Activated Carbon-Adsorption Plant for solvent recovery
The process and explaination of a sample facility utilizing the solvent recovery system is described below.
Conditioning of the exhaust air
The solvent-containing exhaust air first is dehydrated by means of cooling and reheating.
Due to this conditioning the relative humidity of the exhaust air can be reduced to approx. 45%. This is important in order to prevent a diminution of the absorbing capacity for solvents in the subsequent activated carbon absorbers by co-adsorption of steam.
When the relative humidity exceeds 50%, the absorbing capacity of the activated carbon columns reduces drastically especially for low-boiling solvents.
The waste process air is cooled in the process air cooler to approx.13 degrees C (55.4 F) with cold water (5 degrees C / 41 F). Condensed water in this stream is then separated in the mist collector and led into the sump tank. Then the process air is reheated to approx.26 degrees C (78.8 F) in the steam-heated heat exchanger.
Activated carbon pre-filter
An activated carbon pre-filter is installed before the actual solvent recovery to achieved a homogenization and buffering of short-time peak concentrations.
Furthermore, solvent components with boiling point>150 degrees C (302 F) are kept away from the actual solvent recovery plant. This is important as these components (with steam at 140 degrees C / 284 F) are expulsed only insufficiently and thus the intake of low-boiling solvents in the adsorbents will be gradually reduced.
The actual solvent recovery consists of three static bed adsorbents with an activated carbon height of 1500mm (59 inch) and 6000kg (13,228 pounds) activated carbon each.
The adsorbents are operated alternately, for example, first adsorbent bed in `Loading' step, while the second bed in `Regeneration' step or `Regeneration finished' and the third adsorbent bed in `Stand-by' step, ready to take over from the first bed.
The solvent-containing process air (dehydrated and free of high-boiling solvent components) is led through the absorber bed in the `Loading' step by means of the two process air fans.
The process air fans are frequency-controlled via a vacuum measurement.
The solvent-containing process air flows through this adsorbent from bottom to top and leaves the plant via the process air fans to the stack.
During the `Loading' process step the mass transition zone moves in the activated carbon layer from bottom to top. The concentration of the clean air is controlled and recorded continuously by means of a gas measuring device (FID).
When the limit concentration has been achieved, this adsorbent bed is isolated and the next bed; previously in `stand-by' position is activated into `Loading' step. The solvent loaded bed now in isolation is then switched to `Regeneration'.
Switch-over from `Loading' to `Regeneration' is either made automatically via FID-measurement or timed or manually. The regeneration can be explained via four consecutive steps:
The desorption of the solvents is done by an approximately 150 degrees C (302 F) hot steam stream and an overpressure of max.0.5bar (164 inch water). The maximum steam volume is 2000kg/h (73.5 pound/minute).
The vaporization step last for approximately 2hours. The adsorbers are vaporized via a distribution system in countercurrent flow to the process air; from top to bottom.
Due to this “cleaning?scheme it can be ensured that the upper activated carbon layer is practically always free from solvents at the end of the vaporization step.
The activated carbon bed is heated to approx. 120-130 degree C (248-266 F) by the steam in this vaporization step.
Together with the steam the solvent is led to the bottom of the adsorber column where it is discharged from the adsorber and collected.
The steam-solvent mixture is condensed in the regenerate coolers. Cooling water 20 degree C (68 F) is used as cooling medium in countercurrent flow to the operating medium.
The condensed and cooled regenerate flows into the sump tank by gravity from where it is then led to the solvent separator by means of a pump.
The vaporization period is determined via an adjustable time. After having finished vaporization, the steam inlet valve at the adsorber closes.
After vaporization there is a considerable pressure reduction in the adsorber. Release takes place at closed steam valve and open regenerate flap. Release time is approx. 5 min.
After evaporation the activated carbon is hot and humid. When drying and cooling with air, in the presence of ketones, which form thermal instable peroxides there occurs the danger of adsorber burning or at least the formation of “hot spots? In order to avoid this danger, drying and cooling is performed with nitrogen. For economic reasons, this can naturally only be made in a closed circuit.
First, the adsorbers are rinsed with nitrogen.
The nitrogen quantity for the nitrogen rinsing after vaporization is an approx. triple tank volume. The nitrogen drives out the steam and simultaneously strips the residual solvents from the water, which are responsible for the emission peak during drying.
The nitrogen flows to the regenerate coolers via the opened regenerate flap and into the process air via the opened bypass valve.
During nitrogen rinsing of the adsorbers the dry gas inlet and outlet flaps of the closed dry gas circuit are opened for an adjusted time. Thus also creating an inert environment in the piping and components in the dry gas circuit.
The maximum oxygen content in the system may not exceed the concentration of 5%. The oxygen content is monitored by the oxygen measuring device.
The activated carbon is dried in a closed piping circuit by means of nitrogen. The nitrogen is led from bottom to top through the activated carbon bed by means of the dry gas fan. The nitrogen is heated to approx.130 degrees C (266 F) in the steam-heated heat exchanger and the carbon which first was cooled to cooling limit temperature by vaporization, is heated up again and desorbs adsorbed water.
Leaving the adsorber, the hot and humid nitrogen flow is condensed and cooled by means of two subsequently arranged heat exchangers.
The resulting water/solvent condensate is led into the sump tank.
As soon as the temperature rises in the middle of the activated carbon bed, the drying process is finished.
The cooling process is identical to the drying process with the exception that the heater is turned off.
In the beginning of cooling the lower layer of the activated carbon bed is cooled while the upper one is still heated and dried by the nitrogen which was driven out during the cooling process. Simultaneously, further solvents are desorbed from the upper activated carbon layer (the so-called fine cleaning layer) so that there are optimum conditions for the subsequent new loading.
As soon as the carbon is cooled to approx.30 degrees C (86 F), the fully regenerated adsorbent bed goes to “Standby?mode ready for another new cycle.