Black Deposit on 316L Dryer Vessel

[mqas]

We have just installed a new jacketed reaction vessel which feeds into a new pressure filter dryer. Both of these are made from 316L stainless steel.
We are experiencing a black deposit on the dryer which doesn't seem to want to go. The vessel was similar, but we've now managed to clean it, probably by being able to reflux alcohol for several hours. My engineer reported on the dryer as follows:

'It's had several water washes hot and cold. We've dropped alcohol into it from the vessel above, we've tried acid wash/passivation and even scrubbed it with an alcohol damp scouring pad, but we're still picking up black grime
on the Azo-wipes when we swab it.'

Any thoughts?
In the past we have been able to clean vessels by filling with a fairly strong acidic solution (20% nitric) - but this doesn't seem to have worked this time

 

[hollerg]

Seems to me like you have left something out, like chemical compostion or if the source of the black depostion is organic or ??. It would be useful to contrast this system to the old system. e.g. What has changed to warrent the new vessel? Higher pressure steam? New process chemicals, higher throughput per unit volume, etc.

The comments about being black makes me think of what is cooked onto the bottom of a pan or that there is a sulfurous deposit? This is completely spectulative but could you have a reactive chemicals issue, either from decomposion, or small flash or pyrophoric fires that snuff themselves out?

 

[aberta]

hollberg,
Your mrntion of"pyrophoric fires" got me thinking about the black deposits that we have in our odour ccollection system from process tanks that contain light oils (BTX) and wash oil (diesel oil). If we were to get small trace of oxygen in our collection system, would the pyrophoric compounds (which do exist, as we had an explosion some distance away in our tar tank where the roof by blown off..air got into the system when heater port was left open accidentally..no injuries), try to smoulder and create a carbon like residue. This residue plugged our piping system and we have just cleaned it out.
Thanks if you can help. Regards.

 

[SooCS]

Looks like you may have ferrous sulfide in your system. Dry ferrous sulfide ignites spontaneously when in contact with air. I have heard a few incidents of this problem with pipeline pig receivers. To avoid this spontaneous fire, the operator wets the receiver before opening.