Corrosion Control for Vacuum Overheads 1

[petrotrinsmerv]

We upgraded a small vacuum column (26,000 BPSD) a few years ago and changed the overhead from direct contact ejector condensers to shell and tube ejector condeners. We are experiencing severe corrosion problems due to low sour water pH problems (column uses stripping steam). Can we use ahhydrous Ammonia here. The current thinking is that if we can condense water, enough of the Ammonia will partition into the condensed sour water (26 inHg vacuum)to raise the pH. Is this wishful thinking?

 

[orenda1168]

Petrotrinsmerv:

Why not consider using a much more easily controlled liquid neutralizing amine (morpholine, etc.), the application of which for overhead pH control has been done for the past 40 years or so.

Orenda

 

[petrotrinsmerv]

Thanks for the reply orenda1168.
Ammonia is something we use extensively in the refinery and is something we can set up quickly without chemical vendor input. We still have to make a decision on whether we want to go long term with metallurgy changes (we have changed some sections already)over the whole system. However, we want to try to improve the pH using some cheap anhydrous NH3 in the interim until a final decision is made. We have also been talking to some of our chemical vendors but we have only recently started looking into amine for pH control in our refinery. Where NH3 can do the job, thats what we normally stick to. At the very least we would want to try it first. However, NH3 does have partition issues with condensing water, even in the best of circumstances. I am just wondering if under 27 inHg vacuum that may become an even bigger issue. Longer term, we may even decide to use a combination of NH3 and Amine, since we have corrosion in the downstream sour water pipework as well as in the overheads. NH3 is supposed to be good for bulk pH improvement while Amine provides better control. Have you had experience with NH3.

 

[orenda1168]

Hi, Petrotrinsmerv:

No, I haven't any experience with NH3 for overhead corrosion control, probably as much for the reason of unavailability in the refineries I have had business with, as difficulty of control. No doubt there are neutralizing issues suitable for NH3 use (as you indicate, bulk neutralization being a case in point)....in my opinion, however, overhead corrosion control in either atmospheric or vacuum columns is not one of them as reasonably tight control of the neutralizing agent is necessary to minimize metal corrosion loss. I just don't believe NH3 will give you that ability, regardless of what it's partitioning to the water phase may be....in my mind, the question is one of reaction time versus neutralizer concentration, which in the case of NH3 will be far too rapid and far too difficult to adjust the injection of to allow adequate control of pH variances in the overhead.

Whatever you decide to do, I for one would be interested in the outcome.

Regards,

Orenda

 

[MarkHobbs]

Using Ammonia to control the pH works fine and is standard practice in many installations. Problems which have been experienced relate to salt deposition. Design and position the injection quills properly and you should be fine.

 

[25362]

On the other hand ammonia injection may improve a bit the vacuum by reacting with H₂S to form NH₄HS, soluble in water, thus effectively extracting H₂S from the "cracked" gas, unloading downstream jet ejectors which then become able to suck harder.

 

[25362]

I recommend reading the article titled Four steps solve crude-tower overhead corrosion problems by Norman P. Lieberman, on the O&GJ issue of July 5, 1993, under the general heading of Technology.

 

[25362]

As a corollary to what was said above: The best way to control corrosion in the VDU overhead -as well as in topper overheads- is to try and desalt the crude to an efficient removal of Mg and Ca chlorides possibly greater than 60%.

It happens that desalters remove much more efficiently NaCl than Mg and Ca chlorides, these at efficiencies of only about 40-50% per stage vs [≥]90% for NaCl. Apparently if the crude oil chloride salts contain more than 10% of Ca and Mg chlorides, the overall desalting removal efficiency drops.

Many refineries have two desalters in series reaching a 99% chlorides removal efficiency.

Most of the MgCl₂ is decomposed in the topper, but CaCl₂ and the remainder of MgCl₂ are decomposed at the conditions prevailing in the vacuum unit.

Refiners inject caustic solutions in a controlled manner at the desalters' downstream since it helps in combatting corrosion. They do it with utmost care since xs caustic may be deleterious to downstream catalytic units.

 

[0707]

NH4) 2S, is obtained, in the form of micaceous crystals, by passing sulfuretted hydrogen mixed with a slight excess of ammonia through a well-cooled vessel; the hydrosulphide NH4.HS is formed at the same time. It dissolves readily in water, but is probably partially dissociated in solution. The hydrosulphide NH4.HS can be obtained as a white solid, by mixing well-cooled ammonia with a slight excess of sulfuretted hydrogen.

NH3 (g) + H2S (g) ---------- NH4SH (solid crystals)

Synonyms - Ammonium hydrogen sulfide, ammonium sulfhydrate
Ammonium bisulfide, ammonium sulfide, ammonium
Hydrosulfide, ammonium mercaptan (KI-310)

Formula .................................................. (NH4) 2S

The basic Neutralization reaction is

NH4OH + HCL --------------------- NH4CL + H2O

The use of NH3 or organic amine to control Ph produces ammonium or amine chloride as a neutralization product. The desublimation point of NH4CL may take place in the overhead condensing system ahead of water condensation point and lead to corrosion and fouling.

NH3 + HCL ------------------ NH4CL (deposit)

Resulting in under deposit pitting attack – good water wash is required

The required quality of water wash is:

Ph – 5.5-7.5
Total hardness- < or = 50PPM
Total HCO3- and CO3- content < or = 50PPM
Ammonium hydrosulphide (NH4HS) < or = 100PPM
Chloride < or = 2000PPM
Sulphate < or = 200PPM
Oxygen < or = 1PPM

Effects of excess Ammonia in presence of H2S

2NH4OH + H2S ---------------- (NH4) 2S + 2H2O
(NH4) 2S + Fe++ ---------------- FeS + 2NH3 +H2

The overhead H2S corrosion mechanism is as fallows:

H2S «---------------» (H+) + HS-

(Fe++) «--------------» FeSH+

The FeSH+ react further to form a complex sulphide scale: (HS-Fe-S-Fe-S-Fe-SH)

(HS-Fe-S-Fe-S-Fe-SH)

As pH increases toward the neutral region the concentration of bisulphide ion in solution increases. It has been demonstrated by several investigators that there is a sharp increase to the rate of corrosion in the region of pH 6.8 to 7.3. The sharp increase in corrosion rate is apparently the result of faster reduction of bisulphide ions both from the scale lattice and solution. As the scale lattice is altered FeS is released, exposing unreacted iron, and the FeS enters the water phase. In a crude unit this phenomena is recognized as “black water”

The dissociation of H2S in HS- and S++ is minimum at 5 pH.



On sour water systems to minimize corrosion, the sulphur content of your C.S piping and fittings shall be limited to 0.01% max. and shall satisfy the NACE requirements. I suppose that NACE MR 0175 requirements have been recently updated. To PWHT (post weld heat treat) your carbon steel piping would also increase the corrosion resistance of your piping.



A good and proper water wash would probably be the key to minimize corrosion problems. The quality of water wash is very important.


In our crude unit we use continuous water washing of the overhead air coolers. The water from the dessalters is refluxed through a feed water drum to the inlet nozzles of air coolers header boxes. When water is saturated and chloride contents on overhead drum are above 50 PPM we empty the feed water drum and we ad make up water.

The standard wash water quality should be as fallows:

Ph – 5.5-7.5
Total hardness- < or = 50PPM
Total HCO3- and CO3- content < or = 50PPM
Ammonium hydrosulphide (NH4HS) < or = 100PPM
Chloride < or = 2000PPM
Sulphate < or = 200PPM
Oxygen < or = 1PPM

Some years ago because of ammonium chloride under deposit corrosion and difficulties in stabilizing pH we give up injecting liquid NH3 we start controlling Ph with a neutralize inhibitor up stream of the air coolers. On overhead line of the column we have a corrosion inhibitor injection. Down stream of dessalters we have a caustic injection to neutralize the chlorides.


Regards

Luis Marques

 

[25362]

Visit

http://www.eptq.com/Pages/Articles/PDF_Files/PTQ04314.pdf