Q & A > Question Details
I work in a Diesel Hydrotreater Unit. Currently the Naphtha Stabilizer of our unit has a suspected choking problem in the overhead line. The stabilizer top pressure (design value- 7.5 kg/cm2) suddenly increases, top temperature drops and reboiler outlet temperature starts dropping to low value. This happens especially at night or in very cold conditions.

To re-establish reboiling the stabilizer column is drained (both rundown and reboiler side) then after raising level in column reboiling is established again as usual.

The column overhead line has no tracing steam, so we have kept an external steam lancer to keep naphtha in vapor phase.
 
Answers
29/01/2020 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
This sounds very unusual. A sudden increase in pressure is often associated with loss of cooling in columns like this. Is the overhead line free draining? If there is a low point between the tower top and the condenser, you could potentially collect liquid there, which would cause the tower to pressure up and restrict flow to the condenser. With no flow, the measured temperature downstream of the cooler would drop. Not sure why the reboiler duty would fall off unless the tower starts filling up with liquid when the tower pressures up.
28/01/2020 A: Bruce Carr, Suncor Inc, bcarr@suncor.com
Sounds like you might be flooding on the top tray.... Trend the reflux rate v's the Reboiler duty rate.... They should run in parallel... if they are crossing ie reflux going up while reboiler is going down you are passing the incipient flood point and flooding the top tray...
28/01/2020 A: keith bowers, B and B Consulting, kebowers47@gmail.com
There are two potential causes of the system behavior you describe.
1) The formation of hydrates in the overhead vapor line or in the overhead cooler will act as plugging and cause pressure buildup in the column--which reduces boil-up, etc. Of course the hydrates disappear (melt) when the system pressure is lowered and 'nothing found' plugging things up. H2S likes to form hydrates with wet LPGs.
2) If the column reboiler is a 'thermo-siphon' type, the liquid head to it may be marginal. What happens is a slight upset acts to increase reboil rate, but the higher heat input generates more vapor which 'backs-up' into the reboiler feed line--inhibiting the liquid circulation rate, which increases vapor formation in the suction line, which -------. A quick 'fix' is to raise the level in the column bottom, which increases the liquid head feeding the reboiler, which will reduce vapor back-up into the feed line etc.
28/01/2020 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
This sounds interesting. Please send P&ID and tell where you are located, and I will respond. For now, I ask: have you calculated the tray loadings in each section of the column during normal operation and compared those data with the tray loadings from the vendor?

ralph.ragsdale@att.net

Ralph