In conventional Kraft digesters, black liquor is used only in a limited amount, for example, in amounts substantially less than 50% of the total liquid content in the impregnation zone of the digester. The remainder of the externally added liquid usually consists of white liquor. It has now been found that this large addition of white liquor at such an early stage in the cooking process may have an adverse effect on the tear resistance of the fully cooked fibers. Thus, there is a need for a method of digesting pulp which utilizes substantially less amounts of white liquor.
U.S. Pat. No. 3,303,088 (Gessner) discloses a method for continuously cooking cellulose-containing fiber material in a single-vessel system in which:
(1) chips are fed in at a first end of the digester, PA1 (2) white liquor is added at a position at the first end, PA1 (3) the chips are impregnated in a concurrent impregnation zone, PA1 (4) the chips are cooked in a cooking zone downstream of the impregnation zone, PA1 (5) hot black liquor is extracted from at least one screen section, PA1 (6) black liquor is added to the impregnation zone, and PA1 (7) cooked pulp is discharged at the other end of the digester. PA1 feeding chips into an inlet of a digester; PA1 supplying black liquor to an impregnation zone of the digester in an amount such that the black liquor makes up greater than 40% by volume of the total volume of liquid present in said impregnation zone; PA1 extracting liquor from a screen girdle downstream of where said black liquor is added to the digester, in the direction the wood chips flow through the digester, in an amount greater than 50% by volume of the total volume of liquor present at the location of the screen girdle; PA1 maintaining a liquor:wood ratio in the impregnation zone of greater than 3:1; and PA1 discharging cooked pulp from the digester. PA1 an interior chamber defined by a walled structure; PA1 a chip inlet to the interior chamber for supplying chips to the interior chamber; PA1 an impregnation zone in the interior chamber which is connected to the chip inlet, for impregnating the chips; PA1 at least one screen girdle connected to the impregnation zone for extracting liquor from the impregnation zone, the screen girdle being constructed and arranged to extract an amount of liquor exceeding 50% by volume of the total volume of liquor present at the location of the screen girdle; PA1 a cooking zone in the interior chamber for cooking the impregnated chips; PA1 a black liquor recirculation loop constructed and arranged for recirculating the extracted black liquor to the impregnation zone such that black liquor present in the impregnation zone exceeds 40% by volume of the total volume of liquid present in the impregnation zone, the black liquor recirculation loop comprising an extraction screen in the interior chamber, which is connected to the cooking zone, for extracting black liquor from the cooking zone, and means for supplying the extracted black liquor to a location in the impregnation zone such that there is a dwell time of at least 20 minutes for the chips to move from the location the extracted black liquor is supplied to the impregnation zone to the extraction screen; and PA1 at least one cooking liquor recirculation loop comprising a digester screen downstream of the impregnation zone and upstream of the cooking zone for extracting cooking liquor from the digester, a heater connected to the disgester screen for heating cooking liquor extracted by the digester screen, and means for supplying the heated cooking liquor to the digester at a location downstream of said impregnation zone and within 5 meters upstream of the digester screen.
This patent also discloses that the extracted liquor from the first screen section, which is arranged downstream of the position of the addition of the black liquor, is returned to the digester by first being conveyed to a container in which white liquor and the extracted impregnation and cooking liquids are mixed. Due to this recirculation, a high content of volatile sulphur and terpene compounds in the impregnation and cooking liquid can build up. Furthermore, the method disclosed in Gessner does not permit sufficiently rapid heating of the cooking liquid to achieve optimal process conditions. It is also evident that the method of Gessner does not include process parameters which are necessary to achieve optimal conditions, such as, the correct liquor-to-wood ratio for obtaining the desire movement of the chip column in the digester.