Switching from bottled or liquid nitrogen to a pressure swing adsorption (PSA) system as a way to source pure nitrogen (N2) can result in energy and cost savings, as well as reduced CO2 emissions when compared to the conventional air separation process of fractional distillation.
Large air separation plants use fractional distillation of air to generate nitrogen. This is energy-intensive because the ambient air must first be condensed into liquid air by cooling and compressing it. The separated nitrogen must then be purified to the desired level (discussed in Part 1 of this series). Next, the nitrogen is transported to the location where it will be used. Then, the empty tanks must be transported back to the production facility to be refilled. Transportation of the tanks uses a significant amount of energy, which is expensive. Because the process of generating nitrogen is performed on a continuous, large scale basis, large amounts of CO2 emissions are released. CO2 is believed to be a greenhouse gas with a significant unfavorable impact on worldwide climate change.
A PSA system on the other hand, produces nitrogen at room temperature using house compressed air which requires less energy. Additionally, a generator operates on-premises, meaning there is no transportation involved.
A PSA nitrogen generation system separates nitrogen from oxygen based on the preferential adsorption and desorption of oxygen and other contaminants on carbon molecular sieve. Pressurized air is passed through a vessel filled with carbon molecular sieve that adsorbs oxygen while the nitrogen passes through the vessel. Once the molecular sieve is saturated with oxygen, the pressure is lowered and the contaminants which have been trapped (including oxygen, CO2, and water vapor) are released to atmosphere. Carbon molecular sieve has a high degree of microporosity making it ideal for oxygen adsorption. To obtain a continuous flow of N2 and maximize system utility, two vessels are connected in parallel, so that one vessel is providing nitrogen to the system while the other vessel is being regenerated.
PSA systems offer many of the same benefits as hollow fiber membrane systems (discussed in Part 2 of this series) including an uninterrupted nitrogen supply, consistent purity, reduced costs, and freedom from dependence on outside vendors. Generally, a PSA system would be used over a membrane system when the application requires higher purities (>99%).
An energy consumption comparison
Using data from the European Industrial Gas Association (EIGA) as published in EIGA Position Paper PP-33, December 2010, it’s possible to compare the PSA method with the fractional distillation method on the basis of energy consumed.
The EIGA notes that an air separation plant uses 1976 kJ of electricity per kilogram of nitrogen produced (99 percent purity) versus 1420 kJ for PSA — or 28 percent less. For applications only requiring 98 percent purity the contrast is even greater: 1976 kJ versus 759 kJ — or 62 percent less.
This is Part 3 of a 3 part series on a sustainable approach to the supply of nitrogen. Following are links to the rest of the series: