QUESTIONS AND ANSWERS

I.FLUID CATALYTIC CRACKING:

B.CATALYST AND ADDITIVES:

Q-28:How the composition of FCC Catalyst influence its Properties ?

A-28:

Effects of FCC Catalyst Composition on its Properties

Introduction:

A wide variety of zeolite catalysts, having a range of useful properties are being used in different FCC units round the world. These catalysts are tailor-made for specific purposes like selective cracking of the feed stock to produce more olefins or more gasoline as per the requirements. Several catalysts have additives like ZSM-5 mixed with them to enhance the octane number of the gasoline produced or to control the emission of SOx, NOx or CO from the regenerator. In general, as the yield of gasoline increases its octane number decreases, and  this  makes it necessary to add more expensive octane boosting agents to it. As conversion of the feed stock rises, the yields of both gasoline and LCO pass through a maximum and then decrease. But coke formation and gas production show a rising trend  as conversion rises.

Effect of Zeolite Content of the Catalyst:

An increase in the zeolite content of the catalyst leads to an increase in the catalytic activity as expected. For a given conversion, as zeolite input increases the gasoline and LCO yields also increase while hydrogen, C3-C4 olefins and coke yields decrease. The decrease in the unit cell size of the catalyst arising out of the increase in the silica-to-alumina ratio leads to decrease of catalytic activity. Though the USY catalysts are less active compared to conventional  REY zeolites, they retain their activity under more severe operating conditions. For a higher silica-to-alumina ratio there is greater production of LPG fractions, lower formation of coke and lower gasoline selectivity . The unit cell size thus has a major role to play in dictating the selectivity exhibited by the zeolite. Apart from this the decrease in the unit cell size of the zeolite results in an increase in the RON and MON values of the gasoline. This is predominant at very low pore sizes in the region of highly acidic sites. The presence of Na+ ions in the de-aluminated  zeolite framework inhibits the formation of high octane gasoline. This is probably due to the neutralization of the acid sites by the residual free Na ions. Even the preparation method of the zeolites has been found to have an effect on the catalytic activity and hence the final product distribution from the FCC unit.

Effect of Additives:

Several kinds of additives are often added to the zeolite catalysts used in FCC units in order to change the product distribution or to comply with increasingly stringent environmental regulations. Some of these are discussed here.

ZSM- 5

This is a special kind of high silica zeolite which is a member of the pentasil  family. and it is

widely used as an octane boosting additive in FCC. It may be incorpoThis is a special kind of high silica zeolite which is a member of the pentasil  family. It is rated into the catalyst during the preparation or it may be added as separate particles. About 1-3 % of ZSM-5 added to the FCC catalyst can raise the RON of the gasoline produced by up to 3 units. The pore geometry of this additive is such that it prevents the formation of cyclic compounds which lead to subsequent coke formation.

CO Promoter:

This additive consists of metals from the platinum group, present in the concentration of 300 to 800 ppm and distributed over a support, and its use is to facilitate the conversion of CO produced during catalyst regeneration to CO2. Though more uniform burning of the coke is accomplished the additive tends to raise the temperatures during regeneration and also the production of  NOx. Typically 1 to 2.3 Kg of the promoter is added per ton of the fresh catalyst

SOx Additive:

The coke deposited on the FCC catalyst contains sulphur in the form of organic compounds and during regeneration it forms SO2 and SO3. These acidic gases are finally discharged into the atmosphere along with the flue gases. Keeping in mind the harmful environmental effects of the Sox it has become necessary to control their emission from the FCC units. The additive is usually a metal oxide which is directly mixed with the catalyst and it absorbs and bonds chemically with the SO3 in the regenerator. This stable sulphate species is carried to the riser section where it is reduced to H2S and the metal oxide. An excess of oxygen is needed in the regenerator so that the additive may provide the full benefits.

Metal  Passivators:

Metals like Nickel, Vanadium and Sodium are often present in the feed to the FCC unit and they have a detrimental effect on the FCC catalyst as they poison the active sites. To prevent  this metal passivators like Antimony are used. The passivators are injected into the feed and they form an alloy with the nickel. This greatly reduces the dehydrogenation reactions and the resultant production of dry gas and hydrogen.