Chemical Processing - June 2008 - (Page 42)

PLANT INSITES Understand the cold facts about subcooling Systems sometimes can surmount major layout errors UNITS THAT work despite apparent violations of fundamental practices teach us many important lessons. Here we’ll examine hot-vapor-bypass pressure controls that succeed against all odds. Such control on a distillation column requires that liquid entering the overhead receiver be subcooled, so vapor can maintain a pressure blanket with no T1 tower net loss of vapor overhead. One dependable way to get the distillate liquid subcooled is to deliberately oversize the PC condenser, thus ensuring that excess surface area To fuel T1 tower is available for subcooling. The condenser may be mounted above or below the receiver, as long D1 as the liquid from the condenser seals into the PC liquid pool in the drum (Figure 1). Putting the Patially Distillate To fuel E1 ﬂooded product liquid line into the bottom of the drum guaranP1 condenser tees this happens. Pressure balance between the D1 condenser and the overhead receiver (controlled by the hot-vapor-bypass valve) forces the liquid Patially Distillate E1 ﬂooded Figure 1. product into the drum. P1 condenser Oversized condenser ensures that more than enough Now, let’s look at two systems that work in surface area is available for subcooling. spite of major layout errors. You never should build either system deliberately but it’s certainly conceivFree-draining line able that you may run into one. So, let’s see why each manages to work. Figure 2 shows an exchanger mounted above T1 tower the drum with a free-draining line into the drum. Normally, with gravity f low to drain liquid from the condenser to the drum we’d expect PC Restricted that subcooling is impossible. Yet this system outlet E1 To fuel keeps has been shown both by me and other people to T1 tower bundle work. But how? ﬂooded D1 If its outlet nozzle doesn’t let enough liquid PC Restricted pass, the condenser can maintain a liquid level outlet Distillate E1 To fuel despite the piping layout for a gravity drain. keeps product P1 bundle Such f low limitations can stem from selectﬂooded D1 ing too small a nozzle for the service or from restrictions, e.g., caused by buildup of corrosion Distillate products. As long as some liquid level remains on product P1 the tubes, it’s conceivable that the liquid leaving the exchanger could be subcooled. So, here, two mistakes balance out. Figure 2. Or consider what can happen when using Subcooling should be impossible but may occur an air-fin exchanger as the condenser. Figure due to restricted ﬂow from the condenser. 3 shows an end-on view of liquid stratification Stratified liquid inside the tubes in the air-fin. For wide-temperature-range condensing mixtures, low velocity in on the bottom of the tubes. If mixing is limited in the air-fin header box, the liquid may enter the tubes may lead to liquid separating from the vapor. Then external air may subcool the liquid the overhead drum subcooled. The same probStratified liquid 42 Dependable approach While these approaches may work, never count on that. JUNE 2008 CHEMICALPROCESSING.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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