The Energy Costs Associated with Nitrogen Specifications

So you need nitrogen in your plant!

With rising fuel and energy costs, the cost of liquid nitrogen is going up and is making it much easier to justify the purchase of a nitrogen generator in a wide range of purities and pressures.  In a high percentage of cases, generating your own nitrogen using commercially available equipment is a very cost effective alternative to purchasing liquid nitrogen or cylinder nitrogen from traditional supply sources like the industrial gas companies. In some cases, the return on investment (ROI) ranges from six months to 2 years, but ROI can range, depending on several factors, to several years while still being a good investment.

If you are considering purchasing a nitrogen generator for your facility the first question to settle is what nitrogen purity you actually need for your application and facility. Nitrogen purity is generally expressed as a percent, such as 99% Nitrogen (which means 1% Oxygen with the balance nitrogen and other inert gases). In some high purity cases, it may be expressed as PPMv Oxygen remaining in the product gas. 10 PPMV is the same as 99.999% nitrogen. 10,000 PPMv equals 1% O2. The answer to the purity question can make or break a project’s budget and have a huge impact on the amount of electricity used. There are many factors to be considered. First there are some basic dynamics common to self-generated nitrogen gas that should be considered.

The Technology

For the purpose of this article, we will focus on Pressure Swing Adsorption (PSA) Nitrogen Generator technology for generation of Nitrogen Gas. PSA uses compressed air as the feed stock in the production of the nitrogen. Pressure Swing Adsorption (PSA) uses, in most cases, two vessels, packed with carbon molecular sieve (CMS). The CMS adsorbs Oxygen as compressed air flows upward through one of the vessels while the other vessel is depressurized and a small amount of the nitrogen output is flowing downward (counter flow) to drive off or desorbs the oxygen and moisture collected by the vessel when it was online removing oxygen from the air. The vessels alternate this adsorption and desorption process. There are several steps in the process; equalization, pressurization/depressurization, adsorption/purge, then back to equalization.

PSA technology is generally used where higher purities are needed, and in many cases if the system is very large in capacity. Over the last 15-20 years, there have been many advances in pressure swing adsorption (PSA) technologies.  Improvements in carbon molecular sieve (CMS) used in PSA equipment has shown significant increases in flow rates and air to nitrogen ratios (ANR) have come down dramatically.The compressed air purification system treating the compressed air feeding the n2 Generator should consist of a Receiver(s), water separator, air dryer (selected carefully for the application), and multi-stage particulate and coalescing filters, all designed to meet the unique needs of a PSA system.

Nitrogen Purity Specification

The higher the nitrogen purity required, the larger the system to obtain a given flow rate. More compressed air will be required and more electricity consumed. Capital cost, long term maintenance cost, and floor space will increase as well. Whereas, a 95% N2 system will take approximately 1.95 M3 of compressed air for 1 M3 of nitrogen, the ratio of air to nitrogen could be as high as 7.5:1 at 99.999% N2 in some systems. Here are a few “Rule of Thumb” Nitrogen specs for some industries (For Comparison Only)

Nitrogen Quality chart

Is the Nitrogen Purity the right one for my application?

The bottom line here is that most nitrogen systems are “over spec’d” because the information is sometimes hard to obtain and so to be safe, consumers will often specify a higher purity of nitrogen than they need. Sometimes this comes from an equipment manufacturer, who has not done sufficient testing at various nitrogen purities. In the earlier days of nitrogen generators, this information was difficult to come by, but over the years more and more application specific information has become available. This can sometimes be obtained through industry associations and from equipment suppliers, and industry professionals.

 

Energy Cost Implications Based on Varying Nitrogen Purities

In Table 1 below, ANR is the air-to-nitrogen ratio. This is the measure of the efficiency of the technology. Multiply this number by the N2 requirement (m3/min) to get the Volume (m3/min) of Compressed Air required. CCF is the Compressor Correction Factor. Size the compressor 1.15 to 1.2 times the required compressed air m3/min to allow for the compressor manufacturer’s flow tolerance and varying ambient conditions.

Effects of Purity Selection on Energy Required 
                                                                          Generator Inlet 8.0 Bar
System Type N2 Purity M3/min N2 ANR M3/Min Air Req’d CCF Min Comp M3/Min Compressor KW Req
PSA 95% 1.5 1.94 2.91 1.15 3.34 18kw
PSA 99.5% 1.5 2.42 3.63 1.15 4.17 30kw
PSA 99.99% 1.5 4.24 6.36 1.15 7.31 45kw
PSA 99.999% 1.5 7.45 11.17 1.15 12.85 75kw

 

One can see how drastically the effects of the purity chosen for the 1.5 m3/min nitrogen example. The compressor kw ranges from 18.5 to 75kw depending on the purity percentage of the nitrogen.

 

Table 2 below calculates the Overall Air Compressor energy cost for the compressor to produce the required Nitrogen Flow and Purity.

Air Compressor Energy Cost P.a. @ various Purity Levels
                                                                          Generator Inlet 8.0 Bar
Compressor Type N2 Purity Min Comp M3/Min Air Compressor Specific Pwr Total kw for Req’d Volume Hours production P.A. Kw/hrs p.a. Req’d Energy Cost based on 0.10 cent per kw/hr
Rotary Screw VSD 95% 3.34 6.5 kw/m3/min 21.71 8000 173,680 €17,368.00
Rotary Screw VSD 99.5% 4.17 6.5 kw/m3/min 27.10 8000 216,800 €21,680.00
Rotary Screw VSD 99.99% 7.31 6.5 kw/m3/min 47.51 8000 380,080 €38,008.00
Rotary Screw VSD 99.999% 12.85 6.5 kw/m3/min 83.52 8000 668,200 €66,820.00

 

Table 3 below calculates the Energy cost per M3 of Nitrogen produced.

Energy Cost Per m3/N2 @ various Purity Levels 
                                                                          Generator Inlet 8.0 Bar
N2 Purity

 

M3/min N2 M3/P.A. Energy Cost based on 0.10 cent per kw/hr Energy Cost Per M3/N2
95% 1.5 720,000 €17,368.00 €0.024
99.5% 1.5 720,000 €21,680.00 €0.030
99.99% 1.5 720,000 €38,008.00 €0.052
99.999% 1.5 720,000 €66,820.00 €0.092

 

Note: The above calculation does not include the Energy requirement of the system Air Dryer which dries the compressed air. If a refrigerant air dryer is used, then the additional cost is small adding only €0.00096 per m3/N2.

 

On Site Nitrogen Gas Generation

On Site gas generation is an easy and reliable way that can save you money and give you more control over your gas use. Companies who produce their own supply of on-site nitrogen can generate as much or as little nitrogen as they need, at a fraction of the cost of having the gas delivered by an external source.

 

Benefits of On- Site Gas Generation

  • Increased profitability – significantly lower unit gas cost, more up-time. Reduction in Nitrogen costs of up to 95%
  • Increased reliability – gas available on demand 24/7, only pay for gas produced
  • Sustainability – short payback on investment, long service life, low energy use and reduced CO2 footprint

 

ENDS

 

N2 Generation Packages – O’Neill Industrial delivers bespoke, market-leading, high flow & high purity nitrogen generators as solutions for numerous applications within industry sectors, such as Food & Beverage, Pharmaceutical & Biotechnology, Chemicals, Electronics, Metals, Plastics, Rubbers and many more.

Leasing Options – O’Neill industrial can offer Nitrogen generation packages under five-year lease agreements. The advantages of leasing the equipment in contrast to buying can be multiple; you avoid large capital expenditure up front, get complete peace of mind and could potentially install a higher spec Nitrogen System which may have been cost prohibitive if you chose to buy outright.

Contact us today, email Austin[@]oni.ie