QUESTIONS AND ANSWERS

I.FLUID  CATALYTIC CRACKING:

B.CATALYST AND ADDITIVES

Q-27:Briefly summarize the Physical Characteristics of  FCC Catalysts ?

A_27:

 Major Physical Characteristics of FCC Catalysts

Introduction:

Several properties of zeolites have to be considered when selecting them as catalysts for FCC. Some of these properties govern the catalytic behaviour of the zeolites while some others affect the mechanical design and operation of the FCC unit itself . These properties are briefly discussed below:

Attrition Resistance:

During the operation of the FCC unit the high gas flow rates and the high temperature often cause the attrition of the catalyst particles and produce fines. Not all of the fines may be recovered by the cyclone separators before the products can be separated. This leads to catalyst losses and causes emission of  particulate matter into the environment . It also governs the rate at which fresh catalyst make-up must be provided to the unit. As the particle size distribution is affected by this, the conditions of fluidization itself might change.An increase in the zeolite content of the catalyst, reduction of the zeolite crystal size and better dispersion of the zeolite within the matrix leads to improved attrition resistance. The nature and quantity of the binder provided to the catalyst also affects the attrition resistance. For example the boehmite form of alumina is more effective in reducing attrition than gibbsite or bayerite. The type, particle size and themorphology of the clay used as filler has a role to play in the attrition of the catalyst. Kaolin has been found to increase the attrition resistance. The conditions maintained during spray drying of the catalyst affects its future attrition resistance.Catalyst attrition resistance is indicated by the value of the attrition index which is obtained from standard methods.

Pore Size Distribution and Pore Volumes:

The pore size distribution has a major role to play in the catalyst properties of zeolites. If the pores are too small, then they have a greater tendency to get clogged by coke and they also exhibit greater diffusional resistance. If the pores are too large they provide a lower surface area for a given volume and hence lower the efficacy of the catalyst and lead to enhanced attrition. The pore size distribution is also an important parameter for the catalyst matrix as well. The shape of the pores is the most important property governing the shape selectivity of the catalyst which is the most important characteristic used in many applications.

Surface Area

The surface area of the catalyst comes from the zeolite and also the matrix. It ranges from over 800Surface Area/gm for conventional Y zeolites and is around 600 m2/gm for USY zeolites. The reduction in surface area is due to the hydrothermal treatment given to the zeolites to enhance their stability which affects their crystallinity and brings about changes in their pore structure. During the steam stripping step carried out before regeneration of these catalysts the pore structure might collapse and thereby there is loss of surface area. Catalysts in which the matrix is also active generally have higher surface areas

Particle Size Distribution:

Most of the FCC catalysts have particle size ranging from 60 to 80 µm. The actual size distribution depends heavily on the conditions prevailing in the spray drying step of catalyst manufacture. Fines generated during the fluidized catalytic cracking often change the size distribution of the catalyst. This might call for changes in the gas velocities to enhance good fluidization in the riser section and might also make it necessary to make the operation of the cyclone separators somewhat flexible. Particulate emission might also result from the fines.

Thermal and hydrothermal stability

The chemical nature of the zeolites, especially the silica-to-alumina ratio, crystallinity, ion-exchangeand residual sodium ions in the zeolite affect its stability. Maintaining the catalyst stability is essential tomaintaining its activity and selectivity. Rare earth exchanged zeolites are exceptionally stable. Stable catalysts retain their pore structure during regeneration also.

Crystallinity

The crystalline nature of the FCC catalysts as determined from X-ray diffraction studies is an indication of the zeolite content. The X-ray studies of the fresh catalyst and the equilibrium catalyst can provide information about structural loss and the catalyst stability

Microactivity Test (MAT) for FCC Catalysts:

The standard Microactivity test of the FCC catalysts is a valuable tool to evaluate the properties of equilibrium catalysts from the FCC unit . Since the operation of FCC units are cyclic the catalysts undergo major changes in their properties from their fresh state and all these evaluations are carried out after they have stabilized i.e. they have become equilibrium catalysts. At the heart of this test lies a fixed bed reactor into which the hot gas oil sample is injected. The catalyst activity is reported as the conversion of the 221C material. The conversion depends on the catalyst-to-oil ratio, feed space velocity and other factors. The coke forming and gas forming tendencies of the catalyst as well as the above mentioned physico-chemical properties of the catalysts cam be obtained from this useful test. It helps in ascertaining the conditions to which the catalyst has been exposed. This test is simple and not much time consuming.