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Minimize recycle and coke carryover with the coker main fractionator washspray header design

Coker Main Fractionator flash zone design is critical to reliable operation of the column and maintaining product quality. This section commonly experiences problems related to the presence of unstable heavy liquid and carryover of solids. Successful operation often hinges on the design of the wash liquid distributor. Although Sulzer utilizes several styles of wash liquid distributors, this bulletin focuses on the spray header design. The spray distributor design is a careful balance of nozzle type, number, and placement. Optimizing the wash header design minimizes natural recycle, allowing higher flow of fresh feed to the furnace for increased capacity.

Fundamental #1 – Selection of Number of Nozzles
Spray coverage diameter or overlap determines the number of nozzles required. A header with 7 is typical, but more may be required for large diameter towers. Wash spray coverage is fully developed at a vertical distance of 2 ft and no additional coverage can be obtained by increasing the nozzle distance.

Fundamental #2 – Selection of Nozzle Type
Spray coverage diameter depends on the type of nozzle chosen, defined by its desired spray angle, size of orifice, and the nozzle pressure drop. Sulzer designs typically incorporate nozzles that have sufficient pressure drop to maintain the spray pattern and have a cone angle to target the largest spray coverage diameter. Optimized coverage requires less natural recycle. Smaller angle nozzles can be used if coke carryover is an issue, as their greater downward force could potentially improve knockdown of coke. If multiple nozzle types meet the column’s specifications, select the nozzle with the larger orifice passage for increased reliability from plugging or coking off.

Fundamental #3 – Placement of Nozzles
In wash sections with a dual header design, the top distributor is rotated from the lower distributor, ensuring complete wash coverage across the column. The nozzle placement on each header is carefully determined to maximize the amount of coverage. When designing the placement, it is important to maintain some overlap of the spray nozzle coverage, but not so high that the overlapping droplets interfere with and deflect each other.

Other Design Considerations
1. Entrainment: The relatively high pressure drop through a spray nozzle is beneficial to prevent plugging, but the same pressure drop creates smaller droplets that are subject to entrainment, which must be considered when determining the wash header design flow rate.

2. Turndown: At lower than design flow rates, pressure drop through the nozzle is reduced and the spray pattern may collapse. Turndown conditions must be considered during the sizing process, or alternatively in a dual header system, the upper header can be operated with a steam purge (to prevent plugging) while the bottom header operates normally.

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