KNOW MORE ABOUT FCC CATALYSTS

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Introduction

Fluidized catalytic cracking (FCC) technology was commercialized in 1942.One of the most important conversion processes in all modern refineries.Converts heavier, lower valued products from the refining of crude oil to high octane number gasolinethrough the process of catalytic cracking.

Schematic diagram of a typical FCC Unit

Source:www.Lummus.CBI.com

FCC Catalysts

Cracking requires the use of acid catalysts.The earliest FCC catalysts were acid-leached clays and artificial or natural silica-alumina catalysts.Since the 1960s FCC catalysts are exclusively based on Zeolites.The first artificial zeolite was synthesized and commercialized by Union Carbide Co. in 1954.The first commercial zeolitic cracking catalyst was developed by Mobil Oil Co.

Major Components of FCC Catalysts

Zeolites

Catalyst Matrix

Fillers and Binders

Zeolites

Zeolites are highly crystalline, microporous alumino-silicates with a negatively charged macromolecular inorganic framework. Pores accommodate a variety of cations which are easily exchanged by H+

ions.Characterized by uniformity of lattice structure and pore sizes (~ 8 Ao), high specific surface area available.

Various artificial zeolites

Typical Structure of a zeolite

Source: www.himfr.com

Zeolite Chemistry and Structure♦ More than 150 artificial and 40 naturally occurring

varieties.♦ The general chemical formula for zeolites is

Mex/z [(AlO2) x (SiO2) y]. nH2O.♦ The basic building blocks are silica and alumina

tetrahedra.♦ Both Bronsted and Lewis acid sites are present in the

structure,depending on temperature.♦ The nomenclature of zeolites follows the convention set

by the Structure Commission of the International Zeolite Association (IZA).

♦ Typical zeolites used as FCC catalysts are Zeolite X, Zeolite Y, ZSM-5 and rare-earth exchanged zeolites.

Catalytic Activity of Zeolites

The catalytic activity of zeolites stems from the following main facts:

� Thermal stability- Zeolites are stable at even 650 0C, so they are suitable for FCC units. They also remain stable over the numerous deactivation-regeneration cycles.

� Acidity- The density of the acid sites can be controlled by modifying the silica-to-alumina ratio.

�Shape selectivity-Extreme regularity of pore structure and usual pore sizes in zeolites enable shape selectivity to be achieved.

Zeolite Synthesis• Digestion of a mixture of

silica, alumina and caustic soda is digested for 10 hours or more at prescribed temperatures till the appearance of crystallites

• Spray drying of slurry to obtain zeolites

• Exchange of mobile Na+

ions by rare earth elements (Ce,La)

• Steam calcination of the zeolite at a temperature of ~800oC Source: Fluid catalytic Cracking Handbook,

Reza Sadeghbeigi, 2nd Edition

Flowsheet for the synthesis of Zeolites

Role of the Catalyst Matrix

• Matrix generally refers to that part of the catalyst other than the zeolites.

• May or may not possess catalytic activity.• Consist of amorphous silica-alumina gels or silica-magnesia

gels and acid treated clays like kaolinite and montmorrilonite.• Performs various important functions like:

�Catalyst support�Binding agent for micro-spheroidal catalyst particles� Diluting medium�Heat transfer medium

• Matrix activity helps in the pre-cracking of the heaviest molecules in feedstock.

Major physical properties of FCC catalysts

• Attrition Resistance• Pore Size Distribution and Pore Volume• Crystallinity • High Surface Area • Hydrothermal Stability• Particle Size Distribution

The performance of the equilibrium FCC catalysts under different conditions are evaluated from the Microactivity test (MAT).

Typical XRD Pattern exhibited by a Zeolite

Source: Feng H et. al., Applied Clay Science 42 (2009) pg. 442

FCC Catalyst Composition and Properties

• Effect of zeolites content

• Effect of rare earth exchanged zeolites

• Effect of various additives � CO Promoter� SOx Additive � ZSM-5� Metal Passivators

Computer generated image of the pore structure of ZSM-5

Source: www.3dchem.com

Correlation between zeolite unit cell size and structural Si/Al ratio

Source: Octane-enhancing, zeolitic FCC catalysts: scientific and technical aspects, pg 116 by Julius Scherzer

Effect of zeolite unit cell size on gas oil cracking selectivity

Source: Octane-enhancing, zeolitic FCC catalysts: scientific and technical aspects, pg 126 by Julius Scherzer

Catalyst Management in FCC Units

� Involves the following aspects:� Minimizing catalyst losses by improving recovery� Replacement policy for spent catalyst� Ensuring proper catalyst circulation between reactor and regenerator� Stripping of hydrocarbons from the catalyst before regeneration

� This is necessary because of the following reasons:� Regeneration is never 100% efficient � With time attrition of the catalyst particles takes place, leading to the

production of fines which are lost through the cyclones� Hydrocarbons have to be removed from the catalyst as far as possible

before the regeneration step by steam stripping so that coke formation is minimized in subsequent cycles

Recent Developments in FCC Catalysts

• Modelling of FCC Catalyst Pore StructuresPores in zeolites are of regular shape and size, hence attempts are being made to obtain mathematical descriptions of the pore structure.

Simple parallel bundle pore model for supported zeolite A 10 x 10 x 10 3-D stochastic pore network.

Source: R Mann, Catalysis Today 18 (1993) pg. 516 and 518

Recent Developments…..• Special catalysts for short contact time applicationsEngelhard Corporation has developed and commercialized the NapthaMax™ catalyst which enables catalytic cracking of even resids within 1 to 3 seconds.

The patented matrix technology is called Distributed Matrix Structure (DMS). The zeolites are highly dispersed over the matrix and hence even the pre-cracking takes place on the zeolite surfaces. This allows for better selectivity of the catalyst and eliminates the need for a secondary diffusion step to reach the active sites.

DMS Matrix

Conventional Zeolite

Source: Mc Lean J B and Stockwell D M, NaphthaMaxTM Breakthrough FCC Catalyst Technology for Short Contact Time

Applications, NPRA 2001 Annual Meeting, New Orleans

Recent Developments…..

• Special Purpose FCC CatalystsTo maximize the production of specific hydrocarbons like the iso-olefins through the use of proprietary catalyst formulations like IsoPlus™ catalyst. These hydrocarbons are required for production of MTBE or TAME for reformulated gasoline.

The graphic shows the relative Increase in the yield

of Iso-butylene with the use of IsoPlus catalysts in FCC

Source: Mc Lean J B and Witoshkin A, Isoolefins for Oxygenate

Production using IsoPlus™ , NPRA 1993 Annual Meeting, Texas

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