Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND AND SUMMARY OF THE INVENTION .
The invention relates to a method and apparatus for
continuous cellulose digestion that does not require the use
of screens in the digesting zone. ~n present installations,
clogged screens in the digesting zone are a major cause of
downtime for continuous digestors. L~quid usually passes
through the screens under high pressure which has a tendency ;
to cause cellulose particles to adhere to the screen faces, `~
building up and causing complete blockage of liquid flow `
lO through the screens. If such blockage is not eliminated, then ~`
uneven treatment of the cellulose material may result. The raw
product that is treated in modern day digestors often contains
a large amount of leaves, bark, etc., and therefore has a great ;~
tendency to clog conventional digesking zone screens.
According to the present invention, the screens of `;
prior art digestors -- such as shown in U.S. Patents Nos. :
3,007,839, 3,200,032 and 3,298,899 -- are eliminated,
separation of liquid and cellulose fiber chips taking place
in a clarifying portion at the top of the digestor, and thus
there obviously are no hangups due to screen clogging, while
effective liquid-chips separation is effected. The cylindrical
impregnation zone vessel of the continuous digestor according
to the present invention is of smaller diameter and relatively
longer than conventional continuous digestor impregnation zones,
2~ and extends downwardly into the digesting zone to a point well
below the liquid outlets at the top of the digesting zone. The
cylindrical casing for the digesting zone is of significantly ~
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larger diameter than the diameter of the impregnation zone
(the area of the impregnation zone being approximately 1/4-1/3
the area of the digesting zone), and is liquid filled and has
digesting liquid outlets disposed at the top thereof, above the
lower end of the impregnation zone vessel. The inlet for the
digesting liquor must be sufficiently below the end of the
impregnation zone casing so that there is adequate, even -
distribution of the heated liquid through the chips column,
a counter-current flow to the downward movement of the chips
10 being established. This normally requires the inlet for the `;
digesting liquor be located a distance below the impregnation
zone casing of at least the radius of the digesting vessel.
The chips column effectively acts as a heat distributor.
When the chips move downwardly from the impregnation
zone into the digesting zone, because of the increase in
diameter of the container, they tend to flow generally
outwardly and downwardly and establish a generally conical
surface top over the area of the digesting zone. This surface
acts to separate the liquid flowing upwardly therethrough at
slow speed (approximately 5'/min., whereas a velocity of
approximately 50'/min. would be necessary to entrain the chips
particles with the liquid for upward flow therewith3 from the
chips, and thus forms a clarifying zone in the upper portion
of the digestor zone above the bottommost portion of the
impregnation zone vessel. Since the chips are already separated
from the liquid by the time the liquid reaches the outlets,
there is no need to provide screens at the outlets at the top
of the digesting zone.
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In order to further ensure that no chips flow through
the liquid outlets at the top of the digesting zone, a pair of
screened portions connected to suction means are provided below
the liquid outlet, but above the chip column, never being in
contact therewith. The suction means for each of the screened
portions are alternately actuatecl to suck particles adjacent
thereto into contact with the screen face and then to release
the suction to allow the particles to fall downwardly back
into the chips column.
In addition to providing the advantage of no screens,
the present invention also results in lower compaction and a
freer movement of the column, which in turn allow superior
counter-current washing in the washing zone and assists in
cleaning the screens therein. Also, although screenless
washing in a separate and distinct zone may be utilized, there
is really no need therefor, washing in the continuous digestor
vessel itself being practical with reduced chances of the
washing zone screens becoming clogged.
STATEMENT OF THE INVENTION
The present invention therefore pertains to an
apparatus for continuous digestion of cellulose material chips
and the like which comprises:
(a) a cylindrical vertically disposed impregnation vessel
- having a first given diameter, an inlet at the top thereof
and an outlet at the bottom thereofi
(b) means for introducing cellulose material chips and
the like and digesting liquid at relatively low temperature
into the impregnation vessel inlet;
(c) a cylindrical vertically disposed liquid-filled
digesting vessel having a second given diameter, greater than
the first given diameter, a top and a digesting zone and other
treatment zones therein;
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(d) the impregnation vessel extending through the top
of the digesting vessel, concentric therewith, a predetermined
distance so that the impregnation vessel outlet is disposed a
significant distance below the top of the digesting vessel,
chips entering the digesting vessel from the impregnation
vessel outlet forming a column in the digesting vessel below
the outlet from the impregnation vessel;
(e) a liquid outlet formed in the top of the digesting
vessel, a substantial distance above the impregnation vessel
outlet and above the chips columni
(f) means for introducing digesting liquid at relatively
high temperature into the cellulose material chips column at
a point a distance below the impregnation vessel outlet great
enough so that even distribution of:liquid through the chips
. 15 column results and so that short-circuiting of liquid to the
liquid outlet formed in the top of the digesting vessel is
prevented, the quantity of liquid introduced into the digesting
vessel and the first and second diameters being so dimensioned
that the velocity of liquid flowing through the chips column
to the digesting vessel outlet is substantially less than the .
velocity of liquid that entrains a significant amount of
cellulose material, so that substantially no cellulose
particles pass upwardly from the chips column to the liquid
outletj and
(9) means for withdrawing treated cellulose material from
the bottom of the digesting vessel at a yiven rate so that the
chips column is maintained at substantially the same level ~:
while individual particles thereof move downwardly through the
vessel. .`
The invention also pertains to a method for
continuously digesting cellulose material in a vertical digestor ;
having an impregnation zone defined by an impregnation vessel
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having a first diameter, and a digesting zone and subsequent
treatment zones defined by a digesting vessel having a second
diameter substantially larger than the first diameter, the `
impregnation vessel extending downwardly into the digesting
vessel a significant distance, and an outlet being disposed
in the top of the digesting vessel above the outlet of the
impregnation vessel into the digesting vessel, said method
comprising the steps of
(a) introducing cellulose material and digesting liquid
at relatively low temperature into the impregnation vessel at
the top thereof, the material and liquid passing downwardly .:
through the impregnation vessel toward said digesting vesseli .
(b) introducing chips-liquid mixture from the impregnation
vessel into the digesting vessel through said impregnation
vessel outlet so that the chips form a chips column in the
digesting vessel, the highest point of which is at the
impregnation vessel outlet;
(c) introducing digesting liquid at a relatively high ~:
temperature into contact with chips at a point a distance
below the impregnation vessel outlet great enough so that even -
distribution of liquid through the chips column results and
so that short-circuiting of liquid to the liquid outlet formed
in the top of the digesting vessel is prevented,
(d) withdrawing liquid flowing through the chips column
from the digesting vessel through said outlet disposed at the `
top of the digesting vessel without straining, the quantity of
liquid introduced into the digesting vessel and the first and
second diameters being so dimentioned that the velocity of
liquid flowing through the chips column to the digesting vessel
outlet is substantially less than the velocity of liquid that
entrains a significant amount of cellulose material in its
flow so that substantially no cellulose particles pass
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upwardly from the chips column to the liquid outlet; and
(e) withdrawing treated cellulose material from the
bottom of the digesting vessel at a given rate so that the
chips column is maintained at substantially the same level
while individual particles thereof move downwardly -through
the vessel.
BRIEF DESCRIPTION OF THE DRA~INGS
The drawing is a schematic view of an exemplary
continuous digestor according to the present invention
providing a screenless digesting zone.
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DETAILED DESCRIPTION OF THE INVENTION
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A continuous digestor and associated assembly
according to the present invention are shown generally at
10 in the drawing. The assembly 10 includes a conventional -
S steaming vessel 12 or the like from which wood chips or like
cellulose material is fed to the digestor inlet or charging
means 14, which may include a helical ~eed screw therein.
The charging means 14 feeds cellulose material entrained in ~
digesting liquid from feed line 13 or the like into a first ; ~ ;
10 zone A of the digestor, the impregnation zone, wherein the ;~
cellulose material is impregnated with digesting liquid at -
a relatively low temperature. The impregnation zone A is
defined by a cylindrical vessel 16 having a diameter D. It
is preferred that the vessel 16 taper slightly outwardly in
15 the downward direction, for instance, being 9' in diameter
at the top and 9'6" at the bottom, which tapering facilitates
the downward movement of the chips. The diameter D of the
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impregnation zone vessel 16 is smaller than for conventional
continuous digestor impregnation zones, but is also longer.
20 The vessel 16 has a lower portion 18 thereof which extends
downwardly into the digesting zone B a distance F, the bottom
of the lower portion 18 being well below the liquid outlets 24
in the digesting zone B, as will be hereinafter explained.
The digesting zone B and subsequent treatment zone C
25 are defined by a vessel 20 having a diameter E significantly
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greater than the diameter D of the impregnation zone vessel 16.
Preferably, the diameters D and E are chosen so that the cross-
sectional area of the vesseI 20 is ~ times greater than the
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cross-sectional area of the vessel 16. The vessel 20 has a top
surface 22 thereof in which one or more outlets 24 for the hot
digesting liquid are disposed. Spent digesting liquid which
passes through the outlets 24 under the force of the pressure
in vessel 20 may be circulated by circulating pump 26 through
a heater 28 or the like and otherwise recharged and recirculated
back to the digestor for further treatment of other cellulose
material. Inlets for the hot digesting liquid may be provided
by any suitable structure, such as conventional concentric -
inlet pipes 30, 32, leading from a source 29 of digesting
liquid. Regenerated digesting liquid withdrawn through
outlets 24 may be introduced through inlet pipes 30, 32.
It has been found that in order for there to be
proper heat distribution and digesting liquor distribution
in the chips column, the digesting liquor introduced by pipes
30, 32 or the like must be introduced a distance J below the
bottommost portion of vessel 16, the distance J being a
significant distance at least equal to 1/2 E (that is, at
least equal to the radius of vessel 20). As an optimum, it ~;
has been found that if E is 15', J should be about 12', or
about 80% of E. The introduction of the digesting liquid at ~-~
points 33, 34 of pipes 30, 32 also insures that there will be ~!' ''' :
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no "short-circuiting" of the digesting liquid -- that is,
by-passing the chips column G and going directly into the ;
outlets 24; a counter-current flow between the digesting
liquid and the chips column is produced. ~:'!~'. ' '' .
As cellulose material chips and the like move ;-~
downwardly through the impregnation zone A into the digesting
zone B, they form a chips column G being relatively high in
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the center, the point of entrance of vessel 16 into zone B and
tapering downwardly therefrom on all sides. The liquid may
pass through the top surface of chips column G, while the chips
are retained in the column G. T~e area H ~n vessel 20 above
the column G -- having generally the same length F as the
vessel 16 portion 18 -- acts as a clarifier or stillin~ well.
The diameters D and E and the amoun-t of digesting liquid
introduced into vessel 20 are so gauged that the velocity of
liquid flowing through portions of chips column G toward outlets
24 is approximately 5'/min., while the velocity that is necessary
to entrain the chips particles therein and move them upwardly
therewith is approximately 50'/min. This low velocity also ~ -
ensures even chips treatment. Therefore, very few chips will
be moved with the liquid upwardly past the upper surface of `-
15 column G to area H, and therefore, there is no need to provide ~r''.: `'
screens on outlets 24. `
In order to further ensure that any particles that
are entrained with liquid flowing through chips column G toward
outlets 24 are removed before entering outlets 2g, according to
20 the present invention, a pair of screens 36, 38 are provided on
the vertical side walls of vessel 20 adjacent the top surface
22 of vessel 20. Each screen 36, 38 is vertically separated
from the other and connected to a different suction line 40, 42. ~-
Line 40 interconnects screen 36 and suction source 44, while
25 line 42 interconnects screen 38 and suction source 46. Suction
is alternately applied by controlling sources 44, 46 with a -
suitable master control means 48. W~en suction is applied to
screen 36, no suction is applied to screen 38 and vice versa.
When suction is applied to either of the screens 36, 38, some
liquid is withdrawn therethrough, and particles entrained with
the liquid withdrawn therethrough are pulled to the surface of
the screen. Then when the suction is no longer applied to the
screen, the particles against the screen surface will fall
downwardly back into the chips column G. Thus, neither of the
screens 36, 38 will become clogged since suction is applied
only intermittently thereto and only for a short period of time.
Also, the screens are not in contact with the chips column G,
therefore, the tendency for the screens to become clogged is
even more greatly reduced.
Whil~ control of the suction through screens 36, 38
has been shown in the drawing to be by controlling a pair of
sources 44, 46, such control can be affected by other means.
For instance, a single suction source may be provided and
15 valves 50, 52 in lines 40, 42 alternately controlled to hook
up the single suction source with screens 36, 38. Liquid ~-
withdrawn through the screens 36, 38 may, of course, be
recirculated back to the inlet pipes 30, 32, as through line
47, and, of course, an accessory heating means or the like
may be provided for regenerating the digesting liquid before
reintroduction.
During treatment in the continuous digestor according
to the present invention, individual particles in the column G
move downwardly through digesting zone B of the vessel 20 and
eventually into other treatment zones C (for washing, further
digesting, etc.), and eventually will be drawn out through
conventional outlet 58 in vessel 20 with conventional scraper
56 facilitating outward movement of the treated chips. Screens
54 may be provided in washing zone C, if desired (of course,
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the points 33, 34 of pipes 30, 32 must be located significantly ;.
above the screens 54 so that there is no chance of digesting
liquid flowing from the pipes through the screens 54), and
there are reduced chances of the screens 54 clogging since, `
5 according to the present invention, lower compaction and freer .`.
; movement of the column G result, with improved counter-current
washing in zone C, and the freer moving, less compact column G .
having a tendency to scrape the screens 54 clean during ~.
downward movement of the chips in column G. Washing or other ;
10 treatments in zone C are conventional, such as shown in U.S. :
Patent No. 3,209,899. After withdrawal of the treated chips ~ :
from the vessel 20, they may be passed through blow tank 60 :
or the like, or otherwise conventionally treated. Additionally,
a number of level detectors 62 for indicating compaction of the .~ :
15 chips column may be provided, if desired. .
Operation of the apparatus according to the present ; .`
invention is as follows: Cellulose material, such as wood chips
and the like, is introduced into the continuous digestor
charging means 14 from steaming vessel 12, and a first quantity ;~
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20 of conventional digesting liquid at a relatively low temperature ~.
is introduced through line 13 into charging means 14. The chips- :
liquid mixture moves through impregnation zone A, which is --
defined by a vessel 16 which has a diameter D, is relatively
smaller than the diameter of conventional digestor impregnation
25 zone sections, and which extends a distance F into the vessel 20
containing digesting zone B and further treatment zones C. ~
Digesting liquid is introduced into the chips at inlets 33, 34 ~ .
or the like a distance J beIow the bottom vessel 16, and flows
upwardly through portions of the chips column G into outlets 24 ~.
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at the top of the vessel 20. The quantity and velocity of
liquid introduced into vessel 20 and the relative diameters .
D, E of the vessels 16, 18 are adjustea so that the liquid
moves upwardly with a sufficiently small veIocity that very
few chips are entrained therewith, the upper portion H of
the vessel 20 acting as a clarifier or stilling well, and
therefore, no screens for outlets 24 are required. To remove ~;
any chips particles that might be entrained with liquid flowing
toward outlets 24 in portion H above chips column G, a pair of i:
10 alternately operated screens 36, 38 are provided above the ~
surface of chips column G but below the outlets 24. After
treatment in the digesting zone B, the individual chips in.~ :
the chips column G move downwardly through the continuous
digestor vessel 2~, and are further treated according to
conventional practice, the treated chips being finally with~
drawn through chips outlet 58 at the bottom of vessel 20. ~.~
While the invention has been herein shown and `~ :
described in what is presently conceived to be the most :
practical and preferred embodiment of the invention, it will
be apparent to those of ordinary skill in the art that many
modifications may be made thereof within the scope of the
invention, which scope is to be accorded the broadest ~ :`
interpretation of the appended claims so as to encompass
all equivalent structures and methods. :~ :
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