(28b) Moving Gamma Scanning Forward – Quantitative Analysis of Tray Capacity

Improving process operating margins can include maximizing production of higher value products and/or minimizing production costs.  Process plant operations 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 quantitative information is now available about the useful capacity of trayed fractionation towers.

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.  The “inspiration” for this development was taken from a presentation given by Mr. Mike Resetarits, Technical Director of Fractionation Research Inc. (FRI), at the 2011 A.I.Ch.E. Spring Meeting titled “On Distillation Tray Entrainment”.  The author remembered from the presentation that the degree of entrainment is very difficult to quantify.  If the degree of entrainment 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.  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.

A couple of years ago Tracerco had the opportunity 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.  The experimental observations, tray model predictions, and total froth height measurements from gamma scan data using a new detector technology had excellent agreement.

Case studies will demonstrate the beneficial difference of measuring total tray froth heights from gamma-scan data versus the conventional method of measuring the mean tray liquid height.