Value maximisation in FCC units using multispecialty catalyst formulation
Increasing propylene yields while reducing bottoms production.
Somanath Kukade and Pramod Kumar
Hindustan Petroleum Corporation Limited
Viewed : 1854
Fluid catalytic cracking (FCC) is one of the most important conversion processes used in petroleum refineries and has existed for over 75 years. It is widely used to upgrade heavier cuts like vacuum gas oil and residues to more valuable petroleum products such as gasoline and light olefins. It can readily adjust to changes in feed quality through modifications in catalyst and operating conditions.
Catalysts and additives play important roles with respect to activity and selectivity in FCC units. These units typically produce around 4-6 wt% propylene but can go as high as 12%, depending on feedstock type, operating conditions, such as riser outlet temperature, reactor pressure, catalyst-to-oil ratio, and type of FCC catalysts/additives.
A traditional steam naphtha cracker supplies about 57% of global propylene as a by-product of ethylene production. The FCC unit is also an important source of propylene, producing about 35% of world propylene as a by-product of gasoline production. The remaining 8% is produced by ‘on-purpose’ processes, such as propane dehydrogenation, olefin metathesis, and methanol-to-propylene.
However, the shift from naphtha crackers to ethane crackers has seen the gap for propylene increase. Most new steam crackers coming online are designed to use ethane as the primary feedstock, typically producing less than 2% of propylene compared to ethylene production. Propylene demand has increased at an average rate of nearly 4-5% per year.
In 2020-21, the propylene growth rate in India stood at 4% CAGR, and polypropylene (PP) at 4% CAGR. Enhancement of the propylene yield from the FCC, from a maximum of 12 wt% to 20 wt%, is one way to meet the growing demand for propylene. Refiners are integrating with petrochemical complexes, so petrochemical rates increase from the current 7% average to 20%.
Propylene is perhaps the most versatile building block in the petrochemical industry in terms of its variety of end-use products and many production sources. High demand for PP has been a major driver for the rapid expansion in propylene production processes, and many PP units are added by refineries. Worldwide, approximately two-thirds of propylene is used to make PP.
The onus on maximising propylene yields while reducing bottoms warrants a discussion on novel multispecialty catalysts,* such as those formulated and patented by HP Green R&D Centre (HPGRDC). The catalyst formulation is matrix-based and acts as an additive in conventional FCC units and a standalone catalyst in high olefin FCC units, such as deep catalytic cracking (DCC) units for light olefins maximisation.
FCC catalyst design
The FCC catalyst is in the form of a powder in Geldart's group A classification of fluidisation and has a particle size of approximately 80 Î¼m. The important FCC catalyst design parameters are shown in Figure 1. Activity, selectivity, and accessibility convert large feedstock molecules to desired molecules. Attrition resistance is the ability to withstand the impacts of particle-particle and particle-wall collisions during circulation. Hydrothermal stability is the ability to withstand temperature and steam deactivation in the regenerator. Metals tolerance is the ability to withstand the effects of Ni, V, Fe, and Na on the feedstock. Coke selectivity is the ability to give minimum delta coke and should be fluidisable (Geldart A particle). Binder serves as a glue to hold the zeolites (key ingredient for cracking), matrix, and filler (clay) together by giving them sufficient binding strength. Both clay and binder provide critical FCC parameters, such as density, attrition resistance, and particle size distribution.
Catalyst design for light olefins
Catalysts and additives play a vital role in FCC for enhancing light olefins. The proprietary tailor-made catalyst* system has cracking functionality to crack feed molecules to gasoline by use of hierarchical macro-mesoporous and micro-porous functions and increase light olefins by modified shape-selective pentasil zeolite. There is an upgradation of larger molecules by physical transport in macropores (Lewis acid sites) and primary cracking mesoporous sites (medium acid sites) of alumina, which is surface-modified to change the strength of the acid sites. The upgraded molecule diffuses into zeolite pores yielding gasoline, which further cracks to light olefins in the presence of modified shape-selective ZSM-5 additive incorporated in the catalyst* formulation.
The large molecules in the feed prefer to be precracked on the alumina surface. The feed molecules are 370+ boiling range consisting of saturates (C14-C34) and heavy aromatics (C14-C60) in the ranges of 40-60% and 35-45%, with a pore diameter of 12-20 Ao and 12-30 Ao. These hydrocarbon molecules are too large to fit into the zeolite pores. The macropores provide a free path for these molecules to transport and crack on mesopores of active alumina with a pore size of 12-100 Ao. The upgraded molecules viz: LCO range come in contact with Y-zeolite pores with a pore size of 7-8 Ao and convert to gasoline range molecules using strong acid sites, and the gasoline range olefins are converted to light olefins (LPG olefins) through modified shape-selective ZSM-5 with a pore size of 5-6 Ao. The catalyst must have the proper pore size distribution to enable large feed molecules to enter, crack into lighter products, and diffuse out before being over-cracked to coke and gas. Therefore, it is essential to design a catalyst with optimal porosity for effective kinetic conversion. The catalyst architecture and sequential cracking are shown in Figures 2 and 3. The modified ZSM-5 of the formulation incorporates a metal function to increase light olefins.
The catalytic cracking of alkanes occurs via bimolecular and monomolecular reaction mechanisms. If the monomolecular mechanism is dominant, the yield of light olefins (such as ethylene and propylene) is more. Bimolecular reactions are hydrogen transfer reactions, which will saturate the olefins. Hydrogen transfer in FCC is a well-known phenomenon and reduces the gasoline range olefins. The cracking rates of gasoline olefins on ZSM-5 are higher than those of paraffins, so an increase in hydrogen transfer reduces the effectiveness of ZSM-5 additives. As explained, the synergistic alumina and Y-zeolite cracking of feed molecules will provide maximum activity and higher gasoline range olefins for cracking on modified ZSM-5. The ratio of monomolecular to bimolecular for the catalyst* formulation is higher, indicating monomolecular reactions are the dominant catalyst* formulation. The paraffin-to-olefin ratio, a measure of hydrogen transfer reactions, is almost 50% less in the catalyst* formulation, indicating the catalyst design is selective towards light olefins.
Experimental details - Catalyst synthesis
Synthesis of the proprietary catalyst* formulation included RE exchange with USY, surface modification of alumina to change the strength of the acid site, and modification of the ZSM-5 zeolite. The slurry containing various ingredients was spray dried, as shown in Figure 4, to produce the micro-spheroidal catalyst with the typical properties given in Table 1. Slurry solid loading varies from 20-30%. The formed body is calcined at 500-600oC for 2-4 hours.
The catalytic cracking experiments were carried out in a custom-designed FCC pilot unit, as shown in Figure 5. The gaseous products were analysed by Micro GC, and liquid products were analysed in LT SimDis. Conversion was obtained by the sum of yields of dry gas, LPG, gasoline, and coke. Hydrotreated vacuum gas oil (VGO) was used as feedstock with a density of 0.9 g/cc, <500 ppm sulphur, and less than 0.1 wt% CCR. Using this feed, the tailor-made catalyst* formulation was subjected to catalytic cracking at temperatures of 550-560°C. The pilot results showed a C3= yield of 18.96% with gasoline RON >95 and olefin content >55%.
The catalyst* formulation also acts as an additive in conventional FCC units. Refinery trials were taken at three FCC units (see Figure 6) to assess the performance of the catalyst as an additive and commercial demonstration as a catalyst in one of India’s DCC units.
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