Method of improving the homogeneity of lignocellulose containing material

A method for improving the homogeneity of ligno-cellulose containing materials during digestion in a digester is disclosed. Specifically, the ligno-cellulose containing material and cooking liquor are charged to the digester, a portion of the cooking liquor is drawn off from the digester and heated, preferably in a calorisator, and the heated liquor is divided into two portions, the first portion being returned to the top of the digester, and the second portion being returned to the bottom of the digester, and boiling is caused to occur in the digester solely by terminating the flow of liquor into the top of the digester.

FIELD OF THE INVENTION 
The present invention relates to a method of obtaining a homogeneous 
pulping during periodical digestion of a ligno-cellulose containing 
material in a digester. Specifically, the present invention relates to 
such method in which the cooking liquid is drawn off from the digester, 
heated in a calorisator, and returned to the top and bottom, respectively, 
of the digester. 
BACKGROUND OF THE INVENTION 
It is extremely important to obtain uniform digestion in pulp manufacturing 
processes, both from the viewpoint of economy and with respect to the 
quality of the pulp obtained. The reasons why non-uniform digestion is 
obtained vary in accordance with the type of process being utilized. In 
Swedish patent specificaton No. 345,885, some of the most significant 
factors which influence the homogeneity during periodic digestion are 
listed, and a method of eliminating the negative effects of one of the 
most significant of such factors, namely, the circulation of the cooking 
liquid. 
The technique employed in accordance with that application is the heat 
boiling-up, and in accordance with that experiences in mill installations, 
extremely positive effects upon the homogeneity during digestion have been 
demonstrated. In accordance with this method, boiling of the cooking 
liquid is effected by rapidly decreasing the pressure in the digester in 
order to effect upward flow therein, and temperature balances in the 
digester. This is effected either by violent gassing or by the intake of 
cooling liquor to the top of the digester. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, it has now been discovered that 
boiling can be effected in the digester by an entirely new method. 
Specifically, such boiling takes place by periodically heating only from 
the bottom of the digester. Specifically, in accordance with this method, 
the ligno-cellulose containing materials and cooking liquor are charged 
into the digester, at least a portion of the cooking liquor is drawn off 
from the digester, and this liquid is heated and divided into a first 
heated liquid portion and a second heated liquid portion, and the first 
heated liquid portion is returned to the top of the digester, while the 
second heated liquid portion is returned to the bottom of the digester, 
and boiling is effected in the digester solely by terminating the flow of 
the first heated portion into the top of the digester. By employing such a 
method, pressure drops in the digester are prevented, and the heating 
effected by the flow to the bottom of the digester results in boilings-up, 
which also results in distribution of heat to poorly heated zones therein. 
In a preferred embodiment, this heating, when it is to be carried out 
within a reasonable time and without risk of extra incrustation in the 
calorisator for heating the liquid, requires that the conduit for 
returning the second portion of the heated liquid to the bottom of the 
digester has a sufficiently large diameter so that increased circulation 
flow rates during boiling-up are provided for, that is since the total 
amount of cooking liquor drawn off from the digester remains substantially 
constant. 
By employing such a method, a far simpler installation, which is 
considerably cheaper than those previously known methods which employ 
boiling, while at the same time a very uniformly digested pulp, having a 
low shives content is obtained. 
After the initiation of digestion, the first boiling-up is preferably 
carried out when the chlorine number of the pulp is between 30 and 20, 
that is, when the delignification first begins to become substantial, but 
before any substantial delignification has occurred. The total number of 
boilings-up, and the time interval between them, are adjusted in 
accordance with the digestion schedule in question, so that during the 
entire boiling process when delignification takes place, the greatest 
possible heat homogeneity is obtained in the digester. 
The total duration of these boilings may vary, but should be at least about 
ten seconds. Normally, boilings will occur for from about 1 to 30 minutes, 
preferably from about 2 to 20 minutes, and most preferably from about 4 to 
10 minutes. In order to obtain efficient boiling by this process, it is 
most preferred that the conduit which effects circulation of the second 
portion of the heated liquid to the bottom of the digester must be of a 
sufficient size so that the bottom circulation flow rate during boiling-up 
is from about 1 to 15 m.sup.3 /min, preferably from about 2 to 12 m.sup.3 
/min, and most preferably from about 3 to 8 m.sup.3 /min. In addition, it 
is preferred that the digester be degassed during boiling. 
By periodically closing the flow of the second portion of the heated liquid 
to the bottom of the digester after boiling has occurred, an increase in 
the circulation of the first portion of the heated liquid to the top of 
the digester is effected after boiling-up, which result is favorable for 
the digesting process as a whole. 
In a preferred embodiment, in order to reduce the packing of the digester 
contents, the flow of the first portion of the heated liquid to the top of 
the digester can be periodically limited prior and/or subsequent to 
boiling-up.

DETAILED DESCRIPTION 
In accordance with the FIGURE, a digester 1 is provided, having a 
circulation system for indirect heating therein. The circulation system 
thus comprises a strainer girdle 2 which is connected by conduit 8 to a 
circulation pump 3 for drawing off liquid from the digester 1. This liquid 
is then passed through a calorisator 4 for heating thereof, and the heated 
liquid from the calorisator is then circulated to both the top and the 
bottom of the digester 1. This is effected through conduits 5 and 6, 
respectively. Thus, in the conduit 5, for recirculation of the heated 
liquid to the top of the digester 1, a valve 7 is provided which 
preferably is controlled automatically by a timer and/or temperature 
impulses. Similarly, in conduit 6, for recirculation of the heated liquid 
to the bottom of the digester 1, means are provided for controlling the 
flow through this conduit. 
The present invention may be further illustrated by the following examples 
thereof. 
EXAMPLE 1 
Digestion was carried out in a periodic sulphate digester having a volume 
of 140 m.sup.3. The digester was provided with a circulation system for 
indirect heating in accordance with the FIGURE. The digester was initially 
charged with about 25 tons of bone dry chips by means of a liquor packing 
method. Both white liquor and black liquor were charged in such an amount 
so that the total alkali charge was about 220 kg of active alkali (NaOH) 
per ton of absolutely dry wood, and so that the wood/liquid ratio was 
about 1/3.5. During the initial charging of these chips, the circulation 
pump 3 was already started, so that liquid was drawn out of the digester 1 
through the strainer girdle 2. This cooking liquid was then circulated 
through the calorisator 4 for heating, and after the heating, 70% of the 
flow from the calorisator was returned to the top of the digester through 
conduit 5, while the remaining 30% was returned to the bottom of the 
digester through conduit 6. 
After 70 minutes of operation, the digester had been heated to 170.degree. 
C. in this manner by means of steam in the calorisator 4. When the 
temperature in the conduit 5 returning liquid to the top of the digester 
during heating of the charge had risen to about 155.degree. C., an initial 
period of boiling-up was begun. This was effected by automatically closing 
valve 7 with a timer, which had been initially set at the start of 
digestion. In this manner, the recirculation of the heated liquid to the 
top of the digester was terminated, and the digester was thus heated only 
from recirculation through bottom conduit 6, now carrying all of the 
liquid drawn off from the digester by circulation pump 3. When this 
heating was begun, the boiling-up thus occurred solely by the termination 
of the flow of the heated liquid to the top of the digester, and, so that 
heat was spread to poorly heated zones within the digester. After ten 
minutes of boiling, valve 7 was reopened, and heating of the charge 
continued in the normal manner. The total charge was held at the maximum 
temperature of 170.degree. C. for about fifty minutes, and was terminated 
by strongly gassing the top of the digester for ten minutes before 
blow-off. The blow-off time was 15 minutes. The chlorine number of the 
pulp obtained was 6, and the shives content as shown by the screening of 
an average sample from the charge in a Wennberg screen having a slit width 
of about 0.20 mm, was about 1.5%. For a similar charge which was digested 
in a normal manner, i.e., not in accordance with the method of the present 
invention, a shives content of 4.5% and a chlorine number of 6 was 
obtained. 
EXAMPLE 2 
A second digestion was carried out in the same type of digester as in 
Example 1, and at the same charge conditions, except that the wood/liquid 
ratio was about 1/3.0 instead of 1/3.5. The charge was again heated in the 
same manner as Example 1, this time up to 145.degree. C., when the 
recirculation of the heated liguid to the top of the digester was reduced 
by about 15% by partially closing valve 7. When the temperature in the 
conduit 5 for recirculation to the top of the digester during heating of 
the charge had arrived at 155.degree. C., boiling-up was carried out. This 
was effected by terminating recirculation of the heated liquid to the top 
of the digester by closing valve 7, so that again the charge was heated 
only from recirculation of the heated liquid through the conduit 6 to the 
bottom of the digester. After about 8 minutes, valve 7 was reopened to its 
position before boiling-up, i.e., its partially closed position. During 
contingent digestion, this throttling of the flow through conduit 5 to the 
top of the digester was maintained in order to reduce the packing of the 
chip bed. 
The charge was maintained at a maximum temperature of about 170.degree. C. 
for about 50 minutes, and was terminated with a strong top gassing for 
about 10 minutes prior to blow-off. Blow-off time was about 15 minutes. 
The chlorine number of the pulp obtained was about 5.5, and the shives 
content measured by screening an average sample from the charge in a 
Wennberg screen having a slit width of about 0.20 mm was about 1.4%. For a 
similar charge produced by a digester which was not employing the method 
of the present invention, a shives content of about 6.5 was obtained with 
a chlorine number of about 5.5. 
EXAMPLE 3 
A similar digestion was carried out in a periodic sulphate digester having 
a volume of about 125 m.sup.3, again provided with a circulation system 
for indirect heating in accordance with the FIGURE. By employing a liquor 
packing method, the digester was charged with about 23 tons of bone dry 
chips. Both white liquor and black liquor were charged in an amount so 
that the total alkali charge was 200 kg active alkali (NaOH) per ton of 
absolutely dry wood, and the wood/liquid ratio was about 1/3.5. The 
circulation pump 3 was initiated during the chip charging stage, and drew 
off liquid from the digester through a strainer girdle 2. 
This cooking liquid was circulated through the calorisator 4 for heating, 
and was then distributed after heating so that about 75% of the total 
volume of heated liquid was recirculated to the top of the digester 
through conduit 5, while the remaining 25% was recirculated to the bottom 
of the digester through conduit 6. 
Heating to about 170.degree. C. took place within about 150 minutes by this 
indirect heating with steam in the calorisator 4, and when the temperature 
in the conduit 5 to the top of the digester during heating of the charge 
had reached about 135.degree. C., and initial boiling-up was carried out. 
This was effected by terminating recirculation of heated liquid to the top 
of the digester through conduit 5 by closing valve 7, and the digester was 
thus heated only from recirculation to the bottom of the digester through 
conduit 6. After about 8 minutes, valve 7 was reopened, and heating of the 
charge continued in a normal manner. A second boiling-up was carried out 
when the temperature of the charge had reached about 163.degree. C., and a 
similar boiling-up time of about 8 minutes was utilized. The charge was 
held at a maximum temperature of about 170.degree. C. for about one hour, 
and after the intake of about 10 m.sup.3 of cooling liquor, the charge was 
blown off. Blow-off time was about 25 minutes. 
The chlorine number of the pulp obtained was about 5.9, and the shives 
content employed by screening an average sample from the charge in a 
Wennberg screen having a slit width of about 0.20 mm was about 1.0%. A 
similar charge digested without employing the method of the present 
invention had a shives content of about 4.7% and a chlorine number of 
about 5.9.