U.S. patent application number 09/725165 was filed with the patent office on 2001-06-14 for sealing system for cyclone leg.
Invention is credited to Dubois, Aurelio Medina, Fusco, Jose Mozart, Guerra, Eduardo Cardoso de Melo, Huziwara, Wilson Kenzo, Martignoni, Waldir Pedro, Ramos, Jose Geraldo Furtado.
Application Number | 20010003575 09/725165 |
Document ID | / |
Family ID | 4074239 |
Filed Date | 2001-06-14 |
United States Patent
Application |
20010003575 |
Kind Code |
A1 |
Ramos, Jose Geraldo Furtado ;
et al. |
June 14, 2001 |
Sealing system for cyclone leg
Abstract
A system for sealing the legs of two stages of cyclones in
series used in fluid catalytic cracking (FCC) processes, comprising
a terminal leg configuration, which is devoid of moving parts and
makes use of a long-radius curve, and which changes the flow
direction of the descending mass flow of catalyst in dense phase
into a plane orthogonal to the ascending gaseous flow. This
prevents the entry of gaseous phase inside the leg, and promotes
efficient sealing and preventing the "packing-down" of the dense
bed of particulates.
Inventors: |
Ramos, Jose Geraldo Furtado;
(Rio de Janeiro, BR) ; Guerra, Eduardo Cardoso de
Melo; (Petropolis, BR) ; Fusco, Jose Mozart;
(Niteroi, BR) ; Dubois, Aurelio Medina; (Rio de
Janeiro, BR) ; Huziwara, Wilson Kenzo; (Rio de
Janeiro, BR) ; Martignoni, Waldir Pedro; (Sao Mateus
do Sul, BR) |
Correspondence
Address: |
NIXON & VANDERHYE P.C.
8th Floor
1100 North Glebe Road
Arlington
VA
22201-4714
US
|
Family ID: |
4074239 |
Appl. No.: |
09/725165 |
Filed: |
November 29, 2000 |
Current U.S.
Class: |
422/139 |
Current CPC
Class: |
B04C 5/26 20130101; B01J
8/0055 20130101; C10G 11/18 20130101; B04C 5/14 20130101 |
Class at
Publication: |
422/139 |
International
Class: |
B01D 050/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 14, 1999 |
BR |
9905842-1 |
Claims
1. A system for sealing a cyclone separator leg which joins the
lower end of the leg of a secondary cyclone and the leg of a
primary cyclone to form a single primary and secondary cyclone leg
complex where the solids collected by both cyclones are combined,
characterized by the said combined solids being simultaneously
discharged from said single leg by means of a single leg
termination of the long-radius-curve type.
2. A system according to claim 1, characterized in that the
long-radius curve termination is immersed in a fluidized bed of
particles.
3. A system according to claim 1, characterized in that the
long-radius curve of the single leg has a ratio of radius/diameter
within the band from 1.0 to 3.0.
4. A system according to claim 1, characterized in that said
termination is constructed from a succession of straight tube
sections in an arcuate array.
5. A system according to claim 4, characterized in that the total
angle subtended by the succession of straight tube sections at the
centre of curvature of the long radius curve directs the flow
direction of the descending mass flow of dense phase solids into a
plane orthogonal to the ascending gaseous flow.
6. A system according to claim 1, characterized in that, with
respect to the centre line of the inlet to the curved termination,
the junction of the leg of the primary cyclone and the leg of the
secondary cyclone lies on the side opposite the discharge end of
the curve, and higher than the discharge end by a distance in the
range from 3.5 to 5.5 times the diameter of the leg of the primary
cyclone.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a sealing system for the
leg of a cyclone for separating out solids in gas/solid suspension.
More specifically, the present invention relates to a sealing
system for two-stage legs of in-series cyclones used in fluid
catalytic cracking (FCC) processes.
STATE OF THE ART
[0002] The purpose of the fluid catalytic cracking (FCC) process is
to convert hydrocarbons with a high boiling point into light
hydrocarbon fractions, such as gasoline and liquefied petroleum gas
(LPG). The catalyst used in fluid catalytic cracking units consists
of a powder of controlled particle size, which is intimately mixed
with the feed to the FCC unit and subsequently with the cracking
products which are also gaseous, forming a gas/solid suspension
which has to be separated efficiently in order to minimize catalyst
losses from the FCC unit, providing obvious advantages for the
refiner and for the environment.
[0003] In the field of separation of solid particles in gaseous
suspension, it is customary to use cyclones as the mechanism for
separating out the solids in gas/solid suspension. Cyclones of this
type normally have a device at their solid-material discharge ends
or, as is more well-known, at the ends of the legs of the cyclones.
This device acts as a sealing element, preventing the contraflow of
the gaseous stream to the inside of the cyclone, which would
considerably reduce its separation efficiency.
[0004] Typically, in the specific case of fluid catalytic cracking
(FCC) processes use is made of a pair of cyclone separators in
series in order to maximize the separation of the particles--the
process catalyst--from the gas/solid suspension. The use of more
than one set of cyclone separators is also a fairly common
arrangement, depending on the size and model of the FCC unit's
separator vessel. Separation of the greater part of the particles
takes place in the first separation stage (primary cyclone), after
which a much lower concentration is left behind in which the
catalyst particles for the second separation stage (secondary
cyclone) are of smaller average size. In this known method of
operation, the internal pressure in the cyclones is always below
the pressure of the separator vessel, hence there is a need to seal
the lower end of the legs of the cyclones by means of the use of
sealing devices.
[0005] The catalyst collected in the cyclones flows in free fall to
the legs of the cyclones, forming a dense column of solids which
after reaching a specific level establishes a pressure equilibrium
between the base of the cyclone leg and the inside of the separator
vessel. Pressure equilibrium having been achieved, the
cyclone-discharge process begins with the opening of the sealing
devices. It is precisely during the process of discharging the
solids collected in the cyclone legs that significant loss of
solids from this system occurs. Initially, the downward flow of
solids maintains the seal in the system owing to the existence of
the column of solids. When discharging is almost complete, the
column of solids is already significantly smaller and the pressure
differential breaks the seal of the dense phase, and an ascending
gaseous flow originating from the base of the leg is then formed,
returning some particulate material already collected and
prejudicing cyclone operation. When the effect of the
catalyst-discharge movement on the sealing device ceases, the
sealing device returns to its normal position of equilibrium and
another cycle of collection and accumulation of particulates
commences. In practice it has been observed that the return of
particulate material is more intense in the second separation
stage, owing to the greater pressure differential to which this
equipment is subjected and to the smaller average size of the
particles making up the particulate material in this second
separation stage.
[0006] In the light of the above, it has been concluded that a
critical point in the process of separation using cyclone
separators, with direct influence on the efficiency of the FCC
process, is the seal provided by the sealing devices of the cyclone
legs. Sealing devices take various forms, such as valves with a
counterweight (flapper valve), valves involving dripping (trickle
valve) and others in the form of planar or conical plates (splash
plates) and, also, combinations of these.
[0007] These devices, working in fairly severe conditions such as
temperatures in excess of 500.degree. C. and a high solids charge,
do not normally provide efficient sealing so they can allow the
passage of an ascending flow of gas from the separator vessel to
the inside of the cyclone leg. For example, a flow greater than
0.10% of the total volume fed into the input opening of the cyclone
gives rise to the return of finer particles of solid, disrupting
cyclone operation and reducing the cyclone's separation
efficiency.
[0008] With a view to solving the problem of the sealing device,
GB-A-2,212,248 teaches a method for constructing a flapper valve
which allows practically total sealing through the application of a
conical section installed between the base of the valve and the
seat of the cyclone leg. However, it is important to remember that
a small passage of gas fed into the cyclone opening is beneficial,
since it fluidizes the solid which has accumulated in the cyclone
leg, facilitating its discharge. Therefore, the attempt to minimize
the passage of the flow of gas through the valve, as proposed by
GB-A-2,212,248, may give rise to the loss of fluidization of the
solid which has accumulated in the cyclone leg, especially in the
second separation stage in which the solids charge is fairly dilute
and may be of the order of 0.2 to 1.5 grammes of particulate per
cubic meter of gas. With this operating condition, the period for
accumulation of solids for achieving pressure equilibrium, and
therefore the frequency of discharge of the solids from the dip
leg, may be greater than eight hours, and if there is not a minimum
flow of gas through the valve the dense bed of particulates may
become "packed down", with a possible risk of complete obstruction
of the cyclone leg and loss of efficiency thereof.
[0009] As an alternative to the modification of sealing devices,
Brazilian Patent Application PI 9603898 of the applicant (fully
incorporated as reference) teaches that it is possible to prevent
the return of the particulate material to the inside of cyclones
for separating gas/solid suspensions in fluid catalytic cracking
units through the formation of a leg complex composed of the
joining of the primary-cyclone leg and the secondary-cyclone leg,
eliminating one of the two sealing devices, which are the principal
source of problems and leaks in cyclone separation systems. However
this arrangement, although very satisfactory in a variety of
instances, may not afford the desired benefits for certain fluid
catalytic cracking units, principally those in which the physical
arrangement of the primary and secondary cyclones does not allow
the joining of the cyclone legs as taught in Brazilian Patent
Application PI 9603898.
[0010] Despite the attempts described in the literature to solve
the problems of loss of efficiency of cyclones in connection with
the return of particulates during the cycle of opening the sealing
device of the legs of cyclones used as gas/solid-suspension
separators in catalytic cracking (FCC) processes, a definitive
solution capable of maintaining a high cyclone operating efficiency
in a simple, economical and safe manner has not yet been achieved.
However such a solution is presented by the system described
herein.
SUMMARY OF THE INVENTION
[0011] The present invention provides a sealing system for the leg
of a cyclone for separating out solids from a particulate
suspension which joins the lower end of the leg of the secondary
cyclone and the leg of the primary cyclone, forming a single
primary and secondary cyclone leg complex where the solids
collected by both cyclones are combined, characterized by the said
combined solids being simultaneously discharged by means of a
single leg termination of the long-radius-curve type.
[0012] Such a system can be used as a termination system for two
series stages of cyclones used in fluid catalytic cracking (FCC)
processes for separating out solids in gas/solid suspension. The
terminal configuration allows improved, efficient sealing in such a
manner as to prevent the re-entrainment of particulates, and to
reduce or eliminate the risk of the "packing-down" of the dense bed
of particulates collected in the cyclones. It avoids the mechanical
failures which may arise in movable sealing systems, since flapper
or trickle valves are dispensed with. The result of its use is a
substantial increase in efficiency and a consequent reduction of
particulate emission.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The characteristics of the cyclone-leg sealing system which
is the subject of the present invention will be better understood
from the following detailed description, which is given purely by
way of example, in association with the drawings, to which
reference is made below and which are an integral part hereof:
[0014] FIG. 1 shows a representation of the reaction/separation
assembly of a typical FCC unit of the prior art.
[0015] FIG. 2 shows a representation of the separation assembly of
an FCC unit according to the present embodiment.
[0016] FIG. 3 shows a representation of a terminal section of a
cyclone leg according to the present embodiment.
DETAILED DESCRIPTION OF THE INVENTION
[0017] The detailed description of the cyclone-leg sealing system
will be given in accordance with the identification of its
component parts, based on the above-described Figures.
[0018] FIG. 1 shows a standard reaction/separation unit for a fluid
catalytic cracking process. The process comprises promoting
cracking reactions in a charge of hydrocarbons in gaseous phase in
suspension with particles of catalyst over the full length of an
ascending-flow tube reactor which will hereinafter be called
"riser" (1). As a result of the reactions, a carbonaceous deposit
is formed on the surface of the catalyst.
[0019] Rapid separation of the particles of coked catalyst in
suspension in the cracked hydrocarbons is promoted at the top end
of the riser (1). In this manner, the reaction's effluent stream
enters a first cyclone separator (3) where separation of the
greater part of the catalyst in suspension takes place. Through the
action of gravity, the catalyst flows in free fall to the leg (5)
of the cyclone (3) and is retained by the sealing valve (6) which
is shown, by way of example, as a trickle valve.
[0020] The cracked hydrocarbons separated out in the first cyclone
(3), still entraining particles of catalyst, then enter the second
cyclone (4) where they are completely separated out, the gaseous
phase moving on to external systems (10) for fractionation of the
cracked products and the particles of the catalyst descending, as
occurred previously in the cyclone (3), to the leg (7) of the
cyclone (4) where they are retained in the sealing valve (8), which
is shown by way of example as a flapper valve.
[0021] Thanks to the column of particles of catalysts which
accumulate on the caps of the sealing valves (6, 8), at a specific
instant in the process a pressure equilibrium is obtained between
the inner lower part of the legs (5, 7) of the cyclones (3, 4) and
the inside of the separator vessel (2), where the pressure is
normally in excess of those encountered inside the cyclones. As
soon as the valve cap is opened, through the force of the
equalization of pressures the column of solids which has
accumulated in the legs of the cyclones flows to the fluidized bed
(9) which is accumulating in the lower part of the separator vessel
(2). At this time, the conditions under which there is equilibrium
of the pressures cease, causing the valve cap to return to the
closed position.
[0022] This is the most critical moment in the separation stage
since, owing to the period of time required for complete closure of
the sealing valve, it is practically impossible to prevent some
re-entrainment of catalyst particles back inside the cyclone
through the force of an almost inevitable contraflow of gaseous
stream through the inside of the cyclone leg. Therefore,
conventional cyclone separation systems nearly always operate
outside ideal conditions of efficiency and profitability.
[0023] The joining of the legs of a primary cyclone and a secondary
cyclone, such that the solid material is collected by both cyclones
and discharged by means of a single valve at the end of the
combined leg section of the cyclones, is also a known technique. In
this technique, the reduction of catalyst losses grew more
efficient.
[0024] FIG. 2 shows an embodiment of the present invention
consisting of a cyclone-type separation system which comprises a
primary cyclone (21) and a secondary cyclone (22). The legs (23,
24) of the two cyclones are interlinked to form a junction (25)
where the solids, which have been collected, are combined. The
single leg (26) is immersed in the fluidized bed (9) and ends in a
sealing system of the said leg (26).
[0025] FIG. 3 shows, in greater detail, the proposed sealing system
which comprises the use, at the bottom of the single leg 26, of a
long-radius curve termination (31) which is devoid of moving parts.
The said curve (31) has a ratio of "radius"(R)/diameter(() of the
single leg (26) which varies in the region from 1.0 to 3.0 and is
constructed from a plurality of straight tube sections offset by
specific angles of inclination (.alpha., .beta.), these sections
hereinafter being called "buds" (32).
[0026] As shown in FIG. 3, the inlet to the long radius curve
termination 31 is vertical at the foot of the single leg 26 after
the junction 25 between the two cyclone legs 23 and 24. It is
preferable that the direction of discharge from the termination 31
be horizontal, i.e. orthogonal to the inlet direction represented
by the centre line of the single leg 26, so whereas the angle
.beta. represents the angle subtended by the first and subsequent
(in this case second) straight sections 32 of the long radius curve
termination 31, the angle .alpha..sub.1, represents the angle
between the horizontal line through the centre of curvature and the
start of the first straight section 32 and the angle .alpha..sub.2
represents the angle between the end of the last (in this case
second) straight section 32 and a vertical line through the centre
of curvature. Preferably .alpha..sub.1 and .alpha..sub.2 are equal.
Equally it is desirable for the total value of
(.alpha..sub.1+.alpha..sub.2+n..beta.) to be in the range
75.degree. to 100.degree., more preferably 90.degree., where n is
the number of straight sections 32.
[0027] The angular offset which exists between the buds (32)
changes the flow direction of the descending mass flow of catalyst
in dense phase into a plane orthogonal to the ascending gaseous
flow, which prevents the entry of gaseous phase inside the leg
(24), promoting efficient sealing and, simultaneously, preventing
the "packing-down" of the dense bed of particulates which have
collected inside the said leg (24).
[0028] The greater sealing efficiency of the present embodiment is
achieved when, with respect to the centre line of the inlet to the
long radius curve (31), the junction (25) between the leg of the
primary cyclone (21) and the leg of the secondary cyclone (22) lies
on the opposite side vis--vis the discharge end of the curve
termination (31), and at a higher elevation by a distance in the
range of from 3.5 to 5.5 times the diameter of the leg (23) of the
primary cyclone (21).
[0029] The above-described assembly applies to all internal cyclone
systems of FCC units, i.e. reactor and regenerator, since it does
not depend on the level of fluidization of the catalyst bed; its
application includes not only a bubbling bed, which is
characteristic of operation at a low gas-flow surface velocity
(typical of a stripper bed), but also turbulent fluidized beds with
a high fluidization velocity, as are found in FCC regenerators.
[0030] The above description of the cyclone leg sealing system of
the present invention must be regarded only as one of a number of
possible embodiments, and any particular characteristics introduced
therein must be understood as being described only in order to
facilitate understanding. Therefore, they may not be regarded in
any way as restricting the invention which is limited only by the
scope of the following claims.
* * * * *