Minimum flow over Carbonbed adsorber
Minimum flow over Carbonbed adsorber
(OP)
Weve got an adsorber for VOC's in tank ofgas, nothing special. I want to raise the caloric value for a downstream RTO by (partially) bypassing the adsorber. During normal operation, the flow of air and vapour through the carbon helps to remove heat by convection. I was warned that very low air flow rates can result in a buildup of flammable vapour in the bed, which may increase the likelihood of hot spots. How do I know what flow is safe?





RE: Minimum flow over Carbonbed adsorber
Good luck,
Latexman
RE: Minimum flow over Carbonbed adsorber
RE: Minimum flow over Carbonbed adsorber
I believe, if your air flowrate and VOC concn vary constantly,it would be difficult to come at a perticular number of flow...
Best you can do is keep a certain minimum flow based on your temperature reading of the bed.
You can also mitigate the risk by installing water spray around your bed , though it may be a litle costly if your bed is hugh in size.
RE: Minimum flow over Carbonbed adsorber
I'd suggest a different approach. First, at what temperature(s) do bad things start to happen in your system? One example is the auto-ignition temperature of your VOC? There may be other temperatures of concern. You will want the system to stay below the lowest temperature of concern. Calculate the minimum air flow that guarantees that.
Good luck,
Latexman
RE: Minimum flow over Carbonbed adsorber
RE: Minimum flow over Carbonbed adsorber
I have worked on several adsorber systems, and any that had ketones had fires. MEK was the best, but still had fires. detection of fires is a combination of art, experience, and luck. The issue is that a fire starts in a small spot, with low air flow, so that the heat doesn't move readily to the outlet stream until its significant, and the carbon is a fantastic insulator, so you can't detect a hot spot 20 mm away. You may find that the best indicator of a fire before it gets big is a sudden and unusual spike (of just a few percent) in the VOCs in the outlet stream or stack. this is caused by thermal desorption of the carbon near the fire. Humidity control is crucial - make sure you have no superheat in the steam, and that the bed gets thoroughly resteamed every few hours that it is "down" or in reduced flow, and GOOD LUCK!
RE: Minimum flow over Carbonbed adsorber
So let me het this streight. An average carbon bed will not show hotspots (under normal operation and good handling)if we keep average vapour velocity in the bed above 200mm/sec?
Or is there a margin to this?
Thanks anyway (to al you guys for giving me your thougts) becouse I'me starting to see were getting ourselves into a quagmire of problems, just to save some on the gasbill.
Maybe someone has an idea to accieve what i set out to do without the risks involved with lowering gas velocity in the beds
Thanks an greets
RE: Minimum flow over Carbonbed adsorber
A "rule of thumb" for sizing a carbon adsorber is a minimum 200mm/sec superficial velocity. Very high heats of adsorption require additional velocity. Higher heats of reaction (such as with oxidation of ketones) require even higher velocities. The real problem is that uniform flow distribution is assumed, and the geometry of most carbon beds is not favorable to uniform flow (there is added cost to the equipment and piping).
If you want to send more fuel to the RTO, you could reduce the efficiency of the adsorbers by adding outside air to the flow to the adsorber. You should do an analysis of the total operating cost for such an option. Include safety, carbon change frequency changes, steaming cycle changes (more water per mol of VOC to the RTO), and other things. These systems are rather complex.