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Old   September 14, 2010, 00:53
Default fixed bed gasification
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srr
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I am studying a MSc in Colombia, my thesis is on fixed bed gasification modeling, I have used 3D FLUENT but for fluid mechanics (no reactions), so I am confused about this. I checked this: http://www.fluent.com/software/sf_me...torial_cvd.htm and this: http://www.roiengwest.com/Modeling-G...n-Using-Fluent, both give me some ideas but I need more information, besides, the second says something about a gasification calculator for FLUENT.

If someone can help me with information I would be grateful, because I really need to do this in order to finish my Master's Thesis.
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Old   September 21, 2010, 21:30
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Quote:
Originally Posted by srr View Post
I am studying a MSc in Colombia, my thesis is on fixed bed gasification modeling, I have used 3D FLUENT but for fluid mechanics (no reactions), so I am confused about this. I checked this: http://www.fluent.com/software/sf_me...torial_cvd.htm and this: http://www.roiengwest.com/Modeling-G...n-Using-Fluent, both give me some ideas but I need more information, besides, the second says something about a gasification calculator for FLUENT.

If someone can help me with information I would be grateful, because I really need to do this in order to finish my Master's Thesis.
Exactly, what information are you looking after??
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Old   September 22, 2010, 23:03
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Now I finally can simulate heterogeneous reactions on FLUENT, but I am having problems with UDF's because I don't have experience programming them, I have one for devolatilization and one for char combustion, but i don't know how to program the other UDF for heterogeneous reactions, if someone can tell me where I can find a tutorial on how to do this I will be grateful.
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Old   October 13, 2010, 22:56
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I am having problems in stablishing the case
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Old   October 13, 2010, 23:53
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"stablishing the case" do you mean convergence??
and what kind of UDf are using for heterogeneous reactions??
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Old   November 29, 2010, 20:08
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Did you manage to solve your problem. I am working in something similar and I would appreciate help.
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Old   November 29, 2010, 21:22
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antonio,
what kind of problem are you looking at?
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Old   November 29, 2010, 22:43
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Chétan,
I have performed experiments in a cylindrical reacting char bed were a mixture of gases (CO2, H2O and N2) are injected in the top and passes through the reacting bed. Measurements of pressure drop, gas composition (longitudinal direction) and temperature (radial and longitudinal directions) were made. The char bed height was kept to constant cm due to continuous feeding.

I am trying to model the experiment above using Eulerian (granular > fixed bed). I want to be able to calculate:
* Pressure Drop
* Volume Fraction
* Species production and destruction
* Temperature
* Velocity

Does Fluent 12 have the capability of solving this on its on, or UDF's are required?
Do you have any suggestions?

P.S.: I want to model this in steady state
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Old   November 29, 2010, 23:47
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antonio,

yes, you can solve your problem with Fluent 12.

if you wish to use Euler-Granular model to simulate gas-solid packed bed you need to write UDF for heterogeneous reactions.

With Euler granular approach, steady state modelling is not possible. you need to carry out unsteady state solution with very small time steps.

There is another software you can work with is MFIX for fixed bed calculations. i haven't used it, as i prefer to work with Fluent.

all the best, regards
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Old   November 30, 2010, 00:16
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Chetan,

I did manage the problem, as Antonio said, it is possible on FLUENT 12, which was the software used in my thesis, I had to use an UDF for heterogeneous reactions with the Euler granular aproach and the solution was transient (steady state does not work). The turbulence model was k-e standard and the simulation was 2D axis symetric.

I got good agreement between the producer gas composition calculated by the software and that obtained in the lab, but in the temperature profile I found a big difference between them.

All those parameters you mencioned can be obtained from the calculation in FLUENT. The only one I am not sure is the volume fraction, because as I saw in my simulation it is more an input rather than an output (anyway I did not check if I could get that info from the simulation, because I did not need that result; you should check if it is possible)

Hope this helps
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Old   November 30, 2010, 00:20
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Srr,

i agree with you, i am finding similar problem with temperature profile. I am getting higher temperature.

regards
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Old   November 30, 2010, 00:26
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Srr, one thing i am not sure, why did you used turbulence model?? as a packed bed simulation where presence of dense particle phase usually there is no need to implement turbulence model
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Old   November 30, 2010, 00:47
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In many papers I have found that there is important to include turbulence model in numerical aproximation for fixed gasification. Moreover, when I used the eulerian model, I ned to stablish two phases: the continuous, which was the gases mixture, and the one dispersed in the continuos phase, which was the solid particles.

In FLUENT, my char bed was not static, it moved very slowly, its speed was equal to the conversion rate of the reactor, I defined it as velocity_inlet. So both phases were considered as flows whose mixture depends on the turbulence of flows.

PD: my temp is also overestimated.

Regards
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Old   November 30, 2010, 01:02
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Well, i didn't applied turbulent model with assumption that laminar gas-solid flow in dense particles where inertia of the solid particles is much more higher than the gas and role of the gas phase viscosity of in both laminar and turbulent region is negligible ( Agrawal, Syamlal, loezos and Sundaresan 2001).

It will be great if you could share a paper's title where unimportance of turbulence model in numerical aproximation for fixed gasification is elaborated.

One more small question, while defining heterogeneous reaction i used similar reactions and reaction rate equations as in Syamlal and Bissets's MGAS model (1992), in your modelling effort which paper did you followed to adopt reaction rates??

Thanks regards
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Old   November 30, 2010, 10:17
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Gerun, L. ; Paraschiv, M. ; Vˆijeu, J. ; Tazerout, M. ; Gbel, B. ; Henriksen, U.: Numerical investigation of the partial oxidation in a two-stage downdraft gasifier. In: Fuel 87 (2008), p. 1383–1393


This paper proposed a simulation similar to the one I made, and uses the same turbulence model.




For some het reactions I used:



Sudiro
, M. ; Pellizzaro, M. ; Bezzo, F. ; Bertucco, A.: Simulated moving bed technology applied to coal gasification. In: Chemical Engineering Research and Design 88 (2010), p. 465–475



and someone helped me with the char combustion reaction, he used:


Wen at al. (1982), Syamlal and Bissett (1992), Syamlal (1993) Intrinsic rate from Desai and Wen (1978), originally from Sergeant and Smith (1973)




Regards


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Old   November 30, 2010, 17:52
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Thanks Srr, for sharing titles. Well, as your bed is moving slowly (like entrained flow gasifier), it does affect by turbulence. Generally, with dense phase, we can assume flow is laminar. Anyway, thanks again.
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Old   November 30, 2010, 22:56
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Quote:
Originally Posted by srr View Post
Gerun, L. ; Paraschiv, M. ; Vˆijeu, J. ; Tazerout, M. ; Gbel, B. ; Henriksen, U.: Numerical investigation of the partial oxidation in a two-stage downdraft gasifier. In: Fuel 87 (2008), p. 1383–1393


This paper proposed a simulation similar to the one I made, and uses the same turbulence model.




For some het reactions I used:



Sudiro
, M. ; Pellizzaro, M. ; Bezzo, F. ; Bertucco, A.: Simulated moving bed technology applied to coal gasification. In: Chemical Engineering Research and Design 88 (2010), p. 465–475



and someone helped me with the char combustion reaction, he used:


Wen at al. (1982), Syamlal and Bissett (1992), Syamlal (1993) Intrinsic rate from Desai and Wen (1978), originally from Sergeant and Smith (1973)




Regards


I would also add this reference:
American Journal of Applied Sciences 3 (10): 2068-2075, 2006 (The 2D Eulerian Approach of Entrained Flow and Temperature in a Biomass Stratified Downdraft Gasifier)
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Old   November 30, 2010, 23:11
Default Steady state X Transient
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Quote:
Originally Posted by coalgas View Post
antonio,

yes, you can solve your problem with Fluent 12.

if you wish to use Euler-Granular model to simulate gas-solid packed bed you need to write UDF for heterogeneous reactions.

With Euler granular approach, steady state modelling is not possible. you need to carry out unsteady state solution with very small time steps.

There is another software you can work with is MFIX for fixed bed calculations. i haven't used it, as i prefer to work with Fluent.

all the best, regards
Thanks for your help!

Why is not possible to model in steady state? According to Fluent User Guide (24.7.5) The solution method Multiphase Coupled "works very efficiently in steady state situations, or for transient problems when larger time steps are required"

What would be a very small time step?

Best wishes,
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Old   November 30, 2010, 23:27
Thumbs up Bed moving slowly
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Quote:
Originally Posted by srr View Post
Chetan,

I did manage the problem, as Antonio said, it is possible on FLUENT 12, which was the software used in my thesis, I had to use an UDF for heterogeneous reactions with the Euler granular aproach and the solution was transient (steady state does not work). The turbulence model was k-e standard and the simulation was 2D axis symetric.

I got good agreement between the producer gas composition calculated by the software and that obtained in the lab, but in the temperature profile I found a big difference between them.

All those parameters you mencioned can be obtained from the calculation in FLUENT. The only one I am not sure is the volume fraction, because as I saw in my simulation it is more an input rather than an output (anyway I did not check if I could get that info from the simulation, because I did not need that result; you should check if it is possible)

Hope this helps
Quote:
Originally Posted by srr View Post
In many papers I have found that there is important to include turbulence model in numerical aproximation for fixed gasification. Moreover, when I used the eulerian model, I ned to stablish two phases: the continuous, which was the gases mixture, and the one dispersed in the continuos phase, which was the solid particles.

In FLUENT, my char bed was not static, it moved very slowly, its speed was equal to the conversion rate of the reactor, I defined it as velocity_inlet. So both phases were considered as flows whose mixture depends on the turbulence of flows.

PD: my temp is also overestimated.

Regards
Dear Srr,

I would be very interested in read some of your work on this. It would be great if you could send me a copy of your thesis.

One of the approaches I just discussed with my supervisor is to use a slow moving bed.

How did you manage to match the velocity (consumption of the bed) with the conversion rate of the reactor? Where do you set this?

Does the bed keep its height or it reduces proportional to its movement downwards?

Best Regards
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Old   April 6, 2011, 13:45
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Srr,

You mentioned that you had UDF's for devolatilisation and char combustion before you learned how to program the remaining heterogeneous reactions. Did you use UDFs for the devolatilisation and char combustion or the built in combusting particle laws? If UDFs, what macros did you use (VR_RATE, HET_RXN_RATE, DPM_LAW...), and would the remaining het reactions not use the same macro as the char combustion?

I am familiar with the DPM for pf furnaces but am now looking at fixed/fluidised bed simulations using the eulerian/eulerian model. Could you please tell me how you set up your phases - gas mixture/sand/fuel particles for a fluidised bed. It appears that fluid/fluid(granular)/DDPM respectively is the best method as it enables use of the combusting particle laws. However, I am not familiar with fixed/fluidised bed simulations so could someone please confirm?

Many thanks,

Neil

Last edited by neilduffy1024; April 7, 2011 at 09:55.
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