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We have a crude unit with a double drum overhead system. The first drum collects the HC condensed from the hot section exchangers and returns all condensed HC as reflux to the crude tower. The vapor from the first drum (reflux drum) is sent to the cold section where all the HC and water is condensed and collected in the second drum (distillate drum). Any noncondensible is sent to gas recovery section. From the design conception, water should only condense on the second drum, however, we are seeing condensation (and corrosion) in the reflux drums as well as the overhead vapor exchanges heat from a relatively cold crude. In addition, we have seen signs of salt deposition and eventual corrosion in the other exchangers where water dew point is very unlikely. Given these problems in the hot section exchangers, what can we do to address and prolong the life of our overhead condensers? Please take note that the water wash, filming inhibitor and organic neutralizers are only added in the cold section exchangers going to the distillate drum.
 
Answers
04/08/2011 A: Berthold Otzisk, Kurita Europe GmbH, Berthold.Otzisk@kurita-water.com
Replacing the metallurgy to a more resistant metallurgy is a good option to control corrosion in the hot section (1st stage). Changing the metallurgy requires a long planning phase and expensive procurement costs.
The injection of an oil-soluble filming amine via hydrocarbon slipsteam or injection quill is a cost-effective and very efficient alternative to control corrosion. The typical dosing rate for a continuous treatment with a filming amine is in the 3-6 ppm range. The additional injection of a neutralizing amine and filming amine into the 2nd stage is very common.
01/08/2011 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
“From the design conception, water should only condense on the second drum…” I believe that this is expecting too much. A double drum design is intended to retard corrosion in the upper section of the column by refluxing warmer material, and apparently you are achieving that objective. Water will condense in the very first heat exchanger. Consequently, measures to dilute acids and inhibit corrosion should start upstream of the first exchanger. Equally important is reliable desalting operation.
01/08/2011 A: sam lordo, Becht Engineering, salordo@comcast.net
This is a common situation with double drum systems that use cold crude for cooling of the hot ovhd going to the first drum, the design only works on paper and failed to take into account shock condensation (condensation of the water on the tubes). When doing computer simulations the presence of water is not detected, that is because the calculations based on thermodynamic assumptions and do not take into account kinetics of the situation.
The solutions that have been used are:
to remove formation of water in first drum: maintain the incoming cold side approach temperature to with 15-25 F of calculated water dew point
to prevent corrosion and salt fouling: replace metallurgy of piping exchanger shells, and bundles with acid resistant metallurgy. some use of chemicals (filmers and neutralisers) are successful if properly applied