Guarding against contaminants
Analysing spent hydroprocessing catalyst underpins the development of guard bed catalysts to counter contaminants.
MICHAEL Schmidt, Haldor Topsoe A/S
HENRIK RASMUSSEN, Haldor Topsoe Inc
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A very important parameter for the cycle length of a hydroprocessing unit is the deactivation rate of the installed catalyst. The deactivation of a hydrotreating catalyst can be coke induced and contaminant induced. Coke deposition is to a large extent controlled, but also dictated, by the unit operating regime (temperature, hydrogen availability, residence time and partial pressure of hydrogen). This will normally cover most middle distillate units making ULSD or lighter products. However, in units processing heavier feedstocks like vacuum gas oil (VGO), heavy coker gas oil (HCGO), deasphalted oil (DAO), or even heavier cuts, the refinery normally faces a dual type deactivation. The coking reactions here are even more significant because the feedstock is heavy and contains lots of aromatic coke precursors in the high boiling fraction. But heavy feedstocks will often also contain a significant amount of heavy metals and other contaminants. In particular, this includes nickel, vanadium, iron, silicon, arsenic, phosphorus, calcium and sodium. These contaminants are present in processed feedstocks at ppm or even ppb levels, but the effect is significant. Following a non-reversible pathway, heavy metals will be deposited on active catalyst following different mechanisms. The outcome is that activity is lost permanently and will thus not even be regained during catalyst regeneration.
Topsoe researchers have within the past few years analysed and characterised a huge number of spent catalysts retrieved from industrial hydrotreater units operating on heavy feedstocks like VGO and HCGO. This characterisation work tries to understand how these contaminants are deposited to help us develop more effective guard bed catalysts.
Contaminants to deal with
In general, the heavier the hydrocarbon cut the higher the content of contaminants. In a feed stream such as VGO, DAO and HCGO, metals contamination is therefore mainly an issue for FCC pretreaters, hydrocracker pretreaters and lube units.
Vanadium and nickel are mainly found in large, porphyrin-like structures (asphaltenes) in crude oil (see Figure 1). Generally, the metallic species of the crude are concentrated in the resid portion, but some organometallic species are present in the lower boiling ranges with a boiling point above 660°F (350°C).
Introduction of feeds containing vanadium, nickel or iron (such as atmospheric tower bottoms) into VGO hydrotreating could have severe consequences in terms of cycle length. The relatively high space velocity of these units (compared to resid hydrotreating) could result in metal migration into the active, main bed catalyst. To overcome this problem, a highly active demetallation catalyst with a high metal pick-up capacity must be installed on top of the main catalyst bed. Topsoe has commercialised specially designed VGO demet catalysts designated TK-453 and TK-455 MultiTrap.
The large molecules containing these metals require catalyst shapes with a high surface to volume ratio to remove these metals efficiently. Iron will mainly deposit on the catalyst surface, whereas nickel and vanadium will deposit inside the catalyst pore structure due to the very large average pore sizes of TK-453 and TK-455.
Arsenic is a true catalyst poison as it will chemically react with active catalytic sites, for instance transforming the catalyst’s nickel and cobalt into NiAs or CoAs. Poisoned sites will not be reactivated during regeneration, and even small amounts on the catalyst will affect catalyst activity in a critical way. Fortunately, the concentration of arsenic is in the ppb range and can be effectively dealt with by utilising Topsoe’s high capacity arsenic traps TK-49 as an extrudate, and TK-45 and TK-41 in a ring shape.
Silicon found in oil fractions originates from Si-containing anti-foam additives used in coker units, as well as from the use of chemicals introduced during oil transport and tertiary oil recovery. Silicon reacts with the surface of the catalyst and forms a silica gel, hindering access to the active catalytic sites and thereby deactivating the catalyst. Silicon penetrates into the pore system of the catalysts, and deactivation is proportional to the concentration of silicon on the catalyst.
Haldor Topsoe’s TK-400 series catalysts for naphtha and TK-453 for heavier fractions provide the highest silicon pick-up on a volume basis.
Phosphorus species are rarely found in typical crudes; however, some opportunity crudes (and, in particular, renewable feeds) often contain significant amounts of phosphorus. Furthermore, phosphorus-containing anti-corrosion additives can be found in diesel and VGO fractions. Phosphorus compounds are decomposed in the hydrotreater, and the phosphates react with the alumina support (much like silicon), forming very stable aluminium phosphates. Accumulated amounts of phosphates will reduce accessibility to the active sites of hydrotreating catalysts and lower the activity accordingly. Phosphorus compounds are often found to originate from injection of corrosion inhibitors in the form of thiophosphorus compounds like thiophosphate esters, thiophosphites and tributyl phosphate.
Handling of organic phosphorus compounds in the VGO being fed to FCC pretreaters or hydrocracker pretreaters is a major challenge in some refineries. The phosphorus quickly deactivates the conventional catalyst and reduces cycle length dramatically. Topsoe has developed specific guard bed catalysts, TK-31 and TK-455 MultiTrap, to effectively protect the main catalyst. These guard catalysts, with proprietary properties and composition, will be able to prolong the cycle length of the unit.
The main source of sodium in a FCC or hydrocracker pretreat feed is normally poorly desalted crudes. This type of inorganic sodium will not easily enter the catalyst’s pore system. Sodium will therefore tend to deposit around the exterior of the catalyst, forming a solid crust between catalyst pellets, which will harm activity and cause pressure drop issues.
We should not forget to mention inorganic iron, which is a very common contaminant. Iron rust originates from corrosion of upstream equipment and may consist of everything from large flakes to the smallest particle tank rust. Large particle rust is known to be easily trapped in filters, scale catchers and high void catalytic materials. However, 5-10 micron inorganic iron particles are very hard to handle as they will pass feed filters and, without a proper graded bed system, enter the catalyst bed. Inorganic iron will also preferentially deposit in the outer surface of the catalyst, unless a very large pore demet catalyst is utilised as a guard catalyst, and will eventually lead to increasing pressure drop.
To handle inorganic iron, Haldor Topsoe introduced themacroporous particulate trap TK-25 TopTrap a number of years ago and it has proven to be a leading trap material.
The company recently launched an improved particulate trap, designated TK-26 TopTrap, with an optimised daisy shape with three axial holes to provide about 20x higher pick-up capacity as compared to TK-25 TopTrap. TK-26 TopTrap is designed with a 61% particle void fraction as well as a large internal pore volume and macro pores Larger sized inorganic contaminants deposit in the spaces between traps; fines or smaller sized materials enter the pore system and are trapped within the structure of the particle itself (see Figure 2). The internal particle void of TK-25 TopTrap is 25%, so that the total void in this product is greater than 85%.
More exotic metals like calcium, zinc and magnesium are also observed in some units. These can all originate from different additives but are also occasionally found in porphyrin structures in crude. These contaminants are quite common in units upgrading spent lube oils to new base lubes stocks and can be a challenge. They are harmful to the main bed catalyst because they tend to stick to the surface of the catalyst, thereby preventing access to the pore system. Table 1 summarises the mechanisms of contamination.
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