Tuning the parameters of adsorption science to make a lighter, more efficient medical oxygen concentrator.
If you find yourself a bit winded after running up a flight of stairs, chances are you needn’t worry – you’ve probably been spending too much time at your desk or in front of the TV. Once you resume exercising or walking, you’ll be fine.
The feeling of being short of breath, however, can be a steady and unwanted companion for the estimated 16 million Americans who suffer from chronic obstructive pulmonary disease (COPD).
COPD doesn’t receive nearly as much attention as cancer, heart disease or diabetes. But it can be just as serious. Globally, the World Health Organization estimates that COPD afflicts more than 64 million people, causing 3 million deaths a year. By 2030, WHO predicts COPD will be the third leading cause of death in the world.
COPD is a family of diseases, ranging from chronic bronchitis, emphysema and pulmonary fibrosis to less common illnesses like lymphangioleiomyomatosis (LAM). All are characterized by a gradual loss of lung capacity and a progressive reduction in the amount of air that flows to and from the lungs. Many COPD patients eventually require concentrated oxygen in order to survive.
Those patients were once confined mostly to their beds, connected by breathing tubes to bulky cylinders or machines filled with compressed oxygen. Besides needing frequent refilling, the pressurized containers posed a hazard if they were dropped or if they came into contact with a spark or flame. And users could not leave home without carrying a cylinder or hauling a tank on a dolly.
In recent years, battery-operated medical oxygen concentrators (MOCs) weighing as little as six to eight pounds have made life easier for COPD patients. MOCs draw in compressed ambient air containing about 21-percent oxygen, enrich it to a concentration of 90- to 93-percent oxygen and supply it to a user. This is usually accomplished with rapid pressure swing adsorption (RPSA), an air-separation technology employing a zeolite adsorbent, which selectively retains the nitrogen from the air and produces the oxygenenriched gas.
Shivaji Sircar and Mayuresh Kothare, both professors of chemical engineering, are seeking to substantially improve the performance of RPSA and the quality of life for COPD sufferers. They have developed a new technology that could significantly reduce the weight of a portable MOC device while improving oxygen recovery, which lowers the power requirement for air separation and, thus, the compressor size.
The two researchers collaborate with and receive support from one of the half-dozen U.S. companies currently marketing MOCs. In December 2011, they were one of three research teams to win $200,000 in funding from the Science Center in Philadelphia. Fifty groups, many affiliated with research and teaching hospitals, competed for prizes. The Lehigh team was chosen from among 10 groups that had been invited to make business and technical presentations to the Science Center.
Looking beyond cycle speed
Sircar, a member of the National Academy of Engineering, has more than 40 years of R & D experience in adsorption and separation, including 30 years with Air Products and Chemicals Inc., where he served as chief scientist before retiring and joining Lehigh in 2002. He is the author of numerous publications and patents.
Kothare studies microchemical systems, feedback control and neuroengineering and has also published extensively. He is also a visiting professor of biomedical engineering at Johns Hopkins University, where he works with neurologists to improve brain-computer interface technology and help patients regain control of functions lost to brain damage.
The two researchers began their current collaboration about three years ago when Kothare, who was pursuing new applications for microreactors, asked a graduate student to investigate the feasibility of building an adsorption-driven separations system on a silicon wafer. The researchers later determined that reducing the size of a conventional MOC was a more realistic goal.
The new project was conducted by Siew Wah Chai, who earned her Ph.D. in 2011 and now works for Praxair. Chai demonstrated in a lab-scale unit that, by modifying the design of the RPSA cycle and the adsorbent material, it would be possible to make a more compact, lightweight and energyefficient MOC. Chai, Kothare and Sircar published their findings in a key paper in Industrial and Engineering Chemistry Research, a notable journal of the American Chemical Society, and filed a patent application.
The Lehigh team is now seeking to optimize the typical MOC unit, which contains two columns packed with a preferred zeolite for air separation. As compressed air flows through the columns, the zeolite adsorbs nitrogen while oxygen passes through. The flow is then reversed in order to desorb the adsorbed nitrogen by backpurging with a part of the oxygen product gas at a lower pressure. The cyclic adsorption-desorption steps are employed in conjunction with other complementary process steps to obtain the best performance from the MOC.
Researchers have traditionally sought to reduce the total cycle time of the pressure-swing adsorption process in order to decrease bed size factor (BSF, or pounds of adsorbent used by the MOC per ton of oxygen produced per day), to increase cycle frequency and to boost oxygen recovery.
The Lehigh team, after testing different adsorbent particle sizes, adsorption pressures and cycle times, reported in Industrial and Engineering Chemistry Research that, contrary to established belief, BSF cannot be “indefinitely reduced” by lowering total cycle time.
“The conventional wisdom about pressure-swing adsorption technology says that the faster the cycle time, the better,” says Sircar. Consequently, other researchers have sought mechanical solutions, such as devices that switch air flow in subseconds or use complex rotary valves, to speed up the frequency of RPSA cycling.
“Our experiments show that faster is not always better. We’ve demonstrated that beyond a certain cycle speed, you see diminishing returns. We’ve looked at other factors – operating condition, cycle time, material, cycle design. We believe we’ve achieved the appropriate combination of all the parameters involved in order to improve BSF as well as oxygen recovery.”
Greater mobility for COPD patients
The Lehigh team has a patent pending on technology that promises to reduce the MOC device’s weight by 30 percent, while boosting oxygen recovery by 20 percent. The resulting higher energy efficiency and lower power requirement will extend the operating time of a typical MOC, which currently runs for two to three hours before batteries must be recharged, and provide patients with longer periods of mobility.
The team plans next to build and test a prototype RPSA device, demonstrate its performance under realistic conditions, and fine-tune its operating conditions, including pressure, flow, temperature and switch times.
Two designs are envisioned – a more conventional two-column RPSA with an external oxygen storage tank and a novel “snap-on” MOC that has no compressor but can be plugged into one of the existing compressed air lines that are commonly provided in hospitals, ships, airplanes and other facilities.
Smaller, lighter, more energy-efficient MOCs, say Sircar and Kothare, are the wave of the future. As the world’s population ages and more cases of COPD are diagnosed, the global market for MOCs is expected to grow from less than $400 million in 2011 to about $1.8 billion by 2017. Evidence suggests that oxygen therapy might also slow the decline in mental cognitive abilities in people with dementia. And new applications in aeronautics, the automotive industry and even scuba-diving equipment are possible.
Just as importantly, lighter, smaller MOCs will grant greater mobility to COPD patients, enabling them to spend more time on their feet and engage in the cardiovascular exercise that physicians believe promotes pulmonary rehabilitation and could increase life span.
“A project like this makes you feel that your work is very tangible and helpful to society,” says Kothare. “Will we be the group that succeeds in developing the next-generation MOC? We don’t know. But we believe that our approach of producing a better MOC using the fundamental science and technology of adsorptive gas separation will get us there.”