Note: Descriptions are shown in the official language in which they were submitted.
1119033
This invention relates to processes and apparatus for
the treatment of materials with gaseous reagents under pressure,
and more particularly to a method and apparatus for discharging
solid materials in a finely divided condition from pressure
vessels in which gaseous reagents are maintained under pressure.
The need for chemical treatment of solid materials
with gaseous reagents under conditions of elevated pressure
and/or temperature arises in many processing applications.
Examples include high pressure and temperature steam treatments
of wood and wood pulp in pulp and paper manufacture and other
utilizations of wood, the steam treatment of particulate
materials or minerals such as gravel, stones and sand for
cleaning and purifying processes, the treatment of mineral
ores with gaseous reagents such as steam or acids to enrich
the metal content thereof and aid in the extraction of metals
therefrom, treatment of animal products with steam for rendering
purposes, treatment of bitumens, coals, tar sar.ds and other
fuel values with gaseous reagents such as hydrocarbons for
enrichment purposes, plasticizing treatments and the like.
Wheré such processes are to be conducted continuously, and the
product at the gaseous treatment is in a finely divided form,
problems arise with the discharge of the finely divided
product after treatment. Losses of gaseous reagent and gas
pressures and temperatures from the pressurized reactor vessel
as the treated product is continuously discharged therefrom
need to be minimized, for economic reasons among others.
A typical exarnple of such a process is the treatment
of wood chips with steam at elevated pressures and temperatures,
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033
in a continuously operating pressuriZed steam digestor vessel,
e.g. to prepare cellulose fibrous product. The resulting
product is in finely divided fibrous form, and has a variety
of different potential uses (insulation, animal fodder, etc.).
The economics of the process require that the vessel be maintainet
at elevated temperatures and steam pressures, and that the
fibrous product after treatment be continuously withdrawn from
the vessel without excessive losses of steam pressures there-
from. The steam pressures in the vessel may be in the 200-300
psig range. The finely dividedproduct materials must be dis-
charged in such a manner that the physical nature of the product
is not damaged, and that excessive wear and tear on apparatus
parts is avoided.
It is an object of the present invention to provide a
novel means of discharging divided material (e.g. fibrous or
particulate) from a pressurized cooking vessel.
According to the present invention, material after
coo~ing in a pressure vessel under pre-selected, elevated
pressures of gaseous reagents for the required time, is delivered
in finely divided form from the pressure vessel to a discharge
conduit, whilst still under the same gas pressures. The down-
stream end of the discharge conduit has a restricted, valved
exit passageway therein. The cross-sectional area of the exit
passage is considerably smaller than that of the discharge
conduit. A conveyor is provided in the discharge conduit which
moves the material delivered thereto towards the downstream end
and compacts it against the downstream end in a compaction zone,
to form a dense, compressed plug substantially impervious to
the passage of reagent gases therethrough under the pressures
" ~1903~
experienced in the vessel and hence in the discharge conduit
upstream of the plug. The exit conduit contains a valve which
opens intermittently to permit a small amount of the material to
exit. The valve is arranged to have an open position in which
it presents an exit passage of substantially the same cross-
sectional area as that of the exit conduit, to permit unobstructed
materials discharged through the open valve. The valve is
moved very rapidly between its closed position and its fully
open position so that there is effectively no discharge of
material through a half-open, obstructing valve.
These various features operate in combination with
one another to form a discharge and decompression system which
operates efficiently and economically. The formation of the
dense, compact plug by the conveyor in the discharge conduit
maintains the gas pressures upstream of the plug, e.g. in the
pressure vessel, whilst the process is conducted continuously
with intermittent material discharge. Gaseous reagent losses
are effectively prevented at the discharge, thereby enhancing
the economics of the process. The operation of the valve, to
lie in a fully open position or in a fully closed position, with
substantially no time spent in intermediate, half--open positions,
also contributes to this effective elimination of gaseous
reagent losses on discharge of materials. The compact plug is
being continuously formed by the conveyor in the discharge
conduit. Upon each opening of the valve, the downstream wall
or leading face of the compacted plug collapses due to its
sudden exposure to reduced pressure conditions. Since the valve
is moved extremely rapidly to its fully open position, substan-
tially the whole do~mstream face area is exposed at the same time,
and a portion of the plug transverse to the direction of
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~9033
compacting forces and extending over the whole cross-section
collapses for discharge. The overall thickness of the plug is
changed only to a small degree, and the barrier to gaseous
reagent passage maintained. With a gradually opening valve,
parts of the downstream face of the plug would be exposed to
reduced pressure preferentially to and for longer periods than
other parts, with the attendant problem of partial collapse of
the plug in certain areas only, with the same areas being
exposed preferentially repeatedly, on each opening of the valve.
The result would be channelling through the length of the plug
to destroy the s ~ of the plug against gaseous reagent discharge. In
addition, valve operation according to the invention reduces
substantially the amount of wear experienced by the valve and
hence prolongs its useful life. Impingement of particulate
or fibrous materials on moving valve parts and against passage-
way-obstructing valve structures, with consequent wear thereof,
is effectively reduced.
This mode of discharge according to the present
invention has the additional advantage of allowing a greater
degree of control of the processing times of material subject
to reaction or treatment with pressurized gaseous reagents.
With the substantially instantaneous movement of the discharge
valve from its fully closed to its fully opened position, for
a set period of time, small finite amounts of heated material
pass through the valve, and thence to conditions of reduced
pressure, with each valve opening. The discharged quantities
thus decompress and cool by expansion, thereby stopping the
treatment process, very rapidly. The residence time under
treatment conditions of the materials is therefore closely
controllable. This is in contrast to the situation where some
1119033
material passeS through a partly open, restricted valved outlet
and some of the material passes through a fully open, unrestricted
valved outlet. In such case, different portions of the
material have different decompression and cooling times, so
that the bulk of the material as a whole has an indefinite
residence time under treatment conditions.
Thus according to a first aspect of the present
invention there is provided an apparatus for discharging
particulate or fibrous material from a pressurized vessel,
which comprises: a discharge conduit adapted to receive
material from the pressurized vessel and being in pressure co~nication
therewith, the discharge conduit having an upstream end, a down-
stream end and a materials compaction zone adjacent its down-
stream endî a feed conveyor, adapted to feed material towards
the downstream end of the discharge conduit and to cause com-
paction of the material in the compaction zone; a materials
outlet at the downstream end of the discharge conduit, said
materials outlet comprising an exit passage of small cross-
sectional area as compared with that of the discharge conduit so
as to cause compaction of material in the compaction zone by
said feed conveyor; valve means in said exit passage, said
valve means being movable rapidly between a fully open position
and a fully closed position to permit passage of said material
therethrough.
According to a second aspect of the invention, there
is provided a process for discharging cooked particulate or
fibrous material from a pressure cooking vessel, which comprises:
continuously delivering said material, under pressure cooking
conditions, to an exit conduit; continuously compacting said
material into a dense compact plug substantially impervious to
passage therethrough of reagent gases under the cooking
pressures existing in said cooking vessel, said plug having a
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~1~9033
leading face disposed in said exit conduit in a direction away
from the cooking vessel and separated therefrom by at least
the thickness of the plug; intermittently and for pre-
determined brief intervals of time exposing the leading face
of said compacted plug to atmospheric pressure conditions, said
exposure taking place over the entire area of said leading face
substantially simultaneously, so as to discharge the leading
portion of said plug to atmospheric conditions.
The process and apparatus of the present invention
are particularly suitable for the steam pressure cooking of
cellulosic fibrous materials such as wood chips, e.g. in the
preparation of animal fodder for ruminants. The fibrous nature
of the material so produced, which is in finely divided form,
compacts in the discharge conduit to form a suitably dense
mass or plug, substantiaIly impervious to the passage of steam
therethrough. Moreover, the manner of discharge according to
the invention has a beneficial effect upon the degree of
digestibility of the fibrous product by ruminants, believed
due to the resultant surface pore structure of the fibrous
material.
In the preferred embodiment of the present invention,
the rate of movement of the material through the pressurized
treatment vessel and into the discharge conduit, the geometry
of the discharge conduit and its conveyor, the speed of
operation of the conveyor and the frequency of operation of
the discharge valve, are all adjusted in relationship to one
another, to obtain the desired processing conditions for the
material. These parameters are preferably adjusted to achieve
a bulk density of the compressed plug of material in the dis-
charge conduit upstream of the exit passage in the range of from
about 5 to about 20 pounds per cubic foot, preferably from
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3L~19033
about 10 to about 15 pounds per cubic foot, of equivalent
oven dry material. The actual densities experienced in the
discharge conduit will be higher than these values in many
cases, because of the presence of moisture in the material.
The actual maximum should not exceed about 40 pounds per cubic
foot, to avoid placing undue demands on theequipment. The
minimum acceptable bulk density is that which will effectively
prevent gas discharge through the exit conduit on opening of
the valve, at the chosen pressures. This varies to some
extent depending upon the nature of the material being cooked.
As is well-known in the art, permeability to steam and the
like, of a fibrous material such as steam cooked wood, increases
steeply as the void space in the ~,ass of the fibrous material
decreases, in a manner which is characteristic of the particular
material. There is of course a maximum degree of compaction
which should not be exceeded in practice, or the structural
strength of the compacted plug becomes too great for its
collapse and discharge through the opened valve.
Acceptable operating conditions will be found within the
~0 aforementioned bulk density ranges. The speed at which the
valve moves from its fully closed position to its fully
open position is faster than the speed at which the downstream
end wall of the compacted plug breaks down, on exposure to
atmospheric pressure conditions, to ensure that steam losses
are substantially avoided. The speed and frequency of
operation of the valve, the length of the discharge conduit,
the speed of operation of the conveyor therein and the rate
of feed of material from the p~essure vessel to the discharge
conduit, factors controlling the bulk density of the material
immediately upstream of the valve, are ell adjusted with this
~1~9033 ,,
feature in view. As an example, the treatment time of the
material may be within the range of 4-5 minutes, with the
valve opening every ten-fifteen seconds, to balance the
continuous treatment and intermittent discharging of the material.
The preferred form of feed conveyor in the discharge conduit is
an Archimedean screw conveyor, which can cause the necessary
degree of compaction without undue shearing of the material.
Also in accordance with the preferred embodiment
of the invention, throttling of the material, i.e. movement
of the material from high pressure, pro oessing conditions to
atmospheric pressure conditions, with consequent expansion
thereof, takes place at a location downstream of and remote
- from the valve itself. Suitably this is accomplished by the
provision of a throttle tube downstream of the valve, of
similar cross-section to the valve passageway, through which
the material discharges from the valve to atmosphere.
The separation of throttling action and the discharge
valve in the apparatus and process of the present invention
is a further advantageous feature in the successful operation
thereof. At the time of its compaction upstream of the dis-
charge valve, and until it passes through the discharge valve,
the material is still under treatment conditions, and
is very hot. The passage of the material through the discharge
valve effectively terminates the process, and the rate
and frequency of operation of the valve, along with the size
of its discharge aperture, is a significant factor in deter-
mining the residence time under treat~ent conditions of the
material. In other words, the discharge valve modulates
the flow rate in the continuous process. By separating the
.. .. . , ,, ~
two features of discharge rate control, to be done solely by
9033
the valve, and throttling of the material, to be done down-
stream and remote from the valve, faster cooling rates are
attainable. Prior art proposals tended to throttle the treated
material across the discharge valve, with consequent loss of
control and speed of cooling and expansion.
The flow characteristics of finely divided particulate
or fibrous (i.e. semi solid) materials are different from
those of fluid materials. One does not observe the same degree
of continuity of flow over full control ranges of a discharge
valve with semi solid material. Upon ex;t from the discharge
valve, and exposure to reduction in pressure, the semi solid
material will flow readily and easily through a fully open valve
passageway, but not through a restricted valve passageway. 'i
Moreover, a high degree of turbulence in the discharge stream
is created in the process of the present invention, where the
valve moves suddenly and rapidly to its fully opened position
from its fully closed position. This turbulence is desirable,
since it improves the heat dissipation from the material and
leads to a rapid rate of cooling on discharge from the outlet
tube. The amount of turbulence is controllable by changing
the length of the outlet or throttle tube. Preferably, the
length of the throttle tube is about three to five times the
diameter of the valve outlet passageway. The separation of
the discharge valve from the throttle in the preferred form of
the invention separates the function of flow rate modulation,
reserved to the valve, and expansion of the material. Erosion
of the valve by contact with the turbulent stream of expanding
material is thus avoided, further to prolong the useful life
of the valve and reduce eguipment maintenance costs.
9033
The preferred form of valve for use in the present
invention is a ball valve, in which the ball is rotatable by
power means in a housing througha 90D or 180 angle, and has a
diametric passageway extending therethrough, of substantially
the same cross-sectional shape and area as that of the exit
passage and throttle tube. Such a valve can be operated, e.g.
by hydraulic or pneumatic actuating cylinders connected to its
operating lever, or by standard electrical means, with timed
actuation, sufficiently rapidly for use in the present
i~vention. With such a valve, even when it is half way towards
its open position, only ten per cent of its fully open flow
passageway diameter is available for passage of material there-
through. This factor, in addition to its very rapid movement,
between its fully opened and fully clsoed position, makes such
a valve eminently suitable for use in the present invention.
A commercially available form of such a valve is a Kamyr
Stellite-Seat Full ~ore Valve.
In accordance with a further advantageous feature of
the present invention, the creation of the compact plug
immediately upstream of the discharge valve provides a bene-
ficial and convenient location for addition to the treated
material of chemical reagents. Thus, reagents can be added to
the compacted plug immediately upstream of the discharge valve
or elsewhere in the compacting zone of the discharge conduit.
The dense nature of the plug effectively prevents blow back
of the reagents into the pressure vessel, to interfere with
the chemical treatment process therein. Mixing of the added
reagents through the product will occur on passage through the
discharge valve and turbulent flow in the throttle tube, whilst
the material is still hot.
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~119033
As noted above, the process of the invention is
particularly suitable for the steam pressure cooking of wood
chips, to produce cellulosic fibrous product, e.g. for use as
animal fodder. In such case, the cooling of the cooked
cellulosic material is best conducted by rapid adiabatic
expansion of the cooked material, which occurs on throttling
of the material. If cooling of the material from cooking
temperatures to storage temperatures takes place too slowly,
it has been found that the nutritive value and digestibility
of the product decreases. This is believed to be due to
changes which occur in the physical nature of the product on
slow cooling. The pore sizes in the material become so small,
if cooling is not conducted rapidly enough, that the ability of
the digestion en~ymes in the animal's digestive system to
attack the material becomes impaired. Thus, it is desirable
to "freeze" the cooked material in its open-pore, freshly
cooked physical condition. The separation of the discharge
valve and the throttling action are achieved in the preferred
form of the invention by the provision of an outlet tube down-
stream of the valve, of substantially the same cross-sectional
area and shape as that of the valve orifice and the exit
passageway. Throttling thus takes place in the outlet tube
and predominantly at the downstream end thereof.
A result of the steam digestion of substantially any
hardwood material is the production therein of acetic acid.
Ammonium acetate is a synthetic protein of nutritional value
to ruminant animals, although it is not of much use to humans.
It is therefore of advantage to ada a~mDnia to the cooked material
in the vicinity of the compacted plug immediately upstream of
the discharge valve. The addition of ammonia to the cooked
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~19();33
product will thus produce ammonium acetate in situ and enhance
the value of the product. However, it is important to prevent
the added ammonia from entering the cooking digestor, where it
will interfere with the steam cooking process.
A specific preferred embodiment of the present inven-
tion as applied to the steam pressure cooking of wood, is
illustrated diagrammatically in the accompanying drawings, in which: ¦
FIGURE 1 is a cross-sectional view of an apparatus
according to the present invention;
FIGURE 2 is a side view, partly in section, of the
valve of the apparatus of Figure 1, in the open position;
FIGURE~3 is a graph showing the opening and closing
characteristics of the valve of Figure 2.
With reference to FIG. 1, there is diagrammatically
illustrated therein a pressurized cooking vessel 10 in the form
of a horizontally disposed cylinder having inlet and exhaust con-
duits 12, 14, a central rotating horizontal shaft 16 and a screw
conveyor 18 secured to the shaft 16. In operation, wood chips
or similar materials are continuously fed to vessel 10, cooked
under suitable pressures of saturated steam therein, and continu-
ously conveyed horizontally through vessel 10 by converyor 18.
At one horizontal end, the vessel 10 communicates witha discharge conduit 20 in the form of a horizontal tube, which
contains therein a horizontally mounted Archimedean screw con-
veyor 22, the peripheries of the flights of which are disposed in
close proximity to the internal walls of the conduit 20. The
screw 22 is mounted on a central horizontal rotating shaft 24,
power driven by means of a motor 26, gear-train 28, chain drive
30 and sprocket 32. The downstream flight of screw 22 is pro-
vided with axially projecting mastication teeth 2~. Cooked woody
- 12 -
033
material is delivered from cooking vessel 10 into the flights
of screw conveyor 22, and then moved to the left as shown in
FIG. 1, by screw conveyor 22. The discharge conduit 20 is in
free communication with the interior of vessel 10, so that it is
under the same pressures of saturated steam.
The screw 22 terminates some distance from the down-
stream end 34 of conduit 20, leaving a zone 36 of the dis-
charge conduit 20 unobstructed. At its downstream end 34,
conduit 20 communicates with a material outlet comprising an
exit passage 38 of small cross-sectional area, with a conical
entrance 40 thereto. Downstream of the entrance 40, the exit
passageway 38 is provided with a discharge valve 42, with an
operating lever 43 and actuation means 44. Downstream of the
valve 42 is a throttle tube 45 of substantially the same cross-
sectional shape and area as that of exit passageway 38. Throttle
tube 45 terminates in a discharge outlet end 46.
In the operation of the apparatus, the screw 20 feeds
materials delivered thereto towards the downstream end 34 of
conduit 20 and into exit passageway 38. As a result, a dense
compact plug of ~ibrous material is formed in compacting zone
36 of conduit 20 and in exit passageway 38 before valve 42.
Ammonia can be added to the product in zone 36 or passageway
38 if desired. Valve 42 opens suddenly and intermittently to
allow a small amount of the compacted plug of material to exit
to throttle tube 45 on each valve opening. The compact density
of the plug of fibrous material prevents loss of steam pres-
sures through the valve 42 when it opens. The material expands
and cools adiabatically on passing out of outlet 46 from
throttle tube 45, i.e. at a location remote from the valve 42.
Flow rates, conveyor speeds, valve opening durations and
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1119033
frequencies are all adjusted to optimize the cooking process in
vessel 10 and ensure a sufficiently compact, dense plug ahead
of valve 42 to prevent steam losses.
The valve 42 and its operation are illustrated in
more detail in FIGS. 2 and 3. In FIG. 2, the valve is shown
in its open position. The valve 42 comprises a housing 48 of
part spherical form with respective inlet and outlet conduits
50,52 communicating therewith. The inlet and outlet conduits
50, 52 communicate with and are of substantially the same size
as exit passageway 38 and throttle tube 45 respectively. A
rotatable valve element 54 is mounted for rotation in the
housing 48, the valve element 54 having a generally spherical
periphery for rotation in the housing. The valve element has
a bore 56 therethrough, of the same cross-sectional shape and
size as inlet and outlet conduits 50, 52, and turns with
central shaft 58 to bring the bore 56 into registry with
conduits 50, 52 in the open position of valve 42. Operating
lever 43 is connected to cause rotation of valve element 54.
To close the valve 42, lever 43 moves through approximately
90 degrees to its position shown in broken lines in FIG. 2,
with consequent rotation of valve element through 90 degrees,
about the axis of shaft 58, to bring bore 56 completely out
of registry with inlet and outlet conduits 50, 52 so as fully
to close the valve.
Figure 3 graphically illustrates the opening and
closing characteristics of the valve of Fig. 2, with the
percentage of material flow through the valve 42 plotted
logarithmically as vertical axis against percentage of
rotation of the valve element through its 90 degree opening
and closing arc as horizontal axis.
111~033
From this curve it will be seen that, when the valve
element is turned 20 percent through its arc from its closed
to its open position, only 2 percent of the total flow of
material through the valve is permitted. When the actuator
is turned 50 percent towards its open position, about 12 percent
of total flow is permitted. It is not until the valve actuator
has turned about 84 percent of the way to its open position
that as much as 50 percent of the total material flow through
is permitted. These opening and closing characteristics,
together with very rapid actuation of lever 43 through its 90
degree arc and similar such movement of the valve element 54,
ensure that the vaLve is either fully open or fully closed at
substantially all times, and only for very brief intervals is
a partially open valve, with an obstructed valve passage,
presented to the downstream face of the compacted plug of
material. In consequence, the plug face is suddenly presented
with an open valve and reduced pressure exposure across sub-
stantially its whole face area, so that the entire face
collapses to form an exit quantity of material to pass through
the valve. Meanwhile the plug is rebuilt and compacted
forwardly by the screw 20, until the valve opens again to
permit exit of another quantity from the forward face thereof.
The operating means 44 for the valve 42 is preferably
a pneumatic cylinder and piston connected to operating lever 44,
and equipped with an adjustable timing device to cause very
sudden and rapid movement of the piston under high pneumatic
pressure. Such mechanisms are known and do not require a
detailled description herein. Other suitable means can also
be used, provided that they are capable of rapid movement to
cause the valve to move rapidly between its fully open and
fully closed positions.
