Lochte’s spinning band

Distillation is typically a slow and static affair. As Primo Levi put it in his Periodic Table: ‘I love distillation because it is slow philosophical work … that gives you time to think, a bit like riding a bicycle.’ If the connection seems tenuous, in the 1930s there was a convergence between the seeming stillness of lab distillation and the whirling frenzy of a moving bike.

At the start of the 20th century, the growth of the oil refining industry created demand for methods to deal with the mind-bogglingly complex mixtures emerging from oil fields. Contemporaneously, Francis Aston’s discovery of isotopes presented the challenge of separating chemical species that barely differed from one another.

While empirical designs for static fractionators were reported in their dozens, theory lagged behind experiment. Ernest Sorel (Chemistry World, May 2023, p62) was the first to have taken seriously the idea of an equilibrium between material in the vapour and in the condensed phase. He introduced the idea of the theoretical plate, which led him to an effective, but very tedious, mathematical description of the process. His German contemporary Eugen Hausbrand developed a parallel method that focused more on the thermodynamics of the process, considering the energy changes that accompanied vaporisation and condensation.