Troubleshooting premature tower flooding
Thorough analysis of field data and tray hydraulics together with gamma scanning were key factors in the successful troubleshooting of premature flooding in a debutaniser.
LOWELL PLESS, Tracerco
ANDRÃ‰ PERSCHMANN, DAVID BRUDER and THOMAS WALTER, Linde AG
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Flooding is defined as an operating condition where liquid accumulates in a column. It is the most common cause of capacity limitation of a separation column.1 The build-up of liquid in a column can initiate from a variety of mechanisms, for instance jet flooding, downcomer choking, and backup flooding. Flooding is usually revealed by a sudden increase in the column differential pressure. Several reasons for column malfunctions have been reported in the literature. Many column malfunctions related to flooding are caused by fouling.2
This is a case study of successful troubleshooting of the premature flooding of a debutaniser in a steam cracker unit. The column has been operating well for decades and usually did not have serious problems during the typical five-year turnaround cycle. However, during a recent cycle the column had been in operation for only two and a half years since its last shutdown when flooding was detected.
A rapid rise in pressure drop was measured by the plant owner in October 2017. Shortly after that, the bottom product purity deteriorated, followed by the overhead product purity deteriorating and the column operation became unstable. The feed rate to the debutaniser had to be reduced to achieve steady-state operation again. Due to this, the column ultimately became a bottleneck, limiting ethylene production to approximately 80% of plant capacity. Thus, it was necessary to urgently identify the root cause of premature flooding and to develop a solution to overcome the capacity bottleneck.
As Figure 1 shows, the debutaniser is equipped with 20 two-pass standard valve trays in the bottom section below the lower feed inlet, 11 one-pass standard valve trays in the middle section below the upper feed inlet and 17 one-pass standard valve trays in the top section. Each section has straight downcomers on a tray spacing of 600 mm (~24 in).
A C4+ fraction from an upstream pre-separation column is fed to the debutaniser’s lower feed inlet and a C4+ fraction from an upstream depropaniser is introduced to the debutaniser’s upper feed inlet. The feeds are separated into a C4 overhead product and a C5+ bottom product. The column is normally operated at an overhead pressure of 4.75 bara (68.9 psia) and a bottom temperature of 105°C (221°F). The debutaniser is instrumented with two differential pressure (Î”P) measurements: one Î”P measurement covering the top trays, Trays 21-48, and the other covering the bottom trays, Trays 1-19. Polymerisation inhibitor is fed to the upstream fractionators to avoid polymer fouling in the debutaniser.
Root cause analysis
First, the plant owner analysed the operational data to better understand the flooding phenomena. A sudden increase of the differential pressure across Trays 1-19 in the bottom section was noticed in October 2017 (see Figure 2). Shortly after the bottom pressure drop had established a plateau, the pressure drop across the middle and upper sections, Trays 21-48, started to increase significantly. As Figure 2 shows, the pressure drop in the bottom section went from 180 mbar to 350 mbar (72 in H2O to 141 in H2O), followed by the pressure drop in the middle and upper sections which rose from 160 mbar to 370 mbar (64 in H2O to 149 in H2O). After the feed rate to the column was reduced, liquid build-up on the trays appeared to have immediately stopped and then receded. The pressure drop in the middle and upper sections decreased until the initial value was reached again. Shortly afterwards, a reduction in the pressure drop in the bottom section was seen. Steady-state operation and a normal pressure drop were achieved after all the accumulated liquid receded from the trays.
Investigation of the pressure drop data was the first indication that the root cause of the premature flooding was in the bottom section of the column, the reason being that liquid accumulation and hence an increased pressure drop always take place above the liquid flow restriction. The debutaniser is well known for butadiene polymerisation fouling. Hence, this observation was a strong indication that polymer fouling in the bottom section could have caused blockage of the tray active area and/or plugging in the downcomers. Nevertheless, several aspects of the tray design were studied before the entire investigation was focused on flooding caused by fouling.
Second, a hydraulic rating of the trays was performed using plant data from a past high load operation of the column. The results proved that the valve trays have sufficient capacity to accommodate the normal flow rates when no plugging occurs. The calculated overall pressure drop, including the static head and the pressure drop of the overhead nozzle, matched the measured pressure drop of 290 mbar (116 in H2O) for the high load operation and hence proved the validity of the simulation. It could be concluded that the valve tray design was not limiting the capacity of the column. However, there was still the possibility that the trays were collapsed, which could only be verified by gamma scanning.
Gamma scanning is a non- intrusive investigative technique to diagnose malfunctions of process equipment while it is in operation. During the measurement, a gamma ray emitting radioactive source, along with a radiation detector, are synchronously lowered down opposite sides of the column. The radiation beam passes through the process equipment and its intensity is measured by the radiation detector in terms of a count rate. Interaction of the gamma ray beam with the column shell, internals, and the process fluid cause attenuation of the gamma ray, correlating to the average material density.3
A baseline scan had already been conducted by Tracerco on behalf of the plant owner under normal operating conditions in August 2017, three months before premature flooding occurred for the first time. All trays were scanned, but only the western active areas of the two-pass trays in the bottom section were scanned. The orientation of the scanline is shown in Figure 3.
The results of the baseline scan are shown in Figure 4. All 48 trays were holding adequate levels of aerated liquid, hence the good mechanical condition of the trays was confirmed. The one-pass trays in the top section were uniformly loaded, with tray froth levels ranging from 31-48% of the tray spacing. In the lower section, Trays 3 to 20 and Tray 1 were holding uniform froth levels ranging from 33-41% of the tray spacing. But another observation was noticed by Linde AG — a higher froth height was detected on Tray 2 compared to the other surrounding trays, particularly Tray 1. Tray 2 appeared to hold a froth level approximately 66% of the tray spacing. An isolated high or low froth height detected on a single tray is very frequently a result of a measurement anomaly caused by obstructions in the scanline, most typically an external interference such as a support ring, pipe support, weld pad for a manway, or nozzle. However, no notation of potential external interference was made by the scanners in the vicinity of Tray 2. In fact, the same higher froth height on Tray 2 was observed by Linde AG in a previous troubleshooting scan. In this earlier investigation, the cause was physical blockage of Tray 2’s downcomer clearance due to fouling causing liquid to stack on the tray active area.
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