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Moving gamma scanning forward – quantitative analysis of tray capacity

Improving process operating margins can include maximising production of higher value products and/or minimising production costs. Process plants depend heavily on fractionation towers to produce products meeting customer specifications. Tower gamma scanning is well established in the process industries as a qualitative tool to help troubleshoot fractionation towers. Using new measurement detector technology and innovative analysis of gamma scanning data Tracerco is now able to offer quantitative information about the useful capacity of fractionation towers, both trayed and packed.

Trayed Towers - FrothView™
Up to now there are few practiced methods for measuring a liquid or froth level on a tray from gamma scan data. However; no matter the practitioner the conventional methods allocate a tray’s aerated liquid height somewhere in the middle of the layer. This method is adequate for diagnosing tray damage – whether a tray is capable of holding liquid or not. But beyond this determination, conventional tray liquid level measurement has no correlation to any other operating variable. Additionally, to characterise remaining tray space above this position that is seen to be denser than “clear vapour”, subjective terms such as slight entrainment, moderate entrainment or heavy entrainment are typically used. This method is illustrated in Figure 1a.

For trayed towers a new methodology, FrothView™, has been developed and tested whereby the total froth height on a tray is measured from gamma scan data using new detector technology and software interpretation. This method is shown in Figure 1b on page 2. Dividing the total froth height by the tray space provides the percentage of tray space occupied by tray froth. This “% tray space” has been shown to correlate very well with % flood, thus is a way of using a field measurement and directly relating this to specific tray design criteria to determine the useful capacity of operating trays.

The “inspiration” for the development of FrothView™ technology was taken from a presentation given by Mike Resetarits, Technical Director of Fractionation Research Inc. (FRI) at the 2011 A.I.Ch.E. Spring Meeting titled “On Distillation Tray Entrainment” (1). Tracerco determined from the presentation that the degree of entrainment is very difficult to quantify. If it cannot be reliably quantified then it is very difficult to use entrainment in tray correlations particularly to measure tray capacity. Most, if not all, tray capacity correlations use a calculated tray froth height or level. An article written by Mark Harrison with Eastman Chemical explores using tray froth heights to measure tray capacity. (2) Mr. Harrison proposed a graphical method for measuring tray froth heights. However it is unclear to what extent Mr. Harrison was able to test and utilise this method.

During the past couple of years Tracerco has had opportunities to scan a trayed pilot plant tower under varying operating conditions, from weeping to flooding. One objective of this study was to find a method to quantify the scan results to provide a more objective diagnosis of the tray’s hydraulic condition.

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