Iron power found in the process 1

[sshep]

Friends,

Over the weekend we opened a couple towers and took out alot of very fine black solids which were plugging distributors. We just had a shutdown and the same towers were well cleaned and flushed. On drying these solids are magnetic, and there has apparently been a history of such iron containing solids in this plant. The towers and packing are primarily carbon steel, running at atmospheric pressure, and using a solvent containing some 3-5% water.

Years ago in a previous plant (vacuum tower, no water) I encountered the same type magnetic solids on start-up and used rare earth magnets in a basket strainer to capture and mitigate the problem which eventually went away. Since this problem appears to be common in these processes, and the amount of metal loss looks significant, I would like to get a better understanding of the formation mechanism this time around.

Does anyone have any suggestions for where to get some knowledge of this? Is it possible that flash rust or chip scale can be reduced in the process back to a magnetic state? What passivation or treatments might be effective? Is this likely to be from vessels and pipe, or packing, etc. As I know little on the subject at this point, any suggestions would be helpful.

best wishes,
sshep

 

[sshep]

My working theory after further research is that these solids may be conventional orange rust (Fe2O3) formed from steaming, flushing with condensate, and exposing to air; which is then converted to magnetite (Fe3O4) after closing the process back up and applying heat.

Does anyone have any comments or experiences along these lines?

Are any of you using passivation washes on some equipment before exposing carbon steel to the air after steaming or chemical cleaning? What type of passivation washes are you doing (caustic, ammonia, etc)?

any help is appreciated,
sshep

 

[monaco8774]

I am not familiar with your process so this may be well off but in my process when we get black magnetic fine particles it is often iron sulphide or some other iron salt such as carbonate or as you say an oxide. It depends what the process has in it if its oxygen this is the best reducer, if there is no oxygen as it is for me in the oil and gas industry on the hydrocarbon side if you have H2S in the gas or CO2 then these will also reduce the iron. The fact that its black is just dirt and grime etc (although iron sulphide is often black) You can tell if its iron sulphide by dropping a bit in acid and taking a sniff (dont take a big nosefull as it hurts just waft it) you get a smell of rotten eggs. the others as far as I know you can get an XRD analysis in a lab but I dont know a quick an easy way to tell. In all cases this is the product of upstream corrosion and this is what you need to manage.

How to deal with it is the hard thing, you can use chemical inhibitors or you could try material changes or alter some parameters pH/Temp pressure etc but again totally dependednt on your process.

 

[sshep]

Monaco,

Thanks for reminding me not to jump to a conclusion about the product involved.

A story: My previous experience with these type solids was many years back. A younger less experienced sshep was working night shift in a shutdown and we were chemically cleaning an HF alkylation unit to remove iron floride scale prior to opening. The last steps of the procedure called for a rinse, then a caustic wash and then a second rinse. The operators and I believed the caustic wash was conservative to nuetralize any residual acid from the chemical cleaning. Because our first rinse came back with a pH around 7, we did not make a caustic wash. Later I learned that the caustic wash was a passivation step to avoid surface rusting when the equipment was opened. On start-up we had a lot of magnetic solids show up in a stream that was also used for pump seal flushes. A short term crises which eventually went away. This negative experience is still influencing my judgement this time around.

best wishes as always,
sshep