of crop and how many times during a season it needs to be applied. The efficacy of the pesticide, characterized in terms of mg pesticide/acre is known as the functional unit; a very important concept in life cycle inventory/assessment because impacts are normalized to the functional unit; i.e., X number of CO2 equivalents/functional unit.
So, I hope you can see that from a systems perspective, a chemist has a great many opportunities to change the life cycle impact profile and the overall systems effects a chemical might have. In the case of the pesticide, for example, finding a chemical that is exquisitely selective for the intended target such that it does not adversely impact any other living organism, and that doesn’t require a large amount of chemical/acre to protect the crop, will have a huge positive system benefit. Another crop protection strategy might be to employ different approaches like integrated pest management or genetically modifying the plant to have resistance against the pest, and these should also be considered. The chemist also has control of the synthetic path to the final active ingredient through decisions about which framework molecule to start with and how to make and break bonds to functionalize that framework molecule, adding the bits and pieces that take you to the final molecular structure, most likely a molecule with multiple chiral centers and therefore synthetically challenging. And the process chemists and engineers have considerable control over the process that is used to make each intermediate and the active ingredient. Each of these choices has a potential system cost and an overall system benefit. As in all of life, the choices we make matter.
As I’ve said before, the notion that green chemistry and engineering is not good science or that it is not challenging is a frankly astoundingly ridiculous assertion. It is, in fact, amazingly complex, and that complexity is frequently daunting. That’s why systems thinking and life cycle skills are so important for chemists to be aware of and incorporate in their work. I think that chemists are masters of solving complex problems, and I have every confidence they can solve the many challenges faced in delivering a more sustainable world. It’s just going to require them to think about chemistry differently than they currently do, and they are going to need to develop creative, innovative solutions to address the complex problems. The challenges are immense, but if they weren’t, where would the fun be?
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