Nitrogen on demand

Nitrogen is widely used in the chemical process industries — for inerting, blanketing, purging and a host of other applications where it can enhance plant safety. The food and drink industry is another major consumer, again mainly for inerting purposes in the form of modified atmosphere packaging (MAP) of perishable foodstuffs. And even outside the process industries, this basically inert, yet versatile gas is finding increased use in applications such as replacing compressed air for vehicle tyre filling, and even as an aid to beer dispensing in pubs and bars.

At its Etten-Leur plant in Holland, Parker Gas Separation — a division of Parker Hannifin — manufactures advanced hollow-fibre membrane modules to separate nitrogen and oxygen from air. These modules are then assembled into a variety of nitrogen generator units that can be located on site and eliminate the need for regular supplies of gas in bottles, cylinders or tanks.

One of the few manufacturers of hollow-fibre membranes in the world for these applications, Parker has made significant investment in its production processes since 2001 and now says it delivers the most permeable membrane in the world and also one of the most robust.

Unlike other membranes on the market, which are manufactured in two stages — a porous substrate is produced which is then coated — Parker’s HiFluxx nitrogen membrane is produced in one step, the outer coating being formed by carefully controlled crystallisation of the substrate polymer medium, polyphenyleneoxide (PPO).

According to Hans Pluimes, global product manager, ‘one factor affecting both the design of the product and its manufacturer, is the thickness of this gas separation layer in the hollow fibres of the membrane. The function of the membrane is such that if the thickness of this layer is halved, then the capacity of the module is doubled — assuming gas quality is constant.’

Parker Hannifin’s research team had a more ambitious goal, however, and aimed initially at a three-fold improvement that would take the product’s thickness down from 0.25µm to 0.08µm. The production fibre spinning process has a number of critical parameters, such as temperature and flow, but natural variations and batch differences are other important factors.

Parker adopted an unusual combination of statistical techniques and system and control technology to accurately map the existing process. The range within which the process was stable was then assessed. Then the sensitivity of the process settings was established. A process of ‘design of experiments’ then led to a breakthrough improvement that has now resulted in a membrane with a separation layer of just 40nanometers (0.04µm) thick.

The process involves first dissolving the PPO in methanol to produce a homogenous saturated solution. ‘A critical step here is filtration of the polymer before the extrusion and spinning stages,’ says Pluimes. Mixing technology is also critical to ensure a homogenous solution.

The saturated solution then flows through tubular mandrels in the spinning heads, where evaporation of the solvent from the surfaces of the fibre creates a thin skin on the outside. This is the gas separation layer through which oxygen, water vapour and helium will selectively diffuse, leaving a nitrogen-rich retentate as the product. The finished fibres are spun out into a methanol bath (unlike other processes that use water baths at this stage) and then drawn into ‘bundles’ with 2400 fibres in each. The bundles are ‘aged’ for up to eight weeks and then cut to the required lengths.

Such is the improvement in the production process that, whereas four years ago there were 16 spinning heads in use, Hans Wolters, business development manager, says ‘now we only need four to produce the same air separation capacity.’ The Etten-Leur plant is in operation 24 hours day, five days a week.

Quality control of the final fibre is critical, so every 12 hours three test modules are made up to see if the production process is on spec. Every module manufactured has its performance rating tested and each is fully traceable.

The process is fully automated, with only two operators involved. With large amounts of methanol involved, the production floors are classed as Zone 1 hazardous areas. And here, of course, Parker has an in-built advantage in that it uses its own nitrogen generation modules for blanketing the process and the methanol storage tanks.

Parker’s complete range of standard nitrogen and mixed-gas generators covers a wide range of flow and purity requirements. Depending on the available compressed air pressure and the required nitrogen purity, available performances range from 0.1Nm3/h at 99.9% N2, up to over 1000Nm3/h at 95% N2.

The advantages of the higher permeability claimed for the HiFluxx membranes are explained by Wolters: ‘Competitive membranes can require operating pressures of up to 13barg, but because of our higher permeabilities, we can operate as low as 6barg.

This reduces the power consumption of the nitrogen generator system and also means that we can do without an air heater. All our competitors compensate for their lower permeabilities by preheating the air. Some 7% of the compressor installed power needs to be added for the air heater, and for every bar increase in compressed air pressure, operating costs will increase by 7%.’

As mentioned earlier, the applications for membrane-produced nitrogen are many and varied. Wolters says: ‘There is immense interest from oil and gas, because of our low pressure, low temperature concept. We are expecting 30 to 40% increase in business from this market.’

Here, inert gas systems are used to generate oxygen-depleted air needed for many processes such as gas-seal compression, pipeline purging, blanketing storage tanks or purging flare systems. In food industry applications, however, particularly in packaging, Pluimes says that ‘up to 95% of operators do not know that you can make your own nitrogen in the factory.’ To some extent this lack of knowledge is understandable. As Wolters points out, about 70% of the current applications for Parker’s products did not exist a few years ago.

Indeed, such has been the rate of development in membrane production and performance that new opportunities seem to be opening up almost daily. Its latest large diameter module, the HiFluxx ST15020, for example, could soon be put to use as the fuel-tank inerting system on the new Airbus A380. And in Budapest, a Parker generator even protects Hungary’s crown jewels.

More mundane, but potentially far more profitable, is the growing use of nitrogen for filling vehicle tyres. The attraction here is that a car tyre is, in effect, itself a membrane through which oxygen can diffuse, deflating the tyre as it goes. But by replacing some of the oxygen (down to 6.5% concentration) with nitrogen, its partial pressure can be equalised to that of ambient air, so there is no driving force forcing the oxygen through the tyre wall.

Parker offers units such as the TyreSaver and TruckTyre Saver, now being marketed in the UK through Economatics. Regional manager, Stuart Stacey, says major tyre supplier, ATS, should soon be offering the service nationally.