benzene analysis methods

[EastCoastMechie]

I'm looking for some insight on analysing samples for benzene content. The application is a refinery vessel cleaning (via steam purge + chemicals) and we can sample off the top & bottom of the vessel (condensed steam).

My limited knowledge of gas testing tells me that if we're not into the large, expensive gas detectors, my options are hand-held LEL monitors (which will only give me LEL...) and Draeger tubes. If I remember correctly, you also have to acheive a certain level of toluene & xylene before you can get an accurate benzene reading. This may seem like a crude method of analysis, but for this type of applicaton (i.e. monitoring & trending), is it the most economical? Final tests are always done by our customers, so accuracy is not my largest concern.

 

[DedalusEnviro]

I believe that what you're trying to do requires the use of gas chromatography. Portable instruments are available.

The kind of equipment you mention is entirely nonspecific; and cannot be made specific without some means of separating the various compounds. Like, GC.

Dave Wichern

http://www.dedalusenviro.com

Science is a business of empiricism.

 

[EastCoastMechie]

Ok, sounds like GC would be a less time-consuming and more accurate. But I have been at refineries where they still use the Draeger tubes to test for benzene in vessel vapor samples. Is this bad practice or is there a specific application where they are appropriate?

Thanks for the feedback )

 

[DedalusEnviro]

If benzene is all that is present, I could see that working.

I don't know the chemistry behind the particular Draeger tube you're using. I imagine that it is some kind of generalized oxidative reaction, such as dichromate or permanganate oxidation. These reactions go much faster with compounds that have a side chain attached to the aromatic ring. This, I imagine, is the mechanism by which the "prefilters" used with some of these tubes work.

Take a look at this:

device: Detector Tube
manufacturer: Dräger
model/type: Benzene 5/b, order no. 67 28071
sampling information: 20 strokes
upper measurement limit: 50 ppm
detection limit: approximately 1 ppm
overall uncertainty: 25%
method reference: on-site air secondary (manufacturer)

I got it from:

http://www.osha.gov/dts/chemicalsampling/data/CH_220100.html

An uncertainty of 25% could easily make the data you're looking for disappear in the analytical "noise." If I were responsible for this monitoring job, I'd find such a method to be a slender reed to lean on.

Dave Wichern

http://www.dedalusenviro.com

Science is a business of empiricism.

 

[ssn596]

What are you trying to measure LEL or Benzene?
IF it were me in the field I would surely be doing LEL just to keep from getting incinerated.

IF benzene:
First thing I would read the Draeger sheet. These are very informative.

Why are you doing the monitoring? For employee protection, to determine how much benzene is in tank and how much cleanup you got left, how much benzene in plume or what?

From that what level of accuracy do you need and or want?

Do you have to have immediate in field results?

How close is the plant chem lab? What kind of equpment do hey have and can they support you??

IN FIELD GC sounds real neat and all but it has its own set of problems. If you think Draeger tubes are expensive just wait for a full GC setup.

Dan Bentler

 

[JLSeagull]

Several ASTM standards apply to benzene in product.

Your application sounds like an EPA or OSHA issue. Search the web for "EPA method" and benzene. I think that you will find a bunch associated with Volitile Organic Sampling Train, gaseous organic compund immission by GC, VOC;s using Texax adsorption, GC and mass spec, SUMMA passivated canister sampling, air and oil quality improvement research - speciation, ...