Continuous Digester

The continuous digester was developed to improve the large discharge of volatile organic compounds in the waste liquid (black liquor) generated after digestion in the conventional batch-type digester. A substantial energy saving was achieved as a result of the development of a large-capacity treatment process equipped with a heat recovery system in this continuous digestion process. Continuous digester not only saves energy but also contributes to the expansion of production capacity and improves the working environment by reducing volatile organic compounds (VOC). The technology is suitable for upscaling the digester and for use of Conifers as raw materials (NEDO, 2008. p. 157).

There kinds of continuous digesters have been developed:

  • Modified continuous cooking (MCC) digesteris one where cooking is performed with dispersed charging of the white liquor (cooking chemicals) in the middle and lower zones of the digester to realize a countercurrent flow with the raw material flow.
  • Isothermal continuous cooking digester is one where cooking is performed uniformly by raising the temperature in the washing zone at the bottom of the MCC type digester to maintain isothermal conditions in the digester as a whole.
  • Low solid fraction continuous cooking digester has reduced dissolved lignin concentration in the second half of cooking and reduced total alkaline concentration, achieving a selective increase which improves the high sulfide digestion method in the initial stage of cooking (NEDO, 2008. p. 157). - See also Lo Solid Continuos Cooking System
Development Status Products

Continuous DigesterCosts & Benefits

Parent Process: Chemical Pulping
Energy Savings Potential

For Broadleaf trees, steam savings can be by 0.40 t-steam/t-pulp compared to the batch digestor. For Conifers, steam savings can be as high as 0.70 t-steam/t-pulp.

CO2 Emission Reduction Potential

Equipment cost of digester is about ¥3,700 million. Construction cost is estimated as ¥1,000 million approximately. If the elimination of the need for downstream process equipment for recovery of foreign matter and operating energy for such equipment, the payback time of the investment may become less than 10 years.

Continuous DigesterSchematic

Continuous Digester Publications

Global Warming Countermeasures: Japanese Technologies for Energy Savings / GHG Emissions Reduction

This revised 2008 version of the publication from New Energy and Industrial Technology Development of Japan includes information on innovative Japanese technologies for energy efficiency and for the reduction of COemissions.  

Page Number: