October 2020

Catalysts

Maximize FCC product yields with a specialty catalyst formulation

Fluid catalytic cracking (FCC) is one of the most important conversion processes used in petroleum refineries and has been in use for more than 75 yr.

Kukade, S., Kumar, P., Ramachandrarao, B., Hindustan Petroleum Green R&D Center, Hindustan Petroleum

Fluid catalytic cracking (FCC) is one of the most important conversion processes used in petroleum refineries and has been in use for more than 75 yr. It is widely used to upgrade heavier cuts like vacuum gas oil and residues to more valuable petroleum products like gasoline and light olefins, and is considered to be the “workhorse” of the refinery.

The FCC process can readily adjust to changes in feed quality through modifications of catalysts and operating conditions. FCC can operate in LPG, gasoline and middle distillate mode, depending on seasonal demand. Catalysts and additives play important roles with respect to activity and selectivity for switching between these modes. FCC units typically produce around 3 wt%–5 wt% propylene but can go as high as 12 wt%, depending on feedstock type, operating conditions (such as riser outlet temperature, reactor pressure and catalyst-to-oil ratio) and the type of the FCC catalysts/additives. Usage of both ZSM-5 additive and increased operation severity increases the light olefins yield from the FCC unit at the expense of gasoline.

The major source of both ethylene and propylene is the traditional steam naphtha cracker that supplies approximately 57% of global propylene as a byproduct to ethylene production. The FCC unit is also an important source of propylene, producing about 35% of world propylene as a byproduct to gasoline production. The remaining 8% of world propylene is produced by “on-purpose” processes, such as propane dehydrogenation, olefin metathesis and methanol-to-propylene.

Due to a recent shift in naphtha crackers to ethane crackers, the gap for propylene has been growing. Most of the new steam crackers coming online are designed to use ethane as the primary feedstock. Ethane typically produces less than 2% of propylene, compared to ethylene production. Propylene demand has increased at an average rate of nearly 5%/yr. In India, the growth rate for propylene stood at 4.64% compound annual growth rate (CAGR), and polypropylene (PP) was at 7.3% CAGR for 2018–2019. Enhancing the propylene yield from FCC from a maximum of 12 wt% to 20 wt% is one of the options to meet growing demand for propylene. Refinery-petrochemical integration is another.

Propylene is perhaps the most versatile building block in the petrochemical industry in terms of its variety of end-use products and its multitude of production sources. High demand for PP has been a major driver for the rapid expansion in propylene production processes, and many PP units have been added at refineries. Worldwide, approximately 65% of propylene is used to make PP.

A novel catalyst formulationa has been patented for maximizing propylene yields. The catalyst system is matrix-based and acts as an additive in conventional FCC units. The system can be used in a high-severity FCC unit, in a deep catalytic cracking (DCC) unit and for the maximization of light olefins. Laboratory studies and field trials to assess the performance of the catalyst were carried out at Hindustan Petroleum Corp. Ltd.’s Mumbai and Visakh refineries’ FCC units at 10% and 15% inventory changeover. Catalyst performance is discussed in the following sections.

FCC catalyst components

The FCC catalysta is in the form of a powder in the Geldart A classification of fluidization, and has a particle size of approximately 80 µm. Typically, FCC catalysts consist of USY zeolite/ZSM-5 zeolite, a matrix, a binder and a filler. The functions of each component are defined as follows:

  • The matrix is in the active form and has catalytic activity, which provides a path for the transport of large hydrocarbon molecules and pre-cracks them to LCO-range molecules. Alumina is the source of active matrix that is incorporated in the catalyst formulation.
  • Zeolite is a porous, crystalline, acidic component. It is the key ingredient in catalyst that provides activity and is in the pore diameter of 7 A°–8 A°. It cracks LCO-range molecules to gasoline.
  • The filler in the catalyst is kaolin clay, used to dilute the activity of the catalyst and as a heat sink.
  • The binder serves as a glue to hold the zeolite, matrix and filler together by giving sufficient binding strength.

Both the clay and the binder provide required critical FCC parameters, such as density, attrition resistance, particle size distribution and heat transfer medium to carry the heat for endothermic catalytic reactions.

Catalyst design for light olefins

Catalysts and additives play vital roles in FCC for enhancing light olefins. The proprietary, tailored catalyst systema has cracking functionality to crack feed molecules to gasoline through the use of macroporous and mesoporous functions; it can also increase light olefins with its modified, shape-selective pentasil zeolite.

Typically, FCC catalysts have a zeolite-to-matrix (Z:M) ratio of 3 to 5. However, the catalyst has a Z:M ratio of 1 to 2. Upgrading of large molecules takes place by physical transport in the macrospores (Lewis acid sites) and primary cracking mesoporous sites (medium acid sites) of the matrix, which is surface-modified to change the strength of the acid sites. The upgraded molecule diffuses into zeolite pores to give gasoline, which further cracks into light olefins in the presence of the modified, shape-selective ZSM-5 additive incorporated into the catalyst formulation.

The large molecules in the feed are first precracked on the matrix surface. The matrix catalytic function increases the catalyst activity and selectivity toward light olefins. The interactions between the matrix and the zeolite exhibit the catalytic advantages of the zeolite component and also retain the matrix-precracking ability in the proprietary catalyst formulation. These interactions enhance the catalyst activity and improve the product distribution and selectivity.

The feed molecules are 370-plus boiling range, consisting of saturates (C14–C34) and heavy aromatics (C14–C60) in the respective ranges of 40%–60% and 35%–45%, and having respective pore diameters of 12 A°–20 A° and 12 A°–30 A°. These hydrocarbon molecules are too large to fit into the zeolite pores. The macropores provide free pathways to these molecules to transport and crack on the mesopores of the active matrix with a pore size of 12 A°–100 A°. The upgraded, LCO-range molecules come into contact with Y zeolite pores with a pore size of 7 A°–8 A° and convert to gasoline-range molecules, using strong acid sites. The gasoline-range olefins are converted to light olefins (LPG range) through modified, shape-selective ZSM-5 with a pore size of 5 A°–6 A°. It is important that the catalyst 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 sequential cracking is depicted in FIG. 1. Typical properties are given in TABLE 1.

FIG. 1. Sequential synergistic cracking of hydrocarbon feed to LPG olefins using proprietary catalyst.a

Cracking experiments

The catalytic cracking experiments were carried out in a fixed-fluid-bed micro-reactor unit. The liquid product cuts considered were gasoline (C5 at 221°C), LCO (221°C–370°C) and bottoms (370°C and higher). Conversion was obtained by the sum of the yields of dry gas, LPG, gasoline and coke. Hydrotreated vacuum gas oil was used as feedstock (density of 0.9 g/cc, sulfur of < 500 ppm and CCR of less than 0.1 wt%). Using this feed, the catalyst formulationa was subjected to catalytic cracking at temperatures above 570°C. The product yield obtained is comparable to fresh, deactivated USY zeolite with equivalent ZSM-5 of 20 wt%. The results are depicted in FIG. 2.

FIG. 2. Cracking results at constant conversion of 85 wt%.

It is clear from FIG. 2 that the proprietary catalyst formulation is selective toward propylene (18.53 wt%) and butylene (13.87 wt%), compared to the base catalyst with equivalent ZSM-5. The proprietary catalyst gives approximately 1% less bottoms. The modified ZSM-5 of the catalyst has a metal function that provides dehydrogenation activity, thereby increasing the light olefins content. The catalytic cracking of alkanes occurs via bimolecular and monomolecular reaction mechanisms. Both mechanisms occur simultaneously and have a competitive relationship.

The yield of products obtained depends on which products are predominant in zeolites. If the monomolecular mechanism is dominant, then the yield of light olefins (e.g., ethylene and propylene) is greater. Bimolecular reactions are hydrogen transfer reactions, which will saturate the olefins. For example, naphthenes donate hydrogen to olefins and become aromatics, while the olefins are hydrogenated into paraffins and become undesirable.

Hydrogen transfer in FCC is a well-known phenomenon and reduces gasoline-range olefins. The cracking rates of gasoline-range olefins on ZSM-5 are higher than those of paraffins and, therefore, an increase in hydrogen transfer reduces the effectiveness of the ZSM-5 additives. As explained, the synergistic matrix and Y zeolite cracking of feed molecules will provide maximum activity and higher-range olefins for cracking on modified ZSM-5. The ratio of monomolecular to bimolecular for the proprietary catalyst formulation is 0.44, as compared to 0.31 for the base catalyst with equivalent ZSM-5, indicating that monomolecular reactions are dominant in the proprietary catalyst formulation. The paraffin-to-olefin ratio, which is a measure of hydrogen transfer, is almost 50% lower in the proprietary catalyst formulation, indicating that the design of the catalyst is selective toward light olefins.

Demonstration of proprietary catalyst

The catalyst formulation is matrix-based and acts as an additive in conventional FCC units. Refinery trials were conducted at the Mumbai and Visakh refineries’ FCC units to assess the performance of the catalyst as an additive.

Field trial at the Mumbai refinery. Trials were conducted at the Mumbai refinery’s FCC unit through a 10% inventory changeover in 1.5 mos. Unit throughput was 100 m3/hr with a riser outlet temperature of 490°C, feed density of 0.88 g/cc, sulfur of 0.3 wt%, Conradson carbon residue (CCR) of 0.11 wt%, and a unit catalyst-to-oil ratio of 5.7 wt/wt. The test run was conducted to determine the performance of the proprietary catalysta as an additive at a 10% concentration. The catalyst performance showed a 0.6 wt% increase in LPG, a 0.5 wt% increase in propylene, an increase in RON of 0.6 units and a reduction in bottoms of 0.2 wt%. Product yields are shown in FIG. 3.

FIG. 3. Product yields with 10% proprietary catalyst at the Mumbai refinery’s FCC unit.

Field trial at the Visakh refinery. Similar trials were conducted at the Visakh refinery’s FCC unit through a 15% inventory changeover in 1 month. The unit throughput was 130 m3/hr with a riser outlet temperature of 524°C, a feed density of 0.918 g/cc, sulfur of 1.6 wt%, a CCR of 0.62 wt% and a unit catalyst-to-oil ratio of 7.1 wt/wt. The test run was conducted to determine the performance of the proprietary catalysta as an additive at 15% concentration. The catalyst performance showed a 0.3 wt% decrease in dry gas, a 0.48 wt% increase in LPG, a 0.54 wt% increase in propylene, a 0.2 wt% increase in LCO and an increase in RON of 0.3 units, compared to the base case. Product yields are shown in FIG. 4.

FIG. 4. Product yields with 15% proprietary catalyst at the Visakh refinery’s FCC unit.

Monthly average yields before and after the addition of proprietary catalyst are shown in FIG. 5. The proprietary catalyst showed a 0.32 wt% decrease in dry gas, a 0.50 wt% increase in LPG, a 0.54 wt% increase in propylene, a 0.62 wt% increase in LCO and a reduction in bottoms of 1 wt%, compared to the base case.

FIG. 5. Monthly average product yields obtained at the Visakh refinery’s FCC unit.

In both refinery FCC unit field trials, the C3= selectivity for LPG increased by 2.5 vol%–5 vol% from a base value of 34 vol%–35 vol%. During the trial period, there was no observation on incremental catalyst loss from the FCC units due to the addition of the proprietary catalyst; therefore, the catalyst was determined to be compatible with the base catalyst.

Takeaway

Based on the lab study and field trials at the Mumbai and Visakh refineries’ FCC units, the proprietary catalysta can be used as an additive in existing FCC units to maximize product yields and selectivity and to increase RON. In a high-olefins FCC unit, the catalyst improves the propylene yield by 1.3 wt%. HP

NOTES

         a Hindustan Petroleum High Propylene ([HP]2) proprietary catalyst

The Authors

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