Q & A > Question Details
Refineries processing Crack Naphtha often face Naphtha Hydrotreating Reactor high pressure drop problems. At our refinery we do not have NHT Feed filtration. What are the major steps taken by refiners to prolong NHT operation?
11/08/2010 A: Balasubramanian Iyer, Nexen CNOOC Limited, balasubramanian.n@aol.in
Crack Naphtha Produced both from FCC/ Coker Unit should not be stored in Tank for a longer time as they have tendency to polymerize, try to consume almost all the naphtha produced from these units directly as feed from the unit rather storing it in offsite tanks and then consuming. We are processing Cracked Naphtha Boiling Range C5-120 Deg C from coker and not facing high pressure drop problems in our reactors. We have strainers in feed line ahead of the feed surge drums and not witnessed any blockages of strainers. Our plant is running for almost 6 years and we are not facing any high reactor pressure drops, still our catalyst activity is fine and the DP across is reactor is well within the range. If you are forced to store it in Feed Surge drums then do blanket it with Nitrogen and no contact with Oxygen at any cost.
06/11/2009 A: Austin Schneider, Crystaphase Technologies Inc, austin.schneider@crystaphase.com
Cracked Naphtha is one of the industry’s most abundant pressure drop contributors. This is primarily due to the olefinic compounds contained in the feed due to the cracking process taking place in the refinery Cokers or FCCs. The problematic material can be exacerbated if prolonged exposure to an oxygen rich atmosphere occurs. The problem with these materials is that they have the ability to attack the reactor top bed in two ways:
Primarily, these olefinic materials can react in and on the walls of the upstream heat exchanger tubes. Typically this will form coke deposits in the tube side and cause slight to severe pressure drop problems in the heat exchangers themselves. As is natural for all heat exchangers, temperatures and feed rates can wax and wane causing the tubing in the exchanger to “flex”. This flexing process helps to remove deposits from the heat exchanger, while unfortunately passing the fouling problem on to the reactor. The coke deposits formed in the exchanger move into the reactor as a range of particle sizes which are then filtered by the porous bed causing a cake layer to form and an accompanying rise in pressure drop.
If the olefinic compounds make it into the reactor without reacting in the heat exchanger, they can become extremely dangerous to the operational stability. These olefin and diolefin compounds have the ability to react with one another and form extremely long chain polymers known in the industry as “gums”. If received in large enough volume these gums can agglomerate on top of the most porous of top bed materials (like wagon wheel type materials) and form extremely hard layers of coke quickly fouling a reactor. Some instances have been severe enough to involve the use of a jackhammer during the catalyst change.
Aside from these common causes there is always the possibility of iron sulfide deposition.
So, how to solve the pressure drop problem? (Especially if nothing can be done about the influx of particles?)
Install a top bed system in your reactor which can handle a large volume of particles without building a pressure drop. There is a technology in the refining industry called CatTrap® Technology supplied by Crystaphase Products. This material is a reticulated ceramic which comes in large discs (1.5” to 2” diameter). These discs have the ability to filter and store particles inside the discs, while the large dimensions of the discs keep the material from filtering externally. Because the large external pathways stay open, there is no pressure drop build up as the material filters.
Additionally, there is another technology based on the reticulated structure of CatTrap® Technology that carries with it a mild activity meant to aid in diolefin saturation. This technology is called Actiphase® 3D DOS. This technology has the ability to improve top bed distribution, filtration capacity, and supply a mild activity all in the same system.
Crystaphase can custom design their top bed systems to fit the specific problem facing the reactor, be it iron sulfide, coke fines, or gums. There are no chemicals to add or special handling of the feed required. CatTrap® Technology and Actiphase® 3D Technology easily install directly on top of the catalyst bed, sock loaded, just like a standard catalyst loading. These systems protect the bed from pressure drop build up and a host of other problems related to particle deposition in the catalyst bed.
03/11/2009 A: Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
Some supplemental comments:
As pointed out by other responders, feeds to Naphtha Hydrotreaters should be protected from interaction with atmospheric oxygen and cracked naphthas are preferably direct transferred without storage through filtration systems into 'variable inventory' feed surge drum--upstream of high-pressure feed pump operating for extended periods at FIXED gpm or Am3/hr. Traditional horizontal feed surge drums are typically set up for fixed level control and are NOT 'variable inventory' and the HP feed pump is frequently 'hunting'.
Mixture of hydrogen-rich treat gas + naphtha liquids should reach TRUE 100% dry point ahead of the radiant section of the HP Charge Heater and NOT within the first catalyst bed of the NHTU reactor
Coker Naphtha contains linear olefins and cyclo-di-olefins as modest fractions of so-called PONA Olefins and PONA Naphthenes, plus derivatives of silicone antifoam additives [measured as wppm silica] along with significant weight parts per billion of transition elements.
Linear di-olefins + cyclo-di-olefins as well as linear olefins and cyclo-olefins react quite rapidly and aggressively with hydrogen when encountering first layers of active hydrotreating catalyst at hotter temperatures in the first bed of the hydrotreating reactor.
These reactive species and the contaminants all tend to create fouling and pressure drop build up.
If cracked naphtha is more than one-fifth of straight-run naphtha and HDS naphtha, then best practice is to incorporate a first stage selective hydrogenation reactor [operating warm in trickle bed mode].
01/11/2009 A: Virendra Kapoor, Petroleum Refining Consultants, vkkapoor9@yahoo.com
Major steps are:
1- feed filtration 2- magnetic feed filter 3- direct feed from columns 4- Inert Blanketing of storage tanks
for cracked stocks 4-Strainer baskets in top bed 5- Graded bed systems 6- Suitable antifoulants added to feed 7- Suitable catalyst loading in the reactor 7-High hydrogen purity of make up gas etc
01/11/2009 A: Lindsay McRae, Pall Corporation, Lindsay_McRae@pall.com
The most common way to prevent NHT feed exchanger fouling and reactor fouling is to install a feed filter. Older reactor design may use trash screens in the top of the reactor or a graded bed (coarse catalyst acting like a gravel bed media filter) but external feed filter is often preferred these days as this provides optimum reactor protection, and the expended cartridges can be changed out on the run without reactor shutdown plus also external feed filters can protect the feed exchanger from fouling also.
This feed filter may either be a disposable cartridge filter (typically 10 to 20um rated cartridge filter) or an automatic backwash filter. In NHTs a conservatively sized disposable filter is generally a cost effective solution. However, if the suspended solids loading is high (say >10 -20 ppmw) then a backwash filter may be more appropriate. We would recommend an audit to measure the suspended solids loading and particle size distribution in order to correctly size the NHT feed filter. If it is not sized correctly, short filter cartridge life can be quite a maintenance chore plus has a running cost for replacement filter cartridges.
If the high solids loading is only on the cracked naphtha stream, not on the naphtha stream coming directly from the CDU, then a feed filter on the cracked naphtha stream may be used rather than filtering the entire naphtha flow to the NHT.
Pall Corp has laboratory facilities and extensive experience in conducting site audits, and providing tailor made solutions including disposable and automatic backwash feed filters to protect NHT's. We'd be happy to help. I can send you a copy of some published papers or articles on HDT feed filters if you want.
01/11/2009 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
If your cracked naphtha stream is less than 20% of the total stream, nothing special will be required in the design. The most important thing to do is to keep oxygen out of the feed to the unit by having a good gas blanket system on the feed tank. It helps, too, to bypass the feed tank with the cracked feed stream, i.e., pipe to pipe. Some units have an oxygen stripper on the feed in the unit, but that is not yet typical. Oxygen causes poly-olefins to polymerize and foul equipment and beds. Units with high percentages of cracked stock may have a pre-reactor operating at a lower temperature to convert the poly-olefins to mono-olefins and avoid the polymerization.
31/10/2009 A: Sam Lordo, Nalco Company, salordo@comcast.net
Cracked naphtha can have a large amount of olefinic material which then can thermally or catalytically polymerize. A chemical additive solution often used to minimize this is to use a antipolymerant & disperant. If the cracked stock is routed to a tank prior to feeding an NHT and it is not nitrogen blanketed then an antioxidant may also be required.