Material for a coil (serpentine) to heat caustic soda. 1

[Guibar]

Hi.

It is the first time I am writing a question.

I live in South Brazil and I am currently working on a petrochemical Plant as a Maintenance Engineer.

We are having serious problems with a storage tank for caustic soda. In the winter times, temperatures usually drop below zero. So, in the bottom the tank there is a coil with steam at 220ºC and 4,5 kgf/cm² to heat up the caustic soda. We had tried few materials for the coil, but always end up having SCC.

Anyone have any suggestion, because in brazil, we are the only state that have such problem with incrustation of caustic soda.


Thanks

 

[TD2K]

Given you are the only user, have you talked to other users to find out what is different between your system and theirs that you have a problem and they don't? It could your steam conditions, it could be the caustic concentration, it could be impurity levels, etc.

You said you have tried several materials, the specific materials you have tried might get you better suggestions.

What concentration of caustic are you storing? If you are having a freezing issue I'm suspecting it's close to 50 wt%

Dow chemical puts out a caustic soda handbook which you can find with Google. For heating of caustic tanks, they suggest a bayonet heater with suggesting materials being a high nickel alloy.

 

[Guibar]

Well, we are the only one because just in south brazil temperatures drop below zero, and we are the only petrochemical in south Brazil. so in the rest of the country, temperatures dont drop below 12ºC, when the incrustation starts

You are right, the concentration is 50%. The materials we had used are all stainless steel. The original project is from final 70's, and the demand of caustic soda increased in the last few years.

I will give a look in the material you suggested.

thanks

 

[cloa]

Thanks for heeding my advice and hopefully for all future posts- also please try to include much detail in your original post as possible.

 

[moltenmetal]

Be aware that caustic soda can contain as much as 1% chloride.

You need a high nickel alloy or perhaps a super-duplex stainless steel might work.

You also need lower temperature/pressure steam. You want to warm the stuff, not to locally boil it!

 

[TD2K]

The bayonet heater we installed was back in about 2006. I checked with the plant and they haven't had any problems with it. It was a Brown Fintube BTH-3 made out of inconel. 3" finned heating tube with a 6" mounting flange to connect to a tank nozzle. The tank was carbon steel.

We took 250 psig steam, reduced it to about 15 psig through a regulator and then further reduced it through a control valve. The design steam pressure used to size the heater was 5 psig, the 50% caustic was to be maintained at 90F.

 

[Guibar]

We are considering use the similar system we already have in our plant, as you can see in the attachment.
A coil with high nickel alloy will probally solve our problem.

 

[TD2K]

You might want to compare the cost of a traditional coil design with a finned bayonet heater. I'm thinking that's some $$ in high nickel alloy piping and fittings.

Even with inconel construction, one of our senior engineers was concerned with corrosion, that's why we dropped the steam pressure down to 5 psig along with the saturation temperature.

 

[MJCronin]

The people who produce and concentrate caustic soda use pure Nickel 200 for structural parts.

It is the best material to mitigate the effects of caustic SCC.

However, it is very expensive and you may not be able to find it in a tubular shape.

Inconel is the next best choice, if you can find it in the shape you desire.