Q & A > Question Details
In our Vacuum gas oil fractionation column, we have a diesel draw chimney tray. It doesn't have any overflow lines; the pumparound and internal reflux to the bottom trays of the chimney are provided by diesel draw pumps. I just want to know, if both our pumps had issues and stopped, how does liquid from the chimney tray reach down? Even though the liquid level reaches the chimney tray vapour ports, the liquid can't reach below the chimney tray through the vapour ports as the pressure of vapour flowing through the chimney tray vapour ports is more than the liquid pressure. Am I right? If the liquid can't travel down the chimney tray, the bottom trays becomes dry as the liquid flow from the chimney tray is stopped. In such cases, what could happen to the fractionation column top and bottom sections?
11/09/2020 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
If you lose both diesel pumps you will also lose condensing in the top of the tower as soon as the pumparound exchangers flood with condensed diesel. At that point the tower will begin to pressure up to relief conditions. With not reflux your diesel/LVGO fractionating bed will dry up and you will lose fractionation between diesel and LVGO. LVGO and HVGO pumparounds will continue to provide condensing in those zones, so you will at least temporarily get whatever limited fractionation there is between those products. As the tower pressures up from lack of diesel condensing the yield of LVGO and HVGO will fall off dramatically because the increasing tower pressure will cause vaporization in the flash zone to quickly fall way off. This is a common relief scenario done for flare/PSV studies on vacuum towers.
11/09/2020 A: Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com
Three possibilities:
1. The chimney tray may have been designed with an internal overflow pipe. The top of the pipe would be slightly lower than the top of the chimneys.
2. If there isn’t an internal overflow pipe, the chimneys will overflow. If the velocity in the chimneys is low enough, most of the liquid will flow down past the upcoming vapour, while some of the liquid (the smaller droplets) will be entrained and blown up into the bottom of the diesel bed.
3. If the velocity in the chimneys is too high, most or all of the liquid will be entrained and blown into the bottom of the diesel bed. The diesel section will flood until either there is sufficient static head to overcome the upflowing vapour in the chimneys, or liquid will go down the overhead line. In the latter scenario, the ejectors will stop working, the vacuum will decay and the vapour velocity in the chimneys will suddenly drop allowing the liquid in the diesel bed to dump. Once the ejectors and condensers have drained, the vacuum may return the cycle will repeat.