I’m currently a Process Engineer handling gasoil hydrotreaters at our refinery. I've only been a Process Engineer for about 8 months. I know that you need as much details about the problem and I hope this is informative enough. If you need more, I'd be very willing to give more details.
Here's some background about the problem:
We noticed a steady increase in the pressure drop between E-1(HHPS Flash Gas-Treat Gas H/E) to E-2 (Airfin cooler) beginning on August 14, 2011, reaching values as high as 600 kPa (versus a typical operating pressure drop fluctuating between 120-180 kPa). The pressure taps are placed at the inlet of E-1 and outlet of E-2. We first injected water at the inlet of E-2 but saw only a minor drop in dP. We then diverted the water to the inlet of E-1 and noticed the drop more significantly. Only a minute or two of water washing is sufficient to bring the dP down to 90 kPa. After water washing, the dP would continuously increase until we decide to water wash at 350 kPa. It was only after the third wash when the dP stabilized at around 130 kPa. We concluded that it was fouling at the tube-side of E-1. Our initial evaluations using sublimation curves show that the most probable cause of fouling is the presence of chlorides in the vapor. We suspect that this may have reacted with ammonia to form ammonium chlorides in the exchanger’s tube-side. I attached a PFD of the process for your reference.
What we need to find out is where the chlorides may have come from.
Here’s what we did:
- At first we thought that the reformer makeup gas (hydrogen-rich) was having a chlorides breakthrough. Draeger tube tests showed chlorides of about 0.2 to 0.4 ppmv (versus typical 0.0 ppmv). On September 25, 2011, the adsorbent for the makeup gas chlorides treater was replaced with fresh stock but the exchanger was still experiencing a steady increase in pressure drop until our third water wash on around October 19, 2011. Since that third water wash, the dP has stabilized to around 130-200 kPa. Furthermore, the adsorbent’s supplier explained that there was never really any chlorides breakthrough. The readings were due to interference from contaminants. They proved this by pushing makeup gas through a small bed of NaCl and took measurements of the filtered gas using Draeger tubes. These showed 0.0 ppmv.
- We also checked the feed for any chlorides. We sent samples of straight-run feed, light cycle oil (LCO), and light vacuum gasoil (LVGO) to a third party lab to test the samples for total chlorides via UOP 779. The results showed <1 ppm of chlorides in each of the samples (beyond the detection limit of the test).
- We checked if there were any idle lines that could have been passing-thru and may have been injecting raw water, or any other substance with chlorides. There were none.
- We checked our shipping records for batches of crude and plotted the duration of their consumption against the beginning and end of the fouling incident. None of them matched. Furthermore, the chlorides content in the sour water at our pipestills’ overhead accumulator showed typical chlorides content. In other words, there was nothing unusual at the pipestills.
- We checked our slopping from the other units and found that benzene slop from our BTX unit (which uses an ED Sulfolane process) was fed into the pipestills on August 10, 2011. Feeding of the slop, however, ended at around August 22, 2011. This did not coincide with the whole duration of the fouling incident.
We were wondering if you had encountered any similar incidents in other refineries. We would also like to ask the following questions:
- Were we correct to conclude that the rapid increase in dP across the heat exchangers was due to fouling by NH4Cl? We already eliminated NH4SH as a foulant and there were no unusual process parameters during the time. Is there anything else we should look into aside from fouling that may have caused the rapid increase?
- Where could the chlorides have possibly come from? We have already eliminated the hydrogen makeup gas as a source of chlorides based on the adsorbent supplier's "proof" and the timeline of events.
- If the chlorides came from the feed, what mechanisms could have been present that could cause the chlorides to form HCl? How spontaneous are these reactions? At what temperature and pressure will the reaction occur at a significant rate?
- Do you know of any reference material/articles/consultants that can discuss this in length?
Hoping for your kind assistance.
Thank you very much.
Edited by lvroperos, 27 January 2012 - 03:49 AM.