This article is for you if your company is purchasing nitrogen gas at 99.999% purity and you’re not sure why. While there are many applications which do require nitrogen gas concentrated to 99.999%, they are significantly outweighed by the applications that don’t. Rather than relying on a delivery of bulk liquid or pressurized cylinders, many nitrogen users are choosing to produce a custom supply of nitrogen within their facility, and they are doing it at a fraction of the cost. Over the past decade we’ve seen a mass industry shift from delivered nitrogen supply, to nitrogen generation.
With nitrogen being the most plentiful air gas at 78%, the process of separating nitrogen from air is very efficient, relative to the extraction of the other air gasses which only account for 22%, most of which is oxygen. The ability to tailor the nitrogen purity, pressure and flow rate to a specific industry or application is what leads to substantial savings over traditional supply.
The reality (and also the problem) is that no matter how inefficient a system is designed, nitrogen generation will typically be less expensive than purchasing the gas from a third party. This inevitably introduces wasteful and bloated nitrogen generation systems into the market. This article will focus on the key considerations of nitrogen gas generation system design.
Purity Gas premium efficiency nitrogen generation system with modular redundancy.
Why is Nitrogen Generation in High Demand?
Most of our projects are fueled by end users who are motivated to reduce costs and their carbon footprint. When using nitrogen in traditional supply form (liquid or cylinder), the gas is produced at an air separation plant using an electrically intensive process called fractional distillation. The fractional distillation of liquefied air is a process in which air has to be cooled beyond -200 °C and then re-heated to extract the different elements based on their boiling points. This process requires large amounts of energy and can only be done on a large scale to be economically viable.
Once the nitrogen has been ‘produced’, it is decanted into large transport trucks and dispatched from the air separation plant, eventually landing at its final destination where it is stored and consumed by the end user. The nitrogen is analyzed at the air separation plant and registers at an exceptionally high purity, typically 99.998+%.
The alternative solution to delivered supply is nitrogen generation. Nitrogen gas generation is the efficient approach to supplying manufacturing processes with the gas they require. Nitrogen is extracted and concentrated from a supply of compressed air using carbon molecular sieve, or hollow fibre membrane tubes. The purity of the nitrogen is determined by the contact time between the compressed air and chosen separation medium; longer contact results in higher purity. Therefore, higher purity nitrogen requires more input air flow and costs more to generate.
Each application and process using nitrogen will have a maximum allowable tolerance for oxygen. A ‘low purity’ application such as fire suppression may only require 95+% purity, while a high purity application such as selective soldering will typically require 99.995+%. Food and beverage production falls in the middle, with most applications ranging from 98% to 99.5% (or 2% to 0.50% remaining oxygen content). The cost to generate nitrogen at 95% requires significantly less energy and equipment to produce the same volume at 99.999%. Purity selection is a critical component of efficient system design.
As nitrogen purity increases, so does the requirement for input compressed air and energy. Using the national (Canada) average electricity rate of 12 cents per kWh, the annual electricity cost to produce nitrogen at 95% purity will be approximately \$20,957 vs. \$71,256 when produced at 99.999%. Click here to enlarge.
If the process receives no additional benefit past a certain purity, the overpurifying result is waste. An over consuming nitrogen generation system will use much more compressed air and equipment than required. Unfortunately, a blind eye is turned to the wasted capital and overconsumption of energy because it’s still cheaper than purchasing nitrogen from a third party. Many years of compressed air system optimization and energy reduction can be lost in the blink of an eye with a hungry nitrogen gas generation system.
Nitrogen System Assessment Sets the Baseline to Measure the Opportunity
It is critical to understand the nuances of the existing nitrogen supply and how it is used, before designing a new system. Collecting flow, pressure and purity data from the existing supply can help create a baseline for measurable improvement. We are firm believers in the adage “you can’t manage what you can’t measure”. When possible, the nitrogen flow and pressure requirements can be captured using calibrated flow meters and pressure transducers (downstream of the evaporator if using liquid supply).
If a process uses a very large volume of nitrogen intermittently, nitrogen generation may not be the best choice, as the system will need to be large enough to satisfy the peak demand of the plant, but only for a short period of time. Nitrogen generators quickly pay for themselves when they are in operation, not sitting idle. Facilities and processes with consistent flow demands and multiple shifts, typically produce the strongest business cases.
Collecting purity data at the point of use can help reset the expectation that a certificate of analysis from the air separation plant provides. When purchasing nitrogen in bulk liquid or cylinder form, the purity is analyzed at the air separation plant prior to transport, storage, evaporation and process use. Many of our clients are surprised by the purity measured at the point use, or within their finished product after a series of losses. As a part of our detailed nitrogen assessment, we suggest collecting purity data at the point of use with a calibrated oxygen analyzer which measures the remaining oxygen content in PPM or %. We are often instructed that the plant needs 99.999% to support a process, simply because of what the incumbent certificate of analysis reads. More often than not, we will record substantial purity losses throughout the distribution network of pipe and at the point of use, which objectively changes the baseline for purity needs.
Collecting purity data, at the point of use, can help determine the nitrogen purity specification. Nitrogen generators will produce nitrogen gas at a specified purity, displaying the remaining oxygen on the HMI in real time. When gas is not required, the generator will enter energy saving mode preserving valuable compressed air.
Craft Brewery Carbon Emissions Case
Many companies are striving to contribute to global sustainability initiatives and reduce their carbon footprint, Purity Gas included. When purchasing nitrogen in bulk liquid or pressurized cylinders, it’s important to learn where the nitrogen was produced (location of the air separation plant) and how far it was transported to the final destination. The origin of production will typically be indicated on the certificate of analysis. In addition to the greenhouse gasses (ghg) emitted by the electricity intensive fractional distillation process, transportation to the final destination will also need to be considered. The United States Environmental Protection Agency publishes a standard formula for calculating the approximate ghg emitted as a result of freight, via a typical transport truck: Distance x Weight x Emissions Factor. The average transport truck emits 161.8 grams of CO2 per ton-mile as per the EPA. Depending on how your region produces electricity, the delta between emissions associated with transport and the electricity required for nitrogen generation can help determine the environmental gains available through conversion.
A craft brewery in remote, northern Quebec was seeking cost reduction and environmental sustainability opportunities for their business. The brewery was using one bulk pack of nitrogen every two weeks and wanted to learn how the conversion to self generated nitrogen supply would support their environmental goals.
- Using the EPA calculations for freight emissions, we multiplied the distance travelled (120 miles) x weight (1.26 tons) x emissions factor (161.8 grams per mile-ton). Therefore, each bulk pack delivered to site was responsible for approximately 24,464 grams of CO2 emissions. Multiplied by 26 deliveries per year, the brewery’s carbon footprint related to bulk pack deliveries was 636,064 grams of CO2, per year.
- We then calculated the carbon emission associated with nitrogen generation. According to the National Energy Board of Canada, Quebec (most is hydro power) has the lowest CO2 per kWh emissions in Canada at only 1.2 grams of CO2 per kWh. The proposed nitrogen generation system would consume 4,480 kWh annually (mostly from compressed air), for a total impact of 5,376 grams of CO2.
- By implementing a high efficiency nitrogen generation system, the brewery was able to net an annual carbon footprint reduction of 630,688 grams of CO2; a 99% reduction over the carbon emissions associated with delivered nitrogen bulk packs.
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Designing the Optimum Nitrogen Generation System
The efficiency of a system is defined by its compressed air to nitrogen ratio; how many units of compressed air are required to produce a single unit of nitrogen. 95% pure nitrogen may only require two units of compressed air to produce one unit of nitrogen, while 99.999% pure nitrogen may require close to seven units of compressed to produce the same volume. It’s very important to highlight that nitrogen generators do not produce better, or worse nitrogen; it’s just less expensive because it offers a custom purity solution, by application. Typically, the lower the purity, the greater opportunity becomes for savings. A nitrogen system operating at 95% purity can produce 7.5x more nitrogen than that of a system running at 99.999% purity.
As purity increases, the outlet flow rate of the nitrogen generator decreases. A system can become significantly oversized if ‘worst case’ purity is assumed (99.999%) and additional equipment may be required to meet the target flow rate.
Using baseline purity data from a nitrogen system assessment, one can begin to work backwards to the optimum purity. The purity requirement can often be instructed by a quality assurance department, as internal testing may have already been completed. However, if purity is unknown, we often suggest using certified nitrogen & oxygen mixes in a controlled environment to determine the point of diminishing return, instead of assuming a worst case scenario (99.999%). Over assuming the purity will substantially increase the capital and operating costs of the system. There comes a point in each application where an increase in nitrogen purity does not provide additional benefit to the process or result, but will absolutely cost more to generate. When it comes to purity, it is critical to identify the point of diminishing return.
The efficiency of a nitrogen gas generation system can be refined by optimizing ancillary equipment, operating set points and the applying appropriate technology. While this article is not intended to specify compressed air systems, considerations should be made to use supporting technology which is cost effective to own and operate. The nitrogen generator contains very few mechanical components and is largely passive in its operation, relative to the complexity and mechanical involvement of an air compressor. This typically makes the air compressor the most vulnerable point in the system. The electricity cost to run the air compressor, along with system maintenance is what determines the price of nitrogen gas produced by the system. Regardless of the technology, it is imperative to understand the maintenance costs during the evaluation stage.
Example - Some nitrogen generators will use a zirconium oxide oxygen sensor to measure purity, while others will use an electrochemical oxygen sensor with a galvanic cell. The zirconium sensor is more expensive, but is also maintenance and calibration free, with a service life of 5 to 10+ years. The galvanic cell will deplete in the presence of oxygen and requires costly quarterly calibration, and annual replacement. Maintenance costs play a significant role when determining the cost to produce a unit of nitrogen. When evaluating technology options, complete transparency and upfront disclosure of all operating costs are mandatory in order to make an educated decision.
When selecting a nitrogen generator, one has the choice between pressure swing adsorption (PSA) and membrane technology. PSA nitrogen generators use carbon molecular sieve (CMS) and an adsorption process to remove unwanted gas molecules, and can typically deliver purity up to 99.999%. Membrane generators use hollow fibre membrane tubes and selective permeation to remove unwanted gas molecules, and can typically deliver purity up to 99.9%. The selection of which technology to apply is entirely dependent on the application, environment and purity requirements. Whether using PSA or membrane equipment, we suggest using technology that can grow with increasing production demands, without the original installation becoming obsolete.
As stated earlier in this section, the efficiency of a system is defined by its compressed air to nitrogen ratio. The practical reality of manufacturing is that there will likely be periods of variable demand and the full capacity of the system will not be required when designing a nitrogen generation system, it is imperative to meet the maximum demand, while being able to meet the average demand efficiently. A modular technology selection with an appropriate control strategy will allow the system to scale down its gas production duties and isolate the unneeded modules during low demand periods, constantly managing the compressed air to nitrogen ratio. As production increases, the system will scale up by awakening modules from an economy energy saving mode, ensuring that peak demand is satisfied. This control strategy creates an environment where modules will only operate when they are required, avoiding the unnecessary loss of valuable compressed air and stabilizing the measure of efficiency: compressed air to nitrogen ratio.
An existing nitrogen generation system was using antiquated technology and lacked controls discipline. The existing system was retrofitted with energy saving equipment and a new control strategy. This resulted in stabilizing the compressed air to nitrogen ratio and significantly reduced the operating cost of the system.
Next Steps to Self-Sufficiency
A nitrogen generation system with thoughtful and diligent design can be a great way for nitrogen users to reduce costs, carbon emissions and contribute to their financial and operational goals. However, it’s not a catch-all solution all for every application. We always suggest beginning with a condensed preliminary assessment before too much time, energy and capital is wasted; it’s all about efficiency. There are often times when we quickly instruct our clients to not make any changes to their existing supply, even before arriving at the audit or detailed assessment stage. Sometimes the business case doesn’t support the investment in a system. A qualified solutions team will be able to model a detailed capital cost recovery report and pinpoint the savings to the penny. If a viable solution is available, a fully informed discovery and evaluation can quickly lead to self-sufficiency and pulling the plug on a never-ending supply of liquid nitrogen.
About the Author
Alan Hopkins is a Co-Founder & Managing Director of Purity Gas Inc., Email: email@example.com ,Tel: 905.335.6040
About Purity Gas Inc.
Purity Gas is a industry leader in high efficiency nitrogen gas generation design and implementation. Our mission is to engineer and deliver world class solutions empowering nitrogen users to produce their own endless supply of nitrogen gas, while contributing to their economic, safety and sustainability initiatives. For more information visit puritygas.ca
To read similar articles on Nitrogen Generation Technology visit www.airbestpractices.com/technology/air-treatment.