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Problem: Flash Fire Problem

2 weeks ago, Professor Silveston from Waterloo University Canada came to Universiti Teknologi Malaysia for an assignment. While he was around, he gave a workshop on problem solving. It was really a very interesting workshop and we learnt how to think critically on how to solve problems (it can be any problem including chemical engineering operation problems).

I would like to share one problem that he gave us and I want to see some ideas. Please feel free to gave your opinion on the answer for the problem.

Flash Fire Problem

Heavy naphtas are converted into high octane gasoline in platformer. By products of the reaction include LPG and hydrogen rich gas containing 6-80% by volume hydrogen. The products from the platformer reactor (at 700 psig and 500 oC) are heat-exchanged with the naptha feed to the reactor. This preheats the feed. The platforming catalyst is rapidly fouled and must be generated by blowing with steam or waste gas. Thus the process is cyclic. A battery of reactors maintains a steady flow.

In three weeks since the startup, there have been four flash fires along the flanges of the heat exchanger. The plant manager claims that because of the differential expansion within the heat exchanger, because of the diameter of the exchanger and the stream hydrogen is rich these flash fires are bound to occur. The maintainance mean have broken 6 bolts trying to get the flange tighter, but they just cannot get it tight enough. The excahanger costs $200,000 and the whole plant costs #39 million. The technical manager says that the plant must be safe in 3 days.

What will you do?
What are the actions?
Can you think and share with us your action plan...

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posted by zaki yamani @ 2:20 PM,

19 Comments:

At Saturday, November 08, 2008, Anonymous Bobby Baum said...

Something must be wrong with these figures. High pressure gas at 5000C would melt ANYTHING, even rock! That's hotter than rocket exhaust!

 
At Saturday, November 08, 2008, Blogger alzack said...

Bobby,

Its 500 celcius. You've mistaken o with a 0. Anyway, I've put a space between 500 and oC to avoid confusion. Thanks.

 
At Monday, November 10, 2008, Anonymous Bobby Baum said...

Try adding a gasket made of a soft metal (silver?). Make sure it has a melting point over 500C!
Try checking with NASA. The main engines of the shuttle have to deal with similar (worse) conditions.

 
At Wednesday, November 12, 2008, Blogger alzack said...

That gasket will be too costly! Very expensive and may not be acceptable for the procurement.Furthermore, you don't have that type of gasket in your technical store. You must run the plant safely in 3 days time.

What will you do?

 
At Wednesday, November 12, 2008, Anonymous David Andrews said...

From the description, it appears to be an issue with thermal cycling and possibly a secondary effect with the bolts. We've had success using graphite-foil gaskets in this service, and using Bellville (sp?) washers which provide a "springy" action to keep the bolts under more consistent pressure.

However, I'd still like to know what temperature the bolts are getting to. Are they under insulation? If so, and depending on the material, the bolts may be yielding slightly. I would try modifying the insulation where personnel are still protected from burns but the bolts are allowed to get some air-flow.

 
At Wednesday, November 12, 2008, Blogger alzack said...

David,

It appears that they don't use that type of gasket you mentioned, and washer. They don't have it there in their inventory. They don't have the time to get those materials. They need a cheaper solution and fast one.

They've tried a stainless steel spiral wound gasket, but still it's not effective.

The maintenance people never checked the temperature of the bolts. They never thought of that (me to).

 
At Wednesday, November 12, 2008, Blogger priya said...

check out my blog on engineers:

http://www.youknowster.com/jokes/view/224-you-know-you-are-an-engineer-when

 
At Wednesday, November 12, 2008, Blogger alzack said...

Priya,

It's obvious you're spamming The chemical Engineering world blog!!!

 
At Friday, November 14, 2008, Anonymous Anonymous said...

alzack, can't you just post the solution or the best solutions that Professer Silveston chose ! Please... I hate not knowing the (possible) answers to a problem!

 
At Friday, November 14, 2008, Anonymous mosses said...

hello sir.the question quite interesting.sir if you don't bother i would to come to your office by monday.just let me know if you're free.here my email mosses84@gmail.com

 
At Saturday, November 15, 2008, Anonymous Anonymous said...

Boleh cuba balut flange yang berkenaan dengan Tape yang tahan haba.

 
At Saturday, November 15, 2008, Blogger alzack said...

Anonymous,

Hold on there... I can't just give the answer straight away...

Maybe I can give some clue...

It's kind of difficult and impossible to store hydrogen because of its characteristic. It can still penetrate metal tank after some time....

Maybe you cannot replace the gasket...

There's no such tape that can withstand 500 oC.

Maybe you can consider something to dilute the hydrogen....

I look forward to see some responds.....feedback

 
At Saturday, November 15, 2008, Anonymous Anonymous said...

me again.... ok, howabout reacting the hydrogen with another chemical, added to the feed stream, to reduce its volume percent... i assume we can't just release the hydrogen as waste becuase its mixed with the LPG? and we need the heat from it for the feed? sorry if im asking obvious questions, im only a "freshman" in college

 
At Saturday, November 15, 2008, Anonymous Anonymous said...

how about diluting the hydrogen by adsorbtion onto a high surface area of palladium, or maybe a porous frit made of glsss or metal to collect the hydrogen in the intersitial spaces?

 
At Saturday, November 15, 2008, Blogger Hammid_77 said...

Boleh pertimbangkan pengunaan steam samada secara mewujudkan tekanan positif keatas Flange (gelang) atau semburan secara terbuka keatas flange.

 
At Sunday, November 16, 2008, Blogger alzack said...

OK, finally hammid_77 got it.

It's very very difficult to solve the problem. Hydrogen has the properties that make it difficult for us to handle. The gas can penetrate any steel given some amount of time. One cheap and fast solution that can be considered is by diluting the hydrogen with steam. A nozzle of steam will be installed to spray the flange continuously to dilute the escaped hydrogen. Steam is cheaper gas compared to other inert gas available.

 
At Monday, November 17, 2008, Anonymous struggling engineer said...

Well done hammid_77,after a lot of clues...finally u got it...

 
At Monday, November 24, 2008, Anonymous Anonymous said...

Hi,

I am afraid we are concluding wrongly by try to mitigate the risk instead of eliminating/preventing knowing that the frequency of the incident is high !

Does the conclusion mean that there is not preventive measure and we only have to think about the mitigating measures like using the steam ?

We have had a similar problem in our plant but at temperature of 65C and Pressure of 30barg. We investigated to find out to basic causes

1. because of the reverse joule thompson effect, the hydrogen gas generates heat during expansion (unlike the other gases) which can go upto 800-1000C because of which the metal expands on one side releasing hydrogen and hydrogen ignited because of reaching its auto ignition temperature ~600C

2. There was bonding missing on the flanges because of which electrostatic energy been generated (as hydrogen has a low MIE of 0.017) which could have caused the hydrogen to catch fire and increase in temperature would have made the metal/gasket to melt and release (as the hydrogen fire temperature could go upto 2000C)

As far as am concerned, the inherent design of the pipesizing has to be reviewed, bonding practises has to be strenghthened and also could try to maintain the hydrogen composition inside the pipeline within LFL (4%) by mixing with inerts or process fluids itself

 
At Monday, November 24, 2008, Blogger alzack said...

Dear anonymous,

Yes, it's true that the solution is not a preventive measure because of the difficulties to deal with hydrogen. Using steam is so far the best solution i.e. to avoid severe problems from occurring. Maybe, there are other solutions that can be thinked off - used/practiced - or developed to improve the situation.

 

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The Author

zyz

I’m Zaki. I used to be a project, process and chemical engineer. Few years ago I successfully became a Chartered Engineer (IChemE) and Professional Engineer (BEM). I'm now employed as a chemical engineering educator/researcher/consultant. Hope you like reading my blog. I welcome any feedback from you. My email: zaki.yz[alias]gmail.com. TQ!


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