Chemical Processing-August 2008

process puzzler Kayo a drum problem Readers suggest ways to improve the performance of a pot this Month’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 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 (l/D) ratio of 3.4. the vapor passes through a mesh pad demister 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? mist eliminator. This may extend the run of the mist eliminator before it plugs. For some problems, the best you can do is to figure Multiple vapor exits may also be usable. With out how to live with the least bad situation rather the correct valves you could have two or three vapor than being able to find a solution. Your options depend upon the cost the client is prepared to incur. outlets. If each has a run of four months or so, you Some ideas to check the economics of include: vortex have effectively extended the vessel service duration tubes for foam reduction; staged clean water washes; between shut down and cleaning cycles. A single feed in the center of the drum and a vapor outlet at vane mist eliminators in series with the mesh pads; each end would be a good first choice. Of course, if multiple vapor exits; extra vessel elevation; overlapyou have less risk exposure to premature shut-down, ping level instruments; nuclear level instruments; you may also consider making one end of the vessel and weight cells. correctly sized rather than over-sized for the mist Vortex tube clusters (VTC) take advantage of direction changes to help collapse foams in a multiple- eliminator used. Extra vessel elevation may be critical in getting phase stream. If the source of drum foam is the feed, using VTCs may aid in reducing the amount of foam liquid out of the drum. If the contents are foam rather than clear liquid, then the static head available for in the drum. liquid draining is much less. Extra height (and larger If the process will tolerate occasionally using a exit nozzles) will prevent many gravity fluid-draining clean wash, then putting in a second wash system to problems. extend the time between plugging on the mist elimiLevel indication with foam is notoriously difficult. nator pads may be justified. Perhaps one wash in four The first method to deal with liquid level problems or five might use clean water. is to use something like a displacer or a differential A minimum velocity is needed for wire and pressure (dp) cell for level measurement and to use vane mist eliminators to effectively remove mist. The higher the pressure drop the smaller the drop- more than one of them on multiple, different ranges. If the different level instruments show multiple liquid let removed. Fouling occurs from the process liquid trapped by the eliminator or particulate in the levels, the lower instruments are probably completely flooded with foam. Instead of measuring level they wash water. The challenge is balancing pressure are measuring an average liquid density across their drop, fouling tendency and efficiency. You could range. The actual foam level will be in the range of consider using a correctly sized vane eliminator the highest level indicator showing a level. While not followed by the over-sized wire mist eliminator. The vane mist eliminator has a much greater toler- perfect, this can be a vast improvement over a single level instrument. ance for fouling because of its large passages. Of If solids, corrosion or service conditions prevent course, this will require a much fancier water-wash system. The purpose of the vane mist eliminator is multiple level instruments, then a nuclear (gamma to reduce the loading of process liquid in the wire ray) device may work well. While it is much more Address the process issues 41 chemicalprocessing.com august 2008 http://chemicalprocessing.com

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

Chemical Processing- August 2008 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Protect your Plant What’s on Tap for Water? Keep Operations Safe Polystyrene Plant Gains Extra Output and More Process Puzzler Plant InSites Equipment & Services Adlits Product Spotlight/Classifieds Ad Index End Point

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