Note: Descriptions are shown in the official language in which they were submitted.
BFN 7089-C - 1 -
PRQCF.SS AND APPARATUS FOR THE
OXYGEN DE I GN I F I CAT I ON OF PULP
Cross-Reference_to Related Ap ~
This application is related -to co-pending
Canadian Application Serial No. 385,272 filed September 4,
1981, entitled "Process and Apparatus for the Oxygen
Delignification of Pulp". ThiS application is also
related to Canadian Application Serial No. 365,411, filed
November 25, 1980~ and entitled "Apparatus and Method for
Medium Consistency Delignification of Pulp".
Back~round of the Invention
This invention relates to a process and apparatus
for the oxygen delignification of fibrous materials, and
more particularly to the medium consistency oxygen
delignification of bleachable grade pulp and other fibrous
materials using a series of tubular reaction zones.
Conventional processes for chemical pulping of
fibrous raw materials have in the past utilized
sulfur-containing compounds while conventional bleaching
processes have utilized chlorine containing compounds.
Today, environmental considerations have resulted in a
search for nonpolluting processes which can offer 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 delignification of high consistency pulp (i.e~,
20-30% consistency). See, Eachus, TAPPI Volume 58, p.
151-154 (Sept. 1975) and Hasvold, 1978 International
Sulfite Conference, Montreal, Canada ~September 13,
1978)o Other workers have utilized
~æ
, ,
.
BFN 7089-C -2
oxygen delignification in low consistency (i.e.,
1-5~ consistency) pulping or bleaching processes.
See, Paper Trade Journal p. 37-39 (July 15, 1978).
Recently, workers have also investiqated
processes for the oxygen delignification of pulp
mill screen eejects and knots. Such screen rejects
and knots have often been heretofore unusable and
had to be dewatered and then burned or dumped.
However, Kirschner, Paper Trade Journal, p. 32
1~ (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. Hasvold, 1978 Interna-
tional Sulfite Conference, Montreal, Canada
15 (September 13, 1978), has reported an oxygen process
which deligniEies sulfite knots at a 25% pulp con-
sistency.
While most workers have utilized either
high or low consistency oxygen delignification
processes in working either with pulp or with screen
rejects and knots, both of these processes suffer
from several disadvantages. Low consistency opera-
tion requires 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 liquor increases evaporation costs for chemi-
cal recovery processes. Operation at high consis-
tency, on the other hand, usually requires special
dewatering equipment to attain the higher consisten-
cy. It is also known that high consistency opera-
tion of an oxygen delignification system can result
in overheating oP the pulp due to the exothermic
BFN 7089-C -3-
~elignification reaction, as well as pulp degrada~
tion and even comhustion of the pulp. Hiyh consis-
tency operation is also sensitive to the presence of
impurities such as black liquor solids and oxygen
stage solids. These impurities result from incom-
plete washing of the pulp and recycling of the
oxygen stage filtrate, respectively. Even small
concentrations of such impurities may adversely
affect both pulp viscosity and yield selectivity.
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
equipment, is designed to operate in that consisten-
cy range and no special equipment would be required
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
20 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 equipment is not suitable
~or scale-up to handle large tonnages of pulp on a
25 commercial scale. Other workers have encountered
serious problems even on a small laboratory scale.
For example, Eachus, TAPPI Volume 58, pO 151 (1975),
reported that oxygen delignification at medium
consistency was not practical because of a high
30 alkali requirement, o~ygen starvation, and a limited
delignification.
Chang et al, TAPPI 57, p. 123 (1974),
concluded that operation at medium consistency
produced a considerably lower delignification rate
35 than high consistency operation and also resulted in
nonuniform delignification. Although the authors
BFN 7089-C ~4~
suggested that these problems could be overcome
through the use of higher oxygen pressures in the
reaction vessel, use of such higher pressures has
several disadvantages. These include greater costs
for a thicker-walled reaction vessel, greater diffi
culty 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
purposes. (See Annergren et al, 1979 Pulp Bleaching
Conference, Toronto, Canada, June 11-14, 1979,
Kleppe et al, TAPPI Vol. 59, p. 77 (1976) and Kleppe
et al TAPPI, Volume 64, pp. 87-90(1981).) However,
such vertical tube designs have serious deficiencies,
including channeling of gas and pulp up through the
tower and also the requirement for a high speed
mechanical mixer to disperse oxygen into the pulp
slurry. Such high speed mixing can lead to pulp
degradation and additionally requires substantial
power input.
Moreover, during an oxygen delignification
process, the liquid feed to the oxygen stage will
contain dissolved solids as impurities. These
dissolved solids have two sources, black liquor
solids entrainment (BLSE) which result from incom-
plete washing of the pulp and oxygen stage solids
entrainment ~OSSE) which result from recycling
oxygen stage filtrate back to the brown stock
washers upstream from the oxygen stage. A typical
pulp stream from an initial kraft digestion stage
will contain up to 170~ by weight dissolved solids
based on pulp weight.
Prior high consistency oxygen delignifica-
tion systems taught the necessity of thorough
washing of the pulp stream from the digestion stage
to reduce the black li~uor solids content and avoid
BF~ 70B9-C ~5~
their adverse effect on the oxygen stage. For
example, E~illstrom et al, Svensk Papperstidning, nr6
(1977), and Jamieson et al, Svensk Papperstidning,
nr5 (1973), pp~ 187-181, pp. 167-170, indicate that
S a BLSE o~ as little as 1~ has ad~erse effects on
pulp viscosity and Kappa number. See also, 5SVL
Swedish Forest Industry Foundation for Water and Air
Protection-Environmental Care Project, Technical
Summary l1974) pp. 73-76. The medium consistency
oxygen delignification system of Kleppe et al,
supra, also u~ed highly washed pulp stock which was
supplied to the oxygen delignification stage from
the washing stage of a continuous digester or from
an in-line diffuser washer positioned in front of
the oxygen reactor. This need for thorough washing
increases the capital costs for such systems because
of the additional equipment which is required.
As can be seen, there is a need in the art
for a simple and efficient process for oxygen delig-
nification of Eibrous materials including pulp aswell as screen rejects and knots which avoids the
problems which have plagued the prior art.
Summary of the Invention
The present invention meets this need by
providing a medium consistency process and apparatus
utilizing one or more substantially horizontal
agitated tubular reaction zones which produce rapid
oxygen delignification rates at low alkali charges,
minimize oxygen requirements, and yield pulps having
high viscosities. The use of rotary screws or
paddles in the one or more reaction zones provides
the agitation required to enable good mixing of
oxygen with the medium consistency pulp and alkaline
chemicals as well as controlling the pulp retention
time ;n each reaction zone.
BFN 7089-C -6-
By "medium consistency" it is meant that
the consistency of 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 20% and preferably 25-30%) and low
(less than 8~ and preferably 1-5%) consistency
delignification systems. The oxygen delignification
system of the present invention can be used to
delignify any type of pulp includinq mechanical
pulPs, thermomechanical pulps, semichemical or modi-
fied 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 materi-
1~ als for the process are unbleached wood pulps suchas softwood Icraft 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 Kappa number pulp
such as a high yield kraft pulp is utilized, the
pulp may, optionally, be sent to a further refining
stage after leaving the brown stock washers~ In
instances where the pulp has been screened, the
screen rejects and/or knots removed from the pulp
stream may be fiberized in a further refining stage
and then recombined with -~he main pulp stream either
BFN 7089-C -7-
before or af~er the screeninq stage for the oxygen
delignifica~ion process. Alternatively, the pulp
from the brown stock washers may be sent directly
into the oxygen delignification reactor and further
S washed and/or screened aEter delignification. Those
post-delignification screen rejects can be refined
and recombined with the main pulp stream prior to
the oxygen delignification reactor.
A significant advantage of the medium
consistency prccess and apparatus of the present
invention is its ability to tolerate significantly
higher dissolved solids content pulps (both BLSE and
OSSE) than prior systems. The system of the present
invention can be operated using pulp having a BLSE
content as high as 6-20% by weight, preferably
8-10%, and OSSE content of up to 8% by weight with-
out significant adverse effects on the pulp proper-
ties or rate of delignification. This perrnits the
system of the present invention to be operated with
fewer washers, and in the case where the oxygen
stage is retrofitted into an existing system, the
oxygen stage can be positioned so that some washers
which served as pre-bleach stage brown stock washers
can be utilized downstream of the oxygen stage to
remove oxygen stage impurities. Additionally, the
unique system of the present invention is especially
suited to supp:Ly any additional oxygen requirements
in the deligni~Eication stage due to the presence of
increased levels oE dissolved solids.
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
BFN 7089~C -8-
pressure fr~m the vessel and does not severely
compact the pulp so that uniform oxygenation and
delignification can occur.
Oxygen may be introduced into the delignif-
ication s~ste~ either at one injection point or
multiple injection points. Typically, oxygen gas
will be injected 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 recycleO
Additionally, the partially spent gas may be drawn
off and utilized for lime kiln enrichment, waste
water treatmen~, or other suitable uses. Any
organic compounds or carbon monoxide formed during
the delignification reaction may be removed by
passing 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-
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 Eirst
reaction zone and also lubricates the pulp for
easier pumping. An additional portion of the total
charge is added to the first reaction zone from 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
strength of the pulp may be introduced into the pulp
either before or after the thick stock feed pump.
BFN 7089-C -9-
Steam is al50 added to the pulp prior to
its entry into the thick stock feed 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
substantial fraction of the heat requirement.
As the pulp at 8-20% and preferably 10-15%
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 achieve~ 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
utilized to achieve an additional amount of delig-
nification of the pulpo
The reaction temperature, alkali charge,
type of alkaline chemical, oxygen partial pressure,
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 160C, alkaline chemical char~es from 1 to 20%
calculated as Na2o on oven 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 to provide a system for uniformly and
rapidly delignifying pulp at medium consistencies
~FN 7089-C ~10-
while minimizing alkali dosages and oxygen require-
ments to provide a pulp having high strength proper-
ties. It is a further object of the invention to
provide a system which can be operated using pulp
which contains higher dissolved solids levels that
previous systems without adversely affecting pulp
properties~ These and other objects and advantages
of the invention will become apparent from the
ollowing description, the accompanying 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, 2b, and 2c are schematic flow
diagrams illustrating alternative embodiments of the
invention;
Fig. 3 is a graph of pulp viscosity versus
Kappa number Eor medium consistency o~ygen 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 p~lp consistencies;
Fig. 5 is a graph of the change in Kappa
number versus alkaline chemical charge for agitated
and nonagitatecl delignification processes;
Fig. 6 is a graph of alkaline chemical
charge versus Kappa number reduction for different
pulp consistencies;
Fig. 7 is a graph of the effect of black
liquor solids entrainment on Kappa number for medium
and high consistency operation;
Fig. 8 is a graph of the effect of black
liquor solids entrainment on yield selectivity for
medium and high consistency operation;
BFN 7089-C
Fig. 9 is a graph of the effect of black
liquor solids entrainment on viscosity selectivity
for medium and high consistency operation;
Fig. 10 is a graph of the effect of black
liquor solids entrainment on oxygen consumption for
medium consistency operation;
Fig. 11 is a graph of the effect of oxygen
stage solids entrainment on yield selectivity; and
Fig. 12 is a graph of the effect of oxygen0 stage solids entrainment of viscosity selectivity.
Description_of_the_Pre~erred Embodiments
In a preferred embodiment of the invention,
pulp ha~ing a BLSE content of from 4.5-20% by
weight, and preferably 8-10~, and an OSSE content of
up to 8% by weight is introduced into the oxygen
reaction ~essel. Because the system can tolerate
larger amounts of dissolved solids than prior
systems without significant adverse effects on
either pulp properties or the rate of delignifica-
tion, the system can be positioned more closelydownstream from an initial pulp digestion stage than
would otherwise be possible, resulting in potential
savings in capital costs of washing equipment.
As illustrated in Fig. 1, pulp at from
8-20% consistency and preferably 10-15~ consistency
from the brown stock washers is introduced into a
firs~ horizont:al reaction vessel or tube 10 by a
thick stoclc pump 12. Inclined reaction tubes 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, which will interfere with uniform
mixing of oxygen. Additionally, while the reaction
vessel is illustrated as a cylindrical reactor tube,
noncylindrical tubes such as a twin-screw system may
be utilized.
I'g~
BFN 7089-C - 12 -
Pump 12 may be a Moyno (trademark)
progressing cavity pump available from Robbins &
Myers, Inc., Springfield, Ohio. Alternatively, pump
12 may be a Cloverotor (trademark) pump available
from the Impco Division of Ingersoll-Rand Co.,
Nashua, New Hampshire, or a thick stock pump
manufactured by Warren Pumps, Inc., Warren,
Massachusetts.
It 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
and dewatering of pulp so that efficient oxygenation
at the proper consistency eange cannot occur.
Prior to introducing the pulp into thick
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 of 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 p~l when it enters reaction
tube 10~ Alternatively, all of the charge may be
added at this point.
:~)
v~
BFN 7089-C -13-
Generally, the tGtal alkaline material
charge will amount to from 1 to 20~ by ~eight calcu-
lated as Na2o of the oven dry weight of the raw
fibrous material. Examples of alkaline materials
suitable for use in this invention include sodium
hydroxide, sodium carbonate~ sodium borate compounds,
ammonia, oxidized kraft white liquor, and mixtures
thereof although other known alkaline pulping
liquors may also be used.
Once introduced into reaction tube 10, the
pulp 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 200
psi~ .
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
may be used for other purposes such as lime kiln
enrichment or waste water treatment. Any organic
vapors or carbon monoxide produced during the
delignification reaction can be removed by passing
the gas through a catalyst bed.
The delignification reaction is carried out
by mixing the pulp, oxygen, and alkaline liquor
which is in~ected through line 20 and sprayed 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 conventiona
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 liquor is that the exothermic
BFN 7089-C -14~
delignification reaction is more easily controlled
and the risk of localized overheating is diminished.
Satisfactory 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 o reaction vessel 10
and the vessel i5 less than full of pulp. Total
retention times of the pulp in th~ system may vary
depending upon the nature and condition of the pulp
and the desired amount of delignification to be
accomplished. Retention times of between 5 and 12C
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.
In another embodiment of the invention
illustrated in Fig. 2a, where like components are
indicated by like reference numerals, pulp from
wasller 50 is sent through refiner 52 ~or further
fiberization before bein~ fed to thick stock pump
12~ Since the consistency of the pulp lea~-incl
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. Ref iner 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 kraft pulp having an initial Kappa
number greater than about 50.
BFN 7089-C -15- -
Also illustrated in Fig. 2a is the use of
one or more subsequent substantially horizontal
reaction vessels such as vessel 30 to carry out
~urther delignification on the pulp. As shown, pulp
exiting one end of vessel 10 drops into vessel 30
where it is transported along the length of the
vessel with gentle agitation by rotary screw 32
having solid helical flights 34 and driven by a
suitable drive means 33. Steam may be added through
line 48 to maintain the temperature in vessel 30
within the preferred range of 80-160C. Addition-
al oxygen may be injected through line 18a if
requiredO
Yet another embodiment of the invention is
illustra~ed 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 recombined with the main
pulp stream through line 60. This combined pulp
stream is then washed and oxygen delignified as
described above to yield a bleachable grade pulp.
Alternatively, the refined material from refiner 52
may be sent through line 62 and be recombined with
the main pulp stream prior to screens 56.
In another embodiment of the invention
which is illustrated in Fig. 2c, in which like
components are represented by like reference numer-
als, pulp is transported from an initial cooking or
digestion stage through line 54 directly into pulp
washer 50. From there, the pulp is fed into vessel
10 where it undergoes oxygen delignificationl Upon
BFN 7089-C -16-
completion of the reaction, the pulp exits ~essel 10
through outlet 26 and is passed to blow tank 28.
The pulp may optionally be further washed in washer
61 before being screened by screens 56. Accepted
pulp from screens 56 is sent via line 57 to further
processing such as a Eurther bleaching stage.
Rejects from screens 56 are passed through line 59
and fiberized in refiner 52. The fiberized rejects
are then returned through line 64 to be recombined
with the main pulp stream entering vessel 10.
In order that the invention may be better
understood, reference is made to the following
nonlimiting examples.
Example 1
A northeastern softwood kraft pulp having
an initial Kappa number of 29.3 and a viscosity of
26.9 centipoise (cps) was oxygen delignified in
accordance with the process of the invention. The
reaction conditions were 10~ p~lp consistency, 100
psig total gas pressure, and a 3~ sodium hydroxide
dosage by weight based on dry pulpo Retention time
~ in the reaction zone was varied from 8 to 16 to 39
; minutes by varying the speed of the rotary screw in
the reactor. The 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
Initial Kappa No. -
% Delignification = Final Kappa No. 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 a~out 50%
BFN 7089-C -17-
delignification due to severe 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 agitation
of the pulp, more lignin can be removed from the
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 elimina~ion of the
need for a conventional chlorine bleaching stage.
Example 2
Medium ~15~) consistency oxygen delignifi-
cation was carried out on a softwood kraft pulphaving an initial viscosity of 29.5 using the
process of the present invention. The delignifica-
tion reaction wa~ 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 exception
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 rlesults are illustrated in Fig. 4. As
shown by 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 ~as 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
BFN 7089-C -18-
the consistency of the pulp varied from 2% to 15~ to
28~. The results are reported in Table I below.
Table I
Consistency % 2 Consumed % CO Evolved Hydrocarbon
Based On Based On to CO
4Weiaht of PUl~ Weiqht of Pulp Ratio(Moles)
2 3.2 0.010 0.2
lS 1.5 0.015 0.2
1028 ~.5 0.024 0.2
For a working system, it is necessary 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.
Using the data from Table I, material
balance calculations were made to determine the
amount of oxygen required 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 Required*
2 3.2 3.58
1.5 2.06
28 1.~ 2.42
*based on weight of pulp.
The results show that the medium consistency process
has lower oxygen requirements.
BFN 7089-C 19-
Example 4
A high yield sof~wood kraft pulp near the
point of fiber liberation and having an initial
Kappa number of 59.4 was oxygen delignified in an
autoclave at temperatures ranging from 100-130C
and at a total gas pressure of 120 psig. The pulp
was maintained at a medium consistency for an
approximately 15 minute reac~ion time as the charge
of alkaline ~caustic) chemicals was varied from 2-6
~ by weight based on oven dry pulp.
As shown in Fig. 5, the curve labeled A in
which the pulp was continuously gently agitated in
the autoclave shows a greater reduction in Kappa
number (indicative of a greater delignification
rate) than the curve labeled B in which no agitation
was performed. The results show the importance of
gentle agitation of pulp when delignifying at medium
consistency to improve the rate of delignification
of the pulp.
Example 5
; Tests were made using a softwood kraft pulp
having an initial Kappa number of 29.5 to determine
the effect of pulp consistency on the extent of
delignification for a given alkaline chemical
(caustic) dosage and reaction time. The tests were
carried out in a 2 liter autoclave at 110C and
150 psig gas pressure for 20 minutes. Low (2%)
consistency tests were done under conditions of
vigorous agitation (rotation of reactor at 1250 rpm)
while the medium (15%) and high (28%) consistency
tests were conducted without agitation. The results
are shown in Fig. 6.
As can be seen, surprisingly the extent of
delignification for the medium and high consistency
tests were nearly identical at a given causti~
charge. The low consistency ~ests resulted in
~ 3
BFN 7089-C -~~
substantially less deligniEication. Therefore,
longer reaction times would be required for a low
consistency process to achieve the same reduction in
Kappa number as for either a medium or high consis-
tency process~
Example 6
Tests were made to evaluate the effects, ifany of black liquor solids entrainment (BLSE) on
pulp viscosity and yield selectivities in an oxygen
deliqnification reaction. Experimentally, a high
yield northern softwood Kraft pulp having an initial
Kappa number of 59.4 was oxygen delignified for a
reaction time of 20 minutes at 110C at a total
pressure of 120 psig. A caustic charge of 5% NaOH
based on dry pulp was us~d. No protector chemicals
were added. The amount of black liquor solids added
to each test cook is expressed as a weight percen-
tage of pulp as follows:
20weight of total organic
BLSE(%~ = and inorganic black solids x 100
weight of pulp
The range of BLSE studied was 0-40%. The
extent of delignification for both high (28%) and
medium (15%) consistency operation was tested for
comparison purposes. The results are shown in Fig.
As can be seen, surprisingly at medium consis-
tency the final Kappa number drops and then rises as
the BLSE loading is increased indicating an enhance-
ment of the rate of delignification. The maximum
extent of delignification appears to be in the range
of 2-4% BLSE. When the BLSE reaches 16%, the final
Kappa number reached is the same as that reached at
BLSE=0%. Further addition of solids above 20% BLSE
results in a severe retarding effect on the extent
of delignification. By comparison, BLSE has a
BFN 7089-C -21-
severe negative effect on the rate of delignifica-
tion over the entire range tested (0-20%) for high
consistency operation. The data also show that high
consistency processing is much more sensitive to the
negative rate effect of BLSE than medium consistency
operation, over the range of BLSE studied. Se~ Fig.
7.
The effect of BLSE on yield selectivity was
tested. The results are illustrated in FigO 8. The
solid curve represents a base case for 0% BLSE at
15% consistency with caustic charges ranging from
3-6% NaOH by weight based on dry pulp. The data
indicate that BLSE has no effect on yield selectivi-
ty at medium consistency operation for at least up
to 20% BLSE. However, yield selectivity is adverse-
ly affected at high consistency operation at about
10% BLSE and above. The data show that medium
consistency operation is much less sensitive than
high consistency operation to BLSE with respect to
yield selectivity.
The effect of BLSE on viscosity selectivi~y
was also tested. The results are illustrated in
Fig. 9. The solid curve represents a base case for
0~ BLSE at 15~ consistency for caustic charges
ranging from 3-6% NaOH by weight based on dry pulp.
The data indicate that at medium consistency opera-
tion, a BLSE of up to about 8-10% could be tolerated
without any significant loss in viscosity selectivi-
ty. Moreover, the viscosity selectivity appears to
decrease at a faster rate at high consistency opera-
tion than at medium consistency as BLSE increases.
For example, at 20~ BLSE, essentially the same
viscosities were achieved at both medium and high
consistency while the medium consistency Kappa
number was 2.5 points lower.
BFN 7089-C -22-
The effect of BLSE on oxygen consumption in
a medium consistency system was tested using a
softwood kraft pulp having an initial Kappa number
of 60. The results are illustrated in Fig. 10. As
can be seen, the consumption of oxygen increases
substantially linearly with increasing black liquor
solids content. As the results indicate, between
about 0.1~% by weight oxygen (slope M,) and 0.18% by
weight oxygen (slope m3) are consumed for each
percentage unit by weight of black liquor solids
present in the feed liquor~ The average amount of
oxygen consumed was about 0.14% by weight (slope
m2) per unit percentage of BLS.
These results are significant because the
dissolved wood components and spent cooking chemi-
cals which make up black liquor are present in the
pulp leaving a batch digester in an amount which is
about 175% of the weight of the pulp. Before a
conventional bleaching operation, this amount must
be dramatically reduced to a BLSE of about 1-3
based on pulp weight by countercurrent washing
operations. The closer an oxygen stage reactor can
be positioned to the digestion stage, the greater
the savings in capital and operational costs of both
pre-and post-oxygen stage washing equipment. The
data indicate that an oxygen delignification reactor
operating at medium consistency can tolerate a BLSE
of 10% without adversely affecting either the reac-
tion rate or selectivity. High consistency opera-
~ion is much more sensitive to the negative effectsof BLSE on both rate and selectivity.
Moreover, since oxygen consumption
increases with increasing black liquor solids
content, delignification systems in which only
limited amounts of oxygen can be supplied will be
especially sensitive to the presence of black liquor
BFN 7089-C -23-
solidsO The system of the present invention,
however, has provision for adding additional oxygen
as needed at several locations. This also enables
the system of the present invention to tolerate the
presence of increased amounts of black liquor solids.
Example 7
Tests were also made to evaluate the
effects, if any, oE oxygen stage solids entrainment
(OSSE) on pulp yield and viscosity selectivities in
an oxygen delignification reaction. Experimentally,
repulped paper bag feedstock having an initial Kappa
number of 57.5 was oxygen delignified for a reaction
time of 20 minutes at 112C at a total pressure of
100 psig. The pulp was of 15~ consistency, and a
lS caustic charge of 4~ ~aOH based on dry pulp was
used. About 0.3% MgSO4 based on dry pulp was
added as a protector. The amount of oxygen stage
solids added to each test cook is expressed as a
weight percentage o pulp as follows:
~0
weight of dissolved
OSSE(~j = solids from oxy~en sta~e x 100
weight of pulp
The effect of OSSE on yield selectivity was
tested. The results are illustrated in Fig. 11.
The area within the lines labeled ~ = 0.15 and
~ = 0.17 represent the potential theoretical
yield of the reaction. The dashed line indicates
the results for the base case of 0% solids. As can
be seen, surprisingly, the presence of up to 8~ OSSE
improved the yield selectivity slightly over the
base case.
The effect of OSSE on viscosity selectivity
was tested. The results are illustrated in Fig.
12. As can be seen, for medium consistency opera-
BFN 708g~C -24-
tion, the presence of up to 8~ OSSE had no adverse
effect on the viscosity selectivity of the pulp.
While the methods and apparatus herein
described constitute 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 either without
departing from the scope of the invention, which is
defined in the appended claims.
~0
