Refinery Alkylation Unit - Metallurgy Upgrade 3

[bspasov]

From the first start-up of olefin alkylation plant, we are faced with very frequent plugging and bursting of fresh H2SO4 dozing lines, which causes severe process upsets. In such circumstances, fresh acid is routed to the contactor vessel through "fresh acid fill" piping (which normally serves only in the start-up phase of process after turnarounds), untill regular H2SO4 dozing lines are isolated, emptied and replaced (safety and environmental hazard). All acid lines are made of carbon steel.

What are possible reasons for frequent H2SO4 piping (1.5") blockage and leaking/bursting? One of the senior operators is suspecting steam tracing that, in his opinion, causes sulfuric acid to overheat at some spots, making it extremely agressive (fresh acid storage tank temperature is 20C). Calculated velocity of acid through the piping is very small which eliminates erosion concerns. It is interesting that spent H2SO4 lines do not suffer from these kind of problems, or at much lesser extent.

Any experience with this? Do you suggest special metallurgy upgrades, line re-sizing, or something else? I am very interested to hear your opinions.

Regards

 

[ash9144]

Your senior operator is correct. Is this oleum service or just 98 acid service? If just 98 acid then I'd get a crew outside right now to tear that steam tracing off. Higher temps send the corrosion rate exponentially higher. Also you state the line is only used for startup. When taking acid piping out of service you need to have the line drained back to tank and blown with dry nitrogen. Acid left in a line will corrode and the acid strength will drop which increases the corrosion rates.

 

[EmmanuelTop]

It is 98% acid service.
The problem with not having heat tracing becomes obvious when introducing these facts:

1. Since process plant is located in the continental area of Europe (winter temperatures as low as -20C), I am kind of suspicious about total elinination of heat tracing. The fresh acid make-up line is almost 200m long, so just with ordinary insulation ("glasswool") I am not sure if this would be enough to prevent crystalization and plugging of the acid line.

2. Temporary shut-downs of Alkylation unit (because of various process upsets), when acid is trapped inside piping between pump discharge (tank) and the process. Without heat tracing, it would be impossible to prevent crystalization of H2SO4, except during summer conditions when ambient temperatures are high enough.

Also, the additional problem with steam tracing is galvanic corrosion (copper tracing). But still, I do not dare to remove heat tracing because of those two issues I mentioned under point 1 and 2.

What crosses my mind is electrical heat tracing with thermoregulation at 20-30C. Have this been applied somewhere?

 

[Milutin]

Hi all

You can find some useful tips on:

http://www.dupont.com/sulfurproducts/techdata/equipment.html

Regards,

Milutin

 

[moltenmetal]

Consider switching from steam tracing to self-regulated electric heat tracing for freeze protection only.

 

[EmmanuelTop]

Any concerns with galvanic corrosion when electric heat tracing is involved?

Also, it is interesting that great number of gate valves and check valves becomes non-functional after short time of exploatation in Alkylation unit. Is there any recommended practice concerning the valve(s) quality and type in sulfuric acid services, including whole Alky plant?

 

[moltenmetal]

Galvanic corrosion between what and what? Where's the galvanic cell you're concerned about? These traces are fed AC...

 

[Cormet]

Does anybody have reported Corrosion rates of Isobutane in Carbon steel??

Where can i find such data??

Thanks,
Cormet