How ZSM-5 works on FCC gasoline composition
A study of the effect of matrix versus zeolite in the base catalyst on the composition of light and heavy gasoline fractions.
BART DE GRAAF, MEHDI ALLAHVERDI, MARTIN EVANS and PAUL DIDDAMS
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A shift in steam cracker diet, together with a revival of the economy and higher demand, has led to a shortfall in propylene. Steam crackers typically produce a mixture of ethylene and propylene, the ratio depending mainly on the feed type. With the advent of shale gas in the US, natural gas liquids (NGL) are becoming more and more a substantial part of steam crackers’ feed slate in this region. NGL however exhibits a very high selectivity towards ethylene, leaving a shortfall in propylene production. The shortfall is currently being made up in fluid catalytic cracking (FCC) units and propane dehydrogenation (PDH) units. The latter process has only recently begun to gain significant market share in the US. The lion’s share of the shortfall in propylene is made up in FCC units by changing catalyst, using catalytic additives and adjusting operating conditions. Steam crackers provide about 60% of propylene, FCC units provide 30-35% and others (such as PDH) provide the remainder.
Propylene yields from typical FCC units processing vacuum gasoil (VGO) feed are around 4-5 wt%. This propylene yield can easily be increased by moving to more severe process conditions, but use of ZSM-5 containing catalytic additives has the largest impact by far. ZSM-5 additives increase propylene yields and butylenes yields, and improve gasoline octanes. Dedicated loading systems allow the amount of ZSM-5 additive used to be adjusted to ensure that optimal selectivities are achieved at all times.
ZSM-5 additives always increase propylene, butylenes and octane, but the effect of the additive depends strongly on the base catalyst used in the process. ZSM-5 additive effects are generally assumed to be additive to the base catalyst. At low additive concentrations this assumption is reasonable, but at higher ZSM-5 additive concentrations in inventory this assumption becomes increasingly less valid. Base catalyst and ZSM-5 additive exhibit an intricate interplay in cracking gasoline range molecules into LPG and oligomerising LPG olefins back into gasoline range again. Base catalysts are selected for a specific feed and targeted yield profile. The balance between matrix and zeolite, and rare earth on zeolite will determine the hydrogen transfer activity of the catalyst.1,2 Feed composition (aromatics, paraffins, naphthenes, hydrogen content) and unit configuration also play very important roles, as they can affect hydrogen transfer, but in this study we focus on the effect of matrix versus zeolite in the base catalyst on the composition of light and heavy gasoline fractions.
Hydrogen transfer in FCC
FCC catalyst composition is tailored to handle specific feeds within defined constraints in the FCC unit. The main components that determine catalyst activity are: zeolite-Y, rare earth stabilisation of zeolite-Y, and active matrix. These are adjusted to provide a balance between required activity and product selectivities. Hydrogen transfer reactions in FCC are bimolecular reactions in which naphthenes donate hydrogen to olefins and become aromatics, while the olefins are converted into paraffins via addition of hydrogen across their double bonds. These hydrogen transfer reactions mainly take place within the zeolite component (Faujasite/zeolite-Y) of the catalyst. The supercage in zeolite-Y is an ideal place to bring molecules together, and the higher the framework alumina content (the higher the active site density) the more hydrogen transfer reactions take place. Rare earth stabilisation of zeolite-Y further increases the active site density by hindering framework dealumination, and therefore higher rare earth on zeolite increases the rate of hydrogen transfer reactions. Active matrix and ZSM-5 have low acid site density and do not contribute significantly to hydrogen transfer. Increasing hydrogen transfer leads to increased gasoline selectivity and lower LPG selectivity. The gasoline range paraffins formed via hydrogen transfer are more stable than their olefin precursors. This reduces the rate of gasoline cracking, thereby increasing gasoline selectivity. When the activity of an FCC catalyst mainly depends on incorporating high concentrations of rare earth stabilised zeolite-Y, the catalyst will exhibit greater hydrogen transfer than catalysts that contain low zeolite and high active matrix concentrations.
The role of ZSM-5 on LPG and gasoline range olefins
Gasoline range olefins formed in the FCC riser are the key precursors for propylene production. Gasoline range olefins are formed via cracking reactions (beta-scission) of larger molecules and recombination (oligomerisation) of LPG range olefins (see Figure 1). At active sites in zeolite-Y gasoline range olefins are converted to other species via two main mechanisms: cracking to form LPG range olefins; and hydrogen transfer to form more stable paraffins. Not all gasoline range olefins are equally crackable, and susceptibility to hydrogen transfer differs too. A higher degree of branching and higher carbon numbers make gasoline range olefins easier to crack. However, this is not the case in the more diffusionally hindered micropore structure of ZSM-5 where straight-chain (normal) gasoline range olefins are most readily cracked and recombined. In ZSM-5 the more highly branched isomers suffer from diffusion limitations while trying to enter the ZSM-5 structure and are therefore preferentially retained in the gasoline as unreacted olefins. Gasoline olefin composition is a complex and dynamic result of many reactions. In addition to olefin cracking and recombination reactions, ZSM-5 catalyses de-alkylation, trans-alkylation and isomerisation reactions.
Pilot plant study
An ACE pilot plant study was conducted to establish the influence of ZSM-5 additives on light olefin yields and gasoline composition. For this purpose, fresh FCC catalysts with rare earth levels of 1, 3 and 5 wt% were selected. All three catalysts contained high zeolite and high active matrix concentrations. The catalysts were then deactivated between 10 and 20 hours in 100% steam to match equilibrium catalyst activity, and blended with various levels of equilibrated SuperZ Excel (a leading commercially available ZSM-5 additive). ZSM-5 is hydrothermally more stable under laboratory deactivation conditions than the rare earth exchanged ultrastabilised zeolite-Y (REUSY) containing FCC base catalysts, therefore it requires a more severe deactivation to be properly equilibrated. Deactivation conditions recommended for ZSM-5 additives are 815°C for 20 hours in 100% steam, a protocol that would be overly destructive for the zeolite-Y present in regular FCC base catalysts. A fourth catalyst was included in the study to evaluate the impact of active matrix. This catalyst contained 3% rare earth, but had about half the active matrix content of the above three catalysts.
The study was carried out using a VGO feed at a cracking temperature of 1060°F (570°C) to mimic petrochemical FCC operating conditions (equivalent to a riser outlet temperature of about 550°C).
Effects of ZSM-5 and conversion on gasoline composition
Small amounts of ZSM-5 additive added to the base catalyst (say 5 wt%) results in a slight increase in gasoline aromatics (calculated on wt% feed basis)1,2, whereas higher amounts of ZSM-5 additive (say 25%) cause gasoline aromatics (calculated on feed basis) to decrease slightly. This effect is strongly dependent on the hydrogen transfer activity of the catalyst system. High levels of ZSM-5 additive essentially ‘dilute’ the acid site contributions of the base catalyst that contains REUSY thereby decreasing aromatics formed via the hydrogen transfer mechanism.
What is the effect of conversion? Figure 2 shows the effects of increasing conversion from 65% to 71% on gasoline composition. It is well known that higher conversion increases gasoline octanes. Figure 2 clearly shows that an increase in conversion increases gasoline aromatic cores and isoparaffins while gasoline olefins decrease slightly and naphthenes are relatively unaffected. (Please note, this data is obtained close to the point of gasoline over-cracking for this feed and catalyst.)
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