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Avoid unnecessary costs by understanding process problems before taking action

When a piece of process equipment starts experiencing problems and producing off-spec product, every engineer’s reaction is to make a change to “fix” the problem and get it back on-spec. yet, many plants lack adequate instrumentation to completely diagnose the cause of the poor performance. Without a more complete understanding of the root cause of the problem, “corrective action” could have little effect or make the problem worse. Gathering supplemental data can sharpen the “image” of the problem and lead to the appropriate action being taken.
 
Case study 1 -  high differential pressure in H2S absorber
 An ammonia plant engineer found that the H2S Absorber in the unit was displaying a higher than normal differential pressure. The gas stream from the top of the column was still on spec, but the inlet pressure of the gas to the column was higher than normal and climbing. This was putting a strain on the compressor and increasing energy costs. As is typical on most columns, the only pressure sensors on the column were at the top and bottom. High differential  pressure in a distillation column can be caused by several things. These can include fouling, damaged trays, instruments out of calibration, changes in feed composition or conditions, and unexpected contaminants. Being an experienced engineer, an analysis of the amine was ordered to ensure correct concentration and to look for contaminants. The instrument team checked the bottoms level instrument, the temperature probes, and the two pressure meters for proper calibration. The engineer asked the lab to check the calibration of the lab instruments used to perform compositional analysis of the gas feed to the column. All of these tests failed to identify a problem and gave the engineer confidence that there was a physical problem with the internals of the column.
 
It was suspected at this point that the column was either fouled or some internals had been damaged. However, the cost of the corrective action was highly dependent upon the actual cause. Damaged or fouled trays were both going to necessitate an unplanned shutdown, but the duration and costs associated would differ greatly. The engineer decided to perform a Tru-Scan™ on the absorber column.
 
A Tru-Scan™ is performed by positioning a source on one side of a column at the top tangent line and a sensitive detector on the opposite side at the same elevation as shown in the orientation drawing in Figure 1. The scan line through the column does not have to be across a full diameter, but can be across a chord. The chord length does need to be consistent from the top of the column to the bottom. The chord is chosen such that the scan line is across an active area of the trays, avoiding the downcomers. A purposeful scan along the downcomers can also be performed to gain additional operational information, but the tray active area scan is normally performed and analysed first.
 
The source and detector are synchronously lowered down the column incrementally. At each elevation, a reading is recorded of how much of the signal transmission passes through the column. In parts of the column where the average material density is low, such as just under the trays, most of the signal from the source passes through the column. Just above the trays, where the average density of the aerated liquid is highest, the liquid absorbs most of the signal. The Tru-Scan™ results are then provided in a plot of the measured signal counts versus elevation (Figure 2). This plot can be interpreted to indicate the state of operation on each tray in the column.

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