Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
7~
METffQD AND APPARATUS FOR
OXYGEN DELIGNIFICATION
Cross-Reference to Related Ap~
This application is related to Canadian
application Serial No. 365r411, entitled "APPARATUS
AND METHOD FOR MEDIUM CONSISTENCY OXYGEN
DEhIGNIFICATION OF PULP", filed November 25, 1980;
this application is also related to U.S. Patent
No.4,248~662 to Wallick et al, entitled "OXYGEN
PULPING WITH RECYCLED LIQUOR", issued February 3,
19~1 .
Background of the Invention
_ _
This invention relates to delignifying pulp
in the presence of oxygen, and more particularly to
an apparatus and process for the efficient addition,
removal, and recycle of oxygen gas in a pulp
delignification system.
Oxygen delignification can be carried out
on a wide variety of fibrous materials including
wood chips and pulp. When carried out on a
bleachable grade of pulp, the process is generally
referred to as oxygen bleaching. Conventional
apparatuses and processes for the oxygen
delignification of fibrous material such as
cellulosic pulps generally react the materials in a
pressurized vertical vessel. One of the problems
encountered in oxygen delignification systems is
that the partial pressure of oxygen in the vessel is
reduced by the presence of air which enters the
vessel with the pulp and by other gases which are
produced during delignification such as carbon
dioxide, carbon monoxide, and hydrocarbon gases.
Depending upon the purity of the oxygen gas used,
inert gases such as nitrogen and argon may also be
introduced along with the oxygen gas into the
reaction vessel. The reduced partial pressure of
oxygen can have a detrimental effect on
, ~.
i ~i,
BFN 6~98 -2-
deliqnification resulting not only in a slower
reaction rate, but also a reduction in pulp
brightness, strength, and otller properties.
Additionally, the presence of combustible gases such
as carbon monoxide and hydrocarbons can be dangerous
if their concentration reaches or rises above the
lower explosive limit.
One method of increasing the partial
pressure of oxygen in the reaction vessel is to
increase the operating pressure used for the
reaction. However, increased operating presswres
require thicker-walled, and therefore more
expensive, vessels. Additionally, the danger of gas
leakage from the vessel is increased, and the
feedinq of the pulp into the vessel against this
higher pressure becomes more difficult.
Alternatively, the par-tial pressure of
oxyqen can be inreased and the partial pressures of
other qases reduced by bleeding gas from the
reaction vessel and replacing it with oxygen.
However, this procedure increases oxygen usage and
removes heat from the vessel. In order to minimize
the loss of oxygen resulting from bleeding, it is
possible to oxidize catalytically the organic
product ~ases formed during the deliqnification
reaction and recycle at least a portion of the bleed
stream back to the reactor vessel while still
maintaininq the concentration oE combustible qases
below the lower explosive limit.
Temperature control durinq oxygen
delignification can also be a problem due to the
exothermic nature of the reaction. Generally, the
pulp must be preheated prior to its entry into the
reactor to a temperature sufficiently high to
initiate the oxidation reaction. However, once
initiated, the heat evolved during the reaction must
~L~7~
sFN 6~98 -3-
be controlled to prevent pulp deqradation which
results from too much heating. This over-heating
problem is especially acute -Eor processes designed
to generate a ]arqe Kappa number decrease (i.e., 30
UllitS or more) in the pulp.
Circulation and cooling oflthe reactor gas
has been used as a method of ontrollinq the
temperature within the reactor vessel when operatinq
with hiqh consistency pulp. For example, Hillstrom
et al, Svensk Paperstid, ~Jol. 80, pp. 167-70 (April
10, 1977), teach bleedinq qas from the top of a
vertical deliqnification reaction vessel to control
the content of carbon monoxide and organic qases
therein. The carbon monoxide and organic components
of the qas are then catalytically oxidized and the
qas cooled and recycled back to the reactor vessel.
Carlsmith, U.S. Patent No. 3,964,962,
teaches withdrawing a portion of qas from a vertical
delignification reactor vessel and recyclinq it back
to the upper portion of the reactor. It is taught
that the withdrawn qas may be optionally cooled, and
the system provides a means to redistribute and
control heat within the vessel. Laakso et al, U.S.
Patent No. 4,177,105, teaches a similar gas cooling
and recycle system for a vertical delignification
reactor vessel. Finally, Luthi et al, in a paper
entitled "Gas Concentration and Ternperature
Distribution in Oxygen Deliqnification," presented
at the 1977 TAPPI Alkaline Pulpinq ConEerence,
Washinqton, D.C. November 7-10, 1977, studied both
concurrent and countercurrent qas recycle schemes
for a vertical oxgen deliqnification vessel.
~ lowever, there are several problems
inherent in attempting to control both the partial
pressure and temperature of oxygen qas in a
conventional vertical delignification reactor.
BFN 6898 -4-
,
Luthi~et al, supra, found that the use of
countercurrent gas recirculation to achieve adequate
temperature control required large gas flows to
: avoid undesirable hot spots in the vessel and c.ould
result in pulp hang-ups. With respect to concurrent
qas rec.irculation, Luthl et al. concl.uded its use for
purposes of temperature control. is limited by the
compaction of pulp whih occurs in the reactor
vessel. Additionally, in order for concurrent gas
movement to occur at a speed ~reater than the speed
of the pulp, the pulp must be of a high (i.e., 30~)
consistency. It is well known, however, that high
consistency operation can lead to large temperature
inc.reases in the pulp durinq deliqnification because
of the presence of less dilution water to absorb the
heat qe n erated.
Finally, movement of qas throuqh a vertical
column of pul.p such as is present in conventional
hiqh c.onsistency delignification systems ~ay not be
uni~orm. Gas channelinq can OCCUL which can lead to
hot spots and poor qas distribution in the vessel
resultlnq in pulp deqradation and/or an increased
danqer that comb-lstion ~ill occur. Vertical upflow
reactors used.for low or medium consistency oxygen
deliqnification, such as those disclosed by Richter,
U.S. Patent No. 4,093,511, Roymoulik, U.S. Patent
No. 3,832,276, and Annerqren et al, 1979 Pulp
Bleaching Conference, Toronto, Canada, June 11-14,
1979, paqes 99-105, are especially susceptible to
channeling of gas and pulp up through ~he reactor
leadinq to nonuniform qas and temperature
distribution. The channelinq of pulp in this type
of system is illustrated by the residence
distribution curve for the 10 ton/day pilot system
used by Annerqren et al which shows a broad range of
residence times for pulp in the reactor as well as
BFN 6898 -5-
an actual mean residene time substantially less
than the theoretical residence time. This
channelinq problem can be expected to be much worse
for a larqer diameter commercial size reaction
vessel.
Attempt.s have been made to modify vertical
upflow reactors of the type described above to avoid
channelinq problems. However, the equipment used to
accomplish this is extremel~ complex as shown by
Sherman, U.S. Patent No. 4,161,~21. Moreover, these
vertical upflow reaction systems have the additional
disadvantaqe of the inability to circulate qas
throuqh the reactor for temperature control since
the qas is present as a dispersed phase and travels
upwardly at the same speed as the pulp. Jamieson,
U.S. Patent No. 3,754,417, has suqgested other
reactor desiqns for oxygen deliqnification at low
pulp consistency. Ilowever, those systems also have
serious çhanneling problems and require large inputs
of heat because of the low consistency operation.
Accordinqly, the need exists in the art for
an improved means of supply and recirculation of qas
in an oxygen delignification system. The need is
especially acute for those systems in which large
amounts of delignification are desired since the
amount of heat and quantity of combustible and
diluent gases qenerated will be larqe.
Summary oE the In_ention
In accordance with the present invention,
an oxy~qen deliqnificat;on system is provided using
one or more substantially horizontal tubes having
internal screws for pulp transport as the oxygen
reactor. Oxygen qas is introduced into the system
at a point adjacent the pulp inlet and moves in
essentially pluq flow in the same direction as the
pulp throuqh the system. Bleed qas can be removed
. .
BF~ 6~9~ -6-
from the system at a point adjacent to the pulp
outlet ~
A qas space is maintained at the top of
each tube during the delignification reaction so
that free movement of qas in essentially pluq flow
is achieved. The speed of the internal screws
controls the retention time of the pulp in the
reactor and insures that the pulp moves in plug
flow. In order to insure the free movement of gas
at a speed different from the speed of the pulp, it
is essential that the pulp level must be no more
than 90~ of the total tube volume. The gas will be
movinq substantially faster near the pulp inlet than
near the pulp outlet because of the high rate of
oxyqen consumption at the start of the reaction, and
this qas movement flushes the nitroqen and
combustible qases towards the discharqe end of the
system. The gas which is trappecd within the pulp is
exchanqed with the free qas above the pulp as a
result of the action of the conveying screw, which
continuously lifts and turns over the pulp mass in
the tube.
In this manner, the continuous movement of
qas from the inlet to the outlet of the reactor and
the exchanqe of Eree qas and trapped gas prevents
the formation of pockets of qas which have a higher
content oE potentially explosive qases or a lower
content of oxyqen. Simllarly, a temperature
equilibrium is maintained so that hot spots cannot
develop.
Since pure oxyqen is introducecl near the
pulp inlet, the pulp is exposed to the highest
partial pressure of oxyqen durinq the initial stages
of the reaction where delignification is the most
rapid and oxyqen consumption is the qreatest. As
the pulp is advanced by the internal screws, more
BFN 6398 ~7~
oxyqen is consumed and more reaction product qases
such as carbon dioxlde, carbon monoxide, and
hydroearbons are generated. The content of carbon
dioxide in the qas phase increases not only due to
the deliqnification reaction, but also because as
the pH of the alkaline reaction liquor decreases as
delignification proceeds, carbonate and bicarbonate
ions in the liquor can decompose into free carbon
dioxide qas.
As the deliqnified pulp approaches the
diseharqe point at the end oE the reactor, the
partial pressure of oxyqen is at its minimum value
while the partial pressure of other qases present in
the system is at a maximum. ThuS, the qas which is
lost from the system on discharge with the
deliqnified pulp is the qas of lowest oxygen
eontent. Moreover, the practice of the present
invention optionally provides for bleedinq of the
qas from the system at a point adjacent the pulp
discharqe outlet. This bleeding removes from the
reaGtor the gas having the maximum content of
non-oxyqen qases ;.ncludinq potentially explosive
qases such as carbon monoxide and hydrocarbons.
When relatively small amounts of delignification are
desired, bleeding may not be necessary.
The process of the present invention is
applicable to the oxyqen delignification of all
types of cellulosic materials includinq wood chips,
baqasse, straw, other agricultural materials, ground
wood, thermomechanical pulp, chemimechanical pulp,
semichemical pulp, rejects and knots from a pulping
process, and chemical pulps sueh as l~raft, soda, and
sulfite pulps. The eonsistency of raw material
introduced into the reator may be from 1% to 35%,
and preferably from 8% to 20%.
~h~
BFN 6898 -8-
The alkaline liquor used in the
deliqnification reaction may be known alkaline
materials used in the art includinq sodium
hydroxide, sodium carbonate, sodium bicarbonate,
ammonia, Kraft white liquor, oxidized Kraft white
liquor, qreen liquor, sodium tetraborate, sodium
metaborate, and mixtures thereof. The dosage of
alkaline material on the raw material may be varied
over a wide ranqe and is qenerally in the range of
from 0.5% to 30~ calculated as Na2o on oven dry
raw material. Known protector chemicals such as
maqnesium compounds can be used if desired to
preserve the viscosity and strenqth of the pulp.
The temperature and pressure and retention time used
for the delignification reaction can similarly be
varied over ~ wide ranqe. It has been found that
tempera~ures of ~rom 80-160C and an oxyqen
partial pressure oE from 20-300 psiq and retention
times of 5-120 minutes will produce suitable
deliqnification.
In an alternate embodiment of the
invention, a countercurrent flow of oxyqen gas
throuqh the reactor may be utilized by injectinq qas
at a point near the discharge outlet of the
reactor. Because of the pluq flow characteristics
of the gas in both the cocurrent and countercurrent
modes of operation, the ~ormation o~ hot spots and
potentially clanqerous poc~ets oE qas is eliminated.
ln another embodiment Oe the invention, qas
can be bled off and recirculated to each individual
tube to achieve precise control of the reaction
conditions in each tube. Thlls, the recirculated qas
can be cooled or not as required and can be passed
over a catalyst bed to oxidize combustible
components thereof prior to its return to a reactor
tube. For example, in a three tube system it may be
.
~L8~
BFN 6~98 -~9~
desirable to pass any gas bled ~Erom the second tube
throuqh a cooler before reintroducinq it in order to
control the heat from the exothermic delignification
reaction Moreover, the concentration of
combustible gases would be at a ma~imum in the third
tube so that it would be deslrable to circulate that
qas through a catalyst bed prior to its recycle back
into the third tube. Other modifications will be
appreciated by those skilled in the art to adapt the
system to various deqrees of deliqnification.
~ ccordinqly, it is an object of the present
invention to provide a delignification system which
will both rapidly and uniformly delignify pulp and
the like, efficiently supply and utilize oxyqen gas
attain ~Iniform temperature control of the reaction,
and avoid the formation of hot spots and gas
pockets. This and other objects and advantages of
the invention will become apparent Erom the
Eollowinq description, the accompanying drawinqs,
and the appended claims.
B _ f Deseription of the Drawinqs
Fiq. I is a schematic flow diaqram
illustrating the proeess of the present invention;
~ Fiq. 2 is a sehematie flow diagram
illustratinq another embodiment of the present
invention,
Fiq 3 is a schematic flow diaqram of yet
another embodiment of the present inventioll,
Fiq. ~ is a qraph of temperature versus
final l~appa nllmber of pulp, ancl
~ Fiq. 5 is a qraph oE Kappa number versus
pulp viscosity.
Description of the Preferred Embodiments
. . ~
As illustrated in Fig. l, pulp at from 1.0
to 35~ eonsisteney, and preferably 8% to 20
eonsisteney, is introdueed into a first
BFN 639~ -10-
substantially horizontal reaction tube 10 by thick
stock pump 12. The use of substantially hori~ontal
tubes includes the use of inclined tubes. However,
the an~le of incline should not exceed approximately
45 degrees to avoid compression and dewaterinq of
the pulp in the lower end of the tube which will
inter~ere with the uniform mixing of oxygen.
Additionally, while the reaction vessel is
illustrated as a series of substantially cylindrical
reactor tubes, a sinqle vessel having a series of
reaction zones or noncylindrical tubes such as a
twin screw system may be utili~ed.
Pump 12 may be a Moyno progressing cavity
pump available Erom Robbins & Myers, Inc.,
Sprinqfield, Ohio. Alternatively, pump 12 may be a
Cloverotor pump available from the Impco Division of
Inqersoll-Rand Co., Nashua, New Hampshire, or a
thick stock pump manufactured by Warren Pumps, Lnc.,
Warren, Massachusetts.
It has been found that these pumps are
capable of feeding the pulp into the reaction tube
a~ainst the pressure in that tube without severely
compactinq the pulp and without any qas losses from
the tube. Other feeding devices such as rotary
valves or screw feeders are not desirable for use in
this inventionO A rotary valve allows substantial
qas loss from the reaction tube due to the rotation
of valve sections which are alternatel~ exposed to
the hiqh oxygen pressure in the reactor and then to
atmospheric pressure external to the reactor. Use
of a screw feeder results in the severe compression
and dewatering of pulp so that efficient oxyqenation
at the proper consistency ranqe cannot occur.
Prior to introducing the pulp into thick
stock pump 12, steam may be injected into the pulp
via llne 14. The steam aids in expelling excess air
7~
sFN ~898
from the pulp and also raises the temperature o~ the
pulp somewhat. Additionally, it is desirable to add
at least a portion of the total amount oE the charqe
of alkaline material prior to the introduction of
the pulp into thick stock pump 12. This addition of
alkaline material can be made through line 16. The
alkaline material serves to lubricate the pulp for
easier pumping as well as to insure that the pulp
will have an alkaline pH when it enters reaction
tube 10. Alternatively, all of the charge may be
added at this point.
Generally, the total alkaline material
charge will amount to from 0.5 to 30% by weight
calculated as Na2o of the oven dry weight oE the
raw fibrous material. Examples of allcaline
materials suitable for use in this invention include
sodium hydroxide, sodium carbonate, sodium
bicarbonate, ammonia, oxidized KraEt white liquor,
qreen liquor, soclium tetraborate, sodium metaborate,
and mixtures thereof. Other known alkaline pulpinq
liquors may also be used. The temperatures and
pressures used for the deli~nification reaction can
similarly be varied over a wide range. It has been
found that temperatures of from 80-160C, an
oxygen partial pressure of from 20-300 psiq and
retention times of 5-120 minutes will procluce a
suitable level of deliqnification.
The other portion of alkaline liquor is
injectecl through line 20 and sprayecl over the pulp
along the length of the tube. By adding the
alkaline liquor gradually along the length of the
tube rather than all at once as is conventional in
high consistency (i~e., 20-35~ consistency) oxygen
clelignification, better pulp viscosity and strength
is achieved. Another advantage to adding the
alkaline liq~or gradually is that the exothermic
7~
BFN 6898 -12-
deliqnification reaction is more easily controlled,
and the risk of localized overheating is diminished.
Oxyqen qas is added to the system at a
pOillt adjacent the pulp inlet throuqh line 22 where
it is mixecl with the pulp and alkaline liquor. By
"adjacent the pulp inlet", it is rneant that oxygen
is added to the system prior to midway along the
lenqth of the reactor tube. Preferably, the oxygen
qas is of hiqh purity (i.e., typically 95% purity)
although lower purity oxyqen can also be used.
Preferably, the oxyqen is injected at or near the
base of reaction tube 10. Mixing and transport of
the pulp and alkaline liquor is achieved by rotating
- timinq screw 24 by a suitable drive means 26. Screw
24 can be of a desiqn conventional in the art, Eor
example,. a solid helical fliqht desiqn. The speed
of rotation of screw 24 can be varied to control the
retention time oE the pulp in the reactor and
insures that the pulp is transported forward in
-20 essentially pluq flow.
A qas space is maintained at the top of
reaction tube 10 so that the oxyyen qas can freely
move forward in pluq flow at a speed different from
the speed oE the pulp. It has been found that
operation of the system with the reaction tubes less
than full and preferably from 50-90% Eilled,
produces acceptable results. The achiévement oE
plug flow is especially important durirlq the initia].
stages of deligniEication to insure that the pulp oE
highest lignin content is exposed to the gas of
hiqhest oxyqen content. The continuous movement of
gas and pulp alonq the length of the reaction tube
and the exchange between gas trapped in the pulp and
free gas above the pulp prevents the formation of
hot spots or pockets of potentially explosive gases
and enhances the uniform delignification of the
BFN 6898 -13-
pulp. It has been found that maintaining an oxyqen
partial pressure of from between 20 and 300 psig
results in an acceptable level of deliqn;fication.
After traversing the length of reaction
tube 10, the pulp, oxyqen, ancl alkaline liquor
mixture is introduced into one or more subsequent
substantially horizontal reaction tubes such as
reaction tube 30. An internal timing screw 32
driven by suitable drive means 34 continuously mixes
and transports the mixture along the len~th of the
reaction tube. Again, the speed of rotation of the
tirninq screw can be varied to control the retention
time and the level of the pulp and allow for
adequate deliqnification. Further reaction tubes
(not shown) may be utilized if necessary.
As the delignified pulp approaches the
discharge point at the end of reaction tube, 30, the
partial pressure of oxygen is at its minimum while
the partial pressures of reaction product gases such
as carbon dioxide, carbon monoxide, and hydrocarbons
are at a maximum. The pulp is withdrawn from
reaction tube 30 and passed to a cold blow region
where it is contacted with dilution water or liquor
from line 36. Gas may optionally be vented Erom the
system through line 38 at a point adjacent the
discharge outlet of reaction tube 30. In this
manner, qas havinq the least amount of oxyqen and
the greatest amount of di~uent gases is discha~ged
! ` from the system.
In an alternate embodiment of the
invention, a countercurrent flow of oxygen gas
through the reactor tubes can be utilized. As shown
in Figs. 1-3, an inlet 50, located at the base of
reaction tube 30 near its discharqe outlet, can be
used to inject oxygen gas into the systemO The gas
will flow in plug flow through the reaction tubes,
BFN 6898 -14~
but in the opposite direction from the direction of
pulp flow. This countercurrent flow mode of
operation procluces both acceptable delignification
and qood pulp viscosity while avoidincl the formation
oE hot spots and qas pockets. A qas vent 52 may be
provided near the pulp inlet to reaction tube l0 to
bleed qases.
In another embodiment oE the invention
usinq cocurrent gas flow illustrated in Fig. 2,
where like reference numerals represent like
elements, at least a portion of the qases vented
-from tube 30 throuqh line 38 is sent through a
catalyst bed 40. Catalyst bed ~0 acts to oxidize
carbon monoxide and other potentially explosive
hydrocarbon qases produced as a result of the
deliqniEication reaction. The treated qases, which
contain oxyqen as well as carbon dioxide, are then
recirculated to t~be 30 via line ~2 which is in
fluid communication with conduit 44. Gases may be
vented throuqh vent 54 or may be recirculated back
to inlet 22 as shown. If the deliqnification
system comprises a multiplicity of reaction tubes,
gas from each tube may be catalytically treated and
recirculated to the same or other tubes.
Alternatively, simultaneous cocurrent and
countercurrent qas flow schemes are contemplated in
which o~yqen is suppliecl at or near the midpoint of
a reaction tube or series oE tubes. Other possible
arranqements will be apparent to those skilled in
this art includinq treatment and recirculation of
gas flowinq countercurrently to the direction o~
pulp flow.
In the embodiment illustrated in Fiq~ 3,
where like reference numerals represent like
elements, the natural draft created by pulp fallinq
through vertical conduit 44 between tubes l0 and 30
sFN 6898 ~~5
is utilized to draw gas vented from tube 30 throuqh
catalyst bed 40. Since heat is qenerated by the
catalytic reaction, the heated gas will tend to
rise. Thus, qas recir~-ulation lines 38 and ~2 as
well as catalyst bed 40 are inclined upwardly to aid
in the natural recirculation effect. A baffle 46,
or other suitable means, prevents pulp from enterinq
conduit 42. Alternatively, a steam ejector or other
conventional method may be used to recirculate the
treated ~as. A vent tube 58 may be provided
downstream of the catalyst bed to serve as a means
to purqe carbon dioxide and inert gases from the
apparatus.
The invention may be better understood by
reference to the followinq nonlimitinq examples.
Example 1
A softwood thermomechanical pulp was
delignified with oxyqen and alkali at 8% pulp
consistency and 160C. The total reaction time
was 60 minutes and an alkali dosage of 30~ sodium
carbonate was used on the pulp. The reactor was a
horizontal tubular vessel having a horizontal shaft
therethrough equipped with paddle fliqhts and
rotated at a low speed. The partial pressure of
steam at the reaction temperature was 75 psig. To
simulate cocurrent oxyqen qas flow, in Run lA the
partial pressure of oxyqen in the reactor was
yradually reduced from 125 psiq at the start of the
reaction to 75 psiq at the end of the react:ion.
30 Countercurrent yas flow was simulated in Run lB by
increasing the partial pressure of oxygen from 75
psiq at the start of the reaction to 125 psiy at the
end of the reaction.
BFN 6893 -16-
The results of the tes~s are reported below:
Run Kappa No.% Pulp Yielcl Brightness
. ~
1~ 121 70.3 15
lB 136 70.6 12
While Run lA, which simulated courrent gas
flow, had a faster delignification rate, a more
selective delignification (as shown by substantially
equal pulp yield at a lower Kappa number), and a
hi~her brightness compared to Run lB, Run lB
illustrates that a countercurrent oxyqen gas f],ow
scheme in a horizontal tubular reactor will produce
satisfactory delignification.
_xample 2
lS A softwood sulfite pulp having an initial
Kappa number oE 69.2 was deliqnified in the reactor
describecl in ~xample 1 with oxygen and alkali Eor a
total reaction time of 20 min-]tes. The consistency
of the pulp was 15%, the reaction temperature was
120C, and the sodium hydroxide dosaqe was 5.0% by
weiqht based on oven dry pulp. In Run 2A, the
partial pressure of oxyc;en was qradually reduced
from 66 psig at the start of the reaction to 36 psig
at the end of the 20 minute reaction period. In Run
2B, the partial pressure of oxygen was gradually
increased from 36 psiq at the start oE the reaction
to 66 psig at the end of the reaction tiine. In both
runs the partial pressure of steam in the reactor
was maintained at 1~ psig throughout the reaction
period.
The results oE the tests are reported below:
Run Kappa No. % Delignificatlon O Palp Yield Brightness
2~ 40.2 41.9 85.5 37
2B 43.4 37.3 ~5.2 35
13FN 6898 -17-
It is evident that Run 2A which simulated a
cocurrent oxyqen qas flow had a Easter
deliqnification rate (i.e., lower Kappa number), a
higher pulp briqhtness, and a qreater selectivity
than Run 2B which simulated a countercurrent gas
Elow. However, the results reported for ~un 2B
indicate that satisfactory deliqnification is
obtained for a countercurrent oxyqen gas Elow scheme
in a horizontal tubular reactor.
E mple 3_ _
Several tests were performed in a three
tube continuous horizontal tubular reactor using a
countercurrent oxygen qas flow scheme. Each tube
was equipped with a horizontal screw which was
turned at low speed to advance the pu]p. Oxygen was
introduced into the reactor near the discharge end
oE the third tube producing a gas flow
countercurrent to the direction of flow of pulp.
The tests were made with a 10~ pulp consistency, a
3~ sodium hydroxide dosaqe, 100 psig total pressure,
and with a pulp havinq an initial Kappa number of
29.3. The pulp level in the reaction tubes was
maintained at a maximum o-E 55% of the total tube
volume. Retention times were varied from 8 to 39
minutes and production rates were varied :Erom 1.7 to
5.0 ton/day.
The results of the tests are shown ln Fi.gs.
4 and 5.
The resul.ts oE the tests show a rap:id
deliqnification rate and a high pulp viscosity
(indicative of good strenqth properties) over a wide
ranqe of retention times and production rates. The
pulp visc.osity was excellent even when declree of
deliqnification approached 60~. The results show
that oxygen contact with the pulp was good even
thouqh only one gas inlet was used in thi.s multiple
847~
BFN 6898 -18-
tube reactor and even though the tests were run at a
medium pulp consistency. This is the most dificult
consistency ranqe in whi~h to achieve qood oxyqen
contact sinçe the pulp is present as sticky lumps.
While the described apparatus and methods
constitute preferred embodiments of the invention,
it is to be understood that the invention is not
limited to these precise apparatus and methods, and
that changes may be made in either without departing
~rom the scope o~ the invention, which is deEined in
the appended claims.
What is claimed is:
