Cutting the cost of nitrogen
September 1st 2006

Parker Filtration & Separation, part of the Parker Hannifin Filtration Division Europe, explains how the latest membrane technology is cutting the cost of producing nitrogen for industrial use.

Nitrogen is widely used throughout industry as a process gas for blanketing and purging and for preventing oxidisation during production operations and of finished products most notably foodstuffs to prolong shelf life.

Traditionally, nitrogen has been supplied either in cylinders or by tanker, with bulk storage silos being a common sight in many factories and production plants; indeed, these types of supply typically account for a significant percentage of the industrial market for nitrogen.

There are, however, a number of difficulties with these traditional methods of supply and storage including the need to install, fund and maintain often expensive capital equipment and distribution systems for bulk supply; the problems of storing large numbers of gas cylinders and the rising cost of both drop charges and each cubic metre of gas when supplied in relatively small volumes; finally both bulk storage and the use of gas cylinders leaves end users dependent on third parties, and therefore exposed to price fluctuations and potential disruption in supply.

To address these issues and offer an alternative method of supplying nitrogen, a variety of nitrogen generation systems are available that allow end users to produce nitrogen at the point of use. Typically, these systems can be connected to an house compressed air system or standalone compressor and need only a power source to operate and are capable of delivering a constant supply of nitrogen for many different applications, ranging from modified air packaging to pipe purging, chemical blanketing and fire prevention.

Although nitrogen generation systems are available from a number of suppliers each operates on similar principles. For example, those manufactured by Parker are connected to a compressor, which is used to provide a feed of pressurised atmospheric air to bundles of asymmetric hollow fibre membranes contained in modular cylinders. Each hollow fibre is just 0.5mm in diameter and is spun from Parker patented Polyphenylene Oxide (PPO).

The hollow fibres are essentially porous, with a 40 Nanometer thick skin which selectively diffuses oxygen. This allows molecules of oxygen and nitrogen in the pressurised air feed to be separated, with the molecules of oxygen, water vapour and trace gasses being drawn through the walls of the fibres as the permeate and the nitrogen molecules passing along the length of each fibre as the retentate.

This process is extremely efficient and effectively produces two gas streams, both of which can subsequently be used in industrial processes, with the nitrogen stream being up to 99.9% pure at a flow rate of 0.1Nm3/hour or 95% pure at a flow rate of over 1000 Nm3/hour.

The construction of the Parker hollow fibre membranes offer a number of advantages compared with competing membranes. In particular, they can operate at lower pressures of 7 bar, as opposed to around 12 bar, yet produce the same amount of nitrogen.

High operating pressures create a number of potential problems, leading to greater compressor noise levels, elevated air temperatures and increased energy consumption with, for example, every rise of one bar in pressure causing an increase in operating costs of around 7%. In addition, Parker membranes also have less surface area to function efficiently, making the size of the overall system smaller than competitive systems.

Perhaps as importantly, many of the hollow fibre membranes used to date have been extremely thin, making them difficult to manufacture and prone to failure, often caused by contamination or damage from impurities in the air feed stream.

The PPO based hollow fibre membranes manufactured by Parker overcome many of these problems. In particular, they are far more robust and have an extremely high level of permeability, which is between 80 and 100 times greater than that of other materials.

As a result, operating pressures can be reduced to around 7 bar, which is well within the capability of a standard and therefore low cost compressor. In turn, this enables operating costs to be minimised, with less noise, heat and condensate being produced and typically around 20% less power being used when compared with a nitrogen generation system running at 12 bar.

Similarly, these high efficiency fibres are less prone to contamination or damage, while overall system size can be reduced, often by as much as 50%. The Parker hollow fibre membranes have a projected operating life of over ten years and that they can be aged, as their molecular strings relax, in the factory in just eight weeks; by comparison, the fibres used in many other nitrogen generation systems can take anything up to five years to age, so that the performance of a machine in the field gradually declines over time.

Although direct comparisons with other technologies are difficult due to the large number of variables, it is realistic to expect that even allowing for initial capital investment the cost per Nm3 of gas will be up to 25% lower when using a nitrogen generation system instead of liquid tank storage; after depreciation, the cost per Nm3 falls even further by over 50%.

It should also be noted that in addition to significant cost savings an on-site nitrogen generation system offers the benefits of security of supply, minimal use of space as most systems generally have a small footprint of a few square metres, and in many applications the bonus of a free and enriched oxygen stream that can be used, for example, to improve combustion processes.

The latest nitrogen generation systems can be used as stand-alone units, be multiplexed together for high volume applications or be used to supplement existing bulk storage facilities.

More articles from Parker Filtration: