Digester wash extraction by individual screen flow control

In a continuous digester apparatus for washing wood chips for the separation of wood fibers from their bonded state, the digester having a vertical centerline, a substantially circular cross-section, and a wash zone adjacent the bottom end thereof and a plurality of individual wash screens disposed about the inner wall of the digester for the withdrawal of co-current downflow liquor from the wash zone, the improvement comprising

• a conduit connected in fluid communication between each of the wash screens and a collector for co-current downflow liquor withdrawn from the wash zone of the digester,
• a valve interposed along the length of said conduit leading from each of the wash screens, said valve being operable between open and closed positions in response to a signal received from said flow rate meter associated with said conduit leading from each of the wash screens,
• a flow rate monitor associated with said conduit leading from each of the wash screens, said monitor generating a signal representative of the rate of flow of co-current downflow liquor through said conduit and transmitting said signal to said valve, whereby said valve is regulated between its open and closed positions by said signal, whereby the flow rate of liquor through each conduit is altered to substantially equal the flow rate of co-current downflow liquor through each other of the conduits associated with others of the wash screens, the total volume of liquor flowing through all of the conduits collectively being substantially equal to the total volume of wash liquor being introduced to the digester in the wash zone of the digester.

In a digester for a co-current downflow wood chips in a hot liquor and having an upper zone and a lower wash zone, the co-current downflow wood chips and hot liquor defining a chip mass which flows from the upper zone through the wash zone of the digester, the improvement comprising

• establishing and maintaining outflow of co-current downflow hot liquor from the digester through a plurality of screens disposed about the inner circumference of the digester at a location intermediate the upper zone and the wash zone of the digester,
• monitoring the flow rate and temperature of said outflow of co-current downflow hot liquor through each of said plurality of screens,
• comparing the flow rate and temperature of the outflow of co-current downflow hot liquor through each of said screens with the flow rate and temperature of the outflow of hot liquor through each other of said screens,
• employing said comparisons of flow rates and temperatures, as needed, adjusting the outflow of co-current downflow hot liquor through one or more of said screens to bring the rate of outflow of hot liquor from each of said screens to substantially the same rate of outflow of hot liquor as others of said screens or to reduce the rate of outflow of hot liquor through one or more of the screens to a value which permits downflow of hot liquor past said one or more of the screens to sweep said one or more of the screens substantially free of material having a tendency to plug the screens or to fully block the outflow or hot liquor through said one or more screens,
• substantially simultaneously, admitting wash liquor into said digester at the wash zone thereof at a rate substantially equal to the total outflow of hot liquor from the digester through said screens.

A method for enhancing the wash efficiency of a digester for wood chips disposed in a co-current downflow hot liquor, the digester having at least an upper zone and a lower wash zone and a plurality of screens disposed about the circumference of the digester at a location intermediate the upper zone and lower wash zone, comprising the steps of

• introducing a wood chips and hot alkali liquor into the upper zone of the digester to define a chip mass which flows from the upper zone to the wash zone of the digester,
• withdrawing co-current downflow hot liquor from said chip mass trough individual ones of the plurality of screens conveying said withdrawn hot liquor away from the digester,
• monitoring the temperature and flow rate of hot liquor withdrawn through each of the screens, employing said monitor temperature and flow rate of hot liquor withdrawn through each of the screens, adjusting the flow rate of hot liquor withdrawn through each of the screens to a flow rate value which is substantially equal to the flow rate of hot liquor withdrawn through each of the others of the plurality of screens, or to a flow rate value which reduces or substantially fully stops the withdrawal of hot liquor through one or more of the screens,
• substantially simultaneously introducing relatively cool wash liquor into the wash zone of the digester in counterflow to the downward flow of the chip mass through the digester, the inflow quantity of wash liquor being substantially equal to the overall quantity of hot liquor withdrawn from the digester through the plurality of screens.