Watch Out for Trapped Components in Towers

Buildup can prompt erratic performance of distillation columns.

By Bruce S. Holden, Patrick H. Au-Yeung and Todd W. Kajdan, The Dow Chemical Company

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ERRATIC PERFORMANCE
Upon startup of the tower in June 2005, the stripping section was able to remove >95% of the methanol in the wastewater feed with the design flow rate of stripping steam. However, the packed rectifying section was underperforming — far more water remained in the distillate, 3-5% (by mass), than the 0.1% or less expected from computer simulations. In addition, the tower experienced flooding in the top packed section at irregular but frequent intervals. Pressure drop across the packed section went as high as 8 psi (versus the <0.5 psi expected) before Operations was forced to shut down the unit due to degrading control of the tower levels and initiate a restart. Following the restart, the tower would settle back into stable operation for a time until the next flooding episode would occur anywhere from 12 hours to several days later.

Figure 2 shows tower response during a typical flooding episode. Note that pressure drop across the bottom trayed section remains fairly stable while the packed-section pressure drop steadily increases, indicating flooding is taking place mostly in the packed section. The sudden rise in trayed-section differential pressure after the cutoff of wastewater and steam feeds stems from the significant liquid holdup in the tower suddenly dropping into the bottom of the tower and filling it up above the bottom pressure transmitter tap used to calculate the trayed-section pressure drop.

A number of possibilities could explain the flooding: under-design of the tower, improper installation of tower internals, damage to the internals, fouling, and foaming. We verified design calculations and vapor/liquid equilibria. A full gamma scan of the tower three months after startup revealed no mechanical abnormalities, normal liquid holdups, and a possibility of only minor foaming on the trays just below the feed point. However, that scan took place when tower pressure drops were normal, not during a flooding episode. During a subsequent shutdown and opening of the tower, a vessel entry and visual inspection showed all tower internals to be in place, properly installed and undamaged with no sign of fouling of any kind.

Flooding episodes continued to occur. Then, a chemist who conducted periodic analyses of the distillate stream remarked that a partially obscured minor unknown peak in the distillate gas chromatograms frequently appeared. Further analytical work with mass spectrometry identified the unknown as 1,2-dimethoxypropane (DMOP), a component not shown in analyses of wastewater feed samples but one likely (from knowledge of the production process) to be present at low levels. It was hypothesized that the presence of DMOP in the wastewater feed and its possible buildup in the top of the tower could prompt the flooding episodes.

TESTING THE HYPOTHESIS
The lab performed continuous distillation on drum quantities of actual wastewater feed. A 2-in-diameter glass packed column was continuously bottom-fed with wastewater and operated at total reflux, as shown schematically in Figure 3.

With the continuous feed of wastewater to the bottom of the column with continuous drawoff of the bottoms to maintain a constant liquid level in the pot, the lab column configuration roughly matched the top portion of the production tower. In addition, the heat input to the pot was regulated so the reflux-to-feed rate ratio was the same as that in the production tower. Operating the lab column at total reflux enabled testing the potential for DMOP to accumulate in the top of the column because there would be no opportunity for purging any component from the top.

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