Note: Descriptions are shown in the official language in which they were submitted.
B FN 7 0 8 9 1~
PROCESS AND APPARATUS FOR THE
OXYGEN DELIGNIFICATION OF PULP
This application is related to Canadian
application Serial No. 365,411 filed November 25, 19B0,
and entitled "Apparatus and Method or Medium
Consistency oxygen DeligniEication of Pulp."
Background of the Invention
This invention relates to a process and
apparatus for the oxygen delignification o Eibrous
materials, and more particularly to the
medium-consistency oxygen delignification o bleachable
grade pulp and other fibrvus materials using a series o
tubular reaction zones.
Conventional processes Eor chemical pulpin~ of
Eibrous raw materials have in the past utilized
sulur-containing compounds while conventional bleaching
processes have utilized chlorlne containing compounds.
Today, environmental considerations have resulted in a
search or nonpolluting processes which can ofEer the
desired pulp yields and qualities. Much attention has
been devoted to the use of oxygen in combination with
alkaline chemicals to delignify pulp and other fibrous
materials.
For example, several workers have investigated
oxygen deligniication of high consistency pulp (i.e.,
20-30% consistency). See, Eachus, ~APPI~ Volume 58, p.
151-154 (Sept. 1975) and Hasvold, 1978 International
Sulfite Conference, Montreal, Canada (September 13,
1978). Other workers have utilized
~,.,
BFN 7089 -2-
oxygen delignification in low consistency (i.e.,
1-5~ consistency) pulping or bleachinc~ processes.
See, Paper Trade ~ournal p. 37-39 (~uly ]5, 197~).
Recently, workers have also investigated
processes for the oxygen deligni~ication of pulp
mill screen rejects and knots. Such screen rejects
and knots have often been heretofore unusable and
had to be dewatered and then burned or dumped.
However, ~irschner, Paper Trade Journal, p. 32
(November 15, 1978), has reported the use of a
low-consistency oxygen delignification process for
kraft and sulfite screen rejects which produces a
bleachable grade of pulp. }lasvold, 197~
International Sulfite Conference, Montreal, Canada
(September 13, 1978), has reported an oxygen process
whlch deligniEies sulfi~e knots at a ~5~ pulp con-
sistency.
While most workers have utili~ed either
high or low consistency oxygen delignification
processes in working either with pulp or with screen
rejects and knots, both of these processes s~ffer
from several disadvantages. Low consistency opera-
tion re~uires a large reactor volume to maintain an
acceptable retention time for the pulp. Operating
at low consistency also produces large power demands
for pumping large volumes of pulp and a high steam
usage to heat the pulp in the reactor. Additional-
ly, the low concentration of dissolved solids in the
spent li~uor increases evaporation costs for chemi-
cal recovery processes. Operation at high consis-
tency, on the other hand, usually re~uires special
dewatering e~uipment to attain the higher consisten-
cy. It is also known that high consistency opera-
tion of an oxygen delignification system can result
in overheating of the pulp due to the exothermic
delignification reaction, as well as pulp degrada-
tion and even combustion o~ the pulp.
7~
BFN 7089 -3~
Carrying out oxygen delignification of pulp
at medium consistency (i.e., 8-20~ consistency)
would be advantageous in that much existing mill
equipment, including pulp washing and thickening
S e~uipment, is designed to operate in that consisten-
cy range and no special e~uipment would be re~uired
to attain that range. Some workers have reported
satisfactory results operating at medium consistency
on a laboratory scale using rotary autoclaves with
no internal means of mixing (See, e.g., Annergren et
al, 1979 Pulp Bleaching Conference~ Toronto, Canada,
June 11-14, 1979; Saukkonen et al, TAPPI Volume 58,
p. 117 ~1975); and Chang et al TAPPI Volume 56, p.
97 (1973)). However, such e~uipment is not suitable
for scale-up ~o handle large tonnages of pulp on a
commercial scale~ Other workers have encounterecl
serious problems even on a small laboratory scale.
For example, Eachus, TAPPI Volume 58, p. 151 (1975),
reported that oxygen delignification at medium con-
2Q sistency was not practical because of a high alkalire~uirement, oxygen starvation, and a limited delig-
nification.
Chang et al, TAPPI 57, p. 123 (1974), con-
cluded that operation at medium consistency produced
a considerably lower delignification rate than high
consistency operation and also resulted in nonuni-
form delignification. Although the authors sugges-
ted that these problems could be overcome through
the use of higher oxygen pressures in the reaction
vessel, use of such higher press~res has several
disadvantages. These include greater costs for a
thicker-walled reaction vessel, greater difficulty
in feeding pulp against the higher pressure, and an
increased danger of gas leakage.
Vertical tube oxygen reactors operating at
medium consistency have been constructed for trial
BFN 7089 -4-
purposes. (See Annergren et al, 1979 Pulp Bleaching
Conference, Toronto, Canada, June 11-1~, 1979, and
Rleppe et al, T~PPI Vol. 59, p. 77 (1976).)
Ho~ever, such vertical tube designs have serious
de~iciencies, including channeling of gas and pulp
up through the tower and also the re~uirement for a
high speed mechanical mixer to disperse oxygen into
the pulp slurry. Such high speed mixiny can lead to
pulp deg~adation and additionally re~uires substan-
tial power input.
As can be seen, there is a need in the artfor a simple and efficient process for oxygen delig-
nification of ~ibrous materials including pulp as
well as screen rejects and knots which avoids the5 problems which have plagued the prior art.
Summary of the Invention
_
~ ccordin~ to one aspect oE the present
invention, medium consistency pulp at a consistency
of from 8 to 20% along with alkaline materials are
introduced into a substantially horizontal reaction`
zone. Oxygen is added to delignify the pulp while
the mixture of oxygen, pulp, and alkaline materials
is agitated and transported through the reaction
zone. Apparatus for delignifying the pulp includes
a tubular reaction zone, means for introducing
oxygen gas and alkaline materials into the reaction
zone, pump means for introducing pulp into the reac-
tion zone, and means to transport and agitate the
mixture o pulp, oxygen, and alkaline materials
through the reaction zone.
The present invention provides a medium
consistency process and apparatus utilizing one or
more substantially horizontal agitated tubular reac-
tion zones which produce rapid oxygen delignifica-
tion rates at low alkali charges, minimize oxygenre~uirements, and yield pulps having high viscosi-
'76~ 8
BFN 7089 -5-
ties. The use of rotary screws or paddles in the
one or more reaction zones provides the agitation
re~uired to enable good mixing of oxygen with the
~edium consistency pulp and alkaline chemicals as
well as controlling the pulp retention time in each
reaction zone.
By "medium consistency" it is meant that
the consistency o the pulp supplied to and main-
tained in the reaction zone is from 8-20% and
preferably 10-15%. This is to be distinguished from
prior high (above ~0% and preferably 25~30%) and low
~less than ~% and preferably 1-5%) consistency
delignification systems. The oxygen delignification
system of the present invention can be used to
delignify any type of pulp including mechanical
pulps, thermomechanical pulps, semichen~ical or
modified mechanical pulps, chemical pulps, and
secondary fiber. Additionally, straw, flax, and
bagasse can also be delignified as well as pulp mill
screen rejects and knots. Preferably, the starting
materials for the process are unbleached wood pulps
such as softwood kraft pulps having Kappa numbers
between 20 and 50 or hardwood kraft pulps having
Kappa numbers between 10 and 30, high yield pulps
(i.e., 55-60~ yield) cooked to near the point of
fiber liberation such as softwood kraft pulps having
Kappa numbers between 50 and 80 or hardwood kraft
pulps having Kappa numbers between 25 and 50, or
fiberized pulp mill screen rejects and knots.
In accordance with the invention, the pulp
or other fibrous material may be sent directly from
the blow tank of a chip or raw material digester or
cooker to brown stock washers which are typically
operated in the medium consistency range. In
instances where an initially high ~appa number pulp
such as a high yield kraft pulp is utilized, the
~1'7~4~
BFN 7089 -6-
pulp may, optionally, be sent to a further refining
stage before or after leaving the brown stock
washers. In instances where the pulp has been
screened, the screen rejects and/or knots removed
S from the pulp stream may be fiberized in a further
refining staye and then recombined with the main
pulp stream for the oxygen delignification process.
The pulp is then introduced, at a medium
consistency of between 8 and 20~, and preferably
10-15%, into a substantially horizontal tubular
reaction vessel where it is contacted with oxygen
gas and alkaline chemicals. A thick stock pump is
used to feed the pulp into the reaction vessel. Use
of the thick stock pump prevents the loss of gas
lS pressure from the vessel and does not severely com-
pact the pulp so that uniEorm oxygenation and delig-
nification can occur.
Ox~qen may be introduced into the delignif-
ication system either at one injection point or
multiple injection points. Typically~ oxygen gas
will be in~ected on the lower side of the reaction
vessel. Partially spent gas may, optionally, be
removed from the delignification system by venting
to the atmosphere or it may be collected for
recycle. Additionally, the partially spent gas may
be drawn off and utilized for lime kiln enrichment,
waste water treatment, or other suitable uses. Any
organic compounds or carbon monoxide formed during
the delignification reaction may be removed by pass-
ing the gas through a catalyst bed before reuse.
Alkaline pulping chemicals are also intro-
duced into the reaction vessel to aid in the delig-
nification. Examples of such alkaline chemicals
which are suitable for use in the practice of the
present invention include sodium hydroxide, sodium
carbonate, sodium borate compounds, ammonial oxi-
~ ~64~`~
BFN 7089 7-
dized kraft white liquor~ and mixtures thereof.
Preferably, at least a portion of the total charge
of alkaline chemicals is added to the pulp prior to
its passage through the thick stock feed pump into
the first reaction zone. This insures that the pulp
has an alkaline pH when the pulp enters the first
reaction zone and also lubricates the pulp for
easier pumping. An additional portion of the total
charge is added to the first reaction zone rom one
or more injection points along the top of the
vessel. Magnesium sulfate or other known protector
chemicals or catalysts for preserving the viscosity
and stren~th of the pulp may be introduced into the
pulp either before or after the thick stock ~eed
pump.
Steam is also added to the pulp prior to
its entr~ into the thick stock Eeed pump. The steam
aids in expelling excess air from the pulp prior to
delignification. Additional steam may be injected
into the reaction vessel as needed in order to main-
tain the desired reaction temperature, although the
exothermic delignification reaction supplies a sub-
stantial fraction of the heat reguirement.
As the pulp at 8-2~% and preferably lC-15%
consistency is introduced into the reaction vessel
through the thick stock pump, a rotary screw or
series of paddles agitates the pulp, oxygen, and
alkaline chemical mixture. It has been found that a
solid flight helical screw extending the entire
length of the reaction zone produces the gentle
agitation necessary for uniform and rapid delignifi-
cation. Satisfactory delignification is achieved by
rotating the screw at a speed of less than about 15
rpm and preferably 1-6 rpm. In another embodiment
of the invention, one or more additional substan-
tially horizontal tubular reaction vessels are uti-
~FN 7089 -8
lized to achieve ~n additional amount of delignifi-
cation of the pulp.
The reaction ~emperature, alkali charge,
type of alkaline chemical, oxygen partial pres~ure~
and retention time depend on the type of material
being treated and the desired degree of delignifica-
tion. Typically, temperatures may range from 80
to 1~0C, alkaline chemical charges from 1 to 20~
calculated as Na2o on ov~n dry material, and
oxygen partial pressures from 30 to 200 psi.
Appropriate retention times have been found to be 5
to 120 minutes.
Accordingly, it is an object of the present
invention for uniformly and rapidly deligniEying
pulp at medium consistencies while minimizing alkali
dosages and oxygen requirements to provide a pulp
having high strength properties. This and other
objects and advantages of the invention will become
apparent from the following description, the accom-
panying drawings, and the appended claims.
Brief Description of the Drawin~s
Fig. 1 is a schematic flow diagram illus-
trating the overall process of the present invention;
Figs. 2a and 2b are schematic flow diagrams
illustrating alternative embodiments of the inv~n-
tion;
Fig. 3*is a graph of pulp viscosity versus
Kappa number for medium consistency oxygen delignif-
ication of pulp in accordance with the practice of
the invention;
Fig. 4 is a graph of pulp viscosity versus
Kappa number for different pulp consistencies;
Fig. 5 is a graph of the change in Kappa
number versus alkaline chemical charge for agitated
and nonagitated delignification processes, and
* On same sheet as Fig.1
~' !
~'7~
BFN 7089 -9-
Fig. 6 is a graph of alkaline chemical
charge versus Kappa number reduction for different
pulp consistencies.
Description of the PreEerred Embodiments
5As illustrated in Fig. 1, pulp at from
8-20% consistency and preferably 10-15~ consistency
from the brown stock washers is introduced into a
first horizontal reaction vessel or tube 10 by a
thick stock pump 12. Inclined reaction t~bes may
also be employed, but the angle of incline should
not exceed approximately 45 degrees to avoid
compression and dewatering of the pulp in the lower
end of the tube, ~hich will interfere with uniEorm
mixing of oxygen. Additionally, while the reaction
ves5el is illustrated as a cylindrical reactor tube,
noncylindrical tubes such as a twin-screw system ma~
be utilized.
Pump 12 may be a Moyno progressing cavity
pump available from Robbins & Myers, Inc.,
Springfield, Ohio~ Alternatively, pump 12 may be a
Cloverotor pump availa~le from the Impco Division of
Ingersoll-Rand Co., Nashua, New Hampshire, or a
thick stock pump manufactured by Warren Pumps, Inc.,
Warren, Massachusetts.
25It has been found that these pumps are
capable of feeding the pulp into the reaction tube
against the pressure in that tube without severely
compacting the pulp and without any gas losses from
the tube. Other feeding devices such as rotary
valves or screw feeders are not desirable for use in
this invention. A rotary valve allows substantial
gas loss from the reaction tube due to the rotation
of valve sections which are alternately exposed to
the high oxygen pressure in the reactor and then to
atmospheric pressure external to the reactor. Use
of a screw feeder results in the severe compression
~7~4~3
BFN 70~9 -10-
and dewatering of pulp so that efficient oxygenation
at the proper consistency range cannot occur.
Prior to inteoducing the pulp into thic~
stock pump 12, steam may be injected into the pulp
via line 14. The steam aids in expelling excess air
from the pulp and also raises the temperature oE the
pulp somewhat. Additionally, it is desirable to add
at least a portion of the total amount of the charge
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 reac~ion
tube 10, Alternatively, all of the charge may be
added at this point.
Generally, the total alkaline material
charge will amount to from 1 to 20% by weight calcu-
lated as Na2o of the oven dry weight of the raw
fibrous material. Examples o alkaline materials
suitable for use in this invention include sodium
hydroxide, sodium carbonate, sodium borate com-
pounds, ammonia, oxidized kraft white li~uor, and
mixtures thereof although other known alkaline pulp-
ing ~i~uors may also be used.
Once introduced into reaction tube 10, thepulp undergoes an oxygen delignification reaction.
Oxygen gas is introduced into reaction tube 10
through line 18. Alternatively, oxygen may be
introduced at a number of points along the length of
tube 10. Typically, the oxygen partial pressure
maintained in the system is from about 30 to ~00
ps ig .
Spent gas may be removed from the system by
venting it to the atmosphere. Alternatively, it may
be recovered for recycle to the reaction tubes or
~7~8
BFN 708~
may be used for other purposes such as lime kiln
enrichment or ~aste ~ater treatment. Any organiG
vapors or carbon monoxide produced during the
delignification reaction can ~e removed by passing
the gas through a catalyst bed.
The delignification reaction is carried out
by mixing the pulp, oxygen, and alkaline li~uor
which is injected through line 20 and sprayed over
the pulp along the length of the tube. By adding
the alkaline li~uor gradually along the length of
the tube rather than all at once as is conventional
in high consistency (i.e., 20-30~ consistency)
oxygen delignification, better pulp viscosity and
strength is achieved. Another advantage to gradual-
ly adding the alkaline li~uor is that the exothermicdelignification reaction is more easily controlled
and the risk of localized overheating i5 diminished.
Satisactory gentle agitation can be
achieved by rotating screw 22 with drive means 23 at
a rate of less than about 15 rpm and preferably 1-6
rpm. Preferably, the system is operated so that a
gas space remains at the top of reaction vessel 10
and the vessel is less than full of pulp. Total
retention times of the pulp in the system may vary
depending upon the nature and condition of the pulp
and the desired amount of delignification to be
accomplished. Retention times of bet~een 5 and 120
minutes have been found to be satisfactory. Steam
may be injected into the reaction vessel through
line 46 to maintain the temperature within the
preferred 80-160C range.
Upon completion of the delignification
reaction, the pulp exits vessel 10 through outlet 26
and is passed to blow tank 28. The pulp is then
discharged using a conventional blow wiper discharg-
er.
~6~
BFN 7089 -12-
In another embodiment of the invention
illustrated in Fig. 2a, where like components are
indicated by like reference numerals, pulp from
washer 50 is sent through refiner 52 for further
S fiberization before being fed to thick stock pump
12. Since the consistency of the pulp le~ving
washer 50 will be in the medium consistency range,
the pulp can be refined and then fed to the reaction
vessel at the same consistency without any need for
any dewatering~ Refiner 52 may be utilized in
instances where delignification is to be carried out
on pulp having an initially high Kappa number such
as high yield soft wood kraft pulp having an initial
Kappa number greater than a~out 50.
Also illustrated in Fig. 2a is the use of
one or more subse~uent su~stantially horizontal
reaction vessels such ~s vessel 30 to carry out
Eurther delig~ ication on the pulp. AS shown, pulp
exiting one end of vessel 10 drops into vessel 30
where it is transported along the length o the
vessel with gentle agitation by rotary screw 32 hav
ing solid helical flights 34 and driven by a suita-
ble drive means 33. Steam may be added through line
48 to maintain the temperature in vessel 30 within
the preferred range of 80-160C. Additional
oxygen may be injected through line 18a i~ re~uired.
Yet another embodiment of the invention is
illustrated in Fig. 2b in which like components are
represented by like reference numerals. In this
embodiment, pulp is transported from an initial
cooking or digestion stage through line 54 to
screens 56 where oversize slivers, shives, knots,
and other impurities are removed. The accepted pulp
passes through line 58 into pulp washer 50 while the
rejected material is sent to refiner 52 for further
fiberization before being recombinedlwith the main
~7~ 8
BFN 7089 ~-13-
pulp stream thro~gh line 60. This combined pulp
stream is then washed and oxyyen delignified as
described ahove to yield a bleachable grade pulp.
In order that the invention may be better
understood, reference is made to the following non-
limiting examples.
Example 1
A northeastern softwood kraft pulp having
an initial Kappa number of 29.3 and a viscosity of
26.3 centipoise ~cps) was oxygen delignified in
accordance with the process of the invention. The
reaction conditions were 10~ pulp consistency, 100
psig total gas pressure, and a 3~ sodium hydroxide
dosage by weight based on dry pulp. Retention time
in the reaction zone was varied from 8 to 16 to 3~
minutes hy varying the speed of the rotary screw in
tha reac~or. ~he pulp feed rate was set at either
1.7 ton/day (T/D) or 5.0 T/D.
The results are illustrated in Fig. 3.
That graph shows a linear relationship between pulp
viscosity and Kappa number at up to 60% delignifica-
tion, where
% delignification=Initial Kappa No.-Final Kappa ~o. x 100
Initial Kappa No.
This result is surprising because high pulp viscosi-
ties, which are indicative of high pulp strength,
were obtained at a relatively high percentage of
delignification. Commercial high consistency oxygen
delignification systems are limited to about 50%
delignification due to sever~ losses in pulp
strength ~measured as greatly lowered pulp viscosi-
ties) beyond that point.
Thus, utilizing the medium consistency
oxygen delignification process of the present inven-
tion with substantially continuous gentle agitationof the pulp, more lignin can be removed from the
~6~
BFN 708~ -14-
pulp without loss of pulp strength. This can result
in significant reductions in operating and capital
costs over high consistency processes because of
reduced bleaching costs and the elimination of the
need for a conventional chlorine bl~aching stage.
Example 2
Medium (15~) consistency oxygen delignifi-
cation was carried out on a softwood kraft pulp
having an initial viscosity of 29.5 using the
process of the present invention. The delignifica-
tion reaction was carried out for 20 minutes at
110C and at a total gas pressure of 150 psig.
For comparison purposes, the same pulp was deligni-
fied under the same conditions with the exceptiorl
that in one instance the pulp was maintained at a
low (2%) consistency throughout the reaction and in
another instance was maintained at a high (28~)
consistency throughout the reaction.
The results are illustrated in Fig. 4. As
shown hy that graph, for the same Kappa number, the
medium consistency delignified pulp exhibited higher
viscosities than both the high and low consistency
pulp.
Example 3
Softwood kraft pulp having an initial Kappa
number of 29.5 was oxygen delignified in a 2 liter
autoclave at 110C and an oxygen gas pressure of
150 psig for a time sufficient to achieve a final
Kappa number of 18~5. Several tests were run with
the consistency of the pulp varied from 2% to 15% to
28%. The results are reported in Table I below.
BFN 7089 -lS-
Table I
Hydrocarbon
Consistency % 2 Consumed ~ CO Evolved (as Methane)
Based On Based On to CO
~Wei~ht of Pulp Weight of Pulp Ratlo(Moles)
5 23.2 0.010 0.2
151.5 0.015 0.2
281.5 0.024 0.2
For a working system, it is necesary to
provide venting of the reactor gases in order to
remove combustible reaction products such as carbon
monoxide and hydrocarbons. The resulting dilution
of the gas in the reactor with oxygen maintains a
safe condition.
15 Using the data from Table I, material
balance calculations ~ere made to determine the
amount of oxygen re~uired to maintain the reactor in
a safe condition of 30~ of the lower explosive limit
(LEL~ of combustibles. The results are reported in
Table II below.
Table II
Consistency 2 Consumed* 2 Re~uired*
% %
252 3.2 3.58
1.5 2.06
28 1.5 2.42
*based on weight of pulp.
The results show that the medium consistency process
has lower oxygen re~uirements.
Example 4
A high yield softwood kraft pulp near the
point of fibèr liberation and having an initial
Kappa numher of 59.4 was oxygen delignified in an
~764~i~
BFN 7089 -16-
autoelave at temperatures ranging from 100-130C
and at a total gas pressure of 8.~ Kg/cm2 (120
psig). The pulp was maintained at a medium consis-
teney for an approximately 15 minute reaction time
as the eharge o~ alkaline (caustie) chemicals was
varied rom ~-6~ by weight based on oven dry pulp.
As shown in Fig. 5, the curve labeled A in
whieh the pulp was eontinuously gently agitated in
the autoclave shows a greater reduction in Kappa
number ~indicative of a greater delignifieation
rate) than the eurve labeled B in which no agitation
was performed. The results show the importance o~
gentle agitation of pulp when deli~nifying at medium
eonsistency to improve the rate of delignifieation
lS of the pulp.
Example 5
Tests were made using a so~twood kraft pulp
having an initial ~appa number oE 29.5 to determine
the effect of pulp consisteney on the extent of
delignifieation for a given alkaline chemieal
(eaustie) dosage and reaetion time. The tests were
earried out in a 2 liter autoelave at llooc and
10.5 Kg/em2 150 psig gas pressure for 20 minutes.
Low (2%) eonsistency tests were done under condi-
tions of vigorous agitation (rotation of stirrer at1250 rpm) while the medium (15~) and high (28%)
ennsistency tests were condueted without agitation.
Tbe results are shown in Fig. 6.
As can be seen, surprisingly the extent of
delignifieation for the medium and high consistency
tests were nearly identical at a given caustic
eharge~ The low consistency tests resulted in sub-
stantially less delignificationO Therefore, longer
reaetion times would be re~uired for a low consis-
teney proeess to aehieve the same reduction in Kappanumber as for either a medium or high consisteney
proeess.
`8
BFN 7089 -17-
While the methods and apparatus herein
described consitute preferred embodiments of the
invention, it is to be understood that the invention
is not limited to these precise methods and appara-
tus, and that changes may be made in e.ither withoutdeparting from the scope o the invention, ~hich is
defined in the appended claims.
What is claimed is;
