Q & A > Question Details
We are trying to figure out how to improve the feed control to our new Hydrocracking and Hydrotreater Units, since one of the feeds comes from the Coker Unit, we want to know how variable are the quality and flow of the HCGO, Naphtha and LCGO, because we are aware it would be changing while coker cycles are taking place. We don't have tanks to store LCGO and Naphtha as feed to the units, so these streams go to the hydrocracker and hydrotreater directly from the coker stripper, and if there is a sudden change in composition or flow, it could lead on a runaway.
01/03/2014 A: Karthik Codati, IOCL, karthikcodati@gmail.com
We are operating a DHDT which is fed Coker Gas Oil, Coker HVY Naphtha,St Run Gas Oil and FCC total cycle oil. SRGO comes from the tank to the unit and for coker streams there are two provisions either they can be routed directly to DHDT or can be routed to a dedicated CGO tank from where it can be fed to DHDT. There is no problem as such during Coker drum change overs it's mostly smooth but occasionally whn there are upsets in coker our feed filters get choked then it creates problems. Bed Runaways or temp excursions occur at times then temporarily CGO feed to the unit is reduced to control the cracked content in the feed.
04/02/2014 A: keith bowers, B and B Consulting, kebowers47@gmail.com
You are observant of the risks associated with quickly changing feedstocks to a hydrocracker. What most (smart) folks do is have a much larger than normal 'hot feed surge drum' (hydrogen blanketed to exclude any oxygen). A surge volume of 1 hour of feedstock, with a good distributor so contents are well mixed, will dampen the rate of feed quality change. The RATE of feed from the drum should be allowed to 'float' as much as possible to smooth out rate variations--which are another big 'no-no'.
You MAY be ale to monitor spgr. of the feed and develop a feed forward correlation for an Advanced Process Control application to adjust reactor inlet temperature to stabilize reactor product distribution and avoid swinging the recovery system. Otherwise it will be very difficult to keep the recovery system close to optimum conditions and product sulfur levels may vary too much.
You need to be measuring the quality and quantity swings from the Coker NOW to effectively plan mitigation measures.
04/02/2014 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
It is quite common for coker naphtha to connect with the hydrotreater feed line, avoiding tankage in order to minimize the opportunity for di-olefin polymerization. For a hydrocracker, however, it is important to maintain steady flow control to the reactor. With multiple feed streams, and the switching issue with the coker stream, at a minimum there should be a surge drum or tank. From there, the feed to the unit should be on flow control. If this is not done, the reactor bed temperature will not be steady, which will result in variation in conversion and shorter run lengths on the catalyst. The bottoms recycle should also be on flow control, using the level in the bottom of the last column as surge space.