Q & A > Question Details
We would like to reduce the net minimum flow of our Visbreaker unit in order to increase our flexibility. Our concern is that this will lead to shorter runs due to coking in the furnace. Do you have experience regarding the various available options: recycling VSB gasoil or VSB residue? Co-processing LCO or slurry from FCC? Increase the injected BFW/steam to the VSB furnace passes? What would you advise?
09/11/2014 A: Jayaraj Jayam, Chennai Petroleum Corporation Limited, njayaraj@gmail.com
Recycling FCC products such as LCO or MCB may lead to unstabilty in vsb residue. P-value of the final residue may get affected. As residence time is expected to be low in the furnace, extra fouling may not occur. However MAV value( to measure di-olefins which leads to gum formation) may get affected in lighter fractions.
It can cause fouling in soaker drum and in fractionator bottom circuit.
06/11/2014 A: Jayaraj Jayam, Chennai Petroleum Corporation Limited, njayaraj@gmail.com
Your question is not clear to answer. I presume that you are trying to ask the effect of recycling the cracked material in feed stream. Am I correct?
If so, recycling of vsb gasoil or vsb residue may lead to secondary cracking which will be detrimental to the run length of your heater due to quicker coking tendency. Recycling the cracked material in the feed stream will foul the entire circuit such as preheat exchangers, heater coil and soaker.
However, these streams may be introduced at the downstream of soaker, that is at the inlet line of fractionator. This will avoid further cracking but better utilized to distillate further.
Refinery slop (off spec mixture of products) can be fed to fractionator to recover the valuable products from it. But this will affect your fractionator bottom temperature and in turn it affects preheat of your heater.
Introducing recycle at the inlet of fractionator is the better option. But your plan is to introduce at inlet of heater. In that case, maximize BFW injection because volumetrically, water expands much more than LCO or slurry. Sudden expansion of water will help to increase the coil flow velocity dramatically to disturb the boundary layer. N.Jayaraj.
03/11/2014 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Reduce the cut point at vacuum bottoms or increase steam or water injection.