Steam stripper /stripping reboiled-water

In the subsequent product stripping section, the product is stripped free of benzene and water using a live steam stripper or a reboiler column. The effluent discharged from the washing area is steam-stripped to recuperate MNB. 

Each side-product liquid draw goes through a steam stripper, usually equipped with 5 or 10 trays, to prevent inclusion of more volatile hydrocarbons in the product. The side-product stripper may be reboiled for sharper separation. Optimum amounts of stripping steam will often be in the order of 2% or 3% w or more of each product rate. After stripping, the side products are cooled in the preheat train and cooled further in water or air coolers to safe storage temperatures. Heavy gas oil and residuum products often go to downstream cracking processes without cooling. 

One of the questions that 1 am often asked is what can be done to mitigate reboiler tube failures in sour water strippers and the consequent steam leaks into the stripped sour water ... 

The HPS liquid consists mostly of C3 s and heavier hydrocarbons however, it also contains small fractions of Cj s, H2S, and entrained water. The stripper removes these light ends. The liquid enters the stripper on the top tray. The heat for stripping is provided by an external reboiler, using steam or debutanizer bottoms as the heat medium. The vapor from the reboiler rises through the tower and strips the lighter fractions from the descending liquid. The rich overhead vapor flows to the HPS via the condenser and is fed to the primary absorber. The stripped naphtha leaves the tower bottoms and goes to the debutanizer. Usually, at least one draw is installed in the tower to remove the entrained water. 

Many water strippers are initially designed with steam reboilers, rather than with open stripping steam. The amount of steam required is the same in either case. The great advantage of the reboiled stripper is that the steam condensate is recovered, and recycled back to the boilers. When open stripping steam is used, the steam condensate is added to the stripped water, thus increasing the plant s water effluent. Hence, the use of open stripping steam is environmentally unfriendly. 

Referring to Fig. 47.3, for many refinery operations, the circulating thermosyphon reboiler shown can be eliminated and replaced with the same quantity of stripping steam, thus further simplifying the design of your waste water stripper installation. 

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