Membrane technology
The cooling water/air mixture is extracted from the vapor system (gas inlet) by means of a liquid ring compressor (C1) at least. The hydrocarbon/air mixture that is sucked in is combined with the recycled permeate stream from the diaphragm separation stage and compressed to a pressure of approx. 3-4 bar abs. by the liquid ring compressor(s) (C1). The pressure is regulated to a defined setpoint.
The liquid ring compressor(s) (C1) is/are supplied with a partial stream of the used absorbent by means of a liquid pump. A sufficient liquid supply is ensured by monitoring and regulating the flow and the pressure. The hydrocarbon/air mixture is separated from the liquid in the scrubber column (V1). The gas mixture flows through the tank (V1) filled with the packing material from bottom up, whereas a part of the hydrocarbon vapors condenses by contact with the absorbent injected via a nozzle under system pressure and ambient temperature. The necessary quantity of absorbent (= ring liquid for compressor + scrubbing liquid) is provided by others at the terminal point and pump through the downstream filter.
The liquefied hydrocarbons and quantity of absorbent supplied by others are circulated back to an onsite storage tank from the scrubbing column (V1). Depending on local conditions, a liquid pump is employed to provide assistance for this purpose.
- Source: BORSIG Membrane GmbH
The membranes in the modules fractionate the gas/vapor mixture coming from the tank (V1) in a retentate stream depleted of hydrocarbons and in a permeate stream enriched with hydrocarbons.
The permeate stream is recycled via the vacuum pump(s) (C2) upstream of the intake ports of the compressor(s) (C1) and compressed once again, in doing so the hydrocarbon vapors that are sucked in are lubricated before entering the compressor(s).
The retentate stream is released to the atmosphere via the stack or passes (optionally) through a pressure swing adsorption unit (PSA) where the hydrocarbon load is further reduced.
Pressure Swing Adsorption Unit (PSA):
Residual cleaning with strict limits
The second stage consists of two adsorption beds connected in parallel and the corresponding piping and instrumentation. The plant is controlled such that the one bed adsorbs the hydrocarbons (i.e. “cleans” the air stream) and the other bed is regenerated (i.e. cleaned) at the same time. A semi-continuous process is represented in this manner.
The incoming vapor stream (retentate of diaphragm stage) is supplied to the activated charcoal bed in which the majority of the hydrocarbons are adsorbed. The volume flow leaving this bed is released to the atmosphere via the stack with a concentration below the required emission values.
The adsorbers are regenerated using the vacuum pump of the first stage. The vacuum removes the previously adsorbed hydrocarbons from the coal, a small purge air stream (purge gas) which is taken from the cleaned air, transports this via the vacuum pump to the recovery section of the first stage
