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Case Studies

TRACERCO Diagnostics™ Tower Scan

Tracerco Asia was invited to apply the TRACERCO Diagnostics™ Tower Scan technique to perform an on-line inspection of Debutanizer column which was experiencing separation problems.

The objective of the scan was to investigate the cause of the problem. The column was scanned twice, initially at 60% capacity and secondly at 100% capacity, after a steam cleaning process had been carried out to remove suspected deposit.

The scan lines were specially selected to facilitate the purpose of the scanning. It was initially planned to carry out 2 chord scans on the top section of the column, only 1 chord scan could be performed, because of obstructions (Scan 1). The other chord scan was replaced by a diametric scan (Scan 2).
For similar reasons:

The initial scans were performed when the column was operating at 60% capacity. Flooding was expected in view of the high differential pressure and poor separation.

Scans 1 and 2 through the active tray areas, detected no vapour above Trays 27 and 28, which indicated flooding. This condition was further confirmed by Scan 3. This scan also showed that Trays 29 and 30 were flooded and that the flooding extended up into the top dome of the column.

The low counts recorded between Tray 27 and the top of the column indicated that this section of the tower was full of dense liquid, rather than foam. This type of flooding is usually associated with a partial blockage that restricts the down flow of the liquid and causes it to back up in the column. Such a restriction can be caused by the lay down of deposits. This then suggested that there was deposition on Tray 20 and possibly a partial plugging of the Tray 28 downcomer.

Tray 26 appeared to be in good condition and was carrying between 50-100mm liquid.
However, all of the trays from Tray 25 down to Tray 11 were holding significantly less liquid and appear to be more or less “dry”. The worst in this respect was Tray 18.

Though a very low liquid level is often indicative of partial damage or damage to the tray there was insufficient evidence from these scans to confirm this view. It seemed equally likely that the low liquid levels were caused by the much-reduced down flow of liquid, caused by the flow restrictions observed further up the tower.

Scans 1 and 2 from Tray 16 down to Tray 11 showed that that these trays were also holding very little liquid. Again, this may be due to partial damage or maybe caused by the restricted liquid down flow.

Beneath the cone, Scans 4 and 5 covered the region between Tray 10 and Tray 1. The profiles for Trays 10-1 indicated that the trays were in good condition and were carrying reasonable depths of liquid of around
100–150 mm.

The second scan was performed when the column was operating at 100% capacity. Tracerco was informed that a “steaming” process had been carried out to remove any deposit that might have caused restriction to the liquid flow that had been observed at Tray 27 in the previous scan. It should be noted that the temperature at Tray 23 was approximately 40oC lower than for the scans carried out the previous week. This suggested that the top section of the column was the likely problem area.

Scan 1A, 2A and 3A showed that the column was flooded from the top tangent down to tray 22. The density of the liquid in the flooded region was less than that observed in the scans of 15-16 July, which suggested that there was vapour passing through the liquid, indicating that there had been some reduction in the deposits restricting the flow.

Below Tray 22, there was no sign of flooding. Rather, the observed counts in the vapour spaces above the trays coincided with the Free Vapour Count, indicating complete disengagement of the liquid and vapour. However Trays 21-17 were still carrying very little liquid.

The initial scans revealed that the top section of the column, between the tangent line and Tray 27, was flooded. It is believed that this was caused by deposits restricting the flow of liquid down the tower and implies partial plugging of the Tray 28 downcomer.

Beneath Tray 27 there was excellent disengagement of the liquid and vapour above all the trays.

Trays 25 to 11 were carrying very little liquid and Trays 18 and 14 appeared to be “dry”. Though the possibility of partial tray damage cannot be ruled out, it seemed more likely, in view of the flow restriction observed further up the tower, that the low levels of liquid on the trays reflected the greatly reduced flow of liquid down the column.

All of the other trays, from Tray 10 down to Tray 1 appeared to be operating satisfactorily.

The second scans showed that flooding was again observed in the top section of the column but the flood now extended from the top tangent line down to Tray 22. It seems likely that the column cleaning operation had flushed the deposits that were originally causing the restriction at Tray 27 down to Tray 22.

After the Refiner had cleared the flow restriction, either by steam cleaning or by physical removal at shutdown Tracerco Asia’s techniques were able to pinpoint the cause of the column’s separation problems and help the plant to effect speedy repairs and a rapid return to full production of high quality product.

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