In our naphtha hydrotreating unit, stripper heater is being used to heat heavy naphtha coming from NHT reaction section to eliminate all the H2S, NH3, H2O by stripper column. But, currently the heater is not performing well and outlet temperature reduced down from 210°C to 203°C . So, need to know that in order to improve the heater performance, can we conduct only the steaming through the coil to strip out all the dirt layer/coke formed inside the coil surface ?May-2021
Richard Rhead, Refining & Aromatics Consulting, firstname.lastname@example.org
As previously mentioned--feed stock composition (lighter components/lower boiler point) or lower operating pressure could cause the observed drop in outlet temperature from 210 C to 203 C. For a NHT Stripper--the reboiler is to provide necessary heat to generate sufficient vapor/liquid traffic in the column trays to strip out the light components overhead. For me the important criteria--do you have sufficient vapor/liquid traffic (reflux is an approximate indication). In regards to the stripping performance--need to make sure that you are stripping out all the butanes and pentanes--if not this will lower the column bottoms temperature.
While it is possible naphtha service is relatively clean service and not prone to coking--but never say never. First off, I'm making the assumption that your mentioned "stripper heater" is a fired heater and not a heat exchanger. First step would be to determine the condition of the fired heater--what efficiency is it running at. Next perhaps look at longer term trending of the inlet vs outlet temperatures. The delta's between them should be relatively constant (same vaporization ratte--assuming reboiler circulation rates are the same). A narrowing of the delta would be an indication of fouling--inside our outside of the tubes. If firing clean fuel gas--the outside of the tubes should be relatively clean--an no build up of foulants. If firing fuel oil the outside tube surface will get dirtier over time.
If fouling is on outside--the cleaning method would be different than for the inside. I personally have no experience, but have hear of companies offering a proprietary service to clean the outside of the tubes with the heater in service. If shutdown, I have heard to soda ash with wire brushes/wire rope wrapped around the tubes--with opposite direction pulling of the wire rope being used. Inside--will require a shutdown. Possiblly pigging, or steam spalling or steam/air decoking (if heater has these facilities--but for NHT service I doubt it). With forced circulation through the reboiler passes I wouldn't expect that any dirt lay would form.
If this becomes a reoccurring issue, will need to address this issue. One possible consideration would be coating the heater tubes.
Micael Presa, Tubacex, email@example.com
Do you have fouling or coking problems?
If so, have you thought about using protecting coatings in order to avoid fouling and coke deposition?
Adrie Visser, Fluor, firstname.lastname@example.org
What is meant with "the heater is not performing well"? Is it that you are consuming more fuel for the same feed rate? Is the stack temperature or O2 in the stack higher? Is there a problem to keep the reflux rate (or ratio to feed) constant? Some clarification will assist in presenting ideas for troubleshooting.
Normally the NHT charge heater before the reactor is much more prone to coking than the reboiler heater. The NHT feed contains olefins and other coke precursors. These are saturated in the reactor and therefore the reactor effluent would have a lower coking tendency. If you have not had coking problems in the past, then it is more likely that the problem is due to something else. For example, did the 95% point of the NHT feed reduce? Are you operating with lower reflux ratio and still maintaining the same amount of H2S stripping? If any of these is yes, then that can explain the lower outlet temperature of the reboiler.
Have you done a check on the NHT product composition to determine the 50% equilibrium flash vaporisation temperature? Normally fired reboilers are designed for 50% vaporisation at the heater outlet pressure and temperature. If 210 °C provides less than 50% vaporisation, coking of the tubes is also unlikely.
Ruben Hidalgo, DIANAVALEN, C.A., email@example.com
Understanding that the idea is to avoid a plant stoppage, because there is high demand for fuels, the recommendation is:
1. Increase the Hydrogen / Hydrocarbon ratio to maintain the recommended speeds and avoid Coking.
2. Increase the steam flow, stop the plant to make the other suggested recommendations.
Entendiendo que la idea es evitar una parada de planta, debido a que, hay alta demanda de combustibles, la recomendación es:
1. Aumentar la relación Hidrogeno/Hidrocarburos para mantener las velocidades recomendadas y evitar la Coquización.
2. Aumentar el flujo de vapor, hacer una parada de planta para hacer las demás recomendaciones sugeridas.
Vishwanathan Gurumoorthy, Vishwaraj Consultants, firstname.lastname@example.org
For heater performance improvement, decoking is required if it is nearing its usual decoking time or throughput target is achieved.
In case, it has surfaced well before its scheduled time,decoking by steaming of the coil may not help.The problem is not coking but something else.
Steaming out of the coil could be tried first.
Mukesh Patel, Reliance Industries Limited, email@example.com
Looking at the temperature reduction of 7-8 oC in heater, following can be tried: Check whether burner firing pattern is OK? Is any skin temp or tube metal temp are crossing the design limits? If so, fouling may be suspected. If fuel oil is being fired, check FO quality e.g. any particulates, polymeric material ingress etc. Check for any change in feed quality? Whether stored Naphtha is being processed? Steaming may have immediate benefit but need to look for root cause of poor heater performance. Heater cleaning online or external cleaning may be thought off.
Jake Gotham, InSite Technical Services, firstname.lastname@example.org
Mrinal has already commented on the possible influence of composition on the outlet temperature. Operating pressure will also have an effect – lower operating pressure will give lower temperature at the top and bottom of the tower for the same separation. Another variable is the reboiler circulation rate. A furnace reboiler will have a circulation pump and flow control valve(s). If the flow controller is set to a higher flow, the reboiler outlet temperature will reduce for the same duty.
Are you seeing problems with product quality from poor stripping (e.g. H2S in the product)?
You have concluded that the problem is fouling inside the furnace tubes. To confirm this, you should trend (skin point – outlet temperature)/duty. For a reboiler, duty is not easy to measure on the process side, but the fired duty should be relatively straightforward to calculate from the fuel consumption and composition, and the efficiency can be estimated from the excess O2 and stack temperature.
If (skin point – outlet temperature)/duty has increased, it confirms that the problem is likely to be tube internal fouling. Shutting the furnace down to steam through the coil may be partially successful, but I doubt it will be completely effective. Also, consider where you are moving the coke to. If you steam through into the stripper, any coke removed from the furnace will go into the bottoms pump, level control valve or heat exchangers. Maybe it is small pieces which cause no problems, maybe it is larger pieces which cause pump damage or a flow restriction. If the furnace was originally built with steam/air decoking in mind, you should have the ability to steam through the furnace into the decoke quench drum which is obviously preferable, though would require a longer outage in order to swing elbows etc.
If (skin point – outlet temperature)/duty has not increased you need to consider other potential causes, including:
- Operating conditions (composition, pressure or circulation rate as described previously, also stripper feed temperature, reflux rate).
- Poorly optimised furnace conditions (excess O2, draught, flame conditions, air-preheat etc).
- Change in fuel quality.
- Burner plugging or other restriction in the fuel system.
- Fouling of the outside of the tubes – this is mostly a problem in refineries that burn fuel oil, but fuel gas can also cause fouling.
I hope that helps. Please get in touch if you would like more help troubleshooting this problem.
Mrinal Banerjee, Indian Oil Corporation Limited, email@example.com
The following analytical test can be performed:
Have you analysed the operating heater inlet stream composition and compared it with the design inlet stream composition? If the operating composition analysis (through chromatography, for example) returns a summarily lower boiling material than the design, it may be possible that you are experiencing the effect of hightened vaporisation.
In any case, there has to be a precedent of temperature >300°C to indicate cracking and thus coking in the heater. One thing to check there would be – is your naphtha straight run or cracked (FCCU / DCU product)? If there is cracked component in your stream, and it has been taken from a tank with improper nitrogen blanketing, gum formation is a distinct possibility.
Wash water separation efficiency in the three-phase separator is also a point to investigate, if you fear salt deposition.
Hope this gives you some indication of what to do with your stripper charge heater.
Syed Mumtaz, Freelance Individual, firstname.lastname@example.org
Steaming may only remove liquid on hardened coke type material. Consider using a commercial heater cleaning system. You could use a combination of methods, but may need a brief shutdown. Could be worth it if a clean heater can remain onstream for the duration of the turaround .