Flammable off-gas - safety measures?

[PetroBob]

I am interested in general answers to this question, as well as answers specific to this process.

We are doing front end design of a bitumen blowing tower. The off-gas will be incinerated to dispose of toxic gases. The off-gas is in the flammable range (VOC content 30,000 ppm as propane, or about 4% propane). There would only be a short run of piping (max 50m/150 ft) from blowing tower to incinerator.

In this case, we will design for no sources of ignition in the off-gas piping, and will add flame arrestor on off-gas pipe just upstream of the burner. Are there other measures that are normally done? (For example dilute the off-gas with air to bring it below LEL?)

Generally, what design measures are normally taken when a flammable gas stream is produced? (In refinery operations, or any other processes.) It seems to me it would be better to somehow design to remove the flammable mix rather than design around it to prevent ignition.

 

[786392]

For safety reasons inert dilution is preferred however this may tremendously increase the pipeline diameter,I understand

Best Regards
Qalander(Chem)

 

[Rocco8]

First you may be below the Minimum Oxygen of Combustion as the mass of asphalt is absorbing oxygen and it is cross linking/ reacting with the asphalt. At the start and finishing of the cycle you will most likely be in the flammable envelope.

The flame arrester will plug and sometimes after only one blow cycle.

I would recommend an Explosion Isolation Valve (EIV) on the fume line to the thermal oxidizer. Asphalt fumes are unique in that they contain both a pure HC vapor and aerosol droplets in the micron range.

Most of the hydrocarbons are in the C11 to C13 range, however they can go as high as C30.

 

[786392]

With all due respects and weight-age to your justified opinion;

I prefer to differ with oxygen %age decrease hypothesis may not be that valid

since exothermic reactions increase the system temperature(s) as well as oxidation rate is directly propotinal to continuously blown air rate in addition to system temperature.

Explosions do occur either due to oxidation cum polymerization reactions run-offs and/or clogging, blockage of downstream air/ fumes path

which usually contain oxygen very close to combustion suitable range.

Moreover as OP is indicating around 4% propane presence how can we nglect this very light hydrocarbon's presence?

Best Regards
Qalander(Chem)

 

[Rocco8]

Fact 1 - We have never seen the propane levels at that concentration and measured oxygen can go below 5% and this is at many locations.

Fact 2 - Turn the air off and the reaction stops. If you loose cooling water the air blower should turn off automatically. No significant reactions occur even at the operating temperature.

Fact 3 - This is primarily an oxidation reaction but there are many other reactions. In addition as the oxygen cross links the hydrocarbons water is released and that water further dilutes (reduces) the oxygen content.

Fact 4 - Properly constructed Flammability diagrams as per the method describe by the bureau of mines can show when you are in or out of the flammable envelope. The flammability diagram contains the flammability envelope and is bound by the the lines UFL-LFL, LFL-MOC, and UFL-MOC. The x axis is the hydrocarbon concentration and the y axis is oxygen concentration. UFL, LFL and MOC are all composite values for the vapors coming off the still. Lower flammability limits in air are deceased approximately 8 percent by a temperature increase of 100 °C. Upper flammability limits are increased 8 percent by a temperature increase of 100 °C.
Pressure has only a slight effect on LFL. Lower limits are essentially constant down to about 5 kPa generally, below which pressure flame does not propagate. The effect of higher pressures on LFL and correspondingly on MOC is slight. On the other hand elevated pressure greatly increases the UFL.

Fact 4 - We have constructed a proprietary safety program that links the Flammability diagram to a batch process such as the blow still operation. The engine driving this program is ChemCad Dynamics. The vapors discharging from the still are graphed on a flammability diagram and if you have two screens you can watch a dot the travel trough the flammable envelope. The dot is the concentration of HC and oxygen. This excel spread sheet has been used in forensic investigations. It automatically corrects for temperature and pressure.

Fact 5 - NFPA says if you are putting a hydrocarbon into a thermal oxidizer you must be at 25% of the LFL or 50% of the LFL if you have an LFL meter. No one does this in practice.

Fact 6 - Fumes are generated from cracking (another reaction) and the nitrogen stripping HCs from the asphalt.

The above statements are facts not opinions.

An article of interest on flashbacks is:

http://www.rccostello.com/flashback.htm

Thanks,

Rocky Costello, P.E.
Visit us at:

http://www.rccostello.com/explode.html

 

[786392]

Thanks indeed for these Great updating "Facts" Dear Rocky!

Best Regards
Qalander(Chem)

 

[PetroBob]

Thanks folks for the useful replies