ON-SITE generation of nitrogen is used for industrial processes as diverse as winemaking; beer dispensing; extended life packaging for foodstuffs; plastics void production; tyre inflation; lasers; and wave soldering, to name but a few.
This convenient, safe and inexhaustible supply of process gas is also widely suitable for heat treatment of metals.
As industrial applications for nitrogen have expanded, the technology needed to produce it on-site has shrunk in size and complexity. This has helped make it highly affordable and manageable, even for smaller manufacturers and processors.
Older generators using PSA (Pressure Swing Adsorption) and membrane separation technology were usually very large. They were designed to support big customers with a high demand for nitrogen gas.
In the 1980s and 1990s, however, some gas companies realised there was an opportunity to advance the concept of on-site gas generation and develop small-scale systems that would replace traditional nitrogen supplies; i.e. compressed gas cylinders and liquid nitrogen dewars.
One of the leaders of this shift is domnick hunter , which produces MAXIGAS modular gas generators. These extract nitrogen from ordinary factory supplies of compressed air through advanced and compact carbon molecular sieve pressure swing technology for optimum purity and reliability of supply.
The big advantage of such on-site generation is that customers immediately see the benefits of a gas generator, compared with cylinders. They like the cost savings, convenience and increased safety this alternative provides, with OH&S benefits including eliminating the need to store and constantly handle heavy cylinders. It is also a bonus to productivity that production is not interrupted as cylinders are changed, and customers can purchase the gas generator outright if they wish, rather than having to rent it on a long- term rental contract.
One area where this switch to on-site gas generation has been pronounced is in heat treatment of metals. Metals and alloys can be heat treated to enhance their strength as well as resistance to wear and corrosion. These attributes are particularly important for the production of high quality parts at competitive prices. Several heat treatment techniques utilise nitrogen for blanketing to reduce oxidation and absorb hydrogen.
Applications now becoming more prevalent in Australia and internationally include:
Carburising and carbonitriding
A hardening process that involves heating in a controlled atmosphere furnace to the point where alloys absorb carbon and nitrogen. Controlled cooling produces the desired hardened surface characteristics. This controlled reaction normally occurs at around 950°C and uses a hydrocarbon such as natural gas or cracked methanol while nitrogen accelerates the absorption of carbon into the treated metal. The nitrogen to cracked methanol ratio is typically 50:50 or 40:60.
Tempering and annealing
These stress-relieving processes condition stainless steels, carbon steels and non-ferrous metals for further hardening processes. Metals are heated in a controlled atmosphere batch or continuous furnace to avoid oxidation. Nitrogen provides a suitably inert atmosphere that will help prevent exothermic reactions and dangerously overheated furnaces that would otherwise result in distorted components. A nitrogen, hydrogen or hydrocarbon gas mixture can also be used. Hydrogen acts as a reducing agent to ensure a bright surface, while carbon controls decarburisation.
This is an environmentally friendly and more easily controlled alternative to oil and salt baths. Primarily used to speed up cooling, it is widely used in vacuum furnaces but is suitable for all types of furnaces. Nitrogen, hydrogen, argon and helium are suitable gases.
Involves heating components above their transformation temperature, then quenching them in salt or oil baths or in a gas quenching treatment. This style of hardening process requires a protective atmosphere to prevent oxidation and decarburisation.
This process uses nitrogen to gas wipe hot-dip galvanized metals, which achieves an improved surface finish with greater uniformity of the galvanised coating. Nitrogen also minimises zinc oxide formation in the bath, which can cause irregularities.
Re-aligns the molecular structure of 'work hardened' materials to their 'normal' state to avoid differential hardening rates that cause distortion and premature component failure.
Nitrogen at 50ppm provides a blanket that prevents oxidation during slow heating to the normalising temperature of a particular metal. This means components do not require any secondary oxide removal operations.
Carried out in several stages, each sintering stage requires a particular atmosphere. In the first instance an oxidising atmosphere is necessary to remove lubricants. Then a reducing atmosphere is required for decarburising and a good sintered result. Finally a reduced oxygen atmosphere is required in the cooling stage to prevent oxidation and any dullness of the metal surface, so nitrogen gas provides the necessary atmosphere.
Manufacturing costs of composite materials such as kevlar and carbon fibre are high because of their long process times. There is a high oxidation rate at high temperatures; slow heating during carbonisation at 1000-1500°C and graphitisation heat treatments at temperatures up to 3000°C require protection against oxidation in an inert atmosphere to prevent fibres becoming too brittle.
In such applications, MAXIGAS is a cost-effective alternative to other nitrogen gas sources, with no on-going costs such as refills, order processing or delivery charges. It is an effective gas delivery system for applications that require high flow rates and pressure levels. It is also a safer alternative that eliminates manhandling of high-pressure cylinders or cryogenic gas tanks.
Production downtime is minimised due to the permanent availability of an on-demand nitrogen supply. MAXIGAS gives manufacturers increased control over flow rates and requires minimal maintenance. It can also bring valuable space saving advantages.
In one application involving a continuous casting furnace in Melbourne, the customer was able to dispense with the need for continuous supplies of multiple packs of gas cylinders by using a single MAXIGAS installation to generate high purity nitrogen from compressed air.
Using a carbon molecular sieve to separate oxygen from compressed air to produce clean, dry high purity nitrogen, the generator provides an uninterrupted, reliable and safe source of gas 24 hours a day for the Rautomead furnace of NVP Tooling.
The furnace houses graphite crucibles to melt alloys used to produce tapware and building products. Because oxygen would turn graphite into powder at high temperatures, continuous supplies of nitrogen as a protective gas are required to prevent deterioration and maintain high production standards.
NVP managing director Nick van Putten says he switched to on-site gas generation because it is more convenient and ultimately pays for itself.
"Previously I would get a phone call in the middle of the night saying the gas cylinders had run out again. We would have to lift in new ones and get the whole process under way once more. I thought, 'There has to be a better way'," Mr Van Putten said. The better way he selected was a MAXIGAS generator that can be set for gas purities between 97% to 99.999%.
"The lease costs on the generator are much the same as the costs for the gas cylinder rental and refill, but the big difference is that ultimately we will own it and the gas will become very cheap. On-site generation is very convenient too, and eliminates disruptions to production caused by running out of gas cylinder supply," Mr van Putten said.
New CMS technology - how it works
The latest on-site nitrogen generation technologies use carbon molecular sieve pressure swing (PSA) for optimum purity and reliability of supply of commonly used nitrogen.
This domnick hunter MAXIGAS nitrogen generation system - which can be set to supply nitrogen from 97% to 10 parts per million (99.999%) - incorporates a self-regeneration feature to minimise maintenance.
Being introduced to Australia as part of a global launch by domnick hunter (which operates in more than 80 countries) MAXIGAS generators are a proven technology, having been used in more than 10,000 installations worldwide.
MAXIGAS units are constructed in pairs of extruded aluminium columns filled with carbon molecular sieve (CMS) material.
Operating on the pressure swing adsorption principle (PSA), the two columns function alternately, with one side producing gas while the other regenerates itself.
The side of the unit being pressurised by compressed air produces a continuous stream of nitrogen, which passes through the CMS while oxygen and other trace cases are adsorbed by it.
The carbon molecular sieve differs from ordinary activated carbons in that it has a much narrower range of pore openings. This allows smaller molecules such as oxygen to penetrate the pores and be separated from the air stream. The larger molecules of nitrogen bypass the CMS and emerge as high purity gas. Purities are determined by the velocity at which the air passes through the CMS columns.
At a pre-set time, before the online bed is saturated with adsorbed gases, the system switches to regenerative mode, venting the contaminants from the CMS. As this happens, the second CMS bed comes online and takes over the separation process to ensure uninterrupted nitrogen production. An in-built oxygen analyser with alarm function ensures only gas of the required purity is delivered to the storage vessel.
CMS technology is state-of-the-art for purity of on-site gas production, and is largely maintenance-free in operation, provided it is protected against water, oil and oily gas vapours, which are the enemy of sieve and membrane technology alike.
Installation is simple and proven. The MAXIGAS design means that if users want more gas, they simply add extra banks of generators. The technology has won recognition in the Millennium Products Award introduced through the British Design Council by Prime Minister Tony Blair.
*The author of this article, John Davis, is Business Development Manager for domnick hunter Australia, which has doubled the size of its Australian headquarters to cope with expanding demand for its advanced technologies, including gas generation, breathing air purification, sterilisation of process liquids, water chillers, and compressed air treatment products.