In catalyst development literature it is often stated that the particular catalyst under study can be recovered for re-use with full or nearly full activity. I have heard this proclamation at meetings and in conversation as well. Having spent a bit of my adult life analyzing process economics, I would like to comment on this matter.
The world of chemical processing can be coarsely divided into two regimes- continuous and batch processing. Since my hands-on continuous processing experience amounts to less than a year of time, I’ll limit my comments to batch processing.
In this post I’ll define catalyst recycling as an operation wherein a catalytically active substance is recovered from a process stream and made available for another run. There are a great many catalysts and a great many applications, so generalizations are hazardous. Nonetheless, there are a few generalizations to be made.
For a batch liquid-phase process performed in a multipurpose reactor, there are operations that are common to all processes. Charging the reactor with raw materials, heating or cooling, agitation, reflux/distillation, discharging the contents, and cleaning. All of these operations consume resources and plant time. Generally speaking, any change that reduces consumption without harming the product could be considered a process improvement.
For catalyst recycling to qualify as a process improvement, some kind of consumption would have to be reduced over the useful lifetime of the material: i.e., reduction of time and/or materials. Obviously, reuse of a catalyst holds the potential to reduce the consumption expense of the catalyst over the course of the campaign.
Before we draw any conclusions, it is useful to review the requirements put upon any material that might be used in a process. In bulk processing, raw materials are obtained from suppliers who have the necessary experience to provide the material. But of equal importance, the vendor must have the necessary quality control mechanisms in place to warrant that the delivered product meets the promised specifications.
For instance, if you use butyllithium, you must be assured that all of the raw materials going into the process- reagents, solvents, etc.- meet a low water specification. You have to know that the aryl bromide you are using isn’t contaminated with HBr or a polybrominated side product. There has to be assurance that all raw materials going into the pot meet some minimum purity. A chemical processing company must know how to manage change.
Bulk processing is all about stability and predictability. You can’t rely simply on having ordered the proper grade of raw material. You need a certificate of analysis showing that the composition of the lot meets your in-house spec. When a vendor issues a cert, they are warranting the purity and accepting some risk as a result of sending bad product.
Management of change is a business methodology compelling an organization to adopt a standard procedure for the evaluation and approval of chemical process changes. For instance, just because the chemists say that a change should be made to a scaled-up process doesn’t mean that it has to happen tomorrow if ever. The proposed change has to go through a protocol that exposes it to safety and economic scrutiny. Frankly, it also spreads the potential blame for mishaps and economic disasters, so others have motivation to evaluate the process from a fresh view and sign-off.
The re-use of a catalyst brings forth the possibility that the activity of the catalyst could be altered in some way from one run to the next. There could be a downward trend in activity or some kind of variability. This means that a reused catalyst charged into the reactor could be a different catalyst from one run to the next. Potentially, what you saved in catalyst costs you might lose in extra plant hours or lower yield due to degraded performance or from outright process upsets.
Naturally, any kind of catalyst recycle has to be researched and understood by the R&D group and by the cost accountants. Catalyst recycling will involve an operation to retrieve the material from the product or raffinate streams and to prepare it for the next run. Stable activity will have to be demonstrated, preferably under the influence of a variety of off-normal conditions.
Someone- a chemist or engineer- will have to sit down and do the calculations to see if there is a net benefit to the re-use of the catalyst against the backdrop of diminished performance, variability, or added operation costs.
The point is that catalyst recycling isn’t automatically desirable. A recycling scheme that requires many labor hours to purify the catalyst may sour the benefit of the action. Another issue that may arise is the matter of validation of the re-used catalyst. The company will have to decide if or when activity validation is necessary. For a pot full of expensive precursor, a wink and a grin from the analysts may not be enough. A qualification run at the bench may be needed.
Here are my favorite catalyst attributes for batch processing- 1) high turnover number, 2) selective, 3) cheap enough to use once and send to waste disposal, 4) not a PGM (Platinum Group Metal)- PGM’s are subject to large market price variations, and 5) doesn’t contain one of the bad actors that trigger EPA thuggery or public protests- Hg, Cd, Cr(VI), etc. Metals are forever.
Catalyst recycle makes no sense, of course, in a one-time process run. A wise operator will calculate a price to cover the catalyst cost. But it may make sense if a plant is to start an extended run of batches, or if the catalyst is rare or expensive. Sometimes recycle has merit. The point is that a sober cost calculation should be made prior to the implementation of recycle schemes.
At the beginning of the article I stated that some generalizations were possible. I will modify that in saying that PGM’s in the catalyst may necessitate the recovery, though not necessarily the re-use, of the metal for return credit to the supplier.
Lamentations on Chemistry 