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Troubleshooting a C3 splitter tower Part 1: evaluation

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

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Article Summary
The PetroLogistics giant C3 splitter (see Figures 1 and 2) is a heat-pumped, 28ft (8.5m) internal diameter  tower operating at 105 psig at the top. The tower contains four-pass, equal-bubbling-area fixed valve trays with mod-arc downcomers (MOAD) on the outside panels. Open area on the trays was 15% of the active area.

The tower 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. The separation did not improve (if anything, it had become worse) upon turndown. Initial gamma scans through the centre tray panels indicated flooding.

PetroLogistics, Fluor (which was not involved in the tower design), and the tray supplier formed a task force to conduct a troubleshooting investigation to determine the root cause of the poor performance and to propose and engineer a fix. The strategy was to conduct a 
field investigation combining PetroLogistics’ expertise in operating the C3 splitter, Fluor’s expertise in distillation design and troubleshooting, and the tray supplier’s expertise in tray design and modification. Tracerco was later brought in to provide diagnostic expertise in anticipation of extensive use of gamma scanning in identifying the root cause.

The troubleshooting investigation combined hydraulic analysis and detailed multi-pass distribution calculations with the specialised technique of multichordal gamma scanning with quantitative analysis.7 The hydraulic analysis and multi-pass calculations did not identify a reason for the low tray efficiencies, but confirmed that the trays are prone to channelling and maldistribution due to their large open areas. 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 multi-pass maldistribution as the root cause.

The investigation identified the high open slot area (15% of the active area) of the fixed valves to be the prime factor inducing the channelling and maldistribution. A likely initiator of the multi-pass 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. The high ratios of flow path length to tray spacing (2.4 to 3.7), high weir loads, and integral trusses projecting a significant depth (4in) into the vapour space were other conditions that promoted the channelling.

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.

To the best of our knowledge, this is the very first time that field measurements demonstrated interaction between VCFC and inside-to-outside-pass maldistribution. A lesson learnt is that this interaction must be considered when designing and operating large diameter towers. Finally, the investigation highlights that excessive open areas render trays prone to channelling and maldistribution, especially in large diameter towers containing multi-pass trays.

The investigation is described in two parts. Part 1 describes the initial operation, as well as the 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 will describe 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. 

Hydraulic evaluation at initial operating conditions
Figure 2 is a simplified process sketch of the C3 splitter tower and its auxiliaries. Data for typical initial operation were collected at the highest rates at which operation was stable, about 20-30% below design. There is uncertainty about the reflux flow rate due to a metering error that plagued the reflux meter. The propylene product contained 3.4% propane (by mole) compared to the design 0.5%. The propylene content of the bottom stream was a little higher than design. The tower temperatures and pressures were similar to design.

There was a question of whether the trays in the tower were flooding or not. A typical pressure drop for good operation is normally about 0.1 psi per tray, while pressure drops exceeding about 0.2 psi per tray indicate flooding. For the C3 splitter, the pressure drop per tray at operating conditions was about 0.09 psi per tray, which argues against flood. In contrast, the gamma scans concluded that many of the trays were flooded. There was a need to reconcile the two conflicting observations.

To determine whether the tower was flooded, a plot of the measured tower pressure drop against the tower internal vapour traffic was prepared (see Figure 3). The internal vapour traffic is approximately the sum of the reflux and product meters. A point of inflection in such a curve indicates the vapour load at which liquid begins to accumulate in the tower, and is a good indicator of flood.1

In Figure 3, the upper curve is for the entire tower, the lower curve is for the trays between the propylene side draw and the feed. Both curves show a point of inflection at vapour traffic just below 2000 mph, or a total tower pressure drop of 13.5-14 psi. This suggests that the initial operating loads in the tower were right at incipient flooding. While some flooding could have started earlier, the significant accumulation of liquid started above these loads.

With the loads at which flooding initiated in the tower shown to be well below the design loads, it was concluded that the flooding was premature.

A simulation was prepared based on the operating conditions on 31 December 2010. Vapour and liquid loadings from that simulation provided the basis for hydraulic calculations. Table 1 shows the results of these hydraulic calculations. The values in Table 1 were calculated by Fluor. The Fluor values were more conservative than the tray supplier’s, but even Fluor’s calculations do not indicate proximity to any flood limits. This analysis verified the conclusion that the flood observed in the tower was premature.
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