problems with activated carbon bed 3

[JeffH35]

Hi All,

Our plant utilizes an activated carbon bed to filter contaminants and "purify" a stream of hot solution. This solution is then cooled/crystallized, fed through a centrifuge and dried, yielding a high-purity product. We started to see contamination of the product and changed the bed as usual. After we changed the bed, we started seeing a grayish color in the centrifuge effluent and our product has routinely failed inspection due to discoloration. We changed the bed entirely, thinking that the carbon was the problem, but ran into the same problem. Since then, we have been flushing the new bed with hot water to try and alleviate the problem.

It seems that the carbon bed has a lot of ultra-fines in it.

Has anyone experienced similar problems? Can anyone make any suggestions?

Any suggestions are much appreciated.

Regards,
-Jeff

 

[hacksaw]

washing is about the only way with careful velocity control and use of agitation jets.

you can also specify the particle size and dry weight density. some sources of AC produce more fines than others, the expensive ones produce little.

fines production also depends upon your bed design and attrition (lost of particle size) you are experiencing

 

[tickle]

Have you tried filtering the hot solution using a lab 0.2 micron filter. This may prove if the discolourisation is from solids.

 

[JeffH35]

hacksaw and tickle,

Thank you both for your input.

hacksaw, I will contact our supplier and find out how they process their AC and find out what they will tell me. Otherwise, we will just continue to flush the system and keep our fingers crossed, as we can withstand a bit of halted production.

tickle, there is a 10 micron bag filter directly downstream from the bed that has been coming out gray/black, but I will contact our QC lab and find out if they have any filters of such fine quality.

Any other suggestions are greatly appreciated.

-Jeff

 

[aspearin1]

The fines are probably nano-sized particles. A 10 micron bag will come nowhere near reducing the haze. The .2 micron may reduce it some, but not likely enough. I think hacksaw hit it one the nose with his controlled velocity suggestion. Also, check with your suppliers to see if they changed their material. Activated carbon from wood yields a much higher fines content than activated carbon from coconut shell. Also, you may check to see if you can find a supplier of pelletized carbon, (i.e. carbon aggregates bound by resins.). I have seen pelletized zeolite. Carbon cartridges are also available, and may be able to be sized to your needs.

ChemE, M.E. EIT
"The only constant in life is change." -Bruce Lee

 

[gacguy]

Is your adsorber operating in the downflow or upflow mode? If upflow then this problem is usually solved by going downflow. If you are going downflow other causes of fines elution are frequent stopping and starting and viscosity. What is the viscosity of your solution? In systems where the viscosity approaches 5 centipoise, fines elute and the product is gray. It has been refered to as gray sickness in the cane sugar industry. The solution there is to use precoat filters.

If you want to get rid of fines prior to starting you should backwash for 20-30 minutes at a rate of 10-12 gpm/ft2 of adsorber cross sectional area. This rate, using 60-70 degree F water will give you 10-15% bed expansion for 8x30 mesh carbon and ~30% expansion for 12x40 mesh carbon. If you do this, make sure you have sufficient height in the adsorber above the settled carbon bed so you don't lose carbon with the backwash water.

Another major problem with carbon beds is that the user does not properly wet the bed prior to putting it in use. In a carbon bed the pore volume constitutes ~40% of the bed volume. These pores are filled with air which needs to be displaced in order for adsorption to be effective and efficient. This air then forms bubbles in the void space in the bed and, if not removed, causes high pressure drop and channelling. This results in premature breakthrough. This air does not leave the bed during operation in an adsorber operating in the downflow mode.

With water at 70 degrees F it takes ~ 3 days to become completely wetted.

If your adsorber is operating near atmospheric pressure the effect is most pronounced. If it is operating at a high pressure the volume of the air bubbles is obviously less and the channelling is less.
If you are interested in knowing how to wet the bed and get rid of the air contact me at iamaclown@comcast.net

 

[JeffH35]

gacguy,

Thanks for your post. You seem to have some experience with AC beds. I think you gave us some information that is leading to a solution. Our bed already operates in the downflow mode and we don't have the tank capacity to backwash at such a high rate. However, we have done a particle size analysis on some of the AC from our bed and it seems to be breaking down. We were seeing very low(10%) flow rates though the bed, so it seems that you might be right about the occurance of channeling and improper wetting. We are replacing the bed again and I will be e-mailing you about proper wetting procedure.

Thanks for your insight on our situation.

Regards,

-Jeff

 

[Yordif]

Two ideas, I know that Norit carbons has a "slurry" loading technique that might be interesting and should reduce the initial fines. Essentially they water unload and load the carbon. They have a nice video if you can get it from the local norit rep or the Univar guys.

For filter problems there are some support media. These are typically used in refinery hydrotreaters to catch fines and particles to keep the bed from plugging. Unicat offers an material called AFS and someone else offers a "cat trap"

 

[JeffH35]

Yordif,

You hit the nail square on the head, so to speak. Our process is up-and-running and we are experiencing negligible amounts of fine carbon in our product, whereas before there was much more. We changed our bed for, what looks like, the final time.

Using the change-out technique of our most experienced, day-shift operator, who just got back from a 2 week vacation, and had done all previous bed changouts, things look good. Before adding the new carbon, we filled the bed half-way with water, then added the carbon. Apparently, the carbon was added to an empty tank last time. I'm assuming that this allows for the air to exit while filling. Then we ran hot water through the bed to flush it out. Finally, we drained the excess water and started running our hot solution throught the bed.

According to the operator, he usually has to vent off a small amount of gas a few days after the bed change out. He also said that this was the first time that this problem had been encountered that he knew of. He wasn't afraid to point out what "happens when he's gone."

Thanks to everyone who made suggestions.

-Jeff