Page 166 - Fluid Catalytic Cracking Handbook

140 Fluid Catalytic Cracking Handbook

In the pressure balance section, the significance of the pressure balance
in debottlenecking the unit is discussed. Finally, fundamentals of both
"basic" and "advanced" process controls are presented.
This chapter presents the entire procedure for performing heat and
weight balances. The last section of the chapter discusses the use of
the distributed control system and computer in automating the process,

MATERIAL BALANCE

Complete data collection should be carried out weekly. Since changes
in the unit are continuous, regular surveys permit distinction among
the effects of feedstock, catalyst, and operating conditions. An accurate
assessment of a cat cracker operation requires reliable plant data. A
reasonable weight balance should have a 98% to 102% closure.
In any weight balance exercise, the first step is to identify the input
and output streams. This is usually done by drawing an envelope(s)
around the input and output streams. Two examples of such envelopes
are shown in Figure 5-1.
One of the key pieces of data is the composition of products leaving
the reactor. The reactor effluent vapor entering the main fractionator
contains hydrocarbons, steam, and inert gases. By weight, the hydro-
carbons in the reactor overhead stream are equal to the fresh feed plus
recycle minus the portion of the feed that has been converted to coke.
If the feed can contain water, it should be analyzed for and corrected.
The sources of steam in the reactor vapor are: lift steam to the
standpipe, atomization steam to the feed nozzles, dome steam, and
stripping steam. Some units may have other streams and the feed may
contain water. Depending on the reactor pressure, approximately 25%
to 50% of the stripping steam is entrained with the spent catalyst
flowing to the regenerator, which should be deducted.
Inert gases such as nitrogen and carbon dioxide enter the riser
entrained with the regenerated catalyst. The quantity of these inert
gasses is directly related to catalyst circulation rate. These gases flow
through the gas plant and leave the unit with the off-gas from the
sponge oil absorber column. They are not significant for the weight
balance, but they are usually the only source of inerts in the off-gas
and should be deducted.
FCC products are commonly reported, on an inert-free basis, as the
volume and weight fractions of the fresh feed. In a rigorous weight