Chemical Processing - August 2007 - (Page 41) PLANT INSITES < > NMP recovery Precondensor Motive steam ejector After condenser Extract flash tower Steam Raffinate Extract Condensate to water treating Condensed water/ NMP to solvent recovery NMP/ Raffinate NMP/ extract Raffinate flash tower Steam No water-cooled condenser ever could have done the job. der vacuum. the vacuum system included a precondenser and condensate drum upstream of the first ejector stage to minimize ejector installation and operating costs. Figure 1 depicts a portion of a simplified process diagram showing the flash towers, precondenser, drum and part of the vacuum system. Water/nmP condensate from the drum goes to drying for solvent recovery. Wastewater from the vacuum system condensers goes directly to water treating. the plant staff was unhappy with the system due to solvent losses. the extract and raffinate products had virtually no solvent in them. in contrast, the precondenser failed miserably — its duty was so low that no condensate collected in the drum. all the stripped nmP went into the vacuum system and ended up at the water treatment plant. the rest of the vacuum system worked well, though, believed the plant staff. vacuum pressure was maintained at 10 torr (mm Hg absolute) at the flash columns without any problem. this should have raised an immediate alarm. could the vacuum system be working too well? to simplify the analysis, let’s make some extreme assumptions — that the precondenser contains water without any nmP and that the vacuum-side temperature can www.chemicalprocessing.com Figure 1. Stripped solvent went to the water treatment system instead of being recovered as intended. actually match that of the cooling water. Figure 2 shows 3.0 the 140 saturation temperature/pressure curve for water. the cooling water was available at 95°F. so, using that as the 2.5 120 vacuum-side temperature, once the precondenser pressure drops below 42.5 torr, thepressure pressure of water is too 2.0 100 Lower condensing partial limit imposed by cooling low for condensation (Point B). actual operation was 10 water temperature 80 torr, which means to get water condensing the tempera- 1.5 ture 60 must be 52°F or less (Point a). B the downstream ejector had so much capacity that it pulled 1.0 40 down the vacuum pressure to below the water and nmP Cooling water temperature required imposed by actual dewpoint pressure at the available cooling-water temperature. 0.5 20 A operating pressure that’s why the nmP ended up in the vacuum system. 0 the problem wasn’t that the condenser was improp- 0.0 30 50 70 90 110 130 150 erly sized but that the vacuum system pressure was so low Temperature, °F that no water-cooled condenser could ever have done the job. as i’ve frequently pointed out, fundamental limits constrain equipment operation. Pressure control on the vacuum system was needed, not a new precondenser. Ejector pressure control must vary one of the two things that go into the ejector: the load or the driving fluid (steam in this case). Ejectors have relatively narrow operating ranges. steam-driven ejectors require a specific range of steam mass flow to work. inside the ejector the inlet steam nozzle size determines the mass flow rate for any given temperature and pressure. vendors emphasize not to attempt to control the ejector by throttling the steam pressure to the ejector. Torr (mm Hg) august 2007 • 41 160 psia http://www.chemicalprocessing.com
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