Green Chemistry Principle #11: Real-Time Analysis for Pollution Prevention

By Alex Waked, Co-chair for the GCI

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.

In Video #11, Rachel and I discuss the importance of continuously monitoring chemical processes in real-time.

Most of us have driven a car before. Picture yourself driving down the highway in a car that doesn’t have any windows or rearview mirrors. I’d imagine it would be hard to not get into some sort of accident. Now add all the windows and the mirrors. It’d probably be safer to drive now, right?

So what does this have to do with chemistry, or with green chemistry principle #11 in particular? Windows and rearview mirrors provide the driver with means to monitor their surroundings in real time and allows them to react and adjust. This is exactly the idea behind principle #11 – the design of analytical methodologies to monitor chemical reactions in real time and allow for adjustments. We can think of the windows and rearview mirrors as examples of such “analytical methodologies”.

Figure 1. An NMR Spectrometer (left) and a TLC place under UV light (right) [1, 2].

As chemists, we conduct several experiments every day. Depending on the type of chemistry, the goal of these experiments can be to synthesize a novel target compound, design newer chemical processes, or simply study the properties and reactivity of a compound of interest. In a lot of these cases, it is necessary to use various analytical techniques to monitor the reaction. In the case of the simplest chemical reaction, reactants A and B react together to form a product C. How do we know when the reaction is complete? Typically, we can use techniques such as NMR or TLC (Figure 1) to see how far along the reaction has proceeded.

In many industrial settings, it’s crucial to have suitable analytical methods to monitor reactions in real-time. The scale of the reactions performed at these plants are big enough such that issues that we typically consider being only minor ones at the research lab scale can become very problematic.

An example of such a case is an exothermic reaction, in which energy is released as heat. At bench scale (grams), one can use a simple ice bath to cool down an exothermic reaction. And even if the solution’s temperature does end up rising, this usually doesn’t pose a great risk due to the small scale of the reaction.

If we now look at a similar exothermic reaction at an increased scale (kilograms), even a small increase in the solution’s temperature poses a much greater problem. The reaction rate increases at higher temperatures, further increasing the temperature as the reaction proceeds, and hence a rapid increase in the reaction rate. This is called a thermal runaway. At this point it’s nearly impossible to stop the cycle and can result in an explosion. One of the most notable examples is the Texas City disaster in 1947,³ in which a cargo ship containing more than 2000 tons of ammonium nitrate detonated, initiating a chain-reaction of additional fires and explosions in other nearby ships, killing more than 400 people (Figure 2).

Figure 2. Aerial view of the Texas City disaster [4].

Suffice to say, there is currently a huge emphasis in industrial settings to monitor and control large-scale processes in real-time.⁴ Changes in temperature are monitored by internal thermometers, changes in pressure can be monitored by barometers, and changes in pH can be monitored by pH meters. With the help of these analytical tools, it’s easy to verify if a reaction’s conditions exceed the safe limits, and subsequently halt the process before anything gets out of hand.

 

References:

(1) http://researchservices.pitt.edu/facilities/nmr-spectroscopy-lab

(2) https://www.youtube.com/watch?v=HZzA9M0H40U

(3) “Texas City explosion of 1947”, Encyclopædia Britannica. April 9, 2018. Accessed May 2, 2018. <https://www.britannica.com/event/Texas-City-explosion-of-1947&gt;

(4) https://sputniknews.com/in_depth/201509011026442762/

(5) “Green Chemistry Principle #11: Real-time analysis for Pollution Prevention”, American Chemical Society. Accessed May 2, 2018. <https://www.acs.org/content/acs/en/greenchemistry/what-is-green-chemistry/principles/green-chemistry-principle–11.html&gt;