Ringing the changes


It can be a shock for a novice process chemist to see their process running in the plant, out of their hands, for the first time. The scaled-up chemistry is now in a very industrial environment and, if not quite unrecognisable, definitely not what you remember from the lab.

In the laboratory, if you want to look at your reaction, a pair of eyes is all you need. You can, if you choose, press your face against your flask and be mere millimetres of glass (and hopefully a layer of personal protective equipment) away from your compound, swirling away under the power of a mechanical stirrer. When you first look at that same mixture in a 2000-gallon reactor, you peer through a sight glass with the familiar solvent – churning turbulently – up to 10 feet away.

If you’re lucky, the sight glass is a foot in diameter and the reactor has an internal light. If not, you make do with a four inch porthole and a torch, its beam swallowed up by the darkness below. On bad days, the agitator throws the reactor contents onto the sight glass. Unfortunately, most do not come with internal window wipers.

Inspecting reactor contents by torchlight

© David Graham Illustration

Patience, young grasshopper

Even the simplest activities take longer in the plant. Take, for instance, drawing a sample to check the reaction progress. In the laboratory, you reach into the hood with gloved hands, open the stopper and draw out a sample with a Pasteur pipette, powered by your thumb and a little rubber bulb. Or perhaps plunge a syringe needle through a rubber septum.

‘If you’re lucky, the sight glass is a foot in diameter and the reactor has an internal light. If not, you make do with a four inch porthole and a torch’
In the plant, it’s not nearly so simple. Opening the manway (the ‘mouth’ of a reactor) is undesirable, considering the safety risk of exposing a few tonnes of flammable solvent to a 20% oxygen environment. A dip tube allows you to draw up some of the reaction solution from above, pushing it out into an external container with nitrogen pressure. If there is no dip tube, you need an operator to open the bottom valve, drop a small sample of material into an intermediate chamber, then close the valve again. Both these techniques can take up to 20 minutes, which seems like an eternity to the novice process chemist.

There will, inevitably, be errors and oversights; the eponymous Murphy of Murphy’s law works double shifts in the plant, it seems. Equipment will fail, causing delays. Cracks in the reactor lining may shear off pieces of glass into your reaction. If you can identify a solvent in which the product safely dissolves, you can filter out the bothersome splinters. Otherwise, plant workers may spend a week or two carefully sifting through your product, searching for stray shards. Even though this mishap is not the fault of the process chemist, the unfortunate souls tasked with such duties will not look upon you favourably.

For better or worse

There may also be unintended consequences of your ‘positive’ changes. In one instance, I modified a procedure for quenching a reaction, which turned a soft, mushy product into a much more crystalline – and hence more easily filtered – material. In the following step, when it was resuspended, what had been a stirrable slurry became insoluble chunks sitting sullenly at the bottom of the reactor. I’m pleased and only slightly embarrassed to say that it took me an hour of puzzled thinking before I realised that the new, more crystalline material was missing a huge amount of water and solvent that was normally carried through with it!

Among the joys of running a new process in the plant (and let me be clear, it is a joy) is that, in the plant, chemistry really matters. The process is usually running at a scale that costs significant sums of money. Then you get to interact with a team of professionals (engineers, safety officers, production operators, quality control chemists) who all want your process to succeed, and who will offer quite different perspectives. Management gets involved and will occasionally saunter into the laboratory or office and ask: ‘Is your process done yet? Have you checked the quality?’

After all of these travails, it is no surprise that a process chemist can find themselves in front of a chromatograph late in the evening. You watch the peaks on the computer monitor, hoping to see the product peak rise and rise and rise. Then, you can go and sign off the batch record, telling the operator that it’s all right to move on. Walking back, you can take off your hard hat and breathe a triumphant sigh of relief. 

Chemjobber (@Chemjobber) is a process chemist and blogger based in the US


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