The CDU boots water is red-orange color. It has been more than a week now. We are not sure where this problem comes from and how this can affect the process. (When we add corrosion inhibitor in the water, the part of it was turned into brown color.) Could anyone please share the knowledge regarding this issue?Jun-2021
Collin Cross, Suez Water Technologies and Solutions, email@example.com
Orange/red/brown colour changes are most often caused by changes in Fe+3 concentrations and are potentially driven by a number of corrosive species. While it’s possible that some Fe+2 complexes can cause red/orange colour, this is more rare. Common species in refinery sour waters that can cause a red orange colour shift are oxygen, chloride, organic acids, and cyanide (or thiocyanate).
Standard metrics of pH, chlorides and salt point analysis should be a first step. Unfortunately, these metrics are not always sufficient to diagnose the problem as other mechanisms exist. Therefore, thorough analysis surrounding the other species above may be needed to uncover the root causes.
While oxygen is not as common in an atmospheric overhead as it is in a vacuum overhead, oxygen still finds its way into these systems fairly frequently. Oxygenated sources of desalter wash water, supplemental overhead wash water, or a cooling water leak in a trim cooler can contribute here. The presence of hard water ions and/or sulfoxy acid species can be measured in the water and are often indicative of oxygen intrusion. Additionally, higher levels of gaseous nitrogen (N2) present in the off gas can also be used as an indirect measurement.
Many complexes of iron and organic acids can also show an orange/red color in aqueous solution. Glycolic acid is one common example but other light organic acids can present similarly. Running crude diets with high levels of these species can give rise to such issues. Anion analysis can provide a speciation of light organic acids with total light acid content greater than 350-500 ppm as a rule of thumb defining a problematical boundary concentration.
Cyanide in sour waters from cracking units like the FCCU and/or Coker can react with sulfides to form thiocyanide. Iron thiocyanate is famous for causing an orange/red solution. This might be accompanied by corrosion in the overhead of the sour water stripper unit and/or the rundown lines from the cracking units generating the cyanide. Use of ammonium polysulfide as a cyanide scavenger is one common way of directly generating thiocyanate ions, but is not the only way. Cyanide or direct thiocyanate testing of contributing waters can help to reveal these issues.
From a comprehensive perspective, testing and/or inspection around these potential mechanisms can give a good chance of detecting less common drivers of corrosion giving rise to reddish/orange coloration in sour waters.
Jake Gotham, InSite Technical Services, firstname.lastname@example.org
The obvious answer is that you have a serious corrosion problem. Per Chris Claesen’s suggestion, you should do some analysis of the iron content of the water. If you have copper alloys in the overhead system (e.g. monel, cupro-nickel or brass), you should also test for copper. As well as testing the water itself, I’d suggest filtering a large sample and running a AAS metal scan on the solid retentate.
Another possibility if you use cooling water in the overhead system, is a leak of cooling water into the system. Some sites have dirty cooling water systems, particularly open-loop systems bringing in sea or river water, or closed-loop systems with corrosion or bio-fouling problems. With knowledge of your cooling water chemistry, you should be able to test for something in the CDU boot water to identify contamination. Alternatively, you may be able to run a plant trial isolating cooling water to and from individual exchangers, or switching to an alternative cooling water system temporarily.
Good luck with your troubleshooting.
Syed Mumtaz, Freelance Individual, email@example.com
To get the bottom of the issue helpful to analyse the individual crude oils in the feed mix to see the presence of phenolic, cyanides, drilling fluids and other colour causing and organometallic compounds. The corrosion inhibitors just adjust the pH through combining with active acidic or basic species but may not react with all the components present in the crude oil. A detailed in-house analysis can help and can also act as a check of analysis provided by the suppliers.
Chris Claesen, Nalco Water, firstname.lastname@example.org
Do you analyze your boot water? Basic analyses of pH, iron and chloride content can quickly give you a possible root cause. There is a chance that the pH became very low due to a desalter or caustic injection problem, a situation like this can give a boot water appearance like you describe. Once you raise the pH by adding (neutralizing) inhibitor this can create conditions for FeS formation and a colour change.
A more detailed study can be done by detailed analysis of the boot water, identifying all acid and base species and running this trough an ionic model such as Pathfinder. This can best be combined with a detailed evaluation of the desalter and OVHD configuration and operation.