QUESTIONS AND ANSWERS

I.FLUID CATALYTIC CRACKING:

B.CATALYST AND ADDITIVES:

Q-30:

How do you maintain catalyst activity to optimize FCC Operation and Profitability ?

A-30:

                                Catalyst Management in FCC Units

 Introduction:

The zeolite  catalysts used in FCC is the heart of the whole operation. It is therefore necessary to understand the effects of the catalyst on the actual operation of the unit. Catalyst management basically involves recovery of the catalyst and minimizing losses, replacement of the spent catalyst from time to time, ensuring proper catalyst circulation between the reactor and regenerator and stripping hydrocarbons from it before regeneration. The importance of these operations is discussed in the following paragraphs

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 Addition of fresh catalyst:

Regeneration of catalyst, no matter how efficient cannot restore its activity to the level of fresh catalyst. Hence it is imperative to add new catalyst from time to time. It is necessary to maintain proper activity and selectivity and compensate for catalyst losses due to some inefficiency in the recovery cyclones. Also the metals deposited on the catalyst cannot be allowed to increase beyond some level for which some of the catalyst must be withdrawn and replaced by fresh catalyst periodically. This will also depend on the quality of the feedstock. The amount of catalyst added is equal to the amount lost and withdrawn. But sometimes low activity catalyst may be provided as replacement in order to prevent excessive activity which in turn leads to coking. Catalyst replacement is also necessary to ensure that the particle size distribution in the FCC unit is properly maintained, as the particles tend to be reduced to fines in the harsh operating conditions of the FCC. Replacement does not always ensure the same levels of activity as was prevailing before the addition of new catalyst. Generally if the cyclone separators are working properly about 80% replacement efficiency is obtained

 Spent catalyst stripping:

Before the catalyst enters the regenerator it must be freed of the hydrocarbons remaining on it. Generally steam is used as the stripping agent. The optimum steam rate is governed by the mechanical design of the riser, the catalyst circulation rate, the porosity and the surface area of the equilibrium catalyst. The stripping steam rate is optimize control the losses it is necessary to monitor the performance of the cyclones. The size distribution of the equilibrium catalyst as well as the fines needs to be considered. While smaller particles lead to easy fluidization, the requirements for preventing excessive catalyst losses are quite the opposite. Hence there are optimum size ranges for the catalyst particles. The cyclone performance can be affected if it has suffered some kind of mechanical damage like the development of perforations in the dip legs or erosion of the internal lining. Sudden surges in the  vapour  rates in the reactor unit could lead to entrainment of even smaller particles. Too high or too low catalyst levels in the disengagement section of the reactor can also lead to the malfunctioning of the cyclones, thereby leading to catalyst Losses.

Preventing  Excessive Coking Tendencies:

Coke formation is an important part of the FCC operation and the heat balance depends on the exothermic reactions involving the combustion of the coke in the regenerator. But excessive coking diminishes catalyst life and in general affects the working of the unit drastically. Coking tendencies depend on the nature of the feed especially its metal content. To minimize these problems it is necessary to avoid having cold spots in the system, minimizing of heat losses from the transfer lines connecting the reactor and regenerator and improvement of the feed-catalyst mixing system by introducing high efficiency feed nozzles