The color of the methanol in the first tank used for 7-ACA methanol recovery turns yellow
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Description of the phenomenon: In the production of the cephalosporin intermediate 7-ACA, methanol is used for extraction, and this methanol needs to be recovered through a distillation tower. The methanol after extraction is yellow in color; however, after being processed in the distillation tower, the sample taken from the reflux tank is clear and transparent. Yet, once the sample is transferred from the reflux tank to the tank where testing is to be performed, it turns yellow. There is only one plate heat exchanger between the reflux tank and the testing tank, and it is only the sample taken from the first tank immediately after starting the process that turns yellow; subsequent samples remain normal in color. Alternatively, the sample taken from the testing tank after stopping the process for a few hours also turns yellow. All this methanol is reused and repeatedly distilled. What is the reason for this?
Personal opinion: 1. Rust formation inside the plate heat exchanger; 2. After long or short periods of shutdown, the concentration of fusel oils in the distillate product is too high, along with compounds such as isopentanol, butanol, propanol, heptanol, etc. – the exact composition is unknown – and the liquid appears yellow.