Batch Digester Modifications

It may not be operationally efficient to change to larger batch digesters for the smaller mills. Moreover smaller and speciality mills produce variety of pulp that are less suited for continuous digesters. In such cases, several approaches can be considered to reduce energy consumption in batch digesters, such as indirect heating and cold blow.

In indirect heating, cooking liquor is withdrawn from the digester through a center pipe, pumped through an external heat exchanger, and returned into the digester at two separate locations in the vessel, thereby reducing direct steam loads.

In cold blow systems, hot spent pulping liquor is displaced from the digester contents using brownstock washer filtrate at the end of the cooking cycle. Heat is thereby recovered from the spent liquor for heating subsequent cooks, leading to reduced steam requirements for heating the digester contents. Recovered black liquor can be used for preheating and impregnating incoming wood chips or for the heating of other process inputs, such as white liquor or process water (Kramer et al., 2009. p.91).

US flag In the US, cold blow techniques are regarded technically applicable for all kraft pulping, both for softwood and hardwood. The only negative feature is the cost of complete replacement of the existing digesters. Replacement normally cannot be justified unless a production increase or the condition of the present equipment motivates complete replacement of existing digesters (NCASI, 2001. p.62).

Development Status Products
Chemical Pulp

Batch Digester ModificationsCosts & Benefits

Parent Process: Chemical Pulping
Energy Savings Potential

Energy savings of indirect heating may amount to 3.17 GJ/t (Kramer et al., 2009. p.91).
Energy savings of around 2.3 GJ/ADT pulp is estimated with the use of cold blow systems (NCASI, 2001. p. 62).

CO2 Emission Reduction Potential

CO2 emissions saving by 105.6 kg/t is estimated (Martin et al., 2000. p.21).


Indirect heating increases operational and maintenance costs.
For a 1000 tpd plant, reduced energy consumption enabled by cold blow system is estimated to result in savings of $2 million/y. However, capital costs for additional equipment (i.e., additional pumps and accumulators for the recovered black liquor) are quite high for this measure.(Kramer et al., 2009. p.91).

Batch Digester ModificationsSchematic

Batch Digester Modifications Publications

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