Purification technology for the independent distributor

The process, of course, begins with a request from the customer for a desired purity level of the product to be delivered. At this point, the supplier evaluates his/her present production capabilities related to purity, and, if the production technology is inadequate to perform as requested, a financial decision concerning capital investment is made based on the potential sales (and profit) to be realized. This sounds like the typical return on investment analysis to which we are all accustomed. But, like so many scenarios in our business, one size does not fit all. Details to make a good purification decision include:

1) What is the guaranteed product specification necessary to meet the customer’s needs?

2) Is the product needed to satisfy the customer available to you delivered in bulk with a certificate of analysis?

3) Do you have on board the analytical technology (including trained personnel) to verify the customer’s specification requirements have been met?

4) Do you have appropriate cylinder preparation technology that prevents the cylinder from becoming a source of re-contamination during the repackaging process?

5) Do you have appropriate repackaging technology that prevents re-contamination during the cylinder filling process?

Nothing is as important as understanding exactly what your customer will expect when you advertise a guaranteed product specification delivered. Simply put, what purity specification will you guarantee as the product exits the cylinder on its way to the customer’s application? This guarantee is usually expressed in a well written catalog page. For illustration, Figure 1 is an excerpt from the Purity Plus® Specialty Gases catalog describing various grades of helium and is fairly representative of our industry.

Purity Plus® is the registered trademark of a complete line of specialty gases and equipment offered to Independent Welding Distributors Cooperative (IWDC) members for re-sale and produced at various member company locations in accordance with rigid quality specifications. In reading the catalog page you see that the helium products are listed in ascending order relating to assayed purity. The assayed purity is expressed as a two digit number after PurityPlus®, for example PurityPlus® 6.0. The 6.0 indicated that there are six nines in the minimum assayed purity. So 6.0 would equal a helium minimum assay of 99.9999 percent and 5.5 would equal a minimum assay of 99.9995 percent. The difference between the assay and 100 percent would equal the cumulative sum of all contaminants of interest allowed in the helium. Performing the appropriate math and converting percent to ppm would yield a maximum allowable contaminant level of one ppm for grade 6.0 product and five ppm for grade 5.5 product. The lowest grade purity listed on this catalog page is 4.7 which equates to 99.997 percent minimum helium assay and a total combined impurity level allowed of 30 ppm. In addition to the guaranteed minimum assay of the gas is the requirement that individual impurities listed do not exceed their respective maximum allowable concentrations. So the possibility exists where a minimum assay requirement may be met for a particular grade of product, but the product is out of specification because a single contaminant of interest was over its maximum allowable level.

Of most importance to note is that the largest contaminants contributing to the overall purity specification are oxygen, nitrogen, moisture, and hydrocarbons. Since we live in a sea of air, oxygen and nitrogen can easily be added to the gases during the fill process due to inboard leaks in the fill system or inadequate vacuum technology. Moisture is typically contributed via the cylinderitself as a result of inadequate cylinder preparation. Hydrocarbons generally come from oil lubricated compressors or vacuum pump oil back-streaming.

It is not uncommon for a distributor to have available helium delivered as bulk in a gas tube trailer that will meet the specification illustrated as PurityPlus® 5.0, but it may be at a premium price, especially if a certificate of analysis is required on the actual load delivered. However, if available, it may be worth the extra cost as you may find that the purity actually approaches grade 5.5. This is due to the fact that most helium transfill facilities have a large percentage of their load based on liquid helium deliveries to hospitals (MRI applications) or research facilities (cooling for super conductors or magnets in particle accelerators). In the liquid transfill process much gas flash loss will occur if the operation does not reclaim the loss.

The best way to maximize profits is to collect the flash gas and pump it into tube trailers for sale. The serendipity here is that the distributor now has ultra high purity helium available to them for the asking, as this flash gas is very pure. The distributor need only re-package the helium without re-contamination during the process.

The decision to enhance product purity can now begin but a few assumptions will be made here. Simply put, the distributor has already addressed any deficiencies in the fill system and cylinder prep operation and that basic analytical capabilities are on-board. At a minimum, analyzers must be in place to monitor oxygen, moisture, and total hydrocarbons. These are the most important contaminants of interest in ultra high purity gases such as helium, argon, and nitrogen. These are impurities that will increase from their starting values in the bulk material if deficiencies in the transfill process have not been addressed sufficiently.

Types of purifiers
Purifiers can be described by five basic approaches. They are:

1) Ambient getters

2) Hot getters

3) Specific catalytic getters

4) Ambient adsorbers

5) Cryogenic adsorbers

Ambient getters are comprised of a suitable vessel that contains a media, containing a chemical catalyst that effects the chemical reaction between the media and the contaminant to be removed. As the name implies, ambient getters operate at room temperature (Figure 2).

The most common application for this type of getter is for the removal of low levels of oxygen from inert gases such as helium, argon, and nitrogen (deoxo unit). The catalyst effects the combination of oxygen in the gas stream with nickel in the media to form nickel oxide. Oxygen removal continues until all available nickel is used in the reaction, at which time oxygen will break through the purifier indicating the need for bed regeneration. Regeneration is accomplished by passing a regeneration gas, usually containing carbon monoxide or hydrogen, at a sufficiently high temperature to combine with the oxygen component of the nickel oxide and forming carbon dioxide or water.

These reaction products can be vented and the nickel is available for oxygen removal once again. Similar systems can be filled with other getter materials to remove other contaminants. Some common applications are sodasorb for carbon dioxide removal and hopcalite for carbon monoxide removal.

Hot getters use the same type of catalyzed reaction approach but, as the name implies, the reaction only occurs at elevated temperatures (Figure 3).

As illustrated, the unit is equipped with heaters to raise the temperature sufficiently to catalyze the reaction. A common application for this approach is to remove hydrocarbons from air or oxygen. Specific catalytic getters, sometimes called rare metal getters, use media and catalysts for very specific purposes, such as removing nitrogen from argon - a difficult and expensive task. Removal of methane from inert gases can be similarly accomplished. The design approach would be similar to that in Figure 3.

Ambient adsorbers are very simple in design and identical to that in Figure 1. The only difference is that the bed media is not chemically active. The most common media is molecular sieve and is ideal for adsorbing water from an inert gas stream. When the bed soaks up all the water it can hold, the bed is regenerated simply by passing heated gas through the system until the media is sufficiently dry and ready for re-use.

Cryogenic adsorbers are used most commonly to remove many contaminants simultaneously and generally enhance the purity of inert gases. The most common application is to improve the assay of helium (Figure 4).

The bed media is usually activated charcoal or silica-gel, and the bed is kept cold using liquid nitrogen as the refrigerant. The concept is that all the impurities found in helium will remain adsorbed on the bed at liquid nitrogen temperatures and the resultant purified helium will pass through and to the cylinder fill line. The advantage here is that those contaminants that do not react readily with a chemical media (nitrogen, argon, krypton, xenon, and neon) will be detained for a sufficient period of time to purify a sufficient volume of helium. The desired assay is typically 6.0 or better using this type of system. The purified stream needs to be monitored for bed break-through indicating that bed regeneration is necessary. Regeneration is accomplished by warming up the bed and flowing hot gas through the bed until all contaminants have been released from the bed media.

Making the right call
Matching the impurities to be removed with the gas to be purified, you can decide on which purification technologies suit your needs.

After careful consideration of the available technologies, the decision becomes one of financial feasibility. For budget purposes, ambient getters, hot getters, and ambient adsorbers designed for point-of-use installation in a cylinder fill plant will cost about $15,000 each.

These point of use purifiers will accomplish the removal of oxygen, water, hydrocarbons, carbon monoxide, and carbon dioxide. At least two units will be necessary, each with a different bed media. Catalytic getters, depending on the application can easily run into $30,000 each. This would be a typical application for nitrogen removal from argon or helium. A medium sized cryogenic adsorber typical of helium purification to grade 6.0 or better will cost approximately $75,000. This type of unit will also purify hydrogen, but considerations must be taken into account for the explosive potential of hydrogen.

Analytical instrumentation additions to your already existing specialty gas laboratory will require $50-65,000 depending on your present capabilities. These additions are usually in the form of a sophisticated gas chromatograph equipped with a discharge ionization detector. These detectors have lower detection limits below 100 ppb, the level needed for certification at these purity levels.

Before moving forward with a purification expansion, it is highly beneficial to contract the services of a technical consultant to guide you through the process. A detailed investigation into the capabilities on hand, the availability of incoming bulk product of sufficient starting purities, and the potential business to be gained as a result of the expansion is of utmost importance. Your consultant will also assist in the development of purification SOPs and the analytical methods development necessary for quality control.