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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Results showed that with a low but unquantifiable level of DMOP in the wastewater feed, DMOP indeed built up to significant concentrations in the top of the column with a nondetectable level of DMOP in the bottom drawoff from the column. In addition, as the experiment progressed, the concentrations of DMOP and water in the top of the column steadily decreased as the concentration of methanol and other lights built up (Table 1).

The buildup and subsequent falloff of DMOP in the top of the lab column, along with the nondetectable bottom DMOP concentration, indicated that DMOP is easily stripped from water but is less volatile than methanol. This provided some evidence that with a significant stripping section below the feed point, the DMOP entering in the feed couldn't be purged out the bottom of the production tower. In addition, a high reflux ratio in the rectifying section would keep DMOP out of the distillate so that it indeed might be building up in the top portion of the production tower to cause flooding. Every time the tower had to be shut down after a flooding episode, its bottom had to be pumped out to get rid of liquid that had accumulated in the top and dropped into the bottom. This removed the accumulated DMOP.

Upon restarting, the tower worked fine until DMOP again built up enough to cause another flooding episode. Effectively, the DMOP was behaving as a "trapped" component in the distillation tower — insufficient DMOP was leaving in the combined top and bottom streams to compensate for the influx of fresh DMOP in the entering wastewater.

Subsequent vapor/liquid equilibrium measurements indicated DMOP and methanol don't form an azeotrope at atmospheric pressure, and the relative volatility of methanol to DMOP at atmospheric pressure and high concentrations of methanol is 1.4. This verified that DMOP is heavier than methanol in the top of the production tower. Furthermore, measurements showed the relative volatility of DMOP to water at atmospheric pressure and low concentrations of DMOP to be about 70–100, compared to a relative volatility of 8 for methanol at low concentrations in water. These data confirmed that in the production tower DMOP is well stripped out of the wastewater exiting the bottom but can be held down out of the distillate by the high reflux ratio and the tall bed of packing in the top of the tower and thus become a trapped component.

Before the lab distillation and vapor/liquid equilibrium experiments were completed, repeated flooding episodes in the production tower eventually caused damage to its internals, as indicated by very high water concentrations (about 30%) in the distillate and high pressure drop through the condenser. Another gamma scan of the tower revealed that roughly half of the packed bed was missing. So, the tower once again was shut down and reopened. A visual inspection showed that the bed limiter and liquid distributor were damaged and the missing packing was in the vapor line and condenser, which explained the high pressure drop through the condenser. However, the trays were undamaged and in good condition.

THE SOLUTION
In June 2006, after repair of the damaged internals and replacement of the packing, the tower was restarted and its operation was changed to combat buildup of DMOP. Previously, tower control involved manipulating the distillate rate to maintain a representative temperature point in the top section that gave a distillate containing 3–5% water. However, that low a concentration of water didn't allow enough DMOP to exit the top of the tower to prevent it from being trapped and causing flooding. So, the tower temperature set point was raised to allow 5–10% water in the distillate. At that water concentration, sufficient DMOP leaves in the distillate stream to prevent buildup. The flooding episodes ceased.

In conclusion, recovery of low levels of organics from process wastewater by stripping with rectification up to high organic concentrations can lead to unexpected results such as trapped components in a tower. This can occur with complex organic/water-vapor/liquid equilibrium relationships that cause significantly different distillation behavior between the distillate and bottoms. However, methodical investigation into the causes of intractable distillation problems with well-conceived laboratory experiments, thorough data analysis, and understanding of the underlying principles usually will yield viable solutions.


BRUCE S. HOLDEN is principal research scientist in the Engineering and Process Sciences group of The Dow Chemical Co., Midland, Mich., PATRICK H. AU-YEUNG is a research scientist, Engineering and Process Sciences, in Midland, and TODD W. KAJDAN is an associate analytical manager at the Analytical Technology Center in Midland. E-mail them at bholden@dow.com, phau-yeung@dow.com and twkajdan@dow.com.

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