Causes of Inadequate Stripping
Several reasons have been observed for inefficient stripping. Operators note correctly that preheat is improved and steam is saved by minimizing stripping steam. However, far more energy is wasted by unnecessarily running naphtha and kerosene through the FCCU.
Perhaps someone has retrofitted the stripper feed to preheat crude before it flows to the stripper. This is a good energy-saving scheme for the crude unit; however, a steam stripper will not work on subcooled feed. The feed must be at its bubble point (i.e., the tower drawoff temperature). Otherwise, the steam will not be an effective stripping agent.
Using stripping steam upsets the stripper bottoms level, causes erratic fractionator operation, and produces excessively wet products. Conse¬quently, the operators have stopped using it. Such symptoms are indicative of slugs of water in the stripping stream. Check that the steam line has a steam trap. Also, the takeoff for the stripping steam line from the steam header should come off the top of the header pipe.
Increasing steam flow interferes with the ability to withdraw product from the fractionator. The stripper vapor line may be too small or partially plugged. Check the increase in stripper top pressure at various steam rates. This pressure should not increase more than a few psi over the full range of steam flows. Also, the stripper feed line should have a loop seal, as shown in Figure 1-4. This loop prevents steam from backing up the feed line.
Steam is ineffective because of upset trays. At the design steam rate, check the pressure drop across the trays. For a typical four-tray stripper, the pressure drop should be roughly 1 ft of water. If the pressure drop is very much less than this, the stripping trays are likely to be damaged.
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