volatile organic compounds thermal oxidation in the co2 atomosphre 2

[mggkg]

I have a question that Volatile organic compounds (CH4, C2H4, etc Organic concentration is 800vol.ppm, Oxygen 2vol%) can be thermal oxidated in the co2 atomospere?
Reaction temp. is 350~400dC with CTO (Catalytic Thermal Oxidizer, Catalyst is from clariant.
I guess RTO(Regenration Thermal Oxidizer) can not treat this Volatile organic compounds in CO2 atomosphre.
That is why The organic gas concentration is above 500ppm in the RTO Outlet

 

[georgeverghese]

What are the other hydrocarbon components in this stream ? - aromatics ( benzene, toluene and the like) require higher temps for adequate oxidation.

Reaction zone temps have gone up in the last several years, reaching up to 1100-1200degF for uncatalysed oxidisers. Presume your CTO is designed to operate at a lower temp given that this is catalysed. If the exit pollutants concentration now dont meet specifications, would presume this is due to poisoning or plugging of the catalyst bed? A regenerative type TO also may suffer the risk of plugging.

A non catalysed non regenerative type TO may be more reliable. But this stream has the additional complication that CO2 reduction to CO occurs at higher temps (CO is more stable at higher temps)with deposition of carbon. So can the concentration of NOx ( favoured by high temp, high O2 ).

Its a balancing act that these TO vendors need to be well versed at - all I can say is that you'd probably have a better chance of getting to spec if you go up to 1100degF or so with more fuel gas, and take less or no credit for this expensive catalyst bed.

 

[mggkg]

Thanks georgeverghese
No aromatics in this Organic compounds Gas.
And catalyst Vendor Clariant Say 300~400 degC Temp.is OK.
Anyway I tried to heating up more high temp. up to 500 deg.C,
But The result is same.
The volatile organic Gas's composition is CH4 500pppm, C2H4 300pppm, O2 2%, Others CO2 Gas
My question is In low O2 (2vol%) and high CO2 (97% Over)Balance condition, Organic compounds can be oxidized ?

 

[moltenmetal]

You likely have too little oxygen to make this happen. Do a Gibbs energy minimization and see what products you get at equilibrium- the catalyst isn't going to take you anywhere other than to that equilibrium or part way there. It will not suppress other reactions that are thermodynamically favoured, given that some of those will happen thermally (uncatalyzed) to some degree at those temperatures. In particular, I'd guess that CO2 will likely be reduced to CO rather than allowing complete oxidation of your VOCs to CO2 and water. The equilibrium is shifted because you are swamped in one of the products (CO2).

 

[georgeverghese]

Agreed, it does seem like having so much CO2 in the feedstream could suppress the forward oxidation reaction. And any catalyst to aid this oxidation cannot shift the thermodynamic equilibrium. Did the vendor take this into account when they offered you this catalytic TO?

What is the flowrate of this waste stream ? - can you remove most of this objectionable CO2 first before the TO ?

Else, if the total flow is low, maybe route this to an existing fired heater through a dedicated waste gas burner. You'd get much better results with the dilution from the existing combustion air stream.

 

[mggkg]

Thanks...
I agree your answers. But about this question I had pilot test. The result was good. CH4 and C2H4 was removed by CTO. Of course CTO condition was same for example temp. Retention time and gas composition. I have a new experiance. I feed Hrdrogen gas (1%vol of total gas) to the CTO inlet. The result was Hydrogen was removed almost. Hydrogen concentration was 5ppm in the CTO outlet. But CH4 and C2H4 was not. even increased..

 

[georgeverghese]

Another way to remove these organics from the CO2 would to pass this gas through a regenerable activated carbon adsorber. Cold vent the exhaust pure CO2 stream. Regenerate the adsorber with LP steam and then pass this CO2 free stream on to the CTO.