Dilution air to prevent explosion

[op9]

Hope this is not off topic but hopefully a Chem Eng input may help.

I have been working with the commissioning of burners on asphalt plants for many years in Australia on both gas and oil fuels. I am due to commission one on propane in the very near future and have put in all submission documentation for the burner (eg fuel rates, start gas, purge rates and time etc) to the gas authority for approval of commissioning gas. This has up to now sufficed.

This particular inspector was surprised to hear we intended introducing bitumen into the rotary drum as this is a drum mixer. Obviously he has only seen batch plants which only dry the aggregate and mix bitumen externally. He is worried about volatiles in the heated bitumen or asphalt mix exceeding the limit on LEL values. and requires a dilution air calc submitted. This is of the type found in NFPA 86A and copied into our Australian codes. I personally have never done this before but started working through it. I feel however that it isn't applicable to bitumen for the following reason. Bitumen has a flash point of >250degC. Our normal product temp will be 180deg to 200degC max and an overtemp cutout set at 220degC. Hence there should be no combustable vapours given off of any significance. So a dilution air requirement to keep this from reaching say 50% or even 25% of the LEL is unnecessary. Sometimes asphalt plants use RAP a recycled pavement which could contain contaminants of higher volatile compounds but this will not be the case at our plant where only virgin bitumen is used.

Does any knowledgeable person on explosive atmospheres agree or have any comments?

As an aside to this. Other asphalt plants make a product called cold mix. This is made at a low temperature of about 80degC and definitely adds diesel to the aggregate. If it was required to do a calc on minimum dilution air what values would you put in for the equation for dilution air flow in NFPA 86A.Years ago when involved with emission tests on a specific stack they had to calibrate their instrument to check for hydrocarbons from a diesel fired burner. Hexane was used as the calibrating fluid. So would you use values for hexane shown in the tables we have(ie for vapour generated by 1 litre of liquid etc) to insert in the equation. If so there is one equation quantity designated Q which is the max instantaneous evaporation rate in litres/sec at our operating temp. Where would you find that for diesel.
Hope this isn't too long to respond but the first issue on straight bitumen is the most inportant.
Thanks in advance
Rod.


Rod Nissen.
Combustion & Engineering Diagnostics
nissenr@iprimus.com.au

http://www.canded.com.au

 

[Longinthetooth]

I dont know anything about asphalt plants but flash points are measured under standardised laboratory condtions, eg ASTM D93, D56, D1310. The resulting measurement refers only to those standardised conditons and should be only used for comparison of different chemicals or hydrocarbons. The absolute values really dont tell the whole story. You can ignite at less than the quoted flash point if the conditions are right. Also, although your bitumen has a flash point of 250C it will almost certainly be generating significant voulmes of vapour (any light ends for instance) long before you get to 250C.

Regards,

 

[op9]

Thanks Geordie. Where would I find information on distillation temps of these light fractions and the % given off as temperature is raised in a typical bitumen?

Rod Nissen.
Combustion & Engineering Diagnostics

 

[unclesyd]

I don't know if the data for your bitumen is available per say but these numbers can be readily obtained from a Thermal Analysis Lab. This type analysis at one time was done routinely in refineries to classify different feed stocks.

We used this type analysis on several occasions over the years when we tried different schemes to get rid of solid process waste. The last time we used a lab was when we were installing Pyrolysis Furnaces to clean polymer equipment. In our case the polymer melted at 285°C and we didn't see much gas evolved until we got close to the Pyrolysis temperature.

A lab that does Pyrolysis/GC, Thermogravimetric/FT IR,or Evolved Gas Analysis could do the job in minutes. Even a Differential Scanning Calorimeter would tell if you have components evolving at what temperatures.

 

[25362]

At the mentioned operating temperatures the practical LFL may be zero. Thus no air dilution would make the operation safe from the flammability viewpoint.

 

[billstevenson]

What is the heated air temperature coming out of the burner? What is the distance between the burner and the bitumen inside the drum?

Bill

 

[op9]

Hi Bill,
It is a direct flame into the rotary drum (so flame temp at the most) but separated by about 7 mtrs of a dense veil of aggregate and sand. There is no possibility of direct flame onto the bitumen as the burner cuts out if the cold stone feeders stop.The outlet temperature after the area of Bitumen addition as it goes into the ducts is about 230deg. I actually have NATA test reports showing the Bitumen flash point id 344degC.

Rod Nissen.
Combustion & Engineering Diagnostics

 

[billstevenson]

Rod,

It does not sound like you have a credible risk for explosion. To test this hypothesis ask yourself some what ifs. For example: What if you lost power to the drum? What if you lost in-feed of cold product and could not get the bitumen out of the drum during a power interruption? What if the burner malfunctioned and was inputing un-burned gas and carbon monoxide in high concentrations?

If any of the what-ifs that you can dream up could lead to a explosive atmosphere then you will know what to do.

Regards,

Bill