Digester Types
The purpose of kraft pulping is to separate fibers from each other so that they can be used to produce paper and other cellulose fiber based products. This type of chemical pulping is accomplished by cooking wood chips in a chemical solution in large pressure vessels called digesters. Pulping can be performed in digesters running in batch or continuous mode. This module describes the advantages and disadvantages of different types of digesters, including: batch systems, continuous cooking systems with one and two vessels, continuous digesters which are hydraulically filled and those which utilize a vapor phase. Pandia and M&D digesters are also described.
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Learning Objectives
• Describe the purpose of kraft pulping
• List typical configurations of continuous digester systems
• Describe the major steps involved in batch cooking
• Describe the advantages of continuous digesters
• Identify the advantages of vapor phase continuous digester systems
• Describe the advantages of two-vessel continuous digester systems
• Identify the unique feature that the M&D and Pandia digesters share
• Identify the primary use of M&D digesters
Specs
| Course Level | Intermediate |
| Languages | English, Polish |
| Compatibility | Audio, Video |
| Based on: | Industry Standards and Best Practices |
Key Questions
What is the advantage of recirculation in a batch digester?
Recirculating liquor in a batch digester provides an opportunity to control the temperature in the digester by passing the liquor through a heat exchanger external to the digester.
What is a vapor phase digester?
A vapor phase digester is a continuous digester which is not completely filled with liquor during operation. A small area near the top of the digester is filled with steam which condenses on the chips and heats them prior to them entering the liquor.
What is cold blow?
Cold blow is liquor which is well below cooking temperature which is introduced into the bottom of a continuous digester in order to cool the pulp prior to its being blown from 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 are M&D and Pandia digesters used?
These types of digesters are used to pulp wood fines and sawdust that would be otherwise difficult to pulp.
Sample Video Transcript
In some batch digesters, the chips and a liquor are heated by steam which is injected directly into the closed digester. Additional steam is injected as needed into the digester during cooking to maintain the temperature. In other batch digesters, liquor is withdrawn through screens within the vessel, heated externally in heat exchangers and then returned to the digesters during the heating and cooking phases. This technique has the advantages of not diluting the liquor with condensate, recovering the condensate from the heat exchangers for reuse and some improvement in the uniformity of cooking conditions due to the circulation of the liquor.
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