PTQ Q4 2025 Issue

a long-serving operator. His recollections were that initially both columns had the exact same separation efficiency. This is an important point, as some believed that the design of the Still X column internals had an impact on the current performance. However, in the late 1990s, the separation efficiency of Still X degraded, and it no longer performed as an equivalent to Still Y. Significant efforts to recover the column performance at that time were unsuccessful. The operator noted that around the time the performance was lost, the column was unknowingly distilling organic chlo- rides. Two other columns were also involved in these distil- lations and were significantly damaged. However, manhole inspections have not revealed any damage to Still X. To develop a good diagnosis, it was decided to perform a detailed in-situ water test. Water was introduced through the reflux line and meter. All the manholes were opened and observations made at each redistributor. The flow rate was increased from the minimum liquid loading to the max- imum capacity of the distributors. At maximum flow, there was no splashing at the parting boxes, and all looked well there, indicating no issues with the long pipes. However, at the bottom manhole, the water poured right out of one section of the packing (see Figure 4a ). There was definite channelling in the lower section of the packing. Maldistribution below the middle bed was not as obvious as it was below the bottom bed. To get a rough quantitative evaluation, a test using the bucket and stopwatch method was conducted (see Figure 4b ) and clearly highlighted chan- nelling in the middle bed as well. In Round 1, water was fed through the reflux distributor, while in Round 2, it was fed through the feed distributor, both showing much the same results. There was heavy flow towards the centre (position 1), with section 4 being the driest. The in-situ water tests proved that the problem was liquid maldistribution. The in-situ tests also explained why the gamma scans failed to detect the maldistribution. With a + scan (Figure 3b), both scan chords passed through the middle (position 1), where the flow was high. Upon seeing the same high central flow, both chords looked similar, leading to the erro - neous conclusion that distribution was good. In-situ tests provide a much broader picture of the liquid distribution than regular gamma scans. Following this test, all of the packing was replaced in all three beds. The older internals (including all distributors and collectors) were reused, as the similar column (Still Y) had adequate separation and also because these inter- nals had not changed since the tower worked well. Still X efficiency was recovered to the expected and continues to perform well today. Takeaways Properly conducted in-situ water tests are invaluable. Incorporating simple quantitative rough measurements, like the bucket and stopwatch method, is very informative. In-situ water tests view liquid distribution over the entire tower cross section area. Gamma scan chords cover a smaller area. In-situ tests may, therefore, be able to detect some types of maldistribution that gamma scans may miss.

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PTQ Q4 2025