April 7, 2018 at 1:12 am #28406
After a maintenance shutdown of the unit where we have some modifications in this pipe, the AT beetween heater and drum has increased from 18ºC until 27ºC (average values). We process heavy crudes with a 25%w concarbon and 20% C7 asphaltene.
For this cruds, which is the minimum inlet drum T in order to avoid problems with coke formation? (Our current minimal T is around 468ºC)
Which is the maximum coil outlet T to avoid accelerated fouling in the heater? (now we would need to increase up to 500ºC)
We use fiberglass insulation in this pipe. Have you tested any better insulation (e.g. aerogel)?
We are thinking to try an electric heat tracing, do you have any reference of this option being used in delayed cokers? Would it cause fouling in the pipe?
Thank you in advance for you reply
April 7, 2018 at 4:17 am #28407
Anything that increases the residence time in this “transfer” line would increase the temperature drop between furnace and drum: longer line, larger diameter, removal of coke formed, etc. The temp drop is caused more by coking reactions than heat loss. As operation continues, coke will likely deposit in this line reducing residence time and reducing the temp drop. Check your past data after a maintenance shutdown, particularly after one where this lined was cleaned of coke deposits.
Without physical modifications to the line or changing process conditions, the only way to reduce temp drop is to add more flow to the coking coils via more feed, more recycle, or more velocity steam or water. All of these will effect coke morphology in the drum, possibly increasing loose coke formation at the bottom.
I have no experience with insulation “in” this line.
Regards, Steve Sparks
April 7, 2018 at 2:24 pm #28408
You indicate that some modifications were made to the piping between the heater and the coke drums, the heater transfer line. If those modifications reduced the velocity in the line, coke could be blocking the line increasing the pressure in the heater. Excessive pressure drop is another reason for a high temperature drop in the transfer line.
I think a normal drum inlet temperature of 468 C (874 F) is too low. Feed temperature can be this low for about an hour before concerns with unconverted oil and tarry drums are high enough to take corrective action. I prefer a normal drum inlet temperature of 482 C to 485 C (900 F to 905 F). A common heater outlet temperature is 496 C (925 F), which should result in an unquenched drum outlet temperature of 440 C (825 F) which is high enough to ensure conversion of the resid in the drum. Higher heater outlets cause faster fouling of the heater, so some trade off in yields vs run time is sometimes done.
As Steve indicated, the primary reason for the drop in temperature in this line is the continued thermal decomposition reactions in the line due to the residence time and the bulk temperature of the fluid. The other reason for a temperature decrease is a reduction in pressure, meaning that equilibrium flash vaporization is occurring in the transfer line using the energy in the liquid to vaporize some of the fluid as the pressure decreases. A typical temperature drop is 14 C (25 F). Your temperature drop before the 18 C, which is similar to the guideline.
If the velocity is too low in the transfer line, coke will form and will not necessarily be uniform in the pipe. Coke that deposits will form stagnant areas that will allow liquid to collect and coke and cause significant back pressure on the heater.
I have not heard of any use of Aerogel insulation on the transfer line. I have seen it used on portions of coke drums and I have heard that it has been used to insulate entire coke drums. I think a mineral wool or calcium silicate insulation would be adequate for the transfer line. Fiberglass is probably an upgrade. I would not put heating coils on the transfer line as that will encourage coke formation in the transfer line.
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