Arron Farghaly MEng – Lead Application and Business Development Engineer Atlas Copco Compressors
For much of this and last century and throughout UK industry, a dependable supply of industrial gases such as nitrogen and oxygen has been as crucial to critical processes as the supply of electricity, water and gas utilities. Now, major advances in on-site gas generation technology have transformed the way users are able to source industrial gases. Many companies and industries have converted to on-site, self-sufficient generation of industrial gases. As a result, these companies have made substantial gains in productivity, energy efficiency and cost savings.
The development of modern PSA (Pressure Swing Absorption) systems can be regarded as a disruptive change to techniques that originated with cryogenic production back in the late 1800’s and early 1900’s. They are still broadly employed today; up until recent times, the only significant changes were the early developments in membrane and PSA separation techniques that took place in the 1960’s.
That picture has changed dramatically. Progressive industrial managers and operators in many other production sectors have already realised the advantages of using on-site nitrogen generation. They have compared its costs and convenience with those of delivered nitrogen and oxygen bottles, or liquid gas from large cryo plants of which there are very few nationwide.
They have also considered all other negative factors: fuel costs and carbon emissions associated with truck deliveries; wastage from on-site liquid storage; equipment rental fees and, not least, safety concerns. Their conclusion: on-site generation of nitrogen and oxygen represents the most sustainable solution.
These users have seized the opportunity to benefit from a reliable, controllable, continuous source of supply and much more. On-site self-sufficient generation of industrial gases means minimised environmental impact of process operations, increased safety for employees involved in its procedures (handling high pressure bottles and liquid nitrogen at -196o C can be eliminated). And, through these efficiencies, many organisations within growth industries are realising short-term payback on their investment in on-site generation systems.
Power consumption and product loss are significant factors in the supply of liquid nitrogen. Firstly, it requires a great deal of electrical energy to reach the liquefaction point down to -196o C. Also, the delivery procedure to point of use requires the nitrogen to be in liquid form for transportation before being converted back to gas once it’s on site. Inevitably, there is product loss during the transfer and ultimate storage process. Delivered nitrogen, stored on site in a cooled tank prior to use loses some of its volume to venting. This amounts on average to 0.3-3% of its volume each day, dependent on ambient temperatures and consumption demand. Also, if bottled gas is the supply method used, a certain volume of gas, depending on bottle size and final pressure, will be returned to the supplier following every single use. This loss can constitute up to 10% of the delivered gas.
To put the benefits of on-site generation into perspective, we need to look at the needs of specific applications and compare the issues of the traditional approach with the advantages and benefits offered by the latest innovative, on-site sourced solutions. That includes the logistics, product purity levels and overall cost efficiencies involved.
Within the food and beverage sector, and particularly so in breweries and packaging operations, nitrogen plays a vital role. It ranges from purging kegs and tanks to product bottling as well as Modified Atmospheric Packaging (MAP), with the aim of extending product shelf life and protecting stored goods.
The level of nitrogen purity for these operations is commonly optimum around 99.5%. The food safety grade set by the EU (2008/84/EC Purity criteria on food additives) is 99%.
The higher the purity level the greater the product cost. If the source is a liquid nitrogen tank or bottles, the purity level is usually fixed at 99.999 per cent, far higher than is needed and more expensive in terms of total cost of ownership. Furthermore, most of the nitrogen and oxygen uses are as a gas and not a liquid.
It is much the same story for other industrial applications. For instance, in fibre laser cutting processes, where nitrogen is often critically employed as an assist gas which is usually delivered from high pressure cylinder packs. These packs are ten times more expensive in terms of energy requirement than on-site production of the gas. Similarly, the electronics industries employ nitrogen in many roles, primarily as a cooling and blanketing medium in semiconductor, electronics and instruments manufacture. Other major users are on oil, gas, chemical and petrochemical processing sites utilising nitrogen for blanketing to prevent explosive atmospheres in tanks of fuel or chemicals. None of these applications require high purity nitrogen to prevent explosions or fires. Dependent on the minimum oxygen concentration of the flammable substance, a purity level between 95-99% per cent is quite sufficient. What is more, the remote location of some operating sites makes a reliable, continuous, independent supply a necessity in order to maintain process operations.
The primary benefit of generating on site is that the optimum purity level can be selected by the user. The same constraints for nitrogen apply equally to the need for oxygen generation in applications such as anaerobic digestion for biogas production. In this case, a small quantity of oxygen is dosed into anaerobic tanks to decrease the amount of hydrogen sulphide present.
In clean water treatment plants, oxygen is converted into ozone then used to decrease the levels of pesticides. Delivered liquid oxygen is often too pure resulting in an additional energy requirement for mixing in a supply of compressed air to compensate.
There is a growing market for promoting aquaculture growth in fish farms. A constant, reliable supply of oxygen can improve feed conversion rates, prevent hypoxia and maximise production. Many of these sites find deliveries to be logistically challenging, and far from cost effective. They accept that the best option is to generate it on-site.
The cost of delivered liquid nitrogen can range from 10p per m3 to 30p per m3. For bottles, it’s from 60p to £3 per m3. In stark contrast, the product cost of generating nitrogen on site can be as little as 2.5p per m3, averaging at 4.5p, and costing no more than 9p per m3 depending on purity, flow and pressure requirements. Users of on-site systems not only report increased levels of productivity, but also the outstanding cost benefit derived from the installation regardless of location. At start up, many of these growth industries may need capital equipment investment for industrial gas supplies. Whether this funding is designated for liquid storage facilities or for on-site generation equipment, when relative costs are reviewed, the strongest case by far tends to be for on-site generation solutions. For industrial gas users who find themselves without the capex to make the initial investment for their own on-site production, finance deals are becoming more readily available. By switching to a financed on-site solution, many companies will start to save from day one whilst being more productive, profitable and self-sufficient.
In a typical on-site nitrogen or oxygen generation system, an energy-efficient compressor’s dry air output feeds a plug-and-play gas generator, a buffer tank, then there is a separate storage receiver for nitrogen at the required purity level.
The generator’s working principle is based on Pressure Swing Absorption (PSA) technology, whereby a carbon molecular sieve (CMS) separates nitrogen molecules from the compressed air while oxygen, CO2, water vapour and other gases are adsorbed. The result is a guaranteed and continuous supply of nitrogen to feed the process operation.
On-site generation technology is continually evolving to provide innovative features that contribute to energy efficiency and increased productivity, none more so than the principle of Cycle Time Modulation (CTM). It is a control algorithm regarded as the ultimate energy saver in PSA technology.
When demand for nitrogen from a generator decreases or there are changes in the ambient conditions, a control algorithm automatically changes the PSA cycle length to produce nitrogen at the most efficient setting. This results in a longer cycle length which confers a number of benefits, such as the reduction of air consumption in low-load conditions, thereby maximising energy savings.
There is also less wear on valves as there are fewer cycle changes. That in turn, leads to increased time between service intervals and compensation for varying conditions. This allows much more flexibility in the optimisation procedure. In the event that nitrogen demand drops to zero, the machine adopts standby mode, therefore consuming no air or energy.
A nitrogen generator’s air factor defines the compressed air volume required to obtain a specific nitrogen flow. As a leader in the design, development and manufacture of nitrogen generators, Atlas Copco’s latest NGP+ units’ key feature is their industry-low air factor which translates into low total cost of ownership, offering up to 50 per cent reduction in running costs compared to other nitrogen generators. Atlas Copco continually gathers and analyses data from applications around the world to progress industrial gas processes. As a result, it is positioned to help UK manufacturers improve their productivity, processes, energy efficiency, and their bottom line.
A newly installed and highly innovative compressed air and on-site nitrogen generation system from Atlas Copco is providing a key driver in the Outstanding Brewing Company’s on-going commitment to enhance its product quality, productivity and growth. In keeping with the brewery industry at large, the Salford-based company has to rely on a continuous supply of compressed air and nitrogen as a critical element at every stage of the production process: right from purging kegs and tanks, oxygenating beer to promote fermentation, right through to filling, bottling, labelling and packaging the final product. An energy-efficient Atlas Copco rotary screw compressor feeds a plug-and-play NGP+ generator that employs Pressure Swing Absorption (PSA) technology and is fully automated and monitored, with an oxygen sensor as standard. A special off-spec check feature means that nitrogen with a purity that is under the specified threshold cannot reach the sensitive process application.
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