Underflow cyclone with perforated barrel

A cyclone and process for fluidized catalytic cracking of heavy oils is disclosed. Gas and entrained solids are added tangentially around a vapor outlet tube in a cylindrical tube cyclone body. Solids and some gas is withdrawn via a plurality of openings radially and longitudinally distributed in the cylindrical sidewall of the cyclone body. Distributed withdrawal replaces or reduces conventional underflow of solids from an end of cyclone outlet and reduces solids reentrainment. 0-5 micron particle removal is enhanced by reducing eddy formation and particle bouncing near the cyclone sidewall. The device may be used as an FCC regenerator third stage separator.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The field of the invention is fluidized catalytic cracking of heavy 
hydrocarbon feeds and cyclones for separating fine solids from vapor 
streams. 
2. Description of Related Art 
Catalytic cracking is the backbone of many refineries. It converts heavy 
feeds into lighter products by catalytically cracking large molecules into 
smaller molecules. Catalytic cracking operates at low pressures, without 
hydrogen addition, in contrast to hydrocracking, which operates at high 
hydrogen partial pressures. Catalytic cracking is inherently safe as it 
operates with very little oil actually in inventory during the cracking 
process. 
There are two main variants of the catalytic cracking process: moving bed 
and the far more popular and efficient fluidized bed process. 
In the fluidized catalytic cracking (FCC) process, catalyst, having a 
particle size and color resembling table salt and pepper, circulates 
between a cracking reactor and a catalyst regenerator. In the reactor, 
hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot 
catalyst vaporizes and cracks the feed at 425.degree. C.-600.degree. C., 
usually 460.degree. C.-560.degree. C. The cracking reaction deposits 
carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating 
the catalyst. The cracked products are separated from the coked catalyst. 
The coked catalyst is stripped of volatiles, usually with steam, in a 
catalyst stripper and the stripped catalyst is then regenerated. The 
catalyst regenerator burns coke from the catalyst with oxygen containing 
gas, usually air. Decoking restores catalyst activity and simultaneously 
heats the catalyst to, e.g., 500.degree. C.-900.degree. C., usually 
600.degree. C.-750.degree. C. This heated catalyst is recycled to the 
cracking reactor to crack more fresh feed. Flue gas formed by burning coke 
in the regenerator may be treated for removal of particulates and for 
conversion of carbon monoxide, after which the flue gas is normally 
discharged into the atmosphere. 
Catalytic cracking is endothermic, it consumes heat. The heat for cracking 
is supplied at first by the hot regenerated catalyst from the regenerator. 
Ultimately, it is the feed which supplies the heat needed to crack the 
feed. Some of the feed deposits as coke on the catalyst, and the burning 
of this coke generates heat in the regenerator, which is recycled to the 
reactor in the form of hot catalyst. 
Catalytic cracking has undergone progressive development since the 40s. 
Modern fluid catalytic cracking (FCC) units use zeolite catalysts. 
Zeolite-containing catalysts work best when coke on the catalyst after 
regeneration is less than 0.1 wt %, and preferably less than 0.05 wt %. 
To regenerate FCC catalyst to this low residual carbon level and to burn CO 
completely to CO.sub.2 within the regenerator (to conserve heat and reduce 
air pollution) many FCC operators add a CO combustion promoter. U.S. Pat. 
Nos. 4,072,600 and 4,093,535, incorporated by reference, teach use of 
combustion-promoting metals such as Pt, Pd, Ir, Rh, Os, Ru and Re in 
cracking catalysts in concentrations of 0.01 to 50 ppm, based on total 
catalyst inventory. 
Most FCC's units are all riser cracking units. This is more selective than 
dense bed cracking. Refiners maximize riser cracking benefits by going to 
shorter residence times, and higher temperatures. The higher temperatures 
cause some thermal cracking, which if allowed to continue would eventually 
convert all the feed to coke and dry gas. Shorter reactor residence times 
in theory would reduce thermal cracking, but the higher temperatures 
associated with modern units created the conditions needed to crack 
thermally the feed. We believed that refiners, in maximizing catalytic 
conversion of feed and minimizing thermal cracking of feed, resorted to 
conditions which achieved the desired results in the reactor, but caused 
other problems which could lead to unplanned shutdowns. 
Emergency shutdowns cause substantial economic losses. Modern FCC units 
must run at high throughput, and for years between shutdowns, to be 
profitable. Much of the output of the FCC is needed in downstream 
processing units, and most of a refiners gasoline pool is usually derived 
from the FCC unit. It is important that the unit operate reliably for 
years, and accommodate a variety of feeds, including heavy feeds. 
The unit must also operate without exceeding local limits on pollutants or 
particulates. The catalyst is somewhat expensive, and most units have 
hundred tons of catalyst in inventory. FCC units circulate tons of 
catalyst per minute, the large circulation being necessary because feed 
rates are large and for every ton of oil cracked roughly 5 tons of 
catalyst are needed. 
These large amounts of catalyst must be removed from cracked products lest 
the heavy hydrocarbon products be contaminated with catalyst and fines. 
Even with several stages of cyclone separation some catalyst and catalyst 
fines invariably remain with the cracked products. These concentrate in 
the heaviest product fractions, usually in the Syntower (or main FCC 
fractionator) bottoms, sometimes called the slurry oil because so much 
catalyst is present. Refiners frequently let this material sit in a tank 
to allow more of the entrained catalyst to drop out, producing CSO or 
clarified slurry oil. 
The problems are as severe or worse in the regenerator. In addition to the 
large amounts of catalyst circulation needed to satisfy the demands of the 
cracking reactor, there is an additional internal catalyst circulation 
that must be dealt with. In most bubbling bed catalyst regenerators, an 
amount of catalyst equal to the entire catalyst inventory will pass 
through the regenerator cyclones every 15 minutes or so. Most units have 
several hundred tons of catalyst inventory. Any catalyst not recovered 
using the regenerator cyclones will remain with the regenerator flue gas, 
unless an electrostatic precipitator, bag house, or some sort of removal 
stage is added at considerable cost. The amount of fines in most FCC flue 
gas streams exiting the regenerator is enough to erode turbine blades if a 
power recovery system is installed to recover some of the energy in the 
regenerator flue gas stream. Generally a set of cyclonic separators (known 
as a third stage separator) is installed upstream of the turbine to reduce 
the catalyst loading and protect the turbine blades. 
While high efficiency third stage cyclones have increased recovery of 
conventional FCC catalyst from the flue gas leaving the regenerator they 
have not always reduced catalyst and fines losses to the extent desired. 
Some refiners were forced to install electrostatic precipitators or some 
other particulate removal stage downstream of third stage separators to 
reduce fines emissions. 
Many refiners now use high efficiency third stage cyclones to decrease loss 
of FCC catalyst fines to acceptable levels and/or protect power recovery 
turbine blades. However, current and future legislation will probably 
require another removal stage downstream of the third stage cyclones 
unless significant improvements in efficiency can be achieved. 
When a third stage separator is used a fourth stage separator is typically 
used to process the underflow from the third stage separator. The fourth 
stage separator is generally a bag house. 
Third stage separators typically have 50 or 100 or more small diameter 
cyclones. One type of third stage separator is described in "Improved 
hot-gas expanders for cat cracker flue gas" Hydrocarbon Processing, March 
1976. The device is fairly large, a 26 foot diameter vessel. Catalyst 
laden flue gas passes through many swirl tubes. Catalyst is thrown against 
the tube wall by centrifugal force. Clean gas is withdrawn up via a 
central gas outlet tube while solids are discharged through two blowdown 
slots in the base of an outer tube. The device was required to remove most 
of the 10 micron and larger particles. The unit processed about 550,000 
lbs./hour of flue gas containing 300 lbs/hour of catalyst particles 
ranging from sub-micron to 60 micron sized particles. 
We wanted to improve the operation of cyclones, especially their 
performance on the less than 5 micron particles, which are difficult to 
remove in conventional cyclones and, to some extent, difficult to remove 
using electrostatic precipitation. 
Based on observations and testing of a horizontal, transparent, positive 
pressure cyclone, we realized cyclones had a problem handling this 5 
micron and smaller size material. 
We discovered that turbulent vortices grow along the wall of the cyclones 
and then shed into the main tangential flow. This caused the particles to 
hop and bounce away from the wall, reducing collection efficiency. 
We wanted to attack the root cause of the problem, and improve the 
stability of the flow pattern through the cyclone. We discovered that 
perforations in the body of the cyclone could be used to remove minor 
amounts of gas with the solids, and have a major impact on stabilizing 
flow patterns. In addition, by withdrawing some of the gas, and 
essentially all of the solids, from a plurality of radially distributed 
openings we eliminated particle reentrainment. 
BRIEF SUMMARY OF THE INVENTION 
The present invention provides a cyclone separator comprising a cylindrical 
cyclone body having a length and a cylindrical axis; a tangential vapor 
inlet connective with an inlet end of said cyclone body for a stream of 
vapor and entrained solids; a cylindrical vapor outlet tube within said 
inlet end of said cylindrical cyclone body for withdrawal of gas with a 
reduced entrained solids content, said outlet tube having a cylindrical 
axis aligned with said cylindrical axis of said cyclone body; a plurality 
of radially and longitudinally distributed solids outlets for removing 
most of said entrained solids and a minor amount of gas, comprising at 
least two sets of openings, slots or perforations traversing a vertical 
distance equal to at least one half of said diameter of said cyclone body, 
and wherein said sets of openings are radially distributed by at least 
60.degree.. 
In another embodiment, the present invention provides in a fluidized 
catalytic cracking process wherein a heavy feed is catalytically cracked 
by contact with a regenerated cracking catalyst in a cracking reactor to 
produce lighter products and spent catalyst, and wherein spent catalyst is 
regenerated in a catalyst regeneration means containing primary and 
secondary separators for recovery of catalyst and fines from flue gas to 
produce a flue gas stream containing entrained catalyst fines, the 
improvement comprising use of a third stage separator to remove at least a 
portion of the catalyst fines from the flue gas, said third stage 
separator comprising a cylindrical cyclone body having a length and a 
cylindrical axis; a tangential vapor inlet connective with an inlet end of 
said cyclone body for a stream of vapor and entrained fines; a cylindrical 
vapor outlet tube within said inlet end of said cylindrical cyclone body 
for withdrawal of gas with a reduced entrained fines content, said outlet 
tube having a cylindrical axis aligned with said cylindrical axis of said 
cyclone body; and a plurality of radially and longitudinally distributed 
fines outlets for removing most of said entrained fines and a minor amount 
of gas, comprising at least two sets of openings over at least a third of 
the length of said cyclone body and wherein said sets of openings are 
radially distributed by at least 60.degree..

DETAILED DESCRIPTION 
The present invention can be better understood by reviewing it in 
conjuction with a conventional riser cracking FCC unit. FIG. 1 illustrates 
a fluid catalytic cracking system of the prior art. There are myriad other 
FCC units which can benefit from the process of the present invention, but 
the process of the present invention works well with this type of FCC 
unit. 
A heavy feed in line 2 such as a gas oil, vacuum gas oil is added to riser 
reactor 4 via feed injection nozzles. The cracking reaction is completed 
in the riser reactor. Spent catalyst and cracked products discharged from 
the riser reactor pass through cyclones 9 which efficiently separate most 
of the spent catalyst from cracked product. Cracked product is discharged 
via conduit 11 to the fractionator. 
Spent catalyst is discharged down from a dipleg of cyclones 9 into catalyst 
stripper 13 where stripping steam is added via line 41. The stripped 
hydrocarbons, and stripping steam are removed with cracked. 
Stripped catalyst is discharged via line 7 into catalyst regenerator 6. 
Catalyst is regenerated in coke combustor 17 by contact with air, added via 
line 25. Regenerated catalyst is discharged into upper vessel 21. Some 
catalyst is recycled via line 15 to the base 110 of the coke combustor. 
Some catalyst is withdrawn from the regenerator via line 5 and charged 
into the base of the riser reactor 4 to contact and crack fresh feed. Flue 
gas, and some entrained catalyst, is discharged into multiple stages of 
cyclones 19 and discharged via line 10. 
Cracked products removed from the reactor via line 11 are charged into the 
main column 30, which produces a gas product in line 31, a naphtha product 
in line 32, a light cycle oil product in line 33, a heavy cycle oil 
product in line 34 and a main column bottoms product in line 35. 
FIG. 1 does not show a third stage separator. Line 10 in most refineries 
would go to some type of third stage separator (not shown), usually one 
involving 50 or 100 (or more) small diameter horizontal cyclones. Purified 
flue gas would then pass through an optical power recovery turbine (not 
shown) then go to a stack for discharge to the atmosphere, via some flue 
gas clean up devices, such as an SOx scrubber, or elastrostatic 
precipitator. 
FIG. 2 (Prior Art) is similar to FIG. 1 of Improved hot-gas expanders for 
cat cracker flue gas, Hydrocarbon Processing, March 1976, p. 141. This 
article is incorporated by reference. 
Third stage separator 200 receives a fines containing FCC flue gas via 
inlet 210. Gas is distributed via plenum 220 to the inlets of a plurality 
of small diameter ceramic tubes 235 containing swirl vanes not shown. 
Fines collect on the walls of tubes 235 and are discharged from the base 
of the tubes as an annular stream of solids 230. A clean gas stream is 
withdrawn via outlet tubes 239 to be removed from the vessel via outlet 
290. Solids are removed via solids outlet 265. 
FIG. 3 shows a simplified sectional view of a preferred underflow cyclone 
of the invention, while FIG. 4 shows an end view of the same cyclone. Like 
elements have like numerals. 
A flowing stream of gas and entrained solids flows through tangentially 
aligned inlet duct 308 with inlet opening 312. The gas flow spirals around 
outlet tube 320 into region 324, where centrifugal force throws 
particulates to the wall 314 of the cyclone 310. Solids collect in region 
326, a relatively thin, circulating layer of particles, which are 
discharged through a plurality of openings 316 which are radially and 
longitudinally distributed about the cyclone barrel. A minor amount of 
gas, typically in the 2 to 20% range, is discharged with the solids 
through the holes or slots in the cyclone barrel. End plate 330 seals the 
end of the cyclone barrel opposite the clean gas outlet 322. Opposing end 
plate 333 contains outlet pipe 320, forming opening 322. 
FIG. 4, an end view of the cyclone, gives a better idea of the interplay 
between the tangential inlet duct 308 and the outlet pipe 320 in cyclone 
310. It also shows that four sets of openings 316 are evenly radially 
distributed about cyclone barrel 314, and that all cylindrical parts of 
the device have a common axis. 
FIGS. 5 and 6 (prior art) show particle flow in the cyclone barrel of 
conventional cyclones. FIG. 5 shows one type of reentrainment mechanism, 
whereby the bulk flow of gas, indicated as streamline 500, against the 
barrel 514 of the cyclone induces the formation of localized eddies 510 
and 512. This gas phase turbulence, combined with elastic collisions with 
the wall, can lead to an erratic particle "path, shown as streamline 600 
in FIG. 6. These mechanisms will transport particulates from the walls 614 
of the cyclone barrel back into the main body of the cyclone to be 
reentrained by gas flowing therethrough." 
FIG. 7 (invention) shows how particle flow is stabilized by the 
perforations in the cyclone barrel. The gas, and particulates, have some 
place to go rather than back into the gas mainstream. Thus the bulk flow 
of gas, shown as streamline 700, can continue without reentrainment of 
particulates. Particulates, and a minor amount of gas, flow to the walls 
of cyclone barrel 714, but are allowed to exit the walls via a plurality 
of openings 716. Exit streamlines 702, 704, and 706 are able to pass out 
of the cyclone, so the tendency for particles to bounce or for gas to form 
eddy currents or vortices is reduced or eliminated. 
FIG. 8 and 9 (invention) show preferred slot configurations, those which 
optimize removal of gas and solids from the device. The distinctive factor 
in FIG. 8 is that each hole or slot has an axis, or plane of the opening, 
perpendicular to the barrel surface. The slot may increase in cross 
sectional area as it passes from the interior opening 816 to the cyclone 
barrel to the outer opening 818 as it passed through the walls 814 of the 
cyclone barrel, as shown in FIG. 8. The bulk flow of gas is shown as 
streamline 800. 
Alternatively, each slot may have a generally constant cross-section, as 
shown in FIG. 9. Thus the inlet 916 to the slot or opening has the same 
size as the outlet 918, as the perforation passes through the walls of the 
cyclone barrel 914. The bulk flow of gas is shown as streamline 900. 
The perforations should be generally slanted to minimize disruption of flow 
lines of particulates exiting the cyclone barrel through the perforations. 
Ideally, the angle theta shown in FIGS. 8 and 9 ranges from 10.degree. to 
60.degree., as measured from the surface of the hole adjacent to the 
leading edge to a line tangent to the inner wall of the barrel at the 
leading edge of the hole or slot. Thus as shown in FIG. 8 surface 824 is 
the surface of the hole adjacent to the leading edge 820. Line 822 is 
tangential to the inner wall of the barrel at the leading edge. Theta is 
the angle between line 822 and surface 824. 
FIG. 10 (invention) shows a preferred arrangement of slots in the cyclone 
barrel. The perforations 1016, 1018 and 1020 are uniformly distributed 
about the surface of the cyclone barrel 1014. In the arrangement shown 
there is some overlap between perforated slot elements. Elements 1016 and 
1020 overlap with perforated elements 1018 in the central portion of the 
device. Generally about 2-20% overlap will prevent localized stagnant 
regions. 
Having provided an overview of the FCC process and the new cyclone design, 
a more detailed review of the FCC process and of preferred cyclone 
separators follows. 
FCC FEED 
Any conventional FCC feed can be used. The feeds may range from typical 
petroleum distillates or residual stocks, either virgin or partially 
refined, to coal oils and shale oils. Preferred feeds are gas oil, vacuum 
gas oil, atmospheric resid, and vacuum resid. The invention is most useful 
with feeds having an initial boiling point above about 650.degree. F. 
FCC CATALYST 
Any commercially available FCC catalyst may be used. The catalyst can be 
100% amorphous, but preferably includes some zeolite in a porous 
refractory matrix such as silica-alumina, clay, or the like. The zeolite 
is usually 5-40 wt % of the catalyst, with the rest being matrix. 
Conventional zeolites include X and Y zeolites, with ultra stable, or 
relatively high silica Y zeolites being preferred. Dealuminized Y (DEAL Y) 
and ultrahydrophobic Y (UHP Y) zeolites may be used. The zeolites may be 
stabilized with Rare Earths, e.g., 0.1 to 10 wt % RE. 
Relatively high silica zeolite containing catalysts are preferred for use 
in the present invention. They withstand the high temperatures usually 
associated with complete combustion of CO to CO.sub.2 within the FCC 
regenerator. 
The catalyst inventory may contain one or more additives, either as 
separate additive particles, or mixed in with each particle of the 
cracking catalyst. Additives can enhance octane (shape selective zeolites, 
typified by ZSM-5, and other materials having a similar crystal 
structure), absorb SOX (alumina), or remove Ni and V (Mg and Ca oxides). 
Additives for SOx removal are available commercially, e.g., Katalistiks 
International, Inc.'s "DeSOx." CO combustion additives are available from 
catalyst vendors. The catalyst composition, per se, forms no part of the 
present invention. 
FCC REACTOR CONDITIONS 
Conventional cracking conditions may be used. Preferred riser cracking 
reaction conditions include catalyst/oil weight ratios of 0.5:1 to 15:1 
and preferably 3:1 to 8:1, and a catalyst contact time of 0.1-50 seconds, 
and preferably 0.5 to 5 seconds, and most preferably about 0.75 to 4 
seconds, and riser top temperatures of 900.degree. to about 1050.degree. 
F. 
It is best to use an atomizing feed mixing nozzle in the base of the riser 
reactor. Details of a preferred nozzle are disclosed in U.S. Pat. No. 
5,289,976, which is incorporated by reference. 
It is preferred, but not essential, to have a riser catalyst acceleration 
zone in the base of the riser. 
It is preferred, but not essential, to have the riser reactor discharge 
into a closed cyclone system for rapid and efficient separation of cracked 
products from spent catalyst. A preferred closed cyclone system is 
disclosed in U.S. Pat. No. 5,055,177 to Haddad et al. 
It may be beneficial to use a hot catalyst stripper, heating spent catalyst 
by adding some hot, regenerated catalyst to spent catalyst. If hot 
stripping is used, a catalyst cooler may be used to cool heated catalyst 
upstream of the catalyst regenerator. Suitable designs are shown in U.S. 
Pat. Nos. 3,821,103 and 4,820,404, Owen, which are incorporated by 
reference. 
FCC reactor and stripper conditions may be conventional. 
CATALYST REGENERATION 
The process and apparatus of the present invention can use conventional FCC 
regenerators. Most regenerators are either bubbling dense bed or high 
efficiency. The regenerator, per se, forms no part of the present 
invention. 
Catalyst regeneration conditions include temperatures of 1200.degree. to 
1800.degree. F., preferably 1300.degree. to 1400.degree. F., and full or 
partial CO combustion. 
THIRD STAGE SEATOR VESSEL 
Our cyclones are preferably used in a third stage separator removing 
catalyst and fines from regenerator flue gas. In many instances, existing 
equipment may be used, with the cyclones of the invention substituted for 
the prior art small diameter, horizontal cyclones. 
When used as a third stage separator in FCC there will be so little solids 
loading at this point in the FCC process that refractory lining may not be 
needed. 
CYCLONE DESIGN 
Much of the cyclone design is conventional, such as sizing of the inlet, 
setting ratios of ID of the outlet tube to other dimensions, etc. Further 
details, and naming conventions, may be found in Perry's Chemical 
Engineers' Handbook, 6th Edition, Robert H. Perry and Don Green, which is 
incorporated by reference. The nomenclature discussion in Gas-Solids 
Separations, from 20-75 to 20-77, FIG. 20-106 , 20-107 and 20-108 is 
referred to and incorporated by reference. 
The slot area, or perforated area, should be large enough to handle 
anticipated solids flow, and will typically be from 10 to 200% or more of 
the open area of the conventional reverse flow cyclone solids outlet. 
The open area, or the slot area, of the outlets radially distributed on the 
wall of the cyclone may range from perhaps 10 or 20% up to about 100% of 
the conventional solids outlet. Preferably the slot area will be from 1/4 
to 1/2 times the area of the bottom of the cyclone. 
The perforations should be sized so that in use from 1 to 50% of the gas 
exits via the perforations. We prefer to operate with 1.5 to 25% of the 
gas being removed with the solids, and ideally with 2 to 20% of the gas 
exiting the cyclone via the perforations. 
Perforations are preferably uniformly distributed both radially and 
longitudinally. Preferably openings are present in at least every 1/3 
segment of the cyclone barrel, that is, present in every 120.degree. 
segment of the cyclone barrel. Ideally at least 4 sets of perforations are 
provided, distributed at 90.degree. segments. Most preferably, from 6 to 
20 longitudinal slots, or their equivalent, are evenly distributed around 
the circumference of the cyclone barrel. 
The perforations may be slanted to minimize disruption of flow lines of 
particulates exiting the cyclone barrel through the perforations. The 
angle theta, as shown in FIGS. 8 and 9 is preferably between 5.degree. and 
70.degree.. A preferred opening is one where the axis of the opening is 
angled in the direction of the flow within the cyclone, at least with 
regard to flow near the perforations, as opposed to flow within the 
central region of the cyclone, which can be very convoluted. 
Perforations or slots may be offset in the cyclone wall, or one portion of 
the wall of the cyclone barrel may be punched in somewhat toward the 
interior of the cyclone to "peel off" that portion of the rotating solids 
and gas that collects near the wall. Preferably the interior of the 
cyclone is smooth and flush, save for perforations punched or drilled 
through it. While the perforations may be the sole solids outlet, the 
device works well with some solids withdrawn via a conventional reverse 
flow solids outlet. 
DISCUSSION 
The new cyclone is easy to fabricate via conventional techniques. The 
device significantly improves removal of fines, that is, 0-5 micron 
particles. These particles are removed as soon as they reach the 
cylindrical sidewall. In contrast, in conventional cyclones these solids 
must travel the length of the cyclone barrel to the conventional solids 
outlet, where the solids must exit normal to the gas flow. The new cyclone 
design will reduce erosion on power recovery turbine blades, and also 
reduce particulate emissions.