Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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~EAT RECOVERY WIT~ DI~FE~ENTIATED MUTLI-FLAS~
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DURING PRODUCTION OF HIGH ~IELD PULP
BACKGROUND AND SUMMARY OF THE INVENTION
The production of mechanical pulps is of
increasing interest since a higher yield can be
obtained from a given amount of raw material
utilizing mechanical pulping processes as compared
to chemical pulping processes. Mechanical pulping,
in general, refers to refiner mechanical pulping
(RPM), thermomechanical pulping (TMP), chemi-
mechanical pulping (CMP), chemithermomechanical
pulping (CTMP), and board pulping. In each case a
refiner (defibrator) is utilized as one of the basic
components for breaking down the chips (or like
comminuted cellulosic fibrous material) into
progressively smaller bundles in a fibrillation
process. During the refining action, frictional
heat is developed, and that produces steam.
Despite the high yields available for
mechanical pulps, in many areas of the world the
production of such pulps is not economical because
of the energy intensive nature of the fibrillation
process. In such situations, it is necessary that
the process steam generated during the refining
process be effectively utilized, and in that way the
mechanical pulping process can be made more
economical. Typically, the pulp and process steam
in the discharge from the defibrator are separated
in a centrifu~al separator ~cyclone), or like
device, and a significant portion of the separated
steam is passed directly to a chips presteaming
vessel for steaming the chips prior to their
introduction into the defibrator. While such a
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procedure does make the mechanical pulping operation
more economical, it is much less efficient than is
desired, and can result in the eventual build up of
contaminants in the pulp to too high a level.
S The pulp discharged from the cyclone in the
conventional mechanical pulping processes is in
actuality "over qualified" for use in chips
steaming. That is, it has such a high pressure and
heat content that there are potentially many other
more valuable uses for it than presteaming the
chips. Additionally, about 50-90 percent of the
terpenes (as well as other contaminants such as
methanol), from the pulp pass into the stream of
steam being discharged from the cyclone. When the
steam is recirculated to the chip presteaming vessel
to contact chips therein, these terpenes thus are
returned to the pulp stream, and the level of
terpenes in the pulp eventually builds up. For the
production of some paper pulp products, this is
extremely detrimental. For instance in the
production of cardboard designed to hold liquids
(such as for the packaging of milk products), the
smell and taste of the product being packaged can be
affected by the turpentine within the pulp.
According to the present invention, a
method and apparatus are provided which utilize the
process steam inherently produced during the
defibrillation action in the most efficient manner,
and in a manner such that build up terpenes, and
like contaminants, does not occur. The basic
feature according to the present invention is the
flashing of the pulp stream discharged from the
defibrator in a number of different stages, with a
different "quality" of steam produced in each
stage. The steam from each stage is then used in a
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manner to best take advantage oE its pressure and heat content.
For instance the steam separated in a first centrifugal separator
is passed through a reboiler to produce fresh steam, and then the
fresh steam is used to drive the turbine for powering the
refiner, or for a like function suited for steam having that
pressure and heat content. The steam from the second centrifugal
separator (which has very little terpenes since most have been
separated out in the steam stream from the first centrifugal
separator) has a pressure and heat content ideally suited for
chip presteaming. If additional centrifugal separators are used,
the separated steam therefrom is utilized for other purposes
particularly suited to the pressure and heat content of the
separated steam.
By the practice of the present invention, the quantity of the
primary process stream -- the highest quality steam -- can be
increased 20-30 percent compared to conventional single flash
procedures, while the quantity of moisture laden hot air produced
in a cyclotube downstream of the centrifugal separator (which air
is used for hot water production) is reduced. Further, according
to the present invention turpentine from the pulp can be readily
recovered so that it is a worthwhile by-product, instead of being
a contaminant that may adversely affect the quality of the pulp
produced.
It is the primary object of the present invention to provide a
method and apparatus for the energy and cost-effective production
of mechanical pulp. This and other objects of the invention will
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become apparent from the detailed description of the
invention, and from the appended claims.
B~IEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a block diagram showing a
conventional prior art system for separation and
utlization of the process steam produced during
mechanical pulping;
~ IGURE 2 is a block diagram schematically
illustrating the practice of an exemplary method
according to the present invention; and
FIGURES 3 and 4 are schematic diagrams
illustrating other exemplary procedures, and
exemplary apparatus, according to the present
invention.
DETAILED DESCRIPTION OF THE DRAWINGS
Prior Art
In a conventional single flash heat
recovery system in the production of mechanical
pulp, as illustrated in FIGURE 1, chips ln line 10
are optionally pretreated ~as with certain chemicals
in the production of CMP, CTMP, and the like) in
station 11, and then passed to a chip presteaming
vessel 12 wherein the chips are steamed to make them
more malleable, to increase the effectiveness of the
defibrillation proces~. After presteaming in vessel
12, the chips are fed to a defibrator 13 wherein the
refining action takes place, frictional heat being
produced during the refining action changing a
significant amount of the liquid in the chips-liquid
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slurry within the defibrator 13 into process
steam. The mechanical pulp produced by the
defibrator 13~ with steam intimately mixed
therewith, is discharged from the defibrator 13 in
line 14 and passes to a cyclone 15 or like
separating means for separating the pulp from the
steam. The steam is discharged from the cyclone 15
in line 16, while the pulp is discharged in line
17. The pulp discharged from the cyclone 15
typically would have a pressure of about 4.5 bar
(ABS). For a typical operation wherein 1.387 tons
per bone dry metric ton (BDMT) of steam passes in
line 16, about 0.3 tons would be recirculated in
line 18 to the chip steaming vessel 12, while the
remaining 1.087 tons would pass Lo a reboiler 19,
where it would pass in heat exchange relationship
with feed water to produce fresh steam in line 20,
with the dirty condensate from the reboiler 19
sewered. The fresh steam in line 20 typically would
be at a pressure of 4.0 bar, produced at a rate of
1.0 tons per BDMT.
The pulp in line 17 passes through a
cyclotube 21 which is disposed in operative
association with the pulp tank 22. A cyclotube is a
large diameter (compared to that of line 17) tube
(e.g. a vertical tube one meter in diameter and 7
meters high) in which pulp and gas (moist, saturated
hot air) are separated. The moisture laden hot air
that is produced by the flashing that takes place in
the cyclotube passes in line 24 to a heat exchanger
25 and is used in the production of process hot
water, which is discharged from the heat exchanger
25 in line 26. The feed fluid in line 24 is sewered
after it passes through heat exchanger 25.
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In the practice of the prior art procedure
schematically illustrated in FIGURE 1, since the
steam in line 16 has about 50-90% of the terpene~
from the pulp, a significant amount of turpentine is
recirculated to the chips in line 18, ultimtely
producing a build up of turpentine in the pulp so
that the final mechanical pulp produced in line 28
contains significant traces of turpentine.
Additionally, the quantity of high quality fresh
steam produced in line 20 is less than desirable.
The Invention
One exemplary manner in which the invention
may be practiced is illustrated schematically in
FIGURE 2. In FIGURE 2 structures corresponding to
those in the prior art of FIGURE 1 are illustrated
by the same reference numeral only preceded by a
"1" .
In the e~emplary form of the present
invention illustrated in FIGURE 2, essentially all
of the steam in line 116 passes to the reboiler
119. Thus where the amount of steam produced in
line 116 is 1.387 tons per BDMT (same figures as for
the prior art of FIGURE 1), the amount of fresh
steam in line 120, at 4.0 bar, will be about 1.253
tons per BDMT. Also, according to the invention it
is desirable to treat the dirty condensate from
reboiler 119 at station 30 to recover the terpenes,
methanol, or like substances therefrom, and after
recovery of those materials the condensate is
sewered.
In the practice of the invention as
illustrated in FIGU~E 2, the pulp passing in line
117 passes to a second centrifugal separator 32. In
this second centrifugal separator 32 the steam is
discharged in line 33 while the pulp continues in
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the same path 117 to the cyclotube 121. The steam
in path 33 typically would have a pressure of 2.8
bar, with 0.3 tons per BDMT produced. This
"quality" of steam is perfect for presteaming in the
vessel 112, and contains very little turpentine
since most of it has already been separated out in
the first centrifugal separator 115. Should the
amount of steam in line 33 be insufficient to
properly effect presteaming in vessel 112, a small
amount (e.g. up to about 0.05 tons per BDMT) of the
steam from line 116 can be diverted into line 35,
and be introduced into the steam in line 33
utilizing a Venturi injector, or the like.
Since more of the heat content of the
discharge in line 114 is recovered prior to the
cyclotube 121 in the FIGURE 2 embodiment, as
compared to the FIGURE 1 embodiment, the amount of
moisture laden hot air in the cyclotube discharge
124 will be reduced, and thus the amount of hot
water discharge in line 126 reduced. Since turpenes
have not been significantly recirculated, the final
mechanical pulp produced, in line 128, will have
significantly less turpentine than the pulp in line
28 in FIGURE 1.
In an exemplary process for the production
of CTMP utilizing the procedures of FIGURES 1 and 2,
the illustrative data in the following table shows
that an increase in the amount of high quality
process steam on the order of about 25 percent can
be expected by practicing the invention:
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TABLE
Data:
Capacity 14 ADMT/h
Installed electrical motor 15 MW
Motor, efficiency 96%
Reboiler, efficiency92%
1 ton steam/h x 0.8141 MW
Theoretical
Process steam 0.96 x 15 = 17.69 T/h
0.814
Clean steam after reboiler 17.69 x 0.92 = 16.28 T/h
Practical
Single flash system - Fig. 1
Rate of clean steam pro-
duction 1.000 T/BDMT = O.900T/ADMT
Rate x quality 14 x 0.900 = 12.60 T/h
Recovery of energy 12.6 x_100, _ 77,4
(practical ~ theoretical) 16.28
Multi-flash system - Fi~. 2
Rate of clean steam pro-
duction 1.253 T/BDMT = 1.128 T/ADMT
Rate x quality 14 x 1.128 =15.79 T/h
Recovery of energy 15.79 x 100 _ 97 0%
(practical ~ theoretical) 16.28
Difference in recoverv (Fig. 2 vs. Fig. 1)
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Steam 15.79 - 12.60 = 3.19 T/h
Efficiency increase 3.19 x 100 - 2
(Fig. 2 over Fig. 1) 12.-60 ~ 5.3
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While only a single additional separator
has been illustrated in the embodiment in FIGU~E 2,
according to the present invention it is possible to
utilize additional flashing stages between the
defibrator and the cyclotube, depending upon the
initial pressure of the process steam in the pulp
discharged from the refiner. In the FIGURE 3
embodiment, structures comparable to those in the
FIGURES 1 and 2 embodiment have the same reference
lQ numeral only with a "2" as a first digit, and in the
FIGURE 4 embodiment a "3" appears as the ~irst
digit.
In the PIGURE 3 embodiment for the
production of TMP and CTMP, the initial chip
lS treatment apparatus is slightly different than in
the FIGURES 1 and 2 embodiment. Chips can be
initially heated in chips bin 40, washed in washing
vessel 41, impregnated with chemical in vessel 42,
and then passed to steaming tube 43 wherein
presteaming takes place. In this embodiment, an
additional steam/pulp separating structure tsuch as
a centrifugal separator, steam separating conveyor,
or the like) 45 is disposed between the separator
232 a~d the cyclotube 221. Thus there are primary
(from separator 215), secondary ~from separator
232), and tertiary (from separator 45) steam
streams. As in the FIGURE 2 embodiment, the primary
steam passes through reboiler 219 to produce clean
steam in line 220, and the secondary steam in line
233 is utilized for chip presteaming in steam tube
43. The tertiary steam discharged in line 46 from
third separator 45 typically would have a pressure
of about 1. 5 bar (as compared to about 4. 5 bar in
line 216, about 2.5 bar in line 233, and about 0.3
bar for the gas in line 22~), and would be combined
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with heat recovered from the condensate for the
reboiler in heat exchan~er 48, connected to a
scrubber 50 for utilizing the heat and then passed
in line 51 under the influence of fan 52 to chip bin
40. The fluid in line 51 cannot actually be
considered as steam but is moisture saturated hot
air, and performs a valuable function when i.e.
contacting frozen chips in bin 40.
In the FIGURE 4 embodiment, the defibrator
and related structures can be the same as in the
FIGURES 2 or 3 embodiments. The pulp and process
steam in line 314, when the FIGURE 4 structure is
utilized, would typically have process steam at a
pressure of about 6-10 bar. If even higher pressure
steam is produced in refiners in the future,
additional flashin~ stations can be utilized. In
the FIGURE 4 embodiment, three additional separators
332, 345, and 58 are utilized between the first
separator 315 and the cyclotube 321. The steam in
each o~ the lines 333, 346, and 59 is passed to a
user station 60, 61, 62, respectively. The user
station 60, 61, 62 employed is chosen depending upon
the pressure and heat content of the steam in each
of the lines 333, 346, and 59. Each of the lines
333, 346, 59 is preferably optionally connected to
the preceding steam discharge line so that make up
steam can be provided where one of the user statlons
60, 61, 62 requires additional steam. For instance
the line 335 can provide some additional steam to
line 333, which steam would be introduced by Venturi
injector 336. Similarly Venturi injector 65 can
introd~ce additional steam in line 346 from line 66,
Venturi injector 67 can introduce additional steam
in line 59 from line 68, and where it is desirable a
Venturi iniector (69) may even be provided in the
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discharge line 324 containing moisture saturated hot
air from cycltube 321, which introduces steam from
line 70.
It will thus be seen that according to the
present invention an effective method and apparatus
have been provided for increasing the production of
high quality steam during mechanical pulping
processes, and reducing the amount of turpenes, and
the like, in the final mechanical pulp produced.
While the invention has been herein shown and
described in what is presently conceived to be the
most practical and preferred embodiment thereof, it
will be apparent to those of ordinary skill in the
art that many modifications may be made thereof
lS 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
methods and structures.
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