Chemical Processing - June 2008 - (Page 41)

process pUzzler Buried hardware can haunt you Detecting underground pipes and tanks can pose real pitfalls this Month’s Puzzler We’re expanding our old oil refinery. part of the expansion includes replacing our underground settling tanks. Unfortunately, we’re not sure where all the flows are coming from; generations of engineers added to the network, making analysis of this complex of underground piping almost impossible. can you suggest any methodology for identifying these sources? in addition, laboratory tests show that we have naphthalenes in our settling tanks, which confounds our efforts to reduce the size of the new settling tank. This tank will replace one of the larger, older, corroded tanks. how can we reduce the size of this new tank and handle naphthalenes? PrePare for two tough tasks This particular problem consists of two parts: 1. finding underground pipe; and 2. reducing the size of a huge settling tank. Locating the pipe seems the easiest of these tasks. A battery-operated hand-held system can detect metal buried underground. These “radio detection” systems include a transmitter and a receiver called a wand. The signal gets stronger parallel to the direction of the pipe. Most wands have a noise maker but this may not be easy to hear in a chemical plant or refinery. However, radio detection isn’t intrinsically safe and won’t detect pipe made of concrete or plastic. The most critical problem is attaching the transmitter. There is nothing more frustrating than connecting the transmitter and following the wrong pipe underground. If you have pipe constructed of plastic, clay or concrete, detection is even more difficult. The least expensive method may be sonic detection, then there is radar and various tracers. Sonic detection works the same as radio detection, and also isn’t intrinsically safe. Radar offers something not available with the others: a picture of the objects underground. Concrete may pose a challenge to sonic detection and rules out radar unless you’re willing to cut a hole. As far as reducing the size of the settling tank, naphthalenes are notorious for causing stable emulsions that won’t respond to gravity settling. The only ideas that come to mind are polymer addition, ultrafiltration and a pulsating DC current. Polymer addition involves a conventional settler, an oil skimmer and an aeration lagoon. A cationic polymer is added to the emulsion entering the settler and exiting the settler. Oil is skimmed off as it forms before exiting to the aeration lagoon. This method suffers from the cost of the polymer and the size of the aeration lagoon. Ultrafiltration is new and may be appealing but I’m not sure you want it buried out of sight in your refinery. And there’s the pumping cost and the need for pre-filters; then, there’s the question of what is to be done with the spent filters and pre-filters. Using an electric current to separate emulsion phases is tempting because all you need, supposedly, is a couple of electrodes and a two chamber tank. And it will avoid the cost of an expensive chemical. A sludge is a problem for all of the options. No method may be up to the task of dealing with the heavy dirty oil expected as future feedstock to refineries. Dirk Willard, senior process engineer, Ambitech Engineering, Hammond, Ind. august’s Puzzler We are designing a desulfurization process for a client overseas. The client produces a wet coke gas that must be treated for cos, h2s, and hcn before it can be used to make chemicals. a knock-out pot serves double duty — collecting the circulating spent slurry and vapor from the client’s gasification plant. The vapor flow in is 50,000 lb/hr and the liquid flow is 519 gal/min. The pot operates at 200°F and 560 psig with a barometric pressure of only 12 psia. a k of 0.27 was used with a velocity allowance of 0.15 to size the tank for vapor separation. The residence time is set at 30 minutes. The calculated diameter is 13 ft. with a length/diameter ratio of 3.4. The vapor passes through a mesh pad mist eliminator that is only partially used because of low vapor flow. several problems are anticipated with the design: the liquid drains to atmospheric; foaming sometimes occurs, disrupting level measurement; the mesh pad fouls every four months; and the process water used on a timer to keep the pad clean may contain particulates. how would you improve the design? send us your comments, suggestions or solutions for this question by July 11, 2008. We’ll include as many of them as possible in the august issue and all on cp.com. send visuals — a sketch is fine. e-mail us at processpuzzler@putman.net or mail to processpuzzler, Chemical Processing, 555 W. pierce road, suite 301, itasca, il 60143. Fax: (630) 467-1120. please include your name, title, location and company affiliation in the response. and, of course, if you have a process problem you’d like to pose to our readers, send it along and we’ll be pleased to consider it for publication. 41 chemicalprocessing.com JUne 2008 http://www.CP.com http://chemicalprocessing.com

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Chemical Processing - June 2008 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Achieve Model Operations Bolster Your Condition Monitoring Toolbox Particle Analysis Makes Solid Progress Improve Batch Reactor Temperature Control Improve Your Job Security Plant InSites Process Puzzler Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - June 2008

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