The last 20 % of the reaction

It’s common for a kinetics study of a reaction to focus on the first 5 % to say, 20 %, of reaction completion. Usually the study is done at high dilution as well. There are good reasons for this. Ideal solutions are best approximated at high dilution and interferences are not nearly so pronounced. The basic science behind the interaction between reactants can be teased out from the early course of the reaction.

For those of us in the chemical synthesis business the imperative is somewhat different. Our concern relates to the extent to which the reactants go to completion.  In commercial synthesis the desired outcome is to maximize the space yield of a process in the available equipment.  That means that work goes into determining the maximum concentration of reactants and getting the highest yield in the shortest time. The material state in the reactor near the end of a commercial run is quite far away from the conditions one would choose for a kinetic study.

Getting to reaction completion is sometimes rather difficult and may involve whiling away plant hours for the reaction yield to get just a bit closer to the asymptote.  The problem is that the remaining 5, 10, or 15 % reaction completion may consume considerable plant time and bring opportunity costs. Near the end of the reaction the reactants all trend to infinite dilution, so of course the reaction slows down.

Often reaction completion is not simply about getting higher yield. Purification may be greatly complicated by a reaction mass that contains remaining reactant. If chromatography is not an option then one is left with the usual methods of bulk purification. As we all know, some materials crystallize poorly out of a messy solution. This is where the plant chemist has to cancel all appointments and grind through the workup scheme.

I would say that semi-batch reaction completion problems can be a serious matter for a process chemist or engineer.  This is especially true with new processes but older processes can surprise you. My advice is to throw resources at it early. There is a tendency to get the run behind you and move on.  It’s best to work out a detailed analytical profile of the reaction mixture and strive to understand what the components are and what causes their appearance or disappearance. Sometimes changing the stoichiometry helps. Getting to completion and finding a clean work up is where the plant chemist really earns his pay.

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About gaussling

Gaussling is a senior scientist in the chemical business. He occasionally breaks glassware and has been known to generate new forms of hazmats. Gaussling also digs aerospace, geology, and community theatre. View all posts by gaussling

2 responses to “The last 20 % of the reaction

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