2 replies to this topic

[garyc148]

What does your facility do to combat pressure drop in hydrotreaters? I have heard of snake oil that is added to the feed. I have also heard of success with specialized topping material. Some facilities are actually seeing high pressure drop in the second bed of two bed reactor without seeing problem in the first bed. We believe that this might be due to contaminants in the crude that are soluable in the hydrocarbon stream. As the stream is treated in the first bed, the solubility characteristics change and the second bed sees foulants coming out of solution. Has anyone seen this?

[Himanshu Sharma]

What does your facility do to combat pressure drop in hydrotreaters? I have heard of snake oil that is added to the feed. I have also heard of success with specialized topping material. Some facilities are actually seeing high pressure drop in the second bed of two bed reactor without seeing problem in the first bed. We believe that this might be due to contaminants in the crude that are soluable in the hydrocarbon stream. As the stream is treated in the first bed, the solubility characteristics change and the second bed sees foulants coming out of solution. Has anyone seen this?

Dear Garyc ,The information least needed here is process flow diagrams and crude been processed; still i am giving it a try and hope that it might help.

Many different Mechanism acting in situ or independent are responsible for this high del p

1) Solids in the feedstock--rust, sand, corrosion products

2) Source of naphtha stream. Straight run naphtha (SRN) normally do not lead to pressure problems. However, I have observed acute delta P problem when the SRN from Crude Distillation Unit (CDU) was directed in fixed roof storage tank and from there fed to NHDT. A little value of bromine was reported and delta P build up was appreciable in feed effluent exchanger,after cutting the pipe appreciable size reduction by deposits was seen.

3) Coke formed in the reactor caused by insufficient hydrogenation activity, H2 partial pressure insufficient.

4) Existing gum and potential gum contents are to be monitored and kept under control with good design for high value feeds

5) Alphaltenes becoming insoluble as they are dealkylated and reaction mix increases in paraffinicity.

6)Trash materials in naphtha feed, high chloride content in make up H2, recycle gas and NHDT feed need attention.

If only the top bed is developing pressure drop, then # 1 is likely the cause. Coker gas oils can contain dispersed fine coke particles or dissolved coke precursors which quickly 'come out of solution' as soon as the hydrogen is mixed with the feed.
Pressure drop increases below the top bed of a cat feed or coker gas oil hydrotreater is likely #2 if near the top bed, but more likely cause #5 if pressure drop build is in the lower bed, particularly if the reactor is not loaded with enough high activity large molecule hydrotreating catalyst.

i would suggest you to have words with your Process Licensor as they are the one who can suggest you a better troubleshooting option.

[Chellani]

I’ve faced this problem for FCC feed pretreatment. Himanshu has already summarized most of the problems I’ll add something more

1. Solids in the feedstock: most common problem. I also agree with him that coker gas oils (LCGO / HCGO) are the major source of it but rather than dissolved solids, chances of undissolved solids are very high for HCGO as coker main fractionator would always deliver some solids and in most of the cases HCGO is filtered separately in the coker unit. In my case filter never worked because of huge solids contents and finally it was kept out of line.
Finding root cause of the problem is like doing 95% of the work for higher delta P, solving problem wouldn’t take much time / efforts. The best thing to ensure that HCGO / LCGO has got high solid content and which are accumulated in the top bed is to plot trends for first bed dP and filter dP. Here I am talking about filter in your hydrotreating unit (I hope you have filters in your HT unit, if not; you should seriously think of installing it). We had candle type filters and most of the times few of the candles were always found to be distorted during shutdown. If your filter dP doesn’t increase over a cycle (backwash etc.); it means your filter are simply passing solids to your top bed. Apart from this; try and get few samples of the coker gas oil and find out undissolved solids and TDS in that.
2. Source of naphtha stream: are you talking about naphtha only? Things are similar for all the streams. Erosion of the tanks, pipelines and in most of the cases heat exchangers would gift you solids which would kill your first bed
3. Coke formed …: In most of the cases this is really the cause for second / third bed higher delta P as operators tend to put lesser quench gas and most of the H2 in the treat gas is consumed in the first bed itself. If you have some modeling tool (I would prefer KBC’s HTR-SIM) you can actually find out that the H2 supplied in the treat gas is sufficient for the reaction (based on delta T across first bed) and whether you are having access H2 in the outlet of first bed
4. Existing gum and potential gum: Really not a common cause but has been suspected a lot. Just check for the % of cold feed in your total feed. Most of the times, gum formation can happen in the tanks (provided N2 blanketing is not working as intended). I guess lab people can help you in finding the gum content of your cold feed (if any).
5. Not sure about this
6. Trash materials in naphtha feed: In my case (VGO HT), Ni+Si were suspected to be the cause of this and our catalyst vendor also gave the same root cause which I never agreed with. Typically topping material (something like Albermarle’s KG55 / Haldor Topsoe’s TK10 – I hope I remember these brand names correctly) can handle solids and few other catalysts / inerts can handle metals. If you cross your metal limits on few days only (not by great extent); I don’t think it can hamper your bed. Rather than looking at one day / two days analysis I would same that accumulated solids / metal contents should be compared against that guaranteed by vendor; which was very less in my case.

Process licensor would and catalyst vendor usually give some wake ideas rather than RC and ironically they would blame each other. We even thought of going back to our crude basket and tried correlating delP against the crude used in the basket. We got some wake idea that one of the crude may be causing some problem.

As far as topping material is concerned, all catalyst vendors have got their own brands for this. It would help you but in some cases (very few) the volume / depth required is so high that it can affect your run length of the catalyst as active catalyst volume would reduce which would required higher WABT at SOR.

To summarize, I never reached a first RC in ~ 6 months which required test run, analysis etc. etc. if you can reach on a firm decision; please let us know.