We have looked at a number of plants recently where part of the problem is that some of the equipment is oversized, and some of it undersized. The essence of good process engineering is that the equipment is designed to work together across a range of flows and contaminant loadings.
I had a mismatched equipment situation like this last year caused by a decision by a large international civil engineering firm to do without a process designer. This unwise experiment had two severe effects of the effectiveness of the plant. As well as the mismatch in equipment sizes, a number of unproven technologies were selected, and the implications of their not working as advertised were not considered.
There was a secondary effect of the lack of a process engineer's input to the design: the layout was only just large enough to accommodate the specified equipment, which created great difficulties in modifying the poor design.
Why employ a process engineer? To get the right sized equipment, laid out properly to work reliably, cost effectively and safely. Doing without one in the past has proved a false economy for many of my clients.