Wednesday, July 20, 2011

Process Wednesday: Avoiding the 5 o'clock dump

Kilomentor, on planning ahead (and planning on planning ahead):
If a synthetic lab procedure is so long that the reaction and work-up cannot be completed in a single day, chemists can use their experience to extrapolate from similar procedures and guess at what points manipulations can be stopped and under what conditions intermediate solutions or crude solids can be stored without damage. Occasionally there are misjudgments and surprises and a product will be prepared in lower than expected yield or poorer purity. but then even in the worst situation what is lost is no more than a couple of man-days of labor and the price of the starting materials consumed. Also, in the laboratory because the capacities of refrigerators, freezers and evaporators are so much greater than the quantities of material being transformed, there are do-able fixes for the situation where a stoppage is forced at almost any stage. 
There is no room for such risk taking on-scale. For advanced intermediates that are themselves the product of a series of sequential steps, one misstep can be economically disabling. The more points in the process that have been verified as safe to stop, by actual test results, the more confidently the process team can be. Moreover, to be a safe stopping point it must be proven safe not just for the quantity and quality of the product but also for the protection of the processing equipment.
Even in graduate school (where hours are strangely flexible), there's a point at which you need to know when you can leave your reaction stirring overnight. Planning ahead for that point is smart; as Kilomentor suggests, it's a good idea to take a relatively small amount of material and see what happens if it sits for longer than planned.

It's also smart to plan ahead for the end of the day. Starting a 4-hour slow addition at 3 pm on a really crucial intermediate? Well, I hope you have dinner packed, my friend.


  1. "Plant time" vs lab time scales are often dramatically different which does take newbie process chemists a while to learn how to adjust factors accordingly. The concept of "hold times" can seem strange to grad students who use standard settings like 4-8h, overnight, 24h/day and 72h/weekend(rare of course) for experiments. Certainly that's when experienced chemists, engineers and pilot/plant operators can aid education to how things run in a particular facility. Although many claim to operate 24/6 or 7 I don't recall many chem/pharma facilities that are willing to routinely conduct substantial activities during 3rd shift so that where knowing when/length of unit operations critical.

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