corrosion in urea plants 3

[tepcor3]

I am looking for information about the effect of the presence of H2S in CO2 gas feed in urea plants. Specifically, the value at what the H2S content should be lowered to avoid corrosion problems in the urea reactor, stripper, carbamate condensator, etc. Does anyone have information about this subject, or know where I can get it?

 

[mcguire]

Please supply the information on what the material which is being subject to the corrosion is. In these problems all the specifics about the actual situation are crucial:corroded material, temperature,etc., at which the problem occurred.
You are familiar with your process' parameters, but the materials engineers who can help you are not. There are some great materials engineers who frequent this site. Please take the time to spell out your problem in detail and you'll get some very valuable help.

 

[tepcor3]

Mcguire: The commercial synthesis of urea involves the combination of liquid ammonia and gaseous carbon dioxide in a reactor at high pressure (150-200 bar) and high temperature (180-190°C) to form ammonium carbamate, urea and water. Modern energy-efficient processes foresee the decomposition of the carbamate, not transformed into urea, in a stripping equipment working at the same pressure as the reactor, while the off-gas from the stripper is condensed in the carbamate condenser also working at the same pressure. Ammonium carbamate at these temperature and pressure is extremely aggresive to materials. For this reason, SS 316L urea grade, 25Cr22Ni2Mo stainless steel, 22Cr5Ni duplex stainless steel, Ti and Zr are used as construction or lining materials in the reactor, pumps, decomposers, strippers and condensers. Besides this major problem (carbamate), there exist the probability of process contamination with corrosive agents, such as H2S (along with CO2). Then, I am looking for information about the effect of the presence of traces of H2S in CO2 gas feed stream at this pressure and temperature. Specifically, the value at what the H2S content should be lowered to avoid corrosion problems.

 

[mcguire]

A chart I have from "Corrosion of stainless steels" by Sedriks shows H2S having no corrosion accelerating effect below 280C at your pressures. Above 400C each ten fold increase in H2S gives a ten fold increase in corrosion rate at concentrations starting at 0.01 volume %.
Corrosion resistance to H2S is enhanced by chromium content and aluminum content.Based on this I like the high austenitic and duplex, rather than the 316,to the extent H2S is an issue.
I hope that's not too vague, but bottom line, if you have solved the basic corrosion problem, it doesn't appear that the H2S is going to get you.

 

[CoryPad]

One complicating factor here is stress, specifically Sulfide Stress Cracking. With low H2S concentration, general corrosion may not be a problem, but there still can be a problem with SSC, so you may need to watch for this.

 

[mcguire]

This is a high, but possible, temperature for SCC, but unless there is some agent to promote pitting, i.e. halides, it won't be likely. However, this just one more good thing about the duplex grades. That is, they are immune to SCC ( and hydrogen embrittlement ).
Sulfide stress cracking is usually associated with martensitic steels, stainless or not, where sulfides promote the entry of hydrogen and facilitate hydrogen embrittlement. This will not occur over 100C in martensitic steels. Hydrogen in duplex and austenitic steels cannot really do anything worse than slightly decrease the ductility even under high pressure hydrogen charging.

 

[tepcor3]

Mcguire: if the chart which you refer is that of A. J. Sedriks, "Corrosion of Stainless Steels", 1st. edition, Wiley&Sons (ca. 1979), p. 250, figure 10-10, this graph is for pressure range 12-34 bar. The high pressure loop of the urea plant works at pressure in the range 150-200 bar. Anyway, thanks for your information.

 

[mcguire]

Thanks for the correction. I think the conclusion remains the same, however.

I was able to find one more research article froma 1993 conference which showed that pitting attack occurs on duplex SS at a maximum in the temperature range of 80C to 100C when partial pressures of H2S exceed 0.1 bar in a 15% NaCl solution acidified to pH 3.2.
This may help you extrapolate to your conditions. If you want more help, this work was done by Sandvik, the Swedish stainless producer. Avesta is also very good. Try an inquiry directly to them at their web site.

 

[tepcor3]

McGuire: please, let me know full references of the article of Sandvik (pitting attack on duplex SS . Thank you in advance.

 

[mcguire]

The papers are from a conference, Innovation Stainless Steel, Florence October 1993. given by the Associazione Italiana di Metallurgica.
The paper was titled "Cracking behavior of 25% duplex stainless steel weldments in H2S/CO2?Cl media by Gooch.
The auhtor is from The Welding Institute, not Avesta as I erroneously said before. Avesta also has a similar paper on Duplex in organic acid at that conference as well as one on duplex in seawater.

 

AMIChE ran annual conferences [usually in August] on Ammonia and Urea production and safety problems. The safety handle allowed the participating co's to get around co confidentiality. Somewhere in their archives will be the answer or pointer to your solution. Sorry that I do not have a name or contact no. - the presidency rotates through the participants but they should have a permanent staff in the USA. Is your co. a member?

 

[tepcor3]

Volvo90: our company is not a member of AMIChE. Please, let me know how to contact them.

 

Retired some years ago from a German owned co who produced ammonia and urea across the world. Conact BASF in Freeport. They should give you contact peson, etc. Worth looking up past papers at the Ammoni Safety Seminars which AIChe promoted. I went to one in Denver - lots of info exchanged fo free! Good Luck!

 

tempcor3
Should have told you to search "AIChe+Ammonia" in [Google]. It's all there, including this year's programme.e.

 

[ammoniumcarbamate]

I am looking for the effect of CO2 in NH3 at temperature around 150 degree centigrade on carbon steel. We are doing magnetite layer passivation to prevent corrosion due to above chemicals. I want to know the effect.

 

[tepcor3]

Ammoniumcarbamate: see
- C.P. Dillon, "Carbamates can cause corrosion problems", Materials Performance, dec. 1999, p.74
- M.P. Sukumaran Nair, "Control corrosion factors in ammonia and urea plants", Hydrocarbon Processing, Jan. 2001

 

[dhanbad]

We are searching for Proper Material of Construction which can stand Urea Carbamate Solution with NH3:CO2 ratio (wt:wt) arrond 1.6 at 190 C exit first stage distiller operating at 74 ata. Approx. Soln. Composition as:
Urea 51% to 53%
CO2 7%,
NH3 11 to 12%

Proper Passivation method required to protect equipment and pipeline for the above mentioned case.

 

[tepcor3]

Dhandbad: the best materials I know are:
- Superaustenitic stainless steel 25%Cr-22%Ni-2%Mo (UNS No. 31050, DIN 1.4466). Oxygen injection is necessary to induce pasivation in SS surfaces. Normally, 0,5-0,8%vol O2 in the CO2 feed.
- Zirconium (R60702). Oxygen is not needed for the passivation process.