March 10, 2017 at 10:46 am #25025
I would like to know adequate flow of wash oil rate.
As you well know, increment of wash oil will affect the total capacity of delayed coker.
Also we have cold source(cold vr) at the upper side in feed section, which can work as cold wash oil.
As far as I know, cold source through the upper side can reduce the CHGO wash oil rate.
Can you recommend any guide for adequate wash oil rate?
March 12, 2017 at 9:43 pm #25042
The wash zone is used to control the HCGO quality. The amount of wash oil needed depends on the disposition of the HCGO. If it is routed to a hydrotreater, the quality needs to be good to prevent excessive catalyst degradation. This means that the concarbon content should be less than 0.5% by weight and the 95% boiling point should be less than 540 C (1000 F).
A general guideline for wash oil rates is to be somewhere between 0.5 and 1.0 gpm per ft2 to the top of the wash oil section (0.11 to 0.22 m3/h). The quality of the HCGO and the type of wash zone fractionation media will determine the minimum wash oil rate. Sprays with shed decks have the lowest required wash oil rate where a trayed wash zone requires a much higher minimum wash oil rate to prevent coke formation on the trays. The minimum wash oil ratio should be calculated for each unit and is based on the amount of quench oil that is used, the number of theoretical stages in the wash zone, the type of mass transfer equipment in the wash zone, and the overall heat balance of the wash zone. If the calculated wash oil rate is different than the guidance above, then very careful evaluation of the wash zone should be done to ensure it is operating as expected.
Several units have the ability to put feed (vacuum resid) to the top of the wash zone. I do not recommended that configuration. This will minimize any fractionation between feed and HCGO, so the HCGO quality will be very poor and it will increase the recycle ratio as the feed will condense vapors out of the coke drum. The unit will be operating at a higher recycle ratio and will not have the improved HCGO quality to show for the higher recycle ratio.
Cooling the HGCO used as wash oil has been done for high recycle units that have a trayed wash zone. This can help increase recycle ratio without increasing wash oil flow rate. I would recommend against this practice for a low recycle unit using sprays for the wash oil.
March 13, 2017 at 6:06 am #25045
I apologize that my conversion from imperial units to metric units was not done correctly. The wash oil flow rate should be in the range of 0.5 to 1.0 gpm per ft2 of tower cross section. That converts to 1.22 to 2.44 m3/m2-hr. Sorry for the conversion error.
June 5, 2019 at 6:23 am #30258
Dear Mr. Mike,
Thanks for the insights. Can you pls guide us about the design of the spray nozzles for wash oil spray above the shed decks. I mean it should form a hollow cone or a solid cone?
we are having a very high asphaltene content in the HCGO Product. The CCR is also in the tune of 1.3 wt% and Asphaltene is 0.5 wt%.
June 8, 2019 at 8:21 am #30368
Both hollow cone and solid cone spray nozzles have been used successfully. The hollow cone nozzles are less likely to plug as they do not have an insert in them. The solid cone nozzles provide a more thorough wash to the cross section of the tower but they have an insert that reduces the maximum size sphere that can pass through the nozzle.
My preference is to use the solid cone nozzles as that is better from a fractionation and contact standpoint and then use the proper sized strainers in the wash oil stream to prevent the nozzles from plugging. The pressure drop through the wash oil header should be monitored to confirm the condition of the spray nozzles. If the actual pressure drop through the wash oil header is higher than the calculated pressure drop at the wash oil flow rate, the spray nozzles are restricted indicating poor coverage by the wash oil which will lead to poor quality HCGO.
A typical wash zone in a Coker fractionator has shed decks to provide some contact between the wash oil and the vapors from the coke drums. If those are dislodged or non-functional the CCR and asphaltene content of the HCGO will increase.
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