As the intrigue of the Covid-19 pandemic continues to unfold, it can be easy to forget the true horror of this disease. The sight of breathless patients is frightening, and many have needed extra oxygen. The production of this oxygen has been a major challenge of the pandemic, but facilitated by a technique developed in the 1950s by a man specializing in separation.
Charles Skarstrom was born in 1906, the son of a prominent physical education teacher. In 1924 he began a physics degree at his father’s former university, Columbia, in New York, United States. He left in 1933 with a master’s degree to join Jesse Beams at the University of Virginia. Beams had started testing a hypothesis made by Frederick Lindemann and Francis Aston that a centrifuge could be a way to separate isotopes in gases or vapors. Skarstrom joined the group just as difficult and dangerous work on the enrichment of heavier chlorine isotopomers of carbon tetrachloride was being prepared for publication. For his doctoral thesis, submitted in 1939, Skarstrom extended the method.
As part of the Manhattan project to build the atomic bomb, Beams proposed to use 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 develop a system robust enough to do the job, and the isotopes used in the weapons dropped on Hiroshima and Nagasaki in Japan were separated by more conventional gas diffusion through porous membranes.
After the war, Skarstrom got a job with Standard Oil, later Esso (now ExxonMobil), working in their research labs 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 it was home to one of the first catalytic crackers.
In 1956, everything would change as he tried to optimize a method for removing moisture from the air.
Skarstrom worked in the Analytical Separations team and its first patents dealt with gas chromatography and problems associated with the polymerization of olefins and the combustion of jet engines. But in 1956, that was all about to change as he tried to optimize a method for removing moisture from the air. This method passes air over a column filled with adsorbent material. When the material became saturated, the flow was switched to pass the gas through a second, identically packed column. During this time, the first column was heated to desorb moisture, with a small portion of dry gas used as a purge to drive off the water. Thus the two columns were alternated on a 12 hour heating-cooling-adsorption cycle, set on a timer.
Unfortunately, the heater cartridge in one of the columns failed. Reluctant to wait for a replacement to arrive, Skarstrom decided to operate the device without the heaters. Not needing the long cool down time, he set the timer to change every 30 minutes. The apparatus further reduced the humidity in the gas supply. Intrigued, Skarstrom shortened the cycle further, observing that the emerging air was drier at shorter cycles. In this adiabatic process, the heat of adsorption has never been lost, allowing water to be desorbed by pressure drop and purge.
Skarstrom has tried different adsorbents. In his patent, he lists everything from activated carbon to tissue paper, as well as the newly discovered synthetic zeolites that were known for their huge surfaces. Depending on the zeolite chosen, its process could produce a gas that is significantly enriched or depleted in oxygen. Volatile hydrocarbons could also be separated. His method of “non-heating fractionation,” with all its potential to provide medical oxygen and nitrogen for food preservation, was featured in the New York Times weekly patent review. It will later be known as pressure swing adsorption.
Unbeknownst to Skarstrom or its patent agents, by 1953 Heinrich Kahle, working for Linde in Germany, had developed and patented the exact same thing. This was revealed in 1970, the year Skarstrom retired, when Esso sued a competitor for patent infringement. After going through the entire process and dissecting both the principles and the apparatus, the judge rejected Skarstrom’s patent for lack of novelty. What Skarstrom thought about 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 labs and oxygen to glassblowers. It is proposed for the capture of carbon dioxide from cement and steel plants. And in recent weeks, the Indian Air Force has delivered more than 11,000 oxygen concentrators to hospitals across the country to save countless critically ill patients. The idea for Skarstrom might not have been entirely original, but it was quite clever.