Green Engineering Principle #4
Products, processes and systems should be designed to maximize mass, energy, space, and time efficiency.
Contributed by Dr. Michael A. Gonzalez, Senior Chemist, US Environmental Protection Agency, Office of Research and Development, Cincinnati, Ohio
As one reads the twelve principles of Green Engineering, there is one message that stands out and becomes ever increasingly more evident with each and every principle. And, that message is simplicity! It is simplicity that will allow us, as a society, to become more sustainable.
Although I am academically educated and trained as an Inorganic (Catalyst) Chemist, I have had the fantastic opportunity to interact and collaborate with Chemical Engineers during my entire tenure at the US EPA. These daily interactions have been extremely educational to myself and have focused my chemical research to think and design for development and application “beyond the bench”.
So what is meant by “beyond the bench”? Organic chemists are taught to focus on developing chemical reaction schemes that arrive at the desired structure, with all necessary functional groups in the correct positions. But, this has largely been done so with only the correct final molecular structure as the goal. Not, with any consideration regarding the implications associated with the complexity of the reaction, and material, energy and production requirements that will be needed to take this chemical reaction to the next larger scale. This is where “beyond the bench” thinking is prudent and required. As chemists, we can utilize our chemical knowledge, as molecular architects, to influence and accelerate the development of sustainable chemical design, synthesis and production. By making simple changes and decisions on a chemical synthesis route at the bench scale, not only are green and efficient reactions able to be designed. But, these changes have the potential for significant beneficial impacts when taken to the next higher scale of production.
Green Engineering Principle #4 focuses on maximizing efficiency. This is achieved by informing scientists and engineers to create designs that maximize efficiency in multiple areas such as mass, energy, space (i.e. real estate) and time. This is a simple and logical path that should be taken and the benefits gained can be quite significant. However, rather than focusing on these areas individually, by integrating these areas the benefits gained can be further increased. This is due to the high interdependence of one area on another.
In the area of mass efficiency, it is evident that all reactions should be designed to utilize as much of the reactants as possible. Reactions should be designed to catalytic or have stoichiometries as close to what is required for the reaction. As well as have high conversions and be selective to the desired products, with minimal by-product formation.
In the area of energy efficiency, it is desirable to stay as near to room temperature and pressure as possible. The need to heat and cool over large ranges requires substantial quantities of energy and can also be quite inefficient. Especially, if the chemical synthesis route requires a number of heating and cooling cycles. With this in mind chemists need to be cognizant of the subsequent steps in a synthesis sequence and design the route to utilize the heating (or cooling) that has already been committed to the reaction in a preceding step. Additionally, if you can minimize the mass of materials being moved within a chemical process, you are also contributing to an energy savings. This is the result of not needing to pump, stir or temperature control a larger than necessary mass of materials. Designing reactions that produce product streams that are as pure as possible can also experience the same energy gains. Thus, reducing the need for separation steps and recycle loops and the energy that drives these operations.
In the area of space, or in this context real estate, it is prudent to design reactions (and their subsequent processes) to be as small as possible. Not only does this contribute to a smaller physical footprint, it can also lead to a reduced environmental footprint. By having reaction volumes that are smaller, the heating and cooling load demands become reduced. The increased need for expensive materials to construct larger reaction vessels or processing equipment is reduced. And, by processing smaller volumes there is an increase in the safety of operations being performed. You can also imagine benefits in reduced risk of operations, economic gains, and reduced insurance costs to name a few.
And finally, in the area of time. I remember back to my graduate schools days and still see a sign that hung in our laboratories. It read “Time is Money”. As a young adult I had an idea what it meant. As a chemist, it took me a few years to really understand what it truly meant. In this context of time efficiency, the longer a chemical reaction takes for completion, or at least what you determined was the point of completion, the more money that was being consumed on materials, energy and operations. In addition, the longer the reaction runs, it is consuming valuable reactor real estate preventing other reactions from being performed. Which in turn is costing you additional money. When we describe time efficiency, we are identifying opportunities to perform chemical reactions as quickly as possible, while still being as efficient as possible.
Hopefully, I have demonstrated how interdependent these areas of efficiency are with one another. As you put all these concepts together, the opportunities for increasing the sustainability of a chemical reaction or process is increased. And, at the heart of these opportunities lies chemists and chemical engineers, like you and me, who can make smart and simple changes at the onset which can lead to tremendous gains and benefits at the process level. This truly demonstrates that thinking holistically can lead to something beautiful.
So remember, think and design for “beyond the bench”.