Chemical Processing - February 2008 - (Page 41)

PROCESS PUZZLER < > THIS MONTH’S PUZZLER A remote 200,000-gallon storage tank that feeds a stripper column has a side-mount agitator. During start-up, an engineer notices that the side of the tank is moving as if a wave were pulsing inside the tank. Inspection reveals that the agitator is incorrectly installed; the impeller pitch is for clockwise rotation. The solvents in the tank are NFPA Div. 1, Group D with suspended hazardous waste. We can’t afford to delay our start-up for six weeks, the time it would take to get a replacement impeller — a lot of expensive contractors will be waiting around unless we can find a safe way to restore agitation to the atmospheric tank. What can you propose to get our start-up back on track? Use liquid nitrogen Insert liquid nitrogen tubes through the roof vent and boil gently! Depending on tank geometry, more than one sparge line, placed near the bottom of the tank, may be necessary. The advantage of bringing in a liquid for sparging is that it reduces the pipe size and also there is a significant expansion: the density of nitrogen gas at 1 atmosphere and 80°F is only 0.068 lb/ft3. The density of liquid nitrogen at 1 atmosphere is 50.3 lb/ft3, according to the National Institute of Standards and Technology tables for nitrogen from the Internet. The gas expansion will be from 1/50.3 to 1/0.068; a factor of 740 to one. The disadvantage of bringing in a liquid is the heat it will draw may cause some of the heavier organics to crystallize. Crystallized flammable liquids may pose unique hazards. The latent heat of nitrogen is not insignificant: ~48 cal/g (20°C). When compared to other compounds — water, 79.8 cal/g; ethanol, 26.0 cal/g; and carbon tetrachloride, 41.6 cal/g — this could be a challenge. There may be a few ways out of this sand trap. If a heat exchanger is available, it might be possible to recirculate the liquid and avoid problems. Another way might be steam injection into a pump recirculation line, if available. Providing the proper amount of agitation is another problem. Nitrogen will convert to a gas quickly so that empirical equations can be used to estimate the flow needed and the sparger design — see “Mixing principles and www.chemicalprocessing.com http://www.chemicalprocessing.com

Table of Contents Feed for the Digital Edition of Chemical Processing - February 2008

Chemical Processing - February 2008 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Nanoparticle Safety Raises Questions Take the Pressure Off Vacuum Systems Achieve Optimum Centrifugal Pump Performance Rethink Batch-Manufacturing Alarm Systems Dr. Gooddata Orlando Plant Pioneers HMI Migration Strategy Process Puzzler Plant InSites Equipment & Services Ad Index Product Spotlight/Classifieds End Point

Chemical Processing - February 2008

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