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Cdu Overhead Water


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#1 CHEMSTRONG

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Posted 19 February 2022 - 12:47 AM

Dear Sir,

 

For Refinery CDU Overhead Corrosion Mechanisms, we also say that we need to maintain 25 percent more free water in CDU overhead after water dew point. I understand that it is not possible to completely remove water from crude feed so we will have aqueous corrosion anyways. 

 

I am not able to understand why we need free water liquid always present in CDU . We are using Offsite Crude Feed tankages to drain water from tanks before feeding and then also desalter is removing salts, water also from crude feed , then why we need free water in overhead circuit ?

 

Can experts in this field explain the reasons in details. 

 

Also Water Wash added in top section will force water dew point early ? We need to maintain Column top Temperature greater than water dew point temp ( this is for not condensing any water at column top) and also column top temp greater than Salt Formation Temp always ( to avoid ammonium chloride salt corrosion and subsequent fouling ) .

 

Water wash is added to force water dew point or wash away fouling deposits or to maintain 25 percent excess free water in overhead ? Also what should be location of Water wash?

 

Best Regards,


Edited by chetanhm2, 19 February 2022 - 03:21 AM.


#2 PingPong

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Posted 21 February 2022 - 04:29 AM

CDU column uses stripping steam in bottom and side strippers so even if all water would be removed from crude feed there would still be water vapor in CDU overhead.

 

The desalter can't remove all salt completely, so some remaining salt will generate HCl (hydrochloric acid) in the crude heater.

 

When the first droplet of water is condensed in the CDU column overhead condenser the dissolved HCl concentration in that water droplet will be in equilibrium with the HCl partial pressure in the vapor.

So the initial water droplets are the most acidic and cause the most corrosion of the condenser tubes in the zone where the first water condenses.

As more water condenses, and absorbs HCl, the HCl partial pressure in the vapor drops and consequently also the HCl equilibrium concentration in the water, causing less corrosion further downstream in the condenser.

 

By injecting water upstream the condenser part of the HCl from the vapor is absorbed before the condenser, resulting in a lower HCl partial pressure in the vapor entering the condenser, and therefor a lower HCl equilibrium concentration in the free water inside the condenser.

So by using water injection the water inside the condenser is less acidic and therefor less corrosive than if there was no water injection.



#3 SilverShaded

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Posted 22 February 2022 - 03:53 AM

Ah the mysteries of crude unit overhead design :).

 

There are so many different configurations of overhead design mostly aimed at preventing corrosion its fairly mindboggling.  The answer above is spot on.

Corrosion can also occur if top pumparounds are returning below water dew point (e.g. the infamous tip-top pumparound design).  A simulation may show the tray is above dew point but there can be localised corrosion where the pumparound enters the column.



 



#4 CHEMSTRONG

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Posted 23 February 2022 - 12:09 AM

Respected Sirs,

 

Thanks a lot for your expert advice. IT is always great to get your advice as it is from vast experience for so many years. I am doing simulation of CDU Overhead in ASPEN HYSYS for corrosion potential prediction but this is without OLI ionic model  and chemistry. 

 

Regards,

Chetan Chavan



#5 CHEMSTRONG

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Posted 23 February 2022 - 03:03 AM

Dear Sir,

 

So there is only Water Dew Point in system i.e after Condenser ? We always say that Tower top Temp should be greater than water dew point and salt formation temperature etc. 

So do we use Water Wash to force Water Dew Point Temp early ahead of salt formation temperature. Also Only when Aqueous acidic corrosion happens, then only fouling and Ammomium chloride corrosion can happen here.

 

Thanks.

 

Best Regards,

Chetan Chavan 






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