Weird Vacuum Tower dP problem

[lgmarti]

Has anyone out there experienced salt built up in the Vacuum tower? During "normal" operations our LVGO section pressure drop is ~ 8mmHg. Over time, however, the dP increases! When it reaches 14mmHg, the unit goes thru a salt sublimation process. The temperature in the LVGO section is increased to ~400F for ~24 hours (naturally all vacuum is lost). At the end of the procedure the dP is back to "normal". Needless to say, this is costly (and at minimum irritating). I can't think of any reason why there are ammonium chloride salts in this section. Has anyone experienced something similar?

 

[25362]

I haven't had that experience, but I would not only look for ammonium chloride, or any of this salt components, brought in from outside the unit, and would observe whether there is any chance of "in-situ" formation. Sometimes lateral thinking may bring you answers.

 

[25362]

Did you know that the boiling point of FeCl₃ is about 316^oC ? I've heard (not personally witnessed) of one refinery having this salt carried over from the topping side creating delta P problems in the vacuum system.

 

[lgmarti]

25362, no I did not know about FeCl3. Our lab equipment can test for chlorides only. I collected some coke-looking materials out of some strainers and they ended up being 82% chlorides! They were also not soluble in water...at least not tap water.
We have been relying on our vendors to test for whatever is attached to the chlorides. They said it was ammonia coming from amine...As the matter of fact, during the "heat up procedure" we collected samples of the hotwell sour water every two hours. The vendor found a bunch of amines that spiked-up and died-down during the process. These amines were different from one we add for corrosion control at the top of the vacuum tower.
I am still at AWE as to how these amines survive my desalter, a heater, a crude tower, a vacuum heater and come up "alive" at the top of the vacuum tower. I mean is this possible? How come they don't degrade or crack? The boilint point of DEA is 516F, TEA is 680F but shouldn't they stay in the water?
Part of the problem is that we can't come up with how much salts are we taking about. Therefore I do not know the level of amine that I am looking for in the HC. (PPMs, wt%?)
Your suggestion is great. Somehow we will find a way to check for FeCl3.
Thank you!

 

[lgmarti]

We collected more "coke-looking balls" from the LVGO section yesterday. The lab crushed the particles, mixed it with deionized water and place it on a hot plate. After heating, the water turned black (to my annoyance). The slurry was then tested in a ion chromatograph and showed a huge peak of ammonia. We also tested the balls for Fe using the X-ray scan and there were no significant amounts of Fe.

 

[25362]

Since the source of ammonia appears to be the neutralizing amines, one should start thinking about the sources of chlorine.

There are refiners that add chlorinated hydrocarbons to the crude oil. For example, slop streams often injected to the crude oil stream contain:

-spent (additive-rich) lubes;
-chlorinated solvents used for equipment degreasing, or for "dewaxing" of storage tanks and other equipment, when handling "waxy" crudes.

Chlorinated hydrocarbons aren't removed in the desalters and may even reach the vacuum system. They may decompose, releasing chlorine gas, as Ca and Mg chlorides do. Ca and Mg chlorides are remnants of a non-effective desalter operation. Steam or water may help in turning Cl into HCl.

Chlorine can find its way to the vacuum tower also by leaks of saline (cooling) water as in heat exchangers and pumps, or when not using chlorine-free steam condensate to dilute chemicals.

One should strive to find the sources of chlorine, don't you think ?
By the way, it is apparent that amines are succeeding in neutralizing HCl, considered a strongly corrosive agent in crude units.

Good luck !

 

[lgmarti]

Again thank you for your input. I guess that I was going to address the problem backwards. I started looking for sources of amine in the slop/drips. As the matter of fact, we found them at ppm levels. Next I was going to do an "amine" balance in the desalters" to see if they go with the brine or HC. (It still does not make sense to me that the amines will stay in the hydrocarbon.)

I did not know about chlorinated hydrocarbons. Where else can I find information about the chlorinated hydrocarbons? Why aren't they removed in the desalters? Our vendor has not even mention this possibility.

This is a good suggestion. I will keep you posted. FYI our dP in the LVGO section is back to 14 mmHg and holding. Good grief, I need some success with this problem!

 

[25362]

Chlorinated hydrocarbons are not affected by electrical fields, as salts are. Desalters cannot remove them. Some chlorinated hydrocarbons are used as lube additives. Some come in with crudes. Chlorinated solvents are used for dewaxing solvents in lube units. I've heard of one european refinery suffering extensive corrosion in its lube plant by using steam condensate laden with chlorinated solvents leaking from a dewaxing unit. These solvents hydrolyzed and generated HCl.

One "good practice" approach is to take care that the injection of slops into crude oil streams be done in measured and small doses.

I hope you continue enjoying the VDU good performance of late.

 

[RogerH]

I dont know if this adds anything but I experienced a problem with ammonia (going to the WWT plant).
It turned out that it came from the crude via the topcondensate. The source in this case was the the crude producer added an amine (that decomposed in the heater). The additive (amine) was to prevent the crude to release H2S. The crude used was not new, we had been processing this amongst others for years without this problem and suddenly we got this effluent problem. It is appearent that the desalter doesnt remove these amines so the problem will either show up in the CDU or VDU section.

 

[COPChemist]

Did you determine where the amine went in your desalter, with the brine or hydrocarbon? I am trying to track amine in our atmospheric tower. We have found MDEA in two of our LSR stabilizers. We were putting some amine unit slop into heavy slop and running it back to the crude unit. The second source of amine was desalter make-up water. Amine unit slop goes to a light slop tank. Then the water from the light slop tank goes to the sour water stripper. Somehow it appears that the amine is making it pass the desalter, but I do not know if this occurs during desalter upsets or if some of the amine is always carried through the desalter in the HC. In addition, we have found NaCl in our amine slop.