Note: Descriptions are shown in the official language in which they were submitted.
1(~63~09
BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to the treatment of cellulose
material with oxygen or an oxygen-containing gas at elevated
pressure and temperature in a closed oxygen reactor, and the
monitoring of the levels of explosive gases in the oxygen reac-
tor to minimize the chances of explosion, and the dispensing
of oxygen-treated pulp in the reactor to prevent channeling.
While according to the present invention gas monitoring may
take place both in reaction vessels that work completely full - -
; 10 of pulp (wherein gas pockets can form), the methods and appar-
- atus according to the invention are specially adapted for reac-
tion vessels wherein pressurized gas is disposed in a gas
chamber formed at the top of the reactor vessel.
In the past it has been known to treat pulp from
a continuous digestor with oxygen in a reactor having an inner
sub-vessel with open top concentric with an outer chamber sur-
rouding the sub-vessel. Such a reactor is shown in Swedish
`- patent publication 379,069. In such reactors, the oxygen is
introduced before the pulp enters the reactor, and/or while it
' 20 is moving upwardly in the sub-vessel of the reactor, and a
quantity of excess oxygen is released at the upper level of the
pulp in the reactor, along with substances formed in the treat-
, ment and steam. Excess oxygen is introduced during the treat-
~, ment to insure that the explosive gases that will be produced
'5 25 during treatment are diluted by the oxygen at the top of the
`'s reactor. As a result, a much larger amount of oxygen is
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utilized than is necessary for the treatment of the pulp, and consequently
the costs of treatment are relatively high since oxygen is expensive. Also,
in such known reactors, when the pulp is dispensed over the top of the sub-
vessel into the outer chamber, channeling thereof can occur with resultant
non-uniform qualities of the pulp.
- According to the present invention there is provided apparatus for
oxygen bleaching of pulp comprising an oxygen reactor vessel including an
inner concentric sub-vessel having a larger diameter at the open top thereof
than at the bottom ~hereof, and an outer chamber surrounding and completely
enclosing said inner sub-vessel, said outer chamber having a pressurized gas-
filled top portion thereof above the open top of said inner sub-vessel, means
for transporting high-consistency pulp to an inlet in the bottom of said sub-
vessel, means for adding oxygen and steam to said pulp in said means for -
transporting said pulp before entry into said opening in the bottom of said
sub-vessel, means for emulsifying the pulp and the oxygen to thoroughly mix
them together before entry of said pulp into the bottom inlet in said sub-
vessel, rotatable scraper means associated with said inner sub-vessel at the
: top thereof to distribute pulp over the top edge of said inner sub-vessel into
said outer chamber, an outlet for oxygen bleached pulp from said outer
chamber, located sdjacent the bottom of said outer chamber, means for sensing
the concentration of explosive gases in said top portion of said outer chamber,
and means for exhausting explosive gases from said top portion of said outer
chamber upon the concentration of explosive gases in said top portion exceeding
. a predetermined amount and replacing the exhausted gases with compressed air
. or the like to maintain the pressure in said top portion of said outer chamber
. while reducing the potential for explosion therein.
In another aspect, the invention provides a method of bleaching pulp
- with oxygen utilizing an oxygen reactor having an inner, open-top, concentric
sub-vessel with a bottom inlet therein, and an outer chamber having a pres-
~ 30 surized gas-filled top portion disposed above the open top of said sub-vessel,
and a pulp outlet adjacent the bottom of said outer chamber, said method
comprising the steps of transporting digested, washed pulp having a consistency
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between 3-30% toward the oxygen reactor inlet, adding oxygen and steam to
said pulp, emulsifying the pulp and oxygen to thoroughly mix them together,
feeding the emulsified oxygen-rise pulp into the bottom inlet of said sub-
vessel, the pulp moving upwardly in said sub-vessel to the top thereof,
maintaining the pressure in the oxygen reactor at about 1-12 atmospheres and
the temperature at about 100-140C, scraping the pulp at the top of the sub-
vessel so that it is dispensed into said outer chamber from said sub-vessel,
expelling the oxygen treated pulp from the outlet at the bottom of the outer
chamber of the oxygen reactor, sensing the concentration of explosive gases in
the top portion of said outer chamber, and exhausting the explosive gases from
the top portion of said outer chamber when the concentration thereof exceeds
a predetermined level, and replacing the exhausted gases with compressed air
or the like.
The drawbacks inherent in known devices and methods are eliminated,
and a pulp of more uniform quality, and utilizing less oxygen for the treat-
ment thereof, is provided. According to the present invention, only the
amount of oxygen necessary for treatment of the pulp is provided, the oxygen
and pulp being thoroughly mixed with the oxygen in an e~ulsifier (such as a
defibrator) and bleaching occurring at a pressure of about 1-12 atmospheres,
and a temperature of 100-140C. The level of the explosive gases that collect
, in the top portion of the reactor are sensed, and if they exceed a certain
predetermined value ~i.e. 0.7 - 0.8% for turpentine, 3.5% for C0), gases are
exhausted from the top portion of the reactor, and are replaced with compressed
air, or another relatively inexpensive gas that does not have explosive
potential. The compressed air can be metered into the top portion of the
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-`~ reactor in response to a drop in the pressure in the top portion.
Also, according to the present invention, a scraping means is provided
that does not allow channeling, the scraper comprising a rotatable shaft, a
pair of arms extending outwardly from the shaft and each arm having a plur-
~; 30 ality of blade members
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attached thereto. The blade members extend downwardly from
the arms, and the bottom edges thereof are disposed substan-
tially at the top edge of the sub-vessel. The blade members
are arcuate, and the leading edges thereof are closer to the
shaft than the trailing edges thereof. The blade members cio-
sest to the shaft have rounded portions thereof disposed around
the shaft, and the trailing edge of the blade member fu~therest
from the shaft is substantially at the radial extent of the edge
of the sub-vessel.
It is the primary object of the present invention
to provide for less expensive, more uniform oxygen bleaching
of pulp. This and other objects of the invention will become
clear from an inspection of the detailed description of the
invention and from the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a schematic view of exemplary oxygen-
bleaching apparatus according to the present invention;
FIGURE 2 is a detail cross sectional view of the
upper part of the oxygen reactor of the apparatus of FIGURE l;
~ 20 FIGURE 3 is a top view of an exemplary scraper accord-
- ing to the present invention taken along lines 3-3 of FIGURE 2;
- and
FIGURE 4 is a top view of an exemplary caustic liquid
, dispensing means taken along lines 4-4 of FIGURE 2.
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1()63409
DETAILED DESCRIPTION OF THE INVENTION
An exemplary treatment system according to he pre-
sent invention is shown diagrammatically at 10 in FIGURE 1.
The reactor vessel 12 provides for oxygen bleaching of pulp
as the pulp is fed upwardly through the interior concentric
sub-vessel 14, and then is uniformly distributed over the top
16 of the sub-vessel 14, and falls downwardly in the outer cham-
ber 17. The sub-vessel 14 is narrow at the bottom, and gra-
dually widens to the top edge 16. The rotating scraper 18
according to the present invention, at the top 16 of sub-ves-
sel 14 is so desiyned that it prevents channeling of the pulpwhile it is being distributed over the top 16 into chamber 17.
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Also, according to the present invention there is provided a
gas monitoring system, shown generally at 19, in the top por-
~- tion 20 of chamber 17 to control the atmosphere in portion 20
so that an explosive level of CO, turpentine, and like poten-
^ tially explosive gases will not be achieved, and this control
~ is achieved with the minimum possible expenditure.
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According to the present invention, washed pulp is
fed from a conventional continuous digestor 22 or the like
through line 23 to an emulsifiex 24 (a defibrator or refiner).
The pulp in line 23 has a consistency of about 3-30%, about
25% being preferred. The pulp must be washed in order to re-
, move the unoxidizable solids therefrom. Oxygen, steam, and
magnesium sulphate are added to the pulp in line 23 before the
pulp passes into the emulsifier 24, the emulsifying of the
oxygen with the pulp making the oxygen readily available for
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reaction with the pulp, and allowing high consistency pulp to
be treated. O~ may also be added to the pulp at other points
before passage thereof over edge 16 of sub-vessel 14. NaO~ or
a like caustic may also be added to the pulp in line 23, or
alternatively, it may be added by rotating distributor 26 in
the sub-vessel 14, being added to the pulp after it is emulsi-
fied with oxygen. From emulsifier 24, the pulp passes through
line 25 into sub-vessel 14 wherein it flows upwardly until it
reaches the top edge 16 of the sub-vessel 14. The pressure in
reactor 12 is maintained at about l-lZ atms., and the tempera-
ture at 100-140C.
At the top edge 16 of the sub-vessel 14, the rotat-
; ing scraper 18 is provided, the scraper 18 preferably being
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concentric with the rotating caustic distributor 26. The ro-
tating distributor 26 is shown most clearly in FIGURES 2 and
4, and comprises a central channel 29 through which the NaOH
or the like is fed, a pair of hollow arms 30 extending radially
from the channel 29, and a plurality of nozzles 31 disposed
along the length of the arms 30 through which the caustic flows
into the pulp in sub-vessel 14. As the distributor 26 rotates
~; in direction A, the caustic is fed through nozzles 31 and mixed
into the pulp by the rotating action of the arms 30, and by
the natural action of the caustic when it is inserted into
the sub-vessel 14 with a rotational velocity component.
Good, even distribution of the caustic is insured by the dis-
tributor 26.
- The scraper 18 has a central rotating shaft 33 with
- a pair of arms 34 extending therefrom. Each arm 34 has a
plurality of arcuately shaped blade members 35 extending from
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the bottoms thereof ~see FIGURBS 2 and 3 in particular), in-
cluding a pair of blades 35' disposed most closely to the
shaft 33 which have rounded portions 36 terminating the blades
35 on the leading side 37 of the arms 34 when rotating in the
direction of rotation A. No-rounded portions 36 are provided
for the blades 35' on the trailing side 38 of the arms 34
when rotating in the direction of rotation A. The leading
edges 35".~in the direction of rotation) of the blade members
are closer to the shaft 33 than the trailing edges thereof.
- 10 The scraper 18 is so designed that the furtherest point radi-
ally from the shaft 33, the tip 39 of the outer blades 35, is
substantially radially even with the edge 16 of the sub-ves-
sel 14. With the construction of the scraper shown, there is
no channeling of the pulp as it is distributed from the sub- .
vessel 14 into the chamber 17. Any suitable power means 40
may be provided for rotating the distributer 26 and the scraper
18, the channel 29 extending throughout the length of the
.~ shaft 33 to deliver caustic to the distributor 26.
Once the pulp has reached the top edge 16 of the
sub-vessel 14, substantially all the oxygen therein has been
~: consumed in the bleaching process. Thus, there is no need to
introduce pure oxygen - which is e~pensive - into the upper
portions 20 of the chamber 17 to replace explosive components
since the oxygen will not significantly contribute to the
bleaching action. During the treatment process, carbon monox-
. ide, turpentine, and methanol are gaseous materials that may
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1063409
cause an explosion if in large enough concentration, and ad-
ditional unsuitable scum suppressing agents ~e. g. photogene)
can produce explosive gases. The lower threshold for explo-
sion of CO mixed with oxygen is about 12% by volume for tem-
peratures that are normally used in oxygen bleaching (althoughthe threshold is relatively little affected by pressure and
temperature). A reasonable level of safety for the CO concen-
tration is about 3.5%, however.
.
In order to keep the CO concentration at 3.5% or
below utilizing the techniques of Swedish Patent Publication
.
379,069, an excessive amount of oxygen would have to be ex-
hausted from the upper portion 20 of the chamber 17, with con-
~, sequent wasteful addition of oxygen to the pulp for treatment.
Assuming that 420 g CO forms per ton of pulp, and that the
partial pressure of the 2 is 90% of the total pressure, about
~;- 12 kg 2 per ton of pulp would have to be exhausted from the
- gas chamber 20, if the CO concentration is not to exceed 3.5%
' by volume. In turn, this means that a corresponding excess of
`' oxygen must be supplied to the pulp earlier in the treatment,
, 20 as in line 23. Practical experiments have shown that in con-
' s,ideration of the lignin-removing reaction, it is advantageous
'; to conduct the oxygen treatment with a certain excess of oxygen.
'' This excess may be relatively little, however, only about 2 kg
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2 per ton of pulp. In this example then the extra quantity
' 25 of oxygen that must be supplied to keep the exhaust gas CO
concentr-t~on below 3.5~ must thu~ be about 10 kg 2 per ton
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lQ634(~9
of pulp. For a normal size plant with production of 600
tons of pulp per day, this extra quantity of oxygen entails
an extra outlay hundreds of thousands of dollars a year.
Turpentine is found in coniferous wood, especially
in the heart of pine wood. ~he main proportion of the turpen-
tine is removed in discontinuous cooking by degassing and in
continuous cooking in a Kamyr cooker, in the cooker's "~i-
~eat" washing ~see ~.S. Patent No. 4,002,528 for an exemplary
cooker and washer). Depending upon the method of operation
of the cooker, the amount of turpentine in the blow lines can
be between 1.0 and 5.0 kg per ton of pulp. If we assume a
temperature of 120C in oxygen treatment and a pressure of
10 atm, with saturation there will be maximally about 3.3%
by volume turpentine in the gas chamber. The explosion thres-
hold for turpentine in an air mixture is low, only 0.7 to
`- 0.8% by volume. If we assume that the amount of turpentine
:~ in the pulp that is to be treated is 2.0 kg per ton of pulp
- and the 2 partial pressure in the exhaust gases is 90~, -
' about 50 kg 2 per ton of pulp would have to be degassed if
the turpentine content is not to exceed about 0.8~ by volume.
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i To get the necessary margin of safety the turpentine concen-
tration would have to be kept even lower. The consumption of
oxygen therefore will be higher than S0 kg per ton of pulp,
; which would make the process very uneconomical. The above
calculations are approximate because many variables enter into
~ it. It must however be accepted as an illustration of the
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1~63409
dangerous levels that can be reached, and of the need to de-
gass the reaction vessel. The calculations further show that
if we replace the removed gas conventionally by addition of
oxygen, the costs for the extra oxygen will be so high that the
treatment method will become economically infeasible.
According to the present invention, the explosive
- levels of CO, turpentine, and the other explosive gases that
might be present, can be kept well within the safe limits,
while no excess oxygen need be added during treatment to make
up for oxygen that will be exhausted from the top 20 of cham-
ber 17 to keep the explosive levels within the safety range.
Exemplary apparatus according to the present invention is
shown generally at 19 in FIGURES 1 and 2~ The apparatus 19
~ comprises an exhaust line 41 in which an adjustable remote-
- 15 controlled valve 42 is disposed, the valve 42 being controlled
by a sensing means 43 operatively connected thereto. A probe
for the sensing means may be disposed within the chamber 20
as shown in dotted line at 44 in FIGURE 2, or alternatively
gas can be fed through pipe 45 to sensing means 43, and then
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exhausted through valve 42 after analysis thereof. The sens-
ing means may be any suitable conventional type that can moni-
-~ tor the concentration of the explosive gases (espqcially CO and
turpentine) that are expected to be present in the chamber 20
~` such as a flame ionization detector. When the sensing means
43 senses an unacceptable level of any explosive gas within
x) The exhausted explosive gases are discharged to a further
station 42', and they are not returned to the reactor 10,
or otherwise returned to the process.
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la6340s
the chamber 20, the sensing means 43 opens valve 42 to ex-
haust enough gas from the chamber 20 to reduce the explosive
gas concentration.
Cooperating with the exhaust line 41 is an intake
line 50, the line 50 being disposed exactly on the opposite
side of the vessel 12 as the exhaust line 41. The line 50 is
connected to a source of compressed air 51, or other suitable
inexpensive gas that does not have significant amounts of
potentially explosive components therein. Valve 52 is dis-
posed in line 50, and controls the intake of air into thechamber 20 from the source 51. The valve 52 is remotely con-
trolled by a pressure sensing means 53, a probe 54 extending
- from sensing means 53 into the interior of the chamber 20.
When the pressure in the chamber 20 is reduced below a pre-
determined value necessary for proper pressurization of the
vessel 12 for oxygen bleaching, the sensing means 53 controls
valve 52 to ~pen it and allow compressed air or the like to
- be introduced into the chamber 20 until the desired pressure
is achieved. The exhaustion of the gas through line 41 and
the consequent introduction of non-explosive, inexpensive gas
through the line 50, thus insures that an explosive level of
gases in chamber 20 will not be achieved, while not requiring
the utilization of excessive amounts of oxygen to control the
explosive levels. As mentioned above, since substantially all
of the bleaching has taken place before the pulp is dispersed
over the top edge 16 of the sub-vessel 14, there is no reason
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to provide excessive amounts of oxygen in the chamber 20.
When the gas from the line 41 is analyzed, it is found that it
normally has an oxygen content of about 20%, slightly less
than that in normal air (which indicates that all of the oxy-
: 5 gen introduced into line 23, and at any other points during
the pulp upflow in sub-vessel 14, has been consumed.)
~. Alternatively, the sensing means 43 can control
`~ both the valves 42, and 52, coordinating opening and closing
thereof to maintain the proper pressure, the means 53 only
' 10 being used ~if at all) as a back-up device to insure proper
pressure in chamber 20.
After the pulp is passed over the top edge 16 of
the sub-vessel 14, it falls through chamber 17 until it achieves '
:`, a level 60 within the outer chamber 17 of vessel 12. A
plurality of baffles 61 or the like may be provided within the
-chamber 17 to break the fall of the pulp in chamber 17, caus-
ing a cascading of the pulp until it reaches the level 60. The
pulp is fed from the chamber 17 through the outlet 62 of the
.. 'l vessel 12 into an outlet line 63 for transfer to further
~, 20 treatment stations. Suitable stirring or diluting apparatus,
.~ shown generally at 65 and 66 respectively in FIGURE 1, may be
provlded if desired, as may,a recirculation line and pump 67,
, 68 (for start up and for facilitating further stirring or di-
.' lution~, the line 67 selectively being placed in or out of com-
"', 25 munication with outlet line 63 by controlling valves 69.
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Apparatus accordlng to the present invention may
be utilized according to the following example: Sulfate pulp
from conifer wood that has been washed in continuous digester
22 having a Kappa number of 55, was fed to emulsifier 24 under
the pressure of the digester 22. A pulp quantity of 75 tons
per day was provided, and the pulp consistency was 8% tnor-
mally a consistency of 25~ is desixable). 35 kg of NaOH was
added per ton of pulp, and 22 kg of 2 per ton of pulp was
added to line 23. In addition, compressed steam was supplied
to line 22 so that the pulp suspension was heated to about
110C. The defibrator or emulsifier 24 comminuted that part
of the pulp that was not already freed as individual fibers, and
completely mixed the oxygen and the pulp so that the oxygen was
readily available to the pulp and a homogenous suspension was
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provided. The pulp suspension flowed continuously through
` line 25 into the sub-vessel 14, and upwardly in the sub-vessel
14. At the actual production level, the residence time of the
- pulp in sub-vessel 14 was about 40 minutes. At the top 16
of the sub-vessel 14, the pulp suspension was scraped off by
scraper 18, without channeling, and transferred to outer cham-
:
ber-17, falling down to level 60 after passing through baffles
61 or the like. Thus, by this oxygen treatment, the Kappa num-
ber of the pulp was lowered by 30 units to 25. The pressure
in the chamber portion 20 of the chamber 17 was 5 kp/cm2. The
reactor pressure was adjusted by delivery of oil-free compressed
air from source 51 through line 50, and through line 41 gas was
withdrawn from the reactor so that the turpentine content
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thereof did not exceed .25% by volume. The amount of compressed
air added from source 51 to compensate for the gas that was
exhausted amounted to about 30 kg per ton of pulp.
While the invention has been herein shown and de-
scribed 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 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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