Method for discharging a digester

A method for discharging cooked lignocellulose materials from a digester is disclosed. After batch cooking the lignocellulose material in the digester is cooled to a temperature of about 100.degree. C., preferably from 100.degree. to 120.degree. C., a gas is supplied to the upper portion of the digester to provide a gas pressure such that the combination of that gas pressure and the static pressure in the digester created by the height of cooked lignocellulose material therein exceeds by no greater than about 200 kPa the sum of the pressure required to overcome the level difference between the exit port at the bottom of the digester and an entrance port in the upper portion of a receiving vessel and the flow resistance required to transfer the cooked lignocellulose material therethrough.

FIELD OF THE INVENTION 
The present invention relates to methods for discharging lignocellulose 
materials from digesters. More particularly, the present invention relates 
to methods for discharging cooked lignocellulose material from digesters 
after batch cooking therein. 
BACKGROUND OF THE INVENTION 
In connection with sulfate, sulfite and other cooking processes, the 
delignification of lignocellulose material, such as wood chips, is carried 
out. Thus, the lignocellulose material is cooked in a cooking liquor under 
predetermined conditions of time, temperature, pressure, the supply of 
chemicals, and the like. When the cooking has been concluded, conditions 
of high pressure and temperature prevail in the digester. 
Discharge of the digester can be performed by opening a valve at the bottom 
of the digester so that the contents of the digester are blown out by 
means of the pressure prevailing within the digester through a conduit 
into a receiving vessel which is maintained at atmospheric pressure. 
Alternatively, the contents of the digester can be cooled by displacing 
the hot spent cooking liquor within the digester with cooler waste liquor. 
In these processes, the contents of the digester are then blown out by 
means of high pressure air or steam into the receiving vessel. These 
processes are known as the "cold blow" processes. They are exemplified, 
for example, by Canadian Patent No. 1,135,101, corresponding to Swedish 
Patent No. 435,075; and by European Patent No. 100,293. In the Canadian 
'101 patent a system is provided for blowing the pulp from the digester at 
relatively low temperatures, preferably in the range of from 90.degree. to 
105.degree. C., and in which the pulp may be prewashed within the 
digester. In particular, as is shown in connection with the discussion of 
FIG. 3 therein, at the final stage of digesting, the cooking liquor is fed 
to an equalizing tank maintained under pressure. The pressure in the 
digester 1 is then lowered to substantially atmospheric pressure by 
opening a steam valve, and washing liquor is fed to the digester during 
this period through valve 15 as shown in FIG. 3. In this manner, the 
contents of the digester are blown at the above-mentioned temperatures to 
provide such a cold blow process. 
As for the European '293 patent, a further such cold blow process is shown 
with respect to FIG. 1 thereof in which the spent cooking liquor is 
discharged from the digester by using a curtain of compressed, relatively 
cool air at the top of the digester to force the pulp out through a blow 
valve into a blow tank. In accordance with this process, washer filtrate 
liquor from tank 17 is used as the displacement liquid during operation of 
the digester. It is at this point that the relatively cool air is injected 
into the top of the digester, i.e., after displacement, preferably at a 
pressure of about 0.5 to 1.0 MPa. 
In both of these above cold blow processes, flushing or displacement is 
required therewith. Also, high pressure is required in order to achieve 
blowing from the digester in order to guarantee complete discharge of the 
material therein. However, at the same time, discharge is difficult to 
control since the consistency of the material will vary considerably. 
Furthermore, steam or air can penetrate the material through channels 
prior to emptying of the digester. 
Additional problems in these prior art systems concern control of possible 
blow condensers. Thus, problems with potential pressure shocks to gas 
treatment and with transfer to condensate can arise. In order to deal with 
these problems, the gas treatment equipment has thus been extensive and 
quite costly. 
One method of obtaining a more controlled discharge is to relieve the 
overpressure in the digester, and to reduce the temperature to below about 
100.degree. C. The material can then be pumped out of the digester by 
means of a pump. In this manner, the need for gas treatment equipment can 
also be reduced or avoided. This method is known for use with batch 
digesters for sulfite, sulfate and other types of cooking. 
However, the use of pump discharge has a number of concomitant 
disadvantages. A pump is very sensitive to the pressure of coarse 
particles. These raw materials, however, are normally accompanied by 
different metal objects, stone and concrete clods, as well as large pieces 
of wood. Even if very large pump wheels are thus used, there remains a 
considerable risk of sticking and clashing taking place, with resultant 
disturbances in production. Furthermore, pump discharge can also result in 
increased servicing costs and decreased reliability in operation compared 
with the conventional blow discharge mentioned above. In order to 
reconstruct old digesters, problems of sufficient space for pumps and 
associated conduits can arise, and thus the installation can be 
unreasonably high. 
Another method of discharging a digester is to install a rotating discharge 
device. In this manner, the total digester pressure is used as the 
discharge force. Uniform discharge is obtained in this case by dilution of 
the material in a dilution zone, with simultaneous stirring by means of 
the discharge device itself. Such an arrangement is possible in connection 
with a continuous digester having a bulging bottom and employing 
continuous discharge. However, with a batch digester, which is normally 
provided with a conical bottom, such a discharge device is not suitable. 
Moreover, the installation and servicing of such devices in batch 
digesters would require considerable costs, particularly with respect to 
the large number of such digesters in a plant. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, these and other difficulties have 
been overcome by the invention of a method for the discharge of cooked 
lignocellulose material from a digester containing such cooked 
lignocellulose material in an amount providing a predetermined static 
pressure to a receiving vessel in which the digester has an upper portion 
and a lower portion, and includes an exit port at the lower portion at a 
predetermined level therein, and the receiving vessel has an entrance port 
at a predetermined level therein, the predetermined level in the receiving 
vessel being higher than the predetermined level in the digester by a 
predetermined level difference, and the exit port of the digester and the 
entrance port of the receiving vessel being connectable through a conduit 
providing a predetermined flow resistance between the digester and the 
receiving vessel. 
The method comprises cooling the cooked lignocellulose material in the 
digester to a temperature of about 100.degree. C., supplying a gas to the 
upper portion of the digester so as to provide a predetermined gas 
pressure therein, whereby the combination of the predetermined gas 
pressure and the predetermined static pressure exceeds the sum of the 
pressure required to overcome the predetermined level difference and the 
predetermined flow resistance in order to transfer the cooked 
lignocellulose material from the digester to the receiving vessel by no 
greater than about 200 kPa, and connecting the exit port in the digester 
with the entrance port in the receiving vessel so as to transfer the 
cooked lignocellulose material through the conduit. 
In accordance with this invention, the above-mentioned problems are thus 
solved in connection with discharge from a batch digester. According to 
this invention, a method for the control of discharge of a digester at low 
pressure is obtained without the use of mechanical discharge means. 
Specifically, no pump is required and the disadvantages associated 
therewith are thus eliminated, while at the same time the need for gas 
treatment equipment is avoided or at least minimized. 
In accordance with one embodiment of the method of the present invention, 
dilution liquid is supplied to the lower portion of the digester. 
In accordance with another embodiment of the method of the present 
invention, the temperature of about 100.degree. C. is a temperature of 
between about 100.degree. and 120.degree. C. 
In accordance with another embodiment of the method of the present 
invention, supply of the gas to the upper portion of the digester 
comprises increasing the supply of gas during the transfer of the 
lignocellulose material from the digester to the receiving vessel so as to 
maintain the combination of the predetermined gas pressure and the 
predetermined static pressure during transfer at a substantially constant 
level as the static pressure is reduced in the digester. 
According to another embodiment of the method of the present invention, the 
combination of the predetermined gas pressure and the predetermined static 
pressure exceeds the sum of the pressure required to overcome the 
predetermined level difference and the predetermined flow resistance by no 
greater than bout 100 kPa. 
In the preferred embodiment, the lignocellulose material exits the digester 
at a consistency of between about 3 and 10%, and preferably between about 
5 and 10%.

DETAILED DESCRIPTION 
Referring to the Figures, in which like numerals refer to like portions 
thereof, FIG. 1 shows a digester 1 and a receiving vessel 4 connected by a 
valve 2 in conduit 3. After termination of a sulfate cook in digester 1, 
the cooked lignocellulose material is cooled to a temperature of about 
100.degree. C.. Preferably, cooling is carried out to a temperature above 
100.degree. C., and preferentially between about 100.degree. and 
120.degree. C. The cooling process is normally performed by displacing the 
hot spent cooking liquor in the digester by a cooler liquor. Maintenance 
of a temperature just above about 100.degree. C. after the cooling process 
guarantees the presence of a limited overpressure in the digester. Of 
course, instead of a sulfate cook, other types of cooks can be performed 
as well. 
After the cooling step, valve 2 in conduit 3 is opened in order to create a 
connection between the bottom of the digester 1 and a receiving vessel 4, 
where preferably atmospheric pressure prevails. This receiving vessel 4 is 
normally arranged so that the conduit 3 connects the bottom of the 
digester 1 with a point at a higher level in the receiving vessel 4. 
Discharge from the digester is performed by the utilization of a push-out 
pressure which is maintained by the supply of a gas, such as air or steam, 
through a gas inlet 5 in the upper portion of the digester 1. Thus, the 
push-out pressure is actually the combination of that gas pressure and a 
static pressure, the static pressure being determined by the level of 
cooked lignocellulose within the digester. According to this invention, 
this push-out pressure shall not be substantially higher than the minimum 
pressure which is required in order to overcome the flow resistance and 
the level difference in transfer of the lignocellulose material from the 
digester 1 to the receiving vessel 4. This level difference represents the 
lifting height between the outlet of the digester 1 and the inlet of the 
receiving vessel 4. This push-out pressure shall preferably be, at 
maximum, 200 kPa higher than the minimum required pressure, and preferably 
at maximum 100 kPa higher. This is considerably lower than the pressure in 
conventional cold blow systems, where the supplied gas pressure can 
generally be between about 500 and 700 kPa. 
In order to render possible a uniform and controlled discharge, a supply of 
dilution liquid is also required in the lower portion of digester 1. This 
is preferably carried out by means of dilution nozzles 6, which can be 
located in the bottom of the digester, or a small distance higher up 
therein. The reason for this dilution is to prevent channelling, to reduce 
the material consistency, and to thereby reduce frictional resistance, and 
to prevent gas breakthrough. Furthermore, cooling of the lignocellulose 
material is obtained during discharge so that the temperature in the 
material being transferred to the receiving vessel will be below about 
100.degree. C. In this manner, a discharge method is obtained which 
substantially avoids flashing, and reduces pressure variations in the 
receiving vessel, thus simplifying the treatment of both evil-smelling 
gases and condensate. As will be immediately apparent to those of ordinary 
skill in this art, the transfer temperature can be somewhat higher, 
provided that the pressure is such that flashing can still be avoided. 
FIGS. 2 and 3 show the pressure variations in the receiving vessel 4 
following the digester 1 during discharge, as measured in a full-scale 
test. FIG. 2 shows the pressure variations when the method of the present 
invention was used, and FIG. 3 shows the pressure variations when the 
so-called "cold blow" discharge method was used, as discussed in the 
background portion of this specification. These Figures show that the 
present invention produces significantly lower pressure peaks in the 
receiving vessel, in particular, 5 to 15 kPa as compared to 40 to 50 kPa 
with conventional cold blow. Moreover, repeated blows are not necessary 
according to the present method. In this manner, the total discharge time 
was shorter, e.g., from 15 to 17 minutes, as compared to 20 to 30 minutes 
for the cold blow process, which makes it possible to increase production. 
The consistency of the lignocellulose material during transfer is 
preferably between about 3 and 10%, and preferably between about 5 and 
10%. 
In order to obtain uniform discharge, the push-out pressure should be 
maintained substantially constant during the discharge procedure, and 
therefore the supplied gas pressure should be increased as the static 
pressure is reduced. 
Furthermore, the method according to the present invention also provides an 
indulgent treatment of cellulose fibers which results in increased 
strength properties for the product thereof. 
Because of its simplicity, the present method can be utilized in existing 
batch digesters without substantial reconstruction of the digesters and 
their transfer conduits. No new equipment is thus required which would 
complicate the layout and availability of same in a digester. Furthermore, 
the service costs will be low, and the production accessibility will be 
high. Moreover, it should be noted that existing digester outlets and 
transfer conduits can be maintained. it will thus still be possible to 
perform conventional warm or cold blow without the need to switch 
equipment or conduits therein. 
Although the invention herein has been described with reference to 
particular embodiments, it is to be understood that these embodiments are 
merely illustrative of the principles and applications of the present 
invention. It is therefore to be understood that numerous modifications 
may be made to the illustrative embodiments and that other arrangements 
may be devised without departing from the spirit and scope of the present 
invention as defined by the appended claims.