The natural state of the extraction solvent was clear and slightly yellow. With time, however, oxygen would creep into the unit, especially via the vacuum regenerator. Organic acids would form. The acids would eat any carbon steel that they encountered. Iron oxide and carbonaceous materials would accumulate in the solvent, which would turn from yellow to green to brown and then black. Eventually the solvent would look and feel like the three-year-old engine oil in my 1974 Ford Falcon.
One troubleshooting visit was in response to a complaint about reduced recovery. The unit was shut down for the annual turnaround. My colleague, Reese, and I decided to enter and inspect the extractor at three manhole locations, starting with the top manhole. The top manhole was open. According to our drawings, the top tray was only about 3 ft below the top manhole. We inspected the top tray with our flashlights. It looked fine -- no fouling. Then Reese entered the column feet first, on his belly. He almost fell 20 feet.
Unfortunately, Reese and I failed to expect the unexpected. The top tray was there -- but it wasn't. It was paper thin. It crumbled to dust instantly under his weight. Fortunately, he was wearing a harness with a lanyard. It turned out that the top ten trays were similarly thin. Replacement trays were provided to the plant on an emergency basis.One of the things that I'm encountering as a relatively new process chemist in a manufacturing environment is how much focus there is on the materials of construction of equipment. Most of the time in the lab, your compounds are only going to see one material: glass. But in the plant, glass is relatively rare, it seems, and concerns about the compatibility of whatever metal you might be using with your reaction solvent, reagents and byproducts becomes very important.