AirScience Siloxanes & VOC Removal System


For high siloxanes concentration Vacuum Swing Adsorption (VSA) will be recommended if no VOCs are present as VOCs which will be adsorbed by the media will not desorb by vacuum alone.

When siloxanes and VOCs at medium to high concentrations are present Thermal Swing Asdorption will be recommended. The gas purification performance of activated carbon and TSA is similar however as activated carbon is not desorb it needs to be replaced as soon as it reaches close to saturation. The selection of one technology versus the  other is based on the evaluation of capital cost versus operating cost as a function of siloxanes and VOCs concentration.


If the concentrations of both siloxanes and VOCs is low, activated carbon is recommended as it has the lowest capital cost of the three processes that are generally used for this application, however if the concentration of one or both of the impurities is high, the activated carbon will be rapidly saturated, translating into a high operating cost.



Activated Carbon Process


The system consists of a minimum of two towers, each having a bed of activated carbon. The gas travels through the carbon bed from bottom to top. Pressure drop through the bed and outlet concentrations are monitored to alert to a change in performance signaling the need to replace the carbon. To replace the carbon, the tower must be put off-stream, the full flow passing through the remaining tower(s) during the emptying and refilling operation.


Once one tower has been refilled the other tower(s) must also have their carbon bed replaced, so that all towers have carbon with the approximate same loading.



Vacuum Swing Adsorption (VSA) Process


From the outside a VSA unit is very similar to an activated carbon system (several towers in parallel with an inlet manifold and an outlet manifold, each with motorized isolation valves.


The difference resides in the type of adsorbent media (activated carbon for one, molecular sieve for the other) and the fact that a VSA system has a vacuum system connection at each tower.


The vacuum system consisting of a vacuum pump and a series of motorized valves to allow the cycling of each tower from adsorption to desorption mode.


The vent gas is then compressed, chilled to condense siloxanes, then after a separation stage, re-injected at the inlet of the VSA towers.



Thermal Swing Adsorption (TSA) Process


This is the most effective siloxanes and VOCs removal system. It combines the removal efficiency of the activated carbon technology with the low operating cost of the VSA system.

TSA can treat high loads of siloxanes and VOCs and its thermal desorption process assures that the adsorbent will have an extended usable life.

In a TSA system the gas passes through the adsorbent where the contaminants, siloxanes and VOCs are adsorbed. During that time the two other towers are off line, one being heated, the other one cooled. As one tower gets close to saturation, it is taken off-line and switched to regeneration mode while the tower who finished regeneration/cooling mode is brought back on line.

Regeneration gas (usually vent gas from the CO2 removal unit) is first passed through the tower which finished its regeneration/heating cycle to cool the media bed back to the operation temperature, the gas exiting that tower is further heated in an electric heater then passed in a counter-current mode through the quasi saturated tower, slowly heating the adsorbent bed, and desorbing siloxanes and VOCs. The vent gas is then directed to a thermal oxidizer to be incinerated. Once the complete adsorbent bed has been desorbed the hot bed goes through the cooling stage. At that time, the third tower which has completed its cooling stage is brought back on line and the adsorption, desorption, cooling sequence continues providing a continuous VOCs and siloxanes removal from the biogas.

In cases where regeneration gas is not available a three tower TSA arrangement is provided where one tower acts as a regeneration gas generator using a split stream of the raw feed gas. In this case the discharge of VOCs and siloxanes is in the form of a condensed liquid rather than a gas. The adsorption, desorption, cooling sequence is the same as for the TSA with regeneration gas.

In cases of small gas flow is low and low levels of VOCs and siloxanes a two tower TSA system can be used, however as flow and concentration of contaminants becomes higher, the size of a two tower system becomes too large to be practical and a three tower system is implemented.