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In our DHDS plant (Axens licensor, revamped in January 2018 ) with both a hot high pressure separator and cold high pressure separator, we are facing several tube leaks(A179-CS tubes ) in our stripper feed/stripper bottom exchangers (life three years).Corrosion is mainly on the stripper feed side and corrosion is due to localised under-deposit corrosion on the OD side of tubes near the floating head tubesheet, probably due to carry over of water and salts from upstream separators. Our hot separator is operating at 40 ksc and 90 degC operating temperature against the design 100 degC. In the same plant we are facing severe choking issues in our stripper overhead fin fan coolers where a complete header box was found choked with deposits. Around 76 % of the foulant collected is iron, and ammonia is also present. Has anyone faced such issues? Is operating the hot seperator at lower temperature the cause ? Has anyone used Alloy 825 tubes for stripper feed /stripper bottom exchangers?

 
Answers
21/01/2021 A: Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com
Units with hot separators have several advantages over units with only a cold separator – they are efficient in terms of water wash requirement, sour water production and are usually (but not always) more energy efficient. One of the downsides of the hot separator flowsheet is that the hot separator liquid goes to the stripping section without being water washed. In some situations this can contribute to corrosion problems in the stripper feed system or stripper overheads. You are also right that water carryover from the cold separator will flash off and cause corrosion in the stripper feed exchangers. The section of pipework immediately downstream of the cold separator level control valve is also vulnerable and is worth inspecting regularly. It is not clear from your message whether the exchangers you refer to are on the stream from the cold separator, hot separator or whether the two streams combine before these exchangers, so it is hard to be more specific about your problem. 100°C is unusually low for a hot separator. At this temperature ammonium salts will have formed but, assuming your hot separator doesn’t have an upstream waterwash, these salts won’t have been removed so they will carry through to the stripper section causing the problems you have described.

Metallurgy upgrades are one way to address the corrosion problem, but this won’t fix the fouling problem you have also described. It might be better to concentrate on the cause of the problem rather than the symptoms. Get in touch by email if you’d like help troubleshooting this further.
22/12/2020 A: Berthold Otzisk, Kurita Europe GmbH, Berthold.Otzisk@kurita-water.com

The corrosion problems described have also been observed in other DHDS or NHDS. In most cases, the cause of an increase in corrosion was chloride deposits, which were not initially considered critical in the presence of significantly higher sulphide concentrations. The chlorides have a very high corrosion potential and can lead relatively quickly to underdeposit corrosion, especially if no or only little wash water is used for the reactor effluent exchangers.
The lower operating temperature of 90°C certainly also has an influence on stronger formation of under-deposit corrosion. It is the ammonium chloride that precipitates first when the dew point temperature is reached, before ammonium bisulphides can also precipitate later.
In practice, the use of Kurita's ACF technology has proven successful. This is a strong organic base that directly reacts with the chlorides to form a pH-neutral liquid salt with very low corrosion potential. The ACF salt is highly hygroscopic and remains in the sour water (sometimes only a few liters water), which is collected and discharged in the low-pressure separator. In this way, no ammonium salts can be deposited, which could later lead to underdepoisit corrosion. When dosed into the reactor effluent stream, the ACF product usually contains an additional film-forming component to ensure direct corrosion protection on the metal surfaces at the same time.
If a differential pressure increase is observed in the stripper due to ammonium salt deposits or corrosion, dosing of the pure ACF component is recommended. The dosing then takes place in the overhead reflux from the stripper. The dissolved ammonium chlorides then leave the stripper as water-soluble ACF chlorides via the overhead system.
The use of higher quality steels is definitely a way to protect this area. However, this often does not prevent salt precipitation in the stripper if the salts are carried further.
15/12/2020 A: Krishna Nimesh, BPCL-Kochi Refinery, krishna.nimesh@gmail.com
There are various locations where salt deposition might occur:
1. HHPS upstream combined feed preheat exchanger (CFE): if the temperature drops below 204 degC across any heat exchanger then you can expect fouling and in extreme cases corrosion also. 204 degC and 89 degC are considered NH4Cl and NH4SH salt deposition temperatures as per API-932B. As you informed, the HHPS is operated at 90 degC hence initiation of salt deposition can be expected. Basically these salt deposition temperatures are calculated depending upon NH3, HCl and H2S partial pressures in the HC stream.
2. HHPS overhead fin and trim coolers: this is the location where almost complete salt deposition is expected as the temperature drops to 35-40 degC before going to CHPS.
3. Stripper feed preheat exchanger: the HHPS and CHPS HC stream, which is lower than 200 degC, will be preheated up to the stripper inlet temperature using the stripper bottom stream. If salt is not removed in upstream HHPS and CHPS then it will pass onto the stripper and deposit in the exchanger.
4. Stripper overhead: carryover of salt precursors (NH3, HCl and H2S) to the stripper will also move to the overhead and will cause severe fouling and corrosion issues. You can follow the following practices to mitigate the issue:
1. Check your feed HSD composition for N, S and Cl and calculate the quantity of salt formed and the wash water quantity.
2. Increase your HHPS operating temperature to at least 130 degC. This will solve your NH4SH salt deposition issue in upstream CFE but NH4Cl can still deposit. If possible, then increase it to 210 degC. No salt will deposit below this temperature and it will move to the HHPS overhead.
3. Start wash water injection at the HHPS overhead finfan upstream. Axens suggests 5 wt% of design feed. You can also calculate NH4SH (should not be more than 2%) and corresponding wash water quantity (at least 25% remains in the liquid phase after injection) as described in API-932B.
4. You might have an intermittent wash water injection facility at HHPSH upstream the CFE. If a water boot facility is available in your HHPS then start continuous wash water injection in the CFE upstream where the temperature falls below 204 degC ( if the feed contains high Cl and N2) and intermittent injection if the feed has Low Cl and N2. If your HHPS doesn't have a water boot facility (I am expecting your HHPS to be of vertical type with no water boot facility) then you can’t inject water otherwise sour water coming to the HHPS will carry over to the stripper along with the HC stream and your whole LP system will be exposed to fouling and corrosion. In such cases you should clean your exchanger whenever DP is high, or clean while in turnaround. You can expect heavy fouling and corrosion also. You can also expect Reddish-white coloured deposits.
5. Start wash water injection at the stripper overhead finfan upstream. I expect Axens has not considered wash water injection there but still you should provide wash water. It is essential and will resolve your issue (99%) even if salt is getting deposited there.
6. Ask the inspection department to carry out a thickness survey for the stripper overhead dome and lines upto the reflux drum. As you have said that your stripper finfan and trim coolers were choking then you can also expect corrosion and line thinning.
7. Always maintain a stripper top temperature of at least 15 degC above the dew point temperature. Better to provide a calculation on the DCS itself for continuous monitoring.
8. Check for desalter operation in the crude unit if Cl is arriving in the feed. Check makeup hydrogen for any presence of chlorides (if your H2 source also includes CCR).
9. Always maintain stripped water quality as per design. Important parameters are pH (7-9), sulphides (<50 wppm), chlorides (<5 wppm), iron (< 2 wppm).
10. Read API-932B for detailed information.
15/12/2020 A: Nagarathinam S Murthy, Ashphil Consultancy, Chennai, nssvdvr@gmail.com
With a hot separator in the HP loop of DHDS, the feed to the stripper will be liquid streams from the hot separator and cold separator. Operation with too low a temperature in the hot separator can be a source for NH4HS entry into the stripper. However, more likely it could be from (poor water separation) water carryover through cold separator liquid to the stripper, as well as inadequate wash water use in REAC which again pushes higher levels of NH3 and H2S into the stripper. Keep tracking the level of NH3 in the stripper off-gas which could give an indication of any of the concerns mentioned earlier. Also, look at any emulsion seen in the cold separator owing to the presence of surfactants in the DHDS feed. Changing the MOC of thr exchangers should only be a last resort. Further, in most DHDS units the level of NH4HS in sour water from the cold separator should be less than 2-5 wt%. Wash water use should be enhanced if NH4Cl is found in sour water.
15/12/2020 A: Morgan Rodwell, Fluor Canada Limited, morgan.rodwell@fluor.com
This sounds like you have a significant problem with oil/water disengagement in the high pressure separators. Does the HHPS have a water boot? Normally they do not, but at 90°C this is not "hot" and liquid water is highly likely to form (along with ammonium salts). The design/operation of these vessels is critical to separating water to avoid exactly the problem you describe.

The stripper feed/bottoms exchanger you are likely boiling water that contains salts, which would deposit on the tubes. Have you checked if the deposits are chloride or sulfide salts (or a combination)?
15/12/2020 A: Mike Watson, Tube Tech International Ltd, mike.watson@tubetech.com
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15/12/2020 A: Mainak Sarkar, Indianoil Corporation Ltd, sarkarm@indianoil.in
The issue may be related to formation of ammonium bisulfide. Check the wash water injection rate. Another aspect is slippage to sour water from HPS if your HPS is a 3-phase separator and LPS is 2-phase.