Q & A > Question Details
At a Visbreakeing unit(coil type visbreaker) I worked on, the gas oil was drawn from the main fractionator to a gas oil stripper and then transferred via a pump to visbroken bottoms stream. We faced frequent high ampere problem at this pump. The pump returned to its normal amperes after its impeller was cleaned. Coke deposition was found on the impeller, suction strainer and discharge NRV. We had cut stripping steam to the gas oil stripper previously to decrease the overhead naphtha, however it was taken back in service to get the system back to 'design' conditions and doing so, reduced the frequency of high amperage problem. (The pump delivers 2~3 m3/hr gas oil flow. At higher flows it reaches/exceeds FLC(full load current). The pump's design minimum flow is 12 m3/hr.)
Can anyone please suggest possible remedies to this problem, specially the coke deposition at pump's impeller? Is it fairly common to have coke deposits in visbreaker gas oil lines if so what are some practices to reduce/remove the coke?
09/01/2019 A: IPKUMAR V, BPCL, navinvangari@rediffmail.com
The problem can be analysed in 2 ways:
1. The coke formation is taking place in the heater/soaker itself and the small chunks of coke getting into the column along with the product. This means that the VBU is operated a high severity or Quenching is not proper (or not enough to stop thermal cracking). Increase the quench liquid flow to check for any improvement. Or even BFW injection may also be increased to reduce the residence time(by keeping a close watch on the coil pressure) or reduce the back pressure at soaker outlet to reduce the severity.
2. The coke formation is taking place in the VBU fractionator. This is because Quench liquid is not enough for the VBU heater/soaker O/L liquid to stop thermal cracking. reduce the fractionator I/L temperature by increasing the quench flow but at the same time maintain enough enthalpy in the fractionator I/L liquid to flash and strip off gas oil components.
08/10/2018 A: Rajkumar Chate, Sulzer, rajkumar.chate@sulzer.com
If the pump minimum flow is 12 m3/h while pump is running at 2-3 3/h, it means that you have more residence time in the line for further cracking and formation of coke. Lower velocities will increase the risk of coke deposition in the line/strainer and pump. If there is pump minimum circulation line going back to chimney tray of the column, then try to increse the flow of min circulation line back to column. Increased flow will reduce the residence time and increse the velocity of fluid which might help to wash the coke particles deposited in line/strainer/pump.
Quenching is another option but I am not sure you have possibility to send cold gas oil back to chimney tray.
06/07/2018 A: Ravi shankar, HPCL, prskreddy@hpcl.co.in
Try a sort of spill back of cooled gasoil to be used as quench into the GO main take off line off the column (Column take off to Go pump suction ) in order to quench the Go to pump suction by a few degrees (I do not know your GO take off temperature to suggest the actual temp) or simply you may go for an Restricted orifice into such line based on heat balance to suit your needs.
Also there are some commercial coke preventers that can be dosed into the line to prevent Coking.