I’m sometimes asked about chemical engineering by high-schoolers with some science aptitude. Typically they are trying to decide between a major in chemistry or chemical engineering. They’ve typically figured out that chemical engineering must be some practical version of chemistry, but can’t quite figure out how that could be engineering. My key answer here is: unit operations.

If I were a chemist trying to make an interesting product, beer or whisky say, I might start with sugar, barley, water and yeast, plus perhaps some hops and tablets of nutrients and antimicrobial. After a few hours of work, I’d have 5 gallons of beer fermenting, and after a month I’d have beer that I could either drink or batch-distill into whisky. If I ran the cost numbers, I’d find that my supplies cost as much to make as buying the product in a store; the value of my time was thus zero and would not be any higher if I were to scale up production: I’m a chemist.

The key to making my time more valuable is unit operations. I need to scale up production and use less costly materials. Corn costs less than sugar but has to be enzyme processed into a form that can be fermented. Essentially, I have to cook a large batch of corn at the right temperatures (near boiling) and then add enzymes from the beer or from sprouted corn and then hold the temperature for an hour or more. Sounds simple, but requires good heat control, good heating, and good mixing, otherwise the enzymes will die or won’t work or the corn will burn and stick to the bottom of the pot. These are all unit operations; you’ll learn more about them in chemical engineering.

Reactor design is a classical unit operation. Do I react in large batches, or in a continuous fermentor. How do I hold on to the catalyst (enzymes); what is the contact time; these are the issues of reactor engineering, and while different catalysts and reactions have different properties and rates, the analysis is more-or-less the same.

Another issue is solid-liquid separation, in this case filtration of the dregs. When made in small batches, the bottoms of the beer barrel, the dregs, were let to settle and then washed down the sink. At larger scales, settling will take too long and will still leave a beer that is cloudy. Further, the dregs are too valuable to waste. At larger scales, you’ll want to filter the beer and will want to do something to the residue. Centrifugal filtration is typically used and the residue is typically dried and sold as animal feed. Centrifugal filtration is another unit operation.

Distillation is another classical unit operation. An important part here is avoiding hangover-producing higher alcohols and nasty tasting, “fusel oils.” There are tricks here that are more-or-less worth doing depending on the product you want. Typically, you start with a simple processes and equipment and keep tweaking them until the product and costs are want you want. At the end, typically, the process equipment looks more like a refinery than like a kitchen: chemical engineering equipment is fairly different from the small batch equipment that was used as the chemist.

The same approach to making things and scaling them up also applied in management situations, by the way, and many of my chemical engineering friends have become managers.