There are two loops in the typical hydrotreating units;
1. The short loop is from the downstream of the HP separator Hydrocarbon line towards the product stripper. Then the line up of stripper downstream to diesel product header or slop tank. This can also be sent to the feed storage tank. For flushing, straight-run cold diesel is used.
The purpose of short loop circulation during commissioning is to clean, remove debris, checking leakages, etc and make ready the unit before commissioning. This can also be applied after turnaround or after any major overhauling of the stripper section.
Secondly, circulation with cold diesel is applied for cool down and hand over stripper feed exchangers or any hot equipment for a maintenance job.
2. Long loop circulation is through the complete unit. It is carried out after catalyst loading or Turnaround. Its purpose is to perform Pre-wetting the catalyst and remove catalyst dust and any other debris or loose materials from the reactor circuit and then passed through the fractionation section which has already been flushed.
For ~4-6 hours, the circulation is once through then internal circulation is started. After that sulfiding of the hydrotreating catalyst is carried out.