1 reply to this topic

[jigs1980]

Normally in refinery coking found at the shell side of the debutanizer reboiler, but we are facing problem with stripper bottom to DeC4 feed line coking. if any one has experienced this kind of problem, share the root cause of it and corrective action for it.

We found the coke in stripper bottom line in recent shutdown and come to identify the problem. Due to restriction of the flow to DeC4 we have to cut down the throughput.

Parameters for stripper.

Stripper top pressure : 16-16.3 Kg/cm2(g)
Stripper bottom temp : 122-125 Deg C.
Stripper bottom reboiler outlet temp : 122-125 Deg C.
Stripper bottom reboiler Tubeside (Lighr Cycle oil) inlet temp : 200-206 Deg C.

I have dought about the stripper feed high sulphur or water carry over from HP separator. ( pH of the water at HP separator boot might be lower than 7.0.- leads into corrosive nature)

Give your opinion for it.

[Himanshu Sharma]

Sir,

Your question requires a large amount of data for an answer or suggestion that works for you or else our (or at least mine )opinions will be intelligent guesswork based on previous experience in similar units.

The Data required in my opinion is

Feed : VGO/Hydrotreated VGO or Resid
Mode of Operation: C3/C4 maximization or Gasoline
Crude Stock being processed
API gravity of Feed
Operating Temperature of Reactor
PFD showing Process Paramters like Pressure,Temp and flow for reactor section and Gas Plant.


Now let me try n give you a preliminary assessment of the situation.

I would first like to know ,if the feed line coking is the only concern ! as for me it points to serious problems like higher RVP for FCC Naphtha and slipping of C5 in LPG.


Your stripper column is being operated at higher pressure than what i know the conventional figure of roughly in the vicinity of 210 Psig (Rejecting C2 s )in the froth regime for tray operation.

The amount of coking that you refer to can be fouling due to polymerization. Typically, the fouling originates around the feed point of DeC4, on the bottom few trays, and in the reboiler due to the high temperature at the bottom of the column. I believe that this polymer formation can be attributed to vapor-phase combination of olefins including butadienes and pentadienes.

This polymerization fouling is more prominent in case of Resid FCC and Deep Catalytic cracking as compared to conventional FCC operation.