Q & A > Question Details
Does the top naphtha section of the crude column perform distillation as the other sections do?
The Del.P across this section does not seem to be a normal one to me. Top three trays functions at nearly 200mmH2O.
I see this section as a condensing/absorption section more than a distiller.
Am I correct?
04/01/2010 A: Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
I concur with Ralph Ragdale's response. We worked together in the past.
As noted, much depends upon the hydraulic/hardware/metallurgy details of the top three trays and whether the top three trays comprise a 'tip top pumparound' ["TTPA"] or whether the top three trays are seeing 'cold reflux'.
'Cold reflux' for CDU atmospheric tower means that the reflux return temperature is well below the dew point of steam vapours within the overhead vapours. 'Cold reflux'----as popular in traditional designs----can cause fouling and deposition and corrosion of the top three trays and these raise deltaP for the uppermost zone.
In TTPA scenarios, the top three trays should generate at least one theoretical stage vis-a-vis computer simulation modeling--if using best practice hydraulic/hardware design features.
As complex towers, the performance of the CDU atmospheric tower and VDU vacuum tower and associated heaters and transfer lines must be modeled using computer simulation modeling, with the models interpreted by qualified consulting specialists.
03/01/2010 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Good question, as it depends on the configuration. Most atmospheric crude columns have overhead condensing with naphtha as overhead product and jet/kero as the first side draw. In that case the top section of trays fractionate between the two products. If the trays are intact , there would be some pressure drop, but not easy to measure. Some configurations have a top pumparound with no overhead condensing. In this case, the top trays are heat removal only. In process design, we count a heat removal section of this type (pump-up) as one theoretical tray for fractionation allowance, even if there are as many as 10 or 12 actual trays. There would still be pressure drop in that section, however.