JSON file for Solid Carbon

[Daniel Medeiros]

Hi all,

I've created a JSON file for Solid Carbon which you can import and use on your simulations. Download the attachment and import the file on 'Simulation Settings' > 'Compounds' > 'Import (JSON)'.

Regards
Daniel

[Daniel Medeiros]

If you've also created compounds not present in the default databases and would like to share with other users, please use this topic to distribute your JSON files.

[ioncube]

Thanks, so is there a tutorial how to build this JSON file, lets say if I want to do that for DOWTHERM oil

[Daniel Medeiros]

Have you played with the compound creator already?

[ioncube]

Will get back to you after I learn it myself :)

[Daniel Medeiros]

A simple google search for "dwsim compound creator" leads you to http://dwsim.inforside.com.br/wiki/index.php?title=Using_the_Compound_Creator_Utility

[Pavel Hruby]

Hi Daniel,
first of all thank you for the great work you do. I tried to use the Solid Carbon compound that you created, but I'm still puzzled how to set up the simulation properly so that I get the right heat of combustion and the simulation works. ( I know, that the similar question was already raised in the forum before)

If I use your compound for the combustion with oxygen (conversion reactor) for isothermal calculation I get the heat of combustion that doesn't seem to be correct. If I try to do an adiabatic calculation with your compound the calculation ends with error. So I tried to create my own compounds with the following standard enthalpies of formation (C=0, O2=0, CO2=-393 kJ/mol), but the calculation ended with errors as well.

I would have one more general question regarding the compound definitions for the combustion simulations. I tried to do simple hydrogen combustion with oxygen. The heat of combustion was ok, but the adiabatic flame temperature was too high. The reason seems to be that the heat capacity for the compounds is defined only up to 1500 K. I tried to define my own compounds where the heat capacity was defined through expression, but it seems that the limit 1500 K is still there.

I'm sorry if the questions are too basic or I'm missing some obvious solution, but I have a background in civil engineering and this all is still very new for me. Thank you

Regards,
Pavel

[Daniel Medeiros]

Hi Pavel,

Can you attach your simulation file?

Regarding the adiabatic flame issue, there is no forced upper limit for the ideal gas heat capacity. DWSIM simply uses the provided equation coefficients and temperature to calculate the value based on the defined expression. Where did you find this 1500 K limit?

Regards
Daniel

[Pavel Hruby]

Hi Daniel,
please find enclosed the hydrogen combustion simulation file with the supporting files (compounds etc.). I have to admit that the 1500 K limit was a guess, because that is the max. value which is shown in the pure compound viewer as you can see in the attachments. I didn't do the proper recalculation, to check whether the numbers fits.

Regarding the solid carbon combustion, I can't find the simulation file at the moment (I played with that few weeks ago). I will try to recreate the simulation in the following days and I'll send it to you. Thank you for all your assistance

Best regards,
Pavel

[Pavel Hruby]

Hi Daniel,
I recreated the carbon combustion simulation using your carbon compound (and also slightly modified carbon compound - to get right heat of combustion). As I wrote previously there is a problem during adiabatic calculation (converstion reactor). I tried also the Gibbs reactor and I wasn't able to make it work either.

Regards,
Pavel

[Daniel Medeiros]

Thanks Pavel, I'll take a look ASAP.

Regards
Daniel

[Daniel Medeiros]

The adiabatic reactions are not spontaneous at standard conditions. Since they need a huge amount of energy when in isothermic mode, it means that the reactions need energy to occur. You should either increase temperature (https://www.engineeringtoolbox.com/fuels-ignition-temperatures-d_171.html) or add the required energy to the reactor through the energy stream.

[Pavel Hruby]

Hi Daniel,
thank you for the quick reply. I'll need to think about it (and do some calculations).

Regards,
Pavel

[Pavel Hruby]

I thought about it a bit and I'm still puzzled (I'm sorry for that). I believe that from purely thermodynamic point of view both reactions (i.e. solid carbon and hydrogen combustion) are spontaneous (i.e. ΔG°<0) at standard conditions. As I understand it the auto-ignition temperatures that you mentioned are more related to kinetics (activation energy) and I don't know if this appears in simple conversion reactor. Moreover if your argument is correct then the adiabatic hydrogen simulation shouldn't work as well (auto-ignition temperature of hydrogen is 500 C), but it's working fine (the temperature difference that I mentioned previously for adiabatic hydrogen combustion is very likely caused by dissociation of water back to H2 and O2 at high temperatures). What am I missing? Thank you once again for your time

Regards,
Pavel

[Daniel Medeiros]

Solid carbon combustion is spontaneous at standard conditions? Are you sure about that? Don't we need a heat/ignition (= energy) source to make it happen?

[Pavel Hruby]

I'm not 100% sure (as i said, I'm very new in this field), but I think, that it's the case. It's is spontaneous in "thermodynamic sense" (i.e. ΔG°<0).
If I use citation from Atkins (Physical Chemistry):
.....‘spontaneous’ must be interpreted as a natural tendency that may or may not be realized in practice. Thermodynamics is silent on the rate at which a spontaneous change in fact occurs, and some spontaneous processes (such as the conversion of diamond to graphite) may be so slow that the tendency is never realized in practice...
And my point was, that the conversion reactor is probably using this thermodynamic notion. You are definitely right that in reality you need heat/ignition to start the reaction, but I thought that this is more related to kinetics and that the conversion reactor 'doesn't know that it needs the heat to start the reaction'.
Check the enclosed file. It's adibatic simulation of the methane combustion in air. Methane and Air (N2+O2) are at 25 C. You can also argue that when you (in reality) mix air and methane at 25C nothing is going to happen and you will need to heat it or ignite it to start the reaction, but yet the conversion reactor in the simulation is able to solve the reaction. Shouldn't it work the same for the solid carbon?

[Daniel Medeiros]

The difference here is that methane is already vapor, no energy is needed to take it to the "ideal gas state" where the reaction's enthalpy of formation balance occurs.

For solid carbon you have a very negative enthalpy due to its solid state. Not sure if it is correct - I would need to check my compound definitions but, in order to react it with oxygen, you need to take it to an hypothetical ideal gas state (consumes energy) then react it with oxygen. The calculation is failing because such energy would need to come from somewhere and the heat balance tries to take it from temperature. A negative absolute temperature then leads the calculation to a failure.

In DWSIM (and other simulators I believe) there is no such concept of spontaneous reaction (ΔG°<0). What exists is a heat balance (products enthalpy = reactants enthalpy + heat of reaction), if the reaction is exothermic then it releases energy (isothermic mode --> energy stream with positive energy amount), adiabatic mode --> temperature generally increases) and if it is endothermic then the opposite happens. Such calculations depend directly on enthalpies of formation, which is why I'm now displaying them on the reaction editors.

[Pavel Hruby]

Hi Daniel,
do I understand it correctly that in your implementation you need the compounds in the "ideal gas state" in order to solve the reaction?

Usually solid carbon has standard enthalpy of formation 0 and therefore the solid carbon combustion (C(s)+O2) is exothermic. I might be wrong, but I believe that in reality the reaction doesn't go through C(g) (i.e. C(s) -> C(g) -> CO2(g)) it means that you don't need to atomize (sublimate) carbon in order to get CO2. The energy needed to do so is really huge (temperature of sublimation of carbon is 3915 K, which is not usually available during combustion).

If this sublimation of carbon is necessary in your code, then I understand that the energy from reaction (-393 kJ/mol) is not sufficient to maintain the reaction since the enthalpy of sublimation (atomization) of carbon is 717 kJ/mol, but I'm not sure whether it is physically correct.

Thank you once again for interesting conversation, it helped me a lot. I really hope, that you don't take it as a criticism of any kind.

Regards,
Pavel

[Daniel Medeiros]

All formation properties in DWSIM are for the compound in the ideal gas state at 25 C. This is how DWSIM does the reaction's energy balance (and every other simulator, I believe):

If I remember well, I took the enthalpy of formation for solid carbon from cheméo. Maybe it needs some adjustment in order to compensate for the state change from solid to (ideal) gas... you could try that.

[Daniel Medeiros]

Here you'll find the enthalpy of formation for carbon as an ideal gas: https://www.chemeo.com/cid/26-079-6/carbon

If you do the math you'll find that I've indeed used this value (716 kJ/mol = 59666 kJ/kg). Maybe it is wrong???

[Pavel Hruby]

Hi Daniel,
thank you for the information, I'll think about it. I'm currently heading abroad, so hopefully I will be able to come back to that in a few days.

Regards,
Pavel

[Shawn Esquivel]

Hi Daniel, I have been using your JSON file for solid carbon in my simulation. One odd thing I noticed is that in my 800C, 13bar stream, the carbon is acting as a vapour.

I double checked it in another simulation and found that it was acting as a liquid..

This is using Peng Robinson method.

I just want to check if this is normal or if there is some sort of bug in my simulation.

[Daniel Medeiros]

You can force it to stay in the solid phase on the property package advanced settings window.

[Shawn Esquivel]

Hi Daniel,

I did as you instructed and also switched my flash algorithm to Nested Loops SVLLE Eutectic.

I was able to successfully force it to stay in solid phase by using Nested Loops SVLLE method, but now there is a PT Flash: Error calculating amount of vapour phase in the mixture.

I have a heat exchanger modelling a boiler.
Hot stream is vapour/solid carbon, leaves as vapour/solid.
Cold stream comes in as liquid water, leaves as vapour steam.

Am I breaching the one solid, one vapour, two liquid equilibria requirements here?

And my simulation seemed to run fine when I was using carbon as a gas -- what are the implications of using it as a gas other than the volumetric flows being inaccurate?

[Daniel Medeiros]

The heat exchanger model wasn't designed to work with solids. There may be errors in the heat balance.