One of the factors that laboratory and industrial users of nitrogen (N2) may wish to consider when selecting a supply method is the environmental impact of the process used to obtain the gas.
Nitrogen is usually generated through fractional distillation of air. While N2 is not considered a greenhouse gas, it is important to recognize that isolation of the gas from air by fractional distillation is an energy-intensive process. The energy employed to generate N2 produces a significant quantity of carbon dioxide (CO2). CO2 is believed to be a greenhouse gas that has a significant unfavorable impact on worldwide climate change. In addition to the CO2 created by the gas generation process, transportation of the gas from the location where it is manufactured to the location where it is used generates additional CO2, thus providing a further negative impact on the environment.
Once users understand the environmental impact and cost implications of fractional distillation, they may wish to consider an alternative source of the gas, such as an in-house nitrogen generator employing either pressure swing adsorption (PSA) or hollow fiber membrane technology. Both of these technologies produce far less CO2 and are a much less expensive source of N2 over time.
Why is fractional distillation so energy-intensive?
Significant energy is used in the fractional distillation process. The first step involves withdrawing the air from the atmosphere using a compressor. The compressed air is then chilled to approximately 10°C and passed through a moisture separator, an oil adsorber and molecular sieves to remove water vapor, oil, particulate matter and other contaminants. The dried, purified air enters a multi-pass heat exchanger, then a Joule-Thompson type expansion engine to chill it to below the condensation point of -195.8°C at 1 atm and liquefy it. The purified nitrogen is then pressurized and stored in bottles as a gas or directly stored in dewars or delivery tankers as a liquid. The gas or liquid is then transported to the end user’s facility. Once the nitrogen has been consumed, the empty containers must then be transported back to the distillation site to be refilled.
The fractional distillation process is normally performed on a large scale, continuous basis and commercial facilities are designed to generate hundreds or thousands of tons of the gas per day resulting in large CO2 emissions.The environmental cost of transporting the gas to the end-user and returning the empty containers back to the distillation site can also be significant. Suppose a tractor-trailer carrying nitrogen travels 400 miles per day (or 104,400 miles per year). This process will create 360,000 pounds or 163 metric tons of CO2 per year. Carbon Emissions Calculator
How in-house generators compare
Data shows that in-house gas generators are more energy efficient when compared to conventional industrial methods. The European Industrial Gas Association (2010) notes that an air separation plant uses 1976 kJ of electricity per kilogram of nitrogen produced at 99.9 percent purity (EIGA Position Paper pp-33). A PSA system, on the other hand, uses only 1420 kJ of electricity per kilogram of nitrogen. This translates to a 28 percent reduction in greenhouse gases from the emissions. For applications requiring 98 percent purity, such as liquid chromatography with mass spectroscopy detection, the energy requirement drops to 759 kJ of electricity per kilogram of nitrogen produced which equals 62 percent less energy than liquid nitrogen.
Although nitrogen itself is not a greenhouse gas, its production and transportation has a greenhouse effect. That effect, however, can be significantly reduced if users obtain the gas from an in-house generator rather than having it delivered from industrial suppliers.
This is Part 1 of a 3 part series on a sustainable approach to the supply of nitrogen. Following are links to the rest of the series:
This series was contributed by David Connaughton, business development manager - nitrogen, Parker Industrial Gas Filtration and Generation Division.