Q & A > Question Details
Recently a new DHDS Unit was successfully commissioned. At Unit Feed we have 25 micro meter Gas Oil SS cartridge filters. Since start of Unit the filter choke again and again. Some times Unit thruput is reduced as these filter elements have to be manually cleaned and the cleaning interval reduces to less than a day. Our crude composition changes with change in crude tank and sometimes residue is there in Gas Oil. The filter elements are chemically cleaned and the interval increased but problem remained. What are other refiner's experience? What may be other causes of Filter elements chocking? What is the allowable limit of water in diesel Feed to DHDS Unit. Can high amount of residual water can choke filter elements?
04/05/2011 A: Lindsay McRae, Pall Corporation, Lindsay_McRae@pall.com
1) Is this feed filter designed to be a BW filter or is it a cartridge filter using manually cleanable SS filter elements but with no automatic BW facility?
2) Many refiners use disposable filters on DHTU feed , however this depends on the TSS loading. Our experience is the disposable feed filter life is ranging from 1 month to upto 4 or even 6 months. Often at start up the filter element life will be significantly shorter though as corrosion products are flushed though. Weekly or sometimes daily filter changeout can occur during that period after start up for 1 or 2 months. In this application it is critical to size the feed filter on the contamination level present, and the required service life, not just the flow rate. If that has happened the filter will be far too small and short service life will be experienced. Disposable filters having depth filtration mechanism can have higher dirt holding capacity & longer service life than metallic mesh screen filters and do not require cleaning. ( they are disposable). Also it is important that if you are cleaning metallic filter elements, the elements are tested properly to ensure cleaning (chemical and/or ultrasonic cleaning) has being effective and the elements have not been damaged in the cleaning process.
3) Solids (iron sulphide corrosion products) will be the reason generally for filter fouling. Sometimes FCC catalyst fines also when LCO feed component is used as well. In cases where GO is coming from a coker, then there can be a reasonable high level of carbon fines. It varies a lot from what we have seen from 0.2 mg/L to 200 mg/L TSS for GO HDS. If some of the DHTU feed is LCO coming from tanks or imported then it is necessary to have properly N2 blanketed tanks to avoid gum formation which is good for blocking filters too. So to you need to get a feed characterisation study done. We offer such services on a commercial basis.
If the TSS level is high ( say >10 or 20 mg/L ) then we would probably recommend a fully automatic backwash filter, not a disposable or non-regenerable metallic cartridge filter which it sounds like you have installed here.
4) Typical max free water content is 250 ppmw free water, Check with your catalyst vendor on that. Free water should be limited to prevent catalyst deactivation before the catalyst is sulphided. High amounts of water should not affect filter element plugging.
03/05/2011 A: Mark Willingham, Purolator Advanced Filtration, mark.willingham@purolator-afg.com
We have seen this happen with some ss cartridge filters. Our solution to this is a custom designed sintered wire cloth stainless steel filter that is specifically designed to optimize the backwash cycle. We have proven in several different gas oil applications that our media extends the life of the filter with no change in the backwash procedure.