Entrainment issues in vacuum column flash zones
For a vacuum tower to operate effectively, the flash zone and the wash section must work together to provide the best possible feed quality to the sections above
Mark Pilling, Mario Roza and S M Wong
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One of the recurring questions about refinery vacuum tower operation is the issue of flash zone entrainment and the operation of the wash section. As refiners push to optimise yields from the vacuum tower, the limits of existing column internals are often encountered. Although the operation of the vacuum tower needs ultimately to be judged as a whole, this article focuses on the flash zone and wash sections of the column, since they are critical for high-yield operations.
The flash zone serves to transition the high-velocity, two-phase feed from the transfer line into the vacuum column in a manner that separates the liquid and routes it to the bottom of the column, while delivering the vapour uniformly to the upper sections of the column. The feed nozzle’s orientation into the column can be radial or tangential. The number of nozzles can vary, depending on column size and heater configuration. The typical feed internal for a vacuum column is a vapour horn or other device that uses the feed inertia to redirect the stream to contact and remove dispersed liquid particles. The liquids entrained in the upward-flowing feed vapour stream need to be removed because they contain disproportionately high amounts of heavy end contaminants, such as metals and hydrogen-deficient molecules. The metals are catalyst poisons for downstream processes. The hydrogen-deficient molecules tend to form coke and also adversely affect the distillate’s end point â€¨and colour.
The flow configuration of the bottom of a typical vacuum column is shown in Figure 1. Above the flash zone is a relatively short packed bed called the wash â€¨section. The wash section is designed to remove heavier components in the upward-flowing vapour from the flash zone by coalescing entrained liquid droplets and by condensing the heavier vapourised components. The packing provides the surface area that coalesces â€¨the entrained liquid droplets in the vapour stream. A small liquid â€¨gas oil stream is fed to the top of â€¨the wash section, which serves â€¨as a kind of reflux stream and â€¨also acts to wet the packing to prevent it from drying out and coking. It also aids the removal of coalesced liquid from the packing surface.
Liquid leaving the bottom of the wash bed is collected in a slop stream. It consists of coalesced liquid entrainment from the flash zone, the heavy condensed com-ponents, and the heavy portion of the liquid gas oil feed that made it down through the packed bed without being vapourised. Since gas oil is more valuable than slop, process economics dictate that the gas oil feed stream to the wash section be minimised. However, the stream must be maintained at a sufficient rate so that coking is avoided. From a column product draw standpoint, as the entrainment rate to the wash bed increases, the slop draw should increase by the same amount, but the contribution of the slop from the liquid feed to the wash section will remain essentially constant.
Optimising the flash zone and wash section design
When trying to optimise the feed and wash sections of the vacuum column, the goal must be clearly understood. From an operational standpoint, the true goal is to provide the best possible vapour feed to the distillation section above the wash section. The term “best possible” means the lowest possible amount of entrained liquids as well as the most uniform vapour distribution. Again, this is measured above the wash section. However, since the intermediate conditions between the flash zone and the wash section can affect the output of the wash section, we need to under-stand that region as well. Therefore, we need to look at the vapour distribution throughout these sec-tions as well as the corresponding entrainment.
Looking first at vapour distribu-tion leaving the wash section: this should be fairly straightforward. In order for the wash section to function properly with respect to de-entrainment and coking resistance, the vapour feed to the wash section must be very well distributed. Since a packed bed is typically a good distributor of vapour, the vapour flow from the top of a properly working wash bed, by definition, must be excellent. Therefore, when discussing vapour distribution, we need to focus on the region between the flash zone and the wash section.
Now looking at entrained liquids: from a practical standpoint, the flash zone section behaves as a rough vapour-liquid separator and the wash section behaves as a polishing bed, where droplets can be removed and fractionation takes place. Generally, it does not matter how much liquid is removed in the feed section or the wash section independently; it matters how the combined feed and wash section perform as a unit. To some extent, the de-entrainment effects of the flash zone and the wash section are cumulative. In a simplistic approach, if both sections operate at 90% efficiency, the net efficiency will be approximately 99%. For example, if 100kg of entrainment enters the tower, a 90% efficient flash zone will allow 10kg to leave and go to the wash section, which will capture 90% of that entrainment, allowing 1kg of entrainment to leave to the tower above.
However, the efficiency of both sections is based on droplet particle size. If the flash zone or the feed inlet device allow a bigger portion of large-diameter droplets to escape to the wash section, the wash section will be extremely efficient in removing those droplets. However, if a larger percentage of very small droplets leaves the flash zone, the wash section will be less effective in removing that extra entrainment.
Nevertheless, it is desirable to minimise the entrainment from the feed section to the wash section. However, when this comes at the expense of poor vapour feed to the wash section, improved de-entrainment from the feed section usually does not result in a better product from the wash section. The following CFD study provides a review of an industrial column configuration with two types of feed devices. From this study, it can be seen that different feed designs can have unexpected results with respect to vapour distribution and de-entrainment capabilities.
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