Biogas PSA and VPSA systems separate gas components through pressure-swing adsorption, but their pressure duties differ from low-pressure blower service. In addition to methane, it contains a substantial concentration of carbon dioxide together with water vapour, hydrogen sulphide, particulates and process-specific contaminants. Biogas upgrading removes these unwanted components and increases the methane concentration to produce biomethane.
Pressure Swing Adsorption, or PSA, is one of the established technologies used for this purpose. Vacuum Pressure Swing Adsorption, commonly abbreviated as VPSA, extends the pressure cycle below atmospheric pressure to improve adsorbent regeneration. Compressors, blowers and vacuum pumps are therefore essential parts of these systems, but their duties are different and must be evaluated separately.
Roots blowers can be applied in selected low-pressure feed, gas-recirculation or vacuum duties. Roots vacuum boosters may also be integrated with another vacuum-pump technology when high pumping capacity is required at reduced pressure. The correct arrangement depends on the adsorption cycle, gas composition, required pressure ratio and minimum regeneration pressure specified by the upgrading-system designer.
How PSA and VPSA Biogas Upgrading Works
PSA separates gas components according to their different adsorption characteristics. Pretreated biogas is compressed and passed through a vessel containing an adsorbent such as activated carbon, carbon molecular sieve, zeolite or another process-specific material.
Under the selected adsorption pressure, carbon dioxide and certain remaining impurities are preferentially retained on the adsorbent surface. Methane is adsorbed less strongly and passes through the bed as the methane-rich product stream.
Once an adsorption vessel approaches its working capacity, the raw gas is redirected to another vessel. The saturated bed then passes through a regeneration sequence that may include:
- Pressure equalization with another adsorber
- Co-current or counter-current depressurization
- Recovery of methane-rich blowdown gas
- Atmospheric-pressure desorption
- Vacuum regeneration
- Purging with product or recycle gas
- Repressurization before returning to service
Multiple adsorption vessels operate in a programmed sequence so that one or more beds produce biomethane while the remaining beds are regenerated. Fast and repeatable valve operation is essential because the pressure, flow direction and gas composition change repeatedly during every cycle.
In a conventional PSA system, regeneration is achieved primarily by reducing the vessel pressure. Some configurations also apply a light vacuum during the final regeneration stage. When the process deliberately reduces the adsorber pressure below atmospheric pressure as a principal part of the regeneration cycle, it is generally described as VPSA.
The deeper vacuum reduces the partial pressure of the adsorbed carbon dioxide and encourages its release from the adsorbent. This can increase the usable working capacity of the bed, but it also increases the size and energy demand of the vacuum system. The optimum regeneration pressure must therefore be determined from the adsorbent characteristics, required methane purity, methane recovery target and total cycle economics.
Before entering the adsorption beds, raw biogas normally requires:
- Free-water and condensate removal
- Hydrogen sulphide reduction
- Particulate filtration
- Siloxane removal where applicable
- Hydrocarbon or volatile-organic-compound control
- Cooling or drying to the specified dew point
Water and hydrogen sulphide are particularly important. Moisture can reduce the adsorption capacity of materials intended for carbon dioxide separation, while H₂S may be strongly or irreversibly retained by certain adsorbents. Inadequate pretreatment can shorten adsorbent life, increase regeneration demand and contaminate the biomethane product.
Where Roots Blowers and Roots Vacuum Boosters Fit
The term “roots blower for VPSA” should not be interpreted as meaning that one machine can perform every compression and vacuum duty in the upgrading plant.
Traditional biogas PSA systems may operate at several bar of adsorption pressure. A single-stage roots blower is generally intended for a much lower pressure ratio and should not automatically be selected as the main compressor for such a system. Depending on the required pressure, a screw compressor, reciprocating compressor or another multi-stage gas compressor may be more appropriate.
A roots blower can nevertheless be considered for:
- Low-pressure biogas transfer to an upgrading skid
- Feed pre-boosting ahead of the main process compressor
- Low-pressure adsorption processes developed around a small pressure swing
- Recycle of methane-bearing equalization or blowdown gas
- Controlled movement of off-gas to a treatment unit
- Selected vacuum-regeneration duties within the machine’s operating range
A roots vacuum booster operates on the same positive-displacement principle but is configured for vacuum service. It can provide high pumping speed at pressures below atmospheric pressure. For deeper vacuum duties, it is normally installed upstream of a backing pump, which compresses the gas from the booster discharge to atmosphere or to the next process pressure.
Possible backing-pump technologies include dry screw, rotary vane, liquid-ring and other process-specific vacuum pumps. The selection depends on the target absolute pressure, gas cleanliness, condensable-vapour load, methane content, H₂S concentration and allowable contamination of the process gas.
A roots vacuum booster should not be expected to discharge independently from a deep-vacuum inlet directly to atmospheric pressure unless the manufacturer has specifically approved that operating condition. Excessive differential pressure can produce high discharge temperature, increased absorbed power and thermal overload.
In some VPSA designs, the initial depressurization occurs without a vacuum pump. The bed is first equalized with another vessel or vented towards atmospheric pressure. The vacuum system begins operating only during the lower-pressure regeneration stage. This reduces the gas volume that must pass through the vacuum equipment and may lower energy consumption.
The gas removed during depressurization does not always have the same composition. Early blowdown gas may contain enough methane to justify recycling it to the upgrading inlet, while later regeneration gas may be richer in carbon dioxide. The blower and vacuum system must therefore be designed for the complete expected composition range, not merely the nominal raw-biogas analysis.
Vacuum Capacity and Performance Evaluation
Vacuum-system sizing must be based on the adsorption licensor’s complete cycle data. The required pumping capacity depends on:
- Adsorber free volume
- Quantity of gas contained in the bed voids
- Amount and rate of carbon dioxide desorption
- Starting and final regeneration pressures
- Permitted evacuation time
- Purge-gas flow
- Valve and pipeline conductance
- Number of vessels evacuated simultaneously
Glossary
Adsorbent: Solid material that preferentially captures selected gas molecules.
Adsorbent regeneration: Release of captured components so an adsorbent bed can be reused.
Cycle pressure: Pressure condition used during a defined adsorption or regeneration step.
Methane recovery: Fraction of feed methane retained in the product-gas stream.
Pressure-equalization step: Transfer of gas between vessels to reduce pressure differences during an adsorption cycle.
PSA: Pressure-swing adsorption process using pressure changes to separate gas components.
Tail gas: Residual gas stream leaving an upgrading or separation process.
VPSA: Vacuum-pressure-swing adsorption process using both vacuum and pressure stages.