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Having problems setting up surface reaction, catalyst, adsorption and desorption

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Old   March 31, 2014, 13:14
Question Having problems setting up surface reaction, catalyst, adsorption and desorption
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Juun
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(sorry, this is long, just tried to describe my problem clearer)

Hi, everyone

I am doing a simulation on catalyst reaction with ammonia (NH3). The adsorption and desorption rate is important.

I stuck at setting up surface adsorption and desorption reactions. I am not sure about the meaning of ‘selected site species’, so I just copy 2nd NH3 gas from fluent database and add to this site species (with name NH3-S), (the 1st NH3 gas is added to ‘selected species’ and used in inlet mixture).

Then, I set 2 reactions (backward reaction would do it, but I have different Arrhenius rate for each)

Adsorption NH3 = NH3(S)
Desorption NH3(S)=NH3

My first question(s):
Is this the way to set up ad/desorption on the surface in Fluent?
How to use the site-species correctly? (Because I not sure about my understanding, maybe it should be the surface material of the catalyst??)

Then, I want to include surface coverage dependent in desorption reaction, so I tick the option (coverage dependent reaction) at the bottom.

To test it, (only desorption reaction is selected), inlet NH3 is set to 0, and in the reaction mechanisms-site density-initial coverage NH3 is set to 1. I expect to see a small amount of NH3 is released from the surface, but the reaction did not happen during calculation.

However, when I turn off coverage dependent reaction; set initial coverage NH3 to 0, the result shows me NH3 is released from surface, which should not happen.

It seems like the option of coverage dependent reaction controls the ‘on’ and ‘off’ of the desorption reaction, no matter what values is given in initial coverage.

My expectation for this desorption reaction is: surface releases NH3 only when surface coverage > 0, and it stop when surface coverage = 0. The coverage-dependent reaction should do it (by looking at the equation), but I don’t know where is wrong in my model.

The second question(s): why is this happen? Do I have to use UDF to set up anything? Or any other ways can get to the expectation?

Thank you very much for your time and help
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Old   April 1, 2014, 09:52
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Good day!

First, when you turn on the Species Model, FLUENT automatically creates the mixture. Mixture is comprised of several species (gas/fluid phase) and site species. Selected (that means, you select only that you need, you can add a lot of species to materials) site species are the species that are present on catalyst surface. All that you should specify is species molar mass (also, entropy and enthalpy of site species could be specified, but they are, generally, not known). When copying from existing material molar mass is duplicated. So, adsorption/desorption mass conservation seems to be satisfied.
But the second thing to account for is the conservation of number of sites. In your reaction NH3(s) suddenly "appears" from gas. How much NH3(s) can be adsorbed on surface?
FLUENT provides a model (see Theory Guide or "Surface Chemkin manual" in the web for explanation) that treats catalyst like a surface with constant site density - a number of catalytic (active) sites (in kmol) per m2 of surface (let it be active surface).
To specify this, you go into Materials -> your mixture -> Mechanism -> number of sites (1) -> site density (kmol/m2).
Only monolayer adsorption (true for chemisorption, chemical bonding occurs) is under consideration (see "g").
Then for the conservation of site species number, we must write (e.g. take Platinum):
NH3 + Pt(s) => NH3(s)
Molar mass of NH3(s) now should be the sum of NH3 m.m. and Pt m.m. (I think, Pt molar mass can be set arbutrary unless you simulate an etching)
Desorption is described as the reverse reaction.

Second, set a plausible rate parameters (analyze literature, try this article at first).

At third, do you simulate wall surface reaction (catalytic wall - e.g. tube with coating)? Then you should specify coefficient (on wall b.c.) - m2 of active surface per m2 of you wall (look for Surface Area Washcoat Factor).

At fourth, turning on "coverage dependent reaction" must be assisted with additional parameters input ("specify" button will be active).

And at last, when we do a steady-state (I suppose, you do it) calculation, we want to find a steady-state solution. Initial conditions then (in "mechanism" - initial site coverage) are only initial guess for the solution. You can specify 0 for some species and have e.g. 0.7 on your catalyst after simulation. The behaviour of the model, though, can be dependent on initial guess quality.
Note: Check the sum of initial coverages is 1.

Best wishes, Villager.
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Old   April 1, 2014, 14:09
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Hi, Villager, Thank you so much for your help. The answer is very informative and detail.


My problem is a square tube, the complete inside wall is coated with active material (just like a normal catalyst), and mixture goes in from one side, goes out from the other.


Therefore I set Surface Area Washcoat Factor to 1, then "coverage dependent reaction" I used the simplest one, set ‘Mu’ to 1.


But I have further question about the conservation of site species number, as you say: NH3 + Pt(s) => NH3(s)


Is Pt(s) site species? Can I take it like the surface coating? So, I only need to entry a value to absorption capacity (site density) on this site.
Then NH3(s) is the second site species, I guess I don’t need to set site density for this one, as it is a product on the surface. Is my thought correct so far?


The problem here is the molar mass, if NH3 is 1, Pt(s) is 1, and NH3 should be 2.

Will this affect the later reactions, (e.g. NH3(s)+A; NH3(s)+B …)?? because the NH3(s) is 2 molar now, then more reactions happens.
When I set NH3 to 1, Pt(s) to 1, NH3(s) to 1, FLUENT says mass imbalance.
I’m confused here.

At the moment, it is steady state, when this works, I’ll move to transient to see surface coverage % vs time (also with temperature variation). But right now, the surface coverage is either 1 or 0. somewhere is wrong but I havn't figure out yet : (
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Old   April 1, 2014, 15:20
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Catalytic coating is a porous structure, therefore, the surface usually evolves (increase) after coating compared with the wall area. Chemisorption data for active surface determination may be taken into account.

Yes, Pt(s) is site species - imagine e.g. in initial state all surface is free of species - then the surface coverage of Pt(s) (free sites! - but it's indifferently for the model) is equal to 1.
Site density is set for the whole site! Don't mix site and species concepts. This model understands sites as some places (points) on catalyst. Every site can be filled with some species or it can be void.
Different sites = Pt and Ni e.g., if you have both (mixed catalyst). Then different species on different sites (like NH3(s) and NH3(s2) ). I think, it's not your case.

NH3(s) is NH3 linked with platinum, so its "molar mass" is equal to NH3 mass + Pt mass. I checked different m.m. for Pt (both consistent with NH3(s) and not), it seems like having no effect on solution. M.m. are important when simulating etching/deposition. But I prefer to maintain stoichometry balances - for Pt(s) I would input its real m.m. and so on.
but when you import kinetic mechanism from CHEMKIN files NH3 molar mass is important if you use sticking parametrization of adsorption (
see Theory Guide or "Surface Chemkin manual" for more info on that).

Try to move reaction parameters.
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Old   April 3, 2014, 12:16
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Hi, Villager. I am so grateful that you have taken time to help me with the problems. The result is close to my expectation now.
Despite there is an imbalance in the reactions (the strange thing is that, when I use 1 molar mass for NH3 1, Pt(s) and NH3(s) on my laptop’s model, there is no error saying mass imbalance, unless I use 1, 1, 2 or something else. However, when it comes the desktop, the error appears in both case, 1,1,1 and 1,1,2, all other settings are the same. I just ignored it and run calculation, the result looks OK.) Not sure why, but adsorption works. Thank you.
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Old   November 21, 2014, 04:50
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Hello Juun and villager.
I have a problem similar to described by Juun. I want to simulate CO2 adsorption with amine: CO2(bulk)+2DEA(site)-->product1(site)+product2(site). Im doing a transient simulation, the reaction rate looks working fine, DEA concentration reduces with time as products increases. Heat due the adsorption seems to work well.
The problem is that CO2 does not flow when transient+reaction is togheder running. I can stablish a CO2 concentration at fluid volume, but it does not change with time neither varies its concnetration (seems to be 'freezed').
I hope you both can help me with that issue.
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Old   November 21, 2014, 10:03
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Quote:
Originally Posted by asfrallego View Post
Hello Juun and villager.
I want to simulate CO2 adsorption with amine: CO2(bulk)+2DEA(site)-->product1(site)+product2(site).
Do you have an aBsorption or an aDsorption process?
MEA/DEA are traditionally used for aBsorption of CO2.
Adsorption of CO2 is also possible with amine-based adsorbents.
http://aaqr.org/vol12_no5_october201...32_745-769.pdf
So, you have, as I understand, you consider amine-based solid adsorbent with active surface for capturing CO2. Am I right?
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Old   November 24, 2014, 03:57
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Thanks for your reply villager.

The process is an adsorption (amine impregnated adsorbent). My support is a gamma-alumina with DEA. The paper you send me I have read before, but not much newest things it says.
I try to simulate the alumina support as a porous media and the DEA as a surface coverage specie. i have all the input data to feed Fluent (still waiting for real Arrhenius paramethers from the lab) but this seems to work all properly.

But my problem is that I cannot simulate at the same time the surface reaction and the flow for CO2 (and other non-reacting species). I have tried to change the products form site species to solid species (to observ the rate of deposition) but this does not change the problem.
If you have any new suggestion, I am waiting to read it!
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Old   June 7, 2015, 07:11
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Hello Villager,

I am working on a same kind of problem that is as follows:

There is a cylindrical chamber in which some volatile compound being injected from inlet and there is wall inside of this chamber that is made of some material on which I need to simulate Adsorption and Desorption of this volatile compund.

Till this material continues adsorbing VOC compound or act as a sink, injecting VOC continues and once it reaches to Equilibrium when there is no Desorption and Adsorption or it does not act as source or sink,we stop injecting VOC and inject fresh air.So Desorption starts from wall.

And this condition I have to defined as user defined function on this wall material surface as boundary condition.

This whole procedure I need to do in Ansys Fluent.So if you have any idea about how to initiate and proceed for this Problem,Please share your ideas.

Thanks for Suggestions.
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Old   April 26, 2016, 17:35
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Hello guys

Im trying to do species transport to my problem. I chose mixture-template for my reaction and also I defined my needed material. But in the mixture-template dialogue box when I am trying to add my material and remove N2, O2 and H2O (the default ones), I cannot remove them (I can add) and it shows me this error:

"Species n2 is used in one or more reactions." for example when Im trying to remove n2 from my mixture.

If someone have any experience plz help me. I really stuck there
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Old   June 20, 2016, 20:56
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Quote:
Originally Posted by Ebimo View Post
But in the mixture-template dialogue box when I am trying to add my material and remove N2, O2 and H2O (the default ones), I cannot remove them (I can add) and it shows me this error:

"Species n2 is used in one or more reactions." for example when Im trying to remove n2 from my mixture.
I confirm it. it seems at least strange!
I have found a workaround:
File -> import -> Chemkin Mechanism -> specify the Gas-phase CHEMKIN Reaction Mechanism File with your mixture. The file example is listed below:
Code:
ELEMENTS
O H C
END
SPECIES
CO CO2 O2 CH3OH H2O
END
The code defines 5-component gas mixture.
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