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For a reboiler furnace in NHDT, is better:
to have reboiling liquid 34 m3/hr and furnace COT at 208 C or to have reboiling liquid flow at 40 m3/hr and furnace COT at 203 C?
Note: Reboiler furnace is the limitation for maintaining max throughput.
 
Answers
18/04/2018 A: Milorad Popovic, Retaired, milorad.popovic@gmx.com
First try to reduce the stripper pressure as much as product naphtha on-spec to help the heater. Heater re-boiler duty is in direct function to reflux rate for a given separation @ constant Tin.
When a heater in re-boiling service it is better to increase coil flows than COT (°F) or vaporizing rate. Heat transfer is better with more turbulent liquid (lb/sec sqft) against tube wall at given radiant heat flux (Btu/hr sqft).
When increasing throughput and re-boiling liquid flow more heat will be needed to keep COT(°F) high enough for required naphtha vaporization rate and the stripper L/V traffic. Vaporization along tube length is not linear, increased near outlet. Variation of individual coil flows would not help much.
Now the focus is on maximizing the heater duty respecting its limitations: Coil TMT °F, max BWT, max Stack T, max tube hanger T, min. draft,. Adjust burners firing to get uniform coil heating, look in box for "hot spots". Shape flames properly to stay out of tubes, hangers, refractory,check indirect flame impingement by IR scan,.
29/03/2018 A: Rajkumar Chate, Sulzer, rajkumar.chate@sulzer.com
Your goal is to strip some light hydrocarbon gas from the feed, you need certain temperature at bottom of the column which makes sure that your bottom product is on spec. By reducing the flow you may reduce the feed rate to furnace but inside the furnace you will have higher vaporization if you keep the same furnace firing.
Bottom temperature is important to keep naphtha on spec hence better to reduce flow and maintain outlet temperature. If bottom specification is not important for you then you can do which ever is favourable to you.
29/03/2018 A: Ganesh Maturu, ADNOC, maturu.ganesh@gmail.com
I hope you are asking about NHDT stripper reboiler. In Naptha services, I rarely see furnace reboilers since required temperature is very low like you have mentioned which can be managed with steam reboilers. If it is furnace reboiler in your case, ideal reboiling philosophy is to maintain 50% vapor fraction at furnace outlet to avoid polymerization and fouling of tubes. In revamp cases and limitation in NHT stripper bottom pumps, we may go upto 65% vaporization at reobiler outlet by considering some risk of fouling. Higher vaporization means lower circulation and higher reboiler outlet temperature. In your case, if you are limited by hydraulics, you may reduce circulation flow upto 65% vaporization.
For thermosyphon reboilers, no possibility of adjusting circulation flow as it gets adjusted based on hydraulic balance.
28/03/2018 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
If tube skin temperature is your criterion for maximum capacity, then the lower temperature circulation mode should be the best for you. You can find your answer with a test run, collecting the data and observing the skin temperatures.
27/03/2018 A: Marcello Ferrara, ITW SrL, mferrara@itwtechnologies.com
If reboiler limitation is due to fouling ITW Online Cleaning can help in improving performance.
Please contact me privately.
27/03/2018 A: Morgan Rodwell, Fluor Canada Limited, morgan.rodwell@fluor.com
This is not sufficient information to answer this question. The operating pressure and boiling range, vapour fraction leaving the furnace and the boiling point range of the fluid is important, not just the flowrate and temperature rise.
27/03/2018 A: Ashutosh Garg, Furnace Improvements Inc, agarg@heatflux.com
It will be better to have higher flow and lower outlet temperature in the furnace. Higher mass velocity is good as is lower outlet temperature.