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.