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In our vacuum distillation unit, in HVGO section temperature difference between vapors entering into the bed from below and temperature in the bed has come down to less than 10 degc from almost 50 degc after unit emergency shutdown. This has lead to high temperature across bed 1-3. What could be possible reason for this and what are potential impact of this column yield pattern? And what are the remedial measures for controlling this without going for any shutdown?

Dec-2024

Answers

Michael Armstrong, Worley Consulting, mike.e.armstrong@worley.com

Agree that the problem is likely damage to packing. Another possibility is plugging of spray nozzles which would be detectable by higher pressure at the spray header inlet. While plugged nozzles typically could require a shutdown, it is always possible that the plugging is salt which can be eliminated by water washing. This may be worth trying to avoid a full shutdown though there are risks to this procedure as column must be completely dry before restarting vacuum. Monitoring of nozzle pressure drop and thermal scanning are potential checks.

Jan-2025
Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

Considering the information in the question, it's possible that after the emergency shutdown, the bottom liquid reached the feed nozzle which expanded with feed energy and produced damages in the fractionation beds (sudden expansion). A good way to verify this condition is monitoring the pressure drop in the fractionating bed. If the pressure drop is raised, it's possible the coke formation in the bed. Otherwise, if is observed lower pressure drop in fractionating bed after the emergency shutdown, probably occurred preferential channelling or the total destruction of the bed. A good way to confirm this is to open the suction filters of the side stream pumps to identify the presence of pieces of internals of the fractionating bed. Unfortunately, in this case the best solution is shutdown the vacuum distillation column and correct the damages in the affected beds once operate under this conditions will lead to a poor performance of the processing unit impacting the downstream units like hydrotreaters and residue upgrading units and may lead to the worsening of the internal damages of the vacuum tower which can cause another emergency shutdown with even more serious consequences.

Dec-2024