Digester Flash Steam Recovery via Black Liquor Heat Exchange
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Solution Overview
Problem
Existing methods for producing chemical pulp in a continuous digester plant face issues with volatile compounds in flash steam, which are environmentally detrimental and pose an explosion risk, and inefficient use of heat from black liquor, leading to suboptimal energy economy in chemical pulp mills.
Innovation Solution
A method where black liquor is heated in an indirect heat exchanger with live steam to generate flash steam, which is then used to heat fibrous material in the digester, reducing the need for direct live steam and allowing for internal steam generation and recovery of condensate, while also utilizing subsequent heat recovery stages to maximize heat utilization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If flash steam is used for heating fibrous material in the digester, then steam consumption from live steam is reduced, but volatile compounds are introduced into the steaming process causing environmental harm and explosion risk
Solution Approach 1:
The harmful volatile compounds are extracted and separated from the steam generation process by using a dedicated heat exchanger that heats black liquor with live steam indirectly, preventing volatile compounds from entering the steam phase while still generating useful flash steam for heating
Solution Approach 2:
A heat exchanger is introduced as an intermediary device between the live steam and black liquor, allowing heat transfer without direct contact between steam and volatile compounds, thus producing clean flash steam that can be safely used for heating fibrous material
2Power
If black liquor is directly flashed to generate steam, then steam is produced for heating, but heat from black liquor is not efficiently utilized and volatile compounds contaminate the steam
Solution Approach 1:
Black liquor is preheated in a heat exchanger using live steam before being introduced to the flash evaporation process, maximizing the utilization of heat energy from black liquor and increasing the efficiency of steam generation while preventing volatile compound contamination
Solution Approach 2:
The temperature and pressure parameters of black liquor are optimized through controlled heating in the heat exchanger before flash evaporation, ensuring efficient heat utilization and maximizing steam generation while maintaining safety by separating volatile compounds from the steam phase
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 decreases the total steam consumption in the digester plant, recovers clean condensate for reuse, and enhances energy economy by utilizing heat from black liquor efficiently, minimizing the introduction of volatile compounds and reducing the risk of explosions.
Implementation Method 1
the black liquor is led into an indirect heat exchange relationship with steam in a heat exchanger for heating the black liquor
Implementation Method 2
the heated black liquor is led into a flashing vessel, wherein flash steam and flashed black liquor are generated
Data Source
Figure 1
Figure 2
AI summary
The present invention relates to a method of producing chemical pulp at a con¬ tinuous digester plant. The method comprises the following steps: a) leading a slurry formed of comminuted cellulosic fibrous material and liquid into a digester and treating the material with a treatment chemical at a temperature over 130 °C for producing treated material and black liquor; b) extracting at least one black liquor stream from the digester and leading the black liquor into an indirect heat exchange relationship with steam in a heat ex¬ changer for heating the black liquor; c) leading the heated black liquor into a flashing vessel, wherein flash steam and flashed black liquor are generated, and d) using flash steam from step c) for heating the fibrous material in the digester in step a).