Q & A > Question Details
We are operating Full combustion FCC with hydro treated VGO from VGO hydrotreater. Sometimes VGO HDT sends their back was material along with Hot hydro treated feed to FCC. Unfortunately many of the times their backwash filter is in bypassed condition. We commissioned our plant one year ago and till now no issues related bottom circuits propped up. We are having 3 MFC bottom pumps with dual bucket strainers for each of the bottom pump. For one month we are not developing flow though CLO pump around pumps (normal flow 800MT/hr for each pump).For present T'put we operate one bottom pump with one suction strainer in line. But for one month the bottom pumps are cavitating and not developing flow and it is observed that the pumps are not getting proper suction flow from the main fractionator column.We are continuously cleaning our suction strainers and some times we are getting soft coke & catalyst but sometimes finding not much. We lowered T'put due to this issue and running two pumps instead of one pump and all the strainers are in line. Still day to day the condition is deteriorating and not improving. Reason is not known for suction lines chokage issue. We are maintaining MF column levels on lower side but to our surprise the suction lines seem to got choked. Our slurry BS&W didn't cross 0.3wt% at any point of time during the period.
Anyone suggest --
1) How to mitigate the problem while plant is in running condition
2) How to avoid this type of problems what corrective actions to be taken wrt feed, Circuit flow and all.
3) Any system available to clear the MF column bottom circuit chokage issue.
 
Answers
25/01/2017 A: Eric Hennings, Technip Stone & Webster, EHennings@technip.com
1. Concerning backwash from sour VGO filter:
• Bypassing of filter is likely to increase pressure drop in VGO HT.
• Best practice is to not send this material to FCC.
• Solids can contain minerals (such as Ca) which will react within the oxidizing conditions of the Regenerator.
• Active sodium (and other species) will degrade catalyst.
• Tramp iron and clay/sand are not desirable.
2. Best practice is to run more than one bottoms pump.
• Disruption of flow creates coke.
• High bottoms level can damage tray/packing.
3. Soft coke is promoted by higher bottoms temperature and longer residence time.
• Consider best design practices on quenching, cold areas, etc.
• Asphaltenes and other reactive species in bottoms can also promote coking.
• Low reaction severity can lead to soft coke.
• Involuntary heat loss (e.g. poor insulation)
4. Coke already produced in the tower bottom can sluff off.
• Often thermal cycles will accelerate sluffing of coke.
• Best practice and design of “coke catcher” within tower
• Some bottoms pumps are equipped with “coke crushers”.
o Which allows larger opening in elements within suction filters
24/01/2017 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Pump suction strainers should be pulled and left out after a short startup period after shutdown and vessel entry. Discharge filters are used throughout the run.
23/01/2017 A: Peter Marsh, XBP Refining Consultants Ltd, peter.marsh@xbprefining.co.uk
If the bottoms pumps are cavitating and all other operating conditions are similar to the previous 12 months of operations, maybe a flushing oil line has been left open or a restriction orifice in a flushing oil connection has been removed and not replaced allowing a larger than normal flow of light cycle oil into the slurry oil which is then flashing to vapour? Or perhaps the emergency feed diverter valve is passing so that a portion of the feed is bypassing the reactor/riser and flowing directly to the fractionator base?
If the pump appears to be delivering a reduced flow but is not cavitating and the system pressure drop is steady, perhaps a suction strainer fill line or pump warmup line has inadvertently been left open creating an internal recycle around the bottoms pumps? Or perhaps a PSV or bursting disc provided for over-pressure protection of equipment upstream of the slurry pumparound flowmeter is discharging slurry back to the bottoms pump common suction line or to the fractionator base?
23/01/2017 A: William Miller, WorleyParsons, william.miller@worleyparsons.com
I hope I am not suggesting the obvious, but have you been maintaining enough bottoms product flow to stay within guideline for maximum residence time in the tower bottoms pool? This max. residence time decreases as pool temperature goes up. Excessive residence time promotes coking in the pool which could be cause of strainer fouling.