How do chemical engineers think?

The unique focus perspective of this discipline can be represented by an extension ladder, shown in the figure. The two uprights of this very useful tool represent the two primary physical foundations upon which all of chemical engineering rests: chemistry and transport. Here, "chemistry" refers to the rates and extents of transformation among substances; "transport" refers to the movement of mass, energy or momentum.

The rungs of the ladder represent the mathematical balance equations that connect chemistry and transport. The balance equations can be time-dependent or steady-state. Whatever their nature, however, these balance equations are rarely written in their own right; they are almost always written to optimize or control some variable within them. The rungs, therefore, also represent the use of balance equations for the optimization and control of useful commercial processes.

Chemical engineering embraces an enormous range of size scales in a fully integrated way ‒ ranging from atoms to oil tankers. The figure represents this notion by three extension segments, representing length scales corresponding to the microscopic, the bench scale (or "unit operation" in the lingo of the discipline) and the factory. At the molecular level, the balance equations might incorporate variables like temperature or pressure. At the unit operation level, the key variables might be flow rate or controller gain. At the factory level, the variables might be operating cost or overall production rate.

The ladder idea provides more than a simple picture of the conceptual structure of chemical engineering, however. The idea also illustrates an important point about the use of this structure. Consider how a house painter uses a ladder. The skilled painter moves continually up and down the rungs as circumstances dictate. When carrying materials and brushes to the third floor, the painter may climb rapidly, covering a great deal of territory. When scraping the stubborn shavings from an old window, however, the painter may need to stay on one particular rung for a long time. Good painting requires a constellation of climbing skills integrated judiciously: knowing when to climb, when to descend, when to overlap ladder segments, how to lean, how to reach. Although these skills can be described and listed, they cannot be used algorithmically. Judicious ladder use requires judgment and experience (i.e., "ladder wisdom").

In a similar way, when we want to transform chemical substances, the "ladder" of chemistry/transport, balances, and optimization offers a versatile tool. The skilled chemical engineer moves continually over the span of length scales from atomic to factory-level as circumstances dictate. When designing or optimizing an overall process flow, the chemical engineer may move rapidly up and down the span of length scales. When troubleshooting a particular unit operation, however, the chemical engineer may need to stay at that level for a long time with just a few balance equations. Good chemical engineering requires a constellation of intellectual skills integrated judiciously: knowing what kind of balance equation to write, what control volume to use, what terms to neglect, when to overlap tools from different length scales, what mathematics to use. Although these skills can be described and listed, they cannot be employed algorithmically. Judicious chemical engineering requires judgment and experience (i.e., "chemical engineering wisdom"). Thus, chemical engineering has been aptly called the "liberal arts of engineering."

The ability to think quantitatively and interactively about chemistry and transport over many length scales, with wise judgment born of experience, underpins the true value-added contribution of chemical engineering. This ability probably forms part of the reason chemical engineers continue to enjoy high entry-level salaries.