From an economical viewpoint this parameter [CJ's note: productivity (i.e. kilograms/hour-meter3] is usually predominant, but not much in the mind of the chemist who is more focused on a maximal chemical yield. The chemist may ignore the fact that for processes with a higher contribution of the variable costs (raw materials) compared to the fixed costs (labour, equipment, etc.) it is better to optimise on yield, whereas for processes with a lower contribution of the variable costs compared to the fixed costs to the total costs the plant residence time is a crucial factor to optimise and not the yield.
As an example, the condensation of glyoxylic acid and phenol in water gives p-hydroxymandelic acid in 75% yield after several hours, while a 55% chemical yield is obtained already after a period of 2 hours. Thus, the productivity of the process can be considerably higher, while the chemical yield has still not reached its highest value... the chemical yield is after a half-life usually 50%, after two half-lives 75%, after three half-lives 87.5% and after four half-lives 93.8%. So, the gain in yield after 4 half-lives is a poor 19% compared to 75% after two half-lives, whereas the residence time in the plant is twice as long!It seems to me that, for the manufacturing-oriented chemist, it's much easier to focus on understanding the economics of the variable costs (amount of solvent, equivalents of reagent, choice of raw materials) than it is to understand the fixed costs of a chemical plant (labor, utilities, etc.) I've also found that the "cost to run the plant per day" may not be a number that management is interested in divulging, and it may be inaccurately calculated or not really well-understood. Ultimately, though, you have to keep both numbers in mind.