This invention relates to a method for discharging of lignocellulosic material from a digester after batch cooking.

In sulfate, sulfite and other processes of cooking a delignification of lignocellulosic material, such as wood chips, is performed. The material is thereby cooked in a cooking liquor under determined conditions concerning supply of chemicals, time, temperature, pressure etc. After termination of a cook high perssure and temperature prevail in the digester.

The discharge can be performed by opening of a valve in the bottom of the digester so that the content of the digester is blown out by means of the pressure in the digester through a conduit into a receiving vessel where atmospheric pressure prevails. Alternatively, the content of the digester can be cooled by displacing the hot spent cooking liquor with cooler vaste liquor. After that the content of the digester is blown out by means of high pressure air or steam into the receiving vessel, so called cold blow.

High pressure is required for the blowing in order to garantee a complete discharge of material from the digester. However, at the same time the discharge will be difficult to control since the material consistency will variate very much. Further, steam or air can penetrate the material through channels before the digester is empty.

Additional problems concerning control of a possible blow condensor can arise. Thereby problems with pressure shocks to gas treatment and transfer to condensate can arise. In order to take care of these problems the gas treatment equipment has to be extensive and thereby very costly.

One way to obtain a more controlled discharge method is to relieve the overpressure in the digester and reduce the temperature to below 100°C. Then the material is pumped out of the digester by means of a pump. Thereby the need for gas treatment equipment is also reduced or avoided. This method is known from batch digesters for sulfite, sulfate and other types of cooking.

However, pump discharge still has a number of dis- atvantages. A pump is very sensitive to course particles. The raw material will normally be accompanied with different metal objects, stone and concrete clods as well as large wood pieces. Even if very large pump weels are used there are considerable risks for stucking and clashing and thereby disturbances in production. Further, pump discharge will result in increased costs for service and decreased relia¬ bility in operation compared with conventional blow discharge as mentioned above. In reconstruction of old digesters there might be problems to get room for pumps and belonging conduits. Therby the installation costs might be unreasonably high.

Another way of discharging a digester is to install a rotating discharge device. Thereby the total digester pressure is used as discharge force. Uniform discharge is obtained by means of dilution of the material in a dilution zone and simultaneous stirring by means of the discharge device. Such an arrangement is possible for a continuous digester having a bulging bottom and continuous discharge. In a batch digester, which is normally provided with a conical bottom, such a discharge device is not suitable. Moreover, such devices in a ' batch digestery would require considerable costs for instal¬ lation and service with respect to the large number of digesters in such a plant.

The present invention means that the above mentioned problems in connection with discharge from a batch digester can be solved. According to the invention there is obtained a method for controlled discharge of a digester at low pressure

without use of mechanical discharge means. Specifically no pump is needed and the disadvantages assosiated therewith, while at the same time avoiding or minimizing the need for gas treatment equipment. The characterizing features of the invention are defined in the claims.

The invention will be further disclosed below with reference to the drawings where

Fig 1 schematically shows as an example an embodiment of a system for carrying out the method of the invention and Fig 2 and 3 show pressure variations in a receiving vessel after a digester according to the invention (Fig 2) and according to so called cold blow (Fig 3) .

After the termination of a sulfate cook in a digester 1 the cooked material is cooled to about 100 C. Preferably the cooling is performed to a temperature above 100°C, preferen¬ tially between 100 and 120°C. Normally the cooling is performed by displacing the hot spent cooking liquor in the digester by a cooler liquor. By maintaining a temperature just above 100°C after the cooling a limited overpressure in the digester is garanteed. Of course, instead of a sulfate cook other types of cooks can be performed as well.

After the cooling a valve 2 in a conduit 3 is opened in order to perform a connection between the bottom of the digester 1 and a receiving vessel 4 where suitably atmospheric pressure is prevailing. 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 vessel 4.

The discharge is performed by means of a push-out pres¬ sure which is maintained by supply of a gas, such as air or steam, through a gas inlet 5 at the upper portion of the digester 1. Thus the push-out pressure is combined of a gas pressure and a static pressure where the static pressure is determined by the level in the digester. The push-out pressure shall not be substantially higher than the minimum pressure which is required to exceed the flow resistance and the level difference in the transfer between the digester 1 and the

receiving vessel 4. Thus, the level difference is the lifting hight between the outlet of the digester and the inlet of the vessel. Suitably the push-out pressure shall at maximum be 200 kPa higher than said minimum pressure and preferably at maximum 100 kPa higher. That is considerably lower than in conventional cold blow systems where the supplied gas pressure can be 500-700 kPa.

In order to make a uniform and controlled discharge possible a supply of dilution liquid is also required in the lower portion of the digester 1. Suitably this is performed by means of dilution nozzles 6 which can be located in the bottom of the digester or a bit higher up. The reason for the dilu¬ tion is to prevent channelling, reduce the material consis¬ tency and thereby reduce the frictional resistance and prevent gas break through. Moreover, cooling of the material is obtained during the discharge so that the temperature in the material being transferred to the receiving vessel will be below 100°C. Thereby a discharge method is obtained which substantially avoids flashing and reduces the pressure variations in the receiving vessel, thereby simplifying the treatment of evil smelling gases and condensate. Of course the transfer temperature could be a little higher provided that the pressure is such that flashing still can be avoided.

Fig 2 and 3 show the pressure variations in the receiving vessel 4 after the digester 1 during discharge, 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 was used as mentioned in the background portion of the specification. The figures show that the present invention gives lower pressure peaks in the receiving vessel, 5-15 kPa compared to 40-50 kPa with the conventional cold blow. Moreover, repeated blows are not necessary according to the present method. Thereby the total discharge time was shorter, 15-17 minutes compared to 20-30 minutes for the cold blow, which gives a possibility to increase the production.

The material consistency during the transfer should be within the interval 3-10%, preferably 5-10%.

In order to obtain a uniform discharge the push-out pressure should be maintained substantially constant during the discharge procedure, i e the supplied gas pressure should be increased as the static pressure is reduced.

Further, the method according to the invention also means an indulgent treatment of cellulose fibres which results in increased strength properties.

Due to its implicity the present method can be utilized in existing batch digesters without substantial reconstruc¬ tions of the digesters and transfer conduits. No new equipment is requested which would complicate the layout and the availability in a digestery. 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. Thereby it will still be possible to perform conventional warm or cold blow without any switching of equipment or conduits.

The invention, of course, is not restricted to the embodiment shown, but can be varied within the scope of the invention idea.