Method and system for impregnating chips
The withdrawal screen section is for impregnation of chips during the manufacture of chemical pulp. Chips are steamed and impregnated in a low pressure impregnation vessel using pressurized hot spent cooking liquor BL. The spent cooking liquor produces most of the steam BL ST necessary for steaming the chips. To reduce the need for adding fresh steam for chip steaming is a first withdrawal section in the impregnation vessel modified to be used both as a flash tank for warm pressurized wash filtrate and improved wash-out feature for the spent impregnation liquid behind the screen in the withdrawal section. The withdrawal volume is arranged with an upper part located above the liquid level LIQ LEV of the impregnation vessel. Additional pressurized warm wash liquor REC 1 is added to the withdrawal volume and flashed off steam is led in a steam duct to the chip volume above the liquid level.
1. A method for the impregnation of chips during the manufacture of chemical pulp, comprising;
continuously feeding chips without preceding steam treatment to a top of an impregnation vessel,
feeding out impregnated chips from a bottom of the impregnation vessel,
adding hot impregnation fluid (BL), at a first temperature above a boiling point of the hot impregnation fluid, to the impregnation vessel, via a pipe having an outlet end located below a chip level (CHLEV) established in the impregnation vessel and at a distance from walls of the impregnation vessel, the hot impregnation fluid (BL) releasing steam into a chip volume for steaming the chips,
the added impregnation fluid (BL) establishing a fluid level (LIQLEV) in the impregnation vessel,
the chip level (CHLEV) being above the fluid level and the top of the impregnation vessel having a pressure that is essentially at atmospheric pressure,
withdrawing spent impregnation fluid (REC), at the fluid level (LIQLEV), from a withdrawal volume located behind screens mounted in a wall of the impregnation vessel,
adding an additional fluid (REC 1 ) at a second temperature above the boiling point of the additional fluid into the withdrawal volume, the additional fluid releasing steam into the withdrawal volume, and
leading the steam towards the chips below the chip level (CHLEV) for steaming the chips.
2. The method according to claim 1 , wherein the method further comprises establishing a temperature of a fluid-wood mixture that is established at the fluid level (LIQLEV) within the interval 90-115° C.
3. The method according to claim 2 , wherein the method further comprises setting a temperature of the additional fluid (REC 1 ) fed into the withdrawal volume behind the screens in the impregnation vessel to exceed a boiling point of the additional fluid in the withdrawal volume by at least 5° C.
4. The method according to claim 3 , wherein additional fresh steam (ST) is added into the withdrawal volume.
5. The method according to claim 4 , wherein the impregnation fluid (BL) added to the impregnation vessel is obtained from a subsequent continuous digester from a withdrawal position in the digester located in a first half of a digester zone operated at full digester temperature.
6. The method according to claim 3 , wherein the additional fluid (REC 1 ) added is obtained from a withdrawal position located after a digester zone of a subsequent continuous digester.
7. The method of claim 1 wherein the method further comprises providing the hot impregnation fluid (BL) with an alkali level that exceeds 15 g/l.
8. The method of claim 1 wherein the method further comprises providing the additional fluid (REC 1 ) with an alkali level that exceeds 2 g/l.