The pressure drop occurring throughout the packed bed section of the column was higher than the design pressure and samples had been taken down stream of the section which indicated there was poor mass transfer occurring in this zone of the column.
An initial check on the "position" of the packed bed within the column was undertaken using a TRU-SCAN®/Tru-Grid™ Scan . However, due to the diameter and packing density it was impossible to determine the condition of the packing using this technology. The results from this TRACERCO Diagnostics™ Tower scan confirmed that both the bed and the distributor were in their correct positions and there had not been a catastrophic collapse of the packing. The scan also confirmed that the liquid in the base of the vessel was at the correct level and not impinging upon the packing.
A TRACERCO Diagnostics™ Flow Study was carried out using a radioisotope in a suitable chemical form. This involved injecting the radioisotope into the process stream, before entering the packed bed, and arranging a number of rings of detectors on the outside of the vessel (in this case two rings). Each ring consisted of four detectors, each at 90o from the next, connected back to a central data collection system. The results of the a TRACERCO Diagnostics™ Flow Study (Figure 1.), confirm that there was preferential flow occurring down the north side of the column, from the top to the bottom.
Due to the fact that the preferential flow occurred throughout the whole of the vessel, it was suggested that there was a serious problem with the distribution of the process fluid before it reached the packing. However, initial gamma scans had indicated that the distributor was itself in place, which suggested that there was mechanical damage to the integrity of the distributor allowing preferential flow of the fluid down the north side of the column.
A replacement distributor was purchased and the column was opened as soon as was practicable. When the column was opened, the distributor was shown to have a large hole in it in the north side, allowing fluid to preferentially flow through that area. The distributor was replaced and the column re-started. Following this maintenance, the pressure drop in the system returned to normal and the mass transfer increased.
Using a TRACERCO Diagnostics™ Flow Study alongside a TRU-SCAN®/Tru-Grid™ Scan is an excellent way to determine the performance of a packed bed and investigate the mechanical integrity of distributors.
Figure 1: Tracer Study Results
The trace at the top are the results from the top ring of detectors and show preferential flow down the north side of the column (shown in red).
|The second trace at the bottom are the results from the botton ring of detectors and show preferential flow down the north side of the column (shown in red).|