The conservation of mass and energy introduced in Chapter 3 plays a critical role throughout thermodynamics and really engineering itself. Energy in all of its forms, both potential and kinetic, is conserved, at least here on Earth as we aren’t sure what happens at the scale of the universe. Mass is likewise conserved, ignoring nuclear reactions. [Note: It’s mass-energy that conserved when all is said and done as quantified by Einstein’s E = mc2 discovery. But we needn’t go into this discussion right now.]
The mass and energy balance, trademark of engineers, captures each of these conservation laws in the following famous equation:
Accumulation = In – Out
Assumption of steady-state operation, for which accumulation equals zero, leads to
In = Out (steady-state)
The mass and energy balance underpins nearly everything we do as engineers and scientists—whether in the lab, the pilot unit, or a full-scale operating facility. When the balance doesn’t close, we know something’s off: a calculation error, a leak, or perhaps a missing piece of the puzzle. We trust this principle so completely that when the energy balance didn’t hold in beta decay, it led Wolfgang Pauli, in 1930, to hypothesize the existence of the neutrino. The neutrino was eventually discovered in 1956.
In this video, created as part of the lead-up to our 2016 Centennial Celebration of MIT’s School of Chemical Engineering Practice, Carl Bozzuto, now with Electric Utility Consultants, shares his thoughts on the famed mass and energy balance—a foundational concept introduced early in every undergraduate engineering curriculum.
Instead of diving into a philosophical discussion of this topic, I decided to bring it to life with real-world examples. I’m constantly collecting stories from the field that show how engineers use—or depend on—mass and energy balances in their work. Do you have an example from the lab, a pilot plant, an operating facility, or even a modeling project? Any situation at all! I’d love to hear it. Share your story (via email) in whatever length or style you like, and I’ll add it to the collection below. Humor is welcome!
Stories From The Field – Mass and Energy Balance (click link for the stories)
Jim Kempner – Nothing says “mass balance problem” like finding your missing plastic as a new appendage on the extruder.
Markus Preissinger – When Italy beat Germany, my colleague called it the perfect energy balance—happiness conserved, just redistributed.
Thomas Degnan – Years of chasing cleaner mass balances ended when Gordon showed us the results and said, “No fit needed—these are the data.”
Thomas Degnan (Part II) – After visiting big pharma and being told to hire an army of statisticians, we found our data were so precise we didn’t need even the one we had—he retired.
Bill Banholzer – Closing the mass and energy balance separates what’s truly practical from what merely sounds possible.
Bill Borghard – Don’t assume an infinite supply!
Mark Szpila – Sometimes the real value isn’t in completing the model or closing the balance—it’s in the process of trying, because that’s when you truly learn how the system works and how to make it better.
Michael Todd – As someone from the slide-rule generation, I can attest to how painfully tedious it was to solve large mass balances with recycle streams.
Richard Wilcox – Consider all the “ins” and “outs” when doing a mass balance.
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