Liquid-liquid extraction attracts a little poetic licence
Chemistry does not lend itself to presentation in verse. Despite the best efforts of American chemist Roald Hoffmann, most attempts have been more or less extended (and sometimes quite funny) jokes. But perhaps such efforts are misplaced; instead, one can imagine a line of literary criticism in which poems are read as chemical metaphors. Could Hamlet’s dilemma be compared to the sometimes futile attempt to disentangle steric and electronic effects?
Industrial solvent extraction, and particularly the Scheibel column, presents an easier challenge for the critic. Ed Scheibel was born to a working class family in the borough of Queens in New York City. The family business failed in the Great Depression. In the face of these difficulties he won a full scholarship to study engineering at Cooper Union College in New York, graduating with honours in 1937. He continued his education at Brooklyn Polytechnic, funding it by working at a series of small industrial concerns. It was here that Donald Othmer - famous for the Kirk-Othmer encyclopaedia of chemical technology and Scheibel’s degree adviser - got him interested in petroleum processing.
Scheibel’s experience of separation technology got him an engineering position at the pharmaceutical firm Hoffmann-La Roche, while continuing to lecture to graduate students in Brooklyn. His boss, Albert Frey, drew Scheibel’s attention to the company’s main money-earner, vitamin C. Frey wanted to convert the batch process to continuous. The crux was an extraction step that involved squirting a fine spray of a less dense phase into a tower loaded with another, denser liquid. The droplets were supposed to equilibrate as they rose, and coalesce at the top of the tower. The reality was rather less perfect.
After tinkering unsuccessfully for some time with tower packings, Scheibel commented ruefully in one of his lectures about the difficulty of doing extractions in an industrial setting. One of the students, Otto York, who worked for a metal gauze manufacturer, suggested that fine wire meshes might be effective in getting the droplets to coalesce. Scheibel introduced York to his employer and testing began. To their dismay, however, the mesh seemed not to make much difference to the efficiency of the tower. So York went back to his day job.
But it occurred to Scheibel that what might be necessary was to place the gauze above and below a paddlewheel that would swirl the two phases. The gauze would provide a quiescent region where phase separation would occur. Although the separation ratio was not far from experimental error, Frey was intrigued enough to authorise Scheibel to build a bigger column, four inches across, in which 11 paddles alternated with bundles of mesh, along with equipment to monitor composition at each stage.
Their gamble was rewarded by performance not far from the theoretical maximum, and they rushed to patent their invention before anyone else got wind of it. Scheibel now built a more portable prototype, only an inch across, with a view to selling the technique to other chemical firms. However, optimising the agitator speeds and the mesh size and volume proved a much tougher challenge than they had imagined. Scheibel would later comment on how lucky they had been - had they started with a smaller prototype they might have abandoned the idea.
The collaboration with York continued. Frustrated by his employer’s lack of interest in chemical engineering (all they seemed to care about was selling household scouring pads), York set out on his own. With a growing number of companies showing interest in the columns, scale-up now became the problem. The fine gauzes weren’t cheap, and a 12 inch diameter multistage column was too costly to be economically viable. Scheibel therefore introduced plates to separate the mixing stages from the meshes.
Over and over the story repeated itself and the column design, from the shapes of the paddles to the placement of baffles and spacers, steadily improved performance.
In the early 1950s, when inorganic chemists working at the US Department of Energy’s Ames Laboratory began to scale up the solvent extraction method for separating lanthanides, they chose a York-Scheibel extractor. Their paper - entitled ‘The first kilogram of gadolinium oxide’ - is a testament to the herculean task of separating these most similar of elements, and the spectacular efficiency of the columns. By then, both Scheibel and Frey were working for York’s company full time, and were building what were now towers almost 2m across.
In 1963, Scheibel, who had always had a love of teaching, returned to Cooper Union as head of department for five years. He then held various industrial and consulting positions until his retirement, his columns now firmly entrenched in the textbooks.
Perhaps it was the poet Henry Longfellow who best expressed the deep meaning of the extraction process when he wrote:
Ships that pass in the night, and speak each other in passing,
Only a signal shown and a distant voice in the darkness;
So on the ocean of life we pass and speak one another,
Only a look and a voice, then darkness again and a silence.
Andrea Sella teaches chemistry at University College London, UK
- E G Scheibel, US Patent 2493265, 1950