Ar Microwave Plasma Plug Flow Reactor that Pyrolyzes Methane in an Inert Atmosphere(Ar,H2,CH4)

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  • Last Post 04 July 2019
AngelaPaul2019 posted this 14 June 2019

Hello,

I have read the tutorials in the CHEMKIN-PRO manual with respect to the plasma PSR and PFR reactors for plasma etching using Cl gas developed by Meeks and coworkers. Those are similar but very different from what I want to use CHEMKIN for here - analyzing product distributions of an argon plasma driving methane pyrolysis. 

My hope with this post is to get guidance on

1. What my progression should be to develop this model(simplest to more complex)? and

2. What CHEMKIN-PRO(Version 19.2) can do to most closely model this situation(can I use the plasma PFR module or can I tweak the chlorine module)?

It's a non-equilibrium, non-thermal, Ar, microwave wave guide, no-electrode, plasma reactor. The reactor uses this plasma to pyrolyze methane in an inert atmosphere and its configuration is most similar to a plug flow reactor. It does not go through oxidation reactions as the only inputs materials are Argon, Hydrogen and Methane gases. There is also a defined hot zone where the plasma intersects the gas mixture flowing by it after which the radical reactions propagate downstream. As such temperature profiles can be used to model that gradient. The reaction produces solid carbon, hydrogen, aromatics and light hydrocarbon gases and the research group would like to do parameter studies for varying input temperatures/powers and concentrations of the three input gases.

Thank you very much in advance for your goodwill and time.

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jcooper posted this 18 June 2019

Hi Angela:

You mention that your reactor is non-equilibrium and that the desired output is is the product distribution.  Both of these descriptors would make the PFR seem a more appropriate vessel, as a PSR assumes equilibrium  (unless you run a transient) and is 0D. A PSR could be used to start an investigation of the chemistry, however.

The most interesting question here is the Chemical Kinetics. Most mechanisms for methane involve oxygen, so an oxygen-free environment may require a different mechanism, since the reaction rates can depend on other species that are present, but do not necessarily participate in reaction.

If this is a longer-term project, would suggest starting with a thorough literature review to see what other researchers have done to approach this problem.

 

ie: Chemical Kinetics of Methane Pyrolysis in Microwave Plasma at Atmospheric Pressure

Plasma Chemistry and Plasma Processing March 2014, Volume 34, Issue 2, pp 313–326 

 

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AngelaPaul2019 posted this 18 June 2019

Hi!

First of all, thank you for your response Ms.Cooper! I have edited my original responses for clarity, simplicity and consolidation purposes. 

1) Could I please have/be told of any and all resources you have on how the Plasma PFR module on the ChemKin Pro package (a)works and is (b)used? My university email is agp5190@psu.edu.

2) How do I go about creating an Ar Plasma reaction input file like the chlorine chemistry set for the CHEMKIN plasma tutorials(Please see section 4.4.5 and 4.3 in the Tutorials Manual)?

3) Can I pair the Particle Tracking Feature with the Plasma PFR module?

4) How do I go about adjusting the JSR/PFR sample set to be for methane now instead of C2H4? This is with regard to Tutorial 2.7.1. Soot Formation and Growth in a JSR/PFR Reactor

 

jcooper posted this 04 July 2019

Hi Angela:

(1) Unfortunately, we don't have many references, aside from the tutorials and Chemkin theory documentation. The most direct way to get an understanding of how the module works is to go through the tutorial and try to understand how the contents of the mechanism relate to the various Chemkin inputs. (Most Chemkin mechanism files are just text files, although some are encrypted.)  The Chemkin theory manual has a section on plasman systems that will show what calculations are going on behind the scenes.

The  difference between a Plasma PFR and a regular PFR are the inputs relating to plasma and electrons. These inputs, (and the associated reactions) affect how the CH4 reactions progress because the information you enter feeds back to the chemistry calculations.  If you double-click the C1_Plasma_PFR in the tutorial, you will see these inputs.  Mouse-hovering over the text that describes each entry will pop up a description of what it is, so that you can relate it back to the nomenclature used your own studies and any papers you use as background.

(2)  For the thermal and microwave plasma driven pyrolysis of methane (see the two links), people have been combining the gas phase mechanism for methane (combustion chemistry) with a bunch of reactions involving excited species (plasma initiation).  You may need to do a bit of digging to come up with the best reaction set for plasma initiation

https://www.sciencedirect.com/science/article/pii/S0378382017315333

https://www.techbriefs.com/component/content/article/tb/techbriefs/manufacturing-prototyping/8485

 

Pressure in the plasma driven process is much higher than that in a stabilized plasma process (e.g. CVD). Therefore, many users are simply ignoring the separate electronic energy equation and related adjustment to the bulk gas energy equation that is described in the Chemkin Theory Manual - Ch. 8. With that simplification, excited species in methane pyrolysis are treated like other gas phase radicals. The process is driven by the reactions and rate constants in the mechanism. 

The Chemkin Inputs manual describes the reaction mechanism syntax and elements of mechanisms. The reference below, which I found on the web with a search on :"Ar plasma reaction", provides a lead to several other papers, specifically Reference (3) that claim to present mechanisms suitable for modelling:

https://pdfs.semanticscholar.org/1bad/d73c18cd468198e2b20212a968d655676626.pdf

One of these could be combined with the Grimech 3.0 combustion mechanism for methane.

(3)  The dispersed (particle) capability in Chemkin is activated by keywords in the Chemkin mechanism file. Including a DISPERSED material in your Chemkin mechanism will allow Chemkin to recognize particles in a simulation, and is another way of introducing a reactant.  With this, you usually would also declare a corresponding bulk specie that participates in reaction. The mechanism from the tutorial "closed homogeneous particle aggregation" with dispersed phase TiOX is a good example to look at to see how a dispersed material interacts with chemistry. A full description of DISPERSED phase materials is given in Chapter 6 of the Chemkin Inputs manual.

(4)  To adjust the Soot Formation and Growth in a JSR/PFR Reactor tutorial for CH4, load it and then Edit the Chemistry set under Preprocessing,  You will replace chem_abf_1bar.inp with a revised gas phase mechanism that is tuned for CH4 oxidation.  (If you edit the file chem_abf_1bar.inp, you will see that the C1 C2 chemistry is borrowed from Grimech 1.2, which is just an older version of the more recent Grimech 3.0 mechanism, so some reactions may overlap.) Replacing chem_abf_1bar.inp with the Grimech 3.0 mechanism file, grimech30_chem.dat  in C:\Program Files\ANSYS Inc\v19x\reaction\data will take care of the methane chemistry.  Then, run the Preprocessing to check if your Thermodynamics data file will need any additional species.  You can edit the existing therm.dat file and add these by copying from the Grimech 3.0 thermo file grimech30_thermo.dat in C:\Program Files\ANSYS Inc\v19x\reaction\data

 

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