10 replies to this topic

[pacogda]

Hello all,

 

I am trying to estimate pressure drop across fixed bed adsorber (liquid phase upflow direction) but I cannot find Ergun´s parameters for my adsorber. I would appreciate if anyone could help me with this (spherical 1/8" activated alumina based adsorbent). Should I take into account any special consideration for Ergun´s equation or pressure drop estimation for upflow direction? and for regeneration conditions, desorption (downflow), should I consider the same parameters and bed void fraction than for adsorption?

 

Many thanks!!

[PingPong]

but I cannot find Ergun´s parameters for my adsorber

What parameters do you mean?

You need liquid properties (which you should know), adsorbent particle diameter (which you know), and bed void fraction, which is usually given in the documentation of the adsorbant supplier.

 

Flow direction has no impact for single phase flow (only liquid or only gas).

Bed void fraction does not change during regeneration.

[breizh]

Hi,

consider reading this paper to support your work .

 

Breizh

[ankur2061]

pacogda,

 

I am not particularly familiar with Ergun's constants and the basic Ergun equation for pressure drop in an adsorbent bed for liquid flow. 

 

However, Ergun's constants and the Ergun pressure drop equation for natural gas dehydration using an adsorbent bed are readily available in many texts related to natural gas processing. I am not sure whether the information that I am providing below might be of help but nevertheless it might provide you a clue to find what you are looking for.

 

The modified Ergun's equation for gas flow through an adsorbent bed relating pressure drop to the superficial velocity is given by:

 

ΔP / L = B*µ*v + C*ρ*v²

 

where:

 

B, C = constants depending on the adsorbent material particle type

ΔP / L = Pressure drop per unit length, psi / ft

µ = gas viscosity, cP

ρ = gas density, lb/ft³

v = superficial gas velocity, ft/min

 

Constants B & C when used in the aforementioned equation and with the units mentioned have the following values for the different particle types:

 

 

 

Whether the information provide above is in anyway useful for 'your' case needs some investigation from your side.

 

Regards,

Ankur 

Attached Files

•  Erguns_Constants.jpg   25.86KB   13 downloads

Edited by ankur2061, 06 March 2014 - 02:12 PM.

[pacogda]

Hello all,

 

Ping Pong thank you very much for your comments, I am referring to specific constants for that kind of adsorbents, instead of 150 and 1.75, that are average values of experimental data for many diferent systems, or maybe other parameters that also include bed voids fraction or other parameters that depend upon particle nature and size as Ankur2061 shows for Natural Gas (thanks Ankur).

My system ir for liquid propylene purification, I am trying to fix vendor pressure drop data and I am not able to do it with data for bed voids fraction I have collected from articles and literature like Breizh attached (thanks Breizh) and standard Ergun equation. Vendor does not supply that kind of data, I think in the past, as I read in an archived forum topic from 2004, ALCOA (BASF adsorbents now) published that kind of data but it is not online currently. 

 

Thanks!

[PingPong]

I am referring to specific constants for that kind of adsorbents, instead of 150 and 1.75, that are average values of experimental data for many diferent systems, or maybe other parameters that also include bed voids fraction or other parameters that depend upon particle nature and size as Ankur2061 shows for Natural Gas

You are missing the point. Void fraction ε and particle size d are already present in the Ergun equation.

 

Ankur's equation is the Ergun equation, whereby:

 

B = 150(1 - ε)²/(ε³.d²)

 

C = 1.75(1 - ε)/(ε³.d)

 

if consistent units (like SI units) are used for all variables

 

Note that B and C are not dimensionless, but the values posted by Ankur do not give dimensions. In view of the other units of measurement being British units, I suspect that d is to be in inches or maybe even in feet; and moreover the reported values for B and C might include some conversion factor(s) to match the units of measurement on the left and right hand side of the equation.

 

The value of B and C, and consequently the pressure drop, are very much dependent on void fraction ε, and that is where the problem is: we do not know what ε is for your particular adsorber. It is likely somewhere between 0.3 and 0.4 but that stilll gives a big possible range of pressure drop.

 

Moreover: the pressure drop that the vendor has specified is likely much bigger than the theoretical pressure drop, just to be on the safe side.  The adsorbent will contain fines by the time it is loaded into the adsorber vessel, and more fines will form during use and regeneration due to attrition.  Moreover the adsorber bed will act as a filter, accumulating scale and other dirt, thereby increasing its pressure drop over its lifetime.

[pacogda]

Thanks Pin Pong, you are rigth, perhaps I should focus the question on other pressure drop contributions or ε instead of "Ergun´s equation parameters"

What you mention is exactly what I have noted, varying ε I am able to obtain similar pressure drop results to those vendor specified, always much higher than Ergun´s equation estimation (bigger efect of ε).

The problem is that "the modified ε" was quite different from those I have found in literature for that particle form, nature, diameter,... and adsorption conditions (i.e. ε: 0.30 vs 0.40), and is not the same if I vary also flows, L/D ratios for the vessel, etc.. (i.e. ε:0.25 vs 0.30) that is why I am asking because there are other important factors I am not considering and probably are more correct than vary ε.

 

Thanks!

[Art Montemayor]

Pacogda:

 

I believe what you are looking for is the attached Alcoa Alumina design pamphlet.  There are various versions and interpretations of the Ergun equation and I've found this Alcoa version to be good enough for design purposes in the past.

 

I downloaded this pamphlet well over 10 years ago.  I hope it helps you out.

 

 Alcoa_Aluminas.pdf   59.99KB   136 downloads

 

 Estimation of Pressure Drop Across Packed Bed - Alcoa.pdf   54.92KB   160 downloads

 

 Ergun Equation Comparison with Experimental.pdf   238.74KB   126 downloads

[pacogda]

Thank you very much Art Montemayor!!

 

Those pamphlets helped me a lot.  I found the article about Ergun´s equation very interesting.

 

I have estimated pressure drop with the second document equations and the result for desorption conditions (gas phase & high temperature) was quite good; but not for adsorption conditions (liquid phase & low temperature) , for which I estimate around 20% of vendor figures only.  At this point I think I have no more options than ask them again...  

 

Thanks again!

[Vasan_RK]

Art Montemayor

 

Thanks a lot for sharing the valuable documents. Especially the one on Pressure Drop Estimation for Alcoa desiccant, which I was literally not able to find elsewhere.

[Art Montemayor]

Vasan_RK:

 

You are entirely welcome.  These AlCOA documents are no longer available - except from us old timers who used, preserved, and stored them for future use when there was no electronic databases or computers.  ALCOA, a pioneer in adsorbents,  closed down this part of their business and sold out.

 

I hope this information helps you (and others) out as it did me.