rare gas purifier technology

Many industrial and scientific processes require a supply of very high purity gas to operate correctly. Examples of this include:

• Heat treatment, brazing or welding of metals in a high-purity inert atmosphere
• Manufacture of semi-conductors
• Analytical chemistry applications such as gas chromatography, and arc/spark optical emission spectroscopy.

Variability in the quality of gas, or lack of availability of the required grade of gas, has lead to the development of point-of-use gas purifiers able to provide a final purification of the gas stream immediately before it enters the process.

TYPES OF PURIFIER

There are a variety of rare gas purifiers available on the market. Some remove a specific contaminant while others remove multiple contaminants from your gas stream. In order to choose the right gas purifier for your application, there are several factors you need to consider. These include contaminants potentially present in your gas stream, gas purity levels required, flow and pressure limitations, space availability, and desired convenience when replacing consumables and spares. 

A common type of gas purifier is the "Getter purifier" - so-called because the gas is passed over a material called a getter, which attracts impurities from the gas stream. Getters can act in three distinct modes:

1. Absorption:  the molecules of the impurity enter the getter material and are physically trapped within it.
2. Adsorption: the molecules of the impurity "stick" to the surface of the getter material. For the getter to have a useful life it needs to be in a special granulated form which has a very large surface area.
3. Chemical reaction: the molecules of the impurity react with the getter and are chemically bound to it or converted to other molecules which are then absorbed by a subsequent getter.

Getter purifiers are used for purifying rare gases such as argon, helium, krypton, and xenon, as well as common gases such as nitrogen and hydrogen. 

One particular type of getter purifier uses heated getters. The most common application for heated getter purifiers is rare gases since it can remove nitrogen, hydrogen and hydrocarbons. 

GETTER MATERIALS

Metals in Group IV of the periodic table (titanium, zirconium, hafnium) are particularly well suited as getter materials. For more sophisticated applications, tantalum, niobium, thorium and many other materials have been successfully used. 

The amount of gas that a getter can absorb is often referred to as its “getter capacity”. In more scientific terms, it is the number of atoms or molecules of the contaminating species bound up inside the material. Getter capacity is affected by temperature since diffusion rates of the surface-bound gas atoms in the bulk of the getter material increase with temperature. This helps keep the getter surface active continuously and getter capacity rises for gases that bind only to the surface due to chemical reactions. Adsorption also continues for longer periods of time until saturation. 

EXAMPLE OF PURIFIER SYSTEMS

One of the most popular point-of-use rare gas purifiers is Sircal’s MP-2000, which is an example of the heated getter type purifier. As described above, it works by passing the gas through a series of tubes containing chemicals that will remove various contaminants and impurities from the gas stream.

In the case of this purifier, the arrangement is: 

• First the gas is passed through a tube containing ultra-pure titanium in a specific granulated form which heated to around 700°C. This removes oxygen and nitrogen by chemical reaction.

• The gas then passes through a tube containing specially formulated copper oxide pellets, which will undergo an oxidation reaction with any hydrogen and hydrocarbons to produce carbon dioxide and water vapour. 

• Finally, the residual carbon dioxide and water vapour are removed by passing the gas through a tube containing a specially formulated molecular sieve material. 

GETTER EXHAUSTION 

With use, the titanium and copper oxide will become exhausted and the purification process will become inefficient. The tubes then need to be replaced to maintain the efficiency of the purifier. The lifespan of the tubes will vary depending on the quality of the gas you are using, and the amount of use.

Typical contaminants such as moisture, oxygen and hydrocarbons can cause many problems with GC columns and detectors. Capillary and packed columns can degrade when exposed to moisture or oxygen, particularly at high temperatures. Detector performance may also be compromised, since the detector can actually “see” the contaminants resulting in spikes, baseline noise, and drift.

Typical signs of tube exhaustion are: 

Chromatography:

• Abnormal peaks
• Elevated concentrations of nitrogen and carbon on your spectra.

Spark emission spectrometers: 

• Poor quality or “white” spark marks
• Erratic results and loss of precision

 
Replacement of the getter tubes is indicated in these circumstances. Replacement is usually a straightforward process easily performed by the user, and GP Scientific can supply high quality like-for-like replacements to keep your purifier running at peak performance.