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Troubleshooting a C3 splitter tower Part 2: Root cause and solution

Distillation trays are prone to channelling and multi-pass maldistribution in large diameter towers. Multichordal gamma scanning is used for solving such problems

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Article Summary
The PetroLogistics giant C3 splitter started up in October 2010 and had experienced operational difficulties during its initial eight-month run. Tray efficiency appeared to be very low, about 40-50%, compared to a typical 80-90% tray efficiency experienced with conventional trays in a C3 splitter. Due to the low tray efficiency it could not produce on-spec polymer grade propylene. PetroLogistics, Fluor (which was not involved in the tower design), and the tray supplier formed a taskforce to conduct a troubleshooting investigation to determine the root cause of this performance and to propose and engineer a fix.

The troubleshooting investigation combined hydraulic analysis and detailed multipass distribution calculations with the specialised technique of multichordal gamma scanning with quantitative analysis.7 Hydraulic analysis confirmed that the trays are prone to channelling and maldistribution due to their large open areas. It also ruled out several other theories.

The gamma scans showed a maldistributed pattern on the trays, with high L/V ratios on the inside panels and low L/V ratios on the outside panels. The scans showed vapour cross flow channelling (VCFC) on the outside panels. Flooding was observed on the inside panels well below the calculated flood point. The scans pointed at a combination of VCFC and multipass maldistribution as the root cause.

Investigation identified the high open slot area (15% of the active area) of the fixed valves to be the prime factor inducing channelling and maldistribution. A likely initiator of the multipass maldistribution was liquid preferentially flowing to the inside panels from the false downcomers distributing the flashing reflux to the top tray’s panels. This preferential flow is believed to have occurred through the gap at which the reflux pipes entered the false downcomers. Another likely initiator was channelled vapour blowing liquid from the outside to inside panels across the off-centre downcomers.

A short plant outage due to a problem elsewhere provided the opportunity for a quick fix. The key modification was blanking about a quarter of the valves on each tray to reduce the tray open slot areas from 15% to 11%. The gaps at the reflux pipe entry to the false downcomers were sealed and the false downcomer heights were raised to ensure good reflux split to the top tray panels. Anti-jump baffles were added across the centre and off-centre downcomers to prevent the possibility of channelled vapour from blowing liquid from the outside to the inside panels towards the middle. Some downcomer blocks were installed to improve liquid distribution. The modified tower achieved tray efficiencies comparable to those obtained in well-operated, smaller diameter, low pressure C3 splitters.

The investigation is described in two parts. Part 1 (see PTQ, Q4 2014) described the initial tower operation, as well as our hydraulic analysis and how it directed the investigation to focus on the combination of VCFC and multipass maldistribution as the most likely root cause. Part 2, the current article, describes the application of the specialised technique of multichordal gamma scanning with quantitative analysis7 to validate this theory, closely define and map the channelling and maldistribution patterns, and lead to the correct solution. All of the literature references in Part 2 were listed at the end of Part 1.

Gamma scans investigation
To diagnose the nature of the suspected channelling or maldistribution, and to implement an effective fix, we performed extensive multichordal gamma scans together with quantitative analysis of the gamma scans.

The gamma scanning technique normally practised for distillation trays shoots a single chord, or at most one chord per panel, with qualitative interpretation. Often, downcomer chords are also shot. The early gamma scans of the C3 splitter used this simple technique which is excellent for detecting gross abnormalities such as flooding, missing trays, foaming, fouling, and high base levels, but is unable to detect subtle abnormalities such as channelling, abnormal froth structure, and blow-by in unsealed downcomers.

For the C3 splitter, the early qualitative gamma scans established that the inside active areas were flooded. Centre and off-centre downcomers were flooded in some scans, but not flooded in others. This tied up well with the conclusion based on Figure 3 in Part 1 that the trays were at incipient flood at the operating rates.

To identify more subtle abnormalities, multichordal gamma scans with quantitative analysis are invaluable. This technique, seldom applied by gamma scan vendors due to its high costs, was first proposed by Harrison,6 and later developed by Kister.7 The lead author has used it with great success to diagnose a multitude of subtle abnormalities on trays, including various modes of channelling, abnormal froth structure, blow-by in unsealed downcomers, and many others. This technique requires top-quality multichordal scans of each tray panel. Froth heights, froth densities, clear liquid heights, and hydraulic gradients can be calculated as described in references 7 and 8.

Initial multichordal scans confirmed the presence of channelling both on the outside and inside active areas of the trays. The scans showed a similar and quite uniform channelling pattern throughout the tower. There were no signs of unbolted manways.

The uniformity of the channelling pattern throughout the tower made it possible to focus on a relatively small section, map it in detail, and use this section to represent most of the tower. This ‘mapping’ provided a cost-effective way of gaining a concise definition of the nature of the channelling. The costs of multichordal scanning with quantitative analysis rapidly escalate with the number of trays scanned and the number of chords per tray. The small section mapping permitted shooting a large number of chords per tray to give a good definition of the channelling while keeping the costs down by limiting the number of trays scanned.

In this mapping study, five chords were shot on the inside eastern panels, another five chords on the inside western panels, and three more chords on the outside western panels (see Figure 1). The spacing between any two successive inside panel chords was about 6in, and their locations were chosen to minimise interference from the support trusses (also shown in Figure 1). Spacing between successive outside chords was 23-29in. Two of the three outside chords passed through the mod arc downcomers (MOAD). These MOADs are marked as dashed lines in Figure 1 (extending to 63in from the tower end). For each of these chords, froth heights, froth densities, entrainment indexes, and clear liquid heights were calculated. Due to scan quality issues, some chords needed re-shooting to verify repeatability.

Quantitative analysis of gamma scans: results
Figure 2 shows the results derived from the multichordal gamma scans of the active areas. These results are shown on ‘Kistergrams’,7 which are tray sketches drawn to scale with the various measurements also shown to scale. As such, they give a visualisation of the key hydraulic parameters.
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