Thin Chip Digester Cooking via High Kappa Control
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Solution Overview
Problem
Conventional continuous digester vessels face issues with processing thin and small chips, leading to plugging, mass transfer problems, and reduced pulp production rates due to chip agglomeration and inadequate liquor flow, necessitating frequent halts and increased maintenance costs.
Innovation Solution
A method and system for cooking thin chips in a continuous digester vessel where at least 85% of chips have a thickness of 2-6 mm, with most white liquor added in the chip bin or transport passage, maintaining high temperatures and pressures to achieve Kappa numbers of 50 or higher, allowing for uniform downward flow and reduced chip agglomeration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of substance
If thin chips (thickness < 7mm) are cooked to conventional Kappa numbers (20-33 for softwood, 15-20 for hardwood), then delignification is achieved, but chips become soft and easily compressed at the bottom of the digester vessel
Solution Approach 1:
The patent targets higher Kappa numbers (50 or higher) compared to conventional cooking (20-33 for softwood, 15-20 for hardwood). This parameter change in the degree of cooking preserves chip strength while still achieving delignification, allowing thin chips to maintain structural integrity and resist compression at the digester bottom
Solution Approach 2:
The patent uses partial action by stopping the cooking process at a higher Kappa number rather than completing full delignification. This partial cooking approach is sufficient to achieve the desired mass transfer enhancement without over-cooking the chips to the point of excessive softening
2Loss of substance
If thin chips are cooked to conventional Kappa numbers, then delignification is achieved, but chips become densely packed and plug the bottom of the digester vessel
Solution Approach 1:
By targeting higher Kappa numbers (50 or higher), the patent changes the physical state of the cooked chips, maintaining them in a less compressed, more open structure that prevents plugging and maintains continuous chip flow through the digester vessel
Solution Approach 2:
The patent applies preliminary anti-action by preventing chip compression and plugging through controlled cooking to higher Kappa numbers, thereby avoiding the need for subsequent interventions to clear plugged screens and maintain continuous operation
3Loss of substance
If thin chips are cooked to conventional Kappa numbers, then delignification is achieved, but mass transfer problems arise and uniform chip flow is impeded
Solution Approach 1:
The patent targets higher Kappa numbers (50 or higher) to maintain chips in a state that promotes uniform downward flow. This parameter change prevents the formation of agglomerations and ensures consistent mass transfer and reliable chip flow to the discharge outlet
Solution Approach 2:
The patent uses Kappa number as a feedback parameter to control the cooking process. By monitoring and targeting specific Kappa number ranges (50 or higher), the process ensures uniform chip flow and prevents mass transfer problems that would otherwise disrupt reliable operation
4Manufacturing precision
If chip screens are used to select chips of acceptable thickness, then chip classification is achieved, but processing of thin and small chips is still limited
Solution Approach 1:
The patent changes the target Kappa number parameter to higher values (50 or higher), which enables the digester to successfully process thin chips (2-6mm thickness) that would otherwise cause plugging and flow problems. This parameter change expands the range of acceptable chip sizes and increases adaptability to different chip types
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 approach enhances mass transfer, reduces chip agglomeration, increases pulp yield by 3-15%, simplifies equipment, and allows direct use of pulp for brown packaging and bleachable paper products without extensive refining or bleaching.
Implementation Method 1
cooking thin chips in a continuous digester vessel... maintaining high temperatures and pressures
Implementation Method 2
maintaining high temperatures and pressures to achieve Kappa numbers of 50 or higher
Implementation Method 3
enhances mass transfer... to remove used or spent cooking liquor (black liquor) and lignin
Data Source
Figure 1
AI summary
A method to cook thin chips in a continuous digester vessel including: introducing thin chips having a thickness of no more than 6mm, into a chip bin; adding white (cooking) liquor to the chip bin or to a chip transport passage extending from the chip bin to an upper inlet of the continuous digester vessel; injecting medium pressure steam or another heated fluid to an upper region of the digester vessel to elevate a cooking temperature of the chips in the vessel to at least 130 degrees Celsius; cooking the chips in the vessel as the chips flow downward through the vessel without substantial extraction or introduction of liquor in the cooking section of the vessel; injecting wash liquid to a lower region of the vessel; extracting at least wash liquid through a wash liquid extraction screen in the lower region of the vessel and above the injection of the wash liquid, and discharging the cooked thin chips from the lower region of the vessel.