Alternative multiphase reactors for fine chemicals: A world beyond stirred tanks?

Traditionally the intermediate scale and fine chemicals industries have relied on stirred tank reactors, operated continuously or more commonly batchwise, as the work horse of their manufacturing plant to the almost total exclusion of any other type of liquid reactor. The switch to continuous processing has generally been resisted on the basis that this compromises plant flexibility and thus, reduces capital productivity. While the stirred tank and the associated slurry catalyst are of course reliable and flexible to the demands of multi-product plant, it is not always easy to scale up the chemistry from the laboratory and variability in product arises from imperfections in the mixing. This problem is exacerbated for multiphase reactions where the non-uniformity in mixing and mass transfer can lead to significant variance in reaction rate and selectivity, resulting in loss of product quality and productivity. The problem is that while the stirred tank is convenient, it is not in fact a particularly effective mixing device. While historically the stirred tank was one of few options, times are changing. There are many reactor designs now available for multiphase contacting and reaction. Some of these are a proprietary improvement on the simple stirred pot while others are significantly different, such as trickle beds, bubble columns and the use eductors and loop reactors for intense gas/liquid mixing. Interest is increasing into applying process and reactor intensification techniques to batch production through use of small flow channels and structured catalysts, of which monoliths are the best known example. This is however, but one example of work on structured catalysts and reactors that is expected to have an impact on the intermediate scale and fine chemicals manufacturing in the near future.