Why on-site gas generation?
Dr Nicole R Pendini* reveals how on-site gas generation can help laboratories stay green, increase ROI and decrease OHS concerns.
Nitrogen is the most abundant uncombined element on Earth, comprising 78% of the Earth’s atmosphere. It is essential for life as the fourth most abundant element in the human body — primary in amino acids (proteins), RNA, DNA and energy (ATP).
There are several mechanisms by which nitrogen gas can be purified from the air and applications for which it can be used. The classical method of separating the main air gas (N2, oxygen 20.8% and argon 0.7%) is cryogenic separation (aka distillation or liquification). Ambient air is compressed and filtered to remove impurities. Air is cooled to remove CO2, trace hydrocarbons and water prior to liquification. In a rectification column of a cryogenic plant, air is further cooled to -190°C, where air gases are separated in the liquid form. N2 is extracted from the top of the column due to the lower boiling point and O2 is removed from the bottom of the column.
The major usage for these molecules is in the gaseous form, leading to wastage of pure gas during storage (transportation and evaporation). Boil-off is approximated at 0.2–5% per day (depending on insulation/external temperature), equating to wasted energy from gas–liquid–gas phases and safety risks throughout the process to the required point of use.
Common N2 cylinders hold the gas at 2200 psi (150 bar) and weigh 70 kg each. Due to the amount of gas required for various equipment and sites, cylinders are grouped in ‘man packs’ of 12 or 15 cylinders and can require specialist equipment for moving and storage. But on-site nitrogen gas generation has been commercially available for over 30 years in the form of membrane or pressure swing adsorption (PSA) systems.
Membrane principle for high-purity nitrogen generation
Each ‘membrane’ consists of a bundle of hollow fibres in a cylindrical shell. Compressed air supply is filtered and dried prior to entering the membrane. Oxygen, CO2 and water vapour are separated from nitrogen in the air supply due to the differential pressure created between the air supplied at high pressure and the low-pressure end of a membrane. Due to their efficiency, membrane systems provide gas purities of 95–99.5% (O2 gas impurity analysis) and typically smaller gas flows (0.5–1000 LPM).
PSA principle for high- to ultrahigh-purity nitrogen generation
PSA technology is used to separate specific gas species from a mixture under pressure. This is achieved by the molecular characteristics of the molecule and affinity for the adsorbent material. This process operates at near-ambient temperatures. The adsorbent material (eg, activated carbon, molecular sieves and zeolites) acts as a ‘trap’, preferentially adsorbing the target gas species at high pressure. The process will then ‘swing’ to quickly lower the pressure to desorb the trapped species from the adsorbing material.
A carbon molecular sieve (CMS) is often used in a PSA system because of its shape (cylindrical), consistency in size and high surface-to-mass ratio. A CMS is physically tough, chemically inert and non-crystalline; it is specifically treated activated carbon that forms a pore structure of specific size corresponding to the gas molecule(s) that are to be separated, typically <10 Å. N2 molecules are 3–4.3 Å and oxygen 2.8–3.9 Å; the molecular sieves used for nitrogen generation are formulated to a 4 Å ‘opening’. In practice, PSA systems operate with two columns, banks or towers of CMS, allowing compressed air to enter one CMS bank at high pressure (6–10 bar), before oxygen is adsorbed and nitrogen typically passes to a process or storage tank. PSA systems range in purity — 95–99.99% dependent on the gas flow velocity through the CMS — and can provide flowrates up to 5000 LPM.
Concerns have been raised as the vented gas is enriched with oxygen. The oxygen concentration of the desorption gas averages 30–35% (due to the high partial pressure of nitrogen in air at low pressure at which the regeneration process takes place) and dissipates quickly in a ventilated environment.
Nitrogen generation: the complete system
The entire process of producing nitrogen generation by PSA can be broken down into three core components: compressed air; air drying and purification/filtration; and nitrogen generation (process, supply and storage). Various compressors, from oil-lubricated to oil-free, screw, scroll and piston, are typically used in such systems.
Sufficient filtration (particulate and dust), air drying (refrigerant or desiccant to remove water vapour) and air quality (removal of hydrocarbons) must be considered. The type and overall air quality rating required will be dependent on the type of compressor and the ambient temperature of the system’s location. The entire system must be carefully sized and scoped for specific applications, various pressures, location, altitudes, environmental conditions and temperature effects — there is no one size fits all.
Energy and the environment
Purity needed at point of use or application can have a huge impact on capital and power costs, hence carbon costs: liquid nitrogen takes approximately 0.7 kW N-1 m-1 electrical power, whereas this figure is 0.46 kW N-1 m-3 for 99.9% N2 gas purity. A full system at 99% purity vs 99.9% will therefore cost approximately half as much to set up and half the electrical usage for the same flow and production rate.
There is also the environmental impact of constant fuel and emissions due to transport of cylinders and tankers to sites. For example, a piece of equipment or application using 35 LPM equates to over 2000 ‘G’ sized cylinders, 135 ‘man packs’ of 15 cylinders on a skid pallet and two deliveries by truck every week. A single gas generator that can fit under a bench can produce 18.4 ML of nitrogen before requiring a service and can continue producing nitrogen gas for 15 years.
Hydrogen gas production
The traditional method of H2 production is steam reforming or natural gas reforming, which requires high energy consumption. On-site H2 generation is performed through electrolysis of de-ionised water to oxygen and hydrogen via a proton exchange membrane (PEM). Hydrogen ions diffuse through the PEM membrane, whereas oxygen is retained and is then vented to atmosphere. H2 gas is then further purified using a desiccant drier/PSA drier before being supplied to the application (ensure no impurities are introduced into the hydrogen gas).
There are several serious safety concerns when using nitrogen and hydrogen gases, particularly at high pressure and volumes in confined spaces, such as a laboratory. Liquid N2 has an expansion to gas of 1 L = 696 L of N2(g). An oxygen-deficient atmosphere (<19.5%) results in asphyxiation, while an oxygen-enriched atmosphere (>21%) can be a fire hazard.
As a working example, a small lab dewar might contain 50 L of liquid N2, equating to 34,800 L of N2(g). In a lab measuring 5 x 5 x 3 m (75 m3) the volume of air might be 75,000 L, with normal O2 content (21%) of 15,750 L. 11% O2 content is 8250 L (25% of the small dewar), resulting in serious risk of your staff fainting in minutes.
Similar examples can be observed with hydrogen gas. A laboratory measuring 5 x 4 x 2.5 has a volume of 50 m3, or 50,000 L. The lower explosive level (LEL) of H2 is 4.1%. Thus we need 2050 L of H2 to reach the LEL. But a 50 L gas cylinder contains around 9000 L of hydrogen, so releasing just 25% of the contents would reach the LEL. In contrast, an H2 generator produces up to 500 cm3/min and would take 67 h (2.7 days) to reach the LEL, assuming no loss of hydrogen during this time.
The risk associated with cylinders on liquid supply can be decreased by lowering the amount of gas in the room to smaller volumes, moving to a store room and pipe, or looking to alternative gases with lower risk. These options can often be very costly and place more disruptions to the lab and time to personnel to move the gas storage vessel.
On-site gas generation can be a solution as there is a very low stored gas volume of <500 cm3 for H2 and <20 L N2 at 8 bar, compared with 9000 L of gas at 250 bar in a large cylinder. Generators offer automatic shutdown in the case of an external leak, ensuring no more than 10 L of gas leaks into the environment and take 2–3 days to reach lower-explosive or personal harm limits — as opposed to vessels that can reach these hazardous limits in seconds.
Return on investment (ROI)
There are many hidden costs that are driving up gas prices globally, with the cost to operations including energy usage offsite to produce and boost gas pressure; vessel/cylinder manufacture; testing and compliance; logistics/fuel cost; staff training; batch quality variation; PPE; pipework; gauges and regulators; and risk assessment/consultation for explosion and asphyxiation. In addition, there are shortages of non-renewable gas types, so alternative methods must be investigated with renewable gases to continue using certain analytical equipment (eg, helium conversion to H2 for gas chromatography).
In comparison, costs for an on-site generator include initial installation, power consumption and an annual service for the duration of its life. The savings over a three-year period could be as great as 50%, in addition to greater safety. So what are you waiting for?
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