Is there any way to increase or improve the heat transfer rate of the Austenitic Stainless Steel tubes of the VGO hydrocracker reactor charge heater? Chemical cleaning or any mechanical means? Also ensuring temperature doesn't drop below 205C to avoid acid attack.May-2021
Milorad Popovic, North atlantic Refining Limited, firstname.lastname@example.org
First review the hydrocracker charge heater design (API sheets) for normal vs. rated heat duty to avoid over firing. Oil fired is designed with a large duty margin (up to 50 % , with enough radiant tubes BWT _<1300°. 1650°F max., TMT~1220° . If BWT exceeded 1600°F heavy fouling is on tubes: external (oil ash, soot, refractory dust,.) or internal coke layer. Tube external fouling can be visualized , internal per black or silvery spots tube metal temp >1220 °F . In Gas fired thinner layer of metal oxides and dust on tubes indicate less fouling and BWT stays below 1600°F. Rigorous monitoring by tube, firebox TIs PIs ,including IR camera , data logging is necessary.
Oil fired tubes can be cleaned effectively by on-line (CTP Environment, Neutral Low intensity blow out with weak oxidizer ) to avoid austenitic SS 347H sensitization. This is In gas fired heater is less effective (only dust removal while saving metal Cr oxides) and with low cost payback. In both cases heat duty could be restored to near clean design case. Tube internal cleaning is steam/air decoking during turnaround , as here on-line HP steam spalling coil by coil is not in practice. Exchanger train cleaning will help but not resolving problem since the heater is for high-grade heat and the reactor Tin control. Check thermal effect with the Reactor beds Touts ascending vs equally arranged mode.
Rohit Kumar Kumar, HMEL, email@example.com
It's a very interesting question. First, I would like to mention certain operational measures that can be taken to increase the reactor inlet temperature of your VGO hydrocracker. These are:
1. Increase the feed preheat from combined feed exchangers (Breech-Lock). You can do this by minimising the feed bypass across these exchangers. But here you have to be careful of the reactor outlet stream to hot separator inlet temperature; it should not go below 240°C.
2. Try increasing the feed preheat by installing an exchanger that would use the hot unconverted VGO from the fractionator bottoms to preheat your feed VGO. You can install it before your feed filters. If you already have a steam preheater, you can increase steam to increase preheat.
3. Try maintaining excess O2 in the feed heater below 3.0% and above 1.5%, to ensure lower carryover of heat to the flue gases. Keep a close eye on emissions during the operation.
Now let's discuss the chemical/mechanical means:
1. You can carry out decoking of coils using MP steam followed by chemical flushing. During this you need not worry about polythionic stress corrosion because this is a shutdown job.
2. Do not go for oil firing in the heater. Always do fuel gas firing, as it will minimise the external fouling of the tubes. If your H2S in fuel gas is less than 10 ppm, polythionic stress corrosion is less likely. In this case, you will be allowed to maintain the coil temperature of 60-70°C, disregarding the value of 205°C.
Marcello Ferrara, ITW Technologies, firstname.lastname@example.org
Coking of furnace tubes occurs due to degradation of feed in the coils; the coke formation will reduce coil heater heat transfer. Such degradation is accelerated when furnace firing is increased because of, e.g., reduced furnace inlet temperature. Reduced FIT occurs because the preheat is fouled. ITW Online Cleaning can clean an entire preheat train in 24 hours and increase FIT. This will reduce furnace firing and will lead, besides lower fuel consumption and lower CO2 and airborne emissions, to less coke formation and related increase in heater heat transfer.