Vacuum Pressure Swing Absorption (VPSA) is a method of gas separation and is a subset of Pressure Swing Absorption (PSA). Examples of VPSA applications include biogas upgrading, oxygen generation for medical purposes and production of hypoxic air for firefighting.
PSA/VPSA technologies are based on differences in the behaviour of two different gas species with a particular absorbent.
We can classify adsorbents based on two categories. Firstly, those that operate on absorption equilibrium (e.g. zeolites). Secondly, those that operate on absorption kinetics (e.g. activated carbon, carbon molecular sieves).
For absorbents based on absorption kinetics, separation is based on differences in the rates of mass transfer between gas species. For absorbents based on absorption equilibria, we assume mass transfer to be infinitely fast. Moreover, we can achieve separation based on differences in the quantity of gas retained in the absorbent.
Consider the absorption isotherms of nitrogen gas N2 and oxygen gas O2 as seen above. At a total pressure of 100kPa/1bar, O2 has an equilibrium loading of about 0.1mol per kilogram of zeolite. For N2, it is approximately 1.0mol per kilogram of zeolite. As we increase total system pressure to 500kPa/5bar, the equilibrium loading of O2 increases to 0.6mol/kg zeolite. For N2, loading increases to about 2.1mol/kg zeolite.
There is a difference in equilibrium loading for both O2 and N2. For O2, it is around 0.5mol per kg zeolite. For N2, it is around 1.1mol/kg zeolite.
How does this affect the adsorption process?
As total system pressure pressure is increased from 1bar to 5bar, a greater amount (in mol) of N2 is absorbed than O2. In theory, we can use this absorbent to strip N2 from an O2/N2 inlet gas mixture. Such a stripping process would result in a product mixture highly concentrated in O2.
In summary, VPSA/PSA operations usually involve a Skarstrom cycle. A Skarstrom cycle usually consits of 4 main phases.
- Pressurization. Compressing the inlet gas to an elevated pressure
- Feed Stage / Absorption. Passing the gas mixture through an absorption column filled with absorbent material
- Blowdown. The absorbent is saturated with a selected gas species and feed to the column is cut. Pressure decreases to facilitate desorption of the gas molecules from the absorbent.
- Purge. At minimum pressure, the inlet gas is fed through the column and flushes out the desorbed gas molecules via a purge stream.
For biogas upgrading, the gas mixture firstly undergoes pre-treatment with activated carbon filters or condensate traps.
In the absorption column, absorbents are present that have a greater selectivity for carbon dioxide than methane. Carbon dioxide molecules are thus retained while methane molecules pass through the column.
This results in an upgraded gas – or biomethane – with a high concentration of methane and a very low concentration of carbon dioxide. When the absorbents in the column are saturated, the pressure in column decreases and carbon dioxide desorbs from the adsorbents. The system then flushes the columns with inlet biogas and the carbon dioxide is removed via a purge stream.
Examples of absorbents used in PSA/VPSA include carbon molecular sieves (CMS) and zeolites. The design and choice of absorbent is crucial in ensuring optimal performance of the absorption column and the efficiency of the separation process.
VPSA/PSA and Other Methods of Biogas Upgrading
|Technology||Waste||Water||Chemicals||Investment Costs (CAPEX)||Operational Costs (OPEX)||Energy Consumption|
|Vacuum Pressure Swing Adsorption (VPSA)||No||No||No||High||Medium||Medium|
One of the main advantages of VPSA is that it does not require the use of water or chemicals. However, due to its pressure requirements, electricity consumption for a VPSA tends to be higher in comparison to other technologies, resulting in a higher operational expenditure. The efficiency of the process is also limited by the effectiveness of the absorbent and system design e.g. number of columns, length and diameter of columns.
NeoZeo’s biogas upgrading module aims to circumvent the shortcomings of traditional VPSA technologies by keeping high investment and operational costs to a minimum while maximizing process efficiency using innovative zeolite absorbents. Our module is specialized for raw inlet biogas flow rates of 50 to 200 Nm3/h and provides an affordable yet effective technology to allow small scale biogas producers to upgrade their biogas to biomethane.