Green Engineering Principle #2
Prevention Instead of Treatment
It is better to prevent waste than to treat or clean up waste after it is formed.
Contributed by Dr. Martin Abraham, Dean, College of Science, Technology, Engineering, and Mathematics, Youngstown State University
One of the central tenets of all green technologies is to make only the amount that is needed for the process at hand. From a business perspective, this makes absolute sense. If you purchase a quantity of a chemical, bit only use half of what was purchased, you will need to properly dispose of the remainder. You pay for the initial purchase and then pay the disposal costs, paying twice for something you didn’t need in the first place. A related concept can be found in health care, in which we suggest vaccination to prevent disease. Better to get your flu shot in late fall than to suffer the agony of having the flu in the winter.
One can easily extend this to the concept of preventing waste in chemical reactions. For a chemical reaction, the important question to ask is not only how much of the desired product can be formed, but also, how much of the undesired product is formed (and how much will need to be thrown away). We can characterize this parameter as the selectivity, or the ratio of one product relative to another. In a general sense, this can be written as
An alternate definition provides selectivity as the ratio of the amount of the desired product relative to the total conversion
I often use the reaction of ethylene to produce ethylene oxide as an example. This is a standard oxidation reaction performed in the vapor phase over a silver catalyst. Since this is an oxidation reaction, the desired ethylene oxide product can also be converted to CO2. In this process, we pay for the ethylene, we pay for the energy required to run the reactor, and we get no economic benefit from the CO2 that is produced. From a purely economic standpoint, the reaction is best performed in a way to maximize the selectivity to ethylene oxide, keeping the amount of CO2 formed to a minimum. CO2 is also a greenhouse gas that has been implicated in climate change, so it’s transmission to the atmosphere is clearly undesirable.
Fortunately, there are many tools available to engineers and chemists to reduce the amount of waste that is formed in a process. For a reacting system, adjusting the temperature will cause one reaction to be accelerated more so than another, and thus the selectivity to the desired produce may be increased. If selectivity to the desired product is enhanced at lower temperatures, then the reaction may not proceed sufficiently fast in order to be commercially interesting, so the addition of a catalyst may be needed. Also, it is possible that manipulating the concentration of one or more of the reactants may be effective in modifying the relative rates of the reaction. And finally, the engineer might design a process in which the reaction is run to low conversion, a separation is achieved to recover the product, and the unused reactant is recycled back to the entrance of the reactor, allowing higher overall conversion to be obtained. If one includes the cost of waste disposal into the process optimization, then manipulating the process parameters allows will the engineer to adjust the performance of the system to achieve the least costly overall solution, and the one that produces the lowest amount of waste.