Overhead Condenser Corrosion
Crude oil contains various chloride salts. Some of these salts decompose in the furnace to produce hydrochloric acid. The HCI boils overhead in the fractionator and dissolves in the water as it precipitates in the condenser. The resulting low pH water is very corrosive, and frequent overhead condenser tube failures can occur. If such failures become a chronic problem, check the following:
1. The desalter should remove 90+% of the salts in crude. If it does not, check the desalter temperature (usually 270+°F). Steam condensate should be used for wash water. Is the proper dosage of desalter chemical being injected? Is the voltage up to design?
2. Many refiners inject a small amount of caustic in the heat-exchange train downstream of their desalter. The caustic neutralizes the HCI evolved in the furnace. This is a fine way to stop condenser corrosion, but it also promotes fouling in the heat exchangers downstream of the caustic injection point.
3. Check the pH of the water withdrawn from the reflux drum. If it is low (5-6), the operators may not be adding sufficient NH3 or neutralizing chemicals to the overhead system.
Many engineers rely on their desalter and corrosion-inhibitor chemical vendor to formulate a corrosion protection plan for their unit. This is usually a good idea. However, one needs to know enough about this aspect of the unit to determine if the vendor representative is competent. One useful method is to ask competing chemical vendors to come up with alternative programs.
Only the MgCl2 and CaCI2 salts in crude oil decompose to HCI at temperatures above 350°F. This HCI is very hydroscopic. That is, the first drop of water that condenses in the overhead condensers will absorb all the HCI it contacts. This leads to a very low localized pH. The operators see the effect of the hydroscopic nature of HCI as extremely accelerated localized corrosion at the inlets to the overhead crude condensers.
While the use of caustic injection to the preheat exchanger train, improved desalting, and proper use of neutralizing and filming chemicals is important, the key to success in stopping overhead corrosion is water recirculation.
Water from the reflux drum is recirculated into the overhead system just before the point at which water condensation begins. Where there are parallel condensing trains, the water must be uniformly distributed through spargers with a 10-psi to 20-psi pressure drop. Enough wash water must be recirculated to bring the overhead vapors to their calculated water dew-point temperature (see Appendix). Note that the vapor from the overhead of the crude lower is already at its hydrocarbon dew-point.
The recirculated water will dilute the HCI as it condenses and prevent the formation of low localized pH solutions. Injection of water-soluble corrosion inhibitors into the recirculated water is also a good idea.
Categories: Process Troubleshooting | Tags: chloride salts, hydrochloric acid | Leave a comment