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Hydrocarbon Condensation When Relieved At Atmosphere


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#1 Pedro_Kaiser

Pedro_Kaiser

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Posted 23 September 2023 - 06:08 PM

Hello everyone!!
What are the issues in relieving a hydrocarbon (HC) to the atmosphere when this HC is relatively heavy and it is above its boiling point?
For example, consider 120 °C heptane stream being flared. Will the eventual unburned heptane condense and fall down to the ground when it enters in contact with 25 °C atmospheric air? Is it possible to occur a "rain of fire" in an extreme condition?
How could I simulate or calculate this event?
Thank you!

#2 shvet1

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Posted 25 September 2023 - 11:50 PM

Flares have high destruction rate. I would suspect 98%. Experienced forummembers may correct me. This means that unburned portion of relief has low rate and pre-dispersed in large amount of hot air. Some HCs will be carried up by air, some - condense at mist which is not able to be settled down by gravity, some - vaporize and only small portion of this mist coalesce and form some sort of rain. Taking into account that area exposed is large rain can affect surrounding premises but this will rather not cause complaints as it will be in form of e.g. dots on a vehicle windshield.

 

This mist is able to condense and collect on surfaces if comes in contact with but usually this is not a case as surrounding structures are located far enough from plume.

 

Yes - such events (morning dots on windshield) are unpleasant, but who cares. This has a potential to reduce cost of surrounding premises but this is not the end of the world. Speaking of C6 all undistracted relief will vaporize and disperse.

 

Destruction is higher the less relief capacity. Taking in account that max relief has low probability such situations should be rare.

 

Usually combustion products (having low odor/toxic threshold) and smoke pose more problems that destruction rate.

 

If a flare has been designed (liquid separation, elevation, dispersion, radiation, destruction, sound decay, capacity etc.) and works well there is no chance of fire fallout or similar.

 

Hope the core ides is clear.

 

CCPS's Guidelines for Pressure Relief and Effluent Handling Systems 2nd ed.

6.5.6 Flares

...

Flares convert flammable, odoriferous, or selected toxic vapors to less noxious materials. Flares efficiently convert flammable hydrocarbons to water and carbon dioxide and convert highly toxic reduced sulfur materials such as hydrogen sulfide to less toxic sulfur oxides. A primary advantage of a flare is a very high destruction efficiency (greater than 98% for almost all volatile organic compounds and greater than 99.5% for most light hydrocarbons) having large and rapid variations in flows, compositions, and heating values.
The suitability of a flare to handle an emergency relief discharge depends on the availability of space, the properties (particularly heating value) and flow rate of the material to be flared, the location of the flare relative to plant operations and public facilities (considering the noise and nuisance impact of the flare, and potential for flame-out) and capital and operating costs.
...

Edited by shvet1, 26 September 2023 - 05:39 AM.





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