Chemical Processing - May 2008 - (Page 44)

>> PLANT INSITES Cast a cold eye on columns subcooling can complicate and compromise tower performance ContaCting equipment suCh as trays Can handle both mass transfer and heat transfer services. some processes will simultaneously combine the two. however, using a tray for a dual purpose requires care, as examination of a common combined application — subcooled reflux going to the top tray of a tower — will show. many plants will allow the tower overhead temperature to float with the temperature of the utility cooling water or air. often, the overhead duty removed isn’t a major issue because the tower effectively operates with a total condenser overhead. if the pressure control system can handle varying overhead drum temperature, the direct effect of subcooling variation is small. the subcooled reflux returns to the tower. in heating up to the bubble point, the subcooled >> Complex column with subcooled reflux NNF 1 Alternative draw 2 3 Product one Feed 26 Splitter Don’t count trays used for heating liquid for mass transfer. reflux condenses some of the rising vapor. the internal reflux rate inside the tower is higher than the measured reflux rate from the pump to the tower. as long as the tower internals are designed to cope with the required liquid rate after subcooling, the tower won’t flood. also with subcooling, some part of the tray’s vapor/liquid contacting area is being used only for heat transfer and condensing the vapor; no equivalent mass transfer of material from the liquid to the vapor occurs. so, in general, if a tray (or height of packing) is required for subcooling, don’t take any credit for the tray as a mass transfer device. this isn’t always exactly true — however, it’s good practice never to count on such trays to accomplish any mass transfer. in Figure 1, a liquid product is drawn from the sump on either tray 1 or tray 2. Due to complex product blending requirements, a liquid side-stream also is taken from the top of the column (tray 1 or tray 2) and added to the bottoms product. a vapor side-stream is drawn from below the feed, condensed and added to the overhead product. usually, liquid side-draws are some distance down from the reflux return — so changes in reflux subcooling don’t have significant impact. here, however, the correct draw 44 • May 2008 32 34 36 Alternative draws (Vapor) 55 Product two Figure 1. Because the proper top draw-point location varies with subcooling, alternative side-draws are provided. point was only one tray below the reflux and actually varied depending upon the amount of subcooling. taking liquid from the wrong location could result in significant product quality swings. the solution was to provide an alternative draw point on tray 2 that can be switched to as needed. subcooled liquid feeds (and refluxes) can create both hydraulic and efficiency problems in towers. so, take the safe approach and don’t count the necessary trays for heating the liquid as mass transfer trays. CP By Andrew Sloley, contributing editor ASloley@putman.net www.chemicalprocessing.com http://www.chemicalprocessing.com

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

From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor WirelessHART signals a change at plants Avoid costly fabrication mistakes Watch out with variable speed pumping Hot cutover boosts control system migration Plant InSites Process Puzzler Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - May 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.