Chemical engineers in the Netherlands have sparked a heated debate over the necessity of stirring in chemical reactions, following a study suggesting that mixing might not be essential. The team argues that stirring remains a critical factor for reproducibility, selectivity, and scalability, especially in heterogeneous or industrially relevant systems. This controversy has ignited discussions among chemists, with some questioning the study's findings and others defending the importance of stirring. The debate centers around the potential risks of overlooking stirring as a critical experimental parameter, which could lead to hazardous situations and the loss of important findings.
The debate was ignited by a study from Nanjing University of Chinese Medicine, which analyzed 329 classic organic reactions and concluded that stirring had no significant impact on the outcomes. This claim quickly spread through the community and social media, causing a stir among chemical engineers. Timothy Noël, a researcher at the University of Amsterdam, expressed surprise at the rapid dissemination of this idea, emphasizing that it is only partially true under very specific conditions. He warns that scaling up reactions, introducing multiple phases, or dealing with fast or exothermic reactions necessitates active mixing to prevent local hotspots and potential hazards.
In response to the study, Noël and his colleagues published a preprint outlining the fundamental mechanisms of diffusion and convection in chemical species transport and the consequences of poor mixing. They explained how localized concentration gradients, or 'hotspots', can promote side reactions or unwanted precipitation, posing risks in various scenarios. Noël provided a practical example of how an exothermic reaction, when scaled up, can generate excessive heat in the reactor's center, surpassing the capacity of diffusion or passive cooling. Without active mixing, this could lead to runaway reactions and, in extreme cases, explosions.
However, the study's author, Zhong-Quan Liu, stands by his findings, arguing that stirring has minimal impact on the efficiency of most organic chemical reactions. Liu believes that people prefer factual insights over theoretical calculations with assumptions, lacking experimental data. Yet, Noël counters that the study's generalization was misleading, as stirring is often taken for granted in small-scale, homogeneous reactions. He acknowledges that stirring might not be necessary for small-scale, slow reactions but emphasizes the importance of considering the broader context and scale-up challenges.
Alex Speed, a specialist in reaction discovery at Dalhousie University, agrees that stirring is critical for large-scale or highly exothermic reactions. He highlights the risks of screening new reactions without stirring, especially for faster, photochemical, or electrochemical reactions with solid-liquid or gas-liquid interfaces. Speed underscores the importance of stirring in process chemistry, where engineers worry about potential hazards due to poor mixing.
Noël further emphasizes the disconnect between engineering principles and synthetic chemists' daily practices. He suggests that the engineering literature's established knowledge of diffusion, convection, and heat transfer doesn't always translate into chemists' routine work. Differential equations, he notes, are not a common tool for most chemists, but understanding engineering principles is crucial for scale-up considerations.
The heated debate underscores a broader shift in the chemical community, where chemists are increasingly focused on reproducibility, safety, and robustness. Noël believes that revisiting fundamental concepts is essential and long overdue, as it contributes to a more robust and safe chemical industry.
