As the plot of the Covid-19 pandemic continues to unfold, it can be easy to forget the true horror of this disease. The sight of patients gasping for breath is terrifying, and many have required supplemental oxygen. Making this oxygen was one of the main challenges of the pandemic, but it was made easier by a technique developed by a man specializing in separation in the 1950s.

Charles Skarstrom was born in 1906 to a leading physical education teacher. In 1924 he began studying physics at his father’s old university, Columbia, in New York, USA. He left with a masters degree in 1933 to join Jesse Beams at the University of Virginia. Beams had started testing a hypothesis by Frederick Lindemann and Francis Aston that a centrifuge could be a means of separating isotopes in gases or vapors. Skarstrom joined the group as difficult and dangerous work on the enrichment of the heavier chlorine isotopomers of carbon tetrachloride was being prepared for publication. For his doctoral thesis, submitted in 1939, Skarstrom expanded the method.

As part of the Manhattan project to build the atomic bomb, Beams suggested using its gas ultracentrifuges to separate the fissile uranium-235 from the rest. Several patents in the name of Skarstrom, filed long after the end of the war, describe seals and bearings related to this project. However, Beams and his team were unable to come up with a system robust enough to do the job, and the isotopes used in the weapons dropped on Hiroshima and Nagasaki, Japan were carried through by more conventional gas diffusion porous membranes separated.

After the war, Skarstrom got a job at Standard Oil, later Esso (now ExxonMobil) and worked in their research laboratories next to the legendary Bayway refinery in Linden, New Jersey. Bayway was one of the first integrated refineries; the first petrochemical, isopropanol, was produced there and was home to one of the earliest catalytic crackers.

Everything changed in 1956 when he tried to optimize a method for removing moisture from the air

Skarstrom worked on the analytical separations team and his early patents covered gas chromatography and issues related to olefin polymerization and jet engine combustion. But in 1956, everything changed when he tried to optimize a method for removing moisture from the air. In this process, air was passed over a column packed with adsorbent material. When the material was saturated, the flow was switched to pass the gas through a second, identically packed column. In the meantime, the first column was heated to desorb the moisture, using a small portion of the dry gas as a purge to drive off the water. Thus, the two columns were alternated with a heating-cooling-adsorption cycle of 12 hours set on a timer.

An image showing a diagram of an oxygen generator

Disastrously, the heating cartridge for one of the columns has failed. Skarstrom hesitated to wait for a replacement to arrive and decided to run the device without the heaters. Since he didn’t need the long cooling time, he set the timer so that it switched every 30 minutes. The apparatus was still reducing the moisture in the gas supply. Fascinated, Skarstrom further shortened the cycle and found that the exiting air was drier with shorter cycles. In this adiabatic process, the heat of adsorption was never lost, so that the water could be desorbed through a pressure drop and flushing.

Skarstrom has tried various adsorbents. In his patent, he lists everything from activated carbon to tissue paper, as well as the newly discovered synthetic zeolites, which were known for their huge surfaces. Depending on the zeolite chosen, its process could produce gas that is heavily enriched or depleted in oxygen. Volatile hydrocarbons could also be separated. His “heat-free fractionation” method, with all of its potential to provide medical oxygen and nitrogen for food preservation, was featured in the New York Times’ weekly patent review. It later became known as pressure swing adsorption.

Unknown to Skarstrom or his patent attorneys, Heinrich Kahle, who worked for Linde in Germany, had developed and patented exactly the same thing in 1953. This became known in 1970, the year Skarstrom retired, when Esso sued a competitor for patent infringement. After the judge reviewed the entire process and analyzed both the principles and the apparatus, the judge rejected Skarstrom’s patent for lack of novelty. What Skarstrom thought of it will probably never be known. He died in 1984.

Pressure swing adsorption is now routine. It can be used to supply nitrogen to chemistry laboratories and oxygen to glassblowers. It is proposed for carbon dioxide capture from cement works and steel works. And in the past few weeks, the Indian Air Force has shipped over 11,000 oxygen concentrators to hospitals across the country to rescue countless critically ill patients. Maybe Skarstrom’s idea wasn’t entirely original, but it was pretty clever.


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