DIAGNOSIS KEY TO FLOODING PROBLEM SOLUTION

July 16, 1990
Norman P. Lieberman Process Improvement Engineering Metairie, La. When tower flooding occurs, results of field diagnoses of the problem must be included in the solution to the flooding problem. Results of the dianosis can help determine the easiest means to eliminate the flooding. Two examples indicate how good field diagnosis could have led to simpler solutions to tower-flooding problems. One refiner was experiencing flooding of a depropanizer. Based on field investigation, it was determined
Norman P. Lieberman
Process Improvement Engineering
Metairie, La.

When tower flooding occurs, results of field diagnoses of the problem must be included in the solution to the flooding problem.

Results of the dianosis can help determine the easiest means to eliminate the flooding. Two examples indicate how good field diagnosis could have led to simpler solutions to tower-flooding problems.

One refiner was experiencing flooding of a depropanizer. Based on field investigation, it was determined that flooding was caused by corrosion products in the depropanizer feed.

Particulates from corrosion promoted foaming in the effluent from the depropanizer reboiler. High foam level in the bottom of the tower was submerging the reboiler vapor return nozzle, and promoting entrainment and flooding of the bottom tray.

To correct the problem, it was suggested that the source of the corrosion be determined and eliminated. But the refiner decided it would be simpler to build a new, larger diameter depropanizer tower to stop the flooding.

The refiner replaced the 8 ft diameter tower with a new 11-ft diameter tower. The flooding was eliminated because the liquid residence time below the vapor-return nozzle was trippled. This provided sufficient hold time for the foam produced in the reboiler to break.

By ignoring the initial problem diagnosis, the refiner invested more money and time than would likely have been necessary to eliminate the source of the corrosion in the first place.

A second refiner was also experiencing the same type of flooding in a hydrocracker depropanizer. Nitrogen in the naphtha reformer feed was being converted to ammonia in the reformer reactors (Fig. 1).

The ammonia stripped the chlorides off the reformer catalyst. The chlorides then left the reformer in the hydrogen rich off gas.

As a result, the chlorides entered the hydrocracker along with the makeup hydrogen.

In the hydrocracker reactor, the chlorides were converted to hydrochloric acid.

The hydrochloric acid vapor concentrated in the overhead condenser of the hydrocracker debutanizer. Because hydrochloric acid is highly hydroscopic, localized concentrations of extremely low-pH water formed in the condenser, causing rapid corrosion.

Corrosion products from the debutanizer overhead entered the depropanizer. The products caused the foaming in the depropanizer reboiler effluent.

To stop the foaming, the refiner raised the pH of the wash water in the hydrogen off-gas drum.

The refiner reasoned that charging high nitrogen content naphtha from a revamped delayed coker had exceeded the ability of the naphtha pretreater to remove nitrogen from the reformer feed.

In this case also, the refiner chose not to attack the flooding problem at its sources: coker and naphtha pretreater operations. Instead, many manhours were invested in product sampling, field measurements, and laboratory analyses.

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