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Unformatted text preview: 52 www.cepmagazine.org October 2002 CEP Reactions and Separations revious surveys (1, 2) collected case his- tories of tower malfunctions from the open literature. Many reports described simulations that did not reflect what a tower was actually doing. Often, the problem was with the simulation. Sometimes, the problem was that the tower did something unexpected: the simula- tion was actually correct, based on the data fed to it. Finally, there were instances where both mishaps took place — the simulation had some serious prob- lems, but there were aspects of tower behavior that were not fully understood initially and the simula- tion helped to explain. This article focuses on instances where problems were found in the simulation or where the simulation was instrumental in identifying a previously misun- derstood problem (see table). The cases were extracted from surveys in Refs. 1 and 2 and have been updated with some recently re- ported cases. The original numbering for each case has been retained, so that the reader may easily locate the cases in the previously referred to articles. The number of examples presented here is by no means a large enough sample for performing a sta- tistical analysis of the main problems in trou- bleshooting distillation simulations. Nonetheless, the cases provide guidance on what to look for when troubleshooting a distillation simulation — and what to watch out for when carrying out the next simulation. Three major findings This survey revealed three major issues that require attention in using simulations: 1. correctly predicting vapor/liquid equilibrium (VLE) 2. having the simulation match plant data 3. applying graphical techniques to troubleshoot simulations. These three issues are present in about two-thirds of the reported cases. In about 20% of the remaining cases, the process chemistry and hardware efficiency did not match what was true in an actual tower. Other items — correctly modeling feeds, obtaining the true vapor and liquid loads, reliably predicting the hydraulic behavior, and finding and squelching bugs in the simu- lations — were found to be problematic, but to a lesser degree. Problems with VLE data and predictions Most case studies falling into this category involve close-boiling components. The problems can be with two chemicals of similar vapor pressure ( e.g. , hydrocarbons), or due to a non-ideality that pushes the volatilities of a pair close to a pinch. Correctly estimating non-idealities is another trouble-spot when it comes to VLE predictions. A third dilemma is characterizing heavy components in crude-oil distillation. This is a key problem in simulating refinery vacuum towers. Few reports were made regarding other situations. It seems that VLE prediction for pairs of components that have reasonably high volatilities, for ex- ample, methanol/ethanol, is not often troublesome....
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