Fouling in HF alkylation units 2

[sgoel]

I am working in a HF alkylation unit for detergent intermediates. We are facing a typical problem of Increased pressure drop of HF stripper feed side of HF stripper feed exchanger after the start-up.

This phenomena is seen twice after the exchanger was hydrojetted. Problem was not seen when exchanger was replaced with a new exchanger.

Has anybody else faced similar problem? What could be the possible causes for this?

Thanking you in anticipation.

Best Regards

 

[sshep]

sgoel,

Our HF based detergent alkylation unit (UOP design) always has a steady increase in dP on the HF stripper feed preheater due to HF scale. The equipment is a stack of 3 AEU exchangers, with the stripper feed in the tubes. Back when we had an outage every 2 years, a bypass was gradually opened to keep the flow to the stripper without imposing additional backpressure on the HF settler. Now that the time between TARs is longer, we actually have a spare set that can be swapped for cleaning. It has been suggested that the U bends of the tubes do not hydroblast clean as well as straight tubes, but if you want a pullable bundle then straight tube alternatives to U tubes are limited and problematic (i.e. T type rear head will require expansion joint).

Thanks, sshep

 

[sgoel]

SShep

thanks a lot for an excellent information. Could you please share the handing over procedure with me. We have seen this problem particularly after the hydroblasting during shutdown, so handing over procedure is one of the doubts.

You could contact me off the list at shashank_goel@ril.com also.

Thanking you once again.

Best Regards

 

[rex28]

The pressure drop is caused by iron fluoride scale. This is prodcued when HF reactions with the iron. This will always be formed in the HF alkylation unit. You mention that you do not get this pressure increase when the exchanger is replaced rather than cleaned. I suspect this is for one of two reasons.
1) As already mentioned the exchanger is not being cleaned properly and so the scale keeps on building up.
2) After cleaning the exchanger is not being dried properly. Any water present will significantly increase the rate of corrosion and scale build up.