Prehydrolyzed Pulp Alkali Charge to Prevent Pitch Deposits
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Solution Overview
Problem
Traditional prehydrolysis-kraft pulping processes face challenges such as pitch deposits and quality variations due to incomplete hydrolysis and delignification, leading to decreased alpha cellulose content and increased production costs, especially in continuous systems where chemical conditions remain constant, causing strainers to clog and affecting pulp quality.
Innovation Solution
A process involving prehydrolysis at 120-180°C for at least 20 minutes, followed by a strong alkali charge to create an alkaline treatment liquor with a residual alkali concentration above 20 g/l, maintaining the material in this liquor to reduce alkali concentration, and then transferring it to a kraft cooking stage, where the temperature is lowered by at least 10% to enhance alkali diffusion and prevent lignin condensation, thereby improving delignification and alpha cellulose yield.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If prehydrolysis-kraft cooking is used to remove hemicellulose and dissolve lignin, then alpha cellulose content is improved, but pitch deposits occur on strainers causing clogging
Solution Approach 1:
The patent applies preliminary action by conducting a prehydrolysis step before the main kraft cooking process. This prehydrolysis removes hemicellulose and partially dissolves lignin, preventing pitch formation during subsequent cooking. The strainers are protected from pitch deposits by this preliminary treatment, while still achieving high alpha cellulose content in the final pulp.
2Productivity
If continuous cooking system is used for prehydrolysis-kraft pulping, then productivity is improved, but chemical conditions remain constant causing pitch deposits
Solution Approach 1:
The patent applies dynamics by introducing varying chemical conditions into the continuous cooking system. Different alkali charges (weak, medium, strong) are applied at different stages, and the cooking conditions are dynamically adjusted. This prevents constant chemical conditions that cause pitch deposits, while maintaining continuous operation for high productivity.
3Manufacturing precision
If strong alkali charge is added to neutralize acidic hydrolysate, then delignification is improved, but temperature increases causing lignin condensation
Solution Approach 1:
The patent applies parameter changes by carefully controlling the temperature parameter during alkali charge addition. Instead of allowing temperature to rise significantly, the process maintains temperature below 170°C even when strong alkali is added. This parameter control enables effective delignification through alkali treatment while preventing lignin condensation that would occur at higher temperatures.
4Manufacturing precision
If prehydrolysis temperature is increased to enhance hemicellulose removal, then alpha cellulose content is improved, but energy consumption increases
Solution Approach 1:
The patent applies parameter changes by optimizing the prehydrolysis temperature parameter to the range of 100-180°C. This optimized temperature range is sufficient to remove hemicellulose and achieve high alpha cellulose content without requiring excessive energy input. The process finds the optimal balance between temperature and energy consumption.
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 process achieves a more homogeneous cook with reduced rejects and increased alpha cellulose content, preventing lignin condensation and pitch deposits, resulting in higher yield and polymerization degree of cellulose, suitable for dissolving pulp production.
Implementation Method 1
hemicellulose is hydrolyzed into hydrolysate
Implementation Method 2
adding a strong alkali chock charge to the mixture of prehydrolyzed cellulosic material and acidic hydrolysate to such extent that the residual alkali concentration after neutralization
Implementation Method 3
lignin is dissolved by a kraft cooking method for liberating cellulose fibers
Implementation Method 4
the temperature of the resulting alkaline treatment liquor for the prehydrolysed material by at least 10%... to enhance alkali diffusion
Data Source
Figure 1~3b
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AI summary
The invention is related to an improved compact process for producing dissolving pulp in a prehydrolysis kraft cooking process. In order to avoid pitch problems with blocked withdrawal screens and to obtain a distinct ending of the prehydrolysis stage, as well as a thorough alkaline impregnation ahead of the kraft cook stage, alkali is charged to the mixture of prehydrolyzed material to such an extent that the residual alkali concentration after neutralization of the acidic hydrolysate is above 20 g/l EA as NaOH and the temperature of the resulting alkaline treatment liquor for the prehydrolyzed material is lowered by at least 10% in comparison to the temperature in the prehydrolysis stage. The alkali charge will avoid redeposition of hemicelluloses dissolved in the prehydrolyse stage and will abruptly swing the wood material mixture to alkaline conditions favourable for a alkali impregnation stage at reduced temperature ahead of the final kraft cooking stage, which impregnation stage will extract the major part of the hemicelluloses content of the cellulose material.