Automating pressure swing adsorption of hydrogen gas
Why is hydrogen gas purification necessary?
Depending on which production source is used, the refinement and purification of hydrogen gas may be a required postproduction step for hydrogen’s use in various industrial processes. Whereas hydrogen gases generated by electrolysis (including green, yellow, and pink) are typically extremely pure (> 99% H2), other types such as white and orange hydrogen may require separation from other trace gases to become as concentrated.
Pressure swing adsorption (PSA) is a reliable technique to purify hydrogen and other gases (such as oxygen or nitrogen) to high concentrations at ambient temperatures, often above 99%. PSA works by adding mixed gases to selective adsorbents with high affinity for specific gases under pressure. In other words, pressure swing adsorption causes some gases to stick to adsorbents under pressure, allowing for free gases to be separated from the stuck ones.
Alicat’s mass flow meters or controllers can improve the tracking and automation of inlet and outlet gas flow for PSA systems operating under 20 barG while Alicat’s pressure controllers can regulate the pressure or flow of any of these PSA systems under all operating conditions. The following discusses how this works in depth, using the purification of hydrogen gas as an example.
Pressure swing adsorption (PSA) of hydrogen gas
The four main steps in hydrogen pressure swing adsorption (PSA) include adsorption, depressurization, regeneration, and repressurization.
After prefiltering, in the first stage of adsorption, mixed inlet gases are flowed into a chamber containing a series of adsorbents such as zeolite minerals and pressurized with the gas source to a high pressure (typically between 10-40 bar, depending on the specific system). The inlet is closed. Under this high pressure, ambient gases in the mix, or those besides hydrogen, adsorb, or cling to the surfaces of the zeolite minerals, separating them from the hydrogen gas.
In some hydrogen PSA systems, this is reversed such that the hydrogen or other gases you are trying to purify stick to the adsorbent while the unwanted gases remain in gas form in the chamber. Modifications in design include single, double, or multi-chamber PSA configurations.
During this stage, either mass flow controllers or pressure controllers can be used to regulate gas inflow or chamber pressure conditions, providing control loop automation with totalizing and batching features for continuous operation. Alternatively, a mass flow meter such as M-Series running in a control loop with electronic valves could also be used for PSA flow or chamber pressure regulation.
If using controllers, either a MC-Series running in pressure control mode (restricted as a choice only for under 20 barG systems) or a PC-Series are ideal choices for this use, with additional specs and features including:
- Flow ranges from 0.5 SCCM to 5000 SLPM full scale for MC-Series
- Pressure control from 0–3000 PSIG full scale for PC-Series
- Control range from 0.01% – 100% of full scale for PC-Series and MC-Series
- Repeatability of 0.08% for PC-Series and ±0.1% of reading + 0.02% of full scale for MC-Series
- 20 custom gas calibration settings for 98+ preloaded selectable gases, allowing for precise and customizable flow and pressure regulation settings
During the second stage, depressurization, the purified hydrogen gas which is not adsorbed flows through the chamber outlet to storage, lowering the internal chamber pressure and slowly releasing the mixed gases, which are then flowed into another chamber to pressurize it for another adsorption cycle or vented out. Conversely, some systems use the pure hydrogen streams to begin adsorption of other PSA chambers.
Just as at the inlet during adsorption, the outlet flow for all PSA systems can be controlled using a PC-Series pressure controller. Similarly, for those systems operating below 20 barG, a MC-Series mass flow controller running in a pressure control loop is another option. Lastly, just as in the previous step, another option includes a mass flow meter such as M-Series and electronic valves using a control loop (only for sub-20 barG systems).
Choosing any of these Alicat devices allows for PID control algorithm customization to adjust settings such as:
- How quickly or slowly you want the PSA chambers to respond to pressure changes.
- The pressure or flow setpoints which change valve settings
- Developing more complex, multi-chamber PSA systems
Regeneration describes the process of the mixed gases on the adsorbent materials releasing as the chamber is depressurized. When these mixed gases are released, as mentioned previously, they are purged into other chambers to pressurize them or to a separation tank or outlet. Sometimes the pure hydrogen gas stream is used to purge the mixed gases from the PSA chamber.
As in the previous steps, a PC-Series pressure controller is an option to automate the pressure change settings during this step for all PSA systems. Moreover, as previously, either a MC-Series or M-Series combined with electronic valves can be used to automate the pressure change conditions in systems operating below 20 barG during this step.
Repressurization occurs as a regenerated chamber begins a new cycle by increasing pressure and initiating a new phase of adsorption. This is generally accomplished with a split stream from the pure hydrogen line.
All four steps repeat until the desired amount of pure hydrogen gas is isolated using the control loop.
Combining Alicat pressure controllers with mass flow controllers running in pressure control loops or meters running in pressure control loops with electronic valves automated from either a PLC or computer allow for customizable setpoint scripts for continuous operation based on pressure (or flow when using M or MC-Series). Alicat’s extensive analog, serial, and industrial protocol options provide numerous control choices for continuous batching, greatly increasing the value of any PSA system.