Sour water strippers problems

The main corrosion problems in a nonacidified condensing sour water stripper occur in the overhead system. Exchanger tubes in the overhead condenser are often made of commercially pure titanium. The reflux pump is often alloy 20 (CN7M). In very corrosive waters, such as those containing phenols or large quantities of salts, Hastelloy1141 C276 is used. Two API surveys (API Standards 944 and 950) concluded that the location and severity of corrosion vary with the type of unit as follows ... 

For sour water strippers odor or HiS problems at the storage tank 0. 6 to 1 m layer of oil on top of water missing/oil layer exceeds 0.6 to 1 m depth/faulty inert gas operation. 

Pluggng of overhead system top temperature not within the operating window for sour water strippers temperature < 82 °C at which ammonium polysulfides form but temperatures too high give excessive water in overhead vapor causing problems for downstream operation/overhead lines not insulated/insuflicient steam tracing on overhead vapor lines. 

The problem with this sort of approach to tray efficiency is that it is largely irrelevant. The main problems we have with tray efficiency in sour water strippers are ... 

I was working on a smaller crude unit in Alabama to improve desalter efficiency. Their problem was the periodic carry-over of emulsified brine. The depth of the emulsion between the crude and water phases in their desalter was excessive. The pH of the washwa-ter from the sour water stripper was to 9, even though the NH3 content of the stripper bottoms was zero. Talking to the operators, I discovered that they added NaOH to the sour water stripper feed to diminish the NHj content of the stripped water. 

That is, the old back issues of the NPRA Q A (i.e.. The National Petroleum Refiners Associated, Question and Answer) sessions. With this authoritative source as support, 1 asked the operator to stop the NaOH addition to his sour water stripper feed. Two hours later, at the same reboiler duty, the NHj content of the stripped sour water had gone from 0 to about 5 ppm. And the emulsion layer in the crude unit desalter had been reduced enough so that brine carry-over was no longer a problem. The 5 ppm of ammonia was quite acceptable. 

The sour water stripper system represents a very simple separation process, which generally operates in a predictable and straightforward manner. The three main problems encountered in many installations are (1) the presence of fixed anunonia, (2) the presence of phenols, and (3) corrosion. 

Sour water strippers are generally constructed of carbon steel and only minor ctorosion is reported in the tower, trays, and feed-to-bottoms heat exchangers (Gantz, 1975). Materials experience and corrosion problems in sour water strippers are discussed in two API reports (1974, 1976). Significant corrosion has been encountered in the overhead systems of refluxed towers when the overhead systems were constructed of carbon steel. The API reports indicate that titanium is very resistant to cmrosion in such situations. 

American Petroleum Institute, 1974, 1972 Survey of Materials Experience and Corrosion Problems in Sour Water Strippers," API 944, November. 

Special techniques have to be used for processing gas streams containing appreciable amounts of ammonia such as effluents from refinery sour water strippers. The ammonia must be destroyed in the reaction furnace to avoid deposition of ammonium salts on the cataly.st beds. Two methods are available to successfully accomplish this. The first method involves a split-flow reaction furnace design the second requires a high-intensity reaction furnace burner. It is essential that the ammonia be almost completely destroyed because ammonia concentrations as low as SOO to 1,000 ppmv can cause plugging problems (Anon., 1973). 

The problem was not the lack of stripping steam. The problem was also not the excessive pH of the stripper bottoms as my client in India suspected. It is quite normal for the stripper bottoms to have a higher pH than the feed, if the stripping efficiency is inadequate. That is, the acidic H S is stripped out of the sour water more easily than the basic ammonia. The problem of excessive NHj in the stripped water effluent was low tray efficiency. 

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