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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Keeping a chemical process running smoothly is one of the top priorities of plant management and operations. Process upsets and unplanned shutdowns interrupt production, lead to additional maintenance and extra costs, and generally cause headaches for everyone involved. As a result, a unit operation that experiences repeated upsets culminating in its shutdown and curtailed product output quickly becomes the focus of attention for plant personnel. This was the case in a cellulosics production unit at Dow's Midland, Mich., site — a steam stripper for removal of low levels of methanol and other lights from a process wastewater stream was suffering sudden unexplained pressure drop and level fluctuations that required shutdowns and restarts to resolve the tower's erratic performance.

Dow had selected steam stripping because it's a proven technology to remove methanol from wastewater, which reduces organic load on a treatment facility. The cellulosics production unit's tower has a rectifying section to concentrate the stripped methanol and other strippable organics, to lower the cost of incinerating the recovered methanol or possibly allow its use or sale as a byproduct. This turned the steam stripper into a full fractional distillation column.

Wastewater distillation takes place in a 5-ft-diameter tower running slightly above atmospheric pressure with a total of 26 dualflow trays on a 24-in tray spacing and 28 ft of 40-mm ring-type random packing in a single bed (Figure 1). Twenty four of the dualflow trays are located below the wastewater feed point, with two trays immediately above the feed, and the packed bed above those two trays. Dualflow trays were selected for the stripping section for fouling resistance to potential solids and heavies in the wastewater feed, and the two trays above the feed were intended to handle any potential fouling due to entrainment from the feed section before the vapor enters the packed section. Heat comes from direct injection of 25-psig steam into the bottom of the tower to avoid any fouling issues with a reboiler.

A key tower design factor was the dilute nature of the contaminants in the wastewater feed stream. Methanol concentration averages about 2,000 ppm (mass) and ranges from 1,000 ppm to 2,500 ppm; the stream also includes about 200 ppm of other organics lighter than methanol and about 7,000 ppm of heavier non-strippable organics. Because the packed rectifying section would hold most of the water down in the tower, the overhead distillate would consist almost entirely of methanol and other strippable light components and its flow rate would amount to only a very small fraction of feed flow rate.

On the other hand, because methanol removal would require significant quantities of stripping steam, a substantial amount of the condensed overheads would go back into the top of the rectifying section as reflux. This would result in quite a high reflux ratio (reflux rate divided by distillate rate) and would require controlling the tower temperature profile via the low distillate rate rather than the more influential reflux rate. Moreover, regulating the tower this way would give a relatively slow control response because changes in the distillate rate would take time to affect the mass of liquid held up in the overheads system and the packed bed in the top of the tower.

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