FCC process for de-gassing spent catalyst boundary layer

A fluidized catalyst contacting apparatus improves the recovery of entrained hydrocarbon gases by providing a de-gassing zone upstream of a conventional stripping zone. The de-gassing zone has a downwardly increasing catalyst density gradient that reduces the void volume of the fluidized catalyst thereby de-gassing hydrocarbon vapors from the catalyst prior to entering a stripping zone. The de-gassing zone is particularly useful in a vented riser arrangement for an FCC reactor where catalyst concentrates along the wall of the reactor vessle as it flows downwardly into the stripping zone. By providing a de-gassing zone to collect the downwardly descending catalyst and remove hydrocarbon vapors, efficiency of a sub-adjacent stripping zone is significantly improved.

FIELD OF THE INVENTION 
This invention relates broadly to hydrocarbon conversion processes and 
apparatus. More specifically, the invention relates to fluidized catalytic 
cracking (FCC) reactors and the recovery of product vapors. 
BACKGROUND INFORMATION 
Fluidized bed catalytic cracking (commonly referred to as FCC) processes 
were developed during the 1940's to increase the quantity of naphtha 
boiling range hydrocarbons which could be obtained from crude oil. 
Fluidized catalytic cracking processes are now in widespread commercial 
use in petroleum refineries to produce lighter boiling point hydrocarbons 
from heavier feedstocks such as atmospheric reduced crudes or vacuum gas 
oils. Such processes are utilized to reduce the average molecular weight 
of various petroleum-derived feed streams and thereby produce lighter 
products, which have a higher monetary value than heavy fractions. Though 
the feed to an FCC process is usually a petroleum-derived material, 
liquids derived from tar sands, oil shale or coal liquefaction may be 
charged to an FCC process. Today, FCC processes are also used for the 
cracking of heavy oil and reduced crudes. Although these processes are 
often used as reduced crude conversion, use of the term FCC in this 
description applies to heavy oil cracking processes as well. 
The operation of the FCC process is well known to those acquainted with 
processes for upgrading hydrocarbon feedstocks. Differing designs of FCC 
units may be seen in the articles at page 102 of the May 15, 1972 edition 
and at page 65 of the Oct. 8, 1973 edition of "The Oil & Gas Journal". 
Other examples of FCC processes can be found in U.S. Pat. Nos. 4,364,905 
(Fahrig et al); 4,051,013 (Strother); 3,894,932 (Owen); and 4,419,221 
(Castagnos, Jr. et al) and the other FCC patent references discussed 
herein. 
A majority of the hydrocarbon vapors that contact the catalyst in the 
reaction zone are separated from the solid particles by ballistic and/or 
centrifugal separation methods. However, the catalyst particles employed 
in an FCC process have a large surface area, which is due to a great 
multitude of pores located in the particles. As a result, the catalytic 
materials retain hydrocarbons within their pores and upon the external 
surface of the catalyst. Although the quantity of hydrocarbon retained on 
each individual catalyst particle is very small, the large amount of 
catalyst and the high catalyst circulation rate which is typically used in 
a modern FCC process results in a significant quantity of hydrocarbons 
being withdrawn from the reaction zone with the catalyst. 
Therefore, it is common practice to remove, or strip, hydrocarbons from 
spent catalyst prior to passing it into the regeneration zone. It is 
important to remove retained spent hydrocarbons from the spent catalyst 
for process and economic reasons. First, hydrocarbons that enter the 
regenerator increase its carbon-burning load and can result in excessive 
regenerator temperatures. Stripping hydrocarbons from the catalyst also 
allows recovery of the hydrocarbons as products. The most common method of 
stripping the catalyst passes a stripping gas, usually steam, through a 
flowing stream of catalyst, countercurrent to its direction of flow. Such 
steam stripping operations, with varying degrees of efficiency, remove the 
hydrocarbon vapors which are entrained with the catalyst and hydrocarbons 
which are adsorbed on the catalyst. 
The efficiency of catalyst stripping has been increased by using a series 
of baffles in a stripping apparatus to cascade the catalyst from side to 
side as it moves down the stripping apparatus. Moving the catalyst 
horizontally increases contact between it and the stripping medium. 
Increasing the contact between the stripping medium and catalyst removes 
more hydrocarbons from the catalyst. As shown by U.S. Pat. No. 2,440,625, 
the use of angled guides for increasing contact between the stripping 
medium and catalyst has been known since 1944. In these arrangements, the 
catalyst is given a labyrinthine path through a series of baffles located 
at different levels. Catalyst and gas contact is increased by this 
arrangement that leaves no open vertical path of significant cross-section 
through the stripping apparatus. Further examples of similar stripping 
devices for FCC units are shown in U.S. Pat. Nos. 2,440,620; 2,612,438; 
3,894,932; 4,414,100; and 4,364,905. These references show the typical 
stripper arrangement having a stripper vessel, a series of baffles in the 
form of frusto-conical sections that direct the catalyst inward onto a 
baffle in a series of centrally located conical or frusto conical baffles 
that divert the catalyst outwardly onto the outer baffles. The stripping 
medium enters from below the lower baffle in the series and continues 
rising upward from the bottom of one baffle to the bottom of the next 
succeeding baffle. Variations in the baffles include the addition of 
skirts about the trailing edge of the baffle as depicted in U.S. Pat. No. 
2,994,659 and the use of multiple linear baffle sections at different 
baffle levels as demonstrated by FIG. 3 of U.S. Pat. No. 4,500,423. A 
variation in introducing the stripping medium is shown in U.S. Pat. No. 
2,541,801 where a quantity of fluidizing gas is admitted at a number of 
discrete locations. 
As previously mentioned for reasons of heat balance and product recovery, 
improvements in the efficiency of FCC stripping are particularly 
desirable. One way to improve FCC stripping efficiency is to increase the 
contact time between the stripping fluid and the FCC catalyst. This can be 
done by extending the length of the FCC stripping zone. The extended 
length increases efficiency by increasing the relative partial pressure of 
the stripping fluid, typically steam, in the lower portion of the stripper 
from which the catalyst normally exits. Furthermore, higher additions of 
stripping fluid such as steam can also raise the steam partial pressure 
within the stripping zone thereby serving to further reduce the carryover 
of hydrocarbons from the stripping zone into the regenerator. However, 
increasing the length of the stripping zone, or adding additional 
stripping steam to the FCC stripper, increases the cost of the operation 
of the unit as well as burdening downstream recovery facilities by the 
extra circulation and recovery of water. As a result, methods are sought 
to improve the recovery of hydrocarbons from FCC catalyst without 
increasing the length, or adding additional quantities of steam to the 
stripping zone. 
BRIEF DESCRIPTION OF THE INVENTION 
It has now been found that a number of FCC arrangements produce a 
concentrated boundary layer of catalyst and that by catching this boundary 
layer of catalyst in a zone particularly arranged to de-gas hydrocarbons 
from the catalyst steam, stripping efficiency can be improved without 
extending the length of the stripping zone, or adding additional stripping 
fluid. 
A common FCC arrangement, referred to as a vented riser, is one form of FCC 
reactor arrangement that provides a concentrated boundary layer of 
catalyst within the reactor vessel. In the case of the enclosed vented 
riser, the boundary layer of catalyst flows near the wall of the vessel. 
Wherever such a boundary layer of catalyst is formed, it is readily 
collected in a vertically-extended zone having a cross-sectional area that 
is relatively small compared to the cross-sectional area of the reactor 
vessel. The vertically-extended zone increases the density of the catalyst 
that enters from the boundary layer. Increasing the density of the 
catalyst in the restricted zone, de-gases hydrocarbons from the catalyst 
particles. This de-gassed flow of catalyst particles then directly enters 
a stripping zone. The de-gassed catalyst that enters the stripping zone 
has a lower partial pressure of hydrocarbons which in turn increases the 
overall stripping gas partial pressure within the stripping zone, and 
raises the overall stripping efficiency. By de-gassing the hydrocarbons in 
this manner, stripping efficiency can be raised by as much as 35%. 
Accordingly in one embodiment, this invention is a product recovery method 
for a hydrocarbon conversion process that contacts a 
hydrocarbon-containing feedstream with a particulate catalyst. In the 
method, a hydrocarbon-containing feedstream contacts catalyst in a 
confined reaction zone. The confined reaction zone discharges the catalyst 
into a reactor vessel and establishes a localized region in the reactor 
vessel through which a downwardly flowing stream of catalyst particles 
pass at a higher density relative to the average catalyst density in the 
reactor vessel. A vertically-extended de-gassing zone receives at least a 
portion of the flowing stream of catalyst through an inlet. The catalyst 
in the de-gassing zone is maintained with a downwardly increasing catalyst 
density gradient. At least a portion of the catalyst from a higher density 
region of the de-gassing zone passes into a stripping zone. A stripping 
gas contacts catalyst in the stripping zone, and stripped catalyst is 
recovered from the stripping zone. 
In a more limited embodiment, this invention is a process for recovering 
hydrocarbons and stripping catalyst in a reactor and stripper of a 
fluidized catalytic cracking process. The process comprises contacting a 
hydrocarbon-containing feedstream with the catalyst in a riser conversion 
zone. The riser discharges catalyst upwardly from its end into the reactor 
vessel, such that the catalyst passes as a concentrated stream along the 
wall of the reactor vessel. At least a portion of the concentrated stream 
of catalyst enters an annular inlet of a vertically-extended de-gassing 
zone. A fluidizing gas passes into the lower portion of the de-gassing 
zone. The fluidizing gas maintains a downwardly increasing density 
gradient for the catalyst throughout the de-gassing zone. At least a 
portion of the catalyst from a relatively higher density region of the 
de-gassing zone passes into a subadjacent stripping zone. A stripping gas 
contacts catalyst in the stripping zone and displaces hydrocarbons that 
pass upwardly out of the stripping zone along with the stripping fluid. 
The displaced hydrocarbons and stripping gas by-pass the de-gassing zone 
and exit the reactor vessel. A stripped catalyst is recovered from the 
stripping zone. 
Other objects, embodiments, and details of this invention can be found in 
the following detailed description of the invention.

DETAILED DESCRIPTION OF THE INVENTION 
This invention can improve the recovery of hydrocarbon vapors from any 
process that contacts vapors with a particulate catalyst in a fluidized 
manner. This invention will apply where there is an initial separation of 
hydrocarbon vapors from the catalyst that produces a concentrated stream 
of catalyst particles that flows through a region of the containment 
vessel. The de-gassing zone of this invention can have any configuration 
that will receive the concentrated flow of catalyst particles and will 
produce a downwardly increasing catalyst density gradient within the 
de-gassing zone that serves to decrease the void volume of the catalyst 
and drive hydrocarbon vapors upwardly out of the de-gassing zone. A better 
appreciation of this invention can be obtained from the FIGURE which shows 
the application of this invention in an otherwise conventional arrangement 
of an FCC reactor and FCC stripping zone. 
This invention is particularly useful in the operation of an FCC reaction 
zone. Additional information on the operation of FCC reaction and 
regeneration zones may be obtained from U.S. Pat. Nos. 4,431,749 and 
4,419,221 (cited above); and 4,220,623. 
The FIGURE depicts an FCC reactor. The FCC reactor consists of an external 
riser conduit 10 through which a mixture of catalyst and feed enters the 
reactor from a lower section of the riser (not shown). The catalyst and 
feed mixture continues upward into an internal portion 12 of the riser 
from which it exits into a reactor vessel 14. A cyclone separator 16 
receives product vapors, stripping gas and catalyst from reactor vessel 14 
and removes entrained catalyst particles from the product vapors. A vapor 
conduit 18 withdraws product from the top of cyclone 16. Catalyst 
separated from product vapors returns to the reactor vessel through a 
dip-leg conduit 20. 
The top end 22 of the riser is arranged in a typical vented riser 
configuration. The operation and arrangement of the vented riser is well 
known and described in U.S. Pat. Nos. 4,435,279 and 4,070,159, the 
contents of which are hereby incorporated by reference. As catalyst and 
vapors exit the top of riser 12 through vented riser arrangement 22, the 
lighter hydrocarbon vapors turn quiclky along flow path 26 to enter a cup 
24 before exiting through cyclone 16. The higher density catalyst 
particles continue on an upward trajectory along a path 28 and descend 
downwardly along with a substantial proportion of the catalyst travelling 
near the wall of reactor vessel 14 along a path 28'. 
The catalyst travelling along the wall of reactor vessel 14 collects in a 
de-gassing zone 30. As depicted in the FIGURE, a concentric baffle 32 
attached to a bottom cone closure 34 of the reactor vessel forms, together 
with the vessel wall, the annular de-gassing zone 30 and an annular inlet 
36. An inwardly angled section 36' at the top of baffle 32 expands the 
diameter at the inlet of the annular zone 30 to increase the collection of 
catalyst flowing down the wall along path 28'. Catalyst from conduit 20 of 
cyclone 16 can also be arranged to discharge catalyst into the gas 
disengaging zone. Where the location of cyclone conduit 20 would not 
ordinarily overlie the annular inlet 36, the end of conduit 20 may use an 
offset portion 40 to direct catalyst into the de-gassing zone inlet. 
Annular section 30 extends vertically to build a head of catalyst and 
create a higher pressure in the lower portion of the de-gassing zone 
thereby creating a downwardly increasing density gradient which serves to 
decrease the void volume of the catalyst in the lower portion of the 
de-gassing zone. As the void volume is decreased, de-gassing occurs with 
the resulting hydrocarbon vapors flowing upwardly out of inlet 36 along 
path 42. Catalyst from the de-gassing zone now with a decreased 
hydrocarbon partial pressure exits a relatively high density portion of 
the de-gassing zone through an outlet or port 44. The use of fluidizing 
gas in annular zone 30 promotes a free flow of catalyst through the 
de-gassing zone. So as to not interfere with the de-gassing effect of the 
de-gassing zone, only a relatively small volume of fluidizing gas enters 
the de-gassing zone. Ordinarily, the fluidizing gas is restricted in 
volume to provide a superficial velocity of between 0.25 to 0.5 feet per 
second through the de-gassing zone. Fluidizing gas may enter the annular 
de-gassing zone at a location above outlet 44 through a distribution ring 
46, or at a location below outlet 44 through a distribution ring 48. 
Admitting fluidizing gas through distribution ring 48 permits greater 
control of flow through the annular de-gassing zone by increasing or 
decreasing the addition rate of fluidizing gas. Where the flow rate of 
catalyst to inlet 36 exceeds the amount of catalyst exiting the de-gassing 
zone through outlets 44, catalyst overflows top 36' of baffle 32 and falls 
directly into a sub-adjacent stripping zone 50 along a flow path 52. 
Catalyst from outlet ports 44 and any catalyst overflowing inlet 36 enter 
the inlet of stripper 50. Stripper 50 operates in a conventional manner 
and countercurrently contacts the catalyst therein with an upwardly rising 
flow of stripping gas that enters stripping zone 50 through a distribution 
ring 54. A series of baffles 56 cascade the catalyst back and forth in 
order to increase the contacting between the catalyst and the stripping 
fluid. In general, the stripping baffles 56 decrease the average density 
of the catalyst flowing downward through the stripper such that at least 
the higher density portions of the de-gassing zone have a higher density 
than those in the stripping zone. Below distribution ring 54, catalyst 
collects in a relatively dense bed 58 before a spent catalyst outlet pipe 
60 withdraws catalyst for regeneration. Stripping gas and recovered 
hydrocarbons pass upwardly from stripper 50 through the center of baffle 
32 by-passing de-gassing zone 30 and out of the open center of baffle 32 
along flow line 62. 
This invention is applicable to a wide variety of hydrocarbon conversion 
processes that contact particulate catalyst in a fluidized manner. 
De-gassing zone 30 may be located in any portion of a reactor vessel where 
it will receive a concentrated stream of catalyst relative to the average 
concentration across the reactor vessel. For example, where a riser 
separation device produces a concentrated downward flow of catalyst near 
the center of the reactor vessel, the de-gassing zone may have the 
opposite arrangement of that shown in the FIGURE. In such an arrangement, 
catalyst would flow into a central portion of a de-gassing zone while 
stripping vapors and stripped hydrocarbons from a stripping zone would 
by-pass the de-gassing zone through an annular passage located to the 
outside of the central de-gassing zone. Accordingly, the description of 
this invention in the context of a specific FCC process is not meant to 
limit the process or apparatus aspects of this invention to the particular 
details disclosed herein.