Radial Displacement Wash for Continuous Digester Pulp Quality
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Solution Overview
Problem
In overloaded continuous digesters, the conventional wash at the bottom of the digester is often reduced or eliminated, leading to inefficient displacement of free fluid between softened chips, which hampers the delignification process and pulp quality due to low dilution factors and inefficient fluid distribution.
Innovation Solution
Implementing a radially directed displacement wash using nozzles on the shaft of the bottom scraper and strainer surfaces in the outlet tap and outlet line, creating a pressure drop across the blow-valve to exchange cooking fluid with cleaner liquid, enhancing washing efficiency before defibration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If production is increased above original design capacity in continuous digesters, then productivity increases, but the conventional wash at the bottom of the digester is eliminated and dilution factor becomes zero or negative
Solution Approach 1:
The invention applies preliminary action by establishing a displacement wash zone before the final strainer and outlet tap, where washing liquid is added and displaced cooking fluid is withdrawn. This preliminary washing action occurs before the chips are discharged, ensuring that even at high production rates where conventional bottom wash is eliminated, the chips still receive adequate washing to maintain dilution factor and remove cooking fluid.
2Quantity of substance
If washing or dilution liquid is added through nozzles in the wall at the bottom surface of the digester, then washing is provided, but flows or film of liquid form along the inner wall and washing efficiency is reduced
Solution Approach 1:
The invention transitions from wall-based nozzle washing (two-dimensional surface contact) to a centralized displacement wash zone (three-dimensional fluid exchange). By adding washing liquid through a central pipe and withdrawing displaced cooking fluid through the surrounding wall, the system creates a volumetric washing action that efficiently exchanges cooking fluid without forming problematic liquid films along the wall surface.
3Quantity of substance
If large flows of washing fluid are added to achieve efficient wash within short space, then washing efficiency improves, but lifting force acts against motion of chip column and production rate decreases
Solution Approach 1:
The invention utilizes hydraulic principles by establishing a controlled displacement wash zone where washing liquid is added through a central pipe and displaced cooking fluid is withdrawn through the surrounding wall. This hydraulic displacement system efficiently exchanges cooking fluid with washing liquid without requiring large flows that would create lifting forces, thereby maintaining both washing efficiency and production rate.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method improves the washing efficiency by exchanging liquid and releasing bound organic material, increasing the dilution factor, and maintaining pulp quality without requiring additional equipment, thus enhancing pulp strength and yield.
Implementation Method 1
across which blow-valve a fall in pressure of at least 0.5 bar and at most 3 bar has been established
Data Source
Figure 1A
Figure 1B~1E
Figure 2~3
AI summary
The invention concerns a method and an arrangement for the improvement of a wash after completed digestion in a continuous digester (1) for the production of cellulose pulp. According to the invention, the softened chips are exposed to a radially directed displacement wash after they have passed the rotating bottom scraper (36), and which displacement wash has been established in the flow of digested softened chips through the outlet tap (10) before the softened chips are defibrated by the fall in pressure across the blow-valve that is arranged after the digester.