The Refinery eHandbook is an excellent source of information about topics concerning refineries and distilleries. Recently, Chemical Processing tapped WIKA USA’s knowledge on the subject. The result was an article that focused on addressing coke formation in column towers and how it affects production processes. In the article, Robert Torgerson, Director of Technology at WIKA USA / Gayesco, discusses how coke formation, when not controlled, can escalate and seriously affect product yield in vacuum towers and eventually leads to unnecessary and costly shutdowns.
The wash bed in the distribution system of a vacuum distillation column is one of the primary barriers that prevent entrained residue liquids from reaching the heavy vacuum gas oil (HVGO) draw off. This falling liquid stream can be contaminated with metals and hydrogen-deficient molecules and may seriously affect downstream conversion units. If the wash bed packing does not maintain an adequate level of wetness, it cannot de-entrain the vapor stream properly; as a consequence, dry spots will appear in the packing.
Once a section of the packing is dry, condensed entrainment droplets can quickly form coke on the wash bed packing. This coke will then attract and redirect the vapor flow to the already coked areas, creating an above-average flow of the super-heated vapor and liquid entrainment to the already dry and coked section. This process intensifies coke build-up in certain areas and also disrupts vapor distribution, reducing the efficiency of the downstream fractions in the above areas of the vacuum tower.
As vapor distribution gets worse, the vapor flow rate keeps increasing. It reaches a point when the wash bed can no longer properly de-entrain the liquid contaminants. These liquid contaminants just bypass the wash bed and then enter into the HVGO draw-off pool. Eventually, as the issue spirals, the amount of liquid contaminants that bypasses the distribution system becomes critical and the vacuum tower has to be shut down, stopping the feed to downstream conversion units and causing heavy financial losses as the pause in production may reduce the product output of the entire plant.
Coke formation may develop very quickly or over a long period of time, but it can be controlled. Measurements of temperature differentials across the cross section above the wash oil distributor will show uneven vapor distribution caused by inadequately de-trained liquids, a signal of unwanted coke formation. The temperature differential measurement can be used to adjust the wash oil feed to keep an adequate level of wetness in the wash bed and to control the vapor feed rate into the vacuum tower, effectively preventing more coke formation.
To learn more about coke formation and other issues related to vacuum column towers, download the Refinery eHandbook, with up-to-date, relevant, information for refineries and distilleries. The Refinery eHandbook is another example of WIKA’s commitment to collaborate with leading industry companies to answer to the requirements of the petrochemical and refining industries and better serve their needs.