Note: Descriptions are shown in the official language in which they were submitted.
2151893
~ APPARATUS FOR SEPARATING SOLID PARTICLES
FROH A GAS AND FOR INJECTING THE SO-SEPARATED
PARTICLES INTO A REACTION VESSEL
PArK~-~UND OF THE INVENTION
Field of the Invention
The present invention relates to an apparatus
for separating solid particles from a flow of gaseous
products and for injecting the so-separated particles
into a reaction vessel in spite of a higher pressure
that may exist therein.
The invention also relates to the combination
of such an apparatus with a fluidized bed reactor as
used for the gasification of carboneous materials, in
order to achieve a higher degree of conversion of carbon
into a fuel gas.
Brief Description of the prior art
It is known that the gasification of
carbonaceous materials, such as coal or biomass, in a
fluidized bed reactor releases solid particles mixed
with a combustible gaseous product. These solid
particles generally known as "char", contain a large
amount of partially reacted carbonaceous materials, in
addition to ash originating from the feed material and
from the inert fluidizing medium.
In order to enhance the degree of gasification
of the solid feed material and to minimize particulate
concen-tration in the gaseous product, it is necessary
to separate any solid particles entrained from the fluid
bed from the stream of gaseous products. The particles
which are separated, can then be re-injected into the
fluidized bed reactor, so as to cause reaction of the
carbon bearing compounds containing therein with the
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fluidizing gases, and thus produce additional
combustible gases.
As aforesaid, the solid particles carried by
the gaseous stream removed from the fluidized bed
reactor also contain mineral matter such as ash from the
feed materials, inert particles from the fluidizing
medium, or some of the catalyst introduced in the bed to
enhance the rate of reaction.
The separation of such solid particles from
the flow of gaseous products is currently achieved by
means of one or more cyclone separator(s) incorporated
into the gas discharge conduit of the reactor. The
cyclone separators are usually installed outside of, and
separate from, the reactor vessel and connected to the
discharge conduit for the gaseous products. It is also
necessary, for practical reasons, to operate the cyclone
separators at a gas pressure which is somewhat lower
than that in the gasification reactor. The solids
removed from the gases in such separators, are
subsequently fed back to the fluidized bed reactor by
mechanical means such as a screw feeder or star valve,
designed to prevent the back flow of gases from the
reactor vessel to the separator, which flow would impair
its operation.
In order to eliminate the expense of an addi-
tional pressure vessel and of connecting conduits,
attempts have been made to mount the cyclone
separator(s) inside and at the top of the fluidized bed
reactor vessel. Such is currently the practice in fluid
bed catalytic crackers for petroleum products. However,
experience has shown this to be impractical for the
separation of char in the discharge conduit of
gasification reactors. The operation of a high
efficiency cyclone separator at a high pressure drop,
together with the requirement for injection of the char
deep in the fluidized bed, causes a subStantial pressure
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dlfference between the body of the cyclone separator and
the point of discharge of the char in the fluidized bed.
Pneumatic devices such as "L" valves mounted
in the solids discharge conduit or "dipleg" of the
separator do not provide adequate sealing against the
backflow of gases. Moreover, mechanical sealing devices
such as flapper valves, star valves or screw feeders
require extensive and frequent attention when operating
under severe conditions of corrosion and temperature.
Such attention cannot practically be provided inside a
fluidized bed reactor without prolonged interruption of
the operation.
Consequently, the present practice is to
effect the separation of the solid particles from the
discharge gases of fluid bed gasification reactors, in
one or more cyclone separators mounted externally to the
reactor vessel, as was mentioned hereinabove. The solid
particles removed from such separators are then cooled
under an oxygen free atmosphere to prevent overheating
and fusion of the ash, and fed to the body of the
fluidized bed reactor by means of mechanical sealing and
feeding devices. The mechanical complexity of this
system increases equipment and maintenance costs, while
causing losses in carbon and thermal efficiency.
OBJECT OF THE INVENTION
- The object of the present invention is to
provide an apparatus which can be used in combination
with a fluidized bed chemical reactor like that used for
the gasification of carbonaceous materials, which
apparatus improves the separation of the solid particles
suspended in the gaseous products and allows the
injection into the fluidized bed, thereby resulting in
a virtual completion of the chemical reaction with a
simultaneous reduction of the mechanical and operational
complexities of the current method.
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Another object of the invention is to enhance
the production of fuel gas in a fluidized bed reactor by
means of an apparatus, which improves the separation
efficiency of solid particles from the flow of gaseous
product of the gasification reaction and allows for the
injection of the so removed solid particles together
with a portion of the gaseous products as a continuous
stream deep into the fluidized bed, near the point of
entry of the fluidizing gas such as to cause the rapid
volatilization of the carbonaceous solids together with
the release of heat as required for the gasification of
the feed material.
SUMMARY OF THE INVENTION
In accordance with the present invention, the
above object is achieved with an apparatus for
separating solid particles contained in a flow of
gaseous products and for injecting the separated
particles into a reactor vessel, the apparatus
comprising:
(a) a cyclone separator having:
at least one tangential entry port through which said
flow of gaseous products is fed,
an axial exhaust conduit through which the gaseous
products fed into the separator exit from the same;
a collection hopper into which fall solid particles
separated from the gaseous products are directed by
centrifugation and gravity within the separator, and
a discharge conduit having an inlet within said
collection hopper and an outlet opening into the reactor
vessel;
(b) a gas injector for injecting a gas stream into
said discharge conduit so as to cause the solid
particles falling into the collection hopper to flow
continuously out of said hopper through said discharge
conduit and be fed into the reaction vessel in spite of
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any pressure diffe-rential that may exist between the
collection hopper and the reactor vessel.
The apparatus is preferably used in
combination with and within a fluidized bed reactor, on
the top of which it is preferably mounted when the
fluidized bed reactor is used as a chemical reactor.
The incorporation of this apparatus permits to achieve
a high efficiency of separation thanks to a high
velocity of entry of the gaseous products to the cyclone
separator and the continuous removal of the solid
particles from the collection hopper of the cyclone
separator through the discharge conduit which
advantageously penetrates deeply into the fluidized bed.
As aforesaid, the flow of solid particles out
of the cyclone separators is driven by means of the gas
injector mounted in the discharge conduit, which opposes
the pressure difference in the other direction between
the fluid bed and the body of the cyclone separator.
If desired, two or more cyclone separators can
be used, whether in series or in parallel, within or
outside of the reactor vessel. In such a case the
discharge conduit of each separator is equipped with a
gas injector to discharge the solid particles and gases
deep into the fluidized bed.
When applied to the gasification of
carbonaceous materials, the apparatus of the present
invention can achieve the conversion of the carbon
contained in the carboneous feed solids nearly to
completion, in a single pressure vessel, without the
mechanical complexity of external cooling and re-
injection devices used so far. Such a high carbon
conversion is achieved by the combined effect of a high
separation efficiency in the cyclone separator(s) and of
the high reactivity of the nascent char re-injected
together with steam, in the oxidation zone of the
fluidized bed reactor, near the point of entry of the
fluidizing gases.
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The present invention could also be applied to
enhance any other fluidized bed chemical process such as
combustion. For example, in a circulating fluidizing
bed process, a high capacity cyclone separator according
to the transport invention could be mounted within or
outside the transport reactor vessel.
BRIEF DESCRIPTION OF THE DRAWINGS
The structure, advantages and applications of
the invention will be better understood upon reading the
following, non restrictive description of preferred
embodiments thereof, made with reference to the
accompanying drawings wherein:
Figure 1 is a schematic elevational view in
cross-section of a fluidized bed gasification apparatus
provided with a cyclone separator internally mounted
therein and equipped with a gas injector in its dipleg,
as is called for in accordance with the invention;
Figure lA is a cross-sectional view of the
apparatus shown in Figure 1, taken along line A-A of
this figure;
Figure 2 is a schematic cross-sectional view
of the cyclone separator shown in Figure 1, which
incorporates a steam injector in its dipleg;
Figure 3 is a schematic cross-sectional view
of another fluidized bed gasification apparatus provided
with two cyclone separators in series both of which are
equipped with a steam injector, as is called for in
accordance with the present invention; and
Figure 3A is an enlarged view of the outlet of
one of the dipleg of the separators shown in Figure 3.
DESCRIPTION OF PREFERRED ENBODIMENTS
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~ Referring to Figure 1, there is shown a first
preferred embodiment of the invention, comprising a
vertically extending, cylindrical pressure vessel 10
acting as a fluidized bed gasification reactor.
Granulated carbonaceous material to be gasified, such as
biomass, is fed through an inlet 12 into the vessel 10
while an oxygen containing gas such as air is fed
through an inlet 1~ to a plenum chamber 14 located at
the bottom of the vessel and this to the latter through
a fluidizing orifice plate 15.
A fluidizing medium in the form of inert
particles such as sand, i5 maintained as a fluid bed 16
by the upward flow of gas in the bottom section of the
vessel 10. The feed material reacts with the fluidizing
gas in the fluid bed at high temperature and is rapidly
dried and volatilized to yield a char and product gases.
The gaseous products from the reaction rises through the
bed surface 16 to the reaction zone 17 of the vessel
while carrying, in suspension, fine particles of char,
ash and ~and from the bed.
The stream of gaseous products flow through
tan-gential entry ports 18 into the body of a cyclone
separator 19, wherein the particles suspended in the
gaseous products are separated by centrifugal forces and
carried by gravity forces aided by a small flow of gas
to a collection hopper 20, while the larger part of the
gaseous product exits through an axial exhaust conduit
21 located at the top center of the cyclone separator,
substantially free of solid particles. A small portion
(5 to 10%) of the gaseous products flowing into the
separator 19 is allowed to flow down through the
collection hopper 20 and to exit, together with the
separated particles, via a discharge conduit 22, known
as a "dipleg", into the bottom zone of the fluid bed
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close to the fluidizing orifice plate 15. The downward
flow of gaseous products and particles in the dipleg 22
is sustained against the opposing difference in pressure
existing between the discharge point in the fluid bed
16, and the body of the cyclone separator 19, by virtue
of the momentum of a jet of a high pressure gas
preferably consisting of steam, that is injected in a
throat at the entrance of the dipleg 22, by means of a
nozzle 25' fed by a gas supply line 25 installed inside
the axial exhaust conduit 21. Advantageously, the gas
which is so supplied is steam.
An overflow pipe 23 connected to a shut off
valve 24 provides for the periodic removal of excess
solids accumulated in the fluid bed as a result of the
continuous addition of ash by the solid feed materials.
As can be appreciated, the char together with
the gaseous products and steam discharged from the
dipleg 22 are well mixed with the fluidizing gas and
sand in the bottom zone of the bed. The hot and
reactive char that is so recycled is thus brought into
contact with the flui-dizing gas prior to its contact
with the feed material entering in the middle and top
zones of the bed. As a result, the char can react
preferentially with the oxygen in the fluidizing gas,
causing its rapid and complete gasification together
with the liberation of heat which serves to maintain the
temperature necessary for the gasification reaction.
The steam brought in with the char provides a necessary
cooling effect through the water gas reaction and the
temperature in this zone is thus held at a safe level,
below the ash fusion point. In this connection, one may
appreciate that the temperature in this zone could
effectively be regulated by a control device 42 for
adjusting the flow rate of the steam.
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The heat released by the reaction in this
lower zone of the fluidized bed is also transferred to
the sand and fluidizing gas and contributes to the
process require-ments in the middle and upper zones of
the bed where the solid feed material reacts with the
fluidizing gases.
The larger particles of the carbonaceous feed
materials remain in the bed until they are sufficiently
reduced in size to be carried upwards by the gases
flowing out of the free surface of the bed 16 into the
reaction zone 17, and into the cyclone separator 19.
The amount and size of the particles carried upward into
the reaction zone depend on the velocity of the gases
and on the pressure and temperature within the zone.
Thus, the invention provides an effective
means for collecting and re-injecting the char deep into
the bed, thereby permitting substantially higher gas
velocities and flows of char out of the bed than in the
existing processes. This, in turn, allows for a larqer
portion of the heat produced during the process to be
obtained by combustion of the char in the bed.
In addition, a higher concentration of char
and other solids in the reaction zone over the bed,
favors the cracking of tars and higher hydrocarbons, and
thus improves the cold gas efficiency of the process.
Referring now to figure 2, there is shown an
enlarged cross-section view of the cyclone separator 19
shown in figure 1 equipped with a collection hopper and
a built-in gas injector device according to this
invention.
As outlined above, the cylindrical body of the
cyclone separator 19 is equipped on top with one or two
tangentially mounted entry parts 18, through which the
particle-loaded gaseous products are introduced at high
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velocity, causing a cyclonic, rotary and downwards
motion of the gases in the body of the separator, moving
downwards and the separation by centrifugal forces of
the solid particles as in any conventional cyclone
separator.
The solid particles separated from the gaseous
products drop into the collection hopper 20 mounted at
the bottom end of the cyclone separator, and are
subsequently discharged through the dipleg 22 to the
bottom zone of the fluid bed.
In order to insure an uninterrupted flow of
the particles out of the cyclone separator, it is
necessary to induce the flow of a small amount of gas
together with the particles, out of the bottom of the
separator.
As aforesaid, in accordance with the
invention, this flow of gas is obtained by incorporating
a steam injector 25' within the separator body adjacent
the throat-shaped inlet of the dipleg 22. By means of
the steam jet 24 generated by the injector 25', a flow
of gas and particles is induced downwardly into the
dipleg 22 and discharged into the fluid bed.
The provision of such steam jet at the top of
dipleg 22 of the cyclone separator 19 provide a pressure
increase in the dipleg which opposes any back pressure
from the body of the fluidized bed. This permits the
velocity of entry to the cyclone separator and its
efficiency to be raised at will, without concern for any
return flow up the dipleg 22, which would inevitably
spoil the operation of the cyclone separator.
Referring now to figure 3, there is shown
another preferred embodiment of this invention wherein
two cyclone separators 26 and 28 are installed in series
in a reactor vessel similar to the one referred to
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` -- hereinabove, in order to improve the efficiency of
separation of particles. The gaseous products evolved
from the fluidizing bed are introduced into the first
cyclone unit 26, from which they are led by a conduit 27
to a second cyclone unit 28, in which additional solid
particles are removed. The gaseous products leaving the
second separator 28 are fihally discharged through a
conduit 29. Each cyclone separator or unit is equipped
with a collection hopper 30 and 31 and with a steam
injector 32 and 33 as was disclosed herein-above. Steam
supplied by a line 34 is fed to the nozzle of each
injector and the particles collected together with the
entrained gases and steam are discharged in the bottom
zone of the fluid bed through diplegs 35 and 36.
In this embodiment as well as in the other one
previously described, each dipleg is preferably equipped
with a diffuser section 37 and 38 which has an outlet at
the bottom end, which, together with diffusing cones 39
and 40 mounted on the orifice plate 41, insure an even
dispersion of the released streams and particles and an
adequate mixing of the same with the fluidizing gas and
medium.
EXAMPLE
A carbonaceous material consisting of chipped
wood, was fluidized in a bed of sand under a pressure of
1 to 30 atmospheres at a temperature of 1400 to 2000 F,
in a cylindrical vertically mounted, refractory lined
pressure reactor vessel. A high efficiency cyclone
separator according to the invention was mounted inside
the reactor vessel, at its top. The gaseous products
were admitted directly into the separator, while the
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clean gases were discharged by a conduit through the top
of the pressure vessel.
The separated solid particles were discharged
from the conical bottom of the cyclone separator through
a dipleg together with a fraction of about 3 to 10% of
the gases, and re-injected deep in the fluidized bed at
the bottom of the vessel. The pressure in the fluid bed
being higher than in the body of the cyclone, it was
necessary to provide a downward driving force for the
gases and solids in the dipleg. Such a necessary
driving force was supplied by means of a gas injector
mounted in the dipleg, in which a jet of high pressure
steam entrained the gases and the solid particles into
the fluid bed.
Upon their entry into the lower zone of the
fluidized bed, the char, the gases and steam were
brought into contact with the fluidizing air, causing a
rapid reaction while the steam served to enhance the
reaction and prevent overheating of the fluid bed in
that zone and the risk of fusion of the ashes.
It will be understood that the invention and
its operation and applications are not limited to the
two embo-diments described hereinabove or to the shape
and forms shown in the drawings. As a matter of fact,
the invention is applicable to any cyclone separators or
combination of separators, wherein the separated solids
have to be discharged against a back pressure.
