Continuous Digesters – Hydraulic
In a continuous digester, wood chips and liquor are continuously fed to, and withdrawn from, a pressure vessel. Continuous cooking offers some distinct advantages over batch cooking, including reduced space requirements, improved cooking control, and improved energy efficiency. Many variations of continuous digester equipment and process conditions have been used. Some digesters are hydraulically filled, while others utilize a vapor phase. This module covers the basic principles of operation of hydraulically filled continuous digesters.
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Learning Objectives
• Describe some of the advantages of continuous cooking
• Identify the purposes of a continuous digester chip feed system
• Describe how process zones are established in a continuous digester
• Describe the advantages of using an impregnation vessel
• Identify some of the guidelines which maximize the benefits of continuous cooking
Specs
| Course Level | Intermediate |
| Languages | English, French |
| Compatibility | Audio, Video |
| Based on: | Industry Standards and Best Practices |
Key Questions
What are some of the goals of the chip feed system on a continuous digester?
In the chip feed system, air is driven from the chips and replaced with water, the chips are partially heated, and they are transferred from a low pressure environment to a high pressure environment.
Why do chips need to be impregnated with liquor before being cooked in a continuous digester?
Impregnating chips prior to cooking them helps achieve uniform cooking within the chip and it increases chip density so that they will not float in the liquor of the digester.
What is one way chips are heated in a hydraulic digester?
The chips are heated by the liquor which is used to transport chips into the digester.
What is an impregnation vessel?
An impregnation vessel is a pressure vessel which precedes a continuous digester. It prepares the chips for cooking by heating the chips and impregnating them with cooking chemicals.
How is chip level usually determined in a hydraulic digester?
Chip level is determined by a series of vertically spaced strain gauge type sensors which extend through the shell of the digester and detect movement of the chips in the areas of the digester where there are chips.
Sample Video Transcript
By controlling the introduction and withdrawal of liquor at different elevations in the digester, process zones of varying conditions can be established along the length of the digester. The temperature and chemical concentrations of incoming liquor establishes the conditions of the zone between the liquor inlet and the screens where the liquor is withdrawn. In some zones, the liquor is withdrawn at a higher elevation than where it is injected, resulting in the liquor flowing upward or counter currently to the down flowing chips. Counter current liquor flow results in good mixing but can lead to chip column movement problems.
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