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        Last update : September 02, 2003
                  Second Session : The Yield Issue                  
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            9 - Existing pulping alternatives to improve yield and benefits            
M. Sundar, M. Muguet, M. Epiney et G. Homer (Air Liquide)

Logo EFPGIn the Kraft cooking process, two chemicals, namely sodium hydroxide and sodium sulfide, are used to delignify the wood chips. During the course of the reaction, the lignin part which represents some 20-25 % of the wood is solubilised and removed. However, cellulose and hemicelluloses, which are desirable materials, are also attacked. Theoretically, it should be possible to fully retain cellulose and hemicelluloses. The weight contribution of these components varies with each wood species but is usually around 70 %. However, in an industrial kraft cooking process, the amount retained is more in the order of 45-50 %. Typically, 85 % of the lignin, 50 % of the hemicelluloses and 10 % of the cellulose are removed. The hemicelluloses are easily attacked since they are low molecular weight sugars that are more accessible than crystalline cellulose. Hence, one of the goals sought by the industry during cooking and bleaching, is to protect this fraction in order to achieve a better yield (mass output/mass input).

Several opportunities to achieve higher pulp yields are commercially feasible with certain restrictions and limitations based on the selectivity. A major driver for process improvement to enhance pulp yield are average wood cost, pulp production and return on capital spending. The buzzword in today's competitive pulp market environment is low capital spending. It's always anticipated high pulp prices compared to the current trend will prevail making yield improvement processes attractive to pulp producers. However, with diminishing supplies and increasing raw material cost, pulp producers in certain regions of the world are showing more interest to adapt technologies to improve yield and reduce wood compared to increasing pulp production.

Among the possibilities available today to improve yield are the addition of AQ and modification of the cooking sulfide liquor into polysulfide. Air Liquide contributions in analyzing the impact of such modifications are reviewed in this paper along with economic benefits and process impacts on the recovery boiler.


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