Compressed Natural Gas Drying
Using compressed natural gas (CNG) for vehicle fuel is a growing phenomenon in the US. The high cost of gasoline is not the only driving force for CNG growth. New natural gas sources including the massive amounts of natural gas reserves found in the various shale formations all over the US has fueled the CNG growth. CNG stations are being built for many applications near the well sites of Marcellus, Utica, Bakken, Barnett, and Haynesville shale formations. Natural gas producers are converting their fleet trucks to CNG as well as converting their drilling rigs from diesel to CNG. CNG stations are being built for various fleets including school buses, police cars and mass transit buses.
Another natural gas supply source are landfills. Landfill owners are collecting their natural gas from the landfills and are cleaning the gas to pipeline quality levels. Then they are building CNG stations at the landfill and converting their garbage trucks to run off of CNG. This has many economic benefits and also social benefits as the CNG trucks run much cleaner and quieter in residential neighborhoods.
When compressing natural gas for vehicle fuel it is very important to dry the gas and remove as much water as possible. Car engines do not run well with water mixed in the fuel. During cold weather months the gas lines in CNG vehicles will freeze up and the vehicle will be useless unless the CNG gas is dry. Typically a dryer is used prior to compressing the gas at the CNG station. These dryers are built with a single tower or sometimes a two tower design. Inside the towers is CNG dryer desiccant that will remove moisture out of the gas stream.
A CNG dryer is a necessary component to a CNG compressor station. A dryer may be a single tower where the desiccant needs to be replaced manually. When the desiccant becomes saturated it needs to be removed and replaced with new, dry CNG dryer desiccant usually every month or so depending on dryer usage. This is the least expensive way of drying low volume type applications. A second type of dryer is a single tower heat regenerative dryer. This dryer is filled with desiccant and the desiccant can be regenerated overnight with an electric heater and blower that is built onto the dryer skid. With this type, the desiccant typically needs to be replaced yearly due to the desiccant thermally aging with the frequency of heating and cooling.
Per the CNG station and vehicle safety standards under ISO 15403:2000 (E): “The single most important safety requirement of compressed natural gas (CNG) is a very low dew point temperature to preclude the formation of liquid water at any time. Liquid water is the precursor to the formation of corrosive compounds through combination with components in natural gas, namely carbon dioxide and hydrogen sulfide. The combination of corrosive agents, and the pressure cycling, caused by fuel consumption and subsequent refilling of the fuel storage container, can result in crack growth in metals and ultimately damage and failure. Also liquid water itself can be detrimental as it may cause blockages, both liquid and solid, in the fuel system.”
Drying Media – Molecular Sieve 4A
CNG gas typically needs to be dried to a minimum -100 degrees F dew point. Depending on the climate this means that the gas has to contain less than a ½ PPM of water. To dry to this level we recommend our mSORB® family of Molecular Sieve’s typically a 4A.
Contact Gary Byers, Business Development Interra Global. Gary has extensive experience in the air and gas drying industry. He has worked in the air and gas drying industry for over 15 years. Contact Gary for your application questions.