Producing a specialty gas can be a complicated process. It requires knowledge of all grades of gas, of equipment ranging from the simplest component to the most technically advanced analyzer, and diligence to quality control. Not every company will want to assume the tasks and responsibilities required to produce the grades of gases that go beyond industrial. But a facility that handles the gas with quality equipment, processes, and analysis makes all the difference when producing high purity specialty gases that meet the exacting standards the customer requires.
Know Your Source
Before a gas with any level of purity can be produced, you have to know the purity of the bulk source gases. Most facilities that produce specialty gas grades of pure gases (argon, helium, and nitrogen) use the same bulk gas source that they use to fill industrial gas orders. Typically the product being delivered in bulk meets at least 5.0 grade of purity. There is no guarantee of this, however, so the actual purity needs to be confirmed. This can be done by first requesting a specific analysis certificate from the supplier with the next delivery.
Second, fill cylinders with the gases you want to produce, after properly prepping cylinders and checking the system for leaks. Then have them independently tested by an outside lab. The results are then compared to the certificate of analysis (COA) of the source gas.
Thus the purity level and the facility’s equipment dictate what grades can be filled at that plant, or if additional purification equipment is necessary.
One gas that typically will not come in at 5.0 grade or higher is oxygen. Oxygen that is supplied from a bulk supplier does not meet either 5.0 grade or higher, nor does it meet hydrocarbonfree grades. To get high purity oxygen a supply source for hydrocarbonfree oxygen needs to be contacted and a dedicated bulk tank and pumping system needs to be installed.
An obvious yet important fact about gasproducing facilities is that in order to fill high purity gases, they have to have the space for and be setup with the specialized equipment needed. This equipment includes piping, vacuum systems, bakeout area, an analytical lab, and cylinder filling manifolds that will meet the high purity demand.
The piping used in specialty gas filling matters as does all the associated connections. Piping should be orbital welded, as should the materials of construction for the filling equipment. Eliminate as many mechanical connections as possible and use face seal fittings for any necessary connections. Rigid SS tubing is preferred over flexible hoses for pigtails. Quickconnect fittings for the cylinder connections should not be used as these fittings do not have a good seal during the fill process. The fill equipment and piping should be leak tested periodically to ensure there are no leaks.
Next up in the process is the vacuum system. Besides having a high integrity, leakfree filling system, it is also important to have a vacuum system that can pull vacuum levels down to low micron levels. The vacuum systems can make or break a fill operation when trying to achieve high purity gases. The vacuum pump should be able to pull a vacuum down to five microns or less when dead ended. Vacuums should be read in inches in the beginning to make sure a vacuum has started, and then in microns as the vacuum continues to a low level. More than one vacuum will be necessary to produce these higher grades. At least two and maybe three vacuums should be performed; to break the vacuum pressure in between vacuums, use positive pressure from the same gas the cylinder will be filled with later.
In order for the vacuum system to pull a vacuum to less than five microns when dead ended, the right valves are needed. A micron gauge is necessary to ensure a proper vacuum is achieved when evacuating cylinders.The valves used for the equipment must be capable of holding deep vacuums.
Some facilities should consider using residual pressure valves (RPV) for all pure gases. This is especially important if a company is just starting to set up cylinders for 5.0 grades. These companies should consider using RPV valves on all their cylinders. This helps to eliminate the need to bakeout cylinders that come back with the valves open. It also means the vent and vacuum process can be eliminated and the cylinders can just be topped off.
As with the piping, when the gas moves on to the sample system phase, the lines themselves should be specific to this task. Sample lines should be run in 1/8” SS tubing (seamless). The internal volume inside this tubing is small enough to easily purge out the lines but more than large enough for the flow required for instruments. Use high purity SS diaphragm regulators specified for specialty gas service. A constant purge of gas 24/7 (typically nitrogen from the bulk tank) is required for all low ppm analyzers.
The sample system should allow for a method to purge out the previous sample and any upsets in the system that may occur when the next sample gas is connected. Keeping a back purge on the sample line when not in use would be highly effective to keep air and moisture out of the line. As with piping, keep the number of mechanical fittings to a minimum to avoid leaks.
Ready for Analysis
At this stage, the gas is ready for analysis. The types of analytical instruments that can be used are numerous and make up an important section of the facility. The actual space required depends upon how many instruments and gas chromatographs are being used. For labs performing just the analysis for pure gases, a room size of 15’x 20’ may be suitable.
For the past 10 years or more, most of the new analytical instrument models offer a platform system for the unit. The platform system utilizes a PLC that is used to control parameters of testing such as flow rate, pressure, and sometimes temperature. This has made recent instrumentation highly reliable and provides for test repeatability. This has reults in a major improvement in anylsis testing.
For 5.0 grades of gases a low ppm analyzer is required for each of the following contaminants: oxygen, moisture, total hydrocarbons, and trace nitrogen (for argon and helium gases). If a facility is test for 6.0 grades, then a DID type gas chromatograph or mass spectrometer would be necessary.
When analyzing calibration gases, analysis results are only as good as the calibration standards being used. Because these standards can vary greatly from one supplier to another, a safe bet is to use an ISO 17025 certified lab. These labs follow SOPs in producing these gases. A visit to the lab to see what kind of instrumentation they use and how organized they are with their testing is also a good idea.
Once the gas has been analyzed, it is time to bring in the cylinders .The best cylinder candidates for 5.0 pure gases are cylinders that have already been in inert service for the past three years or more. Even if they were previously used for industrial gases they can be converted to handle specialty gases. These cylinders already have been processed a number of times, and each time a cylinder goes through the fill process (vent, vacuum, and fill) the cylinder becomes less and less contaminated. This means that whatever moisture or other contaminants might once have been in the pores of the cylinder shell have been removed over time through purging and evacuation.
As explained above, new cylinders are not good candidates for high purity gases. The same is true if cylinders go through the hydrotest process. Do not put cylinders back into high purity service if they were hydrotested. Any cylinders that come back empty and have the cylinder valve left open also must be baked out prior to putting back into high purity service (unless they have RPV valves as previously mentioned). Bakeout systems must be capable of removing moisture in the cylinders to levels of 0.5 ppm or less of moisture.
Cylinders that do come back empty for refill will need to be properly evacuated and prepared prior to filling. Typically the cylinders are vented, evacuated to levels of 1,000 microns or less, and then the vacuum is broken with positive pressure using the same gas that the cylinder is to be filled with. The process is repeated two or three times to ensure any residual contaminants are removed before filling.
When More Is Needed
While the basic facility set-up described will satisfy most specialty gas requirements, occasionally there is a need for more equipment and processes. If the source of supply gas does not meet the necessary grade, it may still be possible to purify the gas so that it does meet the grade. Units are available for removal of moisture and oxygen. The user needs to make sure the units have the capacity to handle the flow requirements. If using a high pressure pump for argon or nitrogen the purifier would then need to exceed the pump output capacity.
If looking to purify helium, a cryogenic trap is available to freeze out contaminants from industrial grades of 4.8 to levels of 6.0 purity. Keep in mind that besides the purifier equipment needed to produce these gases, additional investments in analytical instruments might be necessary. The typically existing cylinder fill equipment may not be able to produce levels desired, especially if grades of 6.0 or higher are desired.
Consistently producing high purity gases can be achieved with the right equipment for cylinder prep, filling, sampling set up, and analysis. Any issues with these key components can lead to a batch of cylinders that fail to meet specifications. Poor vacuums pulled on cylinders, leaks in the fill system that aspirate air, sample gas improperly purged, and instruments incorrectly calibrated are just a few of the issues that facilities must guard against. These issues all lead to a loss of product and time. However, if many of the recommendations in this article are followed then the success of producing these gases is highly increased. Certainly having a strong quality assurance program endorsed by management is essential, as is having all personnel involved in the operation providing careful attention to details when following procedures. All of this can lead to a successful lab in producing these gases.