PSA in Bio Gas Production

Bio gas is an important source of renewable energy. It has a wide ranging applications including the immense possibility of using this as a Bio fuel. To obtain a pipeline quality Bio gas, it must pass through two major processes:

1. A process of cleaning, where traces of components that are harmful to appliances, the natural gas grid, or end users are removed.
2. An upgrading process in which the different parameters are adjusted in order to match the pipeline specifications.

Upgrading of Bio Gas with Pressure Swing Adsorption:
Pressure Swing Adsorption, or PSA, is utilized for the separation of carbon dioxide from methane. It is generally done by adsorption/desorption of carbon dioxide on beds of activated carbon or zeolites at different levels of pressure. In this process, Hydrogen sulphide is adsorbed by the adsorption material irreversibly. Thus it gets contaminated with Hydrogen sulphide. Exactly because of this a hydrogen sulphide removing step is a compulsory inclusion in the PSA process.

As is seen from the diagram, the upgrading system contains four adsorber vessels that are filled with adsorption material. In a normal operation each adsorber operates in the following alternating cycles namely adsorption, regeneration and pressure build-up. During the phase of adsorption biogas is directed from the bottom into an adsorber. While passing through the adsorber vessel, carbon dioxide, oxygen and nitrogen gets adsorbed on to the surface of the adsorbent material. The gas that exits from the top of the adsorber vessel has around 97% of Methane.

The adsorption phase is stopped as the adsorbent material is nearly fully saturated with the components of the adsorbed feed gas. For achieving continuous operation another adsorber vessel that was regenerated is now switched into adsorption mode. The regeneration of the saturated adsorbent material is done by a step by step process. This means depressurizing of adsorber vessel to atmospheric pressure and then lastly to near vacuum conditions. In the beginning, the pressure reduction is done by a pressure balance by way of a adsorber vessel that was already regenerated. This gets followed by a second depressurization step that nears almost atmospheric pressure. The gas that leaves the vessel during this step has in it considerable amounts of methane. It is then recycled again to the gas inlet. Before another adsorption phase starts, the adsorber vessel gets repressurised to match the final adsorption pressure. The feed gas achieves final pressure build up only after the completion of a pressure balance with an adsorber that was earlier in adsorption mode has been done.

Biogas upgrading system with PSA technology