Q & A > Question Details
We have a problem with our Hydrocracker VGO feed filters resulting in frequent backwash operations due to high Del P. Can you please ascertain the reason for the same as we do not get any FeS or suspended solids in the backwash stream analysis. Is it because of the asphaltenes as we process deep cut VGO (360-580+ degC) along with Heavy gas oil?
15/12/2010 A: Mark Willingham, Purolator Advanced Filtration, mark.willingham@purolator-afg.com
What type of filters are you using- wedge wire, wire cloth, sintered powder metal, or other?
This is a fairly common occurrence in the industry, and is typically thought to be because of sticky asphaltenes binding themselves to the filter media. We have had very good success in retrofitting existing filter elements with our high permeability sintered wire cloth filters. The open pore structure allows for better backwash fluid flow to reach the particles and free them from the media surface.

Our elements are typically designed to remove particles in the 25 micron range, although other ratings are also available.
11/08/2010 A: Balasubramanian Iyer, Nexen CNOOC Limited, balasubramanian.n@aol.in
If you are taking VGO at ambient temperature then plugging of Feed Filters Resulting in high DP and frequent back washing are envisaged. Try to Pre Heat the VGO before taking it to the Feed Filters.
23/03/2010 A: Niels van der Horst, Royal Dahlman BV, n.vanderhorst@dahlman.nl
We have seen this phenomenon more often. You could check the quality of your feed (not back flush liquid) with respect to TSS values over time in order to say something on plugging of your filters. Dahlman can perform on site pilot filtration tests with various types of filter media with an industrial test rig. At this time we are performing identical tests for (imported) VGO filtration. This test filter package can be used to test the behaviour of different types of filter elements beforehand using different pressure for backwashing. The package, equipped for the installation of one (1) single filter element, allows you to determine the right filter element type and modus of operation that is required. Not all types of filtering media are destined to be used in this filtration application.
14/03/2008 A: Charles Cameron, Axens, charles.cameron@axens.net
Feed deposits, such as rust, iron scale, coke or sediment will tend to be captured, at least in part, on the filtering system leading to increased pressure drop and thus increased backwash frequency.
The first item to check is the detailed analysis of feed contaminants. Specifically, the coke and sediment content should be within specified limits. Similarly, the asphaltene content should be low, typically less than 500 ppm. Higher asphaltene content will increase risk of asphaltene precipitation leading to filters plugging. The feed color is also a good indication about feed stability. The typical ASTM D1500 color is to be strictly lower than 8.
Another reason for feed filter plugging arises from non-nitrogen blanketed tank storage. For the case of imported VGO feeds, un-blanketed storage can lead to small amounts of oxidation products, which produce unstable gums-forming compounds, such as olefins, di-olefins, and oxygenates. These products can then induce poly-condensation products upon heating in the heat exchanger train. The formed solids are then partially filtered out by the filters and partially by the top section of the catalysts bed.
06/03/2008 A: Marcello Ferrara, ITW SrL, mferrara@itw.it
We developed a chemical which can prevent asphaltene from precipitating, thereby reducing the possibility plugging can happen.
06/03/2008 A: SImone Robinson, Tube Tech International Ltd, simone@tubetech.com
Highly likely to be asphaltenes as it is a good binder to other deposition like coke or other scale type formation – hence a thorough cleaning regime is required. This could also be an Online solution as well as offline. Contact me if you need anything further.
06/03/2008 A: Lindsay McRae, Pall Corporation, Lindsay_McRae@ap.pall.com
Frequent HCU backwashing can be due to a number of factors.
1) Reverse plugging of filter elements is a common cause when liquid backwash with reverse flow type backwash and wedgewire type backwash filters are used. In some cases up to 10% of HCU feed can be sent to backwash receiver due to frequent backwashing! Dirty backwash fluid can contribute to this. We have measured the solids loading in many liquid backwash fluids and it can vary quite a lot. What I'd ask is where is the backwash fluid is coming from? Is it filtered feed or from bottom of reactor? This may need to be filtered separately to reduce reverse fouling. Best practice is to avoid liquid backwash filters altogether and use gas assisted type backwash in the first instance as backwash fluid generation is typically 5 to 7 times less than with liquid backwash type HCU feed filters.
2) Other possibility is asphaltene precipitation. What the feed filter operating temperature? If too low, this can be very problematic. Pall Corporation developed a solvent soak step as a part of regular backwash cycle to assist in dissolving asphaltenes but feed temp needs to be in the right range too. Not too cold to have asphaltenes precipitate, and not too hot to have coking or polymerisation occur.
Related to this is feed compatibility. If the feed to HCU is from more than 1 source, an Oil Compatibility Test and diene test should be conducted. If feeds are incompatible, a 3rd viscous (asphaltene) phase can precipitate and this is excellent for blocking filters up and causing short cycle time.
3) Another possibility is gums. This occurs especially in case of feed coming from tanks without adequate nitrogen blanketing. Gums can't be dissolved with online solvent soak and only frequent external filter element cleaning can remedy that situation. We'd suggest to also look at N2 blankets to make sure they are working OK and you don't have a leaky roof.
We offer troubleshooting / consulting services to address such issues as you describe. It would likely take some detailed analysis work to determine what the problem is and then define the right solution.
05/03/2008 A: Keng Chung, Well Resources Inc., kengchung@hotmail.com
Yes, very likely. I have seen this, done a comprehensive analysis including split stream filter skid test in the field test, and solved the problem. The culprits are asphaltenes and non-basic nitrogen compounds. Deep cut does not help, but the real problem is entrainment of heavy end. A small amount of heavy end is bad enough to cause all sorts of problem. See if you can minimize the entrainment as the first step. If you give me your email address, I will send you some papers we published on what we did.