10 replies to this topic

[Pronab]

We are using activated charcoal to remove hydrocarbon and degradiation products of MDEA in a lean amine solvent (MDEA). Normal practice is to replace the charcoal when we suffered a high pressure drop and foaming tendency in absorber. Is it fruitfull to backwsh the filter instead of replace.
The activated charcoal which used to remove mainly hydrocarbon, is it absorb or it is a adsorption by the charcoal.

Thankyou.

Pronab.

[Art Montemayor]

Pronab:

Don't waste your time and efforts trying to salvage or regenerate spent activated carbon used in filtering amine solutions. It isn't cost effective.

The Activated Carbon acts as an aDsorbent, removing (as you suspect) degraded amine compounds and other impurities in your aBsorption solution. You haven't stated the size of your acid gas unit (or the gpm of recirculating solution) so it is difficult to grasp or visualize a size or quantity of activated carbon that you are using or consuming and how often. Nevertheless, I would guess you are not consuming 500 lbs of activated carbon per month. But maybe I'm wrong....

If you are consuming a considerable quantity of activated carbon due to amine foaming problems, have you consulted with your licensor? There may be something in the gas stream that is causing the root problem. For example, have you noticed any other changes co-incidently with the high carbon exhaution rate? I presume you are dealing with H2S +CO2 selective absorbtion. I have not used MDEA, but have pumped a lot of MEA with both a re-distillation still and Activated Carbon filters. I never had difficulties with the Act. Carbon, but you can't beat the redistillation still for absolute positive removal of harsh impurities in the solution. I don't know if you can apply a re-distillation still to MDEA solution.

If you have a serious solution foaming problem, I'm afraid you're going to suffer with increased Act. Carbon usage unless you identify the cause. Have you tried consulting your licensor? If there is no help there, you may have to resort to expert consultants if the problem persists in causing you lost profits.

Best of luck.

Art Montemayor

[Pronab]

Art

Thankyou for your reply.
Actually we are looking for alternatives. However still we are able to maintauin a good quality of MDEA solutions.We don't have sever foaming problems. Our flow through the carbon bed is ~40 m3/hr, with 600 m3/hr total solution flow. We have ~ 8 cub. meter of activated charcoal. Licensor recomendation to change over charcoal after 6 months, but still our foamin breakdown and foaming length are in within the range. Pressure drop over the bed also not exceed.
We want to know anybody do backwash their charcoal filter.

Thankyou .

Pronab.

[Art Montemayor]

Pronab:

Let me repeat this, because I don't think you understood my recommendations or you don't understand the principles underlying an adsorption system:

You cannot "backwash" an adsorption bed. This is not a "filtering" operation; it involves van der Waal forces acting on the surface of a solid substance and held there. The only way to re-use the adsorbent is to regenerate it, and this involves ENERGY, no just a simple flushing.

You don't mention what your licensor instructed you to do, but I will bet that they did not tell you to regenerate the adsorbent bed by "back washing". If they did, they're trying to fool you; this simply can't be done. Adsorption is a Unit Operation that is explanined in detail in Kohl & Riesenfeld, "Gas Purification" and other ChE textbooks. There are no alternatives to supplying regeneration energy to the adsorbent in order to re-use it - and even then, the sorptivity of the adsorbent will diminish by a factor. If the system is working as per the licensor's instructions, I would leave it alone and follow instructions. Any thing you change to the licensed process is on your liability, so if you introduce a change in the operating instructions and you suffer from the effects, the licensor cannot be held liable.

Art Montemayor

[coolz_10]

Hi,

I read somewhere (a vendor's distillation handbook) that it is possible to use steam to desorb the adsorbed material (eg solvent) away from the activated carbon bed when it becomes saturated. Subsequently, the stream is carried over to a condensor where further separation could take the form of decanting or distillation.

Is this plausible in practice?

[Pronab]

Dear Coolz

It will be helpfull to me if you can send me the address ( website) of the company that you read . Or can you tell me how can I get it.

Regards.

Pronab.

[coolz_10]

Hi Pronab,

I read it from APV's handbook posted in this website

http://www.apv.inven...ion/default.htm

Hope they have something for you.


Regards

[Pronab]

Thanks a lot Lath after a long day interval .
Still I am in dark.

[sdarone]

Hi Pronab,
I have found your post by chance, probably you are not interested anymore on this issue or you have solved it a very long time ago, but even there just to add some thoughts on this for next time somebody has the same doubts.
Backwashing does not regenerate your bed, all it does is fines removal (pressure drop improves)and cleaning of the bed, removing dust, mud, or other substances that are not adsorbed in the bed but blocks the free adsorbant area reducing your filter capacity.
The counterpart is that during the backwashing you can get channeling of your bed, which means that your pressure drop will be low and remain low, but your filter is not working as supposed since the liquid flowrate will always go through the minimum resistance path which is the channels in the bed, not in contact with the charcoal.
You can do it, with care, and look for very low pressure drops which are a symptom of bed channeling.

Regards

Edited by sdarone, 23 December 2010 - 08:22 AM.

[Pronab]

Hi Pronab,
I have found your post by chance, probably you are not interested anymore on this issue or you have solved it a very long time ago, but even there just to add some thoughts on this for next time somebody has the same doubts.
Backwashing does not regenerate your bed, all it does is fines removal (pressure drop improves)and cleaning of the bed, removing dust, mud, or other substances that are not adsorbed in the bed but blocks the free adsorbant area reducing your filter capacity.
The counterpart is that during the backwashing you can get channeling of your bed, which means that your pressure drop will be low and remain low, but your filter is not working as supposed since the liquid flowrate will always go through the minimum resistance path which is the channels in the bed, not in contact with the charcoal.
You can do it, with care, and look for very low pressure drops which are a symptom of bed channeling.

Regards

Sdarone
I agree with you. Finally we didn't backwash, we replace at a fixed timeinterval.
Thankyou for your interest.
Regards.
pronab.