Stress cracking corrosion from dichloromethane hydrolysis

[SFuller]

Hi,

We are in the process of replacing an obsolete distillation unit on site. One of the requirements for the new unit is that it needs to be capable of recovering a dichloromethane waste stream by atmospheric distillation. The dichloromethane waste stream will contain approximately 0.5% water with the rest of the stream being mainly DCM with some trace organic solvents.

On researching the recovery of dichloromethane I have come across a number of references to dichloromethane causing stress cracking corrosion in stainless steel. This is due to hydrolysis of the DCM releasing chloride ions. A Duplex stainless steel (SAF 2205) is recommended in the literature for this application as giving superior resistance to chloride attack.

Does anyone have any practical experience of recovery of wet dichloromethane by distillation? If so what material of construction was used for the equipment/instrumentation and did you have any corrosion? Alternatively, does anybody know where I could source more information on this problem?

Thanks,
SFuller

 

[moltenmetal]

SFuller: others will chime in soon with a more informed viewpoint soon, so here's some unqualified opinion in the meantime!

DCM boils at roughly the onset temperature of SCC in 316. Yes, hydrolysis of most organochlorine species is a risk. So if you want this thing to last long term, you've got a few choices: either use a more resistant material (2205 or perhaps a 6 moly stainless etc.), or perhaps clad construction (glass-lined carbon steel etc.). Depends on how long you want it to last with good certainty- and how much you have to spend.

It's an atmospheric column so the consequences of cracking aren't as worrisome as they might be in a pressurized system. What about carbon steel with a substantial corrosion allowance for the reboiler and 316 etc. for the rest? Seems to me that hydrolysis would be worst in the reboiler and your bottoms product is headed for water treatment anyway, correct? Won't be much water in your reflux, so not much Cl- there. The trouble is, it doesn't take much Cl- to cause a problem...

 

[SFuller]

Moltenmetal,

Thanks for your help,
We are looking at using the Duplex SAF2205 for the pot, the tower and also the overhead condenser. The parts of the system located after the condenser should be cool enough (with distillate sub-cooling) to eliminate the risk of cracking. Also, as you pointed out there shouldn't be any chlorides in the distillate.

Using carbon steel with a corrosion allowance is a good suggestion and one that I had not considered. The solvent from the column will be used in pharmaceutical manufacture so I'm not sure that our quality people will let use awayt with it.

The column is expected to last 30 years (like the one it is replacing) so a conservative design is probably best.

Thanks,
SFuller

 

[StoneCold]

SFuller
It sounds like your column is a batch unit. I would seriously look at glass lined steel for your "pot" and maybe even the column shell. Then an alloyed condenser. I think that would be very cost competitive compared to 2205. 2205 sounds great but getting it in small quantities has proven to be difficult for me. My two cents.


Regards
StoneCold