Extended delignification in pressure diffusers

Extended delignification of kraft pulp in a pressure diffuser allows a pulp mill with a digester (e.g. continuous digester) to increase its production capacity without building a new line, or to decrease its demand in its bleach plant, or to increase the strength of softwood pulp it produces. Comminuted cellulosic fibrous material is kraft cooked to produce kraft pulp having a blow temperature over about 300.degree. F. The pulp is diffusion treated in the first stage of a pressure diffuser to replace the water around the pulp with delignifying liquor. The liquor has a dissolved lignin concentration of less than 12% (optimally less than about 4%), has an effective alkali concentration of at least 2 gm/l (preferably 8-55 gm/l), and a temperature of at least about 300.degree. F. Treating the pulp with the delignifying liquor significantly reduces the K-number (e.g. on the order of about 5). After extended delignification, the pulp is washed in subsequent stages of the pressure diffuser. The components of the delignifying area of the pressure diffuser are of stainless steel to avoid corrosion from the alkaline liquor.

BACKGROUND AND SUMMARY OF THE INVENTION 
In many facilities with continuous digesters for producing kraft pulp, the 
mills have reached their production capacity yet would like to increase 
their pulp production without the necessity for building entire new lines. 
Other mills would like to decrease the demand in their bleach plants. 
Other mills wish to increase the strength of softwood pulp so that the 
proportion of softwood pulp in paper making, or other applications, can be 
increased. All of these objectives can be achieved according to the 
present invention by providing extended kraft delignification using a 
pressure diffuser. 
According to the present invention, a pressure diffuser is operatively 
connected to the pulp discharged from a continuous digester. No, or very 
little, washing is done in the bottom of the digester, therefore the 
entire digester can be devoted to cooking, and thus production can be 
increased. Any wash liquid that is supplied to the bottom of the digester 
is throttled or heated so that the pulp discharged from the digester is at 
high temperature, typically having a blow temperature over about 
300.degree. F., if extended delignification is to be immediately practiced 
thereafter. 
By providing a particular treatment liquid and by taking advantage of the 
efficient diffusion that is possible using a pressure diffusion washer, it 
is possible to effect further delignification in the pressure diffuser. A 
pressure diffuser is a commercially available piece of equipment, sold by 
Kamyr, Inc. of Glens Falls, N.Y. and Kamyr AB of Karlstad, Sweden. Such a 
diffuser is illustrated and described in Canadian patent 1,173,603, and in 
an article entitled "Pressure Diffuser--A New Versatile Pulp Washer" by 
Knutson et al, SPCI-84 World Pulp and Paper Week Proc., Apr. 10-13, 1984, 
pp. 97-99. 
In the first stage of a pressure diffuser, delignifying liquor is added to 
the pulp. The purpose in adding the liquor is to replace the water around 
the pulp fibers with the delignifying liquor. This is effectively 
accomplished by supplying the liquor to the pulp at a liquor-to-water (in 
the pulp) ratio of about 0.2-1.3. 
The delignifying liquor must have a low concentration of dissolved lignin 
(i.e. a low dissolved solids content). Typically the water in the pulp has 
a dissolved solids concentration of about 12 to 16%. The delignifying 
liquor must have a concentration less than 12%, and preferably about half 
(e.g. 6%) or less of the pulp water concentration, and optimally 4% or 
less. The alkali concentration must be at least 2 gm/l effective alkali 
(Na.sub.2 O), and preferably is at least about 8 gm/l, and about 20-55 
gm/l is particularly effective. The delignifying liquor also must be at 
high temperature. The exact temperature will depend upon the blow 
temperature of the pulp (or other pulp temperature where extended 
delignification is not practiced immediately after continuous digesting). 
The higher the temperature of the pulp actually encountered by the 
delignifying liquor, the lower its temperature need be. Typically, 
however, the temperature of the delignifying liquor must be at least about 
300.degree. F., and for a blow temperature of about 320.degree. F. would 
optimally be around 315.degree. F. 
By replacing the water around the pulp fibers with delignifying liquor, the 
K-number of the pulp is significantly reduced. In actual working examples 
of the invention, the K-number has been reduced, on some occasions, over 
6, a very significant reduction. 
Since mild steel, typical of some equipment that is in pressure diffusers, 
corrodes when subjected to alkaline solutions at high temperatures, for 
extended operation according to the invention it is necessary to provide 
the first wash liquid inlet (and associated components) of the pressure 
diffuser of a material, such as stainless steel, having good corrosion 
resistant properties. For subsequent inlets and related components in the 
pressure diffuser conventional materials may be utilized. A washing liquid 
is applied to the other inlets in the pressure diffuser to effect washing 
of the pulp. 
A pressure diffuser is particularly suited for the extended delignification 
according to the present invention. A pressure diffuser washes pulp at 
very high pressures and temperatures, and the diffusion action has been 
shown to be very effective. The apparatus according to the invention 
comprises a continuous digester with a pulp discharge at the bottom, and a 
multiple stage pressure diffuser having a pulp inlet, a pulp outlet, and a 
screened liquid outlet. As described above, the first stage is made of 
corrosion resistant material, and a treatment liquid inlet is provided to 
each stage. The apparatus further includes treatment means, such as a 
vacuum drum washer, having a pulp inlet, a pulp outlet, and a filtrate 
outlet; a source of white liquor; and liquid heating means. A first 
conduit transports pulp from the bottom of the digester to the pulp inlet 
of the pressure diffuser, while a second conduit transports pulp from the 
diffuser pulp outlet to the treatment means pulp inlet. A third conduit 
supplies a portion of the filtrate from the treatment means filtrate 
outlet, mixed with white liquor from the white liquor source, to the 
heating means. A fourth conduit supplies a heated mixture of white liquor 
and filtrate from the heating means to the treatment liquid inlet to the 
diffuser first stage, and a fifth conduit supplies wash liquid to the 
treatment liquid inlet of subsequent stages of the pressure diffuser, 
including at least some liquid from the filtrate outlet. Optionally, a 
sixth conduit may be provided for supplying screened liquid from the 
diffuser screened liquid outlet to the bottom of the digester for 
effecting a small amount of washing, with throttling valve means disposed 
in the sixth conduit for regulating the flow of screened liquid into the 
bottom of the digester. Any washing done in the digester is merely to 
effect removal of some dissolved lignin, but without reducing the pulp 
temperature to the extent that the desired K-number reduction is 
destroyed. 
It is the primary object of the present invention to provide an effective 
method and apparatus for the extended kraft delignification of pulp. This 
and other objects of the invention will become clear from an inspection of 
the detailed description of the invention and from the appended claims.

DETAILED DESCRIPTION OF THE DRAWING 
A continuous digester 10 is operatively connected by first conduit means 11 
to a pressure diffuser 12. The continuous digester 10 preferably is of the 
type supplied by Kamyr, Inc. of Glens Falls, N.Y. or Kamyr AB of Karlstad, 
Sweden, having pulp discharge 13 at the bottom thereof. The pressure 
diffuser 12 also is of the type supplied by Kamyr and it has a plurality 
of wash liquid inlets and related components, which are illustrated in 
dotted line in the drawing, the first inlet and components being indicated 
by reference numeral 14, and the subsequent inlets and components by 
reference numeral 15. The first liquid inlet 16, and the inlets 17 are the 
subsequent inlets. 
After treatment in the pressure diffuser 12, pulp is discharged through 
conduit means 19 from the top of the diffuser 12 to a treatment means 20. 
The treatment means 20, as schematically illustrated in the drawing, 
comprises a vacuum drum, which provides a washing action to the pulp. 
Alternatively, and in fact preferably, the washer 20 may comprise a 
diffusion washer, or a second pressure diffuser. Vacuum drum 20 is 
supplied with clean wash liquid or the like, and has a pulp inlet 21, a 
pulp outlet 22, and a filtrate outlet 23. 
The apparatus according to the invention also comprises a source of white 
liquor, 24. Third conduit means 26 provide for mixing of some of the 
filtrate from vacuum drum 20 with white liquor from source 24, and supply 
of the mixture to a liquid heating means 28. The liquid heating means 28, 
as illustrated in the drawing, preferably comprises a conventional 
indirect heater in which steam is provided in a closed loop within the 
heater 28 through pipe 29 or the like. A fourth conduit 30 provides heated 
alkali liquor from heater 28 to the inlet 16 to the first stage of the 
diffuser 12, while fifth conduit means 31 provides "cool" filtrate from 
vacuum drum 20 to the subsequent stages 15 of the diffuser 12. 
For the embodiment actually illustrated in the drawing, valves 33 and 34 
are provided. Those valves are provided merely because the system 
illustrated in the drawing is retrofit onto an already existing system. 
The valves 33, 34 are necessary to isolate the first stage of the diffuser 
12 from the others, when extended delignification is being practiced. For 
example when extended delignification is being practiced the valve 34 will 
be closed and the valve 33 will be opened. If it is desired not to 
practice extended delignification using the pressure diffuser 12, then the 
valve 34 is opened and the valve 33 closed (and the white liquor source 24 
is disconnected). 
An optional desirable feature according to the invention is to provide some 
modest amount of washing at the bottom of the digester. For this purpose a 
sixth conduit means 36 is provided for supplying screened liquid from the 
screened liquid outlet 37 of the diffuser 12 to the bottom of the digester 
10, with a conventional withdrawal screen system 39 adjacent the bottom of 
the digester 10 for withdrawing the wash liquid with dissolved lignin. A 
throttling valve 40 is provided in the conduit 36 to control the flow of 
wash liquid into the digester 10 so that the blow temperature of the 
digester 10 will be above about 300.degree. F. The valve 40 will be 
controlled so that some washing takes place in digester 10 to effect 
removal of some dissolved lignin, but without reducing the pulp discharge 
temperature to the extent that the desired K-number reduction achieved by 
delignification in the pressure diffuser 12 is destroyed. Any screened 
liquid in conduit 36 not used for washing, and the spent wash liquor from 
discharge conduit 42 from screen assembly 39, is passed to a series of 
flash tanks or the like, the first flash tank 43 being schematically 
illustrated in the drawing. 
The flow of white liquor and filtrate to the heater 28 will be controlled 
depending upon the desired effective alkali concentration of the 
delignifying liquor, as well as the desired dissolved lignin concentration 
of the delignifying liquor. It is necessary to have the dissolved solids 
content of the delignifying liquor supplied to stage 14 to be less than 
12%, with about half or less of the dissolved solids content of the liquid 
surrounding the pulp in the discharge conduit 13 (e.g. about 6% or less) 
being desired. Optimally, the dissolved solids content will be 4% or less. 
The delignifying liquor is supplied at a ratio of about 0.2-1.3 (liquor to 
water in the pulp), the ratio being lower when the effective alkali 
content of the liquor is higher. 
Delignification occurs according to the invention if the effective alkali 
(Na.sub.2 O) concentration is at least 2 gm/l, but it is desirable to have 
the effective alkali concentration of at least about 8 gm/l, and about 
20-55 gm/l is particularly effective. The heater 28 will be controlled so 
that the temperature of the hot alkali liquor is at least about 
300.degree. F., and typically will be about 315.degree. F. The exact 
temperature will be dependent upon the blow temperature from the digester 
(or other temperature of the pulp at the time of treatment), the purpose 
being to achieve the desired reduction in K-number by extended 
delignification, while minimizing the use of energy. In general, for the 
illustrated embodiment, the higher the blow temperature, the lower the 
temperature the alkaline liquor can have. 
Typical results achieved according to the present invention may be seen 
from Table I which follows. In the following table, columns A and B 
indicate one test, columns C and D a second, columns E and F a third, and 
columns G and H a fourth. "DIG" references the digester, while "PD" 
references the pressure diffuser. The pulp consistency is generally about 
the same throughout the treatment process, from the bottom of the digester 
through discharge from the pressure diffuser, being approximately 8-13%. 
The white liquor used in the test has an active alkali content of about 
101 gm/l. Therefore in test C D and in test E F, the delignifying liquor 
had an effective alkali content of about 51 gm/l. Also in test C D, for 
example, the ratio of delignifying liquor to water in the pulp is about 
0.25 (in view of the relatively high alkali concentration of the liquor). 
In all of the tests, the content of solids (dissolved lignin) in the pulp 
discharged from the digester was about 14.5%, while the solids 
concentration of the delignifying liquor was about 2-4%. 
TABLE I 
__________________________________________________________________________ 
Test A B C D E F G H 
Vessel DIG 
PD DIG 
PD DIG 
PD DIG 
PD 
__________________________________________________________________________ 
Production Rate 
482 
482 
482 
482 
482 
482 482 
482 
(ADBT/D) 
Blow Flow (GPM) 
720 
720 
688 
688 
680 
680 675 
675 
Blow Temp (.degree.F.) 
305 
305 
322 
322 
320 
320 310 
310 
Wash Flow (GPM) 
850 
850 
850 
850 
850 
850 850 
850 
Wash Temp. (.degree.F.) 
166 
166 
164 
164 
164 
164 165 
165 
Extraction (GPM) 
950 
950 
950 
950 
950 
950 950 
950 
Ext. Temp. (.degree.F.) 
280 
280 
288 
289 
287 
287 282 
282 
White Liquor (GPM) 
50 50 80 80 83 83 0 0 
Filtrate Flow (GPM) 
95 95 80 80 83 83 148 
148 
Temp. of White 
316 
316 
315 
315 
315 
315 320 
320 
Liquor/Filtrate 
Mix (.degree.F.) 
K# 34.4 
30.9 
32.3 
26 31.6 
26.6 
29.6 
28.8 
Reduction in K# 
3.5 6.3 5.0 0.8 
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Note in the above Table that although a very significant decrease in 
K-number takes place when practicing the invention (e.g. tests C D and E 
F, reductions of 6.3 and 5.0, respectively), when the flow of white liquor 
is terminated in test G H (by opening valve 34 and closing valve 33 and 
disconnecting source 24) the K-number reduction is not significant (0.8) 
despite the fact that the temperature of the liquid added in the first 
stage of the pressure diffuser is still very high (320.degree. F.). 
According to the method according to the present invention, kraft cooking 
of comminuted cellulosic fibrous material (such as wood chips) is 
practiced to produce kraft pulp having a consistency of about 8-13%. Where 
extended delignification takes place right after continuous digesting, and 
a low wash ratio is used, the blow temperature preferably is over about 
300.degree. F. It is ensured that the temperature is over about 
300.degree. F. by throttling with valve 40 the wash water in conduit 36 
that flows into the digester 10. While some washing of the pulp takes 
place in the bottom of the digester to effect removal of some dissolved 
lignin, it is not enough washing to reduce the pulp temperature to the 
extent that the extended delignification (K-number reduction) desired is 
adversely affected. 
The water around the pulp fibers of the cooked pulp is replaced with 
delignifying liquor having less than 12% dissolved solids, and optimally 
about 8% dissolved solids or less. The delignifying liquor has at least 2 
gm/l effective alkali and preferably over 8 gm/l. The delignifying liquor 
may be provided by combining white liquor (e.g. with an active alkali 
content of 100 gm/l) from source 24 with filtrate from vacuum drum 20, and 
heating the mixture in the indirect heater 28 to raise the temperature 
over 300.degree. F., and preferably to about 315.degree. F. 
The extended delignification in which the water around the pulp fibers is 
replaced with delignifying liquor preferably is accomplished by diffusion 
treating of the pulp with a delignifying liquor by supplying the liquor to 
the pulp at a liquor-to-water in the pulp ratio of 0.2-1.3, resulting in a 
significant reduction of the K-number. 
The temperature of delignifying liquor is adjusted with respect to the blow 
temperature or flow of the pulp so as to optimize K-number reduction. When 
the blow temperature is about 320.degree. F., the delignifying liquor 
temperature is about 315.degree. F., with higher values for the 
delignifying liquor temperature for lower blow line temperatures, and 
lower values for higher blow line temperatures. Where a high wash ratio 
(e.g. 1.2) is used, the blow temperature of the pulp is not as important. 
If batch digesters are used, the pulp might need heating before practicing 
extended delignification according to the invention. 
After the K-number of the pulp has been significantly reduced (e.g. about 6 
in test C D in the Table) the pulp is diffusion washed, in later stages 15 
in the pressure diffuser 12. 
It will thus be seen that according to the present invention a method of 
effecting extended kraft delignification is provided. The method allows a 
pulp mill to increase pulp production without the necessity for building 
an entire new line, by practicing the extended delignification in the 
bottom of a pressure diffuser. Alternatively, a mill can use the extended 
delignification to decrease the demand in its bleach plants. 
Alternatively, the strength of the softwood pulp produced from the mill 
can be increased by decreasing the harshness of the digestion and 
compensating for that by extended delignification, so that the proportion 
of softwood pulp in paper making, or other applications, can be increased. 
While the invention has been herein shown and described in what is 
presently conceived to be the most practical and preferred embodiment 
thereof, it will be apparent to those of ordinary skill in the art that 
many modifications may be made thereof within the scope of the invention, 
which scope is to be accorded the broadest interpretation of the appended 
claims so as to encompass all equivalent structures and procedures.