Chemical Processing-August 2008

process puzzler size is smaller. The vapor enters one end of a horizontal drum and exits the other after passing along its length. The longer it is, the more efficient it will be in settling liquid. Now that the design has been critiqued, let’s consider the three process problems mentioned: de-pressurizing the liquid drained from the drum; reducing the effects of foaming; and avoiding the necessity of cleaning the mesh pad. De-pressurization can be dangerous, especially at high pressure. Another concern is the wear on the orifice plate, the cheapest device available to reduce the pressure. One solution may be a reducing pot. Certainly more than one plate should be used for safety. Foaming could be addressed with an additive but perhaps the best approach is to work with ExaminE thE dEsign carEfully a reliable alternative like nuclear or a bubbler. There are a number of problems in this design Bubblers are the cheapest option and can work starting with the k. A k of 0.27 is probably too high for good separation at 560 psig. The Gas Pro- satisfactorily in foams and slurries if you can find cessors Suppliers Association (GPSA) Engineering a high pressure gas. The best solution for the mesh pad is probably to Data Book suggests de-rating k by 0.01 per 100 be conservative on the tank design, allowing more psig above 100 psig. This would provide a k of about 0.21 not 0.27. Decreasing k will increase the settling time, and replacing the pad with a chevron. Chevrons can serve for many months without cleanarea needed for separation. The velocity allowance (de-rating) of 0.15 seems ing or plugging. Dirk Willard, senior process engineer, correct for a foam problem; this term is multiplied Ambitech Engineering, Hammond, Ind. by the allowable velocity to calculate the area. A barometric pressure of 12 psia corresponds roughly to an altitude of 4,200 ft above sea level. A low October’s Puzzler barometric pressure increases the required separaWe had a fire and resulting explosion in our gas storage tion area less than 1%. facility. the fire turned out to be caused by relief valves The residence time seems a little short. Given the popping off on individual liquefied-propane bottles flow involved, 519 gal/min, additional capacity might stored in the sun on black top. the bottles are standard be desired since this drum appears to be the center of high pressure ones equipped with simple safety-relief the desulfurization process. valves vented to the atmosphere. they typically are Corrosion with H2S, HCN and other compounds stored in steel pallet racks side-by-side to maximize stormay be a problem. A corrosion engineer should review age space. our manager demands that we redesign the the selection because an attack on the metal might facility so that such an incident can never happen again. be anticipated at the phase interface. Review NACE how can we change the layout and operating procedures International standards for H2S service. of the area to reduce the risk to an absolute minimum? The selection of an L/D of only 3.4 seems wrong. send us your comments, suggestions or solutions for “Coulson and Richardson’s Chemical Engineering” this question by september 12, 2008. We’ll include as recommends a ratio of three for pressure up to 165 many of them as possible in the issue and all on cp.com. psig; above this pressure the ratio should be five. The send visuals — a sketch is fine. e-mail us at processpuzreason for the selection is the high cost of a large dizler@putman.net or mail to processpuzzler, Chemical Proameter at high pressure. At 560 psig, a 13-ft diameter cessing, 555 W. pierce road, suite 301, itasca, il 60143. head would be more than four-in thick. Welding a Fax: (630) 467-1120. please include your name, title, locathick plate is more of a challenge than welding a thin tion and company affiliation in the response. plate. If length is a problem because of space, a vertiAnd, of course, if you have a process problem you’d cal separator, though less efficient than a horizontal like to pose to our readers, send it along and we’ll be separator, can work in most applications. Here’s pleased to consider it for publication. another reason for a longer vessel: the capture droplet expensive, the cost may be justified. Properly set up, gamma ray level instruments can separately show liquid level and foam level. Finally, in severe cases, weight cells have been used to infer levels in extremely foaming systems. This is a last ditch effort as weight cells on large vessels can be complex to set up. They can also give false readings due to reactions from piping stresses. You are no longer measuring level but system mass inventory directly. From an assumed foam generation rate, you set a maximum system weight acceptable. I prefer nuclear devices to these but in some situations they do work. Andrew Sloley, principal engineer CH2M HILL, Bellingham, Wash. August 2008 chemicAlprocessing.com 42 http://CP.com 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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